Renxing Li

Phase evolution of AISI 321 stainless steel during directional solidification

Abstract The phase evolution of AISI 321 stainless steel was studied by directional solidification and quenching techniques. Two interfaces, solid/liquid and the peritectic reaction interface, were found to exist in the directional solidification structure. With increasing growth velocity the solid/liquid interface changed in the sequence of planar, cellular, dendritic and the primary phase changed from austenite to ferrite. The phase and morphology selection was verified by the interface response functions (IRFs) and the maximum growth temperature criterion. The ferritic island banding structure was observed, not only in the austenite cellular primary growth condition (3 μm s−1), but also in the dendritic ferrite primary growth one at relatively low growth velocity (5 μm s−1). It is deemed that the former resulted from the nucleation of ferrite in the continuous matrix of austenite phase, yet the latter is the residual primary ferrite attributed to the growth of austenite. Both of them do not come from the nucleation near the solid/liquid interface.

The Solidification Structure Refinement of SWRCH22A Steel Billet under Pulse Magneto-Oscillation Treatment

The solidification structure in the continuous casting of SWRCH22A steel billet (240 mm × 240 mm) with Pulse Magneto-Oscillation (PMO) treatment was investigated. The length of columnar dendrites of the billet decreased from 8 cm to 4 cm after PMO treatment, i.e. the area percent of equiaxed dendritic structure zone increased by 30%. Meanwhile, the center shrinkage of billet was significantly modified. PMO supstantially promoted the nucleation near the solid/liquid interface and drove the equiaxed grains to move to the center of the melt. In addition, the growth of the columnar dendrite was restrained owing to the Joule heat induced by PMO, concentrating on the out parts of the billet. The results indicated that the PMO was a promising technique for solidification structure homogenizing in continuous casting of steel billet.