Case Hardening Simulation of Surface Compacted, Graded Porous Astaloy 85 Mo Components

A new approach for case hardening of powder metallurgical (PM) steels is surface densification prior to heat treatment, hence avoiding carbonisation. Thus compacted surfaces and isolated pores in near surface regions are formed, preventing a capillary effect of the pores but still 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 characteristics as well as the transformation kinetics are depending on the porosity and carbon level. The mechanical characteristics are decreasing with rising porosity and falling carbon concentration. Concerning the transformation kinetics a reduction of incubation 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 sample. As initial conditions for the FEM-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 carburizing was described by a technological mass transfer approach and a effective diffusion coefficients taking local porosity into account. The following quenching was performed using a porosity and carbon dependent kinetic model based on Avrami and Koistinen-Marburger equations. The mechanical characteristics are modelled in dependency of carbon content, porosity, temperature and microstructure. A good correlation can be found for experimental and simulative results verifying the used modelling approaches for carburizing and quenching.