Crystallography of ausformed upper bainite structure in Fe-9Ni-C alloys

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 873 K by 50% compressive reduction after austenitizing, and transformed at 623 K. Lath-shaped upper bainite structures similar to lath martensite are formed at 623 K. 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.