Z-Tuff and CD#1 – The Toughest Knife Steel?

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CD#1 and Z-Tuff History and Composition

The first of these steels to be released was Carpenter’s CD#1 around 2004 [1]. The “CD” stands for coining die, which is an application that benefits from high toughness. The Zapp version, Z-Tuff, came later, though both are very similar. These steels have their roots in the “Vasco Die” series of 8% Cr steels, of which CPM-3V and CPM-CruWear are a part. CPM-3V is a powder metallurgy version of Vasco Die, released by Vanadium Alloys Steel Company (Vasco) of Latrobe, Pennsylvania in the 1960s. CPM-CruWear is a powder metallurgy version of the higher carbon version, Vasco Wear. Vasco Wear had some use in knives in the 1980s, such as from knifemaker Ted Dowell and knife company Gerber. There was also a lower carbon version in this series called Vasco Tuf. You can read more about the history of these Vasco grades in this article. They all combine 8% Cr with vanadium addition for wear resistance, and are known for their good balanced properties. Vasco Die was originally advertised as combining the toughness of A2 with the wear resistance of D2, which are both popular die steels.

CD#1/Z-Tuff are basically modernized versions of Vasco Tuf with reduced carbon and vanadium relative to CPM-3V. Interestingly, there is also 1.5% nickel. Nickel is known for increasing toughness of tool steels, but it is a relatively rare addition to high alloy tool steels, generally being used in low alloy steels like L6 and 15N20.

Microstructure

The reduced carbon means that Z-Tuff has even less carbide than CPM-3V and the powder metallurgy technology keeps the carbides small. Those two factors mean that we expect excellent toughness from CD#1 and Z-Tuff. The vanadium also means that we will see relatively good wear resistance for the steel, given its level of wear resistance. But of course steels with higher carbon and vanadium like CPM-CruWear, CPM-4V, etc. are expected to have higher wear resistance.

Z-Tuff – 3% chromium carbide, 1% vanadium carbide

3V – 5% vanadium carbide

CPM-CruWear – 6% chromium carbide, 3.5% vanadium carbide

Toughness

I’ve got a few new coupons heat treated by knifemaker Warren Krywko where we looked at a range of austenitizing temperatures to see if the toughness changes. Higher temperatures means more carbide is dissolved which is beneficial, but can also mean higher hardness and grain growth which would lead to reduced toughness. In the case of Z-Tuff both the hardness and toughness level off at 1925°F and above. Using 1900°F means a bit lower hardness and toughness. Therefore 1925°F remains my recommendation of austenitizing temperature. The lower temperature is somewhat safer, and may mean slightly better wear resistance in case more carbide is dissolving at the higher temperatures. But apparently the small amount of vanadium carbide is effective at limiting grain growth up to at least 2050°F. Maybe in the future we can see if the tempering temperature can be reduced below 400°F for higher hardness and still maintain good toughness. However, the 61-62 Rc achieved with this heat treatment is quite good for a high toughness steel. We could also make comparisons with the typical 1000°F temper, but previous experiments with steels like CPM-CruWear found the low temperature tempering range to be superior. (Note: we ended up with only two of the 1975°F coupons for toughness testing rather than my preferred three, and one measured 37 and the other 78 ft-lbs, so I am not including it in the chart).

In the following chart you can see where Z-Tuff stacks up compared with other high alloy (non-stainless) tool steels. It has the highest combination of hardness and toughness of any steel I have tested. Below that I have charts for low alloy tool steels, where only 8670 and 5160 reach the heights of Z-Tuff, though at 1-2 Rc lower hardness. AEB-L and 14C28N are the closest for comparable toughness-hardness in the stainless group, though Z-Tuff is still better.

Edge Retention and Wear Resistance

I tested a CD#1 CATRA coupons some time ago. I used 1925°F austenitize with 400°F temper which resulted in 60.9 Rc. As expected it didn’t set any records, but it did manage to have a similar level of wear resistance to steels like A2, A8 Mod, and AEB-L, which is sufficient for many applications, particularly those that need toughness.

Corrosion Resistance

I don’t think I’ve tested the corrosion resistance of Z-Tuff, though the ~8% Cr does give it corrosion resistance similar to steels like D2, CPM-3V, and CPM-CruWear. Perhaps we could put it in the “semi-stainless” group. In an experiment performed by Scott Larimore and Kelly W reported to the Bushcraft USA forum, where steels were heat treated and ground by Scott, and then placed in the elements at Kelly’s house, CD#1 and Z-Tuff did very well. The CD#1 and a few other steels including 4V, CPM-CruWear, and 3V were tested with both a high (~1000°F) and low (~400°F) temper, which resulted in significant differences, which is part of the reason why I prefer low temperature temperiing. CD#1 with the low temper and Z-Tuff saw little or no corrosion, while 5% Cr tool steels like 10V, M2 and A2 all had significant rusting. And of course the low alloy steels like O1, 6150, 8670, 15N20 and 52100 all rusted. From this test it looks like CD#1 may have done somewhat better than Z-Tuff, I don’t know if that can be attributed to its somewhat higher chromium or if it is due to the inherent randomness of a test like this.

Corrosion tests done by Scott Larimore and Kelly W. CD = CD#1, Cru = CPM-CruWear, S30 = S30V, S35 = S35VN. Anything with an “L” after it was given a low temperature temper and “H” means a high temperature temper.

Could Z-Tuff and CD#1 Be Improved?

One thing I notice right away about the microstructure of Z-Tuff is that it has a substantial amount of chromium carbide. We could likely maintain toughness at the same level while increasing wear resistance by re-balancing the composition with increased vanadium. Though I suppose we would end up with something only slightly different than CPM-3V.

Summary and Conclusions

Z-Tuff and CD#1 do indeed have very excellent toughness, among the best for knife steels that reach 60+ Rc. And they balance that with some decent wear resistance as well. For more wear resistance you can move up to CPM-3V or CPM-CruWear. For stainless you can use AEB-L or 14C28N instead at the cost of a little toughness. It was interesting to see that the heat treatment is relatively insensitive to austenitizing temperature, anywhere between 1925 and 2050°F seemed to work fine.

[1] Carnes, Robert and Gary Maddock. “Tool Steel Selection.” Advanced Materials & Processes June (2004): 37-40.