Morphology and formation mechanism of bainite in chromium-molybdenum steel

The morphology of bainite in 12CrMo,20CrMo,35CrMo and 9CrMo steel was observed using metallographic microscope and QUANTA-400 environmental scanning electron microscope after the samples were austenitized and then isothermally quenched at intermediate temperature zones respectively.The forming mechanism of bainite was investigated.The results show that massive or lamellar carbide-free bainite is obtained in the low carbon chromium-molybdenum steel,carbide-free upper bainite and lower bainite containing carbide are observed in the medium carbon chromium-molybdenum steel,while feather-shaped upper bainite and bamboo-leaf-shaped lower bainite form in the high carbon steel.It suggests a significant effect of carbon content on the morphology of bainite.It is considered that carbon-free bainite comes into being if the bainite ferrite laths can not combine together while with the the increase of carbon content,feather-shaped upper bainite and bamboo-leaf-shaped lower bainite will form when the carbide separates out.