Tempering of martensitic 12 percent chromium steels high temperature gas nitrided

Abstract

AISI 410 (11.5%Cr – 0.13%C) and AISI 410S (13,0%Cr – 0.07%C) steels were gas nitrided at 1200o C, in order to obtain surface nitrogen contents between 0.27 and 0.61%. The specimens were direct quenched and tempered, between 200-650o C for one hour. The AISI 420 steel (12,4%Cr-0.37%C) was quenched from 1100o C and tempered at the sames temperatures used for nitrided steels, was emploied as comparision material. Microstructural evolution and tempering kinetics were studied by optical microscopy, scaning electron microscopy, X-ray difracttion, dilatometry and differential thermal analysis. The tempering stages were shifted to higher temperatures and wider intervals in the nitrided steels, probably due to differences in bonding character of the solid solutions Fe-Cr-N-C and Fe-Cr-C. The nitrided steels showed a better tempering resistance probably due to M2X precipitation.