"soft" steel with tempered steel insert or differential hardening?

J

jack43011

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Jul 24, 2015
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Hey everyone! Sorry if this is a stupid question, but a thought came to me.

What would be the benefit of differential hardening vs folding a piece of non-tempered steel over a harder edge or core? it seems like the flexibility would still be intact with the latter method, and a bit more uniform as well. Would this cause a structural flaw of some sort? I feel like I must be missing something here. Thanks!
 
Insert welding was popular, because the steel cutting edge material was rare compared to the iron body

It was cheaper to do back when it was popular.




Good quality modern steel with through hardened HT is strong



Differential hardening get a lot of press by EF and some, but I'd rather have a fully hardened knife with a higher yeild strength than a soft floppy blade

"catastrophic failure" to me is just a buzzword.
 
So for something like say, an axe head, would insert welding be a better option? Hard core to cut the wood and keep an edge, softer everywhere else to prevent chunks of steel chipping off?
 
The two ways of making an axe have been around a lot longer than I have ! Hundreds of years ago "steel" was hard to get so a piece of steel was inserted into an iron head. Later when steel was more common you could make the entire head from steel then differentially harden it. But when inserting a steel piece the head must still be HT'd as the welding softens the steel.There should be videos available to show the two methods.
 
And don't confuse hardness with flexibility. Flexibility is a matter of geometry - not hardness.
Hard and soft steel of the same dimensions flex exactly the same. What is different is the yield point - that point at which the unhardened blade will take a "set", but the hardened blade will return to straight.
Just think of a fillet knife for cleaning fish. They are fully hardened, yet flex to nearly 90 degrees and return perfectly straight. If it was just a hard cutting edge and either a differentially hardened blade or an inserted hard cutting edge it would be bent.
 
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Homogeneous steel, fully hardened and tempered(deferentially, if needed) is what you want in most cases. There is a place for laminated/pattern welded steel and differential hardening but it usually involves specialized/desired attributes, not necessarily related to performance.
 
The advantages will depend on the blade shape more than anything. Some blade shapes have stress concentrations at tangs and such and they benefit from less hard steel in that vicinity.

The first concept to consider is the behavior of material at stress concentrations such as notches or corners. Some high strength materials will be more prone to fracture at notches, much like a piece of glass breaks at the location where it has a scratch. Harden the whole blade and you may have a knife that wants to fracture at a notch, harden it less and it might actually have more strength at the tang.

The second concept to consider is that under lateral bending, the steel farthest from the centerline of the blade has more bending stress. So if the outer layers of steel are softer, meaning less prone to fracture, then the blade can survive lateral bending that would cause another blade to fracture. The way differential hardening works is that the steel near the edge is hardened, where the blade is thinner. So when the blade is bent, the stress in the hardened steel is less than the stress in the thicker less hard steel near the spine. The end result should be somewhat the same whether it is San Mail steel or differentially hardened steel.

If the blade shape is such that it doesn't have big stress concentrations, you might be as well off to harden the whole blade the same amount.
 
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