The Effect of Magnetic Fields on Transformations in Steels.

Studies have been made on the effect of a magnetic field of 16 K. Gauss on the martensitic transformation of two steels (Fe 1C 1.5Cr and Fe 0.3C 2.8 Ni 0.6Cr and 0.6Mo). The transformation was followed metallographically using the Greninger and Troiano technique. A magnetic field of 16 K. Gauss raises the M by ~ 5 C and shifts the entire trans- formation curve to higher temperatures. A thermodynamic model has been proposed to explain the Ms shift, and it is shown how useful thermo-dynamic functions, such as the entropy of transformation S, (Ms), can be derived experimentally from such shifts. An attempt has been made to evaluate the latent heat of marten- site formation for the Fe 1C 1.5Cr alloy from the recalescence effects associated with the periodic application of a field below the Ms. The heat of transformation value combined with the entropy of transformation yields a value of 260 +/- 100 cals/gm mole for the driving force F at the Ms, in good agreement with available theoretical calculations. Hardness measurements on quenched steels (0.4C 1.4C and 1C 1.5Cr) show that quenching in a magnetic field gives an increase in hardness of 20-30 V.P.N. , which is largely associated with the extra martensite formed when the transformation takes place in a field. The effect of quenching in a magnetic field on the subsequent kinetics of the decomposition of martensite during the first stage of tempering has been investigated for two steels (1.4C and Fe 1C 1.5Cr), using dilatometry and differential Hall Probe technique. Quenching in a magnetic field delays the decomposition of martensite during subsequent tempering, and this has been associated with a decreased driving force for the nucleation of E-carbide