CPM SPY27 Experiments – Toughness, Corrosion Resistance, and More

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Spyderco-Exclusive CPM SPY27 

I have an article about SPY27 which I wrote immediately after the announcement of the steel. At that point very little information was available about the steel other than the composition and that it was produced by Crucible steel. The composition of the steel along with some other in the same class are shown below.

For that article I predicted the relative performance of the steel based on its composition as no material was available for me to test. As I said in the article, “I am probably unlikely to get flatstock for experiments due to the steel being exclusive to Spyderco.” However, after publishing I was contacted by Niagara Specialty Metals (company that hot rolls Crucible steels) and Spyderco offering me some steel for experiments. This is a lucky opportunity since it gives me the chance to see what the properties of the steel really are. And it gives everyone else a chance to see whether I am any good at predicting properties of steel based on composition.

Hardness and Heat Treating

I heat treated coupons of SPY27 by wrapping the steel in stainless foil to protect against scale and decarburization. I austenitized the steel at different temperatures for 20 minutes and quenched between two 1″ aluminum plates which provides faster cooling rates than holding in air and also helps keeps specimens flat. The austenitizing temperatures were 1950, 2000, 2050, and 2100°F. After quenching, one third of the specimens were left at room temperature, another third of the specimens were placed in my freezer (a normal household basement variety), and the final third were placed in liquid nitrogen for a cryo treatment. You can read about cold treatments in these articles: Part 1, Part 2, Part 3.

The steel is capable of relatively high hardness, reaching over 65 Rc “as-quenched” before tempering, and nearly 64 Rc even without a cold treatment. I then tempered specimens twice for two hours each time at 300, 400, and 600°F and plotted out the resulting hardness. A 300°F temper is usually the minimum temperature I recommend while 400°F usually gives a good balance of hardness and toughness.

The steel can be heat treated over 64 Rc with a liquid nitrogen treatment and a 300°F temper and a range of lower hardness levels with different combinations of parameters. This provides the steel good flexibility, offering easy heat treatment to 60 Rc with commercial heat treating and also higher levels if desired. In the previous SPY27 article I predicted that it would have “similar hardness to S35VN, S30V, and S45VN” when heat treating with similar parameters. Below shows heat treatments using liquid nitrogen with S35VN which resulted in relatively similar hardness to the experiments with SPY27 shown above:

Corrosion Resistance

In the previous SPY27 article I predicted that it would have “similar corrosion resistance to S30V and S35VN.” I used my previously described corrosion resistance test of SPY27 along with S30V and 420. The test involves heat treating steel, finishing it to 400 grit, and then spraying it every 8 hours either with purified water or 1% saltwater. The purified water is to test if the steel is acting as a stainless. So far only two stainless steels have failed this test, XHP and ZDP-189. SPY27, S30V, and 420 all passed without corrosion after 4 days. I then refinished them and started the 1% saltwater test which was 48 hours long. I performed two heat treatments with SPY27, one austenitizing at 1950°F and another at 2050°F. Austenitizing at a higher temperature means more chromium carbide is dissolved putting more chromium “in solution” to contribute to corrosion resistance. I could not tell the difference between the two heat treatments in this case, however. The S30V and 420 were also austenitized at 1950°F. The two SPY27 specimens look relatively similar to S30V, and all three are significantly worse than the 420. SPY27 and S30V have “good” or “above average” corrosion resistance but cannot match 420. It also appears that SPY27 did somewhat worse than S30V. I am not sure if this is due to variability of corrosion testing or not. There are a couple studies that found a cobalt addition leading to slightly worse corrosion resistance but I’m not sure if this SPY27 study is conclusive enough to confirm that. In my ratings of knife steel corrosion resistance, 420 was rated similarly to S30V. I may have to do some more corrosion testing and modify the ratings.

SPY27 – 1950°F

SPY27 – 2050°F

S30V

420

Toughness

In the previous SPY27 article I said, “I would expect SPY27 to have toughness at least as good as S35VN and probably better, but that would have to be confirmed experimentally.” I tested toughness by austenitizing the steel at 2000 or 2050°F and then tempering at 400°F. All of these specimens included a cryo treatment after quenching before tempering. I compared the toughness results with S30V, S35VN, and S45VN. The toughness ended up being slightly less than S35VN though at slightly higher hardness so in the end the toughness is equal. You can also see that the trend for this class of steels is to have improved toughness with higher hardening temperatures, at least up to 2050°F. You can read more about this in the article on S35VN.

So I was right that the toughness is “at least as good as S35VN” but the “probably better” was a bit too strong. I expected the toughness to be better because of the reduced carbon and carbide content when compared with S35VN. The carbide structure is explored in the next section. In the article on cobalt additions I wrote, I noted that cobalt additions to high speed steels have been known to reduce toughness which may mean that even if the toughness was improved due to reduced carbide, it may be that the cobalt addition reduced toughness back down to the level of S35VN. However, there is another proposed explanation which I will cover along with the micrographs below.

Micrographs

I took micrographs of the steel that tested for toughness, which means the heat treatments were 2000 or 2050°F austenitize, plate quench, cryo, and tempered at 400°F. In the article on S45VN, I found that austenitizing at 2000°F led to a finer microstructure than 1950°F. There weren’t obvious differences in the microstructure of SPY27 between 2000 and 2050°F, however. I have shown the 2050°F specimen below, along with comparisons with S30V, S35VN, and S45VN where you can see that SPY27 has somewhat less carbide than the others.

SPY27 – 2050°F

S30V – 2000°F

S35VN – 1975°F

S45VN – 2000°F

Despite the reduced carbide content of SPY27 the toughness was similar to S35VN. However, as seen in the micrograph the “carbide clusters” in SPY27 are similar in size to the other steels. It may be that those larger clusters are the limiting factor for toughness. It may be that without reducing the chromium carbide content to much lower levels that the toughness remains limited to about 10-12 ft-lbs as is seen with many of these stainless PM steels like Elmax, M390, CPM-154, S35VN, S30V, SG2, etc. To see how common the clusters of carbides are I have a lower magnification image of SPY27 below:

SPY27 – 2050°F lower magnification

Other Tests

I also gave some estimates of edge retention, finishing, and sharpening of SPY27. I don’t have a good quantitative test for finishing or sharpening and I have not yet performed any edge retention experiments. However, you can still see my predictions in that previous article. SPY27 is expected to have slightly less edge retention than S35VN and to sharpen similarly or slightly better.

Summary

Experimental results of SPY27 line up well with the previously provided predictions. SPY27 has similar obtainable hardness to S35VN and similar corrosion resistance to S30V. The toughness is very close to S35VN. The microstructure confirms somewhat reduced carbide content relative to S35VN, S30V, and S45VN. The steel appears to be relatively well balanced and should do well for Spyderco.