Matrix Steels K888 and CPM-1V

Thanks to Roman Kasé for donating the K888 and Niagara Specialty Metals for donating the CPM-1V. Thanks to my Patreon supporters for funding this study, those dollars went toward metallography and CATRA blade grinding. You can support knife steel research by visiting Patreon.com/KnifeSteelNerds

Video Version

There is also a YouTube video of the following information:

History of Matrix Steels

Matrix high speed steels were developed in the 1960’s by VASCO. I previously wrote about historical and current matrix steels in this article. The original steels came out of improved techniques for measuring the composition of the “matrix” of the steel. Some of the carbon and alloying elements go toward forming carbides, what remains is in solution in the “matrix” of the steel. Carbides help with wear resistance but are detrimental to toughness, so the metallurgists decided to make high toughness steels by taking the measured matrix composition of the best high speed steels and using that composition instead. The first was a matrix version of M2 high speed steel called VASCO-MA, and they followed that up with Matrix II, a matrix version of M42 high speed steel.

Jumping ahead to the year 2002, Crucible released CPM-1V, which is a powder metallurgy version of VASCO-MA. The steel was released to offer the highest toughness of any of their available powder metallurgy steels, even CPM-3V. In 2005, Bohler released K890, advertised as a competitor to CPM-3V, it sort of looks like a cross between CPM-1V and CPM-3V, with elevated carbon, vanadium, and cobalt when compared with CPM-1V. K888 was released at the beginning of 2024, and it is essentially CPM-1V with a cobalt addition. Cobalt is added to many high speed steels for higher hardness (with a decrease in toughness). You can read more about the effects of cobalt on steel in this article.

One intriguing thing about K888 is that Bohler is advertising that it has higher toughness at 64 Rc than their prior K890 has at 62 Rc, and that K888 sees only a small drop in toughness between 62 and 64 Rc.

Bohler’s chart of toughness for K888

Toughness drops significantly with higher hardness so the claim that K888 maintains its high toughness at such a high hardness value is exciting. For example, in testing by Hitachi of their tool steels they found that above 62 Rc the toughness of their steels drop significantly:

Hitachi toughness chart for their steels

Experiments

So I was interested to compare K888 and CPM-1V to see the effect of the cobalt addition, and I had not yet tested CPM-1V so this was a good excuse. Knifemaker Roman Kasé was able to obtain a couple small pieces of K888 for me and I got CPM-1V from Niagara Specialty Metals. CPM-1V is also now available at Alpha Knife Supply. I tested the steel for hardness, toughness, and edge retention.

Microstructure

The big benefit of matrix steels is the lower volume of carbide. Carbides are brittle particles so the less carbide found in the steel the greater the toughness. Shawn Houston did the metallography for the two steels:

CPM-1V 2050°F austenitize – 3% carbide volume

K888 2050°F austenitize – 4% carbide volume

K888 has somewhat more carbide as expected, ThermoCalc predicts 1.15% MC (vanadium carbide) and 1.84% M6C (tungsten/molybdenum carbides) in K888; CPM-1V it predicts 0.87% MC and 1.34% M6C. But the big surprise is how much bigger the carbides are in the K888. We will see if this affects the measured toughness later in the article. I have some comparable grades below such as Z-Tuff and CPM-3V, two high toughness powder metallurgy steels, as well as the conventionally produced matrix steel Caldie. CPM-1V has the finest carbides of any powder metallurgy steel I have looked at so far. You can see even more micrographs here.

Z-Tuff

CPM-3V

Caldie

Hardness

Bohler shows that K888 can be heat treated as high as 64 Rc if you use the high end of the austenitizing temperature and temper within the recommended window (blue shaded region):

I made full tempering curves for both K888 and CPM-1V. I don’t typically do full tempering curves (it takes forever), but I thought it would be useful for comparing the effect of the slightly higher carbon in the K888 as well as the cobalt addition.

The hardness I measured for K888 is roughly equal to what was shown in Bohler’s datasheet. You can see that the potential hardness is higher for K888, which is due to the cobalt addition and slightly higher carbon. However, CPM-1V is still capable of relatively high hardness. To get a better feel for the two grades, I have a comparison between the two steels on one plot:

The K888 curve is for an austenitizing temperature of 2000°F and CPM-1V is 2050°F. With the lower carbon in CPM-1V it needed a higher austenitizing temperature to have equivalent as-quenched hardness, and here the CPM-1V was slightly higher as-quenched. However, K888 saw a bigger jump in hardness with a 300°F (150°C) temper than CPM-1V did. This is “precipitation strengthening” due to small carbides coming out during tempering. The same effect happens in the high temperature range around 900°F. You can read more about precipitation strengthening, also called secondary hardening, in this article on tempering. The higher hardness after tempering is then maintained across the tempering range in K888. This is a result of the cobalt addition, which is an alloy that slows the kinetics of tempering. This is what makes cobalt useful for high speed steels, which are designed to hold their hardness at high temperature.

