Lost Knife Steels – Questek Ferrium M60S

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This article has a moderate amount of background metallurgy information in it. I explain briefly the necessary background information and provide links to earlier articles with more complete explanations. However, to get a full picture of steel metallurgy and heat treating, the easiest way is to read my book Knife Engineering.

Lost and Forgotten Knife Steels

Today’s article is a return to the subject of the very first article I ever wrote for this website – lost or forgotten knife steels. That first article was about a modified version of 3V that used primarily niobium rather than vanadium. That steel was patented but never produced commercially. The subject of this article is a steel that was produced and distributed to a limited number of knifemakers but never went anywhere. So first, a little background about the company that was behind it.

Questek

The company Questek specializes in material design using computer modeling. They have previously patented and commercialized several products, primarily steels. They also list titanium, nickel, and aluminum products as being “available for licensing” but do not appear to have been commercialized. The co-founder of Questek, Professor Greg Olson, previously taught at Northwestern, and Northwestern University and Questek are only a couple blocks from each other. So there are many connections between the two institutions, such as shared research between them, and several newly hired employees coming from graduate students who completed their programs at Northwestern University.

Ferrium M60S and S53

Sometime around 2003 [1], Questek began pushing one of their steels as being particularly suited for cutlery applications, which they dubbed Ferrium M60S. M60S is part of the “S-series” of alloys which were patented by Charles Kuehmann, Greg Olson, and Herng-Jeng Jou [2]. Ferrium S53 was commercialized from this patent which is presumably similar except its target hardness is 53-54 Rc rather than the 60 Rc in M60S. Because the “53” and “60” are roughly equal to the target hardness, I believe the naming scheme is based on hardness. All of the commercialized Ferrium grades are produced by Carpenter Technology (maker of CTS-XHP, CTS-204P, and BD1N knife steels), so presumably Carpenter would have been the main target for M60S as well. Ferrium S53 is marketed for structural aerospace applications where a combination of high strength, toughness, and corrosion resistance are necessary. The chromium content is relatively low for a stainless, but it is called a stainless steel by Questek, and said to “provide corrosion resistance similar to 440C.” Landing gear made from non-stainless steels required a coating for corrosion protection. Ferrium M60S and S53 are “precipitation strengthened” steels so it makes sense to describe what those are so that M60S can be understood.

Precipitation Strengthened Steels

High Speed Steel

Precipitation hardening is used in a range of steels, the discovery of precipitation hardening is what started the modern development of tool steels and stainless steels. These were early high speed steels, alloyed with large amounts of tungsten so that very small tungsten carbides (W₂C) would precipitate when tempering at high temperature (~950-1100°F/500-600°C). With high hardness being achieved at high tempering temperatures, the tools can be operated at high speed where heat is built up without losing strength and hardness required for machining operations. Below shows a tempering curve for T1 high speed steel with 18% W, the earliest high speed steel, where you can see that at first hardness decreases with increasing temperature up to about 500°F (260°C), but then increases to a peak hardness around 975°F (525°C). With simple carbon steels the hardness only decreases with higher tempering temperatures. Since the invention of T1, many high speed steels have been developed which use molybdenum (Mo) instead of tungsten (W), or a combination of both, such as M2 and M4 high speed steels. Molybdenum forms Mo₂C carbides instead of W₂C carbides, and Mo-W steels form a carbide referred to as M₂C where “M” can be either W or Mo. There are a few stainless steels like 154CM/ATS34 designed for maintaining hardness at higher temperatures, which is possible thanks to the 4% Mo in those steels.

Precipitation Hardening

The mechanisms of precipitation hardening are described in this article on tempering. Very small particles called “precipitates” strengthen the steel. The particles/precipitates can be a range of different types, most commonly carbides in steels though there are other types used in specific steels discussed below. All mechanical behavior of steels is controlled by defects in the atomic structure called dislocations. For steel to deform, the dislocations have to move so that the atoms can shift around causing a shape change. Without those defects, steel would be much stronger and more difficult to deform. The more difficult it is to move the dislocations, the stronger a metal is. Before the discovery of dislocations it was unknown why metals were so much easier to deform than they should be based on the strength of atomic bonds and the distribution of atoms. All metals have dislocations in them, though the “density” of dislocations varies based on prior processing and heat treating. When there is an array of small particles, they impede the movement of dislocations increasing the strength of steel. The following image shows a dislocation (wavy line) moving through a field of small precipitates.

