Role of internal oxidation and iron penta carbonyl formation on creep life of 9Cr-1Mo steel

Abstract

To understand the cause of discrepancy between predicted creep life through parametric methods using short term creep test data and creep life obtained from long term experiments, 1000 and 10,000 h duration creep tests were conducted on a 9Cr-1Mo grade steel at 550 and 650 °C and the oxide scale, fracture surface and microstructure of the crept specimens were analyzed extensively using scanning electron microscope, electron probe micro analyzer and wavelength dispersive spectrometry. From the creep tests, it is concluded that the discrepancies between predicted creep life and experimental creep life is more pronounced as the stress increases. This is attributed to the sensitivity of scale growth to stress level at both the investigated temperature. Higher scale thickness reduces the effective stress on metal substrate and slows down the creep rate. At 650 °C, significant drop in creep life due to increase in stress from 84 to 92 MPa is attributed to oxygen ingress along grain boundaries, formation of relatively more internal oxide with whisker morphology. At 550 °C, significant drop in creep life due to increase in stress from 196 to 218 MPa is attributed to formation of iron penta carbonyl at the periphery of internal oxide.