Differentially Tempered CPM S30V

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Differentially Tempered CPM S30V

Thomas Haslinger

Another Las Vegas Classic find! I had the pleasure of spending some quality time with Thomas Haslinger at this years show. I was very impressed with what I saw on his table and what he had to say.

One of the more interesting things at his table was a test blade in Differentially Tempered CPM S30V. It was stunning little blade in a good general utility profile with nice looking wood scales and a bolster that I believe was carbon fiber. It was a very attractive knife with a lot of visual appeal in addition to solid design and craftsmanship.

As I looked at the nicely executed kitchen knives, the hunters, and utility blades, I considered the appeal of the differentially tempered CPM S30V. Is the additional cost worth the increased performance? Initially, I think the increased cost is about ~$150USD due to the individual attention and single heat treating process required (versus larger multi-knife batches). This cost estimate is just a guess so, please see the maker for cost specifics as I was not comparing identical knives.

In the end, my initial thoughts are that CPM S30V does not require the advantages the process has to offer at its current expected cost increase. CPM S30V hardened in the tradition way has proven to be an outstanding stainless steel in my use. I do not see myself abusing one of his excellant kitchen knives, or hunters, or utility knives in such a way that a differential heat treat would make the difference between failure or success, in my hands and in my expected use. I am initially inclined to save that money and apply it to other knives but, on the other hand, how many knives do I really need and how many can I really use at one time? ;)
 
How much performance is added to differentially tempered and differentially hardened knives over their more homogenously heat-treated counterparts? This might be something the folks in shop talk can answer, but I believe that it will become a delicate subject once hard numbers come into play and terms are asked to be defined.

I don't know if differentially hardened or tempered blades work better, but they definitely look cooler. Multi-quenched blades, such as Ed Caffrey's, are very stunning to me.
 
One point Thomas made to me was that for repetitive actions like chopping, that your hand will fatigue less because the non-homogeneous hardness of the blade changes how the shock of hitting the cutting board is transmitted to your hand (the total impulse is less as some is absorbed by the springier spine, and it feels to him like it is distributed over a longer period of time since the hard edge and soft spine are going to have different acoustic properties).

I would think this behavior would also be true of any differentially hardened knife blade (I can't imagine why it would only be true of stainless), so it should be an easy behavior to verify with differentially hardened carbon steel, if someone feels like checking it out.
 
Originally posted by Gabe Newell
One point Thomas made to me was that for repetitive actions like chopping, that your hand will fatigue less because the non-homogeneous hardness of the blade changes how the shock of hitting the cutting board is transmitted to your hand (the total impulse is less as some is absorbed by the springier spine, and it feels to him like it is distributed over a longer period of time since the hard edge and soft spine are going to have different acoustic properties).

I would think this behavior would also be true of any differentially hardened knife blade (I can't imagine why it would only be true of stainless), so it should be an easy behavior to verify with differentially hardened carbon steel, if someone feels like checking it out.
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Ah, the spring'iness factor :D In a larger knife, being used harder, I can certainly see the benefit of the non-homogenous blade when you have a really stiff blade geometery with a really hard and inflexible blade steel. That's one piece of information I omitted. In smaller knives though, hmmmmm.......
 
In a smaller blade, maybe it's to alleviate the brittleness normally associated with high-chrome steels? Even there, though, I'd be concerned. I've seen some knives made from 420HC that have had huge edge blowouts (no idea how they occurred, though) by even nonknut standards (.5" inches versus .5 microns). Does softening S30V make it tougher? Could that same purpose be met better by sandwiching S30V between some softer metal?
 
thombrogan :

How much performance is added to differentially tempered and differentially hardened knives over their more homogenously heat-treated counterparts?
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The effect can be very large, as in many to one, in regards to the ability of the knife to flex as well as take heavy impacts on the spine so you will see it for heavy prying knives and blades which are subject to heavy beatings (like hit the spine with a hammer, not hammer in a tent peg which anything should do). However what has to be realized is that there is no gain at all in regards to toughness for the edge.

Some steels like 52100 improve greatly with a spring spine because at full hardness (~58/60 HRC) they will snap with no give when flexed and are prone to brittle failure in general. Some steels like INFI will still take a set and resist fracture very well at a high hardness. Note as well that you can also get differential hardening which leaves the spine dead soft. This is very different than a spring temper as it leaves the blade *very* weak, as you have essentially just made that part from mild steel.

S30V is not very flexible at full hardness nor in general resists fracture well and thus could benefit from a spring spine in those areas. Of course the edge would still be unsuitable for heavy impacts many time to one compared to something like L6.

The damping effect is real in the sense that it produces a result, you can see this by hitting something hard with a file and then with a piece of annealed steel (difference of about 40 HRC points). However for this to be even slightly significant the amount of steel which is soft and the amount of softening very large. Even in those cases this tends to be one of the least significant factors in overall grip feedback. More critical factors are blade geometry, mass, balance and handle shaping and texture.

