|Fig. 1. The Doctor’s boyhood companion|
As a young boy growing up in the neighborhoods of Chicago, one of the Heat Treat Doctor’s most trusted friends was a mythical conjuror by the name of Mandrake, The Magician. If a task seemed impossible to accomplish, or when all else failed, all one needed to do was summon Mandrake, and “voila,” the impossible became possible!
As heat treaters, we often seek answers to processing problems or component part failures from the metallurgical community without fully understanding what they need to do their job properly. What do we need to provide, whether it be accurate background information, a representative set of samples (good and bad) for comparative analysis, or even something as simple as protecting the surface of the component to be analyzed from further damage? Often, we don’t communicate our expectations in precise terms and thus do not know what to expect from an analysis. It’s time for us to learn what we can do to assure that accurate information and reliable facts result from whatever testing or analysis is performed, upon which we can make informed decisions. Mandrake would be proud. Let’s learn more.
The Role of Photography
In this day and age of digital photography, a picture can indeed be worth the legendary 1,000 words (or more). Provide photographs of everything, from multiple angles, and remember to use good lighting and high resolution. Handle parts carefully so as not to induce damage and resist the temptation to refit mating fracture surfaces together. Note part orientation and other salient features.
Processing History/Background Information
Don’t assume that someone knows your process or product, or its intended service application, better than you do. Communicate the history of the part or process; separate assumptions from facts; provide necessary drawings, including mating components if appropriate as well as required specifications; and explain in detail the design requirements. In other words, take the guesswork out of the analyst’s job. Document anything of importance and give this information to the metallurgist, even if it means having a confidentiality agreement in place before you start.
If it turns out that the component is being returned from the field, extreme care must be taken to ensure a representative sample and to avoid further damage (see “The Do’s and Don’ts of Field Failure Analysis,” Industrial Heating, January 2006).
Incoming (Raw) Material Analysis
Too often we are forced to begin an analysis making assumptions about the raw material. Provide the laboratory with a copy of the mill’s material certification sheets. In addition to the chemical constituents, the metallurgist will glean information based on the form of the raw material, its grain size, cleanliness and prior mill processing. Doing an actual chemical analysis is not necessarily redundant. For example, trace-element chemistry can play a significant role when investigating certain phenomenon, such as temper embrittlement.
Discuss with your metallurgist or outside laboratory what type of tests will be conducted and in what order. Understand what will be achieved at each step in the analysis process so that you can ask questions or offer suggestions. (This will also help explain the time or expense involved.) In this way, you will be better able to interpret the final results.
Be aware there is nothing more frustrating in the laboratory than to work hard on a job only to find out that it is not the right sample or that the damage observed was induced by extraneous factors. This translates into lost time and money. Also, be conscious of the fact that once the investigator has begun to work on your project, it is best that the analysis move forward uninterrupted. So be sure to define the scope of work and clarify the boundaries of what he or she is allowed to do once the investigation is under way. (Oftentimes, a “not to exceed” figure works well for this part of the investigation.)
Selecting the proper tests may involve trade-offs due to cost or time. Be sure you understand the cost/benefit relationship of each test and what the expected outcome might be so that the right choices can be made. Insist on specificity to avoid open-ended analysis efforts. Here are some examples of what can be done in the laboratory.
- Nondestructive testing: eddy current, ultrasonic, pressure testing (hydrostatic, pneumatic), surface finish
- Mechanical testing: hardness/microhardness testing, tensile testing, impact testing (e.g., Charpy testing), fatigue testing, torque/torque-tension, shear and double shear strength, torsion testing, creep, stress rupture and stress durability, vibratory testing
- Sample preparation: unetched part examination, etched part examination
- Optical microscopy: microstructural determination, grain size, micro cleanliness, intergranular attack, inclusion characterization, alpha case
- Image analysis: plating depth (layer thickness), defect measurement, grain size
- Scanning electron microscopy: fractography, feature/character recognition
- Energy-dispersive X-ray spectroscopy: qualitative element analysis, inclusion characterization, elemental distribution (dot) mapping
- Corrosion testing
Selecting the Right Laboratory
Not all laboratories are created equal, either in the talent of their researchers or in tools available to do the job right. Talk to people you trust in the industry to help in the selection process. Be aware that many labs are better at some things than others and subcontract certain tasks to other labs. Be sure that you understand when and why this is being done and determine if you are better off going direct.
Comparative Analysis (Good vs. Bad)
If good parts exist, they can be invaluable aids in understanding why a bad part failed. Taking the seemingly extra step (and expense) of testing a good part along with a bad one will yield tremendous insight into the problem at hand. Do this whenever possible.
In an effort to get answers, avoid the temptation to push the lab to the point where steps are skipped or time is not taken to investigate secondary factors that may prove to be major contributors. Ask for verbal reports at key milestones in the analysis work, but avoid taking up valuable analysis time by “checking in” too often. Meeting in person to begin a project is always beneficial.
Metallurgists tend to write reports for other metallurgists, a noble but often frustrating problem for the heat treater. If you need the report “translated” into layman’s terms, be sure to tell the lab. Yes, there is a delicate balance here between the facts and their interpretation, but this can often be handled by placing the interpretation in a “Discussion” section of the report. The trend today, due to liability concerns, is to simply report the facts and rely on the client to interpret them. If necessary, hire outside experts to put the information in the proper context in order for you to determine the right course of action. There is nothing worse than paying good money for a report you don’t understand.
|Fig. 2. Ishakawa diagram – quenching|
“What caused the problem, and how can I avoid its reoccurrence” should be the objective of any analysis effort. There are often multiple contributory factors and removing any one of them might avoid a part failure, even though defects may still exist. While it may or may not be possible to establish the root cause, it should always be the goal. The use of Ishakawa (fishbone) diagrams (Fig. 2) or other diagnostic methods listing all of the variables impacting a successful outcome can be a big help. Sometimes it’s the thought process itself and a discussion among various company departments that leads to the solution to be implemented.
The Bottom Line: To Analyze or Not to Analyze
A cost/benefit analysis should be performed before and after any analysis/testing work. Knowledge is strength, and assumption is weakness. When in doubt, do the metallurgical analysis. It will amaze you what can be revealed. And remember that Mandrake is alive and well, living within each and every metallurgist! IH
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