Over the years, many Heat Treat Doctor columns have featured photomicrographs to illustrate concepts, provide insights or point out why we are able to draw certain conclusions. This is no accident. Most informative technical articles and scholarly works rely on microstructures to provide evidence, confirm conclusions or prove (without many words) certain facts. The key, however, is for the observer to understand the significance of what he is seeing. Therefore, metallographic interpretation becomes an invaluable skill.
Metallurgists secretly like destroying things. If we had our way, we would cut up every part in a load just to confirm they were all good. The problem, of course, is that there would be no parts to ship to the customer. Not our problem, as metallurgists like to say! So, we make the argument that other types of testing only give an indication that all is well but cannot make a definitive statement one way or the other. We must also be careful to draw conclusion only from a truly representative sample. There is no greater sin than spending considerable time and effort only to reach a false conclusion, a red “herring” if you will pardon the pun.
Any analysis starts with a good visual or low-magnification (5-50X) stereographic inspection of a component part (Fig. 1). This helps to set the direction for further investigation and often provides a clue as to the best starting point, which may be missed if we began the analysis at the microscopic level.
Metallography, whether it be via optical microscopy (Figs. 2-5) or through the use of more advanced tools such as a scanning electron microscope (Fig. 6), is oftentimes the easiest, fastest, most direct and most reliable way to determine the acceptability of a component part, solve a problem or determine the root cause of a failure. In the hands of a skilled metallographer, answers come quickly, and positive conclusions can be drawn.
One of the secrets to success in metallographic interpretation is the ability to spot something in the microstructure that is abnormal. Management often doesn’t understand why metallographic equipment costs so much, why more and more sophisticated tools are necessary or why it takes so long to reach a conclusion. The metallographer is akin to a detective constantly piecing together clues until the crime is solved. To the uninformed, however, getting answers is an exasperating experience, as metallurgists never seem to have enough evidence to conclude with absolute certainty.
The Role of Comparative Metallography
It is a practical reality that many companies find themselves without a metallurgist or with fewer metallurgists having less time to support production on the shop floor. As such, technicians or those less skilled in the art are called upon to make daily decisions, and this is where comparative metallography comes in. Having examples of “acceptable” and “unacceptable” microstructures allows non-metallurgists to make decisions. The key is to have these comparisons prepared by a metallurgical laboratory that knows and understands what is needed. For example, there is no need for color metallographic images, if the people doing the work day in and day out do not use this technique, or for providing photomicrographs at a magnification not in use.
A comment about reports is also in order. If you request an analysis from an outside metallurgical laboratory, recognize that their reports are written by metallurgists, for metallurgists. Interpretation by those less skilled is sometimes difficult (if not impossible) in many cases. Outside testing laboratory reports, in particular, often do not attempt to interpret the facts. This is understandable given that, in most cases, not enough background information is available to understand the full scope of the problem under investigation. Asking the right questions of them and, if necessary, bringing in third parties for help in deciphering the results is of critical importance.
A Lost Art?
Courses in metallographic interpretation are offered by universities, technical societies, suppliers of metallographic equipment and independent third-party consultants. These are both important and necessary to teach the basic skills, but they are no substitute for hands-on application of the lessons learned. Metallography is a practical science gained by doing and doing over and over again. While one etchant or method may reveal the desired answer, experimenting with other techniques – perhaps a different edge-retention method, polishing cloth or etchant – may produce new insights. So don’t be afraid to try new things. You will like what you find! IH
1. Mr. Alan Stone, Aston Metallurgical Services Co., Inc., private correspondence.
2. Vander Voort, George, “Microstructure of Ferrous Alloys,” Industrial Heating, January 2001.
3. Vander Voort, George F., Metallography: Principles and Practice, 1984
4. Samuels, Leonard E., Metallographic Polishing by Mechanical Methods Third Edition, 1982
5. Samuels, Leonard E., Light Microscopy of Carbon Steels, 1999
6. Petzow, Günter, Metallographic Etching, 1976
7. Dvorak, Rostoker, Interpretation of Metallographic Structures,1965