Control metal properties

For any component or joint we want to select materials that have specific metal properties and we want to use manufacturing processes that are capable of transforming a material into the desired shape with the desired properties.

This lesson teaches the relationship between the properties of a metal and its composition, microstructure and the manufacturing processes used to form components and joints. Obtaining the desired properties in a metal requires understanding these factors and their interactions.

The concepts and information presented in this lesson applies to all metals.

Click here to watch the lesson

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In metallurgy, the term phase is used to refer to a physically homogeneous state of matter, where the phase has a certain chemical composition, and a distinct type of atomic bonding and arrangement of elements. Within an alloy, two or more different phases can be present at the same time. The images below show the phases in aluminum-copper and iron-carbon alloys.

Al2Cu precipitates in an aluminum matrix. © DoITPoMS Micrograph Library, Univ. of Cambridge
Ferrite (white) and cementite (dark) in steel.

Each phase within an alloy has its own distinct physical, mechanical, electrical, and electrochemical properties. For example, in carbon steel, ferrite is a relatively soft phase and cementite is a hard, brittle phase. When they are present together, the strength of the alloy is much greater than for ferrite and the ductility is much better compared to cementite. Thus, an alloy with more than one phase can be considered to be a composite material.

Want to learn about steel phase diagrams? See our steel metallurgy courses

The phases present in an alloy depend on the alloy composition and the thermal treatment to which the alloy has been exposed. Phase diagrams are graphical representations of the phases present in a particular alloy being held at a particular temperature. Phase diagrams can be used to predict the phase changes that have occurred in an alloy that has been exposed to a particular heat treatment process. This is important because the properties of a metal component depend on the phases present in the metal.

Phase diagrams are useful to metallurgists for selection of alloys with a specific composition and design and control of heat treatment procedures that will produce specific properties. They are also used to troubleshoot quality problems.

Need help engineering the metals in your product? We provide metallurgy consulting to help select alloys for components.

Iron-Carbon Phase Diagram
An example of a commonly used phase diagram is the iron-carbon phase diagram, which is used to understand the phases present in steel. The amount of carbon present in an iron-carbon alloy, in weight percent, is plotted on the x-axis and temperature is plotted on the y-axis. Each region, or phase field, within a phase diagram indicates the phase or phases present for a particular alloy composition and temperature. For the iron-carbon phase diagram, the phase fields of interest are the ferrite, cementite, austenite, ferrite + cementite, ferrite + austenite, and austenite + cementite phase fields.

The phase diagram indicates that an iron-carbon alloy with 0.5% carbon held at 900 °C will consist of austenite, and that the same alloy held at 650 °C will consist of ferrite and cementite. Furthermore, the diagram indicates that as an alloy with 0.78% carbon is slow cooled from 900 °C, it will transform to ferrite and cementite at about 727 °C.

Fe-C_PhaseDiagram

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Aluminum-Copper Phase Diagram
Another commonly used phase diagram is the aluminum-copper phase diagram, which is useful for understanding precipitation strengthening in Al-Cu alloys. The amount of copper present in an alloy is plotted on the x-axis. The phase fields of interest are the Al, θ, and Al+θ phase fields on the left hand side. For precipitation strengthening an Al-Cu alloy, this phase diagram indicates the minimum temperature to which an alloy must be heated to put all the copper in solution. This is indicated by the solvus line on the phase diagram. The maximum amount of copper that can contribute to precipitation strengthening is indicated by the maximum amount of copper (5.45 %) that can go into solid solution in the aluminum.

Al-CuPhaseDiagram

Equilibrium Conditions
Phase diagrams indicate the relationship between the phases present, alloy composition, and temperature under conditions of slow heating or cooling. Slow heating or cooling allows the atoms within a metal to move around so that the alloy is at equilibrium. However, with many heat treatment processes, a metal is exposed to fast heating and cooling. Under these conditions it is possible to have phases missing or present compared to what is indicated by the phase diagram. Therefore, it is also important to understand the kinetics of phase transformations, i.e. the effects of temperature, time, cooling rate, and heating rate on phase changes within an alloy. This will be a topic of another article.

You can learn more about how to read and use phase diagrams in a few of our courses. Metallurgy of Steel and Metallurgy of Steel Heat Treating teach about the iron-carbon phase diagram. Metallurgy of Precipitation Strengthening teaches about the aluminum-copper phase diagram.

This interview is with Rich Nielsen, Director of Engineering at IMS Buhrke-Olson, a metal stamping and assembly company. We discussed various aspects of stamping engineering. The interview is about 17.5 minutes long.

During the interview we discussed:

Rich Nielsen (left) and Mike Pfeifer

 

Interview transcription

Mike P.: Good afternoon. Hi, Rich.

Rich N.: Hello, Mike.

Mike P.: This afternoon, I’m with Rich Nielsen. He’s the Director of Engineering at IMS Buhrke-Olson. They are a metal stamper. Rich has been involved with stamping and fabrication his entire life. He worked for his father, in his stamping shop, and he’s been with Buhrke-Olson for a long time, as well. Thanks for joining me, Rich.

Rich N.: Certainly.

Mike P.: My first question is, what processes do you have at Buhrke-Olson?

Rich N.: We have stamping presses from 30 tons up to 600 tons. Almost all of them, with straighteners and feeders, so that we can run continuous from coil. Most, we’ll use a progressive die stamping method to produce a complete part, with every stroke of the press. These are all mechanical presses. They run off of a crankshaft, with a flywheel and clutch and brake.

In addition to metal stamping, where we can produce parts that are complete, we also have several joining methods available. We will do riveting, welding, toxing, staking, as well as resistance welding of studs or nuts to the stampings that we make. In that way, we can make not only stampings, but assemble them together.  Often, we’ll make five to ten metal stampings, and we’ll rivet, weld, spot-weld, whatever, MIG weld, or join them together.

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Mike P.: You work mainly for OEMs?

Rich N.: 90% of our work is automotive, and we work for the tier one suppliers that will deliver their assemblies direct to the assembly plants.

Mike P.: What materials are you typically working with?

Rich N.: Steels, red metals. Mostly cold-rolled steel. It can be anywhere from .1 millimeters thick up to maybe four or five millimeters, depending on the material thickness. They could be ductile steels that are made for forming. For example, draw steels, they sometimes are harder steels to form.

We do some work with red metals. It’s not a majority of our work, but we also do stainless steels and aluminum. We do a fair amount of work in aluminum, as well – cold-rolled.

Mike P.: Are there any materials that are more or less difficult to work with than others?

Rich N.: Sure, depending on what needs to be done to them, what shapes we need to create. In general, the materials that have a higher percentage of elongation are more forgiving, friendlier to the forming process.

Mike P.: What materials are those? This would be like deep draw steel?

Rich N.: Yeah, the deep draw steels. You just have to look at the percent of elongation, and the yield strength versus the tensile strength, and see what forces are required, and how much springback you can expect to see.

(2:46)  Mike P.: Is there sometimes any negotiation between you and the designer, over strength versus elongation?

Rich N.: Yeah. If we see a problem in forming a part out of a certain material with a low elongation, we will ask for and recommend using a steel with a higher elongation. But of course, that sacrifices some strength, and sometimes they’ve already looked at that and decided that they could not use softer material and more forgiving material, because it would have to be thicker, in order to produce the strength that they need.

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Mike P.: What ends up happening, then?

Rich N.: We have to either change the design to be formable, to be feasible, or in some cases, it’s adding to the design. In some cases, it may be introducing additional offline hits, in order to complete the part, because we would not be able to complete it in the progressive die.

(3:55) Mike P.: This gets to the having to do extra steps that might add to the cost of the part, because the part is not easily manufactured out of the material that’s been selected.

Rich N.:  What we’ve run into in the past year or so have been more and more parts to be made out of high strength and extra high strength materials. These are new products, that we’re not familiar with, so we’re in a learning curve. And often, I can tell that the designers designed it just as though it’s cold-rolled steel or highly formable stuff, and it’s not. Then, we’re all going through a learning curve.

We rely on forming simulation software, as much as possible, but that’s difficult to do, because in order to get a really true simulation, we need to design the tooling. That requires a commitment, and some time and investment right off the bat, before you can even really try it out and see what you can get. We have some forming simulation software that – I call it just a one-shot. It’s not for a multi-stage progressive die type simulation. That tells us some things, right off the bat.

But the problem we have is that in order to really determine if we can successfully make the part, we either have to build a tool and try it, or we have to design a tool, and run those stations and the tools through a simulation that’s capable of doing that high level multi-step processing of the simulation. We don’t have that software. It’s very expensive

But I can outsource it. In some cases, some our tool shops, our tool vendors may have it, and they will do some of that for us. But that’s presenting a problem, because in the case of dual-phase steel, this stuff is designed to harden as it’s formed, so that it absorbs energy in a crash. Well, that’s what we’re doing when we’re forming it. We’re crashing it. We’re crushing it into a different shape.

