Unexplained failure of 17-7PH sheet part 1

[mighoser]

Here's the story. This parts are made using 17-7ph sheet stock (AMS5528 or MIL-S-25043) which is cut into strips and then heat treated to TH1050 IAW Mil-H-6875. The parts are then formed using fine blanking process. The failure occured while theres parts are in opperation. They link together with one another and form a chain. We know that maximum normal opperating loads are 300 lbs and testing has shown these links failure at a minimum of 750lbs. This loading can't happen in reality because there is a weaker part which would fail first. These "links" were brand new so I can't imagine fatigue is the issue. Hardness was slightly high on the links @ Rockwell C 47. AMS 2759/3D says piece parts should be 38-44 HRC which covers heat treat of parts. AMS 5528 covers the raw material and its "responce to heat treatment" They indicate a range of 38-46 HRC. The failure surface is almost parallel with the sheet plane without necking. This is different from links that we tested. Any thoughts from the experts????

 

[mighoser]

Michael,

Would this be possible at a sheet thickness of .032"?

Thanks,

Mark Vose

 

[mcguire]

Sure, rolling only thins layers proportionately. This happens often with martensitic stainless, such as 420,for thin blades, like scalpels. 17-7 is quite similar. The layer may seem thin on a macro scale, but to get rid of the centerline layer by evening out the composition through substitutional diffusion is a tall task. It helps to have a continuous caster with electro-magnetic stirring or to cast with low superheat. You should make the absence of continuous centerline ferrite part of your spec.
Ferrite is the Achilles heel of PH grades. 15-5, 13-8, and Custom 450 are better in this regard.

Michael McGuire

http://stainlesssteelforengineers.blogspot.com/

 

[mighoser]

Michael,

Thank you for the information. I think we're on to something. I'll post something when I get lab results. We get to see if our suspitions are correct!!!

Thanks again,

Mark Vose

 

[NickE]

Mighoser -- See this thread also:

thread330-96298

Nick
I love materials science!

 

[mighoser]

NickE,

Did you find the source who provided the bad material? I wonder if my 17-7 was produced by the same company?

Mark

 

[NickE]

Nope, didn't even look. One of our prototype vendors caught the problem. We sent the parts (except inspection samples)back and the vendor re-made them using a different heat of material. (at least)....

Since we were not buying this directly from a mill or even a secondary house there was no need to pull out the implements on the source...

I'd bet that 17-7 stays on the market a decent amount of time, could be very likely that the heat that made my .050" could have made your .032"

Nick
I love materials science!