Evaluation of Long-term Creep Strength of 2.25Cr-1Mo Heat Transfer Tube in Actual Service Stress Level Range

概要

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The creep data sheet of 2.25Cr-1Mo steel tube by National Research Institute for Metals (NRIM) is widely used for the residual life assessment of boiler tube. The NRIM creep data sheet contains very useful creep test results such as lower stress level than 30 MPa and longer rupture time of over 100 000 h. Therefore, the master curve calculated from this data sheet has been noticed as a highly reliable curve. The influence of oxidation during long term creep test at low stress is very severe. As the tubes specimens of NRIM were tested on 6 mm diameter specimen, these data were influenced with oxidation scale more than 10 mm diameter specimens creep data, and creep rupture time was become shorter.This study was done to calculate the oxidation scale thickness and true stress variation during creep test from scale producing rate equation and to correct the rupture time with linear damage rule. As the difference of master curves between 6 and 10 mm diameter specimens was become very small after correcting scale effect, this correcting method was confirmed to be right.The others creep data sheet, ASTM and ISO, had been also investigated, and these master curves were not so much affected by oxide scale, and they had almost same creep strength level to the NRIM corrected master curve at the lower stress level.