In Situ Microstructural Evolution and Precipitate Analysis of High-Nickel Shipbuilding Steel Using High-Temperature Confocal Laser-Scanning Microscopy

This study investigates the microstructural evolution and mechanical properties of high-nickel shipbuilding steel during thermal processing using high-temperature confocal laser-scanning microscopy (HTCLSM). An in situ observation of the heating and holding processes reveals critical insights into phase transformations, grain-growth behavior, and the formation of precipitates. The experimental results demonstrate that austenitization begins at approximately 700 °C, with significant grain-boundary nucleation. At 900 °C, the formation of black precipitates was observed, and their persistence up to temperatures exceeding 1000 °C was confirmed. Scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS) analyses identified these precipitates as chromium carbides (Cr7C3), which significantly contribute to the material’s strength. A comprehensive analysis using transmission electron microscopy (TEM) confirmed the presence and distribution of Cr7C3 within the grains and along grain boundaries. These findings provide a deeper understanding of the microstructural dynamics in high-nickel steels, guiding the optimization of heat-treatment processes to enhance mechanical properties for maritime applications.