The effect of heat treatment atmosphere on hardening of surface region of H13 tool steel
The cost of an aluminium extrusion die accounts for 35-50% of the total extrusion cost, hence it is important to understand the kinetics of extrusion die heat treatment. The main objective of die heat treatment is to increase the surface hardness so the surface wear property can be enhanced, which extends the die service life. Before the die material is case hardened by processes such as nitriding, the material is first heat treated to certain hardness. Thus the aim of this investigation is to understand the kinetics of carbon diffusion during steel austenitzation because it is significant to steel hardness property.H13 hot work tool steel, a well known material used as a die material for its superior mechanical and hardening properties, was used for the analysis of carbon diffusion during austenization. Samples made of H13 steel were subjected to four different atmospheric conditions: heat treatment without atmospheric control, heat treatment with stainless steel foil wrapping, pack carburization heat treatment and vacuum heat treatment. Three treatment time ranges were also applied for the carbon diffusion modelling. Some samples were further treated by gas nitriding to establish the effect of carbon content on the gas nitriding performance.It was found that decarburization occurred when atmospheric control did not take place during the heat treatment process. Through the carbon diffusion analysis, at austenitizing temperature of 1020ºC, the equilibrium surface carbon content at 1020ºC was 0.157wt%, with activation energy of carbon in H13 steel of 20,200cal/mol, and carbon diffusivity at 1020ºC was 1.97x10-8cm2/s.This study also proved that proper stainless steel foil wrapping on the heat treating material could restrict decarburization process, resulting in a constant hardness profile as vacuum heat treatment does. However the tempering characteristic between this two heat treatment methods are different to each other.For the pack carburization heat treatment, it was noted that the samples suffered from decarburization at early stage of heat treatment as carbon monoxide level was not adequate for carburization process.Results from the gas nitrided samples showed that the thickness and the hardness of the nitrided layer was independent to the carbon content in H13 steel. After 2 rounds of gas nitriding process, further nitriding seemed to have no significant effect on the hardness and thickness of the nitrided layer. Furthermore, the white layer was not observed in the nitrided samples which were heat treated without atmospheric control.