Don State Technical University (Rostov-on-Don, Russia)
V. N. Pustovoyt, Dr. Eng.
Yu. V. Dolgachev, Dr. Eng., yuridol@mail.ru

Isothermal quenching of the lower bainite structure enables higher structural strength values to be achieved compared to conventional quenching and tempering. The intermediate transformation is distinguished by a unique mechanism—the simultaneous occurrence of shear transition processes and diffusional removal of carbon from the moving interphase boundary. The aim of this study was to determine the feasibility of beneficially altering the bainite structure. For this purpose, isothermal quenching processes were studied under an external constant magnetic field with a strength of up to 2 MA/m. The studies were conducted on various iron-carbon alloys, primarily structural alloy steels. It was established that the influence of a magnetic field during the isothermal transformation of austenite in the intermediate transformation temperature range leads to changes in the process kinetics, the morphology of the reaction products, the dispersion of the structure, and the characteristics of carbide formation. The observed changes in the microstructure and hardness already indicated an increase in the rate of austenite decomposition. Data on the volume fraction of bainite under various conditions made it possible to construct kinetic dependences. Fine structure analysis revealed that the influence of a magnetic field manifests itself not only in increased bainite dispersion but also in the internal fragmentation of α-phase crystals. These changes were substantiated based on concepts of the magnetic heterogeneity of austenite and the influence of magnetostrictive stresses in ferromagnetically ordered clusters in the initial γ-phase on the nucleation of the bainite α-phase. An increase in the rate of the bainitic reaction during isothermal quenching in a magnetic field leads to increased dispersion and fragmentation of α-phase crystals, a more favorable arrangement of carbides, and an increased contribution from solid-solution strengthening. The combination of these structural changes should have a beneficial effect on mechanical properties, which is the subject of further research.

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