8620, 9310 vs A2 2

[Tampa]

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

 

[TVP]

8620 is not really for deep case hardening applications, so have you investigated alloys like 4320, 17NiCrMo6-4, etc.? There are some other aerospace-type gear alloys that Corus in the UK offers that also may be of interest, depending on the quantities needed, cost targets, etc. Here are some links to previous Eng-Tips threads and to Corus:


thread330-245718

thread31-189675

thread330-263068

Automotive Components

http://www.corusgroup.com/file_sour.../Engineering steels/Automotive Components.pdf

Aerospace alloys

http://www.corusgroup.com/en/compan...us_engineering_steels/publications/aerospace/

 

[israelkk]

If a roller or needle bearing is running on the pin than maybe your load is to high. If you can reduce the pin diameter and make an inner race made of heat treated AISI 52100 or purchase a standard inner race for your bearing.

 

[Tampa]

I tried 9310 AMS6260 not 6265, I see there is a considerable price difference, over $7 a pound. This is why I was thinking about A2 as it is about half as expensive and relatively cheap to heat-treat compared to the pricing for >40 case. It is not totally out of range however, just looking for alternatives.

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

 

[tbuelna]

Tampa,

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

 

[TVP]

17NiCrMo6-4 is not readily available in the US-- it is a standard European grade, and will only be produced in mill quantities by Gerdau Macsteel, etc. It will definitely case harden to 60-62 HRC at the surface, although a wider specification range is typically used, say 59-63 HRC.

 

[Tampa]

We have tried a larger bearing which provides a couple more contact rollers but it is not a significant improvement with regard to the load carried by each. Yes the subsurface shear stress is close to the maximum and taking into consideration the case may not be uniform after finishing (part distortion) the pin case is thin in some areas. This is why I am thinking about tool steel.

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

 

[tbuelna]

Tampa,

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

 

[JAWright]

Glenn,

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://www.questek.com/filebase/src/Material_Specific_Presentations/QuesTeksFerriumC61C64andC6.pdf

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