Ordering-disordering phenomena and micro-hardness characteristics of B2 phase in Fe-(5-6.5%)Si alloys

The disappearing and forming behavior of B2 ordered phase in high Si steels with Si-levels between 5 and 6.5% (by weight, unless specified otherwise) was investigated by observing TEM and measuring electrical resistivity and micro- and nano-hardness. The critical cooling rate for suppressing the formation of 132 ordered phase increased exponentially with Si-content. The ordered phase coarsening was observed to occur actively above 800 degrees C, where atoms have enough mobility. The removal of solidification segregation is necessary in order to reduce the amount of B2 ordered phase in the as-cast or hot-rolled state, to lower the annealing temperature for the dissolution of B2 ordered phase, and to shorten the annealing time. Micro-Vickers hardness depended only on Si-content even under various heat treatment conditions, and the nano-hardness had a small difference between B2 and A2 phases. In the high Si steels, the short-range order in A2 disordered phase was likely to cause a comparable hardness as that of B2 ordered phase. (c) 2005 Elsevier B.V. All rights reserved.