Microstructure and mechanical property evolution of post weld heat treated bainitic steel welds

Abstract

The achievement of excellent combination of high tensile strength and good ductility has been the key factor that facilitates the Advanced High Strength Steels (AHSS) to be the candidate materials for automotive applications. Carbide free bainitic steels are a class of AHSS that primarily have microstructure consisting blend of fine bainitic laths and interwoven retained austenite in the interlath boundaries of bainite. Solid state welding techniques are regarded to be the most sought after fabrication processes as they result in retention of basic microstructural constituents, especially nonequilibrium phases like bainite. Keeping this fact in view, friction stir welding was carried out on 3 mm thick carbide free bainitic steel plate with varying tool rotational speeds. The temperatures of stir zones were recorded. The microstructural changes and phase analysis from base metal (BM) to Thermomechancally Affected Zone (TMAZ) and Stir Zone (SZ) were studied. The microhardness profiling was done across the weld covering the BM, TMAZ and SZ. The abrupt increase in the hardness of stir zone and microstructure stability i.e., retention of bainitic microstructure along with the retained austenite were observed. The tool rotational speed of 150 RPM has been optimized. Post weld heat treatments were undertaken at different temperatures and increased soaking time on the welded samples of optimized rotational speed to facilitate bringing down the stir zone hardness to the base metal. Basing the phase analysis and EBSD data obtained, the post weld heat treatment cycle conducted at 500OC with a soaking time of 1 h has been optimized.