Refinement of Steel Austenite Grain Under an Extremely High Degree of Superheating

There thermal cycles for the cyclic heat treatment of a bainitic steel were simulated under the different heating rate ranged from 2°C/s to 100°C/s and 1000°C/s followed by subsequent rapid cooling using a Gleeble3500 thermal-mechanical simulator equipped with the self-designed quench system. The microstructural change of the starting material imposed by the different thermal cycles was observed by optical and transmission electron microscope (TEM). The results indicate that a high degree of superheating for the reverse bainitic α→γ transformation was realized as a result of extremely rapid heating (1000°C/s) up to an elevated austenitisation temperature (1250°C), which prevent restoration of the initial coarse-grained austenite and lead to a significant refinement of austenite grain. An ultrafine-grained structure with enhanced mechanical properties then formed during the subsequent rapid γ→α reaction on cooling. The results indicate a novel microstructure refinement route for the bainitic steels.