DEGRADATION OF 0.5Cr-0.5Mo-0.25V STEEL MICROSTRUCTURE DURING EXPLOITATION

Degradation of 0.5Mo-0.5Cr-0.25V material after long-term high-temperature service has been studied by means of microstructure and mechanical properties analysis. Samples were taken from the pipeline with steam leaking found after almost 200,000 h service at 565 °C. The microstructure and properties have been compared with similar materials at similar exposure conditions without any signs of the rupture. Highly tempered ferrite contains secondary particles of MC, M2C and a kind of Cr-rich carbide (M23C6 and/or M7C3) in all materials studied. The difference have been found in the values of hardness. Fractures of samples after the impact test at various temperatures with typical embrittled features are also described. Analysis of the material service history like warm and cold starts-up, conditions of thermal and mechanical fatigue, together with microstruc ture and mechanical properties lead to the conclusion that the material was overtempered. The exhaustion of material deformationability resulted in the rupture-leakage of a component in the place of highest stresses.