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Differential heat treat - chat

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compiled from knife chat Jan 30-09

[markb] How do you Differential heat treat?

[steve sells] I can think of 3 methods off hand: Japanese style of clay the spine areas, Wayne Goddard style of using water to keep spine cool, and the edge quenching, I am sure there are more ways as well.
I clay coating consists of covering the spine and parts of the edge before hardening with a mixture that while still allowing the blade to heat, slows down the cooling in the quench on the areas covered. Allowing the cutting section of the blade to harden, but preventing the rear from conversion to martensite. this has been common for centuries in Japanese style blades, and more modern as well. The physical action will usually cause the blade to warp into a curve, so straight blades in this style are normally counter curved to compensate for this effect due to that side getting conversion, and expanding.
[nts] does it curve up, or to one side?
[mcraigl] Steve, do they warp toward or away from the edge.
[steve sells] the warp is away from the cutting edge

[steve sells] Wayne Goddard, mentions another method, This way the spine never gets hot enough for conversion, as it is kept in water bath while only the cutting edge is heated. this really only works in small blades, but offers the option of combining tempering IN the hardening process. After heating the cutting edges to proper temp the entire blade is plunged into the water mostly cooled, the raised from the water bath to allow residual heat to drift into the cutting portion, then quenched again, and so on, now I only mentioned 3 I am aware of to harden ONLY the cutting edge of a blade.
[Rich Hale] Make notes now Mark and it will help you in research

[steve sells]3rd method good for straight blades of any size is edge quenching, where only the cutting edge is placed into the quenchant to harden, allowing the spine to remain peralite
[markb] why only straight blades
[steve sells] Because the surface of the quenchant is flat due to gravity.
[markb] can't rock the blade?
[steve sells] a blade does curve when hardened this way but if curved prior, then how are you going to get only the edge into the liquid while? I can't rock a blade from one end to the other in less than 0.8 second to beat the nose of the curve needed for quenching. While one larty is in, another is exposed, and not being quenched.

[mcraigl] Oops, I assume that all three of these methods cause some amount of warping then? with Wayne Goddard perhaps the least and the edge quench the most?
[steve sells] all will due to the conversion Only being at one edge. Differential hardening can have limitations. but it insures that only the edge of the blade gets hardened, it is not really usable for everything but there is still full hardening, so weird things like a German Flamberge can be hardened
[Finnr] using a torch for differential temper is one slightly risky way of getting a hard edge and softer back. I also often use a cheap electric hotplate to do a differential temper.
[steve sells] to get the soft spine there we use a differential TEMPERING process to soften the spine more than the edge First temper the entire blade in an oven so its all where ya want the edge, Then using a torch, or heated metal, apply heat to the pine and quench before that color creeps into the edge.
[N8 Knives] what is used for the clay in japanese style
[N8 Knives] does any kind of clay work?
[steve sells] many people have their own thing, I use refractory cement for lining the gasser Once I used fire brick cement. If it can survive the shock of the heating then quenching AND slowe heat transfer. it should work

[mcraigl] The amazing part to me is that ANY sort of clay would stay stuck to the blade upon entering the quenchant.

[mcraigl] I think I've heard of crushing IFB and either making a slurry from it, or adding it to a clay slurry. You ever heard of that?
[N8 Knives] the clay doesnt have to stay on for the whole quench does it, just the initial dunk right
[steve sells] I use wires wrapped around the blade and clay coating. this assists with keeping the clay in place, it needs to stay till after the steel is quenched to below the martensite start point.
[nts] do you have to let it dry, i saw somewhere that it can still be damp when you heat and quench it
[LDW] I did not think you wanted it on the cutting edge
[steve sells] Dry or the damp will explode as water expands 150x when it turns to steam. I covered the entire blade paper thin (to prevent scale) then thicker over the rest others do leave the edge clean. I hung it over night in my house after the first layer, then 2 more nights after the thicker spinal layer was applied, the katana ended up with a smooth sine wave type hamon, even tho I used ashi, and "rolling breaker " type waves on the edge of the clay.
[N8 Knives] I read that each sword smith had their own design of removing the clay from the edge to create a unique hamon line
[steve sells] many have their own clays, pattern of the edge. etc but the hamon does not show the same way as the edge of the clay coat.
[mcraigl] Steve, Is this a process of many thin layers, or after the paper thin, can you go ahead and paste it on there. Also, if many layers, are we talking about a complete drying cycle between layers?

