USA vs Europe – Powder Metallurgy Knife Steels

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Video

I also have a video which covers the same information (more or less) in this article.

Intro

Jim Sutton of Zapp Tooling Alloys contacted me recently to talk about testing their Z-Wear. Zapp has previously purchased their powder steel from a USA company, and they are adding a European steel company as another source. They wanted to compare the properties between the two suppliers to ensure they are equivalent, and/or to determine if one is better than another. Some companies have claimed that their powder metallurgy process is better than other companies, notably Bohler and Uddeholm (of the parent company voestalpine) claim their process is “3rd generation” for a finer powder size and lower “inclusions” in the steel [1]. I previously compared the inclusion content and properties of Bohler, Uddeholm, Crucible, and Carpenter powder metallurgy steels in this article. Bohler-Uddeholm has previously claimed that Erasteel uses “2nd generation” technology, though Erasteel has since made improvements to their process which they call Dvalin which makes similar claims to reduction in inclusions to Bohler-Uddeholm:

Common impurities in steel include sulfur (S), phosphorus (P), and oxygen (O). My testing found P and S to be similar among the manufacturers, but oxygen content to be somewhat lower for Bohler and Uddeholm, Carpenter to be the worst, and Crucible in the middle:

That prior article also has micrographs of polished specimens of each where you can look at the impurities themselves. None of them looked particularly “clean” but you can look at the micrographs and decide for yourself.

However, these differences did not lead to a difference in measured toughness, as M390 and 20CV had the same toughness:

As a side note, I found this austenitizing temperature of 2140°F to be too high leading to inflated toughness, which you can read about here. Also the reported finer powder size did not lead to an obvious difference in carbide size:

M390

20CV

A much bigger effect than powder size is carbide type. The similar CPM-4V (Crucible) and Vanadis 4 Extra (Uddeholm) have only vanadium carbides, and their average carbide size is much smaller than either M390 or 20CV:

Vanadis 4 Extra

CPM-4V

And when testing the toughness of those two grades there was no advantage to the Vanadis 4 Extra. In fact the CPM-4V had greater toughness in the one condition that was the same between the two (1975°F austenitize, 400°F temper, transverse testing direction). However, this was due to small composition differences between the two grades. The CPM-4V had lower carbon so resulted in slightly lower hardness but better toughness.

The only comparison between manufacturers involving Erasteel I have performed was of RWL34 (Erasteel) and CPM-154 (Crucible) which are basically identical. In this case the Erasteel had slightly higher toughness. This was part of a study of Damasteel stainless Damascus which you can read here.

However, all of these tests were done with my normal average of 3 or 4 toughness coupons. Perhaps the difference in toughness would only show up with more coupons tested, looking for one-off coupons with a large oxygen inclusions, so I suggested to Zapp that we test 9 coupons from each manufacturer and determine if there were any clear differences between the two.

Z-Wear

I previously tested Z-Wear, which you can read here. That study was done with knifemaker Warren Krywko, and we found a few key things:

1. A “low” temper of 400°F led to a better balance of hardness and toughness than a “high” temper of 1000°F.

2. Cryo led to an increase in hardness with little or no change to toughness. Increased cryo time beyond 1hr did not affect the properties.

3. The powder metallurgy versions, Z-Wear and CPM-CruWear, have significantly better toughness than the conventional version, CruWear. This is due to the finer microstructure from powder metallurgy.

CruWear (ingot, not the CPM version)

Z-Wear (Powder Metallurgy)

4. When tempering at 400°F, there was no change in properties with more than 2 tempers:

New Z-Wear Testing

So the new test used a 1950°F austenitize for 30 minutes, plate quench, cryo in liquid nitrogen, and a double temper at 400°F for two hours each time. 9 coupons were produced and tested from each of the two manufacturers.

The USA version ended up 1 Rc harder but one ft-lb lower in toughness. I didn’t get any one-off low toughness coupons that would suggest that one version or the other has large carbides or inclusions leading to lower toughness. The standard deviation of the toughness of the European version was slightly higher, but that could just be random, plus higher toughness coupons tend to have a bigger spread than low toughness coupons, so I wouldn’t read anything into that. To determine the reason for the higher hardness of the USA version I measured the composition of the two using OES (optical emission spectroscopy) along with combustion. Combustion tested carbon, sulfur, nitrogen, and oxygen. OES cannot test oxygen, and the carbon and sulfur values from combustion is somewhat better than OES, so both are presented in the table (listed as LECO C or LECO S at the bottom).

The LECO carbon values were a little lower than the OES, but both tests showed the USA version having higher carbon, which is probably the primary reason that steel came out with higher hardness. The P and S content were similar between the two, phosphorus a little lower in the USA version but sulfur a little lower in the European version. The oxygen content of the European version was similar to the Bohler and Uddeholm stainless steels I previously tested. However, somewhat surprisingly the oxygen content of the USA version was the lowest result I have yet had for any PM steel. Perhaps something about the composition of the stainless steels led to somewhat higher oxygen content such as the high chromium content. I am not sure if the USA oxygen content was so low in this case due to variability between heats of steel or some improvement to their process since my last test. Either way I think the results of both of these steels look good.

Because the hardness was different between the steels I wanted to compare against the previous Z-Wear results to see the “normalized” toughness for a given hardness. To see if 1 ft-lb toughness is as much improvement as we would expect based on roughly 1Rc lower hardness. So I plotted them all on the same chart:

The new USA material tests roughly fall on the same trendline as the previous Z-Wear testing, while the new European material falls a bit below the trendline. I believe this is likely due to the small composition differences, or perhaps a difference in annealing (resulting in a different response to the same heat treatment). I would consider these materials to be basically equivalent.

Summary and Conclusions

While there has been significant marketing from the European powder metallurgy companies that they have superior powder metallurgy technology, I have not found a difference when it comes to impact toughness testing. Whether other types of tests would reveal a difference I can’t say, but in general I think performance-wise the composition of the steel matters more than the company making the powder.

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[1] Tornberg, C., and A. Fölzer. “New optimised manufacturing route for PM tool steels and High Speed Steels.” In Proceedings of the 6th International Tooling Conference: The Use of Tool Steels: Experience and Research, vol. 1, pp. 10-13. Karlstadt Sweden, 2002.