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Abstract: The microstructure and the strength of the lath martensite in Fe–0.2C and Fe–0.2C–2Mn alloys were analyzed as a function of the prior austenite grain size. The size of martensite packets formed within individual austenite grains was controlled by the austenite grain size but not affected by the Mn addition. However, the further subdivision of packets into blocks differed significantly in the two alloys, and at a given austenite grain size a smaller block size was observed in the Mn containing alloy. The yield strength of the two alloys was related to the packet size and the block size, respectively, and the results suggested that the block size is the key structural parameter when analyzing the strength–structure relationship of lath martensite in low carbon steels. [Copyright &y& Elsevier]

Abstract: The upper bainite structures were investigated in Fe–9Ni–(0.15–0.5)C (mass%) alloys transformed at temperatures between 723 and 623K. A bainite packet similar to lath martensite is formed and is partitioned into blocks containing lath-shaped bainitic ferrite with the same parallel close-packed plane relationship of the K–S relationship. Its feature is categorized into three types: (A) a packet contains laths of two K–S variants with a small misorientation (sub-blocks), (B) a packet is divided by blocks largely misoriented and each block contains sub-blocks and (C) a packet is divided by blocks containing a single variant of laths. A packet of type (A) is formed at 723K in each alloy whereas the packet type becomes (B) or (C) at lower temperatures. As a result, bainite blocks are refined with decreases in transformation temperature and carbon content. In contrast, blocks and packets of lath martensite are refined with increasing carbon content. [Copyright &y& Elsevier]

Abstract: The change in morphology and substructure of α′ martensite with a volume change accompanying austenite (fcc) to martensite (bcc) transformation was studied using various Fe–Ni–Co alloys. The volume change accompanying martensitic transformation was decreased by the Invar effect of austenite with Co addition to the Fe–Ni binary alloys. In Fe–28Ni–20Co and Fe–24Ni–30Co (mass%) alloys, small and negative volume changes were established (−0.30% and −0.39%, respectively). With a decrease in volume change, the transition temperature of martensite morphology from lenticular to lath increases. The dislocation densities in martensite and austenite decrease as the volume change approaches zero. [Copyright &y& Elsevier]

Abstract: The effect of ausforming on formation of upper bainite structure was investigated in Fe–9Ni–(0.15–0.5)C (mass%) alloys, focusing on the crystallographic feature of bainitic ferrite. The specimens were ausformed at 1073 or 873K by 50% compressive reduction after austenitizing, and transformed at 623K. Lath-shaped upper bainite structures similar to lath martensite are formed at 623K. A bainite packet is partitioned into blocks of all the six variants of the K–S orientation relationship satisfying the same parallel close-packed plane relationship. In both of the non-ausformed and ausformed specimens, bainite blocks are coarsened with increasing carbon content. In the Fe–9Ni–0.15C alloy, the effect of ausforming on the bainite block width is small. For higher carbon alloys (Fe–9Ni–0.3C and 0.5C), bainite blocks are remarkably coarsened by the ausforming. In contrast, the ausforming refines lath martensite blocks in the Fe–9Ni–0.15C alloy but has a small effect in higher carbon alloys. [Copyright &y& Elsevier]