Carburizing Steel Suggestions (Impact)

[mewhg]

I have a pawl looking type of part that is made from either 8620 or 9310 steel. It is carburized to an RC50 of .015" and then cryo treated and tempered back to a hardness of about 90 R15N. After heat treat certain critical sections are shotpeened .

The part is failing in fatigue. After about 30k impact type loadings cracks develop. I need to get the part to last to 100k cycles.

The part geometry cannot be changed but the steel can. Is there a better steel that we can use? Perhaps a particle metallurgy type steel? I have a quality heat treater who vacuum carburizes who will be doing the heat treatment.

Thanks in advance.

 

[4sfed]

Have you done a magnetic particle inspection on the parts before putting them into service? Your heat treater may not be as good as you think.

 

[mewhg]

Yes,

We do spot MP before heat treatment and 100% after heat treatment.

 

[redpicker]

I think you are putting way too much processing in to this part. How big is the part? This should dictate the material used. 8620 is good for thickness up to around 1-1.5 inches. 9310 for 3 inches or more.

Why are you cryo-treating? OK, with the 9310, it is likely necessary to reach 60 HRC on the surface, but with the 8620, the need for cryo-treatment can be eliminated by proper carbon potential control, particularly with a 0.015" case depth. With 8620, cryo-treatment just compensating for poor control of the carbon potential.

Speaking of carbon potential, what potential is the heat treater performing the carburizing with? You should be specifying this. For 9310, you should be no greater than 0.70 %C. With 8620, maybe 0.90 % C. These are maximum and normal operation should be around 0.05%C below these.

If you are going to cryo-treat, temper the parts before the cryo-treatment. You may be cracking the parts at this step (another reason to eliminate it).

Also, 90 R15N (60 HRC) is somewhat high. I'd drop that to 88 R15N (55 HRC) if possible. Do you absolutely need this high of a hardness? Years ago, I was involved with a manufacturer producing parts specified as carburied to 60 HRC on the surface. About 50% of these parts failed just in subsequent processing. Just by dropping the hardness to 50 HRC, the failure during processing was eliminated. Service performance of the parts was exactly the same (it was a wear-protection item and even at the lower hardness, the wear-protection out-lived the tool it was used on).

rp