Effects of thermal plasma jet heat flux characteristics on surface hardening

Two thermal plasma torches with different internal structures, including a two-electrode plasma torch (TEPT) and a multi-electrode plasma torch (MEPT), were designed to produce thermal plasma jets with different heat flux characteristics and used in the experiments and simulations of plasma surface hardening of 45 steel. Experiments with the processing parameters designed by the Taguchi method were carried out and the heat affected zone (HAZ) properties were measured to determine the optimal processing parameters. The HAZ width and depth of the substrate treated by MEPT are 109% and 163% larger than that by TEPT respectively, while the maximum hardness and the average hardness of the HAZ of the substrate treated by TEPT are 30% and 32% higher than that of MEPT respectively. Simulation results show that the heat flux characteristic of the thermal plasma jet generated by TEPT is more concentrated than that by MEPT, which is the main reason for the differences on their performance on the plasma surface hardening. It is suggested that plasma torch with more concentrated heat flux characteristic should be adopted in surface hardening that emphasizes more on the hardness of the treated surface and the processing efficiency. The plasma torch with less concentrated heat flux characteristic should be utilized in surface hardening that emphasizes more on the HAZ width and depth of the treated surface.

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