Effect of DQ and RQ processes on the uniformity of microstructure and properties across the thickness direction of ultra-high strength steel

The effect of two processes, direct quenching and tempering (DQ-T), and reheated quenching and tempering (RQ-T), on the homogeneity of microstructure and mechanical properties in ultra-high strength hydropower steel in the thickness direction was systematically investigated. Comparative analysis reveals that while DQ-T processed steel exhibits superior surface strength, this property shows significant attenuation from surface to core, coupled with progressive toughness deterioration. In contrast, RQ-T steel demonstrates remarkable stability in both mechanical properties and low-temperature impact toughness across the thickness dimension, despite its overall strength being slightly lower than the surface values achieved through DQ-T processing. The reason why the comprehensive properties of DQ-T steel decrease along the thickness direction is first that the thickness of pancaked austenite grain increases, and the lath of martensite and cementite particles coarsen. Crystallographic analysis through EBSD mapping further demonstrates that intensified variant selection promotes structural coarsening at the core region, manifested by enlarged block and packet units. For RQ-T steel, equiaxed austenite grains that replace pancaked austenite extend uniformly along the thickness direction, and the grain size is very uniform. In addition, the size of martensite lath is relatively small and has little change with the thickness. This can be attributed to the superior hardenability during RQ processing, which facilitates diverse martensite variant formation (particularly V1/V2 variant pairs constituting high-density of block boundaries of boundaries) and inhibits preferential variant growth. The uniformity of these structural features ensures stable mechanical performance along the thickness direction.