Reduction of Creep Strength in T91 Superheater Tubes due to Thickening of Steam Oxide Scale on Internal Tube Surface

Abstract

The design life of T91 superheater tubes is based on creep rupture life which depends on operating temperature and stress. However, exposure to high temperatures for a long duration will cause the formation of oxide scale layer on the tube surface which probably will accelerate the tube damage and subsequently shorten its life. The main objective of this paper is to quantify the reduction of creep rupture life in T91 superheater tubes prior to exposure to steam at temperature of 540oC in the oxidation test rig and boiler environments. A specimen that was exposed to a boiler environment for 80,000 hours was found having 191μm thickness of the oxide scale before creep test as measured using an optical microscope scale. While specimen exposed to steam in oxidation test rig at the same temperature for 1000 hours having oxide scale thickness of 35.8 μm. Both specimens were then subjected to a uniaxial creep test at 650oC and 100MPa. Time to rupture for specimens with 191μm and 35.8μm oxide scale thickness is 212 hours and 290 hours respectively. This result shows that a 27% reduction of creep strength in the specimen with 191μm oxide scale thickness as compared to the specimen with oxide scale thickness of 35.8μm. This finding shows the creep strength in the T91 superheater tube is reduces as oxide scale layer thickening. Therefore, it is important to examine the progress of oxide scale thickening to evaluate the current health condition of T91 superheater tubes.