-Cliff
 
Well, acoustical or morphological shock-dampening aside, in my ever-growing chasm of ignorance, differentially hardening stainless seems like pushing with a rope when compared to spring-tempering the spine of a low-chrome steel and applying as good of a finish as possible to the blade (to reduce areas for corrosion to occur) or sandwiching a really hard stainless between two softer slabs of metal.

That's me talking through my hat as my only differentially hardened knives are a bilton khukri and kagas katne (they're teeny tiny khukris). Don't anyone take my words to mean that I have or think I have direct experience with how differentially-tempered S30V behaves. My posts in this thread are just my unstudied opinion.
 
In looking through Crucible's data sheets, it was interesting to see variability in a steel due to hardness (Charpy variation of 1.2 - 2X across RC 55-62) versus variability due to steel selection (L6 or S7 versus S30V gives 4X-10X Charpy variation).

Crucible doesn't standardize the information provided on their datasheets (at least the one's I found) so there's a little bit of head-scratching involved.

I scratch my head more when I try to quantify how much a springy spine would help. Obviously it isn't going to improve the impact-resistance of the edge, but it may reduce the impact itself for a given activity because of the springiness of the spine spreading the impulse over a longer period. I would assume in ordinary impact testing the material is clamped to remove this exact effect.

Cliff?
 
There are various impact tests , Charpy is the most common. However it is sometimes very difficult to translate Charpy values to toughness in actual use .The Charpy V notch test involves a sample that has a V shaped notch.The numbers are the energy in foot pounds necessary to fracture the sample......Differentially hardening or tempering S30V ,to me , seems of questionable value. The Chris Reeve Green Beret knife apparently is good enough for the military so it can't be so bad with ordinary quench and temper.
 
Gabe Newell :

I would assume in ordinary impact testing the material is clamped to remove this exact effect.
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There are two main fracture tests, Izod and Charpy. As an example ("Tool Steels" by Roberts and company), with a charpy v-notch; D2 is 16, L6 is 24 and A2 is 26. With an unnotched Izod, D2 is 41, with L6 is 79 and A2 is 68. So you can see L6 is only 50% ahread of D2 on the charpy, but is double on the Izod.

Izod tests have the material held in place at one end and the other end whacked with a hammer on the side of the notch to crack it off. Charpy tests hit the material on the other side of the notch in the sample and thus tend to crack it open in a more symmetrical manner. Very loosely batoning is like charpy testing while heavy edge impacts is more like Izod.

There can be great differences between notched and unnotched testing, because some steels are much more sensitive to cracking around a stress point than others. This has fairly direct impications to the behavior of chips in the edge under heavy impacts.

Ductility is also a factor in heavy use and correlated but not exactly equal to impact toughness. The ductility of an S30V blade would be greatly enhanced by a spring spine because the thick spine greatly lowers flexibility, this is why thin blades can take very high bends even if they are cheap. The stress that a given bend places on a blade is proportional to its thickness.

The other area that it would benefit is direct impact on the spine. If you are batoning through a car roof for example using a hammer on the spine you will maul the spine of a 60 HRC stainless steel, but drop it down to 45 HRC and you will just see minor denting (the 60 HRC edge won't like this cutting but at least you won't end up with the blade snapping in half).

I would agree with thombrogan though in regards to there are much better steels for that kind of knife, the spring tempered Swamp Rats for example.

-Cliff
 
There can be great differences between notched and unnotched testing, because some steels are much more sensitive to cracking around a stress point than others. This has fairly direct impications to the behavior of chips in the edge under heavy impacts.
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Like Cliff says the notching can make a large difference in the results of an impact test.

The three common types of notching are a simple unnotched sample, the c notch, and the v notch.

Each type of notch will yield different results. Typically for the same steel and condition the v notch will yield the lowest values followed by the c notch and the unnotched sample will yield the highest result. Be carefull when you look at the numbers from different steels and steel manufacturers; for example an unnotched charpy of 45 ft-lb is not necessarily better than a v notch charpy of 35 ft-lb.

To determine which type of test is appropriate for your application take a carefull look at all the stress risers (grooves, notches, changes is section, surface finish, uneven loading etc.) and choose the test that most closely models it.
 
I have had scattered success in finding Charpy test results for different steels - apples and oranges. Any suggestions for the best places to look?
 
Crucible does some impact testing on the steels they made so check the data sheets. The materials section in any close university library will also yield massive quantites of such information. You will get a lot of flak from makers if you quote these numbers as they are not Bos treated steels, which is nothing but hype (not his heat treatment - but the massive inflation it tends to get to such an extent that unless it is Bos it is meaningless).

-Cliff
 
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