Effect of tempering on microstructure and mechanical properties of a non-quenched bainitic steel

The effect of tempering on the microstructure and mechanical properties of a non-quenched (NQ) bainitic steel was investigated by optical microscopy, X-ray diffractometry, scanning electronic microscope (SEM) and transmission electronic microscopy (TEM). The results show that the NQ steel which has been investigated is granular bainite composed of bainitic ferrite lath, retained austenite film and island of austenite and martensite (M/A island) before tempering. The amount of retained austenite decreases with the rise of tempering, slowly before 400 deg. C, sharply at 450 deg. C, and it is close to 0 at 600 deg. C. When tempered at 350 deg. C, this kind of NQ steel has the optimum mechanical properties because of M/A islands partly decomposition, especially for the martensite in the M/A islands. However, when tempered at 450 deg. C, it appears brittleness, which results from carbides distributing along prior austenite grains because M/A island is largely decomposed.