![Sample our Physical Sciences journals, sign in here to start your FREE access for 14 days]({"type":"image" , "src" : "/sda/110819/TOC-Subject-Sample-2021-Physical-Sciences.jpg"})
Abstract
The metallurgy and levels of protective elements in the new P91 and P92 martensitic alloys are quite different to older ferrite pearlite and ferrite bainite steels. This is likely to give problems in the positive identification of failure mechanisms and assessment of the life of superheater tubing. Techniques such as spheroidisation and oxide thickness to assess metal temperatures, or hardness testing and cavitation to estimate the level of creep damage cannot be used with these materials, or are likely to give ambiguous results. These issues are discussed in the paper and were explored in depth in part of a project at the University of Surrey to develop “If-Then” rules for an expert system for power plant failure analysis.
The main aim of the proposed expert system is to identify the root causes of such failures in superheater tubing and will encapsulate current knowledge about superheater problems in the form of “If-Then” rules. Root causes of creep failures include furnace design and operation, overestimation of alloy creep properties, inadequate heat treatment and a non-optimum content of strengthening elements.
A characteristic of these alloys is that oxidation on the steam side of the tubing can induce premature failures due to the insulating effect of the oxide scales raising tube temperatures. In addition, scale spallation could also increase tube temperatures, as spallation debris may collect in the bottom of tubes, blocking steam flow. Attention is drawn to a potential “runaway affect” in which the tube temperature and rate of oxidation increase with time as the oxide builds up. The root cause of this could either be excessive rates of heat transfer or could be due to inadequate oxidation resistance caused by low levels of protective elements. Methods of identifying which of these is the root cause are discussed.