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Heat treatment discolors medical 304SS off and on

We use 304SS tubing in the manufacture of a medical device. The tubing has some EDM work done to it and then a tungsten electrode is welded to the tip of it. It then goes through a ultrasonic wash. Later in the manufacture process the shafts are heated to 650F. Occasionally the shafts turn a gold/brown color and can not used or reworked. Does anyone know why the tubing would discolor and what could be done to prevent this?

Paul Prokop
Medical equipment manufacturing - Andover, Massachusetts
2003

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First of two simultaneous responses --

The stainless is oxidizing. To prevent, it will probably be necessary to use a vacuum furnace or an inert atmosphere or a reducing atmosphere. Invisible "dirt" will agravate the discoloration.

James Watts
- Navarre, Florida
2003

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Second of two simultaneous responses --

How is the heat treatment done? Could you have an air leak in the chamber, or is it contaminated with something?

Trevor Crichton
R&D practical scientist
Chesham, Bucks, UK
2003

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Paul:

Either use an oven with an inert gas instead of oxygen, or wrap the parts in aluminum foil to prevent the formation of the objectionable oxide film.

Dan Weaver
- Toccoa, Georgia
2003