Differential Tempering, a waste of time and money?

Paracelsus

Paracelsus

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Nov 6, 1999
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OK, so the topic is a bit provocative. But it is also a legitimate and serious question. I have had several discussions with ABS mastersmiths who almost never do a diff temper on blades shorter than six inches. Even then, its utility is questionable.

Don't get me wrong here, I LOVE the look of the temper line (hamon) on diff tempered blades, and I have no doubt of its utility in longer, bigger blades meant for chopping. But is the time and energy spent to properly diff temper a blade wasted most of the time?

One of these ABS masters asked this question: "who needs to be able to bend a knife 90 degrees?" Since a knife is made for cutting, not normally for prying, what real advantage is there to applying a diff temper?

There seems to be some disagreement between different mastersmiths on this point. Certainly the ABS cut, chop, and bend tests assess a makers understanding and ability to manipulate the physical characteristics of the steel in a blade, but do these tests actually translate into real world performance?

Note that diff tempering is something done only (mostly?) on carbon steel (low chromium) blades. Most high chromium blades are tempered uniformly (all-hard), even the big ones. Different steel alloys are chosen in order to achieve toughness in large blades intended for chopping. So is differentially tempering just for show, or is it really useful in smaller (6 inches or less) blades intended to cut, not chop. If the blade is really big, wouldn't a slightly lower hardness in a uniform temper achieve the same result?

This thread is an invitation for discussion, not an ideological or terratorial war. I love my differentially tempered blades, and I like my all-hard blades. But are the ABS tests Really measuring something useful to knife users?

Paracelsus, stirring the pot
 
It depends on the piece. A smaller knife would probably not need the differential heat treat. However, a larger knife would probably be used hard and might break without a differential heat treat.
 
Part of my question is whether a diff temper is worth the time, effort, and expense even in larger blades. A large chopper can simply be tempered softer, since it is unlikely that a large blade will be used for cutting tasks requiring extreme edge holding.

Para
 
I think the ABS tests check mastery of the "art", which is obviously important.

Is it relevent? I don't know.
 
Is it a waste of money? probably, but then again so is owning 100 knives just because of whatever reason.
 
Originally posted by Paracelsus
Part of my question is whether a diff temper is worth the time, effort, and expense even in larger blades. A large chopper can simply be tempered softer, since it is unlikely that a large blade will be used for cutting tasks requiring extreme edge holding.
Click to expand...

Even if its not being used for fine work, it still needs to hold an edge. Even if a knife is meant for chopping it will still benefit from a good, strong edge.
 
Depends on the use. The larger the blade, the more likely it is to deliver or receive a blow that could shatter a hard/brittle temper. If you are slicing tomatoes you don't need it, a large battle blade/chopper certainly does. The hard edge holds an edge, the softer spine provides support. A smaller blade probably would not benefit, most users of larger(9" & up) probably would not know the difference, serious users of fighters and choppers would. A machete doesn't matter, but I want a large blade to hold an edge. No, the lower hardness in a uniform temper would not provide the same result. Smiths may not want to bother with the process or raise prices for a proper differential temper, and maybe that influences their position.
 
Actually you have a thought provoking thread here Para.

Under most conditions I agree with your premise.
As far as the A.B.S. test, my thoughts are that the test is to show mastery in knife-making and that does not necessarily coincide with all aspects of what a knife user might need or want.
 
on small blades or many long blades. My khukuri is differentially hardened, but it's a serious chopper. The average Bowie might not denefit in quite the same way. YMMV.

Frank
 
Where is the extra time and expense when differentially heat treating? The extra time for the hammon would be if you wanted one that was not straight. That would be in using clay for the heat treat. If you only heated the part you wanted hard, there would be no difference in the time and money spent on the differentially heat treat. They both are tempered for the same amount of time. Of course, every one will do some things different and still may end up at the same point in performance and blade characteristics.
The use of the knife may determine the tempering temperatures of the blade.
 
I have seen blades shorter than 6" used to split breast bones and chop through leg bones. When they are to be used for these purposes, I think that a differential heat treat is advantageous even in shorter blades.

It is also my opinion that making the whole blade softer would not work as well for these purposes. You will end up with more damage to the edge when chopping through bone with a soft edge (unless you have over hardened the edge when differentially heat treating).

Now, if you were talking about blades under 4" I would be more inclined to agree that differential heat treating would not provide much, if any benefit.
 
I am very new to knifemaking, but the only blades I have heat treated myself were 5" or less. Most of these were differentially hardened. I found that for me and the way I am doing things, I like the looks of the hamon, but I think the fully hardened blades are probably better. I have had two instances where the blade chipped right at the demarcation line. Could be coincidence, could be that the steel was weeker at that point due to the change in grain structure, I really don't know enough about steel to know if that could be true. I have also seen that on a lot of differentially heat treated blades the hard part of the blade doesn't extend into the tang. This results in a very soft tang that is easily bent right in front of the handle where the steel is soft. If the whole blade was hardened and then the back was drawn back this shouldn't be a problem, but my guess is that this is not the way most people are doing there differential hardening. So for me and my knives, I have decided at least for now that it is better to harden the whole blade on smaller knives.
Kyle Fuglesten
 
I do not diff hard blades under four inchs in blade length. I feel its pointless. I cannot find a single reason a knife would need it at that size. For any "knife" task. That's why I use stainless in those sizes, so I can get some added benefits. I diff hard carbon and low alloy for big blades though, always. My .02 cents. :p

-Jason
 
The beauty of a differentially heat treat blade is in both appearance and toughness.