And here is a video of high magnification electron microscopy showing real dislocations moving through a metal, and you can see them running into the small particles:

Maraging Steels

High speed steels gain their hardness through a combination of hard martensite with high carbon content, and precipitation strengthening from the small carbides. Some steels are designed with very low carbon content so that the strengthening comes almost entirely from precipitation strengthening. One common class of these materials are called “maraging” steels, first introduced in the 1950s. In maraging steels the precipitation hardening does not come from carbides, but instead from small nickel-titanium (Ni₃Ti) and nickel-molybdenum (Ni₃Mo) particles. The lack of high carbon martensite and lack of large carbides makes these steels have a very high combination of toughness and hardness. The high cobalt content helps slow the kinetics of particle growth so that the nickel-titanium and nickel-molybdenum particles stay small where they can contribute to hardness. Read more about how cobalt does this in this article on cobalt or this article on 70 Rc high speed steels (which all have high Co content). Below is the approximate composition of maraging “Grade 350” where 350 refers to the tensile strength in ksi (tensile strength described more later in this article).

17-4PH, 15-5PH, and 17-7PH

The most common precipitation hardened stainless steels are 17-4PH and 15-5PH. These steels use a large copper addition to form copper precipitates for strengthening. Both steels are limited to about 47-48 Rc. Another PH steel, 17-7PH, uses aluminum instead of copper, and has even seen some use in knives by Buck for saltwater environments. The steel forms nickel-aluminum (Ni₃Al) particles for hardening. It requires cold rolling to achieve full martensite prior to aging to form the particles. The cold rolling process is required because the steel is partially austenitic, similar to H1 steel which I wrote about here, including why cold rolling is necessary. 17-7PH is limited to about 52 Rc according to the datasheet, though Buck has advertised 54-56 Rc in the past so perhaps they employed some modified processing (such as a greater degree of cold rolling) for a bit more hardness.

AerMet Steels

High speed steels combine high carbon martensite with precipitation hardening, while maraging steels have basically zero carbon and use precipitation hardening. A class of steels somewhat in between those two groups (high speed steel and maraging steel) are the AerMet grades produced by Carpenter, first introduced in the early 1990s [3]. These steels use a low carbon martensite in combination with M₂C carbides from carbon and molybdenum/chromium. There is also a large cobalt addition which provides greater hardness for a given amount of M₂C from restricting carbide growth (in the same way it does for maraging grades).  AerMet also has high nickel content, but not for forming nickel-molybdenum particles.  The patent says that the nickel is to increase hardenability (how fast you have to quench for full hardness). There are other elements that could help with hardenability such as manganese, or even the Mo and Cr that is already in the steel. And certainly a full 12% Ni is likely more than necessary for hardenability. So I think the nickel is mostly a holdover from the earlier maraging grades. However, high cobalt in steels reduces toughness. It has been proposed that these steels have good toughness in part because the high nickel counteracts the negative effects of cobalt [4]. Read more about how nickel improves toughness in this article. Below is the composition for AerMet 340 [5].

Ferrium S-Series

The S-series of Ferrium steels from Questek use a similar alloying approach to the AerMet grades, but are made to be stainless. These were not the first steels to attempt very high strength precipitation hardening stainless, but the inventors claim that previous PH stainless steels which could reach similar hardness levels had unacceptably high amounts of retained austenite (such as 17-7PH which is partially austenitic after quenching).

Retained Austenite and Heat Treatment of Steel

Retained austenite is a phase of steel that is left over after quenching. With conventional steels, the piece is heated to high temperature where the steel transforms to austenite, a nonmagnetic phase where carbides dissolve putting carbon in solution. Read more in this article about austenitizing. The steel is then rapidly quenched to “lock in” the carbon so that when the low temperature phase forms it is high in strength from the carbon. The low temperature phase is called martensite which is what gives the steel its high strength. Read more about martensite and hardening of steel in this article. When the transformation to martensite is incomplete there is austenite left over, or “retained,” in the steel. Martensite formation is primarily controlled by temperature, so if the “martensite finish” temperature is below room temperature then the transformation is incomplete leaving some amount of retained austenite. This is why cryogenic processing is used after quenching, to transform more austenite to martensite by cooling to lower temperatures. The martensite start and finish temperatures are controlled by the elements in the austenite prior to quenching. The following equation shows the approximate effect of different elements on the martensite start temperature:

Ms (°C) = 539 – 423*C – 30.4*Mn – 17.7*Ni – 12.1*Cr – 7.5*Si – 7.5*Mo +10*Co

You can see that most elements reduce martensite start (leading to lower martensite finish and more retained austenite). Carbon is the most important element though others can become significant. For example, Cr does not have as much effect as carbon or manganese but when you have high Cr for corrosion resistance then the effect adds up. This is why highly corrosion resistant steels like Vanax and LC200N are difficult to heat treat above 60-61 Rc, because even with cryogenic processing the hardness is limited by retained austenite from high Cr “in solution” during heat treating. Learn more in this article about Vanax heat treatment. Cobalt is one of the only elements that can raise martensite start and reduce retained austenite.

