Precipitation and hardening mechanisms of an ion nitrided H-12 tool steel

Abstract

In order to investigate the combined effects of tempering and ion nitriding on H-12 tool steel, specimens were tempered at 550 and 580°C and subsequently ion nitrided at 500°C for various times. The effect of the specimen treatments was analysed by microhardness testing, fractography, X-ray diffractometry, and transmission electron microscopy (TEM). A linear increase in the surface microhardness was observed up to 27 ks, however at longer times the surface microhardness decreased. The hardness increase is related to precipitation of iron nitrides, chromium carbonitrides, and tungsten carbides. The maximum hardness reached correlated with the maximum numbers of chromium carbonitrides and tungsten carbides, as demonstrated by the X-ray diffraction analysis. The TEM study shows that the increase is mainly a result of the coherent precipitation of W₂ C in an Fe₄ N matrix. Also, TEM observations reveal that in specimens nitrided for shorter times (9 and 18 ks) martensite is transformed into the nitride Fe₁₆ N₂ (α″). The microhardness decrease observed in specimens treated for longer times is caused by loss of toughness in the grain boundaries.