ZDP-189 and Cowry-X – Super Steel or Overrated?

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ZDP-189 and Cowry-X

ZDP-189 is a steel produced by Hitachi and Cowry-X is produced by Daido. I have not been able to find much background information on the development of these steels. Sal Glesser of Spyderco reported he first heard about ZDP-189 around the year 2000 [1], and the earliest reference I have found to Cowry-X on Bladeforums in 2001 [2]. So both of the steels have been around for some time. The fact that two companies released essentially the same product perhaps indicates that the steel was not patented, which means little information would be available about its development. Both steels have an interesting composition with 3% carbon and 20% chromium along with a few other small additions. There are a few different reported compositions for ZDP-189 in terms of the Mo, V, and W content but below is from Spyderco.

I have been curious about this steel for some time now because of its very high potential hardness, so I was excited when Richard Airey of Barmond Special Steels offered me a piece of ZDP-189 for analysis.

Update 2/4/2020: Knife Steel Nerds reader Yudai sent me links to the patents by Daido and Hitachi. It’s nice having readers that are better at searching foreign language patents.

https://patents.google.com/patent/JPH11279677A/en?oq=11-279677

https://patents.google.com/patent/JP3894373B2/en?oq=09-104954

Hardness

The most intriguing thing about ZDP-189 and Cowry-X is the very high obtainable hardness, 67 Rc or even higher. According to the ZDP-189 datasheet, the maximum hardness is about 70 Rc if given a cold treatment in dry ice.

So one of the biggest mysteries about ZDP-189 is why it is able to reach such high hardness. I have done heat treatment experiments over a range of different stainless tool steels and most max out around 63-65 Rc, so how is ZDP-189 able to make it to 70 Rc? To answer that question we need to discuss what controls hardness so we can see which factors ZDP-189 is exploiting.

Carbon in Martensite

The primary factor that controls hardness in tool steels is the amount of carbon which is in the martensite. During austenitizing, carbide is dissolved putting carbon in solution in austenite, and then the steel is rapidly quenched to “lock in” the carbon in the martensite. Read more about the strength of martensite in this article. With 3% carbon, it is possible to get quite a bit of carbon in solution with ZDP-189 and Cowry-X.

You can see that the max hardness reaches a peak around 67 Rc or so and either levels off or even decreases past a carbon content of about 1%. The reason why the hardness can drop is because of excessive retained austenite. This can be seen in the “without subzero” tempering chart of ZDP-189 where the 1025°C austenitize led to lower hardness than the 1000°C austenitize. The higher austenitize led to more carbon in solution but excessive retained austenite so its hardness was reduced.

Retained Austenite

When steel is quenched from the high temperature austenite phase, the steel forms martensite as it is progressively cooled. Martensite formation is not controlled by time but almost entirely by temperature. So martensite formation is described by temperatures like martensite start (the temperature at which martensite begins to form) and martensite finish (100% martensite). Different alloying elements, including carbon, reduce the martensite start and finish temperatures, and the finish temperature can be below room temperature. When the martensite transformation is incomplete, there is austenite remaining in the microstructure, which is called “retained” austenite. Austenite is much softer than martensite and therefore when it is present in significant amounts it will reduce hardness. Cold treatments like dry ice or liquid nitrogen are used to cool the steel closer to the martensite finish temperature and therefore reduce the retained austenite content and increase hardness. You can read more about cold treatments in these articles: Part 1, Part 2, Part 3. There is some point, however, where even liquid nitrogen no longer converts all of the retained austenite so there is some limit to hardness that can be obtained even with cryo treatments.

Stainless steels have a significant amount of chromium in solution to improve corrosion resistance. However, chromium reduces martensite start and finish temperatures which increases retained austenite. Here is an equation that approximates the contribution of each element on martensite start:

Ms (°C) = 539 – 423*C(%) – 30.4*Mn(%) – 12.1*Cr(%) – 7.5*Mo(%) – 7.5*Si(%)

You can see that chromium does not have the very strongest effect on Ms, but when you have 10-15% Cr in solution it begins to add up. This is the primary reason why most stainless tool steels top out around 63-64 Rc, because with 11-12% Cr in solution that is about the limit in terms of avoiding retained austenite with cryo. Higher corrosion resistance steels with 14-15% Cr in solution have even lower limits, which is likely why LC200N/Z-Finit and Vanax are limited to about 61 Rc. Read more about the hardness limits of those steels in this article.

