Failure AnalysisHave you ever run into the following situation? A component within your product broke or your manufacturing line was producing bad components, and you wanted to determine the root cause of the failure. This required determining the failure mode and failure mechanism and whether there were any metallurgical deficiencies in the metal. So, you sent a sample to a metallurgical lab and got a report, but the report didn't have the information you needed or you didn't know what to do with the information in the report.

Failure analysis results

There are things you can do to prevent these problems from occurring, and improve your chances of determining the root cause of the failure. This article discusses how to work with a metallurgical lab to ensure the likelihood of getting the information needed to determine the root cause of a failure and to ensure that working with the lab is a positive experience.

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1) Provide samples and background information before asking for a quote.
It's difficult for a metallurgist to accurately quote the costs and time required to perform a failure analysis without getting a chance to visually examine the samples and without getting some information about the failure circumstances. The type of failure and the information needed by the client will are factors in determining which analyses will be required. Also, the size and shape of the samples and the materials that comprise the samples will influence the preparation required for the analysis and whether all the required analyses can be performed.

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2) Selecting the analyses to perform.
To many people, metallurgical failure analysis techniques are a mystery. Is scanning electron microscopy or a metallographic exam necessary? Many people don't know what to ask for when submitting a sample for failure analysis. The best thing to do is to provide the metallurgist with detailed background information about the failure and ask him to determine the failure mode and failure mechanism. Also, ask him to determine whether there were any metallurgical deficiencies that might have contributed to the failure. Metallurgical deficiencies include alloy composition, microstructure, tensile properties, and hardness that did not meet specification or were not appropriate for the application.

Let the person performing the failure analysis select the analyses needed to obtain the desired information. It's best not to try to steer the metallurgist in any direction or to select the analyses to perform without getting the metallurgist's input on the analyses required to get the information needed to determine the root cause of the failure. I've seen reports from metallurgical labs that provided the information the client requested, but did not lead to a complete understanding of the failure or its cause. Many labs will do what a client requests. It's best not to constrain the metallurgist by giving too much direction.

Also, ask the metallurgist to determine the root cause of the failure, if it is possible. Many times, it is possible for a metallurgist to determine the root cause of a failure. However, the ability to do this often depends on the background information you can provide.

3) Samples for analysis for manufacturing or assembly problems
For manufacturing or assembly problems, send samples of components or sub-assemblies that meet specifications, along with the samples that do not meet specifications. If needed, analysis results of the “good” samples can be used for comparison. Also, if a metallurgical exam has never been performed on “good” samples, the results will be helpful to verify whether the “good” samples are in fact metallurgically “good.”

4) I don't understand the report
What's transformed austenite? What's dimple rupture or cleavage? What's a grain boundary precipitate? Let's face it, metallurgists have their own language. It makes plenty of sense to us. Unfortunately, many reports require a translator. After reading the report, call the metallurgist and ask him to go through the report with you. Have him explain the results and what they mean. By the way, do this soon after receiving the report, when the analysis and results are still fresh in the metallurgist's mind.

5) Don’t expect the metallurgist to be able to determine the root cause of the failure
Assuming that it was possible to determine the failure mode and mechanism, you still need to figure out the root cause of the failure. It may be possible for the metallurgist to determine the root cause, if you provided enough background information about the failure. However, in many cases, especially for manufacturing and assembly failures, you will probably need to get more information about the circumstances leading to the failure. However, in many cases the information from the failure analysis will point you in the direction of where to look for the additional information.

6) Treat the metallurgist like a member of your engineering team
Find a metallurgist you're comfortable working with and treat her like a member of your team. Invite her to meetings about the failure. The information she gains from participating can be huge for helping her figure out the analyses required, the samples to analyze, and possibly the root cause of the failure. Too often, people keep the metallurgist in the dark, which can slow down the failure analysis and root cause analysis process. Remember, this person is supposed to be an expert

Successful failure analysis

A successful failure analysis results in getting information that leads you to the root cause of the failure. Following the advice in this article will increase the likelihood of getting the information you need, and make the process less frustrating, or maybe even enjoyable, if a failure analysis can be enjoyable.

Need help with a failure analysis? See our failure analysis page

Interested in learning more about failure analysis and root cause analysis? Check out these courses, webinars, and videos that we offer Failure Analysis of Metal Fractures (video), Root Cause Analysis of Metal Problems (video), Metal failure analysis (course), Failure Analysis of Metal Fractures (webinar), Failure Analysis of Metal Problems (webinar),  Root Cause Analysis of Metal Problems (webinar)

Orange peel was present on bowls that were deep-drawn from low-carbon steel sheet. It was present on bowls produced from some batches of steel and not present on bowls produced from other batches of steel. Failure analysis of the bowls was performed as part of the effort to determine the root cause of the problem.

Orange Peel

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Failure analysis results

Orange peel is associated with the grain size of a metal. Therefore, cross-section metallography was used to examine the microstructure of bowls with and without the defect. The images below show the results of the analysis. The bowl without defects had small grains throughout the cross-section. The bowl with orange peel had large grains at the surface.

OrangePeel_grains

For a given amount of deformation, there is a limit to the maximum grain size before orange peel appears. In this instance, the grains at the surface were too large for certain batches of steel sheet.

Root cause

The grain size of metal is controlled through a combination of cold rolling and annealing. Proper control of annealing temperature, annealing time, and cooling methods after annealing are critical to obtain the desired grain size. The top surface of the sample with orange' peel was not properly cooled, which allowed the grains at the surface to grow too large.

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For more information

The roughened surface of large grained metals after forming is called orange peel because the surface has the appearance of the surface of an orange. See the article Orange Peel for more information about the subject, including how to prevent it from occurring.

