Relevance of high-temperature oxidation in life assessment and microstructural degradation of Cr-Mo steel weldments
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INTRODUCTION
MATERIALS aging and degradation are becoming increasingly important in the context of the worldwide efforts for life extension of aging fossil fuel power plants.[1,2] The issue of materials aging is of particular concern for the inservice high-temperature components, such as the steam generating/handling systems. Steam generators/boilers in fossil fuel power plants are constructed commonly from ferritic chromium-molybdenum steels (generally referred to as “Cr-Mo” steels).[3,4] Life evaluation models and codes have been developed[1] for reliability analysis of the traditionally weak areas in the high-temperature components, such as the header ligaments of a typical Cr-Mo steel boiler tube. Successful and popular life evaluation models also consist of provisions for incorporating the inspection results of those parameters that can influence the onset of the creep/fatigue damage and the remaining life. These models consider oxide scale thickness as one of the critical parameters (besides other parameters, viz., crack size, creep/fatigue damage, and hardness/tensile strength), because crack initiation is believed to occur either by oxide notching or by creep/fatigue damage.[5] Surface cracks in the oxide scales formed over steel components can cause cracks in the underlying alloy,[6,7] and propensity of oxide cracking (notching) is reported to depend on the oxide scale thickness.[7–9] As the life extension of in-service components and, hence, the issue of materials aging/degradation are becoming increasingly important, new approaches for life assessment and reliability analysis of aged components have been explored in recent years.[1,2] Besides their role as a critical factor in crack initiation, the oxide scales, which develop R.K. SINGH RAMAN, Research Fellow, is with the Key Centre for Advanced Materials Technology, Department of Materials Engineering, Monash University, Melbourne 3800, Australia. Manuscript submitted March 21, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
over the high-temperature components, have also found applications in life assessment. A. Life Assessment by Oxide Thickness Measurements Life assessment of steam generating/handling systems by scale thickness measurement is an innovative use of existing information in the form of the oxide scale, which is generally present on the in-service high-temperature components. This emerging tool for determining temperature history of the high-temperature components has been effectively used in life assessment of steam generator tubes in fossil-fuel power plants.[1,10] In a typical steam generator tube bank, while the bulk steam temperature may be within design limits, the local steam temperature in some of the superheater and reheater tubing may be up to 100 8C higher than the bulk temperature. Exposure to this increased temperature can cause a rapid loss of tube life. A nondestructive measurement of the oxide scale thickness across the tube bank suggests whether a given tube had developed an excessive scale thickness. A greater-than-ave
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