DICTRA Simulation of Holding Time Dependence of NbC Size and Experimental Study of Effect of NbC on Austenite Grain Growth

Abstract

The effect of austenitizing temperature and holding time on the prior austenite grain size was examined in both the transverse and the longitudinal directions of the samples that were made of the offshore structure steel EQ70. The grain size of the prior austenite was measured by using the linear intercept method. The equilibrium phase diagram was used to explain the abnormal grain growth. The equilibrium precipitation of steel EQ70 was calculated by Thermo-Calc software package, and the relationship between NbC size and the holding time was simulated based on DICTRA. The experimental results show that the prior austenite grain size is initially insensitive to increasing the austenitizing temperature from 1123 K (850 °C) with holding times from 1 to 6 hours, and presents a sudden growth at approximately 1373 K (1100 °C). The growth of the austenite grain size is also insensitive to increasing the holding time while the soaking temperature is lower than 1223 K (950 °C) or higher than 1373 K (1100 °C), and a sudden growth of grains takes place as the holding time is prolonged from 4 to 5 hours at the temperatures between 1273 K and 1323 K (1000 °C and 1050 °C). The results of DICTRA simulation and TEM observation confirm that the abnormal grain growth behavior at 1373 K (1100 °C) was influenced by coarsening of NbC with radius larger than the average and full dissolution of AlN and almost full dissolution of NbC with radius equal to or less than the average, while the same behavior between 1223 K and 1273 K (950 °C and 1000 °C) was caused by coarsening of NbC with radius larger than the average and the full dissolution of AlN but partial dissolution of NbC with radius equal to or less than the average. The present experimental and simulation results can provide a useful reference for determining the austenitizing parameters of steel EQ70.