The interfacial structure and habit plane of proeutectoid cementite plates

Effect of Heterogeneous Microstructure on Refining Austenite Grain Size in Low Alloy Heavy-Gage Plate

The present work introduces the role of heterogeneous microstructure in enhancing the nucleation density of reversed austenite. It was found that the novel pre-annealing produced a heterogeneous microstructure consisting of alloying elements-enriched martensite and alloying-depleted intercritical ferrite. The shape of the martensite at the prior austenite grain boundary was equiaxed and acicular at inter-laths. The equiaxed reversed austenite had a K-S orientation with adjacent prior austenite grain, and effectively refined the prior austenite grain that it grew into. The alloying elements-enriched martensite provided additional nucleation sites to form equiaxed reversed austenite at both prior austenite grain boundaries and intragranular inter-lath boundaries during re-austenitization. It was revealed that prior austenite grain size was refined to ~12 μm by pre-annealing and quenching, while it was ~30 μm by conventional quenching. This is a practical way of refining transformation products by refining prior austenite grain size to improve the strength, ductility and low temperature toughness of heavy-gage plate steel.

Caution regarding ambiguities in similar expressions of orientation relationships

The orientation relationship (OR) between two phases is an essential parameter to characterize microstructure in multiphase crystalline materials. Among the numerous ways to define an OR, the expression in terms of the parallelism between lattice planes and between directions from different phases, referred to as the {hkl}/〈uvw〉 expression, is the most commonly used. However, this expression scheme may be misleading because completely different ORs could be mistakenly regarded as equivalent ORs owing to the similarity of the expressions. In this article, the symmetries of the parallel elements were analyzed to clarify the conditions causing three cases. Namely, two similar expressions may describe two different ORs, two variants of a unique OR or exactly the same OR. This analysis helps not only to avoid ambiguities caused by the {hkl}/〈uvw〉 expression but also to specify how variants are defined with respect to a different matrix phase. Based on the result of the analysis, the expressions of variants for the Kurdjumow–Sachs and Nishiyama–Wassermann ORs between face-centred cubic and body-centred cubic systems for different matrix phases are provided. Examples are given together with stereographic projections to demonstrate different cases.