During the day-to-day operation of heat treat departments, many habits are formed and procedures followed that sometimes are done simply because that’s the way they’ve always 
been done. One of the great benefits of having a community of heat treaters is to challenge those habits and look at new ways of doing things. Heat Treat Today’s annual 101 Heat Treat Tips, tips and tricks that come from some of the industry’s foremost experts, were featured in the 2019 Heat Treat Show Edition, as a way to make the benefits of that community available to as many people as possible. This edition is available in a digital format here.
Today we offer the first Heat Treat Tip from the 2019 edition: Debbie Aliya of Aliya Analytical Inc. on “Where You Measure Matters”, categorized under Materials Testing. Debbie is also one of Heat Treat Today’s featured Heat Treat Consultants. Click here for more information on our Consultants’ page.
Heat Treat Tip #6
Where You Measure Matters
Eugene Gifford Grace (August 27, 1876 – July 7, 1960) was the president of Bethlehem Steel Corporation from 1916 to 1945. He also served as president of the American Iron and Steel Institute and sat on the board of trustees for Lehigh University, of which he was an alumnus. One of his famous quotes is as follows:
“Thousands of engineers can design bridges, calculate strains and stresses, and draw up specifications for machines, but the great engineer is the man who can tell whether the bridge or the machine should be built at all, where it should be built, and when.”
If you check out the additional accomplishments of Mr. Grace, you will see that he was a successful and smart person. Maybe all of us are not capable of reaching such breadth of vision as he articulated above, but as heat treaters, do we simply accept the specification given? Or do we stop to ask if the specification has been properly determined?
With modern computer added stress analysis (FEA), we have at our fingertips a way to move beyond both the “guess and test” and the “copy the historical spec” methods of determining the case depth. Within “guess and test,” of course there are scientific guesses and scientific wild guesses. If you are using a wild guess, chances are that the field is the test lab!
Especially for carburized components, deeper case is more time in the furnace, and thus more expensive. I continue to wonder why, if even back in the 1950s, thousands of engineers were available who could calculate stresses and strains and thus set a quantitative foundation for a case depth, in 2019, so few people take advantage of modern technology to optimize the cost of their products.
If you are not ready to take this big step toward design optimization, maybe you would consider always using effective case depth, based on hardness and thus linked to tensile strength, instead of total case depth, which is not linked to any durability or strength criteria.
Figure 1 shows the metallographic cross-section that was used to measure the hardness. Each white pin point is a Knoop 500 gram hardness indentation. The cross-section of the gear was mounted in black epoxy resin. Figures 2 to 4 show the data collected to determine the effective case depths to the common Rockwell C 50 criteria.
The effective case depth is the depth where the hardness dips below HRC50. For Gear Tooth Flank A, that value was 0.85 mm. For another gear from the same lot, it was over 1.08 mm. But for the root areas, between the teeth—the high-stress area, the effective case depths were only 0.45 and 0.65 mm, respectively. Figure 3 shows the same data as Figure 2, but using a logarithmic scale, illustrating what’s going on near the surface layers more clearly.
In any case, there’s a big difference between the two test locations, and this shows the importance of making sure that all relevant features of the component are adequately characterized!
If you have a heat treat-related tip that would benefit your industry colleagues, you can submit your tip(s) to doug@heattreattoday.com or editor@heattreattoday.com.
