Retained austenite in part subject to impact loading

[UCengno1]

We have strived to reduced RA below 10% all of our high stress alloy steel parts over the last couple years after the discovery of untempered martensite. We are producing a gear component from nickel-chromium-molybdenum steel that is subjected to impact loading following by cyclical loading. We currently carburize and harden per AMS 2759/7 class 1 to 15N 87-89. Case of .012-.017. Acceptable core is 32-45 Rc.

We used to produce the part out of 8620 without special heat treat controls. We ran into trouble with untempered martensite on some other shaft parts and applied experience to this part and we are now using a very detailed carburize and harden process to limit RA. Make a long story short we now experience more complete tensile failures due to impact loading than previous. Should we consider that we have reduced RA too far? Is that possible?

 

[TVP]

Yes, it is possible. Considerable work has been performed by both industry and research institutions regarding the optimum amount of retained austenite. It appears that 10-15% retained austenite allows for better resistance to high stress intensities due to geometry, impact, etc. Have you considered trying to temper twice (no subzero treatment, just 180-220 C) in order to convert the untempered martensite without changing the retained austenite?

 

[Yallapragada1022]

Having R/A (10 to 45%) has been found to be highly beneficial w.r.to improving the mechanical properties in gears, bearing, and many other applications.

What is your current R/A level?

Rao Yallapragada

 

[UCengno1]

We run <10%

 

[UCengno1]

We are also concerned about additional degradation at low temperature. We have experienced a spike in failues during Fall/Winter months but the loading also increases at cold temp.

I have done the obligatory GOOGLE search but is there a good resource for some of the study information concerning this issue?

 

[Yallapragada1022]

You may want to look at these:

http://www.springerlink.com/content/v345381t11105127/

http://www.edpsciences.org/articles/jp4/pdf/2004/08/Jp4120013.pdf

http://www.heat-treat-doctor.com/documents/RetainedAustenite.pdf

Hope this helps.

Rao Yallapragada

 

[TVP]

The following are some references on the subject:

The Role of Nickel in Carburizing Steels, International Nickel (available from The Nickel Institute at

http://www.nickelinstitute.org)

SAE 780772 Impact Properties of Selected Gear Steels
SAE 730142 The Effect of Impact Fatigue Prestressing on the High Cycle Fatigue Resistance of Carburized Gear Steels
SAE 821102 Improvement on the Fatigue Strength of Case Hardened Gears by a New Heat Treatment Process
SAE 860725 Improvement of Rolling Contact Fatigue Life of Carburized Tapered Roller Bearings.
SAE 750050 The Effect of Carburizing Variables on the Residual Stresses in Hardened Chromium Steel

SAE Technical Papers can be obtained from SAE International at

http://www.sae.org

"Some Effects of Retained Austenite on the Fatigue Resistance of Carburized Steel", Metallurgical Transations A, Volume 6A, May 1975
"The Microstructure and Fracture of a Carburized Steel" Metallurgical Transactions A, Volume 9A, November 1978

Met Trans articles can be obtained from ASM International at

http://www.asminternational.org

 

[UCengno1]

Thanks for all.

 

[unclesyd]

Here is another possible source of information, maybe even some quantitative data. This site is sort of convoluted but has a wealth of information.

http://www.autosteel.org/AM/Templat...ONTENTID=6942&TEMPLATE=/CM/ContentDisplay.cfm