MICROSTRUCTURE AND LOCAL BRITTLE ZONE PHENOMENA IN HIGH-STRENGTH LOW-ALLOY STEEL WELDS

Abstract

This study is concerned with a correlation between the microstructure and the local brittle zone (LBZ) phenomena in high-strength low-alloy (HSLA) steel welds. The influence of the LBZ on toughness was investigated by means of simulated heat-affected zone (HAZ) tests as well as welded joint tests. Micromechanical processes involved in microvoid and cleavage micro-crack formation were also identified using notched round tensile tests and subsequent scanning electron microscopy (SEM) analyses. The LBZ in the HAZ of a multipass welded joint is the intercritically reheated coarse-grained HAZ whose properties are strongly influenced by metallurgical factors such as an effective grain size and high-carbon martensitic islands. The experimental results indicated that Charpy energy was found to decrease monotonically with increasing the amount of martensitic islands, confirming that the martensitic island is the major microstructural factor controlling the HAZ toughness. In addition, microvoids and microcracks were found to initiate at the interface between the martensitic island and the ferrite matrix, thereby causing the reduction in toughness. These findings suggest that the LBZ phenomena in the coarse-grained HAZ can be explained by the morphology and the amount of martensitic islands.