So, it’s resisting being formed, and it’s absorbing a lot of energy and giving us a lot of springback, so it becomes very challenging to make parts using that material.

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(6:14) Mike P.: So, someone has to make a commitment to you, for you to be able to get the tooling to do the experiment, so it’s not risk-free for them either then, is it?

Rich N.: Correct. What we’ve done in some cases, when it’s possible, we have made some temporary tryout tools, to just trying forming it, either just in a small press, or even a screw press or something like that, or a hydraulic press, so that we can actually measure the springback in certain conditions. We’ve had to introduce a lot of tricks to fight the springback, and to accommodate that.

We’ve found that for these really super high strength steels, that 90 degree bends are hard enough. But when it’s a complex drawn, highly complex geometry, it’s nearly impossible to predict. We’ve had to add, for example, ribs on a formed flange of a part, in order to form it without having it turn into a potato chip.

This part here is very recent. This is dual-phase 980 steel, very tough stuff. High springback, real high springback. You can kind of see what’s happening along this drawn edge. You see the wrinkling that’s happening there, and the stress and the high pressure marks. What this did was – yeah, we can form it. It doesn’t really come out to a full straight 90. There’s too much springback.

But the result of trying to form that creates an unflat condition, not flat condition, on this     surface, which they need that flat. So, we ended up actually having to put ribs all along here, to gather up some material. It wants to wrinkle anyway, so what we did was we just gave it a place to wrinkle to, and control that, but that took so much extra time and effort and money that was just not at all something, that was planned for.

8:17  Mike P.: If the design engineers had come to you earlier in the project, could some of the problems been avoided?

Rich N.: Yes. We’ve had other customers that have come to us and said “We know this is going to be a tough job, so what do you think? What can we do? What can you hold? What kind of tolerances? Where do you need to open up tolerances, and where can we keep them tighter, and how can we achieve this? Would you recommend any changes?”

This was one that was kind of dropped in on us. They were already months late, so there was no time to do anything, much less have a reasonable discussion. I think one of the biggest things that you need, one of the most important things that’s needed for this kind of part with this kind of material, is an expectation that we may have to change the part, and time to do that. That’s what’s sorely lacking, many times.

(9:10) Mike P.: What’s the typical process for a customer working with a company like yours?

Rich N.: We’re tier two automotive. Our customers are tier one. They’re the ones that have designed the seatbelts or the airbags, or the seat construction , or whatever it is – radiators or whatever the component is. They’re the ones that design that whole component, to be ready at the assembly line, for the car manufacturers. That’s tier one.

Then, they will design all their parts. Then, they have to go to their supply base, which is us, for stamped metal parts, or another supplier for plastics, electronics, whatever it is. They have to contract us to design and build tools to make those parts, and to make those components for them. So, we work directly with those tier one guys.

In some cases, they have good experience in metal forming, and they know where the pitfalls might be. In other cases, they have no clue.

In the best case scenario, they have designed the assembly and the components they want us to make, and they come to us early enough to have us have face-to-face or online evaluations with their designers and their engineering team, to identify and address potential issues with forming the part, like I talked about. If that’s done early enough in the whole process, then there’s time to address those things, and they’re willing to listen, either because they’ve had prior experience and they know they’ll do better if they listen, or because they know their knowledge of it is not as deep as ours.

(10:57) Mike P.: What about the case when someone doesn’t ask for your help or input?

Rich N.: We give them our input, whether they ask for it or not. What happens is when we get a proposal and we have to quote on it, and it looks like it’s getting serious, then we will mark up drawings or prepare a PowerPoint presentation, with views of the part, and maybe some data from a simple forming analysis, a forming simulation, and let them know, or try to give them the data behind why we think there would be a problem in a certain area, why we need to change something, or why we need more tolerance for one item or another.

We’ll do that, regardless of whether we’re asked, just because it gives us a leg up on it. It doesn’t always change their minds. It depends on who you’re talking to. Sometimes we’ll just hear “This is what we need. We have to have it.”

(11:55) Mike P.: In that case, what happens?

Rich N.: If it’s a serious enough problem, we may back away and say “We’re just not capable of making the part the way you want it.” Most often, what will happen is we agree that we will get as close as we can, and we’ll discuss it again after we get the tool made, and we’ve fine-tuned, we’ve debugged the tool to the extent possible. Then, we’ll see them.

“We can’t hold the tolerance you did want, but we can hold this. How about this?” Then, we’ll negotiate a new tolerance, on the basis of that. If everyone is working in good faith, that works okay.

( 12:40) Mike P.: It’s interesting that you mentioned the thing about having to back away from a project, because the ease of fabrication is just not going to be there. When you’re quoting things, and you’re assessing the materials that have been selected and the design, does ease of fabrication influence what you’re going to end up charging per part?

Rich N.: Yes, it does, insofar as we will have an extra charge, or will include extra money, if we know there is going to be a high degree of development, a lot of time to develop the tool, a lot of trial and error, or a lot of outside expense for either making prototypes, trial and error that way, or outside expense for the high level of forming simulation that we might have to outsource.

Another thing that we may do is we try to quote parts as often as possible, on what we call “prog complete.” That is out of a progressive die with many, many stations, that’s progressively pierced and formed and crushed into shape, and finally the part is complete, at the end of the tool. That’s the most cost effective way to make these parts. That way, you’re really making best use of the economy of metal stamping by progressive die method.

If we think there’s a chance that we might not be able to get that tolerance or that bend, or whatever it is, we may include a secondary offline re-strike of the part, which of course, is physically handling it in and out of the tool much slower than getting it complete, off of the progressive die. In that case, we would have the cost of the progressive die, the cost of running it and all the materials, but we would also have the cost of an additional step or multiple steps, that may be needed to complete the part.

If there’s no budging on it, we would have to either – like I said – back away, or include extra steps, to try to make it. We have to be careful what we promise.

(14:36) Mike P.: Does this apply to all materials?

Rich N.: Some materials are more friendly to forming than others, so we really have to watch closely what materials are selected by the customers, and another suggestion might be a different material.

(14:54) Mike P.: Different materials have different ease of fabrication. There’s a tradeoff between strength and ductility or elongation, and also there’s work hardening, too, because different materials work harden at different amounts. Some materials are very easy to work with, because they don’t work harden a lot. They have high ductility. Other materials are more difficult. It could be because of their strength, or it could be because of their composition.

Rich N.: Yeah. And these materials, they’re designed and created for certain purposes, so they suit a certain function. But that doesn’t mean they’re easy to work with.

15:32 Mike P.: Yeah, it’s important for designers to think about not just the mechanical properties for a component’s application, but also think about ease of fabrication, regardless of the fabrication processs. From a project management perspective, it’s important to have discussions with a stamper early in the design process. That can have a big impact on preventing problems and mitigating risk.

Rich N.: Unfortunately, I don’t have an engineering degree. I didn’t go to mechanical engineering school, but I’ve been told that in those courses, in that course of study, they may spend a month or something covering metal stamping. Of course, they’re going to be exposed to some basics, some very basic knowledge. But if you try to design stamped metal parts with only that kind of exposure, you’ll be very limited and you could quickly get yourself into trouble. As opposed to – I’m an old guy. We used to have high schools where you’d make some stamped metal parts. You’d do some casting, you’d mold some plastic and weld some parts.

So, there was a little more hands-on understanding of what was involved in some of those processes. So, I think a little clearer understanding of the limitations, as well as the capabilities, often that’s where we have to fill in the gap.

(16:50) Mike P.: I think that’s where it requires the designer understanding the limitations, or their limitations in terms of knowledge, and reaching out to you or to whatever stamper they’re working with, to get the input required, in order to make sure that things go more smoothly. We can’t be experts in everything.

Rich N.: No, we certainly can’t. Right. So, we have the best results when we do just like you say. We have a good conversation and understanding of what the challenges will be on a particular part.

(17:26) Mike P.: Alright.  I think that’s it, Rich.  Thanks a lot for meeting with me. That was a great discussion.

Rich N.: You’re welcome. I enjoyed it, too. It was good talking to you again.

 

This interview is with Mike Connelly, retired VP of Engineering and Quality at Casey Products, a fastener distributor. We discussed various aspects of fastener engineering and fabrication.

During the interview we discussed:

Mike Connelly (left) and Mike Pfeifer

 

 

Interview transcription

Mike P.: Today, I’m with Mike Connelly. Mike is retired now. He worked at Casey Products for 27 years. He was the VP of Engineering and Quality. Good afternoon, Mike.

Mike C.: Good afternoon.

Mike P.: Thanks for joining me. Today, Mike is going to talk about fasteners. What types of fasteners did you work with?

Mike C.: A huge variety of it. We worked with machine screws, tapping screws. There’s also hex head bolts, or hex head screws. Generally speaking, the difference between a bolt      a screw is a bolt takes a nut. A screw will go into a blind hole, like a casting. There’s also rivets, spring clips, Tinnerman nuts, Spiralock product, pop rivets, aluminum pop rivets, made out of a wide variety of materials.

