differential quench vs differential temper

Yes I've had and excess of time to think lately, but it spent all morning raining so I should be able to fire up my forge soon.
Anyway, I went from thinking on torsion and charpy to wondering about the differences in differential quenching and differential tempering and this led obviously to which results in a tougher knife all else being equal?
As I understand it, differential quenching, either clay-coat or edge quench, will give a hard martenistic edge with a soft perlite spine. (I suppose the spine may be some combination of perlite/martensite and perhaps with a little bainite). A differential temper on the other hand will still have the hard martensite edge with a softened marteniste spine.
So if someone was chasing that elusive ideal knife, which would be a better choice and if you can why?
Ron
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r payne wrote:

Chef knife, fillet knife, bowie knife, marine corp survival knife, brush cutting knife, corn shocking knife? Which are you trying to make? Ideality is a function of requirements.
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Del Cecchi
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[partial austenitizing too? ;]
BTDT by accident one time :) the thin part was ready to quench and the thick back wasn't, but was real close so went for it and quenched it anyway.
That knife couldn't be broken if you wanted to. It bent back and forth like steel shipment strapping ("tie banding" -RR lingo;) but the edge cracked right away since it was "dead hard" (untempered).

I sure as heck don't know. :(

That all sounds right to me. :)

Cool question. :)
Alvin in AZ
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Once properly tempered though it should have been an outstanding blade don't you think?

So let's review a couple of things. Tempering over a metal's ideal temperture - say over 400F and up to 800F I've been hearing all kinds of information lately that says you loose grain cohesion and strength. On the other hand, if the steel has a good grain structure to begin with and you never actually harden it in the quench, it gets left out of the temper equation to a large degree. So you have a hard tempered edge and a dead soft backing... or maybe a slightly hardened backing (I would think this would be better). Seems to me this would be ideal. Of course the best bet is to do the same knife both ways and beat the hell out of them to see :-). Then there is also pulling the knife out of the quench while it is still around 700F. I did this to a sword blade that once cool was plenty hard. I could'nt find a local shop that would hardness test it for me but the blade had a nice springy feel to it after temper. Too subjective for this discussion really but it's an idea to play around with.
GA
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Like Chas sez, "yes for certain uses, but not for others".
So far, I haven't found a need for other than an all-hard knife.

Yeah, "temper embrittlement". :)

Yes, since it'll remain pearlite. That's what my blade did. (above)

At least it'd be ideal to pass some sort of artificial "knife guild type" test. ;) Or survive Cliff. ;)

Yeah, that sucker prob'ly still became mostly martensite anyway, just the same if it hadn't been pulled from the quench. (as long as you got it under that "nose";)
I tend to do my butcher knives that way too. They usually need some sort of "warp straightening" operation :/ and that's a good time to do it since the martensite isn't fully formed. :)
Alvin in AZ
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r payne wrote:

A lot of it is going to depend on what alloy you use. For example, 5160 is an oil only alloy, 1095 is a brine/water/oil alloy. I have a knife of 5160 that I tried water on, little cracks all down the blade about one thickness apart. It cooled so fast that the steel literally tore itself apart from the tensile stresses. I imagine that a brine quench would have literally exploded the work. But... a file won't touch it. I keep it around to remind me to pay attention to the manufacturers recomendations.
Charly
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On Thu, 9 Mar 2006, r payne wrote:

At mid to high fifties, bainite is much tougher than martensite, so ideally you would have a martensite edge and a bainite spine. The fully soft spines are wonderfully ductile and resistant to impacts, but they are so weak the blades bend under little force. You can take a short 1/4" full convex ground blade and jab it into a piece of pine and bend it easier with just wrist strain and it won't break the wood. It acts essentially like mild steel. This seems far from optimal to me. Optimally you also draw the hardness starting at the choil and going to to pearlite in the tang to dampen shock.
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The worth of differential tempering is for weapons use, not particularly for working knives. The optimal attribute is resistance to catastrophic failure in combat- bending is ok, it's snapping that you're trepidatious about. You can afford to accommodate a bend until the affray is over- then you can get a new one, loot his body for another, or pound yours back into shape. Same reason I like pattern-weld; they don't snap on you.
Chas
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Chas wrote:

