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topic 59742

Can Stress relief be done before nital etch rather than after?



November 28, 2014

Q. ARE THERE ANY RESTRICTIONS TO STRESS RELIEVING PARTS BEFORE NITAL ETCH/TEMPER ETCH?
BAC 5436 SPECIFIES THAT STRESS NEEDS TO BE DONE POST NITAL, BUT LGPS 3000 DOES NOT ADDRESS STRESS RELIEF. ALSO, NO BAKING IS REQUIRED AFTER NITAL ETCH WHEN USING JAR3N METHOD IN LGPS 3000.
THIS IS AN INTERESTING DILEMA SINCE ONE DOCUMENT FOR LGPS STATES THAT THE STRESS RELIEF AND THE BAKE REQUIREMENT CAN BE COMBINED AS LONG AS IT IS DONE WITHIN 8 HOURS OF NITAL ETCH, BUT IF ONE COMPANY IS USING JAR3N, BAKING IS NOT APPLICABLE TO THEM. THEN DOES THIS MEAN THAT STRESS RELIEF IS OK TO DO BEFORE NITAL?

MARIA JOHNSON
plating - TORONTO CANADA


January 9, 2015

A. Hi Maria,

The operation being carried out post nital etch is a hydrogen embrittlement relief operation, not a stress relief. High strength steels are susceptible to hydrogen embrittlement when exposed to acidic solutions. Assuming that you are using the standard 2% hydrochloric + 3% nitric acid you are exposing the steel to acid solutions so any baking prior to nital etch will not stop the uptake of hydrogen.

JAR3N is a special case and has been formulated so that it does not cause hydrogen embrittlement, so there is no need to bake after nital etch using this solution.

Now, hydrogen embrittlement relief only applies to high strength steels (normally considered to be steels with a UTS above 160ksi (1100MPa) or a hardness above 36HRC). Each company tends to have their own requirements for the actual strength or hardness that hydrogen embrittlement relief applies, so better check your customer requirements.

Brian Terry
Aerospace - Yeovil, Somerset, UK

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