Preventing decarburization while torch hardening

In my experience, using propane to heat a piece of steel to be quenched always decarburizes it. Although this somewhat works when case-hardening with compounds such as Kasenit, in my opinion it is better to use oxy-acetylene. One can adjust the flame to a slightly carburizing one, and not lose carbon in the steel, maybe add some.

You can get commercial anti-scale mixtures, knife-making suppliers would be one source if your local welding supply didn't have any.

Soap was suggested in a lot of manuals as one coating, you have to use real soap, not a detergent bar. Ivory, I think, is the last of the real bar soaps out there. I tried it once, was pretty stinky.

Any time you use a flame for hardening, you're going to have scaling troubles. If you want to avoid problems, use stainless steel wrap and an electric furnace, you'll have better control, too. Pottery kilns are sometimes available cheap, they'd do the job. IIRC, A2 has some very specific time/temp heat-treat requirements, no way you're going to get anywhere near specs with a weed burner. Might as well have gone with O1 and saved some cash.

Stan

What he said, but I'll just add: the main problem with rough and ready heat treatment of A2 is going to be temper embrittlement: be careful to temper it pretty cool, say 420 degrees max, but probably better around 380. (This is assuming wear resistance is the primary desired result). It will be MUCH less tough when tempered at 520 than at 400. It starts to get tough again at the top of the tempering range, but I don't recall how hot that is, and don't have references here. Probably somewhere around 700 degrees F, but that is a guess, don't rely on it.

Adam Smith Midland, Ontario, Canada

Soft soap is traditional. Ask a pharmacist. A wrapping of iron wire will hold the soap in contact while it heats, but will slow the cooling a bit on smaller items. There are heat treating protectants available from some of the tool supply guys, but I do not know their names.

A soak in vinegar, with as much salt as will dissolve into it, works very well for scale removal, as well as derusting parts. Gotta have total immersion, though, or you will be able to just about watch the etch line grow on the air/surface/part interface.

Cheers Trevor Jones

It's plausible, but I don't have oxy-acetylene just yet, Would oxy-propane work as well?

Joe gwinn

Knife makers. That;'s a good idea.

Soap. I guess I've heard that, but don't recall where. Maybe sugar and soap.

Actually, I was thinking of leaving it untempered, as all it needs to do is not get chewed up by the 5/16" drill bits, and glass hard would be OK.

I've never heard of temper embrittlement. Please tell me more.

Joe Gwinn

I'm looking for a suitable furnace, but not yet. The stainless steel wrap does not work with a torch - it goes orange hot immediately, and one cannot tell how hot the article really is. It would work in a charcoal forge fire, though.

The A2 may no longer meet spec, but it seems to have become plenty hard enough, and without distortion. If I can control the scale and decarb, it will be quite good enough for my purpose. The A2 cost a little under $50 for a piece 18" long, so the 3.6" (rough-cut size) piece is worth ~$10. The labor is worth far more.

I also have a piece of O1, bought for experiments, at about one half the price of A2. I'm interested to see if quench warping of O1 will be a significant problem. I'd guess that it will not. But I will still need to deal with scale and decarb.

Joe Gwinn

Good old fashioned clay based mud works fairly well. Simply pack the part into a clay ball and heat. All you are trying to do is keep oxygen from creating mill scale.

In the old days..they often heated the parts in a clay container with a bit of cloth or paper that would burn as the temperature rose, using up the oxygen inside the container.

Gunner

"Liberalism is a philosophy of consolation for Western civilization as it commits suicide"

- James Burnham

Can you get away with a chunk of shitty old mild steel and a couple pressed in drill bushings? I don't really know what dill bushings are going for, but there has to be some value in the complete lack of post build risk, no?

Cheers Trevor Jones

This sounds workable. Thanks.

Wrap with paper, daub with clay mud, dry, heat. The paper will prevent the clay from sticking as well. A2 needs to be heated to an orange-white heat, so the clay may well become pottery, even though this is a bit cool for real pottery.

I've read that the traditional blacksmith's approach was to make oversize, then grind the surface layers off where it mattered.

Joe Gwinn

I'll just mention that the curve for A2 doesn't look too much like the curve in the cite. The first peak is much closer to the level of the second and the valley is much lower. If it was my part, I'd temper it once at 380-400 degrees for two hours, bring it down to room temperature, then temper it again. The reason is that there is some tendency for A2 to retain austenite from the hardening process. The first tempering will convert some of the austenite to martensite. The second temper will temper the untempered martensite from the first temper.

I think somebody already mentioned that it wants to be hardened hot (relative to the low alloys, anyway), to bring the chrome into solution. My reference shows a hardening range of 925-980 C. I'd try for the bottom of that range, around 1700 - 1725 F. YMMV.

Adam Smith Midland, Ontario, Canada

The problem is that the center-to-center distance is 0.4125", so with

5/16" (0.3125") holes, the metal between holes is only 0.100" thick, far too close for standard bushings to fit. Making a jig plate for every other hole, and moving the plate between sets of three holes is far too fiddly. It's better to make a plate with six holes in it.

Joe gwinn

I'll read this carefully.

I'm sure that this is exactly correct, but until I get a proper heat treating furnace, I won't be able to follow this advice very well, as I don't have anything like sufficient temperature control with my handy little torch. I just get it up to orange-white hot and hold it there for a while, then let it cool on a cold firebrick. I think I will temper it for a few hours at 500 degrees F. However, there is essentially no external stress on this piece, so I suppose the fracture would only come if it were dropped on a concrete floor.

I'm going to try O1 next. It's far easier to harden, as it only needs a nice red heat.

Joe Gwinn

I'm thinking a mixture of soap and powdered charcoal and perhaps clay.

500 would be a bad choice from my point of view: you are up into the temper brittleness range. Much better to go with 420 as a max, as I recommended. Your call though, as you say, brittleness may not be an issue anyway. A kitchen oven is fine for tempering.

Adam Smith, Midland, Ontario, Canada

Yup! I can see how that would work out not so well!

Cheers Trevor Jones

[snip]

Right. I was doing it from memory. It will be 400 degrees F.

Joe Gwinn