Home Forums General Discussion Specifications Specification Conflict

originally posted Fall 2002.

I recently processed a job through our shot peen department that had a conflict in specifications. If someonw could shed some light on this, I would greatly appreciate it.
The parts were Pivot Pins for a medium to small aircraft. They were made of 4130 steel, heat treated to 200-220 KSI. The processing included shot peen, followed by Electroless Nickel Plating.
The plating spec called baking at 350°F for 4 hours for Hydrogen Embrittlement relief, followed by 725-750°F for one hour to harden the nickel.
The peening spec prohibits baking above 475°F, while the plating spec requires shot peen and post-plate heat treatments.
My first thought was that the shot peen should not be rquired since the heat treat to cure the nickel would erase the effect of peening. Because the plating spec clearly required both peening and baking, the parts were processed through all steps.
My question is, "Why does the plating spec require peening if the post-plate cure exceeds the shot peen specification's temperature limitation?"
The parts were plated in accordance with MIL-C-26074E, and peened in accordance with AMS-S-13165.
Thanks for your input.
jlewis@swunited.com

[This message has been edited by webmaster (edited April 21, 2003).]

Most likely the peening is done to assist in plating in adhesion, not to provide a compressive stress layer (i.e., ruff up the surface without being detrimental to it).
shotpeen@hydro-honing.com

originally posted Fall 2002

First, I want to say that hopefully, the designing activity was aware of the temperatures scenarios, which you described very well.
I believe I have an answer why the designing activity, knowing the temperature conflict between the peening and plating spec, still elected to shot peen.
The answer is derived from a stress relief curve, found in Metals Handbook, Volume 4, ninth edition, Pg. 4, which shows that heating at 750°F for one hour relieves only 50% of the residual stress. Thus, 50% compressive residual stress is better than none!
In response to Walter, shot peening before nickel plate, although it does benefit adhesion, is performed primarily for the beneficial residual stress, to mitigate the loss in fatigue strength due to plating.