Identify carburized microstructure
Identify carburized microstructure
(OP)
We carburize small parts (rollers for bearings) in a rotary batch furnace using straight natural gas as the carbon source. We oil quench directly from the 1775F carburizing temperature. We then reheat to 1525F and drop quench in mineral oil. Tempering is performed at 360F. The attached micrograph (picral etch) shows the surface structure we saw in a small roller (approx .5" dia) made from 4320 VAR steel. At the surface, we typically see martensite, significant retained austenite, and fine dispersed carbides. I am not sure, however, about the fine black needles. How would you describe this microstructure?





RE: Identify carburized microstructure
Aaron Tanzer
www.lehightesting.com
RE: Identify carburized microstructure
The amount of retained austenite appears to be excessive; I would doubt you are achieving 50HRC. A high temperature temper (1250F) between the carb and hardening cycles will help reduce this by precipitating carbides int he case. With all the nickel in 4320, though, you will always have a retained austenite problem. A sub-zero treatment will aslo eliminate it.
rp
RE: Identify carburized microstructure
RE: Identify carburized microstructure
RE: Identify carburized microstructure
Maybe a similar mechanism with retained Austenite? A search on this forum or google might reveal something about the delayed effect.
I thought acicular structure was typical only of Martensite but maybe the black needles could be UPPER Bainite and the dark structure under the Martensite could be lower Bainite.
Just my 2 cents.