REMAINING CREEP LIFE STUDY OF CR-MO-V MAIN STEAM PIPE LINES

Identification and quantification of accumulated creep damage to estim ate the remaining life of high temperature pipe lines has attracted wo rld wide attention for the past several years. This paper briefly pres ents NML's experience in this field acquired in the course of the exec ution of an on-going national project ''Component Integrity Evaluation Programme''. This paper mainly deals with the remaining life assessme nt of a main steam pipe line of a 100 MW coal fired power plant. The s teel 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 micros tructural evolution occurred in this steel after 10-year and 15-year s ervice exposure has been extensively analysed and discussed. These sam ples were provided for investigation by the same plant. Contrary to ge neral expectation, damage in the form of creep cavitation was not obse rved even after 15 years service, indeed, some structural/morphologica l 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% di ametrical expansion and a marginal drop in hardness of about 12 HV. Th e 'threshold stress model' earlier proposed by one of the authors base d on the creep data on the 10-year sample is also now validated with r eference to the 15-year service exposed samples. An important feature' of this model is that below the threshold stress the creep rate is pr actically independent of microstructure. And, a unique relationship ex ists between the minimum creep rate and the applied stress, regardless of the initial microstructure; the relationship can be used for the r emaining life prediction. (C) 1997 Elsevier Science Ltd.