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 co ndition through to 190,000 h, have been characterised with analytical trans mission electron microscopy and quantitative X-ray diffraction. Microscopy revealed a number of service-induced changes including M2C development with in pro-eutectoid ferrite, and its dissolution along prior austenite grain b oundaries. Carbide coarsening and development of Mo- and Cr-rich carbide ph ases were also apparent in materials aged for long periods. Microscopy data showed carbide populations that evolved in accordance with the Baker and N utting sequence, with M6C being the thermodynamically favoured end-point ca rbide 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 techn iques. Trends in carbide populations as a function of set-vice exposure der ived using X-ray diffraction showed some scatter, and this probably stems f rom microstructural and compositional variations between the steels studied . (C) 2001 Elsevier Science Inc. All rights reserved.