Rapid Full Annealing under High Magnetic Field

The kinetic effect of a magnetic field on high temperature phase transformation during cooling in 42CrMo steel is investigated. Results show that the magnetic field can considerably increase the driving force for the transformation from austenite to ferrite by enhancing the Gibbs free energy difference between the two phases. Even at the high cooling rate of 46 °C/min, the decomposition of austenite in a 14 Tesla high magnetic field is still ferritic and pearlitic, instead of being bainitic as is usually observed. A microstructure of fine and randomly distributed ferrite grains and pearlite colonies is obtained. Therefore, this rapid magnetic annealing process can also effectively prevent the formation of the banded structure that occurs commonly during the conventional full annealing due to previous hot working. Image analyses and hardness tests show that the amount of ferrite obtained in this way is almost the same as that obtained by the conventional annealing and the hardness is still within the optimum range for the subsequent machining. Magnetic annealing has thus the merits of improving the microstructure by avoiding banded structure and also optimizing the process by greatly shortening the cooling time. Therefore, it is a promising approach for the innovation of conventional processes. Probing into this issue is of both theoretical significance and technical interest.