Tohru Arai1, Hironori Fujita1, Masayoshi Mizutani1 and Noboru Komatsu1
1Toyota Central Research & Development Laboratories, Inc., Nagoya
When steels containing carbon are heated at 850∼1050°C in a fused borax bath containing Fe-V, Fe-Nb, Fe-Cr or metallic Cr powder, the carbide layers such as VC, NbC, Cr7C3, etc. are formed on the steel surfaces, respectively. Effects of the treating condition and steel composition on the growth rate and composition of such carbide layers were investigated with the aid of microstructure observations, electron probe microanalysis and X-ray diffraction analysis.
The results obtained are as follows:
(1) The carbide is supposed to grow on the front surface of the carbide layers by the reaction between carbide-forming elements dissolved in the fused borax and carbon atoms successively supplied through the layer from the matrix.
(2) The growth rate of the carbide layers was controlled by the diffusion rate of C in the carbide layer and the C content in the matrix.
(3) The activation energies for diffusion of C in the carbide layers were about 45000, 50000 and 40000 cal/mol for VC, NbC and Cr7C3+Cr23C6, respectively.
(4) The carbide layers did not or scarcely contain Fe and alloying elements such as W, Mo, Cr or V.
(5) The carbon content in the VC or NbC layer was about 90% of the stoichimetric values.
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