I think that on a 7" blade that is a 1/4" thick, it is overkill from a functional standpoint. I don't know anyone personally who would be able to break a blade of this size without assistance. Still looks cool though.

On the other hand, a 10" or 12" blade made from thinner stock may benefit from this process when being used as a chopper or while being used for heavy limbing. Probably even this would be from the standpoint of glancing blows and flex.

Cosmetically, I definitely prefer it over a mirror polish blade :D
 
generally a smaller blade would not need diff. treating, but a larger blade, intended for hard use, such as chopping could very well break without it (eventually, also depending how its used, ect.).
 
I don't think the differential hardening is necessary. If you want a super tough blade that can handle almost anything, forge a blade out of L6 and marquench it in salt and cryo treat it. But, I gotta admit, I sure like the way the blade looks with that visible temper line though.
 
The initial question was about differential tempering. The majority of the blades with "temper" lines or hamons, are actually diferential hardened. IMO, 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. You'd be better of with a fully hardened blade that would resist bending in the first place.

However, I feel that differential tempering of the spine and tip(to spring hardness only) is an advantage on a large fighting knife (bowie) or large chopping knife. The maximum tensile strength of most steels is not at the usual knife edge hardness, but lower. By having a spine and tip spring tempered (not dead soft), you increase the tensile strength of the steel, making a blade that will resist bending, but if bound, will bend instead of break, and then hopefully return to straight.

So, IMO, differential tempering is worth effort on large blades(9"+), but diff. hardening is no advantage on any size blade.

David
 
Great responses guys! My intent was to create a discussion, and we certainly have one here. However, I feel compelled to respond to the last post for clarity. I use the terms differential tempering and differential hardening to refer to the same thing, the final result of a blade having a harder edge and a softer spine.

Of course Drummer is correct in pointing out that hardening and tempering are different parts of the heat-treatment process. However, in my experience the word 'temper' usually refers to the final hardness of the blade, not just the process of softening the steel after the quench.

Perhaps I should have been more precise in my word useage. It is true that a blade which has a heat-treatment applied to produce relatively hard and sofft areas is actually differentially Hardened. By that I mean that the differential treatment is done in the quench, or hardening stage of heat treatment. Not the final tempering stage where the hardness is drawn down by gentle heat cycles.

My bad, I should have been more explicit about the meaning of the word. When I refer to a differentially tempered blade I also mean a differential HARDENED blade. It is the final result that I want to see discussed, not the many different methods that will produce a differential tempered (hardened) blade.

Para
 
A few comments:

kile commented that he's seen blades have problems at the demarcation line between hard and soft. That's the one complaint I've heard about differentially-treated blades, that they can be weak to lateral stress at the temper line. Can anyone with more experience comment?

The other note is that there's an occasional undercurrent about bigger blades needing differential temper. I know that Busse knives are very well-tested and even the big ones hold up to very tough use. Are they differentially tempered? If not, have we just proven that there's no need for a differential temper?

If you think Busse's knives hold up because of the steel, then explain this. As I recall (someone correct me if I'm wrong), when Strider started out, they were using ATS-34, even for very big blades. People were going ga-ga about how strong and tough these knives are. If even ATS-34 -- plain and simple, not a tough steel -- can hold up to hard use by just using a slightly thicker edge, couldn't a tougher steel hold up fine to hard use even being fully hardened with an even better edge profile?

I'm not, despite appearances, arguing that differentially-tempering a big knife doesn't make sense. Para just gave me an excuse to ask aloud some questions I'd been keeping to myself.

Joe
 
Differential heat treatment will always result in a stronger blade. As a knife nut, I cannot imagine not doing it on a low alloy blade myself unless you really had a business need, like selling knives to make a house payment, to offer your customers a less expensive product. Then it is the customer's choice.

There are some steels that are so wear resistant that super hardening of the edge offers little benefit. You can temper the entire blade to 58 and have a very tough blade with absurd edge holding. No point in differentially heat treating something like that. I am thinking of the CPM steels, but this may apply to more common high-alloy steels such as 154CM as well. Besides, as I understand the situation, air hardening steels are difficult to put a differential HT on that will be meaningful.

There are a lot of use specific performance aspects as well. Do you want the blade to fail by bending or breaking, for example.

Do a search on a couple of threads we had here featuring Ed Fowler describing the Extreme Performance blade, I think it was. In there, he described what you can accomplish in a real world working blade with differential heat treatment.
 
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