Static and dynamic fracture toughness of a medium carbon microalloyed steel of three different microstructures

A multiphase ferrite-bainite-martensite (F-B-M) microstructure was developed in an automotive grade V-bearing medium carbon microalloyed steel, 38MnSiVS5. It was characterized using optical, scanning, and transmission electron microscopy. The tensile, Charpy impact, and static and dynamic fracture toughness behaviors were evaluated. The results are compared with those of ferrite-pearlite (F-P) and tempered martensite (T-M) microstructures of the same steel. Although the tensile properties of the multiphase microstructures were superior, the Charpy impact and static and dynamic fracture toughness properties were inferior compared with those of the other two microstructures. The F-P condition displayed the highest plane strain fracture toughness value (K_IC), while the T-M condition was characterized by the highest dynamic fracture toughness (conditional) value (K_IDQ). The Charpy impact energy of the T-M condition was greater than that for the other two conditions. An examination of the surfaces of fractured samples revealed predominant ductile crack growth in the F-P microstructure and a mixed mode (ductile and brittle) crack growth in the T-M and the F-B-M microstructures. Although the Charpy impact energy, plane fracture toughness (K_IC), and conditional dynamic fracture toughness (K_IDQ) of the multiphase microstructure were inferior to those of the T-M and the F-P microstructures, the toughness properties were comparable to those of medium carbon low alloy steels having bainite-martensite (AISI 4340) or tempered martensite microstructures.