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Stainless steel turns black after post weld heat treat

We have a 304/304L stainless steel that is post weld heat treated and when it is returned to us it is completely black. this black can be wire brushed off. how do I correct this problem.

Dave Amaro
Quality inspector - Stratford, Connecticut, U.S.A.
April 12, 2010

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Dave,

Can you give some details of the heat treatment that is called up? Currently we can only make wild guesses about what the black is.

Brian Terry
Aerospace - Yeovil, Somerset, UK
April 23, 2010

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First of two simultaneous responses --

If it is a purple-black, I will guess that the oven is filthy, or is being run with dirty parts.

James Watts
- Navarre, Florida
April 23, 2010

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Second of two simultaneous responses --

The furnace is vacuumed down, then the material is heated to 1950 °F and held for 20 mins. Then nitrogen is introduced to cool to below 800 Deg. in 20 mins time. After 800 Deg. is reached it is left in the furnace to cool till below 300 Deg. Then parts are removed.

Dave Amaro
- Stratford, Connecticut, USA
April 26, 2010

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April 27, 2010

You need to do a burn out cycle and then a leak down check.
Make sure that the parts are clean.
You might also want to do an analysis on the purity of your nitrogen. Although it seems impossible to me, nitrogen system CAN venturi outside air into the line.

James Watts
- Navarre, Florida