Effect of Carbon Content on Bainite Microstructure of Chromium-Molybdenum Steels

The microstructure evolution of bainite of ultra-low carbon and high purity steel,12CrMo,20CrMo,35CrMo,9Cr2Mo and GCr15 steels,was studied by QUANTA-400 electronic microscope after their medium temperature phase transformation took place during the quenching.The results show that,with increase of the carbon content,the medium temperature transformation of overcooled austenite in the steels were as follows: transformation from massive microstructure to low-carbon,medium-carbon and carbon-free bainites,and further to high-carbon feathery upper bainite and bamboo-leaf-shaped lower bainite,which indicated that the carbon content of steel had great effect on the morphology of bainite.In the meantime,the mechanism of the effect of carbon content on the microstructure of bainite was investigated.The main points of view are described as follow:(1) There is a affine relationship between massive transformation and bainite transformation.In bainite transformation,carbon-poor area forms by way of composition fluctuation and bainitie ferrites nucleate in carbon-poor austenite.Bainite transformation has no essential difference with γ→α transformation in massive transformation and belongs to incoordinate transformation without diffusion.The interface moves by means of thermal activation transition of atoms and new phase can grow continuously.(2) Carbon-free bainite will come into being when ferrite pieces do not mix together because of carbon-rich austenite film left between bainite ferrite pieces with increment of the carbon content.Fea-thery upper bainite will form if carbide precipitates from carbon-rich austenite between bainite ferrite pieces.Bamboo-leaf-shaped lower bainite will be obtained if carbide precipitates from phase interface of BF/γ so that bainite ferrites are divided into many fine subunits.