Toughening of necklace-type M/A constituent in the ICGHAZ of HSLA steels used for wind turbine towers through thermal cycle temperature control

The toughening effect of the triple thermal cycle peak temperature on the highly brittle necklace-type M/A constituents in the intercritically reheated coarse-grained heat-affected zone (ICCGHAZ) formed by the secondary thermal cycle during girth welding of high-strength low-alloy (HSLA) steels used for wind turbine towers was systematically studied. It was found that controlling the third thermal cycle peak temperature within 500-600°C significantly enhances the -20 °C impact toughness of ICCGHAZ, while the -60 °C impact toughness remains difficult to improve. The necklace-type M/A constituents formed in ICCGHAZ exhibit blocky morphology with high dislocation density and incomplete reverse austenitic transformation, resulting in elevated hardness and internal strain. After the 500-600 °C thermal cycle, most M/A constituents decompose into cementite (θ), reducing internal stress and providing deflection resistance to brittle cracks, thereby improving -20 °C impact toughness. Additionally, undecomposed M/A constituents experience reduced dislocation density and size due to the tempering effect of thermal cycling. However, peak temperatures outside this optimal range (500-600 °C) fail to optimize M/A morphology or enhance low-temperature toughness, as excessively low temperatures prevent effective toughening while excessively high temperatures may reform brittle M/A constituents. The limited improvement in impact toughness at -60 °C arises because decomposed cementite maintains localized aggregation along prior austenite grain boundaries according to the original M/A morphology. Therefore, for HSLA steel girth welds, optimizing interpass spacing and heat input can regulate the peak temperature of subsequent thermal cycles to refine pre-existing brittle microstructures and enhance low-temperature toughness of the joint.