A286 stake studs problems

[GunnyD]

I am with a cold heading company, we have been experiancing an issue where our customer cannot stake an M10 stake stud into the their exhaust flange (won't fully seat). We have been providing this stud for a number of years and just since 11/05 have had this issue. We have investigated and cannot find any process that have changed either in our process or the customers. We did have a the steel analized metalurgically and discovered a condition that we were told was called Eta phase. Our knurls are "rubbing" away during the stake operation. We have found the surface hardness to be slightly softer than the core after the studs have been precipitation hardened per ASTM A453M. We have tried different steel suppliers and different heat treat facilities, knurl diameters, but have hit a dead end every time. Any help in pointing me in a direction to finding a root cause to this would be greatly appreciated.

 

[CoryPad]

When you say the studs won't fully seat, do you mean at any force, or just the force that has been used in the past?

This sounds like a geometry/force problem, not a materials one. Even if both the stud and flange are harder/stronger, a larger force will seat them (unless debris has become trapped between the flange and stud).

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[GunnyD]

They wont seat at the force that has been used in the past in their automated assembly process (1100psi). My customer can take the parts off their assembly line and use a larger press with much more pressure and the parts will seat fully. We have pressed out some of the assemblies that have not completely seated and noticed the knurls have been 90% or so "rubbed" away. They should be maintaining more integrity than that are after being staked (i would think). Due to the need to maintian production rates my customer has aquired some studs from one of our competitors and has no problems getting them to seat fully in the assembly (in the automated line at the 1100psi). We have done a dimensional layout on both ours and our competitors studs and have found no difference dimensionally between them. Because of this we don't think it is a geometry issue. But we are not ruling it out. I am currently waiting for another metalurgical lab analysis of these studs with etched photo micrographs to see if this "Eta" phase is present in these studs. Has anyone had any prior issues with this so-called "Eta" phase? If so what does it do to the performance of this material?

 

[CoryPad]

I didn't fully understand your "rubbing away" comment, although I thought that was a possibility so I mentioned debris. You are correct, the splines (not knurls) should not disintegrate during seating.

You should have microhardness values taken at the splines. Also, do some lab installations of the splined studs into the flanges. Look to see if the splines are sheared due to low fracture toughness, or grossly deformed due to low strength.

Eta is Ni₃Ti, one of the possible phases in this system. I don't have a lot of experience with that, so I can't comment much.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[GunnyD]

Cory, thanks for the info. We are having the micro hardness checked at the splines as well as having some studs sectioned and grain structure looked at. I will pass on to the lab that is doing this your recomendations. I will also attempt to secure some flanges and ask for the lab to perform the installations you recommended. Again thanks,
If anyone else has any more info on the Ni3Ti, it would be helpfull i think to better understand this possible phase.

Thanks,

GunnyD