Tempering cooling rate plays a significant role in the impact toughness of 2CrMoV weld metal. Three different tempering cooling rate experiments were carried out; it is found that the impact toughness of weld metal improved from 44.61 to 117.49 J as the cooling rate increased from 5 to 40 °C/h. Microstructure characterization revealed that the large blocky M–A constituents and cluster precipitation were considered to act as stress concentration sources and cleavage fracture initiators at a cooling rate of 5 °C/h. Under the cooling rate of 20 °C/h, the decrease of blocky M–A constituents as well as homogeneous distribution of precipitation induced the transition from cleavage to interfacial decohesion. The chance of crack propagation in intragranular ferrite matrix was increased, which needed to absorb more energy and improve impact toughness. When the tempering cooling rate reached at 40 °C/h, the cracks mainly propagated in the ferrite matrix; meanwhile, fine and homogeneous distribution of precipitation greatly inhibited crack propagation and led to higher impact toughness.

This paper details an investigation into the effect of martensite–austenite (M–A) constituent on impact toughness of intercritical heat-affected zone (IC HAZ) in Q690 steel, a low carbon bainitic steel. Large samples with uniform microstructure similar to that of the actual IC HAZ were achieved through combinations of heat treatments. The samples were heated to temperature between 750 and 830 °C for 30 min and then quenched in water. After these heat treatments, hard martensite islands distribute along the boundaries of soft matrix, and the size of martensite islands increases with heating temperature. The microstructure is quite similar to that of the IC HAZ. Using these large samples, the impact toughness could be measured conveniently. It was found that the embrittlement of the IC HAZ depends on the deformability and size of the M–A constituent. When the size of the M–A constituent whose deformability is low exceeds a critical value (2.0 μm), the embrittlement of the IC HAZ occurs.