Embrittlement of 1Cr-1/2Mo pressure vessel steel during long-term service

• Microstructure evolution is correlated with mechanical properties degradation for 1Cr-1/2Mo steel for up to 40 years. • Strengthening is due to nanoscale precipitation within ferrite; embrittlement is due to grain boundary particles. • A practical guidance for in-situ metallography of aged pressure vessels to look for larger grain boundary particles. Assessing the mechanical properties of in-service pressure vessels is essential to ensure their safe and continued operation. 1Cr-1/2Mo steel samples are collected from pressure vessels with different years of service exposure. Sub-size flat tensile specimens are tested, and the results show that with an increasing service exposure time, samples exhibited an increased strength, but a decreased ductility compared to the original material. Precipitation of nanoscale carbides within the ferrite/bainite grains has contributed to strengthening the material while the decrease in ductility is believed to be caused by larger size discontinuous carbide particles on the grain boundaries.