Tempering temperature control of medium carbon steel according to the parameters of the saturation magnetic hysteresis loop

The main magnetic parameter sensitive to the structure of steel is the coercive force (H_c). However, the coercive force of steels with a carbon content more than 0.3% appeared unsuited for control of the tempering temperature (T_t), hardness (HRC), and mechanical properties of hardened products made of such steels due to non-monotonous character of H_c dependence on the tempering temperature after their quenching The goal of the study is to develop a method for control of the structural changes of medium-carbon steels that occur upon heat treatment proceeding from the information parameters generated using the coercive force H_c and the ratio K_S of their residual magnetization (M_r) to the magnetization of technical saturation (M_s) in a practically important temperature range. The advantage of the developed approach compared to those based on measuring the relaxation magnetic parameters of steels is that the parameters H_c and K_S can be measured using standard methods with the minimal relative errors of measurement (2 and 1%, respectively). We consider three different combinations of the parameters H_c and K_S – H₀, H_c1, and H_c2 — and analyzed their dependence on the tempering temperature and hardness of medium carbon steels C30 and C45. The parameters H₀, H_c1, and H_c2 monotonously change as the hardness of medium-carbon steels decreases. The correlation coefficient between the values of H₀, H_c1 and H_c2 and HRC £hardness of steel 30 in the range of 32  HRC  41 is 0.959, 0.965 and 0.978, respectively. The results obtained makes it possible to abandon the complex and inaccurate measurements of the relaxation magnetic parameters of steels when developing the methods and devices for magnetic structuroscopy, and focus on enhancing of the accuracy of measuring£ H_c and M_r/M_s ratio of the material.

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