Evaluation of the structural degradation and damage of steel of steam pipelines subjected to the long-term action of thermomechanical factors
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We demonstrate the efficiency of the method of J-integral for the analysis of the kinetics of structural degradation and accumulation of microdamages in the metal of steam pipelines after different periods of operation. The results of the tests carried out at room temperature show that, as the operating time of the metal increases, its short-term static crack-initiation resistance J1c increases. The operating time of steam pipelines produces, in fact, no influence on the value of J1c measured at a working testing temperature of 540C. By applying the parameter j = dJ/da under the conditions of high-temperature tests, one can show that the difference between the characteristics of crack resistance of the intact metal and the same metal after a long period of operation is significant. As the operating time increases, the values of j decrease. The observed drop of crack resistance means that the structural degradation of steel caused by its in-service embrittlement decreases its resistance to subcritical crack growth. This fact is explained both by the formation of grain-boundary precipitations of complex carbides and by the accumulation of damages in steel in the process of long-term operation. 1998 Plenum Publishing Corporation.
