Comparison of Creep Damage Appearances Among Low Alloy Steel Pipes

Abstract

To establish a remaining life assessment standard for aged thermal power boilers, internal pressure creep tests of representative low alloy pipe steels were conducted. The tested materials were 2.25Cr-1Mo steel, 1Cr-0.5Mo steel, and 0.5Mo steel. Specimens with and without welded parts in their trunks were made of each material and were φ 70 mm-L400mm-t15mm in dimension. Creep test conditions were 570–620°C × 59MPa and internal pressures were applied by Argon gas or water vapor. Creep tests were interrupted at intervals to acquire data on the crept material as creeping progressed. The observations of replicas from the specimen surface showed the characteristic features of each and the following results were obtained from the comparison of them. (1) The specimens of 2.25Cr-1Mo steel and 1Cr-0.5Mo steel suddenly expanded at about 80% of creep life ratio, and that of 0.5Mo gradually expanded from 60%. They finally ruptured with 15–20% expansions in diameter. (2) Although 2.25Cr-1Mo steel witnessed few creep voids near the end of its creep life, 1 Cr-0.5Mo steel and 0.5Mo steel witnessed many voids such as microcracks from mid-creep life onward. (3) Deformation of ferrite grains contributed to the expansion of 2.25Cr-1Mo steel specimens. On the other hand, partial separations of grains seem to have been owed to the expansions of 1Cr-0.5Mo steel and 0.5Mo steel. In 2.25Cr-1Mo steel, the strength of grain boundary versus grain interior seems to be higher than those of 1Cr-0.5Mo steel and 0.5Mo steels. (4) The amount of creep voids in the HAZ of 1.5Cr-0.5Mo steel was roughly three times that of 2.25Cr-1Mo steel.