In Situ Observation of Austenite Grain Growth and Phase Transformation of A517GrQ Rack Steel for Jack-Up Offshore Platform

Abstract

Austenite grain size plays an important role for quenched and tempered A517 GrQ rack steel. In order to investigate the austenite grain growth behavior, in-situ austenizing experiments were conducted at different temperatures with varied holding time. Austenite grain growth, and subsequent bainite and martensite transformation were observed by a confocal laser scanning microscope (CLSM). The average austenite grain size increased rapidly from 32.7 μm to 137.9 μm with increasing temperature from 1100 °C to 1250 °C, and abnormal grain growth occurred at 1250 °C for 15 min. While holding from 5 min to 20 min at 1200 °C, the grain size grew gradually from 70.2 μm to 84.8 μm. The grain size distributions under different conditions all obey lognormal function. After austenizing, the rate of subsequent cooling has an significant impact on transformation process. The transformation duration time decreased from over 142 s to 11 s with increasing cooling rate at 1 °C/s and 5 °C/s, which could be corresponding to bainite and martensite transformation respectively.