VOLUMETRIC CHANGES AT HEATING IN STEEL 60Si2CrV SUBJECTED TO Q&P TREATMENT

The paper presents results of the investigation of phasestructural transformations and volumetric changes that occur during heating in high-silicon spring steel 60Si2CrV subjected to Q&P(quenching and partition) treatment. Chemical composition of the steel was: 0.53  %  C; 1.46  %  Si; 0.44  %  Mn; 0.95 % Cr; 0.10 % V; 0.016 % S; 0.013  %  P. Steel samples were subjected to Q-n-P treatment as follows: a) austenitization at 880  °C; b) quenching with the cooling stop at 120, 160, 200 and 240  °C; c) subsequent holding at 220, 250 and 300  °С with duration from 10 to 3600  s; d) final cooling in water. The volumetric changes during heating were studied using an optical differential dilatometer at a heating rate of 1  K/s. As a reference, a sample of the same steel stabilized by high tempering was used. The amount of retained austenite was determined by X-ray diffraction using a diffractometer DRON-3 with Fe-radiation. It is found that on the heating curves of Q&P samples, the sections corresponding to different transformations during tempering are clearly identified. On dilatograms of the Q&P samples, dilatometric effect corresponding to the second transformation during tempering (270  –  430  °C) was found to be increased dramatically, indicating an increase in retained austenite amount compared to the quenched state as a result of Q&P treatment (as confirmed by Xray study). At the same time, value of the effect corresponding to the third transformation during tempering was found to be decreased. To obtain the maximum amount of retained austenite in 60Si2CrV steel, the partitioning temperature should be of 260  –  300  °С, while the quenching completion temperature should be of 160  –  240  °С. The amount of retained austenite rises substantially as the quenching temperature increases. Duration of the partitioning stage should be selected taking into account the extreme character of austenite dependence on the partitioning time. As a result of the work, an effective applicability of the dilatometric method for analyzing the steel structural state and choosing the optimal mode of Q&P treatment was demonstrated.

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