In Situ Observation of Phase Transformations of Pearlite Heat-Resistant Steel during Solidification

The phase transformation of pearlite heat-resistant steel during solidification is observed using high-temperature confocal scanning laser microscopy. The precipitation of the & delta;-ferrite (& delta;) phase proceeds in a cellular manner at cooling rates of 5 and 15 & DEG;C min(-1), whereas precipitation occurs in a dendritic manner at a cooling rate of 100 & DEG;C min(-1). Furthermore, a change in the solidification path is observed at high cooling rates. When the experimental steel is cooled at a low cooling rate of 5 & DEG;C min(-1), the sequence of continuous solidification is L & RARR; L + & delta; & RARR; L + & delta; + & gamma; & RARR; & delta; + & gamma;, which is consistent with that of equilibrium. However, when the cooling rate increases to 15 and 100 & DEG;C min(-1), the sequence of continuous solidification is L & RARR; L + & delta; & RARR; L + & delta; + & gamma; & RARR; L + & gamma;& RARR;& gamma;. Furthermore, at various cooling rates, the interface of the peritectic transformation of & delta; & RARR; & gamma; presents three different modes: planar and cellular modes controlled by solute diffusion and massive transformation controlled by the processes that occur at the transformation interfaces. The different & delta; & RARR; & gamma; transformation modes and the associated different degrees of volume shrinkage are hypothesized to lead to uneven strand shell growth during continuous casting.