Toughness testing – Cru-Wear, Z-Wear, Upper vs Lower temper, Cryo vs No Cryo

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I recently completed some toughness tests on samples that were heat treated by knifemaker Warren Krywko. The steel was donated by Chuck Bybee of Alpha Knife Supply. The samples are subsize unnotched charpy specimens with dimensions as specified on the bottom of this page: http://knifesteelnerds.com/how-you-can-help/ If we can get more people to make toughness specimens we can have more comparisons between steels, hardness points, heat treatment parameters, etc. Patreon dollars are for the purpose of paying for machining, shipping, testing, etc. for tests like toughness and CATRA edge retention, so if you are able to contribute that way please visit the Knife Steel Nerds Patreon page. 

Warren likes to use CruWear and PM Z-wear in his knives because he likes its combination of good toughness and wear resistance along with high hardness. Both are copies of the older VascoWear developed by Vasco, the company that James Gill worked for who I wrote about here: The Development of High Vanadium Steels, M4 and the First Tool Steels Book.  James Gill didn’t develop VascoWear but I won’t be writing about the history of VascoWear in this article. Maybe another time. It is also sold by Carpenter as PD-1 and probably other companies under other names.

Warren and I discussed comparing low vs upper tempers (400 vs 1000°F), with and without cryo, and the ingot version vs the powder metallurgy (PM) version of the steel. Crucible shows a significant increase in toughness for the PM version [1][2] so we wanted to see if our toughness testing shows similar behavior. We also saw a big difference between them:

This makes sense because of the great refinement of the carbide structure from the powder metallurgy process. You can see a comparison between micrographs of ingot and PM 154CM in this article: Micrographs of Niolox, CPM-154, and AEB-L. I also wrote an article about toughness; the literature indicates that PM steels are better at resisting crack initiation but not crack growth. Therefore, the improvement in toughness in Z-Wear relative to Cru-Wear is likely due to resistance to crack initiation with these unnotched specimens.

Also results are lacking when it comes to comparing low and high tempering temperatures when retained austenite is maintained at the same level. The reason is that using the upper temper converts the retained austenite to martensite but the lower temper converts very little in high alloy steels (see my tempering article). So generally higher toughness is found by using a low temper but that is attributed to the difference in retained austenite. For one example you can see these toughness numbers from Uddeholm Caldie [3]:

I am aware of one fracture toughness study of M2 high speed steel where they compared both cryo and non-cryo heat treatments which you can see here [4]:

There is a big drop in toughness around 540°C (1000°F) and then it increases above that. It’s a bit hard to tell in the figure but it appears that even with higher tempering (>540°C) to bring down the hardness the lower temper with cryo is still superior. In our study it also appears that toughness was higher when using 400°F temper rather than 1000°F:

There is also some research showing that extended hold times in liquid nitrogen for cryo leads to an increase in wear resistance but a decrease in toughness [5], so we looked at different times in cryo:

Somewhat surprisingly there is little change in toughness by using cryo, regardless of length of time. Due to variability, however, if there is any “real” difference in toughness the resolution of the test may not be high enough to tell without more repeated specimens. However, there was an increase in hardness with cryo so using the liquid nitrogen treatment led to a higher balance of hardness and toughness. Because of the overwhelming amount of research showing that cryo tends to somewhat lower toughness means that we probably can’t take this result as a hard and fast rule. Perhaps the retained austenite in the non-cryo version of Z-Wear didn’t have the right level of stability to improve toughness or there was too little of it to make much difference.

Warren uses 4 tempers with his Z-wear and he wondered if he needed all of those tempers. Usually there is an improvement in toughness by using 2 tempers instead of 1 one because any retained austenite that transforms to martensite during the first temper needs to be tempered in the second step (see the tempering article). An example of the improvement can be seen in this chart with Caldie [6]:

Warren used a minimum of 2 tempers:

There was less variability in toughness with more tempers but it is hard to tell if that is due to random chance or because of a real improvement. All other tests used 4 tempers and some of those showed higher variability than the heat treatment showed on the chart. So 2 tempers is probably enough, though if we go off just that chart then we might say that 3 tempers has a chance of a slight improvement in toughness.

Warren also used a small range of austenitizing temperatures in an attempt to keep the hardness consistent with different cryo/temper combinations. In our previous CruForgeV toughness testing it was found that austenitizing temperature was very important for toughness, and this aligns with the literature which was described in Austenitizing Part 2. In the Z-Wear testing we performed, the lowest average toughness value of all conditions was found for the highest austenitizing temperature used, so again austenitizing temperature is a very important parameter for toughness:

To see where Z-wear and Cru-Wear fit in with the other steels tested so far here is the big summary:

Z-wear was very flat for toughness relative to hardness. Perhaps some more variation in austenitizing temperature and tempering temperature would lead to higher values at lower hardness. However, the toughness at 62 Rc appears to be quite good. I’m looking forward to more steels and heat treatments being added in the future.

Summary

A low temperature temper (400°F) led to higher toughness than a high temperature temper (1000°F), even when using cryo so that each heat treatment has a similar level of retained austenite. Extended time in cryo had no effect on toughness, and in fact adding a cryo step did not affect toughness despite an increase in hardness with Z-Wear. Conventional CruWear had lower toughness than Z-Wear/CPM CruWear as expected due to larger carbides. An increase in austenitizing temperature led to higher hardness and reduced toughness. More than 2 tempers did not lead to improved toughness though 1 temper was not tested to see if there was a difference.

A summary of the raw data described in this article is shown here:

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[1] https://www.crucible.com/PDFs/DataSheets2010/dsCPM%20CruWear2015.pdf

[2] http://www.crucible.com/PDFs/DataSheets2010/dscruwearv12010.pdf

[3] Rehan, Muhammad Arbab, Anna Medvedeva, Berne Högman, Lars‐Erik Svensson, and Leif Karlsson. “Effect of Austenitization and Tempering on the Microstructure and Mechanical Properties of a 5 wt% Cr Cold Work Tool Steel.” steel research international 87, no. 12 (2016): 1609-1618.

[4] Horton, S. A., and H. C. Child. “Relationship between structure and fracture behaviour in 6W–5Mo–2V type high-speed steel.” Metals Technology 10, no. 1 (1983): 245-256.

[5] Zurecki, Zbigniew. “Cryogenic quenching of steel revisited.” In Heat Treating: Proceedings of the 23rd ASM Heat Treating Society Conference. 2005.

[6] Rehan, Arbab, Anna Medvedeva, Lars-Erik Svensson, and Leif Karlsson. “Effects of austenitisation temperature and multiple tempering on the microstructure and impact toughness of a 5 wt.% Cr cold work tool Steel.” In 10th TOOL Conference, Bratislava, Slovaki, October 4-7, 2016, pp. 1-10. 10th TOOL Conference, 2016.