Vacuum level for annealing 8620

[dgallup]

We have a magnetic component that is blanked from AISI 8620 which we vacuum anneal for magnetic properties prior to gaseous nitrocarburizing to give it a hard case. This whole process was developed initially for similar parts made from 2.5% silicon iron which has worked well for 30+ years. However, I designed a new application that is more sever and the silicon iron proved to be too brittle, pure electrical iron was no better. So I changed the steel to 8620 for the new application and it has passed all our testing and been in volume production for a couple of years with no failures. I made no changes to the anneal and heat treatment specifications which we do in-house. I should note that staff metallurgists who wrote the specs have long since retired and the company has not seen fit to hire any replacements.

Recently we have increased production and the plant manager has asked purchasing and manufacturing to get outside heat treatment sources approved. The outside anneal supplier did not follow our spec which calls for 1 micron of mercury vacuum during the heating and soaking periods of the cycle. Instead, they used 100 microns of mercury vacuum. I can see no change in properties or performance from this but would appreciate someone with a bit of formal training in this area to opine before I approve it.

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[EdStainless]

Why in vacuum at all?
Since the magnetic properties aren't critical (if 8620 is just as good as the other materials) have you thought about testing parts with no anneal? Or maybe a simple neutral atmosphere anneal?
The only issue that I can see is the surface before your N-C treatment. If they respond fine then you are OK.
Though I am a little suspicious, 100um is a crappy vacuum, I would have expected 10um maybe.

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P.E. Metallurgy, Plymouth Tube

 

[MagBen]

In terms of magnetic properties, I expect very little/practically none difference using 1 vs 100 micron vacuum (others equal) for this alloy.

 

[dgallup]

Thanks for the replies. I did not see any performance effects from this vacuum discrepancy. Now I have to decide either to change the spec (and justify that to our end customer who is a royal PITA) or reject the parts and find a different supplier.

I would not say 8620 is as good magnetically as the electrical steel or silicon iron but it's good enough (for the new application) and the main thing is it is much tougher. We could break the silicon iron parts in about 100 million cycles, the 8620 goes over one billion, never seen one crack.

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