[mcraigl] So you can put the thicker part on in a single thick layer then if I understand you correctly. Good, I was afraid this was a month long process to get the clay on there, and I'm not that patient.
[Rich Hale] Half hour here

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  • 4 months later...

While the above are three methods, there are (I think) only three ways to obtain a differential result.

1) Differential heating for quench

2) Differential quenching

3) Differential tempering

Of the three, differential tempering wood appear to offer the best possible qualities (IMNSHO). Instead of a relatively soft spine, you can have a hard edge with a "spring tempered" spine, that adds considerable strength to the blade.

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I have used the differential heating mentioned by nakedanvil for some types of blades. It can be a very useful strategy in the right situation. An example is a rabbet plane that I bought which came with an iron so soft that it needed resharpening after only one or two strokes. In the field I used a small propane torch to heat the edge and quenched it in water. With such a limited heat source I could not have done a full harden and temper cycle but I was able to exploit the quicker heating of the thinned edge to get a nicely controlled differential heat which when quenched hardened my edge for about 3/16" to 1/4" back from the sharp edge (adequate for many years of use in this case). In this case I was very happy with the result and able to get my work done (which had started to become a bit frustrating). I still use this strategy sometimes... even when I am at my forge. As example I may only harden the edge area or tip section of a punch or chisel, leaving the handle and striking end soft. This is easy to do just by how I place the tool in the fire and time the heat. With a knife blade it can be more complicated but a partial grind before HT can allow a torch (especially if skillfully manipulated) to heat only the thinner edge area to critical temperature. Such blades as ordinary chisels are very good candidates for this type of hardening strategy as they usually only need hardening near the edge and given their long tapering shape zoned heats are easily achieved. Depending on the steel they may also need tempering however.

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One other thought on this subject... a full heat and partial quench will often result in a stress weakness at the point where the immersion in quenchant stops. I have not had that sort of problem when using the differential heating strategy as it seems that the steel naturally feathers the heat for a gradual enough transition zone to avoid serious weakening. I could see how it might be a problem but in ordinary practice it seems not to happen... at least not often.

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I think you misunderstood me though Charlotte. I believe that the pictures you saw likely referred to a clay shielded quench. A different kind of differential quench than the crude and dangerous method which I was referring to (just partial immersion in the quench lengthwise). I am not intending to cast aspersions on any of the traditional and tested methods of HT... simply to add another type of strategy for those who might not yet have it in their personal bag of tricks. My own experience is limited too... but I have had some successes with this and think it worthwhile to discuss.

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Charlotte's post has reminded me that there is another way to obtain a differential result. Carbon steel has to quench in a little less than 1 second to get fully hard. This's why it's rated as a "water quenching" steel. Well, thin parts have no problem quenching this fast. Very thin sections will even harden in oil. So, if we quench a nice high-carbon like W-1 or 1095 in oil we will get an edge that is very close to fully hard and a spine that is not.

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  • 2 weeks later...

My bladesmith teacher that I am learning from, has a small one burner gasser that he can use for differential results. He will heat the spine up, and then take the air hose and blast the knife but the blade area, hits the spine, tang, everything. It reaches about 45RC that way, then lets it cool. Then heats up just the edge area and quenches. We did that on my last knife. Edge area 62RC, middle of blade 53RC, spine 44RC. Thought it was a pretty cool trick that works, and gives you the spring steel, and hard edge.

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