Crystallography of upper bainite in Fe-Ni-C alloys

The upper bainite structures were investigated in Fe-9Ni-(0.15-0.5)C (mass%) alloys transformed at temperatures between 723 and 623 K. 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 723 K 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. (c) 2006 Elsevier B.V. All rights reserved.