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17-4ph discolors upon heat treatment

We manufacture 17-4ph metal targets for our electromagnetic sensors. For the best magnetic properties we do not do a solution anneal but just an H1050 heat treat. The problem is the parts come back discolored. We tried passivation before heat treat and they were still discolored. Is there any process we could do before heat treat that would make it so they are not discolored after heat treat.

Kevin Woolsey
Electromagnetic Engineer - Lynnwood, Washington, USA
2004

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First of two simultaneous responses --

What you are probably looking at is oxidation from the high temperature. Methods of avoiding or minimizing ar using a vacuum furnace. Using the stainless steel baggies or wrap that are sold specifically for that purpose. Possibly using a reducing atmosphere furnace. I am not aware of the ramifications of this possibility.

James Watts
- Navarre, Florida
2004

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Second of two simultaneous responses --

The discoloration is a normal product of oxidation of the surface. Heat treatment in either inert atmosphere or vacuum will avoid the discoloration, as there will be no oxygen. Both these heat treatment facilities are well established commercial activities that have been around for many decades, so you should be able to find a suitable shop without too much difficulty.

Bill Reynolds [deceased]
consultant metallurgist - Ballarat, Victoria, Australia
We sadly relate the news that Bill passed away on Jan. 29, 2010.
2004

We H1050 in a vacuum and then cool with a nitrogen backfill. The heat treater swears there is no leaks. They stated that they put our parts next to other parts and ours come out discolored and the other parts do not. It seems like it is a surface contamination issue.

Kevin Woolsey
Aerospace - Lynnwood, Washington, USA
2004

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Perhaps a film of coolant or lubricant from a machining or other forming operation? If there's no oxygen in the furnace, the discoloration isn't from any aspect of the metal itself, especially when you're giving it only the low temperature ageing treatment, not the usual prior austenitise.

Bill Reynolds [deceased]
consultant metallurgist - Ballarat, Victoria, Australia
We sadly relate the news that Bill passed away on Jan. 29, 2010.
2004

You can get tinting from a dirty vacuum furnace. Ask your heat treater when the last time was that he did a burn out cycle. It may be alloy specific. Try adding machined bar stock of the same alloy and of a different alloy in the same run. It will tell you a lot as to where to look for the solution.

James Watts
- Navarre, Florida
2004

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Passivation may not be the thing to do before age heat treatment, especially for steels, this gets up to 1050 °F for a certain time. The material needs to be thoroughly cleaned (possibly pickled) prior to this thermal processing. Passivation, to my understanding is basically a protective sealed coating for stainless steels. Talk later. Interesting question.

Regards,

Brad Huddleston
- Haysville, Kansas, USA
2004