Creep Rupture Strength of Heat-Resisting Alloy Tubes (15-15N) under Internal Pressure

Abstract

The effects of deformation in pipes and environmental differences around them on creep rupture strength are discussed after conducting creep tests in the uni-axial stress state and other tests under internal pressure of water by using a heat-resisting alloy, 15-15N.
The analysis of the deformation in pipes was made based on the uni-axial elongation curves with consideration for the reduction of wall thickness. The environment tests were carried out by using pipes with both non-corroded and corroded innner surface by aqua-regia solution.
Although the experiments covered a relatively short period of time, the following results have been obtained.
(1) The uni-axial result of 15-15N shows a similar behaviour as that of an ordinary austenitic stainless steel.
(2) The result of non-corroded pipes is not in agreement with the uni-axial results calculated by any of the existing creep formulas, but for the corroded pipes, the mean-diameter formula or the creep-common formula represents the result at elevated temperature (700°C) well. This shows that the crack initiation and propagation behaviours of both corroded pipes and bars are alike.
(3) The results of numerical calculation of external diametral strain for the creep strain curves agree fairly well with the values obtained in the uni-axial experiments.
(4) The notch-strengthening effect of corroded pipes in a short life range are well proven.