Spyderco-Exclusive CPM SPY27 Steel Analysis

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Update 3/10/2020: We did a range of experiments with SPY27 to supplement the estimates in this article. You can read those experiments here.

History

On January 15, 2020 Spyderco announced [1] knives being produced in a new steel: CPM SPY27, a steel produced by Crucible. The steel is reportedly exclusive to Spyderco. The SPY refers to Spyderco and 27 is the atomic number of cobalt [2]. The composition [3] is as follows:

I don’t have any of the steel for analysis, and given it is exclusive to Spyderco I may not be able to get any. However, we can look at the composition and make some educated guesses on its performance which I will do below.

Design

The steel has the same Cr and Mo content as S30V and S35VN. The Cr and Mo addition provides corrosion resistance, hardenability, and secondary hardening. The Nb content was bumped up to 1% from the 0.5% of S35VN and S45VN which will of course increase the amount of niobium carbide. Niobium carbide is very hard, similar in hardness to vanadium carbide, so it makes a significant contribution to wear resistance. Niobium carbides have also been found to help refine the microstructure of steel. You can read more about niobium in this article. The vanadium was reduced to 2% relative to S35VN. There is also a small nitrogen addition which is made to increase hardness and corrosion resistance, as described in this article. Some more information on nitrogen additions to these types of PM stainless steels can be found in this article on S45VN. Speaking of S45VN, it is somewhat surprising that this steel has been announced so shortly after that steel being announced and released. Crucible seems to be coming out with more steels lately. The cobalt addition increases secondary hardening when tempering in the upper temperature range, which I have written about previously here and here. Contrary to some information available out there, cobalt does not improve toughness or corrosion resistance.

Hardness

The carbon content is of SPY27 is balanced to result in a similar hardness to S35VN, S30V, and S45VN, as indicated by carbon in solution estimates from Thermo-Calc:

Therefore we would expect the steel to reach a similar hardness to those other CPM stainless steels. Each can reach approximately 64 Rc though are generally tempered down to the 58-61 Rc range depending on the manufacturer and heat treatment.

Corrosion Resistance

I have previously performed corrosion resistance experiments and related it to predicted chromium and other alloying elements “in solution,” as you can read here. Based on the predicted alloy in solution for SPY27 it should have similar corrosion resistance to S30V and S35VN. You can see my corrosion resistance rating in the table below:

This is a pretty decent corrosion resistance, similar to Elmax (7.8), but is below steels like S45VN (8.4), M390 (9.1), and Vanax (10). You can see more ratings in the corrosion resistance article I linked to above.

Edge Retention

I used carbide contents from Thermo-Calc to estimate CATRA edge retention relative to 440C at 58 Rc (100%). I estimated edge retention for a hardness of 61 Rc. You can read about edge retention and predicting it based on carbide content and hardness here. The edge retention of SPY27 is expected to be a bit below S35VN, similar to Super Gold 2, though higher than steels like VG10, 440C, and CPM-154.

Here are some experimental results of different steels for CATRA slicing edge retention to get a feel for where SPY27 may sit:

Toughness

Toughness of steel can be difficult to predict based only on composition or carbide content. In general, lower carbide volume means higher toughness. However, it is also controlled by carbide size and I do not have any micrographs of the new steel. The 1% niobium content may help refine the overall carbide structure when compared with 0% or 0.5% Nb PM stainless steels. It may be that the reduced carbon content when compared with S35VN leads to superior toughness. See the edge retention prediction chart to see the expected carbide volume of SPY27 which came in under S35VN, S30V, S45VN, and Super Gold 2, and is also lower than Elmax and M390 which are not shown in the table. Therefore I would expect SPY27 to have toughness at least as good as S35VN and probably better, but that would have to be confirmed experimentally. Here is a chart of other steels to compare with S35VN:

Sharpening

With similar MC carbide content (NbC + VC) to S35VN and S45VN, SPY27 would be expected to sharpen similar to those two steels. Though it should be a little easier to abrade away steel because of the lower chromium carbide content (M₇C₃). The MC carbides are harder than most common abrasives apart from diamond and CBN, so lower MC content leads to easier sharpenability. The ease or difficulty in burr removal would be more based on the heat treatment, particularly when it comes to retained austenite, as lower retained austenite makes burr removal easier.

Conclusions and Summary

CPM SPY27 is a steel produced by Crucible exclusively for Spyderco. The steel looks somewhat similar to S35VN but with increased Nb and N, decreased C and V, and a 1.5% cobalt addition. The resulting steel is expected to have similar hardness and corrosion resistance to S30V and S35VN. The edge retention is likely a small step down from S35VN. The toughness is probably a bit better than S35VN. And sharpening is probably a bit easier to S35VN. Hopefully we will be able to learn more as knives are released. Unfortunately, I am probably unlikely to get flatstock for experiments due to the steel being exclusive to Spyderco.

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[1] https://catalog.spyderco.com/Reveal4/?page=10

[2] https://forum.spyderco.com/viewtopic.php?f=2&t=85769

[3] https://www.spyderco.com/edge-u-cation/steel-chart/