Does ZDP-189 have low chromium in solution to achieve its high hardness? According to Thermo-Calc estimates the answer is yes, predicting about 6.5% chromium in solution at 1025°C. I was quite shocked by this number as stainless steels are expected to have at least 10% chromium in solution. However, when looking at the Cr:C ratio this makes sense. Below I have shown the Cr:C balance for a range of steels that have little other alloying elements to muddy the analysis:

You can see that even the non-stainless D2 steel has a higher Cr:C balance than ZDP-189. This is not a perfect way for estimating chromium in solution but it gives us a simple check of the Thermo-Calc estimate. What does this mean for the corrosion resistance of ZDP-189? We will get to that later, but first…

Tempering Carbides

In general, hardness is primarily controlled by the strength of the martensite and then limited by the retained austenite content. However, carbides also affect hardness. When tempering, very tiny carbides are formed, and at certain tempering temperatures these carbides are the right size to increase hardness. With stainless steels there are two peaks, which you can see in the ZDP-189 hardness curves posted above. One is around 100°C (212°F) and the other is around 525°C (975°F). You can read more about this “precipitation strengthening” in this article on tempering. The ZDP-189 datasheet recommends a tempering range of 100-150°C (212-300°F) which are the lowest tempering temperatures I’ve ever seen recommended in a datasheet. This recommendation is apparently to utilize that precipitation strengthening peak with the low tempering temperatures, without regard for toughness or other adverse effects from very low tempering temperatures. This recommendation is odd to me because the hardness is still quite high even with a 200°C (400°F) tempering temperature. Apparently they are seeking hardness over all else.

Primary Carbides

The larger “primary” carbides that form during casting and contribute to wear resistance can also affect hardness, at least when present in very large amounts. Below I have a comparison between Vanadis 4 Extra (8% carbide), 10V (16% carbide), and 15V (23% carbide) which are relatively similar steels but with different amounts of vanadium carbide. This is the “as-quenched” hardness for each steel after austenitizing, plate quenching, and then a dip in liquid nitrogen for an hour. You can see that the peak hardness was higher when there was more carbide in the steel:

ZDP-189 has a very high content of carbide, about 30%. This very high carbide content probably helps to increase the hardness of the steel. Below is a micrograph I took of my ZDP-189 and it has more carbide than any steel I have photographed other than Rex 121 which I have shown below as a comparison. You can compare with other steels in this article.

ZDP-189 – 1850°F austenitize (31% carbide volume)

Rex 121 – 1925°F austenitize (32% carbide volume)

Hardness Summary

Therefore ZDP-189 maxes out hardness in several ways: 1) high carbon in solution, 2) low retained austenite from the low chromium in solution, 3) low recommended tempering temperatures for precipitation strengthening, and 4) high carbide volume. I only performed one heat treatment with ZDP-189 which used 1850°F austenitize, liquid nitrogen, and 400°F temper. The datasheet shows about 67 Rc but I got 65 Rc with that heat treatment. I’m not sure what caused the discrepancy. I did not perform a range of heat treatments to see its max potential hardness. However, a 2 Rc deviation is not incredibly large, and at least 67 Rc should be possible by reducing the tempering temperature I used. Perhaps a more optimized austenitizing temperature could further increase hardness.

Toughness

I performed a toughness measurement using the same heat treatment: 1850°F, plate quench, liquid nitrogen, and 400°F temper for 65 Rc. With its high carbide content and high hardness the toughness of ZDP-189 is not expected to be high. And indeed that was found in the toughness measurement. I have the broader view of the stainless chart and also a zoomed-in view to see where it fits better:

ZDP-189 had the lowest toughness of any other stainless apart from maybe the 62.5 Rc N690. However, no other stainless steel was tested above 64 Rc. AEB-L and CPM-154 both had significantly better toughness at 64 Rc. Rex 121 and Maxamet were tested with even lower toughness but those were 67 Rc or higher. So there aren’t many comparisons in a similar hardness range. Regardless, the toughness is not particularly high as we would expect from a steel at 65 Rc with a high volume of carbide. Again, this toughness measurement is not a major surprise, as even Hitachi did not measure the toughness of ZDP-189 as being very good:

Edge Retention

I have an experimental result for ZDP-189 from CATRA testing, with a value of 162%. That percentage is relative to 440C at 58-59 Rc (with identical edge geometry). So 440C is set to 100% and everything else is compared to that value. This edge retention of ZDP-189 is relatively high but still below stainless steels like S90V. This is because ZDP-189 is made up of the softer chromium carbides which do not contribute as much to edge retention as vanadium carbides such as in S90V.

