17-7PH CH900 delamination problem

[pipsqueek]

Our manufacturer has just had some .080 thick bent up channel sections heat treated to CH900 (239000 psi)
They all have a delamination crack all round half way through the thickness.
This temper is not shown in MILHBK but is shown as acceptable on Matweb.
Both material and heat treat have the relevant certs, can anyone throw any light on this problem??

 

[jetmaker]

pipsqueek,

For some reason, this problem makes me think of a stress corrosion issue, where cracks form perpendicular to the short transverse direction.

So, it could be that during forming, the material developed residual stresses (too tight a bend radius, too rapid a bending operations, etc...). If a stress relief bake was not done, these stresses may have lead to a premature cracking during the final heat treat process. Have you tried a stress relief bake prior to final heat treat?

jetmaker

 

[monkeydog]

Did you bend the material before or after heat treating it? What did you end up with for elongation? Have you performed a section to look for inclusions or impurities?
Also, just because someone has given you a certification, it does not mean they did a good job verifing elements. Check the certification yourself to verify the material elements are correct.

 

[tbuelna]

pipsqueek:

17-7 cond H900 does not show up in MIL-HDBK-5 because that material condition is not acceptable for use in aerospace structural applications. The only condition shown for 17-7 is cond H1050.

17-7 cres in an H900 cond would be very susceptible to stress corrosion and that is why it is not used for structures.

Regards,
Terry

 

[pipsqueek]

Thank you all, this is not a flight component but a piece of ground handling equipment, that is why I chose to use a non MILHBK temper, obviously I made a mistake even though Matweb quotes it, many thanks

 

[HSThompson]

Pip,

I have seen this situation before and this may come as a shock.

I was involved in a case where we purchased cress material from a supplier who used a somewhat unethical yet perfectly legal manufacturing technique. Specifically, during the rolling or mill process of the sections and sheets, the mill was taking two thin sheets, while still near molten, and running them through the rollers thereby forging them together. The intent was that two thins make a thick. Well, that ain't the case. In our situation, during final assembly, parts were delaminating right down the middle. Talk about a mess. People were fired and companies were sued. In a nutshell, the material is not homogeneous. Without getting into allot of detail, what you end up with in making raw material this way is a product that has a funky residual stress layer running right down the middle of the part. Not good.

My advice, check out your supplier, if you don't like what you see, get a new.