I consider myself fortunate that combat applications are not high on my list of important knife attributes.
And to your list of what to do when "the affray is over" I would add "buy more ammo" :-)
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On Fri, 10 Mar 2006, Chas wrote:
[weapons]

I am understand this and I would prefer similar for a survival knife. However I would also argue that a bainite spring spine would be of far greater benefit than a pearlite spine.
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Supposedly, the tang is to be dead soft, the edge quite hard, and the spine at a spring steel temper. Of course, much of it is predicated on Japanese practice which uniformly uses water as the quenching agent, and except for some heating to straighten a warp, the heat-treatment is done in one movement.
Chas
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On Fri, 10 Mar 2006, Chas wrote:

Yep that is exactly how I would want it ideally. I had a large blade made up, forged from spring steel to exactly those specifications. It was a big parang styled knife, very nice. I had it recurved as an experiment, if I had it to do over I would have it as an upsweep instead, it works better on brush that way, live and learn, same general curvature found on a katana.

Yeah, it is kind of amazing what they do by hand, the residual heat from the spine autotempers the blade after the quench, very nice balance and efficiency there. I like a lot of japanese blades and wood working tools in general, nice use of hollow grinds in general. Have not used any of the larger blades, but would expect them to be quite solid.
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Cliff Stamp wrote:

Which raises the question of how one would go about getting a bainite spine with a martensite edge. As I see it the problems are keeping a hard edge while holding the spine at the required temp for the necessary time or not losing the bainite when heat treating the edge to form martensite.
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In rec.knives r payne wrote:

Yeah, I'd like to know that too.
Alvin in AZ
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Wasn't that the intent/result of the really heavy quenching media? Molten salt, mercury, lead, blood; all that?
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Chas
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No kidding, I haven't got a clue on how to do that. :/

http://www.panix.com/~alvinj/graphTTT.jpg
I need to steal a better graph? ;)
Take 1400F 1080 steel, cool it in an instant to below 800F, then within a few seconds, bring it down to 600F and hold for 10,000 seconds (2.8 hours) and it'll transform into bainite.
That's what the salt-pots and lead-pots are for.
Mercury is supposed to quench faster and so even lower carbon steels can be hardened without expensive alloying.
Blood isn't all that much different from sea water (according to a Bell Telephone movie I saw in junior high;).
Alvin in AZ
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Best I can come up with (based on what Alvin has said here to refresh our memory on making bainite, as well as what he's said in the past about getting things full hard, and some other sources on that subject) involves some complicated fixturing which needs to be actively heated and cooled, assuming you can get things headed more or less the right direction during quench based on thickness (ie, drop the edge down far enough while leaving the spine hot enough - then slap it into a custom-fit rig which applies (clamps) 600F copper to the spine, and iced or better copper to the edge, and keeps heating the one and cooling the other despite the fact that heat will be travelling from one to the other through the blade (and how does that in-between part turn out?). Once the edge portion was adequately iced (or colder-than-iced) it could be permitted to come to whatever desired tempering temperature, reducing the heat flux a bit.
Complex, and costly in terms of energy, but it should do what is asked for. Then there's the simpler option of making a mechanical joint between the two parts, heat-treating them separately and press-fitting them together.
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wrote:

Sounds like what the saw blade folks do for "bi-metallic" blades. weld the two pieces together.
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Yeah, how do they do that and make it turn out right? :)
Alvin in AZ
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That one's pretty simple, as nobody's specifying bainite in the back. Same applies to all the other examples of welded-bit tooling others have put forward - high-carbon welded to low carbon - not the bainite you were asking for in the back.
Bandsaw blades, IIRC, are generally laser or electron-beam welding of a high-carbon edge to a low carbon back in a very automated process - similar approach to the heat treat (that's often induction, as far as I recall, but might also be done with a laser depending on the setup & tooling of a particular production line).
If you could put up with the HAZ and had a precise, high-power laser or electron-beam setup you could weld bainite to martensite, but I don't think you'd like the properties of the HAZ even if it was small, which is why I suggested a press-fit earlier.
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