Edge Retention Testing of Seven More Steels – XHP, SPY27, Maxamet, Rex 45, 420, T15, Rex 76

Thanks to Knife Thoughts, Nicholas Rossi, Jason D. Stone, and Ken McCurdy for becoming Knife Steel Nerds Patreon supporters!

Update 1/5/2024: I neglected to mention in the original writeup of this article that the Z-Max was retested along with the new Maxamet and Rex 121 tests. The total cardstock cut was updated from 909 in the original testing to 948 mm.

CATRA Testing

I wrote about my first large batch of 48 different steels (some with multiple heat treatments) in this article. I had another set of blanks cut out ready to heat treat basically right after that article was released but it has taken some time until they were all ready to test and now they are finally completed. The knives were ground and initially sharpened by knifemaker Shawn Houston. First I will give a rundown on what was tested.

The Steels

The selection is a bit random here though there are some themes. Maxamet, CPM Rex 45, CPM Rex 76, CPM Rex 121, and CPM T15 are all powder metallurgy high speed steels. 420, XHP, and SPY27 are all stainless knife steels. XHP and SPY27 are both powder metallurgy steels while 420 is produced conventionally. 420 has better corrosion resistance than the other two.

CPM T15

T15 is the oldest of these high speed steels, having been released in the 1940s. For many years it was the highest wear resistance steel available, period. With conventional steel production, its 5% vanadium was pushing the limits of what could be produced. When Crucible first introduced powder metallurgy tool steels in 1970, T15 was one of the first steels that was produced with the technology [1]. Crucible metallurgists soon discovered that vanadium alloyed powder metallurgy steels had excellent properties. In fact, vanadium carbides would be smaller than other carbide types in PM steels rather than the largest like in conventional production. CPM-T15 has relatively high wear resistance with its 5% vanadium content, and is also capable of about 68 Rc. Toughness is also pretty good given its high hardness and wear resistance.

CPM Rex 45

CPM Rex 45 is a powder metallurgy version of the high speed steel M3:2 plus cobalt. This grade is relatively common in Europe, being produced as Erasteel ASP 2030, Uddeholm Vanadis 30, and Bohler S590. And Hitachi HAP40 in Japan. When Crucible was pushing CPM M4 as a well rounded PM high speed steel early on, Stora was pushing ASP 30. Rex 45 has a bit less wear resistance than M4 (see the vanadium content) though is capable of a bit more hardness due to the cobalt addition. M4 actually also has better toughness, due to the lack of cobalt and the higher vanadium content (remember vanadium carbides are smaller).

CPM Rex 76

CPM Rex 76 is the first grade that Crucible patented specifically for production using powder metallurgy. They wanted a steel that could reach 69-70 Rc in combination with increased vanadium when compared with previous ~70 Rc high speed steels like M42. This was only practical using powder metallurgy. Rex 76 doesn’t look that different, really, from Rex 45, but with increased tungsten and carbon content. This gives Rex 76 higher potential hardness. Read more about the history of 70 Rc high speed steels in this article.

Maxamet

Maxamet is a more recent ~70 Rc high speed steel, with high vanadium content to go along with it. It was introduced in 2000 as a “carbide replacement’ high speed steel due to its combination of high hardness and wear resistance. It isn’t the only steel in this general category, see the 70 Rc high speed steel article linked above. However, it may be the best one, with a relatively fine microstructure and good toughness given how high its wear resistance is. Maxamet appears to be a modification of T15, with more carbon and cobalt for higher hardness, along with a small increases in Cr, W, and V.

Rex 121

Rex 121 is the most wear resistant steel available for purchase. I tested it in the previous CATRA article. However, as noted in that article this was the only steel that we had difficulty getting a good edge on it. So the initial sharpness/cutting ability was not good enough to show its full potential. The reason for the difficulty in sharpening was high retained austenite. The reason for high retained austenite requires knowledge of some specific elements of heat treatment so for knife buyers the following information may not mean much to you. The heat treatment I had picked was originally intended to be tempered at 1000°F to reduce RA. However, after tempering all of the knives at 300°F I checked the hardness and found 71.4 Rc and I decided we would test it since the hardness was so extreme. However, it was apparent in sharpening that the austenitizing temperature of 2050°F was too high for the low temper regime. Something like 1875-1925°F would have been more appropriate in that case. The new heat treatment uses the 1000°F temper and this knife was not difficult to sharpen, confirming the retained austenite issue. The hardness was somewhat lower at 70.5 Rc but the tradeoff was definitely worth it, and I think everyone can agree that 70+ Rc is somewhat insane anyway.

