Carbon diffusion measurement in austenite in the temperature range 500 degrees C to 900 degrees C

Carbon diffusion in austenite plays a critical role in phase transformation in steel. However, it can only be estimated in the fully austenitic range and has then to be extrapolated to the temperature range of the phase transformation. Therefore, published data are limited to temperatures above 750 degrees C. In this study, new experiments are carried out to determine the carbon diffusion coefficient in austenite at temperatures as low as 500 degrees C. Carburization experiments are performed in the austenitic range for a Fe-1.5 pet Mn 0.13 pet C and a Fe-31 pet Ni alloy (wt pet). Composition profile measurements, which are done using glow discharge optical emission spectrometry (GDOES), show that the surface composition is not constant with time. A methodology has been developed to assess the diffusion coefficient of carbon in austenite combining the measured carbon profiles and a numerical method to compute the diffusion profile taking into account the time evolution of the boundary condition. This method is first validated on the Fe-C-Mn steel. Carburization experiments are carried out on a Fe-31 pet Ni alloy at 900 degrees C, 800 degrees C, 700 degrees C 600 degrees C, and 500 degrees C. The carbon diffusion coefficient is assessed using the method described above and fitted with the following expression (T in Kelvin): D = 1.23 center dot 10(-6)center dot e 15,050/T(K)(K) (m(2)/s). The new expression is compared with previous experimental results measured for comparable nickel content at higher temperatures, and it shows a reasonable agreement. The model proposed by angstrom gren for carbon diffusion has been modified to take into account the thermodynamic contribution of nickel. This model also shows good agreement with the present experimental results, even if it was fitted to experiments performed at higher temperatures.