Effects of M‐A constituent on fracture behaviour of weld HAZs: Deterioration and improvement of HAZ toughness in 780 and 980 MPa class HSLA steels welded with high heat input (5th report)

Summary

The effects of M‐A (martensite‐austenite) constituent on the initiation and propagation of cracks have been investigated by observation of the fracture morphology and cracks around the M‐A constituent. Crack initiation and propagation energies were estimated using an instrumented Charpy impact test machine. The concentration and distribution of the tensile plastic deformation introduced into the soft phase around the hard constituent were also qualitatively analysed by the finite element method (FEM) to explain the effect of the M‐A on crack initiation and propagation. The results show a sharp decrease in the crack initiation and propagation energies with increasing area fraction of the massive M‐A constituent. The FEM analysis suggests that the greater amount of hard phase reduces the crack initiation energy in the surrounding soft phase, which explains the observed decrease in the crack initiation energy with increasing area fraction of the massive M‐A constituent. It was also shown that the decrease in the crack propagation energy is due to an increase in the interfacial area between the massive M‐A constituent and matrix, since the crack was observed to propagate preferentially along the interface between the M‐A constituent and matrix.