420

420 stainless is on the opposite side of the spectrum from the high speed steels, as it is certainly not capable of 70 Rc. It is, however, the first stainless steel that was ever introduced commercially, first announced in 1915. And it was developed for knives. 420 now has a reputation as a low end steel, because of its typically low hardness and use in many cheap knives. 420 can come in a range of carbon contents, however, with the spec only requiring that it have a minimum of 0.15% carbon. So some of the cheapest 420 has only 0.2% carbon, and is capable of only low hardness. There is also 420 available at around 0.38% carbon which is capable of hardness as high as about 57-58 Rc. When the steel has over 0.4% carbon it is typically called 420HC instead. 420 has excellent toughness and corrosion resistance due to the low carbide content as well. I got a lot of requests that I test “cheap” steels and I was able to get some 420 for testing.

CTS-XHP

We had an XHP knife ready for testing for the initial report but it was very thin material and we ended up losing it during grinding. So we produced another for testing. XHP was developed to be a combination of 440C and D2, with the hardness capability of D2 but the corrosion resistance of 440C. I wrote about the steel in this article. XHP offers high hardness, good wear resistance, and good toughness. And having only chromium carbides (as opposed to vanadium) makes it relatively easy to grind and polish with standard abrasives. Unfortunately, its corrosion resistance is relatively low and is on the border of even qualifying as a stainless steel. In fact, I don’t think it should be called stainless.

CPM SPY27

SPY27 is a steel produced exclusively for Spyderco. I wrote an initial article after its announcement where I wrote about the design and the expected properties. And then a follow-up article where I tested it for hardness, toughness, and corrosion resistance. Now we can finally round out those experiments with some edge retention as well.

The Tests

The steels were heat treated as shown in the following table. In general we tried to use pretty standard heat treatments for each steel, though cryo was used for each which is used by some knife manufacturers but not others. In the previous round of testing we compared cryo with no cryo in D2 steel and found no difference. Low end 420 knives can be as low as 50-52 Rc so this 420 will be better than those, of course. Not all 420 knives are so low, however.

Results

You can see the results of our testing in the chart below. Note that the dotted lines represent the approximate effect of hardness. So the edge retention of steels at different hardness levels can be estimated by the slope of those lines.

Commentary

420

Despite being at the bottom of the chart, 420 did somewhat better than expected. It did as well as many low alloy knife steels at higher hardness, such as 1095. The small amount of chromium carbide must be enough to give it some wear resistance to improve its performance. The edge retention is still not much to get excited about, of course. As noted previously in the article, my 420 being of the 0.38% carbon variety makes it better than some of the very low carbon stuff. And the 55 Rc I heat treated to is higher than some cheap knives.

XHP

XHP did relatively well, a bit higher than Elmax, S45VN, and S35VN, but a bit below S30V. This is about where it was predicted to fall with its 22% chromium carbide based on the following charts that were shown in the original article:

SPY27

I originally predicted that SPY27 would be a bit below S35VN because SPY27 has 2% V and 1% Nb while S35VN has 3% V and 0.5% Nb, which should give S35VN more of the high hardness vanadium/niobium carbides. However, in this testing SPY27 was roughly equal to S35VN when compensated for hardness. This is the second surprise I have had with SPY27 testing, as I originally expected it to have slightly superior toughness to S35VN but instead it also ended up being about the same. SPY27 behaves surprisingly similarly to S35VN despite the somewhat different composition.