Retained Austenite, Yield Strength, and Hardness

Excess retained austenite drops the bulk hardness of steel. However, even with equal hardness, higher amounts of retained austenite reduce the “yield strength” of steel. The yield strength is the amount of stress a piece of steel can take before it starts to permanently deform. This is different than the “ultimate strength” of steel which is the amount of stress prior to fracture. Hardness correlates more strongly with ultimate strength than yield strength so hardness doesn’t always tell the whole story of how “strong” a piece of steel is. Read more in this article about what hardness does and doesn’t tell you.

Design of S-Series Steels

So the S-series of steels were designed to be a stainless PH steel which could achieve relatively high hardness while avoiding excess retained austenite. This is somewhat difficult to do with high nickel, chromium, and carbon, though is helped somewhat by the cobalt addition. So the Ferrium S53 grade can reach about 53-54 Rc along with good corrosion resistance through careful composition control by minimizing the required carbon, chromium, and nickel necessary for achieving the hardness and corrosion resistance targets. Another interesting point brought up in the patent is the claim that the high cobalt helps “partition” the chromium to the surface to form the chromium oxide film for corrosion resistance. This effectively improves corrosion resistance for a given amount of chromium. It is an interesting claim in part because cobalt is rarely added to stainless steels, and also because I have read another study that found a reduction in corrosion resistance from cobalt [6]. Read more about how corrosion resistance in stainless steel works in this article. Ferrium S53 and M60S also have a small vanadium addition which can form small VC carbides to contribute to precipitation strengthening as well. And VC can help pin grain boundaries for a small grain size.

M60S Composition and Hardness

The composition of Ferrium S53 is given in its datasheet, but the old datasheets of M60S do not provide a specific composition. The primary difference between S53 and M60S is likely the carbon content though there may be minor deviations of other elements as well. The patent shows how hardness changes with carbon content in a simple chart:

So we can estimate the composition of M60S but not the exact chemistry. But in the end we don’t need to know the exact composition to know how the steel works, as has been described above. The hardness, according the datasheet, can reach as high as 62 Rc, with a general recommendation of 60 Rc

Yield and Tensile Strength

Sometimes steels are characterized by their “yield ratio” where the yield strength is divided by the ultimate strength. M60S is shown to be 250 ksi yield strength and 350 ksi tensile strength, for a yield ratio of 0.71 which is relatively low. AerMet 340 at the same ultimate strength (350 ksi) has a much higher yield strength of 305 ksi, for a yield ratio of 0.87. Ferrium S53, with an ultimate strength of 288 ksi (54 Rc), has a yield strength of 225 ksi for a yield ratio of 0.78. A lower strength S-series steel shown in the patent had an ultimate strength of 247 ksi (51 Rc) with a yield strength of 211 ksi for a yield ratio of 0.85. So it seems that for these S-series steels the yield ratio is going down as the ultimate strength (hardness) goes up. This makes sense, as the carbon content is higher with the higher strength grades, likely increasing the amount or retained austenite and therefore dropping the yield strength. Cryogenic processing is recommended in the datasheet to reduced retained austenite but it may not be enough. Perhaps if the Cr and/or Ni content was reduced the retained austenite could be lower, as those elements drop the martensite start temperature.

This drop in yield strength is not only of theoretical importance, as the yield strength will control how easily the steel deforms. So a 60 Rc steel with a low yield ratio may deform as easily as a steel of much lower hardness. The 250 ksi yield strength of M60S would be equivalent to about 52 Rc for S5 tool steel [7], and about the same hardness for AerMet 340 [5], which as I described before is a similar type of steel to M60S but non-stainless. Therefore a knife in M60S may behave more like it is 50-52 Rc rather than 60 Rc when it comes to resistance to deformation.

This low yield ratio was also seen in knife testing by Jerry Hossom, who reported results comparing knives in 154CM and M60S with the same edge geometry [8]. He chopped the knives through nails where he observed chipping in the 154CM (top), but major deformation in the M60S knife (bottom). Presumably the M60S knife that Hossom tested would have been heat treated with cryogenic processing as recommended in the datasheet to help with yield strength. If it wasn’t that would exacerbate the problem of low strength. However, according to the datasheet the yield strength of M60S would still be low even with cryogenic processing.

