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 degrees C and 400 degrees 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 similar to 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.

Automated summary

The novel use of double austempering treatments in a multiphase steel has been studied to refine and homogenize the microstructure and improve material strength. The two-step heat treatments effectively reduced blocky-type austenite and fresh martensite while retaining a high volume fraction of retained austenite. The microstructural characteristics, including reduced bainitic plate thickness and film-like retained austenite, led to a significant increase in hardness (50 HV) and yield strength (180 MPa).