Rex 45

I get asked relatively frequently whether CPM Rex 45 or CPM M4 has better edge retention. M4 has somewhat more wear resistance but they are close enough that it greatly depends on the hardness of the two knives. CPM M4 is heat treated to a relatively wide range of hardness, all the way from 60 Rc up to 64+ Rc. We chose ~61 Rc for our initial study because a fair number of knives are produced around the value, and gives a more 1:1 comparison in terms of wear resistance with other knife steels that are typically around that hardness. With Rex 45 we decided to do a standard heat treatment which results in higher hardness because the Spyderco knives produced in the steel are in the upper end of hardness. Otherwise I will be dealing with emails questioning our heat treating for the next several months. With the higher hardness, Rex 45 had superior edge retention to our CPM M4. However, if compensated for hardness the M4 appears to have better wear resistance.

Rex 76

Somewhat surprisingly Rex 76 was only about 1 Rc harder than Rex 45 despite the higher carbon and cobalt content. And the edge retention is only higher due to the increased hardness and does not seem to have benefitted from the higher carbide content that would result from the higher carbon content. So Rex 76 would primarily be useful for applications where very high hardness is desired.

CPM T15

The higher vanadium content of T15 gives it higher edge retention than either Rex 45 or M4. In combination with the relatively high hardness (~65.5 Rc), T15 has excellent edge retention. Unfortunately CPM T15 has never really caught on as a knife steel, though I don’t see why not. It has an interesting set of properties.

Maxamet

Maxamet is a notch higher in edge retention over Z-Max/Rex 86 with its higher vanadium (6 vs 5%). This is about where we expected the steel to be based on the position of Z-Max. It has very high edge retention of course.

Rex 121

With the new Rex 121 knife with a superior heat treatment it retains its edge retention crown due to better sharpening and therefore better initial sharpness.

Toughness 

For these comparisons I also did toughness experiments on CPM-T15, Rex 45, Rex 76, and Z-Max/Rex 86. Somewhat surprisingly all of these CPM high speed steels had relatively similar toughness if compensated for hardness. If you draw a line from the CPM-M4 hardness trend those other steels all fall along it. Even with the 5% vanadium steel CPM-T15 and Z-Max. And Z-Max has somewhat better toughness than Maxamet at comparable hardness which is also impressive. The toughness differences between steels do become smaller at higher hardness, however.

Our measured toughness of Maxamet is rather good considering how high in wear resistance and hardness it can be, however. At 66.7 Rc it had toughness equal to 15V at 65.3 Rc. At 68+ Rc the toughness becomes somewhat less impressive, however.

XHP has similar toughness to S35VN, SPY27, and CPM-154 but with better wear resistance. It is a bit surprising how good the toughness was with all of the carbide that is in XHP. Sometimes I wonder if an “intermediate” carbide size such as with chromium carbide powder metallurgy steels can provide better results in an impact test than the absolute smallest carbides possible. Crack initiation is more difficult with smaller carbides, but crack growth takes more energy if the distance between carbides is greater (as is seen with larger carbides). The unnotched charpy test that I use requires both crack initiation and crack growth to occur so it is a combination of both factors. Another possibility is that the softer chromium carbides are less detrimental to toughness than the harder vanadium carbides. Typically the hardness of the carbides doesn’t matter much for toughness, with carbide volume and size drowning out the effect of carbide hardness. But perhaps with specific comparisons you start to see the effect. If you compare CPM-D2 and Vanadis 8 you see something similar; those two steels have a similar amount of carbide but the carbides in CPM-D2 are larger because they are chromium carbides as opposed to the vanadium carbides in Vanadis 8. The toughness was actually slightly better in the CPM-D2. If the carbides are larger than CPM-D2 the toughness is reduced, as is seen when looking at PSF27 (sprayform D2) or conventional D2, which both have lower toughness than the PM version. Whether the toughness differences come from carbide spacing or the softer carbides I’m not sure. Comparisons with three-point or four-point bending tests (crack initiation and strength) and fracture toughness (crack growth) might be revealing in this regard but I don’t have an easy/inexpensive way to do those comparisons. You can compare the microstructure of these different steels I discussed in this article.

SPY27 has a very similar edge retention-toughness balance to S35VN. As I noted before, the two steels behave surprisingly similarly both in terms of toughness and edge retention despite relatively significant composition differences.

