Some Thoughts on Stainless Steel and Heat Treating

There’s always going to be debates among knifemakers and knife enthusiasts about the “best” kind of steel. There are purists who feel that high-carbon steels are the only real steel for serious knives. There are also those for whom a stainless steel knife is the only kind that makes sense.

Personally I like both simple carbon steels and modern stainless steels for knives. I learned that if I have to share knives in my kitchen at home, they have to be easy enough for my whole household to take care of, which means stainless steel.  With proper steel selection and careful heat treating I can fashion a stainless kitchen knife that cuts just as well as a typical high carbon knife with much less maintenance.

 

Stainless Steel Heat Treat: What’s so special about it?

To me the heat treat is the most critical part of the process. If I’m putting my name on it, it has to be done to my standards. I can make a more affordable knife by machine finishing, or upscale a knife by doing a high-grit, hand-rubbed finish, but everything starts out with the same high quality heat treat.

What makes heat treating stainless challenging is the higher temperatures and longer soak times required.  Additionally, the margin for error in hitting the correct temperatures is much smaller.  On top of this, to properly harden most stainless you need to subject it to a cryogenic cycle.  Once you have gone though all the steps the only way to verify you did it right is with a Rockwell hardness tester.  In total this makes for a rather large investment in equipment and time, which is why many knifemakers send their blades out to one of the very reputable heat treating firms.  While I could save some time, save some money, and reduce the hassle by sending my blades out to be heat treated, then each blade wouldn’t get the level of attention and care required for me to be comfortable putting my name on it.

 

Kitchen Stainless of Choice: CPM 154 CM

I’ve chosen CPM 154 CM to be my primary stainless steel for kitchen knives. The CPM means “crucible particle metallurgy,” a process that ensures a consistent alloy throughout the material. CPM 154 CM is a particle metallurgy grade of 154-CM, which has a long history of being a very good knife steel. The “PM” grade improves on this steel by reducing carbide size and providing a more even distribution of carbides.  One of the side benefits is increased toughness for the hardness level when compared to standard 154-CM.  I find this steel provides a good balance between hardness and toughness while retaining excellent stain resistance.

I also work in S35VN, but I prefer that steel for utility knives. In testing both steels in my own kitchen, I found that S35VN may cut better than the CPM154, but the S35VN stainless steel tended to stain slightly with some of the more acidic fruits and vegetables, like strawberries and tomatoes.

 

Heat Treating CPM 154 CM

After the blades are profiled (AKA cut out) but before the bevels are ground,  the blades get run through a stress relief cycle. The reason for this is that there is sometimes stress left in the steel from forming it into sheets and the stress relief cycle helps prevent the blades from warping later. For CPM 154 CM, this means a 2 hour soak in the digitally controlled kiln at 1275°F.

Next the blades are hardened. This is the most important part, and the most time-sensitive. I wrap the knives in stainless steel foil so they’ll have less exposure to the oxygen inside the kiln, place them in the kiln, and bring the temperature to 1400° F. The knives soak at this temperature for 15 minutes to ensure the blades are preheated all the way through.  The kiln is then ramped up to 1975° F, and the blades allowed to soak for 45 min. This austenitizes the steel and puts the carbon and other alloy constituents into solution. The knives are then pulled out of the kiln and quenched between thick slabs of aluminum, which brings the temperature down to under 400° F within seconds.

When the blades reach room temperature, they are put in a 35-liter Dewar filled with liquid nitrogen for at least 8 hours. This cryogenic quench lets the blades reach their maximum hardness. After the blades are removed and reach room temperature they are individually tested with the Rockwell Hardness tester. I target a hardness of 63-65 on the Rockwell Hardness C scale at this step.

At this point, it is time to temper the blades to reduce the brittleness. If I left the blades so hard, they’d be in danger of chipping out or cracking. The blades are subjected to 2 temper cycles of  2 hours each, at a temperature around 700° F.  This brings them to a better hardness for final use, between 61 and 62 on the Rockwell Hardness C scale.

 

 The bottom line.

By the time I verify final hardness, each blade has gotten hardness-checked at least 3 times. That might be excessive, but I’m a detail-oriented person. I want each knife I make to be as useful as it can possibly be – and that has to start with careful, thorough heat treatment.

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