Fracture toughness of cold worked and simulated heat affected structural steel

Abstract

Deterioration of fracture toughness due to both cold working and welding are studied to investigate the combined effect which is seen in the welded part of cold worked members. It is known that fracture toughness is degraded in cold-worked steels by strain aging and is also degraded in the heat affected zone (HAZ). In this experimental study, CTOD tests are carried out using simulated HAZ specimens which are pre-strained before heat treatment. Although multi-pass welding produces a complex distribution of microstructures in HAZ, two types of thermal cycles are used to simulate the HAZ. One is to simulate intercritically reheated coarse grain HAZ (IRCG HAZ), the most degraded part in the local brittle zone. Another is to simulate subcritical HAZ (SC HAZ), the least degraded part in HAZ. In these two types of simulated microstructures, the effects of cold working are discussed based on fracture mechanics. It can be observed that the fracture toughness in IRCG HAZ is fairly low and the variation of fracture toughness with increasing pre-strain does not exist in that particular region. The fracture toughness in SC HAZ is not so crucial as seen in IRCG HAZ, but is strongly affected by pre-strain before heat treatment.