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Introduction to Quantitative Metallography (part 1)

May 7, 2012
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With this blog, we begin a multi-part discussion of quantitative metallography. We hope to provide information that will be usable by many in their metallurgical investigations.  

For most of its history, metallographic observations have been largely qualitative in nature. The structure might be described as being relatively coarse or fine, layered or uniform. Particles might be labeled as globular or spheroidal, lamellar, acicular or blocky. Microstructures were single-phase or duplex and so forth.  

Thirty years ago when I entered industry, chart ratings and visual examinations were the main approach toward quantification. I can well remember the mill metallographers looking at spheroidized-carbide tool-steel structures and stating that it was, for example, 90% spheroidized (many raters would never say 100%, just as some teachers would never grade an essay at 100%!) or that it was 60% spheroidized and 40% lamellar tending to spheroidize. Or, without looking at the chart (a seasoned rater never did), they would pronounce that the grain size was, for example, 100% 6 to 8 or perhaps 70% 8 and 30% 3 to 5 if it was duplex in appearance.  

As a novice metallurgist, I was quite impressed by these pronouncements and tried to repeat the practice on my own. But I found myself unable to repeat such ratings, if done several weeks apart, unless I made my estimates very broad. Later, I tried submitting the same specimens to different metallographers or to the same person at different times. I quickly learned that their repeatability/reproducibility wasn’t that much better than mine. (Don’t let a mill metallographer know that you are checking him/her!)  

The greatest mystery to me, however, was inclusion-chart ratings. I was very impressed that they could scan the 160-square-mm area in a few minutes and, without taking any notes, jot down worst-field ratings. Absolute black magic! I never could do this, and I still cannot. I had to use a scale and measure stringer lengths or count inclusions when I saw a field that looked like it had a high inclusion content. And I had to keep notes. But I was a college graduate, and some of these men did not complete high school! Was I overly dense?  

Naturally, I tried testing their ability to reproduce test results, and I found out that maybe I wasn’t as slow as I thought. They didn’t do that well reproducing their own results. Later, when I became active with ASTM Committee E-4, I found that inclusion ratings on E 45 round-robins, which had been tried numerous times, were notoriously non-reproducible. Well, that was good for soothing my shattered ego, but it did not solve my problems of describing structures.  

Stay tuned.
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So true !

Martin
May 9, 2012
Hello, I can absolutely confirm this! Reading this remembered me on my first steps on this field. I watched the mill metallographers doing inclusion-chart ratings at incredibly speed and at first it was impossible for me to figure out how they did that trick. It took me years to figure out what you pointed out in this article - and I am feeling a bit relived that others made the exact same experience. But, for myself, I never could do it this way. I am working with parts relevant for the security of human lives and therefore I am doing it the slow way. It is impossible for me to work against my conscience when it comes to this point. Thank you for this article Martin

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