| The morphology and fracture mode of the bainite in a Fe-C-Mn high-strength hypereutectoid steel have been examined by means of optical, scanning electron, and transmission electron microscopy, and also the isothermal decomposition of a Fe-1.35% C-2.06% Mn austenite has been investigated in connection with its hardenability. It is shown that the bainite transformation has taken place both by columnar type growth along grain boundaries forming the grain boundary allotriomorphic form and by nodular type growing into the grain, which are distinctly different from the usual bainite appearances. It is also shown that there may be no transition temperature from upper to lower bainite. It is found by the fractographic study that fracture is predominantly transcrystalline showing cleavage facet. But, there are the small amount of quasi-cleavage and dimple fracture in bainite phases transformed at lower temperature, and also the fracture unit in them is relatively finer. It seems the inferiority of fracture ductility in this steel is attributed to proeutectoid cementites along austenite boundaries or a Widmansta¨tten type of cementites grown into bainite grains, which can be removed by the proper heat treatment alternative, rather than to its peculiar bainite. |
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