Remaining creep life study of Cr-Mo-V main steam pipe lines

Abstract

Identification and quantification of accumulated creep damage to estimate the remaining life of high temperature pipe lines has attracted world wide attention for the past several years. This paper briefly presents NML's experience in this field acquired in the course of the execution of an on-going national project "Component Integrity Evaluation Programme". This paper mainly deals with the remaining life assessment of a main steam pipe line of a 100 MW coal fired power plant. The steel for the pipe line was 1 Cr-0.3 Mo-0.25 V (Russian grade 12 Kh 1 M Phi) steel which had widely been used in the older plants. The microstructural evolution occurred in this steel after 10-year and 15-year service exposure has been extensively analysed and discussed. These samples were provided for investigation by the same plant. Contrary to general expectation, damage in the form of creep cavitation was not observed even after 15 years service, indeed, some structural/morphological changes in the carbides had occurred. Also, any conclusive evidence of Mo-depletion as earlier suggested by some workers in similar steels, was not found. Intesestingly, the 15-year sample showed only 0.3% diametrical expansion and a marginal drop in hardness of about 12 HV. The 'threshold stress model' earlier proposed by one of the authors based on the creep data on the 10-year sample is also now validated with reference to the 15-year service exposed samples. An important feature' of this model is that below the threshold stress the creep rate is practically independent of microstructure. And, a unique relationship exists between the minimum creep rate and the applied stress, regardless of the initial microstructure; the relationship can be used for the remaining life prediction. (C) 1997 Elsevier Science Ltd.