Mike P.: What are the most common alloys that you worked with?

Mike C.: As a fastener distributor, we touched on a broad base of alloys; low carbon steels, low alloy steels, stainless steels, super alloys. The most common alloys we worked with are low carbon steel; 1008, 1010, 1020. You know, alloy steels, such as 4037, 4140, 8640, 15B41. Those are the big materials.

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Mike P.: How are machine screws fabricated?

Mike C.: What you’re going to have is material that looks like wire. If you’ve ever been on the highway     you see these giant coils of steel that look like giant slinkies, that’s usually fastener wire. The first step will be to do a spheroidized anneal on it, to get the material with the proper grain size and a proper carbide distribution, so that they can form the material.

After that’s done, they’ll pull that through a straightening die set, that brings it to its final size. Then it goes through a heading operation. A heading operation is described as it’s a cold forging operation that puts the head of the fastener on, and it also reduces the shank of the bolt to the pitch diameter, so that they can roll the threads.

Now, on a machine screw, all of the strength comes from this cold-working process. They’re not hardened, they’re not heat treated, they’re not a class fastener. That is something that’s going to go into hex bolts. Then, they’ll run them through thread dies. Another way of doing this is to have a bolt maker, where the thread rolling process and the heading operation are all In one machine. It comes out, you have a completed part, ready to be packed up and shipped.

Mike P.: So, the final microstructure is spheroidized steel that’s been cold-worked?

Mike C.: Yes. If you were to do a cross section on that, you would see the deformation from the thread rolling process, as the material flows into that thread shape.

Mike P.: They’re relatively low strength screws.

Mike C.: Yes.

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Mike P.: How are the self-tapping screws made, and what alloys are used for that?

Mike C.: Self-tapping screws will be heat treated, because the screw has to be harder than the material that it’s going into. It pretty much follows the same processes, in that you’re going to be using – let’s say a 1020 steel, a little more carbon. It gets spheroidized annealed, it gets drawn to size. Then you get the heading operation, the trimming operation, and then they roll the threads.

The pitch on the thread, it’s a larger pitch. There’s more spacing between the threads. After the thread form is made, then these will go out to heat treat, and they’ll either be carburized and hardened, or carbonitrided and hardened. The material will become harder, and it will reach a hardness of somewhere around 50-55 Rockwell C. So, we have a hard skin, and underneath we’ll have a tough core.

Mike P.: How are larger fasteners (bolts) fabricated?

Mike C.: Once you get past like one inch, 1-1/8th inch in diameter, you know, 30 millimeters – when you get above that, you start going into the realm of where they’re hot heading the bolt. On a high production run for bolts, the way they’re made is that once again, we get wire. Even though the diameter of the wire could be 30 millimeters, 36 millimeters, it’s stilled called wire.  It goes through a spheroidizing process, to get that cementite in a nice round spheroidal shape, so it takes less energy to displace it. I can use a smaller press or a smaller header.  Then it goes through the drawing process. Then, it will go into a bolt maker. The  bolt maker does the heading operation and the trimming operation   – you know, trim the length.

And the head part is multiple steps, in that you start out with a round button. They heat up the head in a forge furnace or an induction coil, and then they upset the head, to give the head configuration. Then, what they’ll do is machine the washer face on the head and the shank.  Then they’ll machine the threaded end, down to the pitch diameter. Then, depending upon what the requirements are, they may either roll the threads or cut the threads. You want rolled threads.

There’s a set of dies in there, and they’re called flat dies. The flat die rollers will put the threads on. Once again, it’s rolled threads. Now, when they go through heat treating, all the beneficial cold-work stresses that you’ve got, they go away. Once it comes out of that furnace, it goes into the quench.

That as-quenched hardness is going to come out right around 50 Rockwell, 55, somewhere in there. Then, we run it through a tempering furnace.

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Mike P.: Why are rolled threads better than cut threads?

Mike C.: The thread profile, when you roll the thread, is going to be a radius. So, when you get down into the root of the thread, you’re going to see a radius. There are actually thread forms called UNJ or MJ, where that root radius is controlled very tightly, to improve the fatigue life. When you cut the threads, you wind up with a sharp V down at the bottom of that root. and you get some tearing in there, and you wind up throwing material away, because you’re making chips now.

Mike P.: So, cut threads have lower fatigue resistance than rolled threads?

Mike C.: Yeah, they do.

Mike P.: Why would someone get them cut, instead of rolled?

Mike C.: Convenience. If the quantity required is small, if the size of the bolt is large, it becomes easy to just cut the threads in, as opposed to rolling the threads.

Mike P.: It seems that for a fastener that will be used in a fatigue application, even if only a small quantity will be purchased, it’s still worthwhile to have the threads rolled, rather than cut.

Mike P.: What’s the difference between rolling the threads and then heat treating the bolts versus heat treating first and then rolling the threads.

Mike C.: If you make those threads before heat treatment, any of the beneficial cold-work that you’ve got, it goes away once you stick it in the heat treat furnace. If parts are heat treated, and then they’re rolled. Then, you’re looking at bolts where the stress level is going to be 180,000 PSI, or like 1,070 megapascals.

Mike P.: They’re cold-working it after heat treating, to get the additional strength associated with cold-working.

Mike C.: Right, and better fatigue life.

Mike P.: Are there things a designer can do to make sure that they get what they really need for their application?

Mike C.: What I would do is get proficient in standards. Invest some time in looking at the consensus standards, and see how it aligns with what you’re trying to do. Try not to reinvent the wheel.

A good place to go for that is the Industrial Fastener Institute. The IFI has several volumes. One is for metric and one is for inch. It’s the Inch Fastener Standards, and it will have those standards in there. Another good book is An Introduction to the Design and Behavior of Bolted Joints, by Bickford. The other standards I would get close to me are the ASTM standards, SAE, the ISO standards. ISO is becoming the predominant fastener standard for metric fasteners.

Take a course. There are folks out there that train people in understanding fastener standards.

Mike P.: What are the common standards?

Mike C.: The common fastener standards will be SAE J429. That’s the grade 5, the grade 8, the grade 2. You also have ASTM standards. Within those ASTM standards, usually they’ll have the stress located in the title for the fastener, so that you can get a grasp of what’s going on.

In the metric world, you’re going to look at ISO or DIN or JIS, which is the Japanese standards, but everybody is going with the ISO. The ISO standards are truly global. Even in the United States, when we make metric fasteners, instead of using ASTM standards or SAE standards, they go right to the ISO, because the product can be used worldwide. 

Mike P.: What do you recommend as the being the process for designing a fastener and what should be included in the specification for the fastener?

Mike C.: Land on a standard. If you’re going to stay specifically to sourcing here in the states, then go to SAE J429, if you’re using that type of thread.  and that will define what the hardness should be, what the material should be. In material selection, there will be more than one type of steel that you can pick from. The testing requirements will be in there.

If you’re doing metric, go with the ISO standards, and become proficient in them, and look at all the fine print in those things, for exclusions.

Understand your alloy systems. If you’re going to make a hex head cap screw out of Inconel 718, understand what that alloy is. It’s expensive to begin with, and it’s not like an SAE Grade 8. Because it’s 718, it’s going to be difficult to roll the threads. The setup time is going to be amortized over those 12 pieces. If it’s an aircraft part, you’re not going to cut the threads.

You have to understand those things during the quoting process, and when you’re putting that on there. I’ve seen where an Engineer will come back and we would send a price in. “Okay, they want 50 pieces,” or something like this, and they’re going to cost $50 apiece. The design review Engineer goes crazy, because “Oh my God! That’s just too expensive!”

 Mike P.: What are the problems with not referencing a known standard, in a fastener specification?

Mike C.: That’s a great question, Mike, because if I’m doing an incoming inspection, and I have on this print, let’s say a 1/2-13 thread on a hex head cap screw, and I want it hard 33 to 39 Rockwell C. Well, there’s nothing in there about what the required stress is, or the tensile test. There’s no decarburization limits that are in there. There’s nothing in there about thread laps, what kind of surface imperfections.

There’s nothing about the material composition. It’s just “Okay, make it out of this stuff, and I want it this hard.” Well, that’s what you’re going to get, then. If you take the standard, you’ve got all of those things are going to be covered in that standard.

The other thing that comes into play is if it’s a standard fastener, the cost is going to be lower. The availability is going to be there, because there are distributors all over the place. You should always start with a standard.

Mike P.: Use the standard as a base, and just modify that standard as needed.

Mike C.: Yes.

Mike P.: What happens when a customer has a problem with a fastener? Let’s say it fails during manufacturing or fails during testing, or during use. What would help the root cause analysis process go more smoothly?

Mike C.: Number one, go pull the blueprint. Pull the engineering standards. Get the print, and articulate the problem as best you can. You would have to get a really good definition of the problem, before you can proceed.

I would actually make a list of questions on my callback. “Okay, you broke this while it was being assembled. What was the torque that you used? Are you lubricating the bolts? Did you do a hardness check? How did you check the hardness?” As much as hardness testing is a very popular test, and it’s easy to do, a lot of people do it wrong.

