Controlling the formation of reverted globular austenite and the as-transformed austenite grain size in low-alloy steel based on cementite

Austenite reversion has been widely used in the traditional heat treatment of steels, and recently, it has been used in the fabrication of advanced high-strength steels. The microstructure of reverted austenite significantly influences the final microstructure and properties of steel; thus, it is crucial to understand the formation of globular austenite to accurately grasp its reversion behavior. In this paper, an Fe–2.5Mn–1.5Si–0.35C alloy was chosen as the research object, and the evolution of intragranular globular austenite and finally transformed austenite grain size were studied under different pre-tempering conditions using a metallographic optical microscope, scanning electron microscope, and electron backscatter diffraction. It was found that as the pre-tempering temperature was increased from 350 ℃ to 650 ℃, the volume fraction of intragranular globular austenite first increased and then rapidly decreased. At the pre-tempering temperature of 400 ℃, the volume fraction of intragranular globular austenite initially increased and remained stable thereafter, when the pre-tempering duration was increased from 1 to 10 h. Fine cementite particles were primarily formed immediately before the reversion in the non-tempered or low-temperature pre-tempered initial structures. This provided less effective nucleation sites for the formation of intragranular globular austenite. Therefore, lesser intragranular globular austenite grains were formed, thereby resulting in relatively coarse finally transformed austenite grains after reversion. The cementite particles were gradually coarsened as the pre-tempering temperature was increased to 550 ℃, thereby increasing the number of effective nucleation sites for the formation of intragranular globular austenite. Conversely, when the martensite samples were pre-tempered at a high temperature of 650 ℃, Mn is seriously enriched into the cementite particles before the reversion, largely reducing the driving force for reversion. This resulted in the growth of intragranular globular austenite under the partitioning local equilibrium mode, with a slow growth rate, resulting in a low volume fraction. Therefore pre-tempering can effectively promote the formation of intragranular globular austenite. Owing to its multiple orientations, increased intragranular globular austenite formation resulted in significantly refined austenite grains after reversion. This study provided a new strategy to regulate the formation of intragranular globular austenite and finally transformed austenite grain size by controlling the size and composition of cementite particles through pre-tempering without changing the chemical composition of the steel.