8620, 9310 vs A2
8620, 9310 vs A2
(OP)
Have issues with 8620 deep case carburized pins, spalling by bearing rollers and have tried 9310 with better results because of the higher core hardness but 9310 barely meets the hardness spec of 60/62 Rc (after finish grinding). Thinking about a tool steel with high toughness like A2 which would easily meet the hardness but am somewhat worried about the core hardness and loss of fatigue strength; any thoughts?
Glenn





RE: 8620, 9310 vs A2
thread330-245718: EN36A carbursing core hardness
thread31-189675: USA steel equivalent
thread330-263068: Material Substitution 9310 steel
Automotive Components
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Aerospace alloys
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RE: 8620, 9310 vs A2
RE: 8620, 9310 vs A2
I have used 4320 for another application, I didn't consider it for this pin but now I will. I did an analysis of a pin from Germany and they made it from 15crNi6 and it seems to perform well, I believe this is close to 4320H so this makes sense.
I will also look at 17NiCrMo6-4, since I live in the States I don't know how available it is here. Will this alloy go to 60-62 Rc?
Glenn
RE: 8620, 9310 vs A2
As israelkk noted, if you're experiencing case spalls then you likely have excessive contact stress. Before you go through the effort of changing materials, I would first recommend that you do a thorough contact analysis. If the analysis shows that your point of max subsurface shear stress does not lie well within your case depth, and/or that shear stress level is excessive for your material's fatigue allowables, then a material change won't help.
If your contact analysis shows that your materials and heat treatment are marginal, then you can get better fatigue results by using a vacuum melt quality material instead of an air melt stock.
Hope that helps.
Terry
RE: 8620, 9310 vs A2
RE: 8620, 9310 vs A2
Using an inner race as suggested by israelkk introduces other assembly problems as the pin is pressed into an assembly and the engagement is compromised to a certain extent. I will look into this, however.
The pin finish is also an area that is suspect. Too fine of a polish finish isn't necessarily good for a rolling element bearing - in my opinion. I would think a honed finish would be better (roller traction?) somewhere in the 2 to 5 RMS range. This is a high speed application.
Glenn
RE: 8620, 9310 vs A2
If you want a carburized case with a tough core, then 9310 VIM-VAR (AMS 6265) is your best bet. It can be carburized with sufficient case depth for most any application. Controlling quench distortion and subsequent stock removal during finish grinding are very important.
As for the effect of surface roughness on rolling element bearing performance, a smoother surface is not detrimental except with regards to cost. Your race surface needs to be smooth enough to allow your roller/race contacts to be in a hydrodynamic regime for as much of your operating conditions as possible. Achieving hydrodynamic contact depends greatly on your contact oil film lambda ratio (the ratio of surface asperity height to oil film thickness). A smoother surface with lower mean asperity heights will be able to maintain hydrodynamic contact conditions with thinner oil films.
Your last post mentions this is a high speed roller bearing. Designing high speed roller bearing systems is a very complex task. With high speed roller bearings (ie. dN>1,000,000)things like roller skidding, skewing and oil churning can cause excessive heat build up. This heat build up can easily result in local oil temps in excess of 450degF or more, and can lead to oil flash and scoring failures, or even de-temper of your race surfaces (which produces spalling). If you have the resources available, I would recommend a detailed analysis of your roller bearing system, including a heat transfer study. It may provide some very useful insights into your problems.
Good luck.
Terry
RE: 8620, 9310 vs A2
You may want to consider Ferrium C64, which is a new, high-performance gear steel alloy. I presented a technical paper about the alloy (and the similar C61 alloy) at AGMA 2009 Fall Technical Meeting.
http://w
It has a case hardness of ~62-64 after vacuum carburizing with ~48 Rc in the core at ~80 ksi sqrt(in) fracture toughness. It was custom-designed for helicopter transmissions.
Because of the higher surface hardness it might solve the contact fatigue issue you're experiencing. The raw material costs more than typical VIM/VAR 9310, but because it has much higher hardenability, it can be directly gas quenched from vacuum carburizing, resulting in process cost savings and reduced quench distortion.
Jim