Case hardening simulation of surface compacted, graded porous Astaloy 85 Mo components

摘要

Surface densification prior to heat treatment is a new approach for case hardening of powder metallurgical (PM) steels which avoids carbonization. Thus compacted surfaces and isolated pores are formed at near surface region, thereby preventing a capillary effect of the pores but nevertheless leading to an increase in carbon penetration, since the diffusive mobility of atoms along the pore surface increases. As a result of the process chain, the mechanical properties as well as the transformation kinetics depend on the porosity and carbon level. The mechanical properties decrease with increasing porosity and decreasing carbon concentration. Concerning the transformation kinetics, a reduction of both the incubation period and overall isothermal transformation time can be detected for decreasing relative densities. The phase transformation is retarded for an increasing carbon concentration.In the present study a case hardening simulation is performed for a surface compacted graded porous Astaloy 85 Mo fatigue strength bar. As initial conditions for the FE-simulation a porosity depth profile was experimentally determined and mapped to the geometry. The actual simulation was carried out in two different sequential parts, namely carburizing and quenching. The carburization was described by a technological mass transfer approach and effective diffusion coefficients taking into account local porosity. The following quenching was performed using a porosity and carbon dependent kinetic model based on Avrami and Koistinen-Marburger equations. The mechanical properties are modeled as a function of carbon content, porosity, temperature and microstructure A good correlation can be found between experimental and simulation results, hence verifying the used modeling approaches for carburizing and quenching.