XHP Steel – History and Properties

Thanks to John Bates, Kenny Lazarus, Robert Abel, and Matt Danielson for becoming Knife Steel Nerds Patreon supporters! I have some exclusive Patreon supporter content this week. Pete of the Cedric and Ada Youtube channel posted this past week a video summarizing his rope cut tests on 14 Spyderco Mule knives in different steels. This is a fun test because all of the knives are nearly identical and only the steel and heat treatment is different, so it is a better steel comparison than some of the others between different knives. I already have a previous article where I compared CATRA testing to rope cutting so this new test didn’t warrant a whole new article, but I offered an analysis of his test for Patreon.

440XH and CTS-XHP Steel

XHP is a powder metallurgy steel produced by Carpenter Technology with the following composition, along with the similar 440C and D2 steels:

XHP started out as a steel named 440XH which was patented in 1994 [1] by Paul Novotny, Thomas McCaffrey, and Raymond Hemphill. XHP was originally produced as a conventionally cast “wrought” steel rather than as a powder mteallurgy steel. The patent and other development documents by Carpenter talk about 440XH as a steel designed to combine the best properties of D2 and 440C. 440C has good corrosion resistance but the max hardness is a bit low. D2 has excellent hardness but its combination of 1.5% carbon and 12% chromium does not have sufficient corrosion resistance to be stainless despite the 12% chromium. Learn why D2 isn’t stainless in this article. Adding 0.5% carbon and a small vanadium addition to 440C makes 44XH. Or adding 4% chromium to D2. So 440XH, which is now called XHP, can be seen as either a high hardness 440C or a high corrosion resistance D2.

The researchers looked at a range of alloy combinations by producing 17 pound ingots in the laboratory which were then forged and tested [2]. They focused on hardness and corrosion resistance to find the optimal composition for the desired properties. Hardness was tested “as-quenched” after austenitizing at a range of temperatures from 1750-2000°F. The corrosion resistance of each steel was tested by machining a cone specimen and heat treating it by austenitizing at 25 minutes and air cooled. The specimens were then tested in a 95% relative humidity environment at 95°F for 1-200 hours. The samples were then rated for the percentage of the surface with visible rust. Results of the hardness and corrosion resistance tests are shown below:

Alloy codes 88, 90, 89, and 84 met the hardness target, though 90 and 84 had worse corrosion resistance than D2, and alloy 88 was only marginally better than D2. Alloy 91 had similar or superior corrosion resistance to 440C but did not meet the hardness target. Alloy 89, however, was similar to 440C in corrosion resistance and also had high hardness. The compositions of all of these alloys are not shown in the presentation but alloy 89 was similar to the final composition of 440XH.

Properties of Conventional 440XH

Along with improved hardness relative to 440C, the new 440XH was found to have superior wear resistance as well (lower is better):

However, the toughness of 440XH was reduced relative to 440C:

The relatively low toughness of 440XH is explained by comparing the microstructure between 440XH and 440C, where 440XH has significantly more carbide and much larger carbides:

Micrograph of 440C steel [2]

Micrograph of 440XH [2]

Powder Metallurgy 440XH

The relatively low toughness of 440XH somewhat limited the broad use of 440XH outside of applications that are limited by wear resistance. Therefore, a powder metallurgy version of 440XH was explored. The properties of the powder metallurgy version were also compared to powder metallurgy 440C, named 40CP. It was discovered that the toughness of 40CP was only marginally better than the conventional 440C, but the PM 440XH was actually found to be superior to 40CP:

To determine why the PM 440XH had superior toughness to PM 40CP, they compared the microstructure of the two steels:

PM 440C (40CP) [2]

PM 440XH (XHP) [2]

The microstructures look relatively different. The 40CP has more “clumping” of the carbides together with relatively oddly shaped carbides as a result. The PM 440XH has more spherical carbides that have a relatively large diameter for powder metallurgy, consistent with chromium carbides in PM steels. They measured the diameter of the carbides and did a full count of different carbide sizes. The 40CP has more of the largest carbides, which probably explains its poorer toughness:

Therefore the PM 440XH was found to have superior toughness, wear resistance, and hardness when compared to PM 440C (40CP), both with similar corrosion resistance. So it appears to be a superior choice overall.

Properties of XHP Steel

The above discussion is related to the development process as seen by Carpenter metallurgists with the tests that they performed. Below I have a discussion of XHP properties when compared to other knife steels.

Microstructure

The micrograph I took of XHP looks relatively similar to the one provided by Carpenter. It has relatively spherical carbides that compare relatively closely in size with a steel like CPM-154. You can compare the microstructure with other steels in my article of my micrographs. Through analyzing the micrograph with “point counting,” I found the steel to have about 21% carbide volume which is a relatively big number, similar to steels like S90V, 20CV/M390, Maxamet, and 15V. The high carbide volume means relatively high wear resistance but relatively poor toughness when compared with lower carbide volume steels.

