Can you anneal and harden steel indefinitely?

crossthread · Sep 4, 2019

Last night I was in the final process of tempering a machete I am making for my grandson. I am using a light leaf spring for the blade. I was explaining to him the process of hardening and tempering and he asked me a question I am not sure how to answer. I did some internet searching and couldn't come up with anything definitive. The question was can you anneal steel, harden and temper it indefinitely or at some point does it become slightly "fixed" in one state and require higher heat or greater shock to work the magic? Thanks very much for any practical insight.

BSCustoms · Sep 4, 2019

In theory yes you can go back and forth forever. In reality your composition will be changing a little with each heat. Especially if you are using an open atmosphere furnace where you may be losing some carbon content due to decarburization.

Dumpster_diving · Sep 4, 2019

Cue the Viagra jokes...

crossthread · Sep 4, 2019

Thanks BScustoms. I never thought about losing carbon. Yes I am using an open furnace (forge). In my fifty years of doing this I have burned the carbon out of quite a few things by mistake. It has generally occurred with white hot forge weld temperatures however. There could very well be some carbon loss at normal hardening and tempering temperatures however. Thanks again.

HuFlungDung · Sep 4, 2019

Might also cause grain growth each time you anneal it, so I've read. But if you hammer forge it each time, it might break those down.

Limy Sami · Sep 4, 2019

Dumpster_diving said:
Cue the Viagra jokes...

Swallow with water or you'll get a stiff neck

LKeithR · Sep 4, 2019

Limy Sami said:
Swallow with water or you'll get a stiff neck

Just had to play along didn't you?

garyhlucas · Sep 4, 2019

So if any of you guys want make knives and stuff the wife of an old friend who passed away recently has a gas forge sitting in her shed. It was next to the foundry furnace she gave to my grandson and I’ve been looking for a new home for it to help her out. Right after her husband died she fell breaking both legs and her wrist and needed multiple operations and 3 months in rehab!

lr harner · Sep 4, 2019

even at 1500f you will be burning out a bit of carbon in your forge. if you have the gas running rich it helps big time. using a kiln and foil you can reset grains and even somewhat carbides. you are at least normalizing a few times before you do your heat to quench run right

Limy Sami · Sep 4, 2019

LKeithR said:
Just had to play along didn't you?

At my age it's about all I've got left

4GSR · Sep 4, 2019

In past lives, we had problems with investment castings not responding to induction heat treating. The surface to about .010" deep would not get hard. Had to do with with the diffusion of carbon atoms from the metal when cooling from casting temperature. We would have to send out the castings and have them carbon restored. Essentially put in a gas atmosphere, heated to around 1400-1500 deg. F and furnace cooled to near ambient, before removing from the oven. Then we would carry out the induction heat treated required. Later ditched the induction HT method and went to straight carbonizing HT process. Yes, you can loose carbon from reheating material.

sfriedberg · Sep 4, 2019

Several people have mentioned decarburization, which used to be an issue with tool steels straight from the mill back in the day. Sufficiently old books on toolmaking will recommend taking as much as 1/8" off the outside of a newly supplied bar, to remove the decarburized skin. Pack hardening is exactly the process of re-carburizing the skin of a bar or part.

HuFlungDung mentioned grain growth, which is another problem, along with the usual remedy. Most steel alloys do not have a heat treat schedule that will restore small grain size, and for those hot work (forging or rolling) is about all you can do to restore grain size. Some alloys have alloying elements that either strongly resist grain growth or will restore a small grain size through a normalizing (not always the same as annealing) treatment. It's been decades since I studied that stuff, so please don't ask me which alloys or alloying elements those are.

And of course, if you mistreat your steel by overheating (blacksmith "burning"), that portion is pretty much irreparably damaged from a metallurgical perspective.

adh2000 · Sep 4, 2019

As others have said, grain growth. At high temp the grains grow and the material becomes more brittle. For the same reason the high temp soak time should not be excessive. For heat treating one and done is best.

Sent from my iPhone using Tapatalk

crossthread · Sep 5, 2019

Thanks very much for all your insight. This is all pretty much theoretical. I don't intend to anneal, harden and temper more then once (hopefully). My grandson was intuitive enough to ask the question, so I felt a need to give him a good answer. I have had to re-harden casing a few times for some strange reason but I always got it to work on the second try.

CarbideBob · Sep 5, 2019

Is there a limit to grain growth? At some point does it quit?
One can pack or toss in the carb furnace to replace the lost carbon.
If you go back to melt I think a lot of the steel we now use is not virgin but the thought of draw back and retreat a 100 times adding the lost carbon is interesting.
I've never seen this done. I will guess somebody has tried it.
Bob

adh2000 · Sep 5, 2019

CarbideBob said:
Is there a limit to grain growth? At some point does it quit?

In theory no. Impurities in steel, like C, Mn, Si, P, etc all tend to precipitate out at the grain boundaries when the material is at high temperature. The accumulation of impurities at the grain boundaries prevents the grains from continuing to grow. So it depends on the quantity and type of impurities. Sometimes impurities can be added to intentionally discourage grain growth. Larger grains always equals more brittle which is generally not a desirable property. Also, as the impurities precipitate out at the grain boundaries the bulk property of the steel changes since the impurities are no longer uniformly distributed inside the grains. For these reasons it is not a good idea to keep the sample at high temperature longer than necessary. Best is to heat the sample until it changes to austenite all the way through (non magnetic) then quench. Don't keep doing it over and over. That's not good. The sample will become more brittle.

Ron Hofer · Sep 7, 2019

garyhlucas said:
So if any of you guys want make knives and stuff the wife of an old friend who passed away recently....

I've been teaching grammar for too long. Gary, I read your post incorrectly. It appeared to me you were suggesting that 'guys' (subject) was followed by a compound verb and direct objects: 1.'want to make knives' & 2. 'want to...stuff the wife....' My apologies to you and your late friend. Oh, the vagaries of language.
Hofer

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