Development of Ultrafine Bainitic Structure in AISI 431 Stainless Steel

The development of ultrafine bainitic structure in AISI 431 stainless steel was the goal of this study. For this purpose, the AISI 431 specimens were austenitized at 1100 °C for 60 min followed by low-temperature austempering treatment at different temperatures and times. Austempered samples were characterized using field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD) and tension test. According to achieved results, the microstructure of AISI 431 steel after austempering treatment consists of ultrafine bainitic ferrite plates and retained austenite with two morphologies of micrometer-block and fine film. At austempering temperature up to 300 oC, the micrometer-blocky morphology of austenite was vanished completely from the microstructure. The maximum values of strength and ductility of about 1664 MPa and 9.5 % were achieved in austempered sample at 350°C for 24 h. By enhancement the austempering temperature and time, the strength and elongation percent were reduced simultaneously as a result of change in shape and size of bainite phase.