Evaluation of Microstructural Changes in Forged and Cast Low Alloy Steels under Aging and Creep Conditions

Creep studies were performed on as-received and aged 1Cr1Mo1/4V steels with an objective to develop methodology for estimating remaining life of CrMoV steel components under creep conditions. The microstructutal condition of the service exposed material was simulated by accelerated aging carried out at 873K (600 degrees C) for 3648 hours which is equivalent to the material condition exposed at 813K (540 degrees C) for 200,000h. Creep testing on as received (normalized and tempered) and aged 1Cr1Mo1/4V steel was carried out at 773, 813, 848 and 873K (500,540,575 and 600 degrees C) at four different stress levels in each material condition in the range of 110 to 350 MPa. Extensive scanning electron microscopic (SEM) transmission electron microscopic (TEM) studies were carried out to evaluate the microstructural changes as a result of aging. The as-received steels offered higher creep-rupture life, which may be attributed to the inherent creep strength that the material possessed. Gradual fall of creep strength at intermediate aging times is due to recovery in ferrite, gradual depletion of solid solution carbides from the ferrite matrix, metastability and transitional character of precipitated carbides. In aged steels, rupture life is slightly less but is comparable to as-received steel. It is higher than that for the steels aged at intermediate stages. Evolution of more stable microstructure such as coarsened MC, precipitation of M23C6, formation of 'H' carbides in the ferritic matrix containing low density dislocation sub-structure together with freshly formed Mo2C and VC, are responsible for improvement in creep-rupture life.