Differential Tempering, a waste of time and money?

Before we can discuss the strengths of differentially hardened blades versus fully hardened blades we have to define strength. So, the question is, how do we determine the strength of steel?

Strength in steel is normally measured in tensile or impact strength. Tensile strength is roughly defined as a continous pressure on the blade (like in prying). All steels, depending on the alloy, will have a maximum tensile strength at a certain hardness. As you move away from that hardness, either higher or lower, the tensile strength of the steel drops. Some steels reach their maximum tensile strength at or around 60 rc. But, for most steels it is lower. For the blades where the maximum tensile strength is less than the hardened edge, having a softer spine will increase the toughness of the blade. But, if the hardness of the spine drops too much, then the tensile strength drops accordingly. In some steels, the tensile strength of an edge quenched blade (dead soft spine) could be less than the fully hardened blade. So, all diff. hardening did was weaken the blade.

On the other hand, impact strength is the amount of impact a blade can handle. For the most part, diff. heat treating a blade will almost always increase the impact strength of a blade because the softer the steel becomes the more ductile it becomes. Therefore, it will bend instead of breaking.

Keep in mind that there are very many variables, like steel thickness, hardening method, tempering method, quench method, steel alloy and its hardening depth, etc.

David
 
For background information, the thread I think Steve Harvey is referencing in which Ed Fowler dicusses differential heat treatment is: What are the attributes of a high performance knife?

Another good thread on the topic from the Shop Talk forum is: Differential Tempering Or Not?

And for those who need some really basic understanding of heat treatment, I wrote a response in this thread a while ago which I think will help explain the basic steps of heat-treatment and their purpose: Heat Treatment, What Is It?

Now, let me thank everyone for your responses. It is pretty clear to me that there is no correct answer to my original questions. I want to interject a couple of other observations into this discussion.

One is that some ABS masters seem to think the differential heat treatment will not necessarily improve real world performance, and may even degrade it. Depending on the blade size and its geometry, an all-hard heat-treatment may be better than a blade with a diff heat-treat. I am not defending this idea, simply putting it out there for discussion.

The second observation is that few Mastersmiths apply a differential heat treat to their pattern welded blades. I have been told this is because it is much more difficult to to properly in a pattern welded blade, and that the temper line would be obscured by the pattern in the steel. It seems to be generally accepted that a pattern welded (damascus) blade will At Best be no better than a blade made of the tougher of the two (or more) steels used in its construction. If differential heat-treatment always results in a superior working blade, why are damascus blades rarely differentially heat-treated? It seems to imply that a differential heat-treatment is more about aesthetics (that cool looking hamon) than performance. Or is a differential heat-treatment NOT applied simply because makers of damascus blades do not expect their blades to be actually used?

Since Ed Fowler has been mentioned, whose work and philosophy I admire very much, it is worth discussing his blades. Ed is constantly striving to perfect his art, ever improving the performance of his blades. As I understand it, Ed believes very strongly in differential heat-treatment for his 52100 blades, and does not forge pattern welded steel anymore. Ed is seeking the highest level of performance that he can achieve in the materials he chooses. And many folks actually use his blades in the real world. If a customer is seeking the highest level of real world performance he can buy, then I would certainly recommend a Fowler knife. But you will pay for that performance. These knives are not cheap by any standard, even though they are made from relatively simply and inexpensive materials. You have to pay dearly for Ed's expertise. There is nothing wrong with that and I am not claiming that Fowler knives cost too much. I know Ed wants his customers to use their knives, but I am sure the expense prevents many from actually doing so.

Certainly a strong case can be made for the utility of a differential heat-treatment in low-alloy high carbon blades, but since the cost of steel is only a minor part of the cost of a finished knife, and there are many new steels with outstanding toughness and strength which do not seem to need differential heat-treatment , can the whole business of differential heat-treament be regarded as something which made sense in the past, but is no longer of an real world utility?

Anyway, I have asked enough questions, and probably pissed off more than a few folks with these questions. But I wanted to start and interesting discussion, and I think I have achieved that goal with this thread. Thanks everyone for sharing your knowledge, experience, and opinions.

Para
 
You wrote:

"The second observation is that few Mastersmiths apply a differential heat treat to their pattern welded blades. I have been told this is because it is much more difficult to to properly in a pattern welded blade, and that the temper line would be obscured by the pattern in the steel. It seems to be generally accepted that a pattern welded (damascus) blade will At Best be no better than a blade made of the tougher of the two (or more) steels used in its construction. If differential heat-treatment always results in a superior working blade, why are damascus blades rarely differentially heat-treated? It seems to imply that a differential heat-treatment is more about aesthetics (that cool looking hamon) than performance. Or is a differential heat-treatment NOT applied simply because makers of damascus blades do not expect their blades to be actually used?"