Wear Resistance and Edge Retention

Comparing the dry sand rubber wheel wear resistance of XHP to other Carpenter steels, it is very good (35 mm^3), though still not to the level of a steel like 10V with its very hard vanadium carbides.

This good wear resistance also translates to edge retention. XHP has similar edge retention to S35VN or S30V. See in the chart below and compare 3.5% V (S35VN) to the 22% chromium carbide value (XHP):

The lack of vanadium carbide does mean that XHP will not have the same level of edge retention as a steel like S90V, however (9% VC). To learn more about what controls edge retention read my articles here: Part 1 and Part 2.

Toughness

I planned on performing independent toughness tests on XHP before writing this article, but I have had difficulty purchasing any in an appropriate size, so we will have to wait on those results. Cold Steel Knives decided to move away from XHP due to availability issues [3], so perhaps this is a common problem. However, we can compare with other steels tested by Carpenter and get an idea of where it sits:

The XHP toughness value above is a bit lower than earlier in the article. This value came from the datasheet while the earlier value came from the development powerpoint. As you can see on the chart, in the broader picture XHP does not have particularly great toughness. It can perhaps be said to have “good” toughness. In our toughness testing it would likely be in the 6-10 ft-lb range around 60 Rc:

Hardness

I did some limited heat treatment experiments with XHP, austenitizing for 15 minutes, plate quenched, liquid nitrogen for 1 hour, and then tempered twice for 2 hours. XHP can reach very high hardness values, up to about 65 Rc.

Those measured values are very close to those presented in the datasheet for XHP [4]:

Corrosion Resistance

I have an article where I calculated the chromium and molybdenum in solution for different knife steels to estimate the approximate corrosion resistance. In that article XHP was estimated to have the lowest chromium in solution of any stainless steel apart from ZDP-189. It makes sense that increasing the carbon content of 440C by 0.5% would lead to a significant reduction in corrosion resistance. You can read about carbon-chromium balance in stainless steels in this article. In my article on D2 corrosion resistance, I estimated that D2 needs about 17.6% chromium for good corrosion resistance using an austenitizing temperature of 1950°F, which is significantly more than the 16% in XHP. The high attainable hardness of XHP is also a clue to low chromium in solution for reasons spelled out in this article. All of those estimates indicate that XHP has relatively low corrosion resistance, which seems to contradict the result from Carpenter showing similar corrosion resistance to 440C. Perhaps that is because of the corrosion testing that was employed (humid environment) rather than in salt water or different acids. XHP may have sufficient corrosion resistance to prevent broad rusting in a humid environment for 200 hours, but differences with other stainless steels are perhaps not seen except for in longer exposure or more extreme environments. However, I have not done any independent corrosion testing on XHP.

Update: I have since tested the corrosion resistance of XHP and found it to be poor.

Finishing and Polishing

The low vanadium content of XHP means it is made up of chromium carbides which are softer than typical abrasives used for polishing and sharpening of steel. That means that XHP should have relatively good ease in finishing, particularly relative to its degree of wear resistance and edge retention. XHP is likely easier to hand finish than S30V, Elmax, M390, S90V, and other high vanadium stainless steels.

Summary and Conclusions

XHP started out as a combination of 440C and D2, an effort to combine the high hardness of D2 with the high corrosion resistance of 440C. The steel had relatively poor toughness, however, due to having very large carbides. A powder metallurgy version, however, had better toughness than PM 440C (40CP). The edge retention of XHP is good, matching S30V and S35VN. The toughness is good as well, likely being similar to other powder metallurgy stainless steels, though independent testing has not yet been performed. XHP can attain rather high hardness for a stainless steel, about 65 Rc, which gives it some versatility in heat treatment. Despite the results of corrosion testing by Carpenter, I have reason to believe the corrosion resistance of XHP is relatively low compared to other stainless steels. XHP should be easier to finish than other PM stainless steels with high vanadium contents, giving it a good balance of ease in finishing and edge retention.

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[1] Novotny, Paul M., Thomas J. McCaffrey, and Raymond M. Hemphill. “Corrosion resistant, martensitic steel alloy.” U.S. Patent 5,370,750, issued December 6, 1994.

[2] A PowerPoint summarizing the development of XHP was sent to me by Paul Novotny, I’ve attached it here: Development of Carpenter Micro-Melt 440XH Alloy

[3] https://www.coldsteel.com/blog/S35VN-steel-change/

[4] https://cartech.ides.com/datasheet.aspx?i=102&E=343&FMT=PRINT