Welding thermal cycle effects of S960 steel butt-welded joints: Microstructure and hardness

This research focuses on predicting the microstructural evolution and hardness distribution in the heat-affected-zone (HAZ) of S960 steel butt-welded joints. Robotic butt-welding was conducted for S960 steel plates, and the corresponding WTCs were recorded using thermocouples and numerically simulated. Vickers hardness tests were employed to assess the hardness distribution of the HAZ. The grain sizes and phase volume fractions of HSS HAZ were then characterized by the SEM and EBSD. It is observed that with increasing distance from the weld fusion line, the WTC cooling time ( t 8/5 ) shows a slow rise but acceleratingly increases when the peak temperature ( T max ) is close to the austenization-finish temperature (Ac 3 ). The fine-grained HAZ with microstructures of martensite and bainite exhibits a hardness peak at a T max value around 1025 °C. The lowest hardness within HAZ occurs when the T max value approaches the Ac 3 temperature, resulting from the formation of soft phases of pearlite and ferrite. A novel hardness distribution prediction method, which considers the effects of phase volume fraction and grain size in the estimation of hardness of vital positions delineating HAZ subzones, and utilizes the linear interpolation between these crucial locations, is proposed for the HSS HAZ.