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"Tempering after grinding for stress relief?"
In reworking landing gears for a client (steel), we grind off the existing chrome plate. As a Next-Step, the Standard Overhaul Practices calls for a heat treating at temperatures near that of tempering. This is in addition to a low temperature heat-treat employed to eliminate embrittlement - where hydrogen has been picked-up.
Why would the Standard Operating practices call for this Surface Tempering? ASM International and the Machining Excellence Guide from the Institute of Advanced Manufacturing Sciences state that "Post heat treatments following material removal are of limited usefulness". In total, stress relief treatments, used to soften hardened layers produced during grinding of steels, do not restore the hardness of over-tempered layers which are present immediately below the damaged surface layer.
If not for stress relief, what would be the reasoning behind this tempering? Any assistance would be appreciated.Dave Reim
- Worthington, Ohio
Grinding damage can result in either softening from lower temperature overheating or re-hardening from higher temperature overheating. I can only surmise that the tempering treatment is to protect against possible brittleness from re-hardening.
One reason is liability. No one wants to be liable for a landing gear faiure, so will go to great extremes to do anything that might prevent a failure.
More than one aerospace specification calls out for a bake cycle after grinding on very hard high strength materials.
Mr. Hanke is infinitely more knowledgeable on the subject, but I believe the bake is to reduce the stress in the outer layer of the metal. If you grind a few thousandths off a 1/8 bar, it will normally have a slight bow afterwards. If you grit blast a thin metal on one side, it will always have a deflection in it the opposite direction of the blasted side. Has to do with the stresses.
Bake the part as advertised. I do not care to ride on any aircraft who's gear was not done according to established procedures.James Watts
- Navarre, Florida