Effect of M-A Constituent on Fracture Behavior of 780 and 980MPa Class HSLA Steels Subjected to Weld HAZ Thermal Cycles(Materials, Metallurgy & Weldability)

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In order to explain the embrittlement of 780 and 980MPa class HSLA steels undergoing high heat input weld thermal cycle, the effects of the M-A constituent on the initiation and propagation of fracture have been investigated metallographically and theoretically. The M-A constituent was observed both at initiation sites of cracks in the cleavage fracture and at the bottom of dimples in the ductile fracture. This suggests that the M-A constituent acts as an initiation site for both cleavage fracture and dimple fracture. FEM analyses of strain in the α phase involving M-A constituents show that under a tensile load the concentration of tensileplastic strain occurs over a wider area around massive M-A constituents than around elongated ones having the same length. This can explain the result from the instrumented-Charpy impact test that the massive M-A constituent reduced the fracture-initiation energy more significantly than the elongated. The instrumented-Charpy impact test also suggested that the fracture-initiation and propagation energies were lowered more significantly with an increase in the massive M-A constituent. Since cracks in the Charpy impact test were observed to propagate preferentially along the M-A constituent/matrix interface, it can be considered that the increase in the area of the interface is responsible for the decrease in the initiation and propagation energy of fracture because of the increase in the amount of the massive M-A constituent.
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Effect of M-A Constituent on Fracture Behavior of 780 and 980MPa Class HSLA Steels Subjected to Weld HAZ Thermal Cycles(Materials, Metallurgy & Weldability)

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