We want to replace a 4130 steel forging with a PH17-4 stainless machining but need to prove equivalency for fatigue. The PH steel is 150% stronger than the 4130 in static tensile (Ftu) values. I have been searching all over the net to find fatigue values (curves) for forgings, anyone point me in the right direction please.
Maybe best to find a metallurgy forum of some sort for this one.
Still, one thing to consider is surface treatment of the machined part. Surface finish can contribute to a part's toughness. You may want to consider shot-peening your machined part.
All this, of course, without having seen your application! The usual caveats and disclaimers apply.
Mil-Hdbk-5?
Can't offer anything, but maybe it would be best if you consult whoever you are using as structures / fatigue / damage tolerance DER (or equiv) on this?
... Google search [fatigue machining forging] turns up some interesting reading, but I'd still get with whoever it is you'll have to convince. They may have an "off the top of the head" idea of what they will be comfortable with.
Hi Jeff, we are the DER on this (DAO actually). Mil-Hdbk does not differentiate between forged or drawn plate in its values. Our certification authority has the notion that forgings are better in fatigue, I also believe this and all forging companies espouse this notion, but no-one can prove it to us. The part we wish to replace was originally made 40 years ago and forgings were used also to save expensive maching operations rather than for fatigue issues. This is not the case today especially when replacing one off parts. What we really need is to find a controlled fatigue test between a forged and a machined part and be able to apply the delta (if there is one) to our machined part. We might not solve this one, but it would be interesting to find substantiating data to back up the forged part assumptions rather than just being told that 'grain flow of the part substantially increases crack and fatigue life'.
Sorry Jeff, above post is mine, used another puter to send it,
Phil
Thanks, we shotpeen all machinings automatically. But that raises another question, we all know shotpeening improves the surface toughness against crack propagation by work hardening but prove it!
Understood. Have you searched NASA / Lerc / etc.?
Phil,
There are a lot of variables here.
You may find some fatigue data in MilHDBK 5. Ftu doesn't alway translate into better fatigue properties. The forging has been compressed by the forging process reducing the possibility of a small crack starting. Depending on the process used to make the 17-4, the environment and temperature you may get better or worse fatigue life.
You may very well have to do some testing. I would also get the services of a metalurgist familiar with both materials.
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