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Stress Relief Bake prior to phosphate coating - what is it for?

December 17, 2009

We are phosphate (manganese or zinc phosphate) coating a variety of (non cres) steel parts to the MIL-DTL-16232 [from DLA] specification. We perform a Stress Relief bake prior, per the spec requirements, at 350-400 °F for 1 hour per inch of metal thickness.

My question is, if the stress relief bake were not being performed at the correct temperature, what would the result be? (Our oven uniformity is ±25 °F)

Many of these parts also receive a Hydrogen Embrittlement Relief Bake at 210-225 °F for 8 hours after phosphate.

Ilene Weisberg
Metal finisher - Ravenna, Ohio

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If you bake at 375 °F nominal, you will be ± 25 of the 350-400 requirement.
Uniformity of ± 25 is really very poor for an oven to bake parts for MIL spec requirements. Normally, something can be modified to tighten this up. I have spent a good bit of time changing air flow hole openings on some moderately large ovens to bring them into customer requirements. You might have to add a high temp fan to get a more uniform circulation of the heat.

James Watts
- Navarre, Florida
December 21, 2009