Want to learn how to perform a failure analysis? See our metallurgy courses

Interested in learning more about failure analysis and root cause analysis? Check out these courses, webinars, and videos that we offer Failure Analysis of Metal Fractures (video), Root Cause Analysis of Metal Problems (video), Metal failure analysis (course), Failure Analysis of Metal Fractures (webinar), Failure Analysis of Metal Problems (webinar),  Root Cause Analysis of Metal Problems (webinar)

rivet failure analysisProblem
Brass rivets were cracking during assembly, when the rivets were being set. Failure analysis of the rivets was performed as part of the process to determine the root cause of the cracking.

The rivets were manufactured by machining them from wire stock. The main requirements for the rivet material were:

  1. Easy to machine rivet from wire.
  2. Easy to form rivet during assembly.

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Meeting these two requirements put mutually exclusive constraints on the brass composition. Lead is added to improve machining, however lead reduces brass cold forming properties. The alloy selected had 0.9 to 1.5 % Pb.

Rivet Failure Analysis
Stereo zoom microscope (up to 70x) examination revealed that rivets were cracking at the portion deformed during the rivet setting process, when two components were being joined.

Composition analysis of cracked and uncracked rivets was performed using atomic absorption spectrocopy. The results indicated that uncracked rivets had less than 1.1% Pb and the cracked rivets had more than 1.3% Pb.

Conclusions
The root cause of the cracking was poor design. A new alloy was selected that had lower Pb. This required slowing down the machining process, but eliminated the cracking.

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The problem could have been prevented by conducting a failure modes and effects analysis (FMEA) when the product was being designed. An FMEA involves reviewing components, assemblies, and subsystems to identify failure modes, and their causes and effects. Had the design team performed a design FMEA, they may have identified the mutually exclusive behavior of brass with Pb. Information about how to perform an FMEAs is available in the publication "Potential Failure Mode & Effects Analysis", which is available from AIAG at www.aiag.org.

Want to learn how to perform a failure analysis? See our metallurgy courses

Interested in learning more about failure analysis and root cause analysis? Check out these courses, webinars, and videos that we offer Failure Analysis of Metal Fractures (video), Root Cause Analysis of Metal Problems (video), Metal failure analysis (course), Failure Analysis of Metal Fractures (webinar), Failure Analysis of Metal Problems (webinar),  Root Cause Analysis of Metal Problems (webinar)

A failure analysis is performed for these situations:

This video is a short discussion of the circumstances when a failure analysis is performed, the goals of a failure analysis, and the steps of a failure analysis.

Need help with a failure analysis? See our failure analysis page

Interested in learning more about failure analysis and root cause analysis? Check out these courses, webinars, and videos that we offer Failure Analysis of Metal Fractures (video), Root Cause Analysis of Metal Problems (video), Metal failure analysis (course), Failure Analysis of Metal Fractures (webinar), Failure Analysis of Metal Problems (webinar),  Root Cause Analysis of Metal Problems (webinar)

cracked chuck failure analysis

Problem
Determine the root cause of a tool chuck that was cracking during product assembly.

Description
About 30% of the tool chucks were cracking. The chuck was made of a stainless steel alloy that was similar to PH 13-8 Mo.  The component was fabricated from wire stock that had been 70% cold drawn.  The following fabrication processes were used to make the chuck:

  1. Machine hole through wire center
  2. Machine other features inside the hole
  3. Machine a slot into the side of the chuck
  4. Heat treat at 900 °F to precipitation age the steel to a final hardness of >53 Rockwell C, which was the peak hardness of the steel.

The cracks initiated at the root of the slot when a mating part was inserted into the chuck.

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Analysis
To start the failure analysis, cracked samples were split open completely to allow the fracture surface to be examined with a scanning electron microscope. The analysis indicated that the fracture mode was dimple rupture, indicating that the metal was exposed to a stress that exceeded its tensile strength.  However, there was no plastic deformation of the material around the crack, which indicated that the metal had very little ductility.

The heat treatment that was used resulted in a steel with high yield and tensile strength and poor fracture toughness.  Using a higher heat treating temperature of 1000 or 1050 °F was recommended.  This heat treatment resulted in overaging, with a slight reduction in strength, but almost double the fracture toughness.  The reduction in strength was acceptable, so the recommendation was implemented, and the cracking no longer occurred.

Conclusions
The root cause of the cracking was poor design.  The heat treatment that was specified resulted in a material that had poor fracture toughness.

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Could the problem have been prevented?
Yes.  Completing a failure modes and effects analysis (FMEA) when the product was being designed would have probably identified cracking as a potential failure mode.  An FMEA involves reviewing components, assemblies, and subsystems to identify failure modes, and their causes and effects. For each component, the failure modes and their resulting effects on the rest of the system are recorded in an FMEA worksheet.  There are both design and process (Manufacturing and Assembly) FMEA analyses.

A successful FMEA activity helps to identify potential failure modes based on experience with similar products and processes - or based on common physics of failure logic. It is widely used in development and manufacturing industries in various phases of the product life cycle.

While performing an FMEA can be laborious, the output is often very powerful for giving design and manufacturing teams clear direction on engineering issues to resolve during product design and manufacturing process development.  Ultimately, the impact is improved product reliability, more capable manufacturing and assembly process reliability, and fewer problems and surprises.

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Had the design team for the product considered here performed a design FMEA, they may have identified the susceptibility of the chuck to cracking, and considered an analysis of the effects of different aging heat treatments on the cracking.  With this information, the design team could have optimized the mechanical design and metal properties requirements.

Information about how to perform an FMEAs is available in the publication "Potential Failure Mode & Effects Analysis", which is available from AIAG at www.aiag.org.

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