Degradation of a Cr-Mo steel by carbide precipitation over long-term service

Microstructure of the 40-year service-exposed 1Cr-0.5Mo steel extracted from pressure vessels through the boat sampling technique was characterized using x-ray diffraction, field-emission scanning electron microscopy, energy dispersive spectrometry, electron backscatter diffraction, focused ion beam, and transmission electron microscopy. The microstructure degradation during long-term service was identified. Statistical crystal plasticity models were developed to correlate the microstructure and mechanical properties quantitatively. The steel is found significantly strengthened but slightly embrittled. Nanoscale Mo2C carbides have nucleated within the ferrite matrix over the long-term service, resulting in significant strengthening of the matrix. The coarser grain boundary Cr23C6 carbides, on the other hand, act as the damage accumulation sites and lead to the degradation.