Novel Heat Treatment Of Advanced High-strength Good Toughness Roller Steels

The aim of the present dissertation is to investigate the formation, microstructures,and mechanical properties of conditional bainitic transformation and multi-stepsisothermal bainitic transformation in60CrNiMo steel. The microstructural morphologywas studied by optical microscope，scanning electron microscope, and transmissionelectron microscope, quantitative X-ray analysis was used to determine the volumefraction of retained austenite．In addition，mechanical properties were also measured．The experimental results showed that the amount of bainite increased, and thecontour became clear with the transformation time increasing during the bainitictransformation in250℃to350℃, but the hardness deceased. At350℃, with thetransformation time increased from2h to8h, the quantity of bainite increased untilfinally there were only bainite and retained austenite, the hardness fell from381HV1to365HV1. Under the condition of the same time4h, the bainitic plate became more tiny,the amount of retained austenite decreased from36.1%to28.6%, meantime, thehardness increased from378HV1to560HV1while the content of retained austenitedecreased.This article used60CrNiMo steel to do the experiment in order to eliminate theblocky retained austenite. The processes were after austenitization at850°C, sample1was isothermally transformed at300°C for6hrs. Sample2was isothermallytransformed at300°C for0.5hrs, then was immediately moved to another furnacewhere it was kept at250°C for24h. The first and second thermal cycles of Sample3were identical to those of Sample2but it had a third stage where it was kept at200°Cfor50hrs. The results showed that in the multi-step bainitic transformation, the firststep was essentially a conventional bainite transformation to obtain a partiallytransformed bainitic microstructure. The following steps were carried out immediatelyafter the first step in order to transform the retained austenite to nano-structured bainite.The average plate size decreased, the blocky retained austenite was refined. The hardness increased from456HV1to539HV1, and then increased to600HV1. Theimpact energy increased from79J to83J, the tensile strength increased from1195MPato1230MPa. Thus, the multi-steps bainitic transformation could changed the blockyretained austenite to thin film retained austenite, the strength increased while theductility and the toughness improved significantly.