A quantitative X-ray diffraction and analytical electron microscopy study of service-exposed 2.25Cr–1Mo steels

2.25Cr–1Mo power plant headers, with exposures spanning the as-installed condition through to 190,000 h, have been characterised with analytical transmission electron microscopy and quantitative X-ray diffraction. Microscopy revealed a number of service-induced changes including M2C development within pro-eutectoid ferrite, and its dissolution along prior austenite grain boundaries. Carbide coarsening and development of Mo- and Cr-rich carbide phases were also apparent in materials aged for long periods. Microscopy data showed carbide populations that evolved in accordance with the Baker and Nutting sequence, with M6C being the thermodynamically favoured end-point carbide in the bainitic regions, and M2C in the pro-eutectoid ferrite. X-ray diffraction showed that microscopy-based identification of carbides on the basis of composition alone was a valid approach, and good agreement between assessment of relative carbide populations was achieved with the two techniques. Trends in carbide populations as a function of service exposure derived using X-ray diffraction showed some scatter, and this probably stems from microstructural and compositional variations between the steels studied.