Actually, it is the other way around. Differentially hardened blades, whether damascus or not, etch differently. That is why you get a visible hardening line (temper line is a misnomer,it has nothing to do with tempering). In damascus, most makers want a uniform etch to bring out the pattern. Thus, NOT differentially hardening damascus is about aesthetics, and not the other way around.

As for the discussion in general, it is a good one. Good arguments can be made either way. I have certain reservations about differential hardening IF it produces a so-called "wimpy" blade (that is, very tough, but not hard). I also don't think a knife is a crow bar, it is a knife. But it also may be the only thing you have with you at a bad time and may need to be used for more than cutting. Since only the edge and tip need to cut/pierce, having the middle and spine LESS HARD (notice I did not say SOFT) is not such a bad thing. An insurance policy if you will.

Differential hardening is not a simple matter of "hard edge, soft everything else." By controlling the depth of the quench the maker can control the qualities imparted in the blade. On my JS test blade I went for a deep edge quench, nearly half the width of the blade. Since the steel was 5160, I also got some hardening above the oil. This was not an aesthetic decision, as the blade was never polished or etched. It was a performance decision. I wanted a blade that would bend 90 degrees without breaking, but absolutely not stay bent to 90 like the shallow quenched wimpy blades do. It took a 3' long cheater bar and all of my 175#s to make that blade go to 90--I could not have done it with the leverage supplied by my two hands and the knife handle. The blade then sprang back to 25 degrees.

Nothing about this blade was soft. On the other hand, I could have pried with it if I needed to. I also could have hammered the spine through logs without worrying about breakage, etc.

Finally, asking if a knife should be differentially hardened is like asking if a knife should have a single or double edge, serrations, natural or synthetic handle materials, etc.--in other words it comes down to the practical intended use of the knife, and the more personal preferences of makers and users. Variety is good.

John
 
Paracelsus: An excellent thread and I thank you for your kind comments. I did not see this thread until tonight, glad I did!
The nature of the differentially hardened blade varies as greatly as any other aspect of knives from maker to maker, steel to steel. There are many variables that contribute to the performance potential of the differentially hardened blade. While the steel we are using is fairly cheap, the time and knowledgable labor developing the full potential of that steel renders it very expensive. I am not complaining, the investment is well worth it in the perfromance arena.

One of my greatest fears is a tool that fails when needed. It doesn't matter if the blade is 2 or 10 inches, if I am going to invest in it, I demand quality. WE never know what the future holds, a sunny day can easily turn to a wreck, when this happens, they only tools that matter are the ones you have with you. This is the primairy driving force behind the investment Bill Burke and I have put behind the knives we make. We take knifemaking very seriously, they can be a critical tool. (We each make our own knives, but colaborate intensly)

Some steels respond to differential hardening as individuals. When the table is set to push the steel to its limit, differential hardening is of great benefit, for many reasons. When combined with multiple quench the differences become exponetial. The more the knife maker puts into his knives, the greater the rewards for all concerned.
 
Thanks for the honorable mention Ed.

I have just one Question for everyone; If you were stranded upside down in a vehicle with no way out but through the sheet metal and the only tool you had with you was a knife with a four inch blade that you had to use to cut yourself free then which would you rather have a knife that was made to be as nearly indestructable as the maker was capable of or one that was not made to be used for anything but cutting?

If you have an easy answer for that, then what if you had a fractured arm and fuel leaking into the passenger compartment at the same time?
 
Thanks very much for your reponses Ed and Bill! I have no doubt that the knives you guys make are pushing the boundaries of knife performance, and a critical component of your art is differential tempering and other refined aspects of your heat-treatments. But is your process something that is peculiar to your material of choice (52100)?. Do you think that some of the newer steel alloys can come close to performance of your knives without differential tempering?

Folks are not always seeking the ultimate in blade versatility when choosing knives, Bill's question aside. I respect what you guys are doing, and would suggest that anyone looking for the highest level of performance in a knife look at your work. I admire folks who are always seeking to create the best work that they can.

I know that there is a lot more to your art than just the heat-treatment (edge geometry, ergonomics, etc), and I will always have a fondness for simple carbon steels, but in the modern world of outstanding new CPM steels, I am not yet convinced that your heat-treating methods will necessarily produce a superior blade compared to simply choosing another blade material.

Why don't we see any cutting competitions between forged and non-forged blades? Are the ABS cutting competitions open to any blade makers? If so, where the heck are these guys? Maybe a big part of the problem is that is no generally agreed on method for testing blade performance.

