OT Forgings vs machinings in fatigue

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The part we wish to replace was originally made 40 years ago and forgings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'.

 

We might not solve this one, but it would be interesting to find

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.

 

Why don't you comission some physical testing of both designs? Fatigue is
notoriously difficult to predict using CAE - get an example of each design
on a physical fatigue test rig and see what happens to each with the
appropriate test cycles? You must have the characteristics of the
environment available, otherwise you cannot prove *any* equivalency of
anything!

Just my thoughts!

Regards,

Horse