Study on JIC Fracture Toughness in the Weld Heat-Affected Zone of a Structure Steel

Abstract

The fracture toughness in a simulated weld heat-affected zone (HAZ) of the high tensile strength steel (HT80) were evaluated through JIC values using a Charpy-size three point bending specimen at a temperature range between 18°C and -150°C.The effect of the microstructure on crack initiation was investigated by a fractographic method using the scanning electron microscope (SEM). In the tempreature range from about -50°C to 18°C, for the base metal and HAZ whose cooling time from 800 to 500°C (Tc) was 17 sec, the JIC values increased with decreasing test temperature. The SEM examination showed that the area fraction of martensite-austenite constituent (M-A) in the microstructures mentioned above was less than 4 %. The fracture occurred by stable crack extension after the formation of the stretched zone at the precrack tip. On the otherr hand, the JIC values decreased with decreasing test temperature on HAZ microstructures whose Tc were 89 and 223 sec. In these microstructures, which had more than 8 % of area fraction of M-A, cleavage fracture occurred immediately after the formation of the stretched zone. J_IC values in the microstructures which included more than 8% of area fraction of M-A were nearly a quarter of the one in the microstructures whose M-A were less than 4%, at about -50°C.
The critical stretched zone width (SZW_C) and J_IC were found to be related as follows:
J_IC=α⋅σ_ySZW_C
where -σ_y is the yield stress of steel and α is an experimental constant which was found to be 8.5.