Bainite plate thickness reduction and microstructure tailoring by double austempering of Al-rich 3Mn steel

The novel use of double austempering treatments in a multiphase steel to refine and homogenize the final microstructure and thus improve the material strength have been studied in the 3.3Mn-0.17C-1.6Al-0.23Mo-0.22Si alloy. The microstructural features developed after conventional isothermal austempering treatments at 450 °C and 400 °C were compared with those obtained after two-step heat treatments. These treatments consisted of a first isothermal holding at a temperature slightly above the initial M_s, that was interrupted at 25 and 50% of transformation, followed by a second stage treatment at a lower temperature to complete the bainitic transformation. One- and two-step treatments were performed in a high-resolution dilatometer, and the critical transformation temperatures and phase transformation kinetics were determined from the longitudinal changes recorded during these tests. It was shown that blocky-type austenite was almost completely eliminated after the two-step treatments, which in turn positively reduced the amount of fresh martensite from ∼6 to «1%. It was possible to keep the volume fraction of retained austenite above 10%, while reducing both the thickness of the bainitic plates and the film-like retained austenite by 20% and more than 40%, respectively. These microstructural characteristics made it possible to increase the hardness of the alloy by approximately 50 HV and yield strength by 180 MPa.