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Austenitic nitrocarburising has been carried out at 620 deg C with NH3(35% ) , CO2(5%) and N2( 60%) and then tempering at 350 deg C. Cracks are noticed on the inner radius of bend portion. Any recommendation to overcome cracking problem ?

Nitrogen austenite has a Ms of 35°C. Thus, it is still austenite upon cooling from nitrocarburazing temperature.

Upon tempering austenite trasforms to martensite, tends to expand and causes cracks owing to constrained expansion.

Nitrocarburizing has to be performed below 590°C

Austenitic nitrocarburising has been carried out at 620 deg C with NH3(35% ) , CO2(5%) and N2( 60%) and then tempering at 350 deg C. Cracks are noticed on the inner radius of bend portion. Any recommendation to overcome cracking problem ?

Austenitic nitrocarburising has been carried out at 620 deg C with NH3(35% ) , CO2(5%) and N2( 60%) and then tempering at 350 deg C. Cracks are noticed on the inner radius of bend portion. Any recommendation to overcome cracking problem ?

I am not sure why you are carbonitriding this grade of steel. This grade is a through hardening grade that can reach high hardness without carbonitriding, a process that is very prone to surface cracks. I would recommend that you austemper. If you need further high hardness, it is recommended that part can be nitrided via salt bath, gas or plasma nitriding. The plasma nitriding process has advantage in that it can use very low temperatures for nitriding.

Ratnesh Dwivedi, Ph.D

WWW.rkdengg.com

Austenitic nitrocarburising has been carried out at 620 deg C with NH3(35% ) , CO2(5%) and N2( 60%) and then tempering at 350 deg C. Cracks are noticed on the inner radius of bend portion. Any recommendation to overcome cracking problem ?

Austenitic nitrocarburising has been carried out at 620 deg C with NH3(35% ) , CO2(5%) and N2( 60%) and then tempering at 350 deg C. Cracks are noticed on the inner radius of bend portion. Any recommendation to overcome cracking problem ?

This discussion is very confusing. Is the topic austenitic nitrocarburizing or ferritic nitrocarburizing or carbonitriding. They are three completely different heat treat processes.  The can be conducted successfully in gas atmosphere furnaces (and perhaps in vaccum furnaces) at temperatures very much above those mentioned in previous comments.

Al Swiglo, scssaas67@sbcglobal.net 651-490-7752

Austenitic nitrocarburising has been carried out at 620 deg C with NH3(35% ) , CO2(5%) and N2( 60%) and then tempering at 350 deg C. Cracks are noticed on the inner radius of bend portion. Any recommendation to overcome cracking problem ?