Mike P.: Does it help to have pictures?

Mike C.: Absolutely. The most important photo you can get when you’re looking at a potential failure, is of the head marks. On the head mark, you’re going to see the manufacturer, you’re going to see the class. You’re going to get a grasp of what that bolt is supposed to be. You can’t send too many pictures.

I can actually sit there and count the threads, and see “Do we have the right pitch? Did somebody put a metric bolt in, where it’s supposed to be an English bolt?”, because there’s a couple of sizes that kind of overlap, like 3/8” and ½”, and 10 millimeter and 12 millimeter.

Also, looking at the fracture surfaces, If it’s a ductile fracture, I’m already looking at it’s either not heat treated properly or they’ve overloaded it.  The other thing is where did it fail? Did it fail in the head? A fastener should never fail in the head. It should always fail in the threads.

Mike C.: Yeah, and for us, we’d ask for some samples.” We can tensile test, look at the microstructure, check the dimensions. We can do all of that stuff.

Mike P.: Well, I think that’s it. Thanks, Mike, for giving your time and sharing your expertise.

Mike C.: Thank you for having me.

Abstract: Annealing is a heat treating process used to modify the properties of cold-worked metal. This article discusses the reasons for annealing, the metallurgical changes that take place within a metal during cold working and annealing, the effects of these metallurgical changes on the properties of metals, and the effects of annealing temperature and time on the final microstructure and properties of annealed metals.

Many metal fabrication processes involve cold-working, such as cold rolling sheet and plate, wire drawing, and deep drawing. Due to metallurgical changes that occur to a metal during cold working, the ductility of a metal decreases as the amount of cold-working increases. There comes a point when additional cold working is not possible without causing the metal to crack. At this point, it is necessary to anneal the metal if continued cold-working is required.

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During an anneal, metallurgical changes occur that returns the metal to its pre-cold-worked state. These changes result in a reduction of the metal’s yield and tensile strength and an increase in its ductility, enabling further cold working. In order for these changes to occur, the metal must be heated above its recrystallization temperature. The recrystallization temperature for a particular metal depends on the metal’s composition. This specific annealing process is sometimes called a recrystallization anneal, though other names like process anneal are also used.

Metallurgical effects of cold working
During cold-working there is an increase in the number of dislocations in a metal compared to its pre-cold-worked condition. Dislocations are defects in the arrangement of atoms in a metal (discussed in Principles of Metallurgy).

The increase in the number of dislocations causes a metal’s yield and tensile strength to increase and its ductility to decrease.  After a certain amount of cold work, a metal cannot be cold worked further without cracking.  The amount of cold working that a particular metal can withstand before cracking depends on its composition and microstructure.

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Metallurgical effects of recrystallization anneal
During a recrystallization anneal, new grains form in a cold-worked metal.  These new grains have a greatly reduced number of dislocations compared to the cold-worked metal.  This change returns the metal to its pre-cold-worked state, with lower strength and increased ductility.

With continued time at the annealing temperature, some of the newly formed grains grow at the expense of neighboring grains.  There is some further decrease in strength and increase in ductility as the average grain size increases during the grain growth phase of the annealing process.

The final grain size depends on the annealing temperature and annealing time.  For a particular annealing temperature, as the time at the temperature increases the grain size increases.  For a particular annealing time, as the temperature increases the grain size increases.  A piece of metal with large grains has lower strength and more ductility than a piece of metal of the same alloy with smaller grains.

The figure shows micrographs of a brass alloy that was cold-rolled to 50% of its original thickness and annealed at two different temperatures.  Figure (a) shows the microstructure of the cold rolled sample.  Figure (b) shows the microstructure of a sample that was cold rolled and then annealed at 1022 °F (550 °C) for 1 hour.  Figure (c) shows the microstructure of a sample that was cold rolled and then annealed at 1202 °F (650 °C) for 1 hour.

The cold-rolled sample had a yield strength of 80 ksi (550 MPa).  The sample that was annealed at 1022 °F (550 °C) for 1 hour had yield strength of 11 ksi (75 MPa).  Many small grains are present in this sample.  The sample that was annealed at 1202 °F (650 °C) for 1 hour had yield strength of 9 ksi (60 MPa).  Fewer, large grains were present in this sample compared to the sample in Figure (b).

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Other reason for recrystallization anneal
In addition to enabling additional cold-working, recrystallization annealing is also used as a final processing step to produce metal sheet, plate, wire, or bar with specific mechanical properties.  Control of the annealing temperature and time, heating rate up to the annealing temperature, and amount of cold-working prior to anneal is important for obtaining the desired grain size, and therefore the desired mechanical properties.

Rockwell and Brinell hardness tests are common metal characterization methods used to determine whether metal stock or a metal component has the required properties. The reason for this is that these tests are simple and quick to perform, in addition to being inexpensive. However, while these tests do provide useful information, there is a danger to the common practice of specifying only hardness and alloy composition on component design drawings.

There are circumstances where a metal can meet the composition and hardness requirements, and still be unsuitable for use in the intended application. This can occur when the metal microstructure is deficient in a way that is undetectable by Rockwell or Brinell hardness testing.

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ferriteatgbsConsider the microstructure of a steel component. The microstructure consisted of martensite, with ferrite on what were austenite grain boundaries (arrows) before the austenite transformed to martensite. This was the result of improper quenching during heat treatment. The sample had a hardness of Rockwell C 38. However, the ferrite on the prior austenite grains compromised the mechanical properties of the metal by allowing a fracture path through the relatively soft ferrite. Without specifying that the microstructure should be completely martensitic, the component designer risked using components that met the spec, but were potentially unreliable.

Other examples where it is possible to have a Rockwell or Brinell hardness value that meets specification, but still have a component that is unreliable or unacceptable are:

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The main thing to take away from this discussion is that specifying composition and hardness is often not enough. In many cases, it is important to specify the microstructure of stock metal or a component to improve the likelihood of consistently getting the desired metal properties. The importance of this increases as the expected component performance and reliability increases. Other analyses, such as tensile testing may also be required.

More information about the effects of microstructure on properties and the effects of processing on microstructure is in our Principles of Metallurgy, Metallurgy of Steel Heating, and Corrosion of Metals courses. There are also many books that discuss the relationship between processing, microstructure, and properties.

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Residual stresses are locked-in stresses within a metal object, even though the object is free of external forces.  These stresses are the result of one region of the metal being constrained by adjacent regions from expanding, contracting, or releasing elastic strains.  Residual stresses can be tensile or compressive.  In fact, tensile and compressive residual stresses co-exist within a component.

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Causes of residual stress

Residual stresses arise whenever a component is stressed beyond its elastic limit and plastic deformation occurs.  Plastic deformation occurs when the stress exceeds a metal’s yield strength (discussed in Tensile Testing).  This can be as a result of...

This article discusses the first three causes.

Residual stress due to temperature variations

In parts cooled from elevated temperatures, residual stresses are caused by temperature variations in the metal during cooling.  Cooling from elevated temperatures occurs during heat treating and welding.

Temperature variations in a metal during cooling from an elevated temperature result in localized variations of the amount of thermal contraction.  Thermal contraction develops non-uniform stress due to different rates of cooling experienced by the surface and interior of the metal.  During cooling, the outer portion of a component cools first and that portion of the metal contracts, compressing the hotter inner metal.  As the inner portion of the component cools, the metal tries to contract, but is constrained by the already cooled outer portion.  Consequently, the inner portion will have a residual tensile stress and the outer portion of the component will have a residual compressive stress.

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Residual stress due to phase transformation

A phase transformation is a change in the metallurgical phases present in an alloy.  For example, the transformation from austenite to martensite in steel during through hardening is a phase transformation.

Residual stresses that arise during a phase transformation are due to the volume difference between the newly forming and initial  metallurgical phases.  The volume difference causes expansion or contraction of the metal.

For phase transformations that occur during cooling from an elevated temperature, such as in steel, the outer portions of the metal cool first and undergo the phase transformation first.  If the volume of the new phase is different from the volume of the initial phase, then the transformed volume of metal will change as the new phase forms.  As the interior of the metal cools it will also try to increase or decrease in volume.  However, the volume change of the metal interior will be constrained by the cooler outer layer of metal that has already transformed.

When the volume of the new phase is larger than the volume of the initial phase, the center portion of the component will be under compression and the surface will be under tension.  When the volume of the new phase is less than the volume of the initial phase, the center portion of the component will be under tension and the portion of the metal at and near the surface will be under compression.

For example, during through hardening of steel during a quench, austenite transforms to martensite, with the martensite having a volume that is about 4% greater than austenite.  During the quench, the steel at the surface transforms to martensite first since the surface cools the fastest.  As the metal at the interior continues to cool, it transforms to martensite.  However, its volume expansion is restricted by the hardened, cooler surface layer. This restraint causes the interior to be under compression and the outer surface under tension (see Metallurgy of Steel Through Hardening).

