04-18-2003, 01:22 PM
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Senior Member
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Join Date: Sep 2002
Location: So. Calif.
Posts: 323
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Quote:
Originally posted by samiam4
Ref Juvinall and Marshek “fundamentals of machine component design”
Snip “shot peening is more effective with high strength steels… particularly beneficial in steels with tensile strengths above 200 ksi”
“Chrome and nickel plating, even though good for the surface in providing corrosion protection, can substantially reduce the endurance limit of the steel parts. This is well known as “hydrogen embitterment.” This damage can be minimized by taking special care in using low plating current densities and baking the parts at 600-900 F after plating.”
Note this minimization of effects does not gain any benefits over an endurance limit of a non-plated part. So, if they do it properly- your still loosing 50% of the endurance strength.
Endurance strength= the highest value the part can sustain for a reversing load 10+8 cycles, or infinite life.
Thermal and chemical surface hardening treatments:
“The purpose of thermal and chemical surface-hardening treatments is usually to provide surfaces with increased resistance to wear: however, they also serve to increase the fatigue strength, and for this reason are considered here.”
“The strictly thermal processes of flame and induction hardening of steel parts containing sufficient carbon produce surface residual compressive stresses as well as surface hardening. Nitriding is adding nitrogen to the surface layer, together with appropriate heat treatment.”
Endurance limit comparison (via ASME Handbook of metals)
Without notch 90 ksi nitrided vs 45 ksi Not Nitride
With notch 87 nitrided vs 25 ksi Not Nitrided
So, If you grind a metal shaft perfectly, it would could only take 29% of the alternating force as the nitrided part for an unlimited life. If your hard-chrome plating is properly baked, it would also be a 29%. If you shot-peen the area before plating this would be somewhat helpful, but it is a industry standard to keep the plating 1/8 inch away from the tangency of the fillet. And the stress peak is in the fillet(k=2+).
I don’t think it’s a good rule of thumb to decrease the fatigue strength by 2/3, period.
Michael
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Yer really good at the ol' CUT n' PASTE ! ! !
NASA would be IMPRESSED ! ! !
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