Volumetric Changes and Structural Stresses after Quenching in a Magnetic Field

Introduction. As is well known, the process of steel hardening is accompanied by volumetric changes due to the difference in specific volumes of transforming phases. These volume changes result in structural stresses within the steel. The presence of these stresses in a hardened product negatively affects its resistance to brittle fracture, leading, for example, to decreased safety during operation of steel structures. In this regard, it is essential to improve heat treatment (HT) methods that reduce quenching stresses. One promising method involves applying a permanent magnetic field during phase transformation, which affects the kinetics of transition and resulting transformation products. However, there is a lack of data on volumetric changes during quenching for this method. The aim of this work is to investigate the effects of permanent magnetic fields on volumetric changes and structural stresses during steel hardening. Materials and Methods. The research was conducted on technical iron and carbon steel 35, 45, U8, U10, U12. Magnetic fields with strengths of 1.4 and 1.6 MA/m were generated in an interpolar gap of the FL-1 electromagnet, designed by Moscow State University. Volumetric changes after conventional and magnetic quenching were quantitatively assessed by measuring the specific volumes using hydrostatic weighing method. Results. Concentration dependencies of changes in specific volumes of carbon steels during quenching in a magnetic field at temperatures of 800 and 1 000°C were obtained. There were no changes in the volume effect of martensitic transformation in iron and U10 steel when quenching at temperatures higher than 800°C. Different changes in the volume effect were observed in steels with carbon content: from 0 to 1% — reduction in specific volume; from 1.0% to 1.2% — increase in specific volume. Calculation data showed that after quenching without a field, the level of structural stresses increased with an increase in the carbon content in austenite and an increase in the heating temperature for quenching. The influence of the magnetic field was reduced to a decrease in structural stresses in low- and medium-carbon steels and their increase in high-carbon steels. At low tempering temperatures, the level of structural stresses after quenching in a magnetic field was lower for medium-carbon 45 steel, and higher for U12 steel, than after quenching without a field. Discussion and Conclusion. The data obtained for low- and medium-carbon steels can be explained by the increased degree of martensite decomposition “in statu nascendi” upon cooling in a magnetic field and an increase in the amount of martensite phase in high-carbon iron alloys. The change in the volume effect caused by the increase in the amount of martensite phase under the influence of a magnetic field prevailed over the change in the volume effect caused by its decomposition during the quenching cooling process. The magnitude and sign of the observed effects were determined by the carbon content in the original austenite, and there was a narrow range of concentrations for which magnetic hardening had virtually no effect on the level of structural stresses. The effect of a magnetic field during tempering somewhat slowed down the reduction of residual stresses in the temperature range of martensite decomposition. Structural stresses after heat treatment in a magnetic field, without taking into account the temperature gradient across the cross-section, were mainly determined by the effects obtained during quenching in a magnetic field. The intensification of the phenomena of martensite decomposition caused a decrease, and an increase in the completeness of the martensite transformation, an increase in the level of structural stresses.