In some conditions, the volume changes can produce residual stresses large enough to cause plastic deformation, leading to component warping or distortion.  With severe quenching the quenching stresses can be so large that they cause cracking.

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Residual stresses caused by mechanical treatment

Residual stresses also arise when plastic deformation is non-uniform through the cross-section of an item being deformed such as during bending, drawing, rolling, and extruding.  When a metal undergoes plastic deformation, a portion of the deformation is elastic (discussed in Tensile Testing).  After the load causing the deformation is removed, the metal tries to recover the elastic portion of the deformation.  However, the elastic recovery is incomplete because it is opposed by the adjacent plastically deformed material.

Consider a metal item that has been bent.  Regions adjacent to the bend will have been only elastically deformed, and this region will try to recover, a phenomenon known as springback.  After removing the external force, the regions which have been bent prevent the adjacent regions from undergoing complete elastic recovery to the non-deformed condition.  These regions are left in a state of residual tension and the regions which were plastically deformed are in a state of residual compression.

In general, the sign of the residual stress produced by non-uniform deformation will be opposite the sign of the plastic deformation which produced the residual stress.

Effects of residual stress

Residual stresses can be beneficial or detrimental, depending on whether the stress is tensile or compressive.  Tensile residual stresses can be large enough to cause component distortion or cracking.  Also, fatigue and stress corrosion cracking require the presence of tensile stresses.  Because residual stresses are algebraically summed with applied stresses, surface residual tensile stresses combined with an applied tensile stress can reduce the reliability of components.  In fact, a residual tensile stress is sometime sufficient to cause stress corrosion cracking.

Surface residual compressive stresses are generally helpful because they reduce the effects of applied tensile stresses. In most cases, surface compressive stresses contribute to the improvement of fatigue strength and resistance to stress-corrosion cracking.

Controlling residual stress

Controlling the type and magnitude of residual stress is important for applications in which components will be exposed to fatigue or stress corrosion cracking conditions or if the residual stresses are large enough to cause component deformation or cracking.  This can be achieved through mechanical treatment, stress relief heat treatment, control of heat treating processes, and alloy selection.

Mechanical treatments such as shot peening, light cold rolling, stretching, and small amounts of compressing are used to intentionally induce a compressive residual stress at the surface of a component.

Because metal yield strength decreases as its temperature increases, metals can be stress relieved by heating to a temperature where the yield strength of the metal is the same or less than the magnitude of the residual stress. At this temperature, the metal can undergo microscopic plastic deformation, thus releasing at least a portion of the residual stress.  After stress relieving, the maximum residual stress that can remain is equal to the yield strength of the material at the stress-relieving temperature.

From a component processing perspective, residual stresses can be minimized by using reduced cooling rates to reduce temperature variations and allow for phase transformations to occur more uniformly throughout a component’s cross-section.  Also, alloys can be selected that alloy for slower cooling rates to be used, while still getting the desired phase transformations to occur.  For example, for carbon steel components to be through hardened, low-alloy carbon steels enable the use of slower cooling rates compared to plain carbon steels.

Measuring residual stresses

X-ray diffraction is used for measuring residual stress nondestructively.  With this technique, strains in the metal’s atomic crystal lattice are measured, and the residual stresses are then calculated based on the strain measurements.

Bibliography

Abstract: Orange peel is a cosmetic defect most commonly found on sheet metal components after forming.  This article discusses the causes of orange peel and engineering approaches for preventing it from occurring.

Orange peel is a cosmetic defect associated with a rough surface appearance after forming a component from sheet metal. It is called orange peel because the surface has the appearance of the surface of an orange.

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During sheet metal forming the individual grains in the metal tend to deform independently of each other. Consequently there are differences in the amount of metal thickening or thinning between neighboring grains and within each grain.  As a result, the grains stand out in relief on the surface. The larger the amount of deformation and the larger the grain size, the more apparent is the effect. With a small-grained material, there is less variation in the amount of thickening or thinning between the grains and the individual grains are too small for the eye to detect the surface variations.

Orange peel grains

These figures show two samples of a sheet metal component.  One component had small grains throughout. The other had large grains at the surface. The component with the large grains at the surface exhibited orange peel.

Orange peel photos

Preventing orange peel

Orange peel can be prevented from occurring by identification of the maximum grain size that is tolerable for a component and implementing measures to prevent the grain size from exceeding the maximum identified.

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Identifying the maximum grain size that can be tolerated without cause orange peel to appear requires engineering studies to compare the effects of different grain size on surface appearance and ability to easily perform the forming operation. This evaluation is required because there are competing requirements for appearance and ease of forming a component as large grained metal is easier to form compared to small grained metal. For many components there is usually a compromise grain size that will optimize formability and cosmetic appearance.  The grain size can be evaluated by cross-section metallography of the different samples according to ASTM E112 Standard Test Methods for Determining Average Grain Size.  The maximum grain size that does not cause orange peel is the desired grain size.

Alternatively, an engineering team can deep draw samples from a few different batches of sheet metal and measure the grain size for a sample from each batch. If there is any variation in grain size between the samples, the maximum grain size that does not cause orange peel is the desired grain size.  With this type of testing, it is possible that there is a larger grain size that can be tolerated than that identified. However, the testing is simplified compared to the first type of study mentioned.

Controlling grain size

Grain size is controlled through a combination of cold rolling and annealing. Obtaining the desired grain size requires proper control of the amount of cold rolling, annealing temperature, annealing time, and cooling methods after annealing (See Principles of Metallurgy). For the example that was shown earlier, the top surface of the sample with orange peel was not properly cooled after annealing, which allowed the grains at the surface to grow too large.

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Controlling grain size also requires specification of the maximum grain size acceptable for sheet metal so as to communicate the requirements to sheet metal suppliers. The grain size specifications can be on the mechanical drawing for a component or on the purchase order for the sheet metal stock. The specifications should include the metal composition, thickness, tensile strength or hardness, grain size, and method for evaluating the grain size.

Abstract: Recrystallization annealing is a heat treating process used to modify the properties of cold-worked metal.  This article discusses the reasons for preforming a recrystallization anneal, the metallurgical changes that take place within a metal during cold working and annealing, the effects of these metallurgical changes on the properties of metals, and the effects of annealing temperature and time on the final microstructure and properties of annealed metals.

Many metal fabrication processes involve cold-working, such as cold rolling sheet and plate, wire drawing, and deep drawing.  Due to metallurgical changes that occur to a metal during cold working, the ductility of a metal decreases as the amount of cold-working increases.  There comes a point when additional cold working is not possible without causing the metal to crack.  At this point, it is necessary to anneal the metal if continued cold-working is required.

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The specific annealing process used is called recrystallization anneal.  During this annealing process, metallurgical changes occur that returns the metal to its pre-cold-worked state.  These changes result in a reduction of the metal’s yield and tensile strength and an increase in its ductility, enabling further cold working.  In order for these changes to occur, the metal must be heated above its recrystallization temperature.  The recrystallization temperature for a particular metal depends on its composition.

Metallurgical effects of cold working

During cold-working there is an increase in the number of dislocations in a metal compared to its pre-cold-worked condition.  Dislocations are defects in the arrangement of atoms in a metal (discussed in Principles of Metallurgy).  The increase in the number of dislocations causes a metal’s yield and tensile strength to increase and its ductility to decrease.  After a certain amount of cold work, a metal cannot be cold worked further without cracking.  The amount of cold working that a particular metal can withstand before cracking depends on its composition and microstructure.

Metallurgical effects of recrystallization anneal

During a recrystallization anneal, new grains form in a cold-worked metal.  These new grains have a greatly reduced number of dislocations compared to the cold-worked metal.  This change returns the metal to its pre-cold-worked state, with lower strength and increased ductility.

With continued time at the annealing temperature, some of the newly formed grains grow at the expense of neighboring grains.  There is some further decrease in strength and increase in ductility as the average grain size increases during the grain growth phase of the annealing process.

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The final grain size depends on the annealing temperature and annealing time.  For a particular annealing temperature, as the time at the temperature increases the grain size increases.  For a particular annealing time, as the temperature increases the grain size increases.  A piece of metal with large grains has lower strength and more ductility than a piece of metal of the same alloy with smaller grains.

The figure shows micrographs of a brass alloy that was cold-rolled to 50% of its original thickness and annealed at two different temperatures.  The figure on the left shows the microstructure of the cold rolled sample.  The center figure shows the microstructure of a sample that was cold rolled and then annealed at 1022 °F (550 °C) for 1 hour.  The figure on the right shows the microstructure of a sample that was cold rolled and then annealed at 1202 °F (650 °C) for 1 hour.

Annealed brass grains

The cold-rolled sample had a yield strength of 80 ksi (550 MPa).  The sample that was annealed at 1022 °F (550 °C) for 1 hour had yield strength of 11 ksi (75 MPa).  Many small grains are present in this sample.  The sample that was annealed at 1202 °F (650 °C) for 1 hour had yield strength of 9 ksi (60 MPa).  Fewer, large grains were present in this sample compared to the center sample.

