Effect Of Austenitizing Temperature On Bainite Transformation In The Fe-0.88C-1.45Si-0.95Cr-0.37Mn Steel

It was studied that the influence of the microstructures and mechanical properties of bainite obtained austenitizing in different temperature in Fe-0.88C-1.45Si-0.95Cr-0.37Mn experimental steel by observation of optical and transmission electron microscope, X-ray diffraction analysis, and measurement of hardness, impact toughness and tensile strength.The experimental results indicate that the lath-bunchiness upper bainite forms after austenitizing at lower temperature which is close to Acm and isothermal holding at 300℃, but only acicular low bainite forms after austenitizing at a far higher temperature than Acm and isothermal holding at 300℃. This result shows that when the austenitizing temperature is close to Acm, upper bainite transformation is still outstanding, even isothermal holding at low bainite transformation region. The upper bainite is of expanded lattice, formed under austenitizing at lower temperature (850℃or 880℃) which is close to Acm and keep at 300℃within short holding time (less than 20min), and also low bainite is of expanded lattice, formed under austenitizing at higher temperature (950℃) which is far away from Acm and isothermal heating at 300℃within short holding time (less than 20min). The degree of lattice expansion of low bainite is higher than that of upper bainite, and lower than that of martensite.The observation of fine microstructures shows that the upper and low bainite formed in the early stage of bainite transformation, such as isothermal heating at 300℃for 20min after austenitizing, are of the similar lath morphologies. The lath composes of ferrite super-saturated with carbon, and the sub-structures of bainite ferrite are of dislocations. So those bainites are called atypical upper bainite and low bainite. The intersection among the atypical low bainites leads to the change of the boundary of bainite obviously, but there is no obvious change in the upper bainite. The experimental results show that the growth of atypical bainite is of the shear transformation characteristic outstandingly, but this characteristic in upper bainite is weak. So the transformation of atypical bainite in the experimental steel cannot be explained by martensite transformation mechanism totally.The atypical upper bainites provide to material higher toughness. But comprehensively considered toughness and strength, the atypical low bainites provide to material excellent comprehensive mechanical properties.