Paracelsus, respectfully asking questions
 
Paracelsus,
As far as I know, the only requirement is that you have made the blade yourself. Jerry has tried a few times to have some of the stock removal makers of the "hot" steels join in the cutting competition. So far there have been none that I know of.
As far as a generally agreed upon test goes, not one of the contestants will know what at least 3 of the 5 cutting tests will be. There has always been the 2x4 chop and the rope cut. The other 3 will test the blade for point, edge retention, blade geometry, and edge holding ability. What difference does it make what they are, as long as all the knives can perform? If the cutting contest was for only one item, then all could build a knife to do the best they could at that particular job of cutting. The ABS cutting has a variety of cutting chores to do and the "best" score of all will win.
I have often wondered why some will not participate and have come to the conclusion that either they were unable to attend, were physically unable to compete, or were not designing knives for overall performance and did not wish to abuse them. This is only my idea of why and I am sure that there are other reasons that I don't know about. Hopefully there will be some in the future to show how the super steels are in overall performance.
 
Thanks for your response Ray! I guess it is up to some of the stock removal guys to show up at one of the competitions sometime and demonstrate their stuff. Or organize their own competition. Maybe such an event could be arranged on 'neutral' turf at next years Blade Show. It sure would be interesting to those of us who would like something more objective to compare than individual makers claims for performance.

I also appreciate the idea that not all of the tests are announced ahead of time. That make sense. Maybe it is the idea of all-around performance that scares the stock removal people away. I dunno. I was thinking of Jerry when I wrote the post above since he seemed so keen on joining one of these cutting competitions earlier this year. I wish he had come himself even if he couldn't get anyone else to participate.

I love forged carbon steel blades, but I am increasingly curious about the claims being made for the new 'super' steels. I know ABS members routinely do destructive and non-destructive testing, but that does not seem to be common in the stock removal world. Too bad....

Para
 
Paracelsus: The last I read, the ABS competiton was for forged knives only. I always wondered why they would not let stock removal makers compete.

As to our methods. I wrote all of the theory and our findings up in Blade some time ago. As a summary, we believe and our research indicates that the greater the reduction by forging, the further we can push the performance levels of this steel. We start from 5 1/2 inch round bars and reduce it to knife size, all with low temp forging, below 1625 f. The steel is subjected to numerous thermal cycles and normalizing cycles during the forging operaton. At no time does a hammer hit the future spine of the blade, all hammer blows are to the sides and future cutting edge. As a result of the large degree of reduction by forging, the low temp forging and the subsequent thermal and hardening methods we have developed a grain structure that surpasses what used to be the theoritical limit.

The experimental method that supports and enhances our work is that all blades are etched, there is a thread in the archives that will greatly enhance this discussion if others what to read it. I don't know how to mention it, so will leave it to you if you choose. Asd I remember it was called the tell tale etch.

There may be some steels that can out cut our blades, I dont know of any. If we wish to shoot for cut only, we can achieve very high levels of cut. We shoot for total high performance, cut, tough, strong. Taking all together in one package it is hard to get much better, but we are trying.

I strongly believe that many steels can be improved as knives by the multiple quench. The hard part is getting over the traditions that keep others from trying it. I do not have the time to experiment on all steels, it is up to bladesmiths who what to explore to do their own homework. I always offer to help, and will assist any who ask.

I have been writing about the virtues of low temp forging for over 15 years. It is not new, many others knew of it in the 1800's. Neither is Multiple quench new, it is as old as the art of the blacksmith. Wellman's book, The Iron Mistress mentiones it.

The problem is that there are no shortcuts to high performance blades. Many can't make a living making knives taking enough time to push the envelope.
 
differential hardening serves no purpose on any knife, as I can not think of a single scenario where the ability to bend a blade 90 deg. is advantageous.
Click to expand...

Perhaps not quite 90 degrees. But, there are plenty of light flexible blades (e.g. fishing knives) that would benefit from the ability to bend and return to true.

n2s
 
Thanks Ed. I must have missed that thread while I was hibernating last Spring. Looks I've got a lot of reading to do :)

For anyone else who wants to see the thread Ed just mentioned, it is here: The Telltale Etch

Thanks for all the replies gentlemen. I'm learning some new things and I hope others are as well. If we are lucky, maybe we can see a cutting competition sometime soon that will compare forged high performance carbon steel blades against blades made by stock removal makers using some of the new 'super' steels. I know I would love to see that happen. Maybe the ABS could issue an invitation to one of its competitions to more than just the Smiths. I can understand how such an event might seem threatening to some, but if blade makers are really serious about proving performance, it would sure be interesting for all.