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Other reason for recrystallization anneal

In addition to enabling additional cold-working, recrystallization annealing is also used as a final processing step to produce metal sheet, plate, wire, or bar with specific mechanical properties.  Control of the annealing temperature and time, heating rate up to the annealing temperature, and amount of cold-working prior to anneal is important for obtaining the desired grain size, and therefore the desired mechanical properties.

Abstract: Metal strength and fracture toughness are important mechanical properties for structural components.  This article explains the trade-offs between strength and toughness and designing for applications requiring high strength and toughness.

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For structural components, strength and fracture toughness are two important mechanical properties.  Yield strength is the measure of the stress a metal can withstand before deforming.  Tensile strength is a measure of the maximum stress a metal can support before starting to fracture. Fracture toughness is a measure of the energy required to fracture a material that contains a crack.

As a metal's yield strength increases, the amount of stress the metal can support without deforming increases.  Alternatively, as yield strength increases, a smaller cross-section of metal is required to support a given load without deforming. As tensile strength increases, the amount of stress a metal can support without cracking and fracturing increases.

As a metal's fracture toughness increases, the energy required to cause a crack to grow to fracture increases. For a component with a crack of a certain length, as the fracture toughness decreases there is a decrease in the component’s ability to support the load without fracturing. Conversely, for a certain load, as fracture toughness increases, a component can tolerate a longer crack before fracturing.

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As shown in the figure, metal toughness decreases as strength increases. For any particular alloy, mechanical and/or thermal treatments are used to modify the alloy's strength. For many alloys, it is possible to use different processes to get different toughness vs. strength curves

Designers are often tempted to use a material that is as strong as possible to enable them to minimize component cross-section. However, this can inadvertently lead to using a material with insufficient fracture toughness to withstand fracturing if a crack forms in the component during manufacturing or use.

There are a few options when a component, made of any particular alloy and fabrication process, does not have the toughness and strength needed - use a different alloy and/or use different fabrication processes. Fabrication processes include the mill or casting processes used to make the metal, subsequent mechanical processing, and heat treating. For example, the toughness vs. strength for a cold-rolled carbon steel alloy is different than for the quench and tempered alloy.

One common source of cracking during use is exposure to fatigue condition. When designing components that will be exposed fatigue conditions, knowledge of metal fracture toughness is required to determine how long the component can remain in service before a crack grows too long and the component fractures. This applies to aerospace components and pressure vessels such as boilers.

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For structural components exposed to fatigue conditions, designers must be concerned with both the strength and the toughness. The strength must be large enough so that the material can withstand the applied loads without deforming. The toughness must be sufficient for the metal to withstand the formation of fatigue cracks without failing catastrophically.

For more information read Deformation and Fracture Mechanics of Engineering Materials by R.W. Hertzberg.  This book contains lots of information about the relationship between fracture toughness and strength.

This article outlines an eight-step process to help you identify and repair the root cause of a problem that arises when working with suppliers.

I have consulted with manufacturers in many industries. Regardless of the products being produced, they all face the same general challenges and frustrations about product development and manufacturing.  Each of them is concerned about the following questions that impact their profitability:

1.    How can we reduce the time it takes to develop new product?
2.    How can we reduce the costs to make our products?
3.    How can we improve product quality?

With this in mind, let's look at the following all-too-common scenario with a new, low-cost supplier:

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A manufacturer decides to have a component of his product manufactured by a new lower-cost supplier.  The manufacturer receives prototypes from the new supplier and proceeds to build the product using the prototypes.  He performs some tests on the product and decides to change to the new, lower-cost component.  Unfortunately, quality problems start to appear along the way.  For example

The ripple effects of these problems are:

  1. The product launch is delayed.
  2. The production line is shut down or limps along while the quality assurance staff sorts through batches of components looking for acceptable samples.

For both situations, the manufacturer’s engineers must drop their other projects and fix the new problem quickly, before it becomes too costly.  Meetings, analyses, tests and overtime follow in rapid succession while other projects are delayed.

What follows is a discussion of the steps to take to identify the root cause of the problem and fix it.  The strategy is based upon the understanding that the shape, dimensions, properties, and performance of a component depends on its materials and the response of the materials to the component fabrication processes.  The same concepts can be applied to a sub-assembly and the formation of the joints (welded, soldered, brazed, adhesive) between the sub-assembly’s components.

Fixing the Problem

A component will not perform as required or will have poor reliability due to one or both of the following reasons:

•    The component’s shape, dimensions, or cosmetic appearance do not meet specifications.
•    The component’s materials do not meet specifications for the composition, microscopic structure, properties, or type and number of defects that are present.

In some cases, problems with shape, dimensions, or cosmetic appearance are related to a problem with the materials.

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Identifying the root cause of a quality problem requires analysis of the component and its materials in order to obtain the information needed to quickly focus on the potential root causes of the problem.  Without this information engineering teams end up guessing at the possible causes, and their effort is usually not productive.

The following eight-step approach should be used to determine the root cause of the problem so that it can be quickly corrected:

1.  Gather all known requirements for the component. These include the component’s shape and dimensions, its materials, performance requirements, and reliability requirements.  Reliability requirements refer to the use conditions to which the component will be exposed during its use and the allowable performance degradation after exposure to the use conditions.

2.  Get samples of the component for analysis of the component and its materials. Samples of both good and bad samples should be obtained. The analysis data from the good samples will be used as a benchmark against which the data from the bad samples can be compared.  In addition to analyzing the bad samples, it is also helpful to evaluate good samples to verify that the materials do indeed satisfy their requirements.  Sometimes, something works even though it doesn’t meet its specifications.  This can make problem solving very difficult if it is not known what the supplier was doing right in the first place.

3.  Gather information about the manufacturing processes that were used to make the component. Find out the process steps and process conditions.  This information is important to help understand the results of the analyses discussed in the next step.

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4.  Identify the analyses that need to be performed. Analysis of the shape, dimensions, and cosmetic appearance is fairly straightforward.  Analysis of the materials that comprise the component can include analysis of a) composition, b) microscopic structures, c) properties, and d) defects in the material.  The specific analyses to perform depend on the component’s design requirements and the types of materials that make up the component.  Work with the lab analyst to get her input about the analyses that should be performed.  This will require providing as much information about the problem as possible to the analyst.

5.  Perform the analyses.

6.  Review the analysis results and identify possible root causes of the problem. The root cause can be related to the materials used to make the component, an error by the operator on the component production line, the manufacturing equipment, or the manufacturing steps or process parameters used to fabricate the component.  The data from the analysis should help focus on a few potential root causes.  Sometimes, the root cause is obvious.  In other cases, some work is required to verify a specific root cause for a particular problem.

7.  Provide feedback to the supplier. The supplier may need help figuring out what to do to control or change the manufacturing processes in order to produce a component that meets all of its requirements.  The materials analysis information will help the supplier understand what is required.  After the supplier has made the necessary changes get new samples of the component and have them analyzed to verify that the problem was fixed.

8.  Update the component's specifications. A potential root cause is that the specifications for the component did not have enough detail, resulting in a component that met the specifications, but did not work as required.  If this was the case, then the specifications must be updated to include the missing shape, dimensional, cosmetic, and/or materials requirements.

Suppliers of metal stock and components are critical to a company's success. They depend on their suppliers to provide materials and parts that satisfies engineering requirements and is delivered on time.  However, companies do not always use a methodical approach for evaluating and selecting suppliers.  Instead, they are lured by the promise of a low piece part price, only to find that the costs due to poor quality and delayed product launch quickly overshadow the planned savings.  This article explains how to select suppliers that will enable production at the lowest total cost and highest quality.

Michael Pfeifer, President, Industrial Metallurgists, LLC
Herb Shields, President, HCS Consulting

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Consider the company that used a low-cost supplier to fabricate a plastic sub-assembly for a brand new product.  The sub-assembly, which consisted of two plastic components that were joined by ultrasonic welding, was a critical part of the product.  Soon after the product was launched failed samples started coming back from customers.  The weld joint was failing because the welds were poorly formed.  To make matters worse, the supplier was not capable of fixing the problems with its manufacturing processes, so the problem dragged on and overwhelmed the original excitement with the new product.  Ultimately, the problems with the weld joint caused the product to be unsuccessful.

Maximizing the likelihood of selecting optimum suppliers requires that design and purchasing teams have the resources, expertise, and discipline to do the following

This article discusses the considerations required for improving the likelihood of properly performing these three tasks.  These considerations are based on the materials engineering and supply chain perspectives.

Materials Engineering Perspective

The materials engineering perspective is based upon the following three considerations:

With respect to supplier selection, the first and third considerations are significant.  Both considerations help design and sourcing teams focus on selecting suppliers that have the expertise and capabilities to provide a component or sub-assembly whose materials enable the component or sub-assembly to satisfy its design requirements.

