Identify carburized microstructure 1

[rizzy77boy]

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?

 

[mrfailure]

I think the dark needles are plate martensite.

Aaron Tanzer

http://www.lehightesting.com

 

[redpicker]

Acicular martensite, combined with retained austenite.

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

 

[metman]

An LPS for redpicker

 

[rizzy77boy]

Thanks to everyone for your insights. Even though there is a lot of retained austenite, we still reach hardnesses over 60 HRC. This product is given a deep freeze, but there seems to be little visual effect on the structure. We cannot freeze in our own facility, so the lag between hardening and freezing can stretch into several days. This may be the reason we see little visual effect on retained austenite. We do perform a high temperature temper (1100F) but only on our higher alloy grades. It may be worthwhile to consider the temper for the 4320 grade as well. One potential problem that is difficult to investigate is the effect of Vacuum Arc Remelting (VAR) on this microstructure. I am wondering if the VAR process causes some local segregation of alloying elements (especially Mn). The local segregation could cause some lower bainites to form (this was my best guess for the dark needles in the picture) or promote grain growth. Larger prior austenite grain size seems to promote retained austenite as well. Thanks again for your comments.

 

[metman]

There is a mechanism that I have not heard an explanation for the cause but the symptoms are; Cr,Mo and Ni,Cr,Mo Steels if straightened immediately after quenching will plastically deform with no problem. However, if there is a day or two time lapse before straightening (maybe only hours), then the Martensite "locks up" and the parts break during straightening. This was my astute metallurgist friend's rude description of his experience where he managed heavy industrial heat treating and in depth failure analysis.

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.