Para
 
This is certainly one of the most fruitful and substantial discussions in recent months. Thank you, gentlemen. :)
 
The rope cutting competion at the Oregon knife show has been entered by one or two guys with stock removal blades for the last four years. The most noteable being a blade made of the "new s30v super steel. this knife was made and swung by Ed Severson from crucible metals. Ed has been in the competition for the last three years so I'll assume that he knows how to cut. The most that I remember him cutting is six pieces of free hanging one inch hemp rope if I am wrong on this please correct me. I also don't know if The knife is giving up or Ed is. Just thought that I would mention it.
 
Let me hop in here....

This is a great thread!

Here is a question to pose here: Are there any disadvantages to a differentially hardened blade?

The reason I pose this question is because the process of edge quenching is no more difficult than the process of fully hardening the blade...be it short or long. So unless there is a disadvantage in the method....why not just do it and go with the advantages?


Greg Covington
 
Originally posted by not2sharp
Perhaps not quite 90 degrees. But, there are plenty of light flexible blades (e.g. fishing knives) that would benefit from the ability to bend and return to true.
n2s
Click to expand...

Ragnar has some nice fishing knives; I think some of them are laminated. Do they really need to be differentially tempered also?
 
The only disadvantage to differentially hardened blades I know of, they do not lend themselves to folders as the tang wears too much.

As to competition, It only tests what it is meant to test. For the High performance knife it is a test to absolute destruction. You cannnot infer anything from a knife a knife that cuts a pop can in half a general use senario. Rope Cutters are Rope cutters. They are thoroughbreds.
These competitions are fun stuff and should be recognised as such.

As I propoosed in the thread Paracelsus mentioned. Cutting a rail road tie, slicing rope, edge flexes, full 90 degree bends back and forth, strength. These are not the kind of competitions most wish to participate in. These are absolutely critical in the bladesmiths development of my vision of the High Performance knife.

It is another realm, similiar but much more serious as they predict, as close as we can estimate, the preformance of a knife when you have no choice. The single rope cut, the 2x4 followed by a 90 degree flex are the most basic tests, true an achievement, but only the start to the development of High Performance knife.
 
Ed is dead on about the 2x4s and rope. When I was preparing for my JS test that's all I could think about. Consequently, I began to believe it was the goal rather than the beginning. Luckily, J.D. Smith straightened me out. He told me 2 2x4s are nothing and that a good knife would do many more. Now I don't know if I'm ready for a railroad tie, but I got through 10 2x4s and the knife was still shaving over its entire edge and cutting cigarette paper over about 70% (digital photos of the aftermath by request). My arms got tired, I ran out of wood, and I ran out of arm hair, otherwise I'm sure I could have got a few more. Oh yeah, the blade is a 1095 bowie with a deep (about 1/2 its width) edge quench.

To answer Greg's somewhat rhetorical question, the only potential disadvantage of the edge quench is the possible lack of strength (propensity to bend easily) on a very shallowly quenched blade. But this problem is with a poor application of the method, not with the method itself.

Just as a proper tempering temperature is calculated for ideal working hardness, so should a proper depth of quench be calculated for the intended use of a blade.

Finally, going back to what Ed and Bill were saying/implying, I was once told by J.D. Smith, quoting Jimmy Fikes, that the primary consideration for a tool is that it hold and edge and that the primary consideration for a weapon is that it not break. I now see that it is more complicated than that. First, a knife is both a tool and a weapon; in fact, all weapons are simply a sub-category of the larger category of tools. Second, that only works if you are using your tool in your garage, etc. If you are using your tool in the bush, and it's all that you have, then we're back to square one--the primary consideration is that it does not break. Now, if it holds an edge as well, and there is no reason it should not, that's even better.

John
 
Hi John,
Yes we are basicly in agreement. Let me clarify one point however, by tool I meant gouge, plane, chisel, whittler, thing like that. While your point that weapons are subsumed under the class of objects known as tools, the converse under extrordinary circumstances may be true as well. I think we all can agree that all of these things should hold together well within the resonable parameters of their intended purposes. To this end, we seem to be about the business of applying the principle know as "Headroom", the 8 cylinder engine, the 200watt stereo amp, to an extent these things are some what overbuilt for most end user usage patterns. What it does though,is allow the thing to be used in normal to moderate extremes without approaching taxing limits, e.g. a200watt amp @100db is operating at maybe 20% peak capacity whereas a 50 watt amp @ 100db is maxed out and distorting badly, get the picture? so I guess what we're doing is applying that same thinking to the gig we're on. in short the tool/weapon thing gets a bit sketchy without a bit of clarification. say hi to your girls for me.
 
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