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Suppliers must be capable of developing manufacturing processes that result in the proper properties for the materials that make up their products.  This means that suppliers must understand the materials and manufacturing processes being used in order to ensure that their products meet the performance and reliability requirements specified by their clients.  This applies to the materials the make up individual components, components within a sub-assembly, and joints between components in a sub-assembly.

A supplier must be able to control the variation of the materials used to make its product and the variation of its manufacturing processes so that the materials in the process output have consistent properties from sample to sample.  If the properties of the materials in the process output vary too much from sample to sample, then the performance and reliability will be inconsistent from sample to sample.  Furthermore, inconsistent materials properties can hurt the manufacturability of the product made using the component or sub-assembly.

Supply Chain Perspective

The supply chain perspective is based upon the following three considerations:

  1. Suppliers must be selected based on their ability to provide components and sub-assemblies that minimize the total cost to build a product.
  2. Suppliers must be selected based on their ability to maximize assurance of production supply.
  3. Suppliers must be selected based on their ability to provide components and sub-assemblies that consistently meet their design requirements.

Total cost includes all costs, not just the purchase price.  Transportation, inventory, quality, import duties, etc. all need to be included in the cost analysis.  The best supplier should have the ideal balance between all three factors.  This may or may not be the supplier with the lowest piece price.  Focusing on only the unit price of the item being supplied will possibly result in increased costs due to poor quality or service.  Purchasing and design teams should focus on reducing the overall risk to product success.

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Assurance of production supply relates to delivering quality products on time and in the correct quantities.  Also, there must be some assurance from the prospective supplier that they have sufficient capacity to meet your requirements if demand grows.  This commitment probably cannot be open ended, but suppliers should have some contingency plans in place.

In today’s global economy, high quality must be a given with any prospective supplier, otherwise there is no reason that they should be considered.  A supplier must be able to control its process within specified parameters in order to produce an item that consistently meets its design requirements.

Assessing the Technical Importance of Components and Sub-Assemblies

Technical importance refers to the impact of a particular component or sub-assembly on the performance and reliability of a product.  Within any product the components and sub-assemblies cover a spectrum from low importance to high importance.  A high importance item has significant contribution to the performance and reliability of one or more of a product’s
functions.  A low importance item has little contribution to the performance and reliability of any of a product’s functions.

For example, consider a lightweight, high performance bicycle.  Examples of high importance items are the frame sub-assembly and the sub-assemblies within the gear shift sub-assembly.  Examples of low importance items are the seat and reflectors.  This is not to say that the seat and reflectors are not necessary parts of a bicycle.  However, they do not influence the bicycle’s performance in a way that provides substantial differentiation compared to other bicycles.

Identify Supplier Selection Criteria

Selection of a capable supplier involves consideration of many different criteria that can be placed in the following categories:

  1. Technical expertise and capabilities.
  2. Manufacturing capabilities
  3. Manufacturing capacity.
  4. Manufacturing competence.
  5. Ability to provide technical assistance.
  6. Supply chain organization.
  7. Production planning process.
  8. Shipping and logistics.
  9. Geographic location.
  10. Company size and financial stability.
  11. Delivered cost.

Each category and the supplier evaluation process will be discussed in the next article.

Suppliers of materials, components, and sub-assemblies are critical to the success of a product.  This is the second part of a two-part article that discusses the considerations for selecting suppliers that will enable production at the lowest total cost and highest quality.  The first article discussed the materials engineering and supply chain perspectives that must be considered when evaluating and selecting suppliers, and ended with a list of supplier selection criteria.  This article explains the considerations for each of the selection criteria and discusses the evaluation process.

Michael Pfeifer, President, Industrial Metallurgists, LLC
Herb Shields, President, HCS Consulting

Identify Supplier Selection Criteria

Selection of a supplier involves consideration of the following different categories of criteria:

  1. Technical expertise and capabilities.
  2. Manufacturing capabilities
  3. Manufacturing capacity.
  4. Manufacturing competence.
  5. Ability to provide technical assistance.
  6. Supply chain organization.
  7. Production planning process.
  8. Shipping and logistics.
  9. Geographic location.
  10. Company size and financial stability.
  11. Delivered cost.

Each of these categories is discussed next.

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Technical expertise and capabilities. Is the supplier knowledgeable about the materials to be used in the sub-assembly or component?  Does the supplier have experience with the materials and processes required to make the item under consideration?  Does the supplier have the technical expertise to work out manufacturing problems?

From the materials engineering perspective, design teams should be concerned with a supplier’s experience with the materials used in the item being provided and with the manufacturing processes used to manipulate the materials into the item to be made.   A supplier must also have the technical knowledge required to identify, assess, and mitigate risks to meeting the development schedule.

Manufacturing capabilities. This refers to the different manufacturing processes that are available, and the capabilities of the manufacturing equipment.  Things such as the capability of fabricating very large or very small components, or the capability to perform different primary and secondary processes are a few of the considerations when assessing manufacturing capabilities.

Manufacturing capacity. This refers to the number of components or sub-assemblies that can be produced in a period of time.  From the supply chain perspective, how much capacity is available for this product and is there an alternate plant or equipment that could be utilized if needed?  Is the supplier prepared to commit the necessary capacity on a long term basis?

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Manufacturing competence. Is a supplier capable of controlling its manufacturing processes in order to produce a component or sub-assembly that consistently meets its design requirements?  Does the supplier have manufacturing experience that is directly applicable to the item under consideration?
It is important to understand the process variation, and compare it to the specifications for the item to be made.  If the supplier’s standard deviation is more than what is required for your component or sub-assembly to work, then this must be discussed before going into production.  Even if there appears to be no issue, it is important to understand how the process is controlled on an ongoing basis.

Ability to provide technical assistance. A design team may determine that it does not have all of the expertise to make certain decisions about the design of its product, and may want outside expertise.  Many suppliers have a great deal of technical expertise.  In fact, an experienced supplier will have in-depth knowledge in their area of expertise combined with experience working on a wide range of different products for different customers.  Suppliers like this have the knowledge and perspective that enable them to offer design suggestions about the options of materials and manufacturing processes that can be considered.

Geographic location. Is it important for the supplier to be located nearby for ease of communication, ease to visit, and speed of delivery?  Or is it acceptable for the supplier to be on the other side of the world.  From the materials perspective this decision will affect the ability to assist with the supplier’s product and process development and to assist with product quality problems that arise during production.  This is an important consideration from the supply chain perspective.  Off shore suppliers will mean longer lead times for shipments of components or sub-assemblies.  Inventory costs and transportation costs will be higher with a global supply chain.

Supply chain organization. The supply chain extends beyond the supplier to its suppliers as well.  It is important to understand what commodities are used by your supplier, who the key sources are, and what type of relationship exists between your supplier and its sources.

Production planning process. Is the supplier able to schedule production in lot sizes and frequency that match well with your projected requirements?  If you are going to produce your product in 1000 piece lots every week, then does the supplier have sufficient available capacity and can its supply chain support weekly production of raw materials and components?  What is the lead time that the supplier needs to produce products on a regular basis?  Does the lead time exceed the forecast or ordering information that is available from customers?

Shipping and logistics. You should agree on frequency and lead times associated with shipments of product from your suppliers.  Does the supplier have a back up plan if weather or other issues affect the primary mode of transportation?

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Company size and financial stability. All other things being equal, the size of a company and its financial stability are indicators of the resources that can be brought to bear to solve problems.  Also, for long term projects the financial stability is an indicator of the likelihood that a company will be able to meet its obligations to its customers over the entire length of the project.  If this is likely to be a long term relationship, meeting with the supplier’s management team is an important step.  Establishing a good relationship from the start will make problem resolution easier to handle going forward.

Delivered cost. Delivered cost includes all costs, not just the purchase price.  Transportation, inventory, quality, import duties, etc. all need to be included in the cost analysis.  The best supplier may or may not be the supplier with the lowest piece price.

Supplier Evaluation

Evaluation of suppliers involves an audit of suppliers to assess their ability to satisfy the critical items from the list above.  The assessment should include a questionnaire and evaluation of the suppliers’ product and be conducted by a cross functional team that includes representatives from purchasing, quality, engineering or R&D, manufacturing, and scheduling.  With a rating scheme developed by the team it is possible to objectively rate suppliers based on all the information.

A questionnaire is useful for obtaining basic information about the general capabilities of a supplier and its systems for ensuring that its product is delivered on time and with the desired quality.  The team should visit and administer the questionnaire to potential suppliers of high importance items.  For lower importance and overseas suppliers it may not be practical to send the entire team, which is acceptable provided that the audit team has a well documented process for obtaining the desired information.  The team can meet with the people who conduct the audit and make a decision.

Evaluation of a prospective supplier’s product is important for verifying its ability to provide an item that consistently meets its design requirements (i.e., performance and reliability).   This is information that cannot be obtained through a questionnaire.  From the materials engineering perspective the evaluation involves analysis of the materials in the suppliers’ products, which will help determine if a supplier is capable of controlling the materials and manufacturing processes used to make its product.  The analyses to perform include those for the materials’ composition, various microscopic features, significant properties, and defects.  This approach makes sense since the performance and reliability of any item is directly related to the properties of the materials that make up the item.  The properties depend on the materials’ composition, various microscopic features, and defects.

