Influence of magnetic-field gradients on the pearlitic transformation in steels

Carbon steels of 1 wt%C were subjected to the pearlitic transformation in a helium-free superconducting magnet in search for some influence of the magnetic field gradient on the transformation. It was found that steels containing Mn and/or Cr got hardened proportional to the field gradient present during the transformation at 560degreesC. The hardness increase amounted to 8% at the field gradient of 50 T/m, the magnitude depending on the steel composition. It was confirmed that the hardening was irrelevant to the sign of the gradient and absent for homogeneous magnetic fields. This kind of hardening was not observed for a binary Fe-C alloy. The hardened specimens exhibited age hardening by annealing at 150 similar to 200degreesC. TEM observations and internal friction measurements suggested that supersaturated carbon atoms were introduced into the ferrite layers of the pearlite phase under the field gradient. Such extra carbon atoms should have combined with Mn and/or Cr atoms during the quenching to impurity complexes responsible for the solution and age hardenings. Plausible arguments are made on the action of magnetic field gradients for the introduction of supersaturated carbon atoms in the pearlitic ferrite.