Effect of austempering time on bainite plate thickness and variant selection in a high carbon low alloy steel

Abstract

The study investigates the microstructure and crystallographic orientation relationship in a high carbon nanostructured bainitic steel that is austempered at 250 degrees C for different durations. The resulting microstructures are examined using optical, scanning, and transmission electron microscopy (TEM). Quantitative and qualitative analyses of dislocation density are performed using X-ray diffraction (XRD) and electron-backscattered diffraction (EBSD) respectively. The study finds that longer austempering time reduces the dislocation density because of slowing down transformation kinetics due to the enrichment of carbon concentration to a level corresponding to T0 ' temperature in the retained austenite phase. The bainite plate thickness in the longer-duration sample shows abnormal growth rate at the edges of the sheaf. The abnormal thickening rate is attributed to the annihilation of dislocations after prolonged austempering because of slow kinetics. The thickness of the bainitic plate is predicted using an available empirical relation and critically compared with experimentally measured values. It is found that the empirical equation used to compute thickness overestimates the plate thickness due to the omission of factors such as dislocation density and austenite block size. The crystallographic orientation relationship between bainite and austenite is analyzed locally by TEM and over a broader area by EBSD. The results reveal that both the Kurdjumov-Sachs and Nishiyama-Wasserman relationships are present in the material in equal fractions and some specific variants are formed in the sample subjected to longer austempering.