Toughness

I used a few different combinations of austenitizing and tempering to compare CPM-1V and K888. For K888 I compared the low and high temperature tempering with 2050°F (1120°C) along with 1000°F (540°C) and 350°F (175°C). Based on the K888 tempering chart from Bohler, the 2050-1000 combination is presumably the one they used for the 64 Rc toughness datapoint, so it was an important one to test. I also tried a lower austenitizing temperature, 2000°F (1095°C) with 400°F (200°C). For CPM-1V I used all low temperature tempering with 2000-400, 2050-300, and 2050-350. The resulting hardness and toughness is below:

The first thing that sticks out is that the CPM-1V tested significantly better than the K888. The 2050-300 treatment led to 64 Rc and 22 ft-lbs, an excellent combination of hardness and toughness. 2050-350 led to 62.5 Rc with 36 ft-lbs, again an impressive combination. The 2000-400 heat treatment, however, did not lead to an improvement in toughness when compared with 2050-350 despite the lower hardness.

For K888 the 2050-1000 heat treatment, which is presumably the heat treatment used for the 64 Rc toughness datapoint in the datasheet, had the best toughness, but it was still below the result for CPM-1V at comparable hardness. The other interesting thing was that the low temperature tempering at 350-400°F did not seem to be high enough to give the steel good toughness. This may be from the cobalt addition delaying tempering.

K888 does not look particularly impressive on the chart, though it did manage to have somewhat higher toughness than CPM-CruWear at comparable hardness. The CPM-1V was again somewhat higher than K888 around 64 Rc, showing a clearer improvement in toughness at that high hardness. At 62.5 Rc, the CPM-1V continues to look impressive, having comparable toughness to high toughness steels like Caldie, CPM-3V, and A8 mod when they are at significantly lower hardness. Z-Tuff still remains alone at the top of the chart since the 2000-400 heat treatment of CPM-1V did not lead to an improvement in toughness. However, if the CPM-1V is extrapolated to lower hardness we would expect similar toughness to Z-Tuff:

It would be interesting to test other heat treatments to see if this extrapolation to lower hardness could be achieved. Such as a 2050°F austenitize with a higher tempering temperature such as 400-500 or 1000°F to see if the toughness could be further improved in the 60-62 Rc range. Because the low tempering range was not successful for K888, modified heat treatments for lower hardness would explore lower austenitizing temperatures with the same 1000°F, or increased tempering temperatures beyond 1000°F (540°C). Presumably the toughness would remain below CPM-1V due to the larger carbides. Alternatively it would also be fun to test Z-Tuff with a lower tempering temperature of 300-350°F and see what its toughness is like at higher hardness. So far I have only tested it after tempering at 400°F.

To show how good the toughness of CPM-1V is compared to low alloy steels and stainless steels I created charts with 1V overlaid on those:

Edge Retention

I tested one CATRA knife each, Shawn Houston ground the blades after I heat treated them. Each were heat treated using an austenitizing temperature of 2050°F (1120°C) and a tempering temperature of 350°F (175°C). This resulted in 63.1 Rc for CPM-1V and 63.9 Rc for K888.

As expected the two steels are near the bottom of the chart, following the approximate edge retention of AEB-L and ApexUltra. If compensating for hardness, the two steels also approximately match CD#1 (nearly identical to Z-Tuff) and A8 Mod. So I think overall the two steels did about as well as could be hoped for high toughness powder metallurgy steels with low carbide volume and only 1% vanadium.

Summary and Conclusions

These are two matrix high speed steels based on the original matrix steel VASCO-MA, though K888 has slightly higher carbon plus a cobalt addition. The cobalt addition led to somewhat lower toughness in the K888 when compared with CPM-1V. The cobalt did lead to more tempering resistance and higher hardness, though that tempering resistance also affected its toughness, especially in the low temperature tempering range (350-400°F). CPM-1V has an excellent combination of hardness and toughness in the 62-64 Rc range, though it may be possible to achieve higher toughness in the 60-62 Rc range if we try higher tempering temperatures. K888 and CPM-1V have similar edge retention, close to AEB-L, ApexUltra, and Z-Tuff/CD#1. Overall after testing these steels CPM-1V is my preference for a steel that maintains very good toughness at high hardness. I don’t know of any knife supply companies offering K888 yet, and is apparently difficult to obtain from Bohler right now, though CPM-1V is currently available at Alpha Knife Supply.