Wednesday, October 18, 2006

Differential tempering

Something I don't think I've mentioned before is differential tempering. I have mentioned hardening this way, where- as shown by the line on that blade- you only bring part of the blade up to critical temperature. I've always done large blades this way, and some small ones.

There is another way to get this effect, by doing the tempering from the back of the blade with some heat source that bleeds heat into the blade from that point only. This way you wind up with the cutting-edge area hard with the area further back being softer and tougher, more flexible.

Four ways to do this, all starting with a blade that has been fully hardened, then polished so the surface is smooth and shiny. This has to be done in a place with good light so you can see the colors moving through the blade(which is why this method sucks with either stainless or some highly alloyed steels; the colors don't show the same or don't correspond to temp the same way) and a container of either water or oil handy to cool it off at the right time.

First is to use a heavy block of steel or iron, or a heavy rod, brought up to a high temperature. You hold the blade with tongs and lay the back of it on the block. You'll need to move it back & forth to keep the heating even and not let it get too hot near the point. You keep working it and watching the color, and as the chosen color/hardness approaches the edge you quench it.

Second is to put a piece of heavy steel pipe over the forge, say 8-12" long and 4-6" diameter, cut in half lengthwise. The idea is the fire heats the pipe and you work the back of the blade over it.

Third is to use a torch. Use a fine flame you can work along the back of the blade, again watching the colors as you go.

Fourth involves a torch also, but with a different method. In this case you temper the blade in the oven to desired hardness, then set it up in a shallow pan of water with the edge beneath the surface. I've used hemostats and vise-grips to hold one in place for this, they allow a fair amount of adjustment of angle, plus can help stabilize the piece so it doesn't fall over.
When it's set up, you work the torch along the back. You still need to watch the colors- don't want to get the back too soft- but unless you're using a very shallow pan and an oxy-acetylene torch it's almost impossible to draw the temper of the edge down too much.

All work, all have their drawbacks. The torch is simpler, but it's real easy to get an area too hot. If you catch it in time you can quench the piece, then shine it back up & start over.

The block & pipe methods are slow, but you're a bit less likely to overheat the piece that way.

Any of these methods, you need to temper at least twice and preferably three times; it makes for the most even result. All will give you a blade with a hard edge and a softer, tougher back.

No comments: