Need some help re-hardening/tempering vintage steel

[David Weaver]

You can fully anneal it to get it relaxed again, but the schedule for it would probably have it at a temperature that it could decarburize. You'd need steel foil to do it, and all of the W1 schedules (and probably O1, too) want the steel soaked at 1400 or so degrees for a half hour, and then very slowly reduced.

Temper temperature is going to depend on how much carbon is in the steel. I incorrectly guessed that the temper temperature would be as low as O1 - if it's 1%, it can get to 500 degrees and still be almost 60 hardness.

Proceeding at small intervals for an hour per is a better idea, though, in case it has more carbon than that.

[Ron Hock]

Ron, thanks very much for answering. I think there is a distinct possibility of this. It was the first time using my forge, and I was having a tough time judging color accurately. I may have erred on the side of too high.
If I did get it too hot, will annealing and re-hardening solve the problem, or is the grain growth a one-way street?

You can try to anneal: heat to "just starting to glow" then pack it in ashes or sand and let it cool Very Slowly. Then reheat to 1450F (non-magnetic), quench and temper to 325F. I'd say it can't hurt to try but without atmosphere control, it actually can. As the steel starts to glow, it will start losing carbon to the surrounding air -- oxygen's tempting nature being too much for weak-willed carbon to resist -- forming a de-carburized skin on the steel. The core may harden but the soft skin will have to be ground off before you'll be able to use the blade. I'd say give it a try. It's not working now so you have little to lose and a lot to gain in the education category. If it was easy, everyone would do it. Good luck!

[Steve Voigt]

I'd say give it a try. It's not working now so you have little to lose and a lot to gain in the education category. If it was easy, everyone would do it. Good luck!

Thanks for the advice, Ron. I've tried everything, and nothing's working, so it might be time to accept defeat and move on.
I forgot to mention, I hardened an old dead-soft mortise chisel in the fire at the same time, and that turned out pretty well. So either I got the plane iron too hot, or maybe it was defective from the start.
Regardless, I learned a lot from the experience and from this thread, so I'm not disappointed. A few modifications and I'll bet the next time goes better. Thanks for the help everyone!

[Warren Mickley]

Steve, here is what you need to do to reduce crystal size, if that becomes a problem and the steel is too soft again. First heat the steel above the critical temperature. At the critical temperature the steel completely changes crystal structure from body centered cubic to face centered cubic. All memory of previous crystal size is wiped away by this structure change. If you then cool it very slowly it forms large orderly crystals as it changes back to body centered cubic structure; this is annealing. If you cool it very quickly (quench it) small crystals form in a jumble because they don't have a chance to form an orderly structure. This is hardening. Subsequent heating of the steel to temperatures below the critical temperature will ease stresses and soften the steel; this is tempering.

You can go through this cycle several times, but heating the steel to high heat burns off small amounts of carbon eventually degrading the steel. I have occasionally treated the same tool three or four times before being satisfied. That can happen when one is not treating regularly or with a known steel. There is no need for you to anneal unless you want to reshape the tool with hacksaw or file or something.

[george wilson]

Heating to just above glowing is not hot enough to cause decarbing. In fact,the only steel I wrap in stainless steel is A2 air hardening. I have had no problem with W1 or 01 decarbing enough to be noticable.

[Steve Voigt]

… I have occasionally treated the same tool three or four times before being satisfied. That can happen when one is not treating regularly or with a known steel.

Warren, that's good to know, and it fits with what Ron said, that if it were easy everyone would do it. My first dovetails weren't too good, so I wouldn't expect this to be any different.
Actually, this is not my first time--when I was a machinist, I did heat treating from time to time. But as I discovered, using a DIY forge is a whole different ballgame from using a commercial heat-treating oven.
Anyway, thanks for the tips, I will keep at it…

[george wilson]

Steve,I assure you that if you do not anneal after the first couple of tries at hardening,the steel will start doing unpredictable things. I experimented with this myself,using a high class Versitron hardness tester to test the hardness each time. You might get the steel hard after each quenching,but exactly how hard is very iffy. Then,the tempering becomes very iffy too. Certainly the steel does not have its best performance after being tortured thus.
If I recall,the first hardening yielded 67 Rc. The tempering yielded an expected 58 RC. The 3rd yielded a hardness of 60 RC. The 4th yielded 50 RC. This was not due to decarbing: I test filed the surface of the sample after each quench.The sample was the same hardness all the way through. Of course,after the hardening,the tempering was entirely dependent upon the initial hardness achieved. I have tested hundreds of blades of various types,and of different types of metals in my very long career.

Repeatedly re hardening without annealing is risky business.

The Versitron is one of the finest hardness testers available.

The steel I tried my little experiment with was W1,which is the type of steel in your antique plane iron.

[Steve Voigt]

George, I appreciate your advice. I think I'm done with this blade, but I'm planning to anneal some old, shot files soon.
I'm wondering if you saw the question I asked you in post #9? Here it is again:

Follow-up question: I am never exactly sure where/how to do the "file test" for hardness. On the edge (I assume not), the side, or flat across the back? I tried flat across the back (but not right at the edge) with a new-ish file. It scratches a little but just barely. So I'm thinking it is plenty hard.

I *think* I answered my own question--filing flat across the back on several different irons gave me interesting feedback about their relative hardness--but I'd still appreciate your thoughts.

[george wilson]

Instead of scratching up a larger surface,I'd just file as if you were trying to sharpen the cutting edge. Or,you could file the side of the blade,and still save making scratches on larger surfaces.