Microstructure Characterisation of Bainitic HSLA Steel for Pressure Vessels

Abstract

The microstructure and mechanical properties in the midsection of a high thickness HSLA bainitic steel, was investigated in this master thesis. A homogeneous microstructure and good mechanical properties can be difficult to achieve in high thickness steels, due to slow cooling rates in the midsection. The different cooling rates at the surface and in the midsection of the steel can cause a difference in microstructure and mechanical properties through the thickness. The microstructure of the base material was investigated using EBSD and light microscopy. A homogeneous bainitic microstructure was detected. The mechan- ical properties were investigated using Vickers hardness measurements as well as longitudinal tensile tests. The mechanical properties showed little variation and were found to be in accordance with what is common for a bainitic microstructure. Three steel samples were heated in a dilatometer and cooled at different cooling rates in order to assess the microstructure and the mechanical properties within these cooling rates. The cooling rates were chosen to be 5◦C , 1◦C and 0,5◦C per minute. The results proved the formation of ferrite for all cooling rates and that a faster cooling rate was needed to achieve a homogeneous bainitic microstructure. A reduction in Vickers hardness with increasing ferrite content was also detected.