Effect of Silicon Content and Prior Structures on Microstructures of Nitrided Hot Working Tool Steels

Abstract

Microstructures of nitrided hot working tool steels SKD61, with various silicon contents and prior structures, were investigated by transmission electron microscopy of extraction replicas, X-ray diffraction, electron probe microanalysis and microhardness measurements. Plasma nitriding was carried out at 773K for 126 ks.
Any alloying nitride precipitates such as CrN that was previously reported for Cr-alloyed steels were not identified for nitrided specimens. After reheating to 973K, hardness of nitrided layer was decreased accompanying CrN precipitates. Effect of silicon content on hardness was relatively small both of as-nitrided and after reheating. Depth of nitrided case and hardness after 973K reheating decrease with silicon content. Nitrided hardness of quenched specimen was higher than that of annealed one. Grain boundary carbides in diffusion zone and a carbonenriched zone ahead of the nitriding front were also observed for both of quenched and annealed specimens.