Translated from Élektricheskie stantsii, No. 12, December 2007, pp. 22 -25. Coils of 12Kh18N12T steel become magnetized during operation because the surface layer of the metal at the scale boundary is depleted in chromium and enriched in ferromagnetic nickel. A large-scale study of the relationship between the thickness of the altered layer and the operating temperature and individual service life is underway at Mosénergo's TÉTs-26 heating and electric power plant. Ferritometry and selective replacement have made it possible to reduce the amount of strain-induced and embrittlement damage and to extend the service life of steam superheaters. This could not be done by traditional methods, in particular ultrasonic monitoring of the wall thickness. The data obtained here can serve as a basis for creating a new technique for monitoring thermal inhomogeneity in steam superheaters made of austenitic steels. The advantages of ferritometric monitoring include the following: special preparation (cleaning) of coils is not required, the measurements are convenient, and segments with the highest temperature can be detected.The existing technique for evaluating the individual service life of steam superheater coils made of austenitic 12Kh18N12T steel is based on calculating the equivalent operating temperature of the outer surface based on the amount of the ó-phase in the metallic structure, the stress in the metal owing to internal pressure, and the time to failure corresponding to its high-temperature strength [1]. These data are obtained by laboratory study of a limited number of samples, as a rule 3 -6, which have been selected for excision based on ultrasonic measurements of the residual wall thickness immediately in the boiler. The choice of segments of the coil for excision of representative samples is often not formal, but is a fundamental operation. The validity of the service life estimate depends on making the correct choice.Nonuniform thinning of the wall and weakening of the metal in coils along the length, width, and height of the gas duct is a characteristic feature of all heating surfaces. The intensity of this process depends on the thermal inhomogeneity and the kind of fuel being burnt. The rate of overall corrosion of the metal under isothermal conditions is 5.5 times greater when a boiler is run with high sulfur fuel oil than with natural gas. Information on the thermal inhomogeneity of steam superheaters based on thickness measurements is not reliable, especially in the early stage of operation and when a boiler is run on natural gas 92 -97% of the time. A majority of the boilers at Mosénergo are currently running under just these conditions. The absence of new high speed methods for nondestructive monitoring of thermal inhomogeneity based on the state of the metal makes it impossible to determine the service life and amount of repair work reliably, to correct the temperature regime as a preventive measure, to extend the service life, and to reduce the number of shutdowns owing steam superheater damage....
This paper examines the effect of fabrication procedure and metal quality on the service properties of pipes fashioned from chromium-magnesium steel DI59. The kinetics of the development of creep strain of the metal in the temperature region from 650 to 730°C is investigated. It is indicated that vigorous strain begins to develop after 90% of the expected life has been exhausted. It is established that in the steel under investigation, the content of ó-phase over the entire period corresponding to 30 -90% exhaustion of expected life varies negligibly from 1.8 to 2.5%, and cannot be a criterion for estimation of exhaustion of the metal's expected life. The investigations that we conducted on steel DI59 tubes fabricated in accordance with a new procedure indicated a significant increase in the long-term ductility of the steel, which dictates the high quality of the metal.Keywords: quality and expected life of metal, steel DI59, service properties, tubes, metal strain, ductility of steel.
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