420 stainless likely has very high toughness. I have tested the toughness of Buck 420HC before and it did very well but I have never added it to the chart before since it was done as part of a consulting job. However, I recently talked to them and they are fine with the result being public. The 420HC does excellent, as expected, due to the small carbides and low carbide volume that is found in the steel. 420 has even less carbon, and is at lower hardness, so it would also have very good toughness. The strength would be low, of course.

Toughness vs Edge Retention

The holy grail is to have both toughness and edge retention, of course. Normally these two properties are opposed to each other. So the best steels are those that have the highest edge retention for a given level of toughness, or vice versa. The best balance of properties are typically found in the non-stainless PM steels such as 3V, 4V, Vanadis 8, 10V, etc. The values below are normalized to the hardness of the nearest-hardness toughness specimen that was tested. The effect of hardness on edge retention is more predictable than on toughness. Orange dots indicate non-stainless and blue dots are stainless.

Rex 45 and Rex 76 are somewhat low on the chart, being close to stainless PM steels in a similar edge retention range. This is because of the high hardness which limits the maximum possible toughness. There may be certain applications where the higher strength is a benefit for thin edges in cutting tasks that are more limited by deformation than by chipping. If they were heat treated to lower hardness they would still be similar to other stainless steels, they would just be closer to steels like SPY27, XHP, and S35VN. CPM-T15 shows a better property balance with its higher toughness at the somewhat lower hardness along with superior edge retention. Vanadis 8 and 10V in the 60-64 Rc range offer a bit more balanced properties than high speed steels at high hardness. Z-Max and Maxamet fit the trendline better with others of the best non-stainless steels though of course at the higher end of edge retention. Somewhat surprisingly Z-Max has better toughness than 15V for similar edge retention, all while being at a higher hardness for better strength and resistance to deformation. It is hard to say what a reasonable cutoff is for “too much” wear resistance or too little toughness, and it obviously depends on the knife and the end customer. Maybe Maxamet is a good choice for some customers that want wear resistance over all else. Rex 121 might be too extreme for me.

SPY27 and XHP end up in a similar place on the chart, especially because the toughness coupon for XHP was somewhat lower hardness (60.8 Rc) than the CATRA knife (62.6 Rc). They are competitive with S35VN and Vanax in terms of property balance, other than the fact that XHP has poorer corrosion resistance than the others in that list.

I am not that confident in my ability to predict exactly where 420 would end up, but it would certainly be 40+ ft-lbs (see 420HC) along with its ~330 mm in the CATRA test. So it would be much higher and further to the left than the other steels that were tested in this batch. The property balance wouldn’t look too bad in comparison to other grades, really. And its corrosion resistance is also excellent. Again, it is primarily the low strength that holds it back. Choices like 12C27, AEB-L, or 14C28N would be better for the superior hardness potential at the cost of some corrosion resistance. 12C27 and AEB-L are essentially very high carbon 420 steels, and 14C28N is a modification with higher chromium and nitrogen.

Summary

OK so another batch of CATRA blanks is complete. And I included a lot of mostly unnecessary commentary and analysis so hopefully that is useful to someone. I was probably most impressed with Z-Max and Maxamet in terms of their combination of toughness, hardness, and edge retention they have, though of course still at the lower end of the toughness spectrum. Maybe there is something to putting “max” in the name for steel performance. 420 is not as awful of a steel as its reputation sometimes indicates, though it would require more companies hardening it to 56+ Rc to turn that around at all. Rex 45 and Rex 76 are not really my favorite high speed steels, as the toughness-edge retention balance is not as impressive as other options. CPM-T15 is kind of in the middle; I think it could be an interesting choice for some knives but it doesn’t particularly stand out when placed on the toughness-edge retention chart. But I would prefer CPM-T15 to Rex 45 and Rex 76 if I wanted a high speed steel with 65+ Rc but better toughness than Z-Max. SPY27 behaves very similarly to S35VN despite the differences in composition. XHP has a competitive toughness-edge retention balance with other stainless steels, though its corrosion resistance is worse.

---

[1] Kasak, A., and E. J. Dulis. “Powder-metallurgy tool steels.” Powder Metallurgy 21, no. 2 (1978): 114-123.