Electron Microscope Study of the Tempered Martensite and the Bainite in 5% Ni-0.5 ro Mo Steels

Electron microscopy were made to examine the structural features of the tempered martensite and the isothermally transformed bainite produced in 5%Ni-0.5%Mo steels with the following varibles; austenite grain size (fine and coarse) and carbon content (0.15 and 0.30%).The tempered martensite and the lower bainite were found to be composed of the lath packet of single orientation the lath packet of mixed orientation and the large martensite or bainite. Lowering of carbon content or coarsening of austenite grain size was found to cause the increase of lath width in the martensites. While the characteristic side-by-side growth of bainite lath was found to be dominant mode in the lower and upper bainite transformed from the fine-grained austenite of 0.15%C steel, the lower bainite from the fine-grained austenite of 0.3%C steel was composed of lath bundles randomly nucleated and the upper bainite were granular. The lower and upper bainites from coarse-grained austenites were considerably different from those from fine-grained austenites; the initial austenite grains were partitioned by the networks of straight lath packets which enclosed several massive bainite. The transition from lower bainite to upper bainite, which occurred at 350°C for those of 0.3%C steel and 400°C for those of 0.15%C steel, caused a drastic increase in both lath width and carbides size.