Static and dynamic fracture mechanics parameters of material in the heat-affected-zone

Abstract

The behaviour of heat-affected-zone (HAZ) in welded joints of structural steels with different chemical composition and different strength class (yield strength ranged from 450 to 700 MPa) has been evaluated, applying two approaches. In the first approach two types of specimens were used, one taken from simulated different microstructural regions in HAZ, and the other from the real welded joint with crack tip positioned in different microstructural regions. Impact toughness was determined by instrumented Charpy test and different fracture mechanics parameters were tested including single specimen method applied to measure J integral. The results of simulated specimens testing indicate that M/A microconstituents significantly reduce toughness and crack resistance in HAZ. The results obtained on specimens made from the real welded joint indicate higher crack resistance, due to multi-pass welding procedure and to the size effect as well, since the brittle microstructure region containing M/A microconstituents are surrounded by regions of higher toughness, capable to arrest crack growth. In the second approach the crack properties of parent metal, weld metal and HAZ in real welded joint are compared. Superior properties in HAZ compared to weld metal can be contributed to the high quality of parent metal, but also this can be an indication that crack tip in HAZ was in more ductile, and not in brittle microstructure. Extension of investigation can be achieved by numerical method, as it is proposed by J integral model for HAZ.