Edge Retention

A CATRA edge retention test is reported in the M60S datasheet as well. It is reported as being 146% of 440C, which would put it at similar edge retention as S30V [9]. However, I think this number is questionable. Edge retention in the CATRA test comes mostly from hardness of the steel, volume of carbide, and the hardness of carbides. M60S has very little carbide, the very small carbides that form in precipitation hardening contribute little to edge retention as the total volume of carbide is very small. In a previous CATRA test I performed, I compared CPM-4V with a low temper (no precipitation hardening) and high temper (precipitation hardening), to see the effect of of M₂C carbides on edge retention, and found very little difference. To get to a similar level of edge retention to S30V, the M60S would have to have much more carbide, such as through significantly higher carbon and vanadium content. And while those types of modifications to the composition are not impossible, it would be a whole new development, not part of the “S-series” of steels that were patented. Instead, M60S would have edge retention similar to other low carbide steels at 60 Rc, such as 1095 or 8670 in the chart available here, which would put the steel around 65% of 440C. I don’t know why such a high number for edge retention was reported in the datasheet. It is very important that the knives tested have identical edge geometry and sharpening, of course. Perhaps the comparison to 440C used a standard value from CATRA rather than a 1:1 comparison between two identical knives.

Toughness

Because of the low carbide content, the toughness (resistance to chipping and fracture) should be very high for M60S. These precipitation hardening steels are designed specifically for high toughness after all. Ferrium S53 is reported to have 18-24 ft-lbs in a charpy v-notch toughness test at 54 Rc [10]. V-notch toughness tests are rarely used for tool steels and stainless knife steels because it usually results in low single digit values. L6 steel, known for good toughness, is only 5 ft-lbs at 52 Rc in the same test [11]. So while M60S would have a lower value than S53 because of higher hardness, it should still have very high impact toughness compared with other knife steels.

Ease in Grinding, Finishing, and Sharpening

The lack of a significant volume of hard carbides would likely have meant that M60S was easy to grind and finish. Similar to simple steels at 60 Rc.

Why Wasn’t it Commercially Produced?

I contacted Greg Olson, one of the inventors of M60S, and asked why the steel wasn’t commercialized. He confirmed that the low yield strength from retained austenite was a major issue. Though he did say that Al Pendray had more positive things to say about the steel (Roman Landes sent me a picture of an Al Pendray knife made in M60S, below).

Charles Kuehmann, another of the inventors on the Questek patent also provided his perspective on why the steel wasn’t commercialized. He said that there wasn’t enough of a market for the steel and that without uses beyond cutlery the companies weren’t interested in the small amount of production they would get from it. Since Questek licenses their designs, I imagine that convincing companies to take on the products can be challenging unless there is a clear path to significant revenue.

Al Pendray knife in M60S

Summary and Conclusions

Questek Ferrium M60S was a precipitation hardening stainless steel promoted for knives around 2003-2005. It used a similar alloying approach of Co-Ni-Mo as non-stainless AerMet grades but with increased Cr to be stainless. It was tested by a few knifemakers, but was ultimately never commercialized. At least one knifemaker reported low yield strength of the material, though the lack of commercialization was possibly in part due to lack of interest by steel companies rather than Questek believing the steel did not have sufficient properties for the application. The steel would have been easy to grind and finish and have very high toughness. Despite a high reported edge retention value, I don’t believe the edge retention would have been particularly spectacular, though it would be comparable to plenty of other knife steels. The steel is very different than other knife steels in common use, so it would have been interesting to see what consumers thought about it.

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[1] Questek Ferrium M60S datasheet, I have archieved it here

[2] Kuehmann, Charles J., Gregory B. Olson, and Herng-Jeng Jou. “Nanocarbide precipitation strengthened ultrahigh-strength, corrosion resistant, structural steels.” U.S. Patent 7,967,927, issued June 28, 2011.

[3] Garrison, Warren M. “Cobalt and the Toughness of Steel.” In Materials Science Forum, vol. 710, pp. 3-10. Trans Tech Publications, 2012.

[4] Hemphill, Raymond M., David E. Wert, Paul M. Novotny, and Michael L. Schmidt. “Age hardenable alloy with a unique combination of very high strength and good toughness.” U.S. Patent 5,866,066, issued February 2, 1999.

[5] https://cartech.ides.com/datasheet.aspx?i=101&e=338&c=TechArt&FMT=PRINT

[6] Peissl, S., G. Mori, H. Leitner, R. Ebner, and S. Eglsäer. “Influence of chromium, molybdenum and cobalt on the corrosion behaviour of high carbon steels in dependence of heat treatment.” Materials and Corrosion 57, no. 10 (2006): 759-765.

[7] Roberts, G A, and Robert A. Cary. Tool Steels. Beachwood, Ohio: American Society for Metals, 1980.

[8] https://www.bladeforums.com/threads/s30v.485714/page-2#post-4754204

[9] http://www.crucible.com/PDFs/DataSheets2010/dsS30Vv1%202010.pdf

[10] https://www.questek.com/wp-content/uploads/2020/05/FerriumS53CarpenterDataShe.pdf

[11] https://www.alphaknifesupply.com/Pictures/Info/Steel/L6-DS-Latrobe.pdf