Effect of Tempforming Temperature on the Impact Toughness of an HSLA Steel

The fracture behavior of a high-strength low-alloy steel subjected to tempforming at 873 K, 923 K, or 973 K was studied by means of impact and bending tests. A decrease in tempforming temperature promoted the grain refinement resulting in the ultrafine grained lamellar-type microstructure with finely dispersed particles that led to significant strengthening along with an increase in the impact toughness. The tempformed steel samples exhibited Charpy V-notch impact energy well above 100 J at temperatures of 183-293 K due to delamination along the rolling plane during bending, which was attributed to high anisotropy of cleavage fracture stress. Delamination owing to easy cleavage crosswise to the impact direction blunted the primary crack and resulted in the zigzag crack propagation, leading to high impact toughness. Depending on tempforming temperature, three types of the delamination behavior were observed in the V-notch specimens upon three-point bending tests at room temperature. Namely, the early, restrained, and late delaminations took place in the samples after tempering at 873 K, 923 K, and 973 K, respectively. On the one hand, a decrease in the test temperature promoted delamination, and on the other the strengthening by lowering tempforming temperature is accompanied by a suppression of ductile fracture. The sample tempformed at 873 K exhibited the highest impact toughness at room temperature, whereas the samples tempformed at 923-973 K were characterized by the higher impact toughness at 183-233 K.