Home Forums General Discussion Ask Dr. Peener R&O and overpeening

We do repairing and overhauling of load compressors at our facility. The impellers of these machines (which are made of Ti-6Al-4V) sometimes require repairs or rework. Afterwards they are shot peened. I have two questions regarding this:
1) I know there has been research regarding coverage and intensity vs. fatigue life of Ti-6Al-4V parts. But this seems to be applicable only to new parts. What criteria should we use for R&O parts? How many times can one repair a part, shot peen it, place it back into the field, get it back after a number of operating hours and repeat this procedure? Or to put it more concretely: how many times can we shot peen a part before the shot peening itself becomes detrimental to the part, taking into account that the part experience operating conditions between each session? Is there any literature available on this topic?

2) As said these parts have a number of operating hours between each R&O shop visit. During operating conditions these parts experience centrifugal and thermal loads. Is there any literature available regarding the influence of operating loads on the shot peen layer? Will the compressive stress slowly decrease? If so, what is the effect of this on the number of times such part can be shot peened?

1. Impellers are rotating parts. They are lifetime limited. Normally, shot peening during the repair is limited up to 4x. I heard from cases that more than 5 x shot peening over the lifetime destroyed the part during service. The coverage should not exceed no more than 1000% in total except special areas like holes which are peened under impingement angle of 45°.
2.The operating temperature up to 500°C.
During service the compressive strees will be reduced, but not to zero. The rest of it and the other effect of shot peening will be good enough.

I don't think there is one answer that fits all parts and all alloys and all service conditions. Holger's answer sounds like a good practical limit, though. I have seen examples of a high strength alloy that has coverage limits when peened at high intensity and an other alloy peened to 5000% coverage without life damage.

Holger:
4x indeed sounds like a good practical limit. Do you know any literature which we can refer to for this? An specification that limits shot peening to 4 times without any basis is a bit tricky. Rejection of part solely based on the fact that it already has been peened 4 times is probably not acceptable, unless there is research that backs up such limit.

Regarding the operating temperature, we know this will affect the compressive stress somewhat, but we like to quantify this. Any literature available? Anything known about the impact of this stress reduction on the number of the times a part can be peened?

This is a interesting question, and the answer can not be clear. The titanium alloys are sensitive to surface integrity and the key or important factor is the surface roughness and thichness of parts. Ti-6Al-4V can be employed to manufactue compressor blades of engine and link joints in aircraft. For compressor blades, the thichness and surface roughness are factors which should be reviewed when choosing coverage. And in Specification of shot peening for aeronautical parts which I proposed, the coverage of parts of Ti-6Al-4V should not exceed 400% when shot peening new parts, and the times of repairing and overhauling is less than 4 and the coverage of each repairing and overhauling is not large than 200%.
I hope it can be useful because our investigation of shot peening of Ti-6Al-4V illustrates sensitiveness of surface integrity to fatigue and the strehgth of Ti-6Al-4V is not high compared to high strength steels, so my advice is a less coverage will be benifical to fatigue.