Strengthening and Toughening Mechanisms in Martensitic Steel

Low carbon martensitic steels are often produced by reaustenitizing and quenching (RA/Q). Direct quenching (DQ) has gained interest in the past few decades and requires quenching immediately after working above or below the austenite recrystallization temperature to form martensitic microstructures. In the current study, microalloyed ASTM A514 steel is used to produce martensite from either equiaxed or pancaked prior austenite grain (PAG) microstructures. The equiaxed PAG conditions simulate microstructures produced by RA/Q and the pancaked PAG conditions simulate microstructures produced by controlled rolling (CR) before DQ. Controlled rolling followed by DQ was simulated with double hit compression in a Gleeble® 3500. The prior austenite grain size (PAGS) was varied between 9 and 75 μm prior to controlled rolling. The strengthening and toughening mechanisms are being investigated in the as-quenched (AsQ), low temperature tempered (LTT: 200 °C), and high temperature tempered (HTT: 600 °C) conditions. The equiaxed PAG condition has a Hall-Petch (H-P) relationship between yield strength (or microhardness) and PAGS in the AsQ condition. There is not a H-P relationship between PAGS and microhardness in the CR-DQ conditions. The CR-DQ conditions generally exhibit higher microhardness than the RA/Q conditions with similar PAGS, with the most significant differences in the larger PAGS conditions. Toughness was only measured in the equiaxed PAG conditions. The smallest PAGS has the lowest ductile-to-brittle transition temperature (DBTT) with the highest strength in the AsQ and LTT conditions. The smallest PAGS has the lowest DBTT and the lowest strength in the HTT condition.