In the chart above looking at the trendline for chromium carbide ZDP-189 looks a tad low. If this is due to experimental variability (such as somewhat different edge geometry or sharpening leading to a lower value) that would take ZDP-189 closer to S90V, around 190%. Or perhaps this is indicating that there is some saturation of the effect of carbide volume past some amount. Hopefully we can conduct some CATRA experiments in the future to look into this more. However, a steel like S90V gets higher edge retention with less overall carbide which presumably means that it would have superior toughness for its given level of edge retention.

Corrosion Resistance

As indicated in the hardness section, ZDP-189 appears to have low chromium “in solution” which is primarily what controls corrosion resistance. However, Hitachi presented corrosion experiments with ZDP-189 that show the steel having comparable corrosion resistance to 440C and ATS34:

I previously wrote about corrosion resistance in this article, where I gave ZDP-189 a very low rating for corrosion resistance and predicted that it did not qualify as “stainless.” In that article I finished a range of steels to 400 grit after heat treating them and sprayed water on them. All of the steels were rust/corrosion free except for XHP and so I deemed that it does not qualify as a “stainless” steel though that definition is surprisingly dubious among metallurgists. I repeated this experiment with ZDP-189 along with Takefu SG2 and VG10 at the same time. The two Takefu steels passed without corrosion but the ZDP-189 steel saw significant rusting after only 8 hours.

I have no good explanation for why Hitachi found decent corrosion resistance with ZDP-189 while I did not. Perhaps those mass-loss acid tests do not predict rusting well. Or maybe they had poor testing procedures or exaggerated their results; I cannot say. However, Thermo-Calc predicts low Cr in solution, the Cr:C ratio is very low, and the high obtainable hardness all point to the Cr in solution being low. Furthermore, there are reports from users about ZDP-189 corrosion resistance being relatively poor [3]. So I trust my corrosion test over Hitachi’s. ZDP-189 is not a stainless steel.

Sharpening and Finishing

ZDP-189 has all chromium carbide which is good from a sharpening or finishing standpoint because those carbides are softer than aluminum oxide. ZDP-189 is used in many high hardness Japanese kitchen knives which are often sharpened with aluminum oxide-based waterstones. Vanadium carbides in steels like S30V and S90V are harder than aluminum oxide which can make sharpening them more difficult with aluminum oxide abrasives. Not impossible, but more difficult. This fact likely makes sharpening ZDP-189 a bit easier compared to other steels with a similar level of edge retention. Finishing or polishing the steel would also be easier thanks to the lack of vanadium carbide.

Replacing ZDP-189 with a Real Stainless

There are many non-stainless steels that can achieve 66+ Rc making ZDP-189 a bit less special. However, what if a maker wants a steel to match the performance of ZDP-189 but wants one that is actually stainless rather than a “semi-stainless” steel? First we should list off the properties we want to match:

1. High hardness
2. High edge retention
3. A lack of vanadium carbides to help with sharpening

CPM-154

To achieve high hardness we want the chromium in solution to be on the lower side but still be stainless. One way to improve corrosion resistance without chromium is with molybdenum, as shown in this chart from my corrosion resistance experiments:

You can see that CPM-154 has the lowest Cr of the steels on the chart, but has decent corrosion resistance thanks to its high Mo content. CPM-154 also passed the “is it stainless?” test with distilled water. Molybdenum has less effect on retained austenite than Cr (see the Ms equation) so this may be a way to achieve relatively high hardness despite being stainless. I also tried low tempering temperatures (<300°F) to see the boost possible from precipitation strengthening. I don’t normally recommend tempering below 300°F but Hitachi does with ZDP-189 so I figure it’s not cheating. CPM-154 also has the benefit of being made up of chromium carbides so sharpening is not affected by the harder vanadium carbides.

