Annealing fubar

[dgallup]

We had a load of magnetic parts in our vacuum furnace run part way through the "burn out" cycle before the operator realized his mistake and shut it down. Then he ran the proper anneal cycle. These parts are either 8% or 12% chrome core alloys from Carpenter. Our normal anneal is 1450 F for 2-4 hours, they got up about 2000 F in the burnout cycle. The parts look OK and are meeting the hardness spec. I'm getting some critical dimensions measured.

Carpenter has this to say about heat treatment:
Due to its relatively low chromium content, Chrome Core (8)(12)-FM alloy will
form austenite if heated to too high a temperature, and some hardening
will occur if the austentized part is rapidly cooled. Consequently the best
heat treatment for improved soft magnetic properties is to subcritically
anneal.

So did the overheating do anything permanent that the subsequent anneal can't reverse?

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[EdStainless]

No, maybe some grain growth, but that shouldn't hurt.
They were cooled between the two treatments? They should have been.
If they weren't then re-run your standard sub-critial anneal.

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P.E. Metallurgy, Plymouth Tube

 

[dgallup]

Yes, they were cooled down and then the proper cycle run. Dimensions look OK and hardness is OK. Looks like we won't have to scrap them.

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[MagBen]

Did you notice any hardness increase? if not, you are probably OK. 2000F has the biggest chance to form Gamma phase, if you slow cooled, and held at 1450F for couple of hours, most probably the gamma phase disappeared. My only concern was that there might be some spots of pearlite structure formed for which you can easily verify with a microstructure analysis.

 

[dgallup]

They did not check hardness between the cool down and the re-anneal.

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[MagBen]

dgallup, I meant to compare with regularly heat treated part, i.e. historical hardness data if available.
Could you test magnetic performance on parts/in the assemble? Let us know in case you messed up, I may have a solution for you.

 

[dgallup]

Hardness was fine after proper anneal cycle compared to regular parts. We are keeping this lot separate until they get build so we can see if there are any problems in assembly & test.

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