Microstructure and properties of simulated heat affected zone of Q690D low alloy high strength steel

Gleeble-3500 thermal simulator was used to simulate the welding heat of Q690D low alloy high strength steel, and the microstructure of the heat-affected zone under different peak temperatures and cooling times during the first and second welding thermal cycles was obtained. The microstructure observation, hardness test, impact property test and fracture morphology analysis were carried out. The result showed that in the first welding thermal cycle, with the increase of the peak temperature of the welding thermal cycle, microstructure of samples became gradually coarsened, the granular bainite transformed to upper bainite and lath martensite, the hardness increased and the impact performance deteriorated. When the thermal cycle peak temperature was 900 ℃, the maximum impact energy absorption was 78.95 J. When the peak temperature was 1 350 ℃, the minimum impact energy absorption was only 17 J. The impact fracture changed from ductile fracture to dissociative fracture. At the same peak temperature, as the cooling time t8/5 increased, the hardness of the sample decreased and the impact performance deteriorated. In the second welding thermal cycle, microstructure of samples was coarse grain, mainly lath martensite, the hardness was higher and the impact performance continued to deteriorate. The lowest value of impact energy absorption was only 24.99 J, and the impact fracture was mainly cleavage fracture and quasi cleavage fracture, which indicated that the performance of samples deteriorated due to the second welding thermal cycle.Highlights: There were few researches on the microstructure and properties of HAZ of Q690D low alloy high strength steel, especially multi-layer and multi-pass welding, which would lead to more complex microstructure and properties. In this paper, the method of welding thermal simulation was used to study the microstructure and properties of HAZ of Q690D steel.