Effect of magnetic field on microstructure and mechanical properties of austempered 70Si3MnCr steel

The effect of magnetic field on bainite microstructure and mechanical properties of 70Si3MnCr steel was studied. Microstructure observation shows that introducing magnetic field during the austempering process, the microstructure was significantly refined and the volume fraction of the bainitic ferrite was increased. The high nucleation rate for bainitic ferrite caused by the high magnetostatic energy should be responsible for this positive effect. As compared with samples without magnetic field treatment, the magnetic samples also show a simultaneously increased strength and ductility. The ultra-fine bainitic ferrite with higher dislocation density, in magnetic samples increased the tensile strength, and carbon-riched filmy retained austenite and small blocky retained austenite in magnetic samples have a tremendous effect on ductility. High magnetic strength in magnetic samples increases the strength of the bainitic ferrite, which acts as a stress shielding source to prevent the retained austenitic phase from higher stress and increase the plasticity. Moreover, high magnetic strength can alter the spins of the free radicals between the dislocations and obstacles from S state to T state which had weak bonding with dislocations. Hence, depinning of dislocations is facilitated which results in increased plasticity. Therefore, due to the magnetoplasicty, high magnetic properties, and ultra-fine microstructure, magnetic samples show a more excellent strength and ductility as compared with the normal samples.