For a custom-made item the analyses can be performed on a similar item that the prospective supplier already produces.  For and off-the-shelf item the analyses can be on samples of the actual item.  The audit team must identify the significant materials that should be analyzed and the significant materials properties, composition, microscopic features, and defects to evaluate.  Any problems uncovered by the analysis will likely be present in the component or subassembly to be made for the design team.

As an example, consider a sub-assembly consisting of welded metal components.  The evaluation team should have the weld joint analyzed for its strength, microstructure, presence of defects like voids and cracks, and weld shape.  Samples can be taken from an item that is already in production at the supplier’s factory.

Money, time, and effort must be spent to properly evaluate suppliers. However, it is a relatively inexpensive way to mitigate the risks of selecting a supplier that is not capable of properly designing and manufacturing the component or subassembly. Furthermore, the analysis is much less expensive than missed deadlines or low process yields related to poor supplier quality.

Finally, the complete audit may indicate that a supplier has strengths in areas that offer a significant competitive advantage, but is weak in other areas.  For example, a supplier may have special manufacturing capabilities or technical competence; however, the variation of the manufacturing processes is excessive.  In this case, the design team can work with the supplier to help it reduce the variations.

Product Success

The methodologies and considerations discussed in this article are critical for improving the likelihood of selecting suppliers that can contribute to the success of your product.  None of what is discussed here is new.  It just requires focus and discipline.  However, there are many companies that struggle with problems that are a direct result of using incapable suppliers because their supplier selection processes are inadequate.

One approach to cost reduction is to re-engineer products to use less expensive materials or reduced quantities of higher priced materials.  This approach can have a significant impact on a product’s costs, since the materials used in a product account for a large part of its total cost.

With appropriate risk assessment and design verification, it is possible to make the desired product changes and preserve the required performance and reliability.  In fact, sometimes a product’s performance and reliability is also improved, providing an additional competitive advantage.

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The materials engineering approach discussed here is applicable to all types of products, all types of materials (metals, plastics, ceramics, composites, coatings, and adhesives) both common and uncommon materials, and components and materials used to make joints between components.  The trick is to identify the products, components, and materials that offer the greatest opportunity for cost reduction in the shortest period of time.

This article provides ideas and examples for re-engineering the materials used in products.

Opportunities for cost reduction

The following situations offer opportunities for cost reduction through materials re-engineering:

Each of these situations is discussed below.

Over-Designed Products

In some instances, products may be over-engineered using materials that provide way more performance and reliability than necessary.  Such over-engineering can add significantly to the bill of materials cost and to direct labor cost.

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Consider the case where steel alloy 4142 was specified, but a 1045 steel would have sufficed.  The 4142 steel was selected when the material was not too expensive, and the design team could afford to use a material that provided way more reliability than needed for the product.  How about the situation where gold plating is used, but tin plating would be sufficient for the desired performance and reliability.  Or the component made of polycarbonate, but could have been made of hi impact polystyrene.

In some designs, the material used in a component is the correct choice but the physical dimensions of the component are overkill, resulting in the use of more of the material than is necessary to meet the performance and reliability targets.

Product over-design is a fairly typical situation in older products that were designed when there was less customer price pressure and manufacturers could afford to have more cost in their products.  It also occurs in new products when design engineers use the same materials repeatedly, without considering other options of materials that might be suitable.

In all these cases re-engineering can be a fairly straight-forward process that starts with a review of the product performance and reliability requirements and identifying the options of materials that can be used.

Consolidation of Materials

Another opportunity for realizing significant cost reduction is the economy of scale realized through the consolidation of materials used across product lines.  Do your different products use common components made from different materials?  If so, this may offer an opportunity to use the same materials in the different components.  This enables cost-reduction through volume discounts, and the need to carry less inventory of a particular material or type of component.  In some cases, consolidating materials also leads to improvements in manufacturing yields by selecting the materials that are easiest to work with.

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For example, a company was using three different grades of polycarbonate for similar types of components.  The company changed to a single grade of polycarbonate for all components and got a price reduction of 10% per pound of plastic resin.  Furthermore, of the three resins, the one that was selected was the easiest to use during injection molding process.  Its use led to a 7% increase in manufacturing yields.

Or consider a large manufacture that used three different solder pastes to build electronics assemblies.  Changing to a single, newer solder paste resulted in 25% reduction in material cost and improvements in manufacturing quality.

Reducing Manufacturing Costs

Manufacturing costs can be reduced by using materials that enable higher yields or higher throughput.  Poor yields are often due to using materials that were selected with only performance and reliability in mind, and not optimized for manufacturability.  Or, the materials may be inherently suitable, but there is too much variation in their properties because the supplier has poor process control.  In either case, inadequate materials make it difficult to make components and form joints that consistently meet their design requirements.  In some cases, this shows up as poor yields.  In others, production line throughput is reduced because the process requires extra attention.

Revisiting Existing Designs

It is beneficial to hold periodic design reviews to determine if there are opportunities to revisit previous materials selection decisions.  This approach is applicable to both long-standing products and designs that are still under consideration.

The design review approach provides an opportunity to consider taking advantage of materials that were not initially available when the product design was contemplated.  It also allows the manufacturing team to take advantage of new, improved methods for processing and joining materials.

The goal of the design review is to look for significant opportunities to reduce bill of material costs, reduce labor content, reduce overall process time, and reduce yield loss.  Addressing any of these issues involves the methods already discussed, namely -

As mentioned earlier, re-engineering is a straightforward process that starts with a review of the product design requirements and identifying the options of materials that can be used.

Why Isn’t Everyone Doing This?

There are a few reasons why companies do not look to re-engineering of materials in their products for cost reduction.  First, engineers often do not see materials as an aspect of the design for optimization.  Typically, they first look to modify the mechanical or electrical aspects of a product.

Second, engineers often believe that changing materials will turn into a big research project that will take a long time to complete.  However, this is typically not the case if the proper considerations are made when evaluating and implementing the materials.

The final reason is that engineers may not know of all the options of materials to consider, especially when trying to optimize all of the performance, reliability, cost, and manufacturability requirements.

Many Opportunities

There are ample opportunities in most product designs to achieve significant cost reduction if one knows where to look and what questions to ask.   These techniques are applicable to simple and complex products and to common and high-tech materials.  In many cases it is possible to find materials that not only reduce costs, but also improve product performance and reliability.

Whether you are through hardening steel, annealing a cold-worked metal, or solution treating an aluminum alloy, the heat treating temperature is critical for obtaining the desired microstructure, and therefore, the desired metal properties.  Microstructure refers to such things as the metallurgical phases present in a metal and the grain size.

Using a temperature that is too hot can result in a metallurgical transformation that proceeds too quickly or the formation of undesired phases.  Using a temperature that is too low can result in incomplete metallurgical transformations, cold worked metals that do not soften sufficiently, or insufficient stress relief.

Want to learn more about steel and aluminum heat treating and metallurgy? See our metallurgy courses page for training options.

Steel heat treating

For example, during the through hardening heat treatment of a carbon steel, the steel is heated to transform all the ferrite and cementite to austenite and then quenched to form martensite.  If the steel is not heated to a high enough temperature, then there is the risk that all the ferrite and cementite does not transform to austenite.  If this occurs, then when the steel is quenched, the remaining ferrite and cementite will be present along with the martensite.  These ferrite and pearlite remnants can weaken the steel.

Annealing

Another example is cold-rolled sheet metal that is annealed to improve its ductility, and reduce its strength and hardness.  If the annealing temperature is too high, then excessive grain growth will occur.  This will result in the metal having lower strength and hardness than intended.  Also, if the metal is to be formed, there is the risk of orange peel, a cosmetic defect in heavily formed metals with grains that are too large.

Why temperature might be too high or low

So, why might a heat treater use a heat treating temperature that is too high or too low?  To save money, to save time, or just sloppy.  To reduce energy costs a heat treater might try to run its furnaces at the low end of the required temperature range.  However, normal temperature variations throughout a load and normal composition variations within the metal can result in the temperature being too low to cause the desired metallurgical transformations.

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To save time, a heat treater might operate a furnace at the high end of the specified temperature range to try to move the metallurgical transformations along as fast as possible.  Again, with normal temperature and composition variations, the temperature may end up being too high, resulting in excessive or undesired changes in the metal’s microstructure.

As for a sloppy heat treater, who knows what you will get from batch to batch of metal stock or components.

To learn more about the effects of temperature control on steel microstructure and properties, take our Metallurgy of Steel Heat Treating course or read Practical Heat Treating by J.L. Dorsett and H.E. Boyer or Steels: Processing, Structure,and Performance  by George Krauss.  Also, the two courses mentioned in the introduction above will discuss the effects of temperature control on precipitation strengthening and annealing cold-worked metals.

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