Does direct hardening after carburisiong is a good idea?

[harsuda]

I have been hardening the case carburised material by reheating from room temperature to hardening temperature after carburisation process for materials like 8720, 9313, 4718 etc. Now I am planning hardening by bringing down the temperature to hardening temperature from carburising temperature for cost saving. I suspect that I may end up with high retained austenite. Please advise.

 

[metengr]

Yes, I would expect that the level of retained austenite would be higher by consolidating the post-carburizing heat treatment. The level of retained austenite in the above steels is based on austenite grain size, carbon content in the case, and austenitizing temperature. However, you are going to have to decide how much retained austenite you can tolerate in service.

If you determine you need low levels of retained austenite, an option after the carburizing quench is to perform a cryogenic treatment to assure as much transformation of retained austenite to fresh martensite, as possible.

 

[redpicker]

Yes, you can and, yes (as metengr mentions) the level of retained austenite will go up. I did this in the 80's with drilling tools and was able to keep the retained austenite to a minimum by keeping tight control on carbon potential. Particularly for the last half of the cycle, you want to bring the potential down to your surface carbon target. Of course, this will be different for the different grades with the 93XX being lowest and the 87XX being the highest. For the 93XX steels, you may find it easier to just use a cryogenic treatment since it forms retained austenite at comparatively low carbon levels.

If I remember correctly, we would drop the carbon down to around 0.60%-0.75% (depending on grade) for the last 3 or 4 hours, but that's been 20 years ago.

rp

 

[dbooker630]

I direct harden 8822H gears after carburizing. Typically I will lower the carbon potential by .10 to .90% in the diffuse zone and then quench from that temp which for me is 1550F. The RA ranges from 10-20%. Occasionally on a part requiring a deeper case I sometimes see up to 30% and yes, I have used a deep freeze, followed by a second temper (I have an in-line temper) to reduce the RA as well as raise the hardness a few points.

 

[swall]

redpicker and dbooker630 are right on the money. Standard practice to do it that way.