Stress Relief 1

[sbozy25]

I am looking for someone to settle a discussion between my self, our lead purchasing agent, and our head metalurgical engineer (engineer by trade, not education).

They are trying to switch a powdercoat process on us to go with a cheaper powder. The only issue is that this powder cures at 350°F. Now as it is we have springs made of chrome silicon that get powder coated after it is shot peened. They are run through an oven at 450°F which not only cures the current powder, but it also acts a stress relief for the part. They are in the oven a total of 5 minutes and then are allowed to air cool as the line moves around.

Our purchasing guy and metalurgy guy are arguing that the 100°F difenrence will not affect anything and also, they find it hard to believe that 5 minutes @ 450° really even makes a small bit of difference.

My arguement is that it is just enough to remove the risidual compressive stresses that could be detrimental to the parts life. Any thoughts on this?

Oh and FYI. the part is stress relieved after coiling almost imediately. So the only stresses I am talking about are from the peening process.

 

[TVP]

sbozy25,

Can you please confirm the details on the stress relief that takes place after coiling? A typical stress relief for Cr-Si steel is in the range of 315-425 C (600-800 F) for 5-20 minutes. This is the important step for relief of coiling stresses. The last low temperature treatment (after shot peening) is important for increasing the yield strength or elastic limit, and is most properly called a strain aging treatment. Strain aging will be at a maximum when aging occurs around 200-250 C. If you change to only 175 C, the yield strength will be lower, and the spring may be more susceptible to stress relaxation. I suggest you quantify the results with some test pieces before moving forward with this request.

 

[sbozy25]

The initial stress reliefe takes place at 750°F on a moving conveyer that takes 15 mins to complete the trip. So the parts are being properly relieved after coiling.

Personaly I want to veto this whole project because of the danger to the parts. These would constitute a 10 on a FMEA, so they are very critical. I think that changing their final relief temperature from 450 to 350, even if for only 5 minutes will cause detrimental failures. These parts spend 90% of their life in a compressed state, so they can not have any relaxation.

I just thougth I would see what others thought of this situation.

 

[TVP]

Without knowing the stress vs. deflection characteristics it is difficult to give anything more than subjective feedback, but based on your information, I would not change without a thorough evaluation of the parts and the final assembly. And if you don't control the final assembly, then good luck convincing your customer about the requested change.