Interview: Metal Fasteners - Industrial Metallurgists

May 18, 2021

Interview: Metal Fasteners

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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:

  • Machine screw, self-tapping screw, and bolt fabrication
  • Things designers can do to improve the likelihood of getting fasteners that work as needed
  • Common fastener standards
  • Information to include on fastener drawings
  • Problems associated with not referencing a standard fastener
  • Root cause process for fastener problems

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.

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