I already knew the approximate austenitizing temperature for peak hardness from previous heat treating experiments so I restricted my analysis to 2000, 2025, and 2050°F austenitizing temperatures with a 20 minute hold. I then plate quenched, dipped in liquid nitrogen for 12 hours or so, and then tempered twice for 2 hours each time at the following temperatures:

It looks like we did not quite reach 66 Rc but we almost got there. The drop in hardness above 2025°F is from excess retained austenite with all that carbon and chromium in solution. I did a heat treatment of 2025°F with 300°F temper that resulted in about 64.1 Rc for toughness specimens that you can see in the previous toughness diagrams. The ~64 Rc CPM-154 had significantly higher toughness than ZDP-189 so it has a toughness benefit over the ZDP. It would be interesting to see what the hardness is like with the 250°F temper and 65+ Rc but perhaps 64 Rc is enough for most people.

S90V and S110V

So I think CPM-154 is a decent option given its relative ease in sharpening, potential for 65+ Rc, and good stain resistance. However, it is likely a step down from ZDP-189 in terms of edge retention even when heat treated to that high hardness because of the reduced carbide content. There are two other options to look at if we drop the requirement of avoiding vanadium carbides, which leads us to S90V and S110V. These steels exceed the edge retention of ZDP-189 but are somewhat more time consuming to finish or polish because of the hard carbides. I did a set of heat treatments with these two in an attempt to max out the hardness and found that they could reach 66+ Rc:

Those steels are helped in achieving high hardness because of the high carbide content, significantly higher than CPM-154. The S110V has more chromium in solution which would lead to more retained austenite, but the cobalt in S110V reduces retained austenite so the chromium is offset. Unfortunately I do not have toughness results for either of these steels, whether at high hardness or not. They will come in the future. But these steels have high hardness capability, high edge retention, and good corrosion resistance. Therefore I recommend CPM-154 for toughness and ease in sharpening and S90V or S110V for highest edge retention and hardness.

Could ZDP-189 be Redesigned to be Stainless?

ZDP-189 could be redesigned to be stainless, depending on the level of hardness that would be acceptable and still meet the target properties. To maintain a similar chromium carbide volume for wear resistance but increased corrosion resistance we need higher chromium but lower carbon. Increasing chromium alone would lead to higher corrosion resistance but even more carbide which we don’t want. Reducing carbon alone would lead to reduced carbide and hardness but improved corrosion resistance. But increased chromium and reduced carbon can maintain carbide volume while increasing corrosion resistance. For example, according to Thermo-Calc, a steel with 2.28% C and 24% Cr would maintain a similar chromium carbide content of 30% while increasing the chromium in solution to 11% at 1875°F. That would also mean a significant reduction in carbon in solution to 0.4%, reducing the hardness, probably still 63 Rc or so given all that carbide and a cryo treatment. With higher austenitizing temperature it would be heat treatable to at least 64 Rc if not a couple points higher. Just like with CPM-154 and many other stainless steels, one of the primary limiting factors for hardness would be excess retained austenite. But it seems unlikely that any modified version of ZDP-189 is coming any time soon.

ZDP-189 vs Cowry-X

As you might expect, the small alloy addition differences between ZDP-189 and Cowry-X are not expected to make a significant difference in properties between the two. Perhaps the higher Mo and W in ZDP-189 somewhat improves the corrosion resistance but as seen in this article it’s not enough. I would be surprised if there was much measurable difference between the two steels.

Summary and Conclusions

ZDP-189 is an interesting steel because of its high hardness capability despite being advertised as a stainless steel. However, it has low toughness, and corrosion resistance experiments confirm that the steel is not actually very stain resistant. I do not consider it to be a stainless steel. Its edge retention is good but gets that edge retention by a very high content of carbide which reduces toughness and fine edge behavior. There are many other steel choices that can achieve high hardness and/or edge retention if stainless levels of corrosion resistance are not required, making ZDP-189 much less special. Because of the false advertising of this steel as being “stainless” I give ZDP-189 the Knife Steel Nerds “Most Overrated Steel” award.

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[1] Hashew, Mike. “The Ferrari of Blade Steels?” Blade Magazine February 2005, pp. 66-69.

[2] https://www.bladeforums.com/threads/vg-10-steel.180486/

[3] https://www.bladeforums.com/threads/zdp-189-corrosion-resistance-compared.992801/#post-11297843