Study of Normalizing and Tempering Processes on Microstructure and Properties of Low Thermal Expansion Pipeline

The 9Cr-1Mo ferrite-martensite (FM) heat-resistant steel is one of the main candidate materials for sodium-cooled fast reactor process pipeline and steam generator. Compared to austenitic stainless steels, FM steel exhibits higher thermal conductivity, lower thermal expansion coefficient, anti-irradiation properties and lower cost. As the designing lifespan of commercial reactors extends to 60 years, higher requirements of microstructure stability under high-temperature environments are proposed to ensure the long-term creep strength of FM steel. Through multi-scale characterization and mechanical testing, this study investigates the effect of normalizing and tempering processes on the microstructure and mechanical properties of 9Cr-1Mo FM steel used in sodium-cooled fast reactor process pipelines. By comparing the microstructure evolution and mechanical performance under varying normalizing temperatures (1 040, 1 060, 1 080 ℃), tempering temperatures (730, 750, 770, 800 ℃) and tempering time (1-4 h), combined with scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD), the mechanisms of heat treatment on grain size, precipitate distribution, and dislocation density were elucidated. The results show that increasing the normalizing temperature from 1 040 ℃ to 1 060 ℃ enhances the dissolution of alloying elements, which facilitates the precipitation of dispersed M23C6 carbides with the average size decreases from (150±30) nm to (138±19) nm after tempering, thereby improving precipitation strengthening and elevating both room-temperature and high-temperature strength by 20 MPa. However, further increasing the normalizing temperature to 1 080 ℃ shows limited influence on mechanical properties and prior austenitic grain size. Besides, higher tempering temperatures (730 ℃→770 ℃) promotes the coalescence of martensitic laths into blocky, which reduces total grain boundary length by 34% and dislocation density. This leads to a decline in strength but enhances long-term creep resistance due to optimized grain boundaries and homogeneous precipitates distribution. In addition, extending tempering time promotes the precipitation of M23C6 and MX phases, which leads to the decrement of matrix strength and the increment of impact energy. The mechanical properties are not changed significantly since the tempering time extended to more than 2 h or the post-welding heat treatment time extended to more than 4 h. The mechanical properties tend to remain stable confirming the engineering applicability of the optimized process. Based on above studies, the quantitative relationships between heat treatment parameters and microstructure evolution in FM steel are established, and the recommended heat treatment regime of FM steel used in processed pipeline is 1 060 ℃×1 h normalizing followed by 770 ℃×2 h tempering, which effectively promotes the dispersion of M23C6 and MX phases, reduces grain boundary length and dislocation density and balances high strength with long-term microstructure stability. This work provides theoretical support for determining heat treatment processes and engineering application of low thermal expansion pipeline.