Phase Transformation Behaviors in the Heat-Affected Zones of Ferritic Heat-Resistant Steels Enabled by In Situ CSLM Observation

Fossil-fired thermal power generation has dominated China's electricity production for a long time, contributing to around 70% of the total capacity. Developing long-life ultra-supercritical thermal power units is essential for improving coal-fired power generation efficiency, reducing harmful gas emissions, and achieving national energy conservation and emission reduction targets. The assembly and manufacture of advanced heat-resistant steel grades are required to address the above demands, serving as crucial components driving the technological advancement of thermal power units. Heat-resistant steel grades P11, P22, and P91, which are Cr-Mo based ferritic, possess a range of highly attractive properties, such as excellent mechanical properties, excellent corrosion resistance, and relatively low construction costs. These steel grades are widely used in pressure vessels and pipelines focused on high-temperature applications. Fusion welding techniques are invariably necessary to weld such heat-resistant-grade steels before they are positioned in high-temperature service. However, it is worth noting that drastic solid-state phase transformations in the heat-affected zones (HAZs) during thermal welding cycles can profoundly influence the heterogeneous microstructures of welded joints, determining their final mechanical properties to a large extent. Furthermore, it seriously threatens the safe and stable operation of thermal power plants. High-temperature confocal scanning laser microscopy (CSLM) revolutionized traditional metallographic experiments, enabling real-time morphology and quantitative analysis tracking. This innovation has facilitated investigations into the kinetic phase transformation process and microstructure evolution in steels at high temperatures. In this work, the kinetics of phase transformation and microstructural evolution in the HAZs of P11, P22, and P91 ferritic heat-resistant steels during continuous cooling processes were systematically investigated using CSLM. The results revealed that bainite laths preferentially nucleate in the order of increasing difficulty in the energy barrier on austenite grain boundaries, inclusions, internal grain distortion areas, previous bainite laths, and grain interiors. Meanwhile, the growth characteristics of bainite/martensite laths were documented as the phase transformation progressed. It is revealed that bainite laths attach to prior austenite grain boundaries and the previous bainite, while martensite laths grow radially inside the prior austenite grains. Both bainite and martensite laths cease growing when they encounter grain boundaries or other laths, eventually forming an interlocking microstructure. Additionally, the growth rates of bainite/martensite laths in the HAZs of P11, P22, and P91 ferritic heat-resistant steels exhibited considerable variations as the temperature decreased. The analysis revealed that as the temperature decreased, the growth rate of laths in the coarse-grained heat-affected zone was considerably higher than that in the fine-grained heat-affected zone, which can be attributed to the increase in the degree of supercooling and prior austenite grain size.