Formation of Residual Stress Diagram after Quenching in a Magnetic Field

Abstract

Introduction. After hardening, a product has residual stresses: structural and thermal. The magnitude of the total stresses in the finished part determines its crack resistance under the influence of operational loads. Quenching in a constant magnetic field affects the process of martensite nucleation, and the kinetics of martensite transformation, as well as the processes of martensite decomposition. However, there is currently no data available on how these changes in structure affect the stress diagram in a heat-treated product. The aim of this study was to investigate the influence of a constant magnetic field during hardening of iron-carbon alloys on the stress distribution across the cross-sectional area of parts.Materials and Methods. The studies were conducted on samples of technical iron, steel 45, and ferritic malleable cast iron. Cylindrical samples with a diameter of 16 mm and ring samples with an outer diameter of 20 and 55 mm were used. The samples were heated in an electric furnace or an induction heating lamp generator LZ-13, and quenched in water or mineral oil. A constant magnetic field with strength of 768 to 1600 kA/m during hardening was created in the bore of a FL-1 electromagnet. Residual stresses were determined using the original method developed by V.A. Blinovskii based on measuring bending deformations in hollow bodies of revolution.Results. The change in temperature on the surface, in the core, and the temperature difference across the cross-section of a cylindrical sample during cooling in water with and without a magnetic field was obtained. The distribution of stresses over the cross-section after quenching with and without a field for industrial iron in still water was studied. The stress distribution over the cross-section was studied after quenching in a field and without a field in calm water, as well as during spray cooling of steel 45 and ferritic ductile cast iron at different rates.Discussion and Conclusion. The obtained calculated and experimental data allowed us to evaluate possible changes in the residual stress diagrams under the influence of a magnetic field after quenching with volumetric and surface heating. A study of the kinetics of cooling in water under the influence of a magnetic field showed that the temperature difference across the cross-section remained practically unchanged, but there was a decrease in the cooling capacity of the water, which contributed to a reduction in the level of thermal stress. Hardening in a magnetic field led to a reduction of residual stresses in iron-carbon alloys. The change in the distribution of total residual stresses during magnetic tempering was due to a change in their structural component. The magnetic field influenced the distribution of structural, thermal and total residual stresses. The reason for the observed effects was the change in the structural state of steel and cast iron and the cooling ability of water-based quenching liquids under the influence of a magnetic field. The reduction of the level of residual stresses during heat treatment in a magnetic field reduced the likelihood of brittle fracture and cracking, led to a decrease in deformation and warping of hardened steels, and created favorable conditions for the operation of parts under conditions of alternating loads and abrasive friction.