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Differntial tempering question?

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I read awhile back that drawing the temper back on the spine and tang of a knife with a torch can soften only the superficial layers of the metal and leave the deep layers still hard--prone to failure.

This weekend I was working on a full tang knife. I had normalized and triple quenched the knife. I tempered in an oven at 420 for 3 hours. I drew the tang back to Dark Blue/Gray with a torch and same on spine of knife. I then attempted to drill the holes in the tang for the scales. The drill bit worked well for the first 1/16" of depth but then stopped. I sharpened the bit with a drill doctor and re-tryed. Same thing-no movement. I can only believe that the outer layer softened and the inner metal remained hard.

I used a MAPP gas torch. THe steel is 0.25" thick 5160.

I then used the torch to heat the tang up to red heat and let air cool. Suprise, the drill bit cut like butter.

I had to requench and temper in the oven again.

My question, how should I draw the temper back? More time with the MAPP torch? I spent probably 15 minutes drawing the temper back the first time.

Also, I have problems with drawing the temper back when the edge of the blade is in water. Seems as if it doesn't get hot enough to draw back the temper.

Any help appreciated for this newbie.


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Get yourself a 2-3" piece of angle iron about 10 inches long. Weld plates on the ends that act as supports as well as "caps". Fill the void with water and place the edge of the knife in the water to keep it cool while you heat the spine. Make sense?
If you do need to soften a tang after heat treating, suspend it in a tube of water while you heat the tang. When heating the tang, in a semi-dark room, heat until you go through all of the colors and it JUST starts turning the slightest bit red, and stop! Let it cool all the way back down. Do this three times. After that you can drill it with a dull drill bit! I watch so many guys keep posting questions about what kind of drill bits to buy and use to drill their tangs, and all they have to do is soften them. It's not so much a matter of NOT quenching them, because even oil/water hardening steels will harden partially in air just by bringing them up to critcal. Always, always, always soften those tangs prior to drilling. I welded a plate to the bottom of a scrap piece of tail pipe I got at my local muffler shop. Welded on a little rod along the side that sticks about a foot above the top of the pipe. I hang my blades blade-down in the tube filled with water and soften the tangs.
As mentioned before, one way of getting what you seeks is to limit the depth of your quench in a horizontal tank and quench only the edge.

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