Retained austenite in carburized gear

[houseman]

There are some literature about retained austenite role on pitting resistance. As I know it works increasing superficial compresive stresses due to its transformation into martensite induced by contact stress.
ISO 6336-5 allows up to 25%, but such levels lower hardness.
Anybody has a recomended level (standard?) for this issue?.
In the other hand what could be its impact on surface distress failure mode?
What kind of heat treatment proccess parameters could I play to control retained austenite amount?

Regards,

 

[dbooker630]

For the gear applications at my plant 30% is max but I try to keep it around 15-20%. There have been debates for quite a while on what is the optimum RA level especially in gearing.
You could verify what temperature you are quenching from and what are your carbon potentials for starters. I have seen hardness problems when the structure got too saturated that only a deep freeze (and temper) was able to rectify but I would focus on the furnace first.

 

[TVP]

houseman,

This is a fairly complicated issue, as there are many interrelated variables (retained austenite, non-metallic inclusions, grain boundary oxidation, prior austenite grain size, grain boundary segregation, non-martensitic transformation products, residual stresses, hardness, lubrication, surface roughness, resistance to thermal softening). I think the industry consensus is 10-30% retained austenite in carburized gears is the optimal range when considering tooth root bending and tooth flank rolling contact fatigue. One reference for this is "Temperature – the key to optimize cost and result
in carburizing vehicle driveline parts" by Stefan Hock, et al, Steels in Cars and Trucks 2005 Conference, pp 245-255, Steel Institute VDEh. Dr. Hock is the head of the materials group at ZF, and one of the foremost experts on gear materials. Another technical paper that discuss the benefits of retained austenite on pitting resistance is SAE Technical Paper 1999-01-2792 Development of Long-Life Needle Roller and Cage Assembly with Special Specification and Shaft with Unique Heat Treatment, by Osamu Kodama, et al.

Just to show how complex this subject really is, there is new research from the Colorado School of Mines that indicates cold (sub-zero) treatment of typical carburized steels improves contact fatigue resistance by transforming retained austenite into martensite, etc. This data is presented in SAE Technical Paper 2006-01-0896 Examination of Pitting Fatigue in Carburized Steels with Controlled Retained Austenite Fractions by Ryan Wagar, et al.

 

[houseman]

Thanks dbooker630 and TVP,
I understand this is not a easy issue. I have found other reference about it in Gear technology, May/June 2003: "characterization of retained austenite in carburized geras and its influence on fatigue performance".
I'm really grateful for your help. I think this forum has a high level of experience and knoledge.
Regards,