The composition-dependent diffusion coefficient of carbon in austenitic rolling bearing steel SAE 52100 was determined at the common austenitising temperature of 1123 K by application of the isothermal powder-pack decarburisation experimental technique, which ensures a diffusion-controlled process. Mathematical analysis of the concentration-distance curve, obtained by means of a novel measuring method based on secondary ion mass spectrometry (SIMS), was performed in the framework of an automatized evaluation tool. A finite element code, which permitted simulation of the austenite-ferrite phase transformation occurring in the rim zone of the sample by a step in the diffusivity-concentration relationship, was utilized for recurrent calculations of carbon distributions covering defined value ranges of the fitting parameters. In this computation cycle, usual linear and exponential expressions for the concentration dependence of the diffusion coefficient were used. Comparison with the SIMS data was carried out by automatically rating each calculated depth profile according to the least-squares method. The best fit quantitatively described the measured decarburisation profile. The following expression for the concentration (c_c in wt.%) dependence of the carbon diffusivity in austenitic SAE 52100 (D_γ in cm~2/s) was derived: D_γ=4.4+0.6x c_c. This result can be compared with data on the binary Fe-C system reported previously.
展开▼
