مشخصات پژوهش

Conventional welding processes destroy the unique microstructural characteristics in nanobainitic steels. A novel method, i.e. tailored friction stir welding (FSW) and postheat treatment, was employed to overcome this problem. The postprocessing austempering was carried out on friction stir welded high-carbon steel to obtain nanobainite, and obtained microstructural characteristics were compared to that achieved by simple isothermal bainite transformation. Bainitic sheaves and austenite microblocks could be achieved in both production procedures. However, bainitic ferrites and austenite films within the bainitic sheaves were thinner, and austenite microblocks were finer when implementing the post-processing heat treatment after FSW. FSW enhanced the hardness of the primary steel from 385 HV to above 800 HV, which was maintained at about 520 HV even after post-heat treatment, higher than that of after implementing the isothermal bainite transformation (490 HV). This achievement can be related to FSW that reduced the mean diameter of austenite blocks, the thickness of bainitic ferrite, and the thickness of austenite films from 3820, 138, and 122 nm to 2200, 92, and 78 nm, respectively. Moreover, the microstructure and hardness of various microstructural zones of friction stir welded sample before and after heat-treatment were analyzed and discussed in detail. The outcome of this study can be applied in academic and industrial areas to propose a cost-effective method to weld nanobainitic steels.