In situ observations of the austenite grain growth and martensite transformations in developed NM500 wear-resistant steel were conducted via confocal laser scanning high-temperature microscopy. The results indicated that the size of the austenite grains increased with the quenching temperature (37.41 μm at 860 °C → 119.46 μm at 1160 °C) and austenite grains coarsened at ~3 min at a higher quenching temperature of 1160 °C. Furthermore, a large amount of finely dispersed (Fe, Cr, Mn)₃C particles redissolved and broke apart at 1160 °C, resulting in many large and visible carbonitrides. The transformation kinetics of martensite were accelerated at a higher quenching temperature (13 s at 860 °C → 2.25 s at 1160 °C). In addition, selective prenucleation dominated, which divided untransformed austenite into several regions and resulted in larger-sized fresh martensite. Martensite can not only nucleate at the parent austenite grain boundaries, but also nucleate in the preformed lath martensite and twins. Moreover, the martensitic laths presented as parallel laths (0~2°) based on the preformed laths or were distributed in triangles, parallelograms, or hexagons with angles of 60° or 120°.

In-situ observation on the austenite grain growth and martensite transformation in a developed NM500 wear-resistant steel were conducted on a Confocal Laser Scanning High-temperature Microscope (CSLM). Results indicated that the martensite nucleation did not synchronously proceed during quenching process. The selective prenucleation dominated in martensite nucleation, which dividing untransformed austenite into several regions, and resulting in the larger size of fresh martensite. Martensite can not only nucleate at parent austenite grain boundaries, but also nucleate in the preformed martensite laths and twins. Smaller constraint in the martensite growth was achieved in a larger parent austenite grain size, leading to the longer fresh martensite and secondary martensite. Besides, the transformation kinetics of martensite was accelerated in a higher quenching temperature. Moreover, the martensitic laths presented in parallel laths (0~2°) based on the pre-formed lath or distributed in triangle, parallelogram or hexagon with an angle of 60° or 120°. Interestingly, martensitic lath also traversed the unstable new parent austenitic grain boundaries. In addition, the size of austenite grains increased with the quenching temperature, and austenite grains coarsened in a short time at a higher quenching temperature of 1160 ℃. Furthermore, a large amount fine dispersed particles redissolved and ripped at 1160 ℃, resulting in the many large and visible carbonitrides.

Because steel plates of 150~300 mm cannot reach the ideal cooling rate under the industrial cooling condition, the development of a super thick plate for the seat ring and guide vane of hydropower stations cannot be successful. In this paper, the influence of the original microstructure and the quenching temperature on the austenite grain size and the analysis of the quenching temperature on the austenite transformation amount were carried out through experiments. The effect of quenching temperature on the content of austenite carbon was calculated by a phase diagram, and then the effect of quenching temperature on the cooling characteristics of the steel was simulated by JMatPro, and the microstructure changes and performance advantages of the QLT heat treatment process were studied and analyzed. Finally, the QLT heat treatment process of SXQ500/550DZ35 steel for the seat ring and guide blade of hydropower stations was researched and formulated (925 °C + 830 °C + 640 °C), and the heat treatment process optimization of a thick steel plate under insufficient cooling conditions during industrial production was solved.