Study On Growth Behavior Of Sheave-like Bainite In The High-Carbon Silicon Steels

This paper has investigated the effect of austenitizing temperature on the transition startpoint, microstructure, and growth behavior of sheave-like bainite ferrite in the new designedhigh-carbon low-alloy silicon-containing steel isothermally transformed at200-300℃forshort time after austenitization at880-1000℃, by means of X-Ray Diffraction, metallurgicalmicroscopy, and transmission electron microscopy.The results show that the fast bainite is generated in the form of sheaves duringlow-temperature isothermal transformation of the high-carbon steel,which can be finishedonly in about30min with a bainitic ferrite volume fraction of80%, and anincomplete-reaction phenomenon. The microstructure evolutions of upper and lower bainitein present study are similarly consistent, known from conventional bainite. The typicalmicrostructure of sheave-like bainite gives a most elegant fine scale of an intimate mixtureof austenite films and slender ferrite plates with a width of just30-50nm and a length in μmscale. These ferrite plates are carbon-supersaturated and lattice-distorted, indicating thecharacteristics of diffusionless transformation. The isothermal cooling curve of theexperiment steel gradually moves to the lower left, with increasing the austenitizingtemperature, which leads to a gradual increase of austenite grain size and is responsible forthe larger length of bainite ferrite. The bainitic ferrite in the sample corresponding toaustenitization at980℃has the best bundle phenomenon with high density, hardness of62-64HRC, or in excess of800HV, and excellent impact toughness.Bainitic ferrite nuclears in different forms at the austenite grain boundaries or defects inaustenitic grain. The sheave-like bainite can only grow inside the austenite grains, and noevidence has been found that bainite plates traverse the austenite grain boundaries. Finestructure observation shows that every ferrite plate in the bainite sheave is composed ofseveral paralleling ferrite platelets with the same orientation relationship, and each platelet iscomposed of several conterminous sub-platelets with high-density of dislocation. Thickeningof the bainite ferrite plate is a consequence of new platelets nucleation and growth by shearmechanism at sides of the already formed platelets. Due to the yield strength of austenite,thicken of platelets has a limited side, and total number of which is also limited to more than adozen. Lengthening of the bainite ferrite plate can be regarded as a consequence of newplatelets nucleation and growth by shear mechanism at leading tip of the already formedplatelets.