Change in the Strength of Steel Grade 11 Loaded at Room Temperature

Abstract

Steel grade 11 is widely used to fabricate superheater pipe in the thermal power plant. Since the steam has a high temperature (above 300oC) and pressure (max. 20 MPa), lifetime of the tube/pipe decreases or the failure occurs during the operation. Problems of the steel include micro-crack, creep degradation, pit corrosion, mechanical failure, etc.; and increasing working time raises the failure potential and decreases the economy if the thermal plant has to stop operating. This research investigated the change in the mechanical properties of steel grade 11 (ASTM A335) loaded constantly at room temperature. The steel specimens were cut from a new pipe P11, hanged under various stresses (6.45 – 9.68 – 12.9 MPa) for 2160 and 4320 hours. After the loads were removed, the steel specimens were tensile tested and observed by optical microscopy and SEM. The results shown that strengths of the steel increased with increased the stress for 2160 hours, but slightly decreased for 4320 hours. There was no significant change of the microstructure including 87 vol.% ferrite and 13 vol.% pearlite, and the grain sizes were estimated as about 18 μm for all conditions. It was considered that a deformation hardening occurred in the beginning stage of loading, but the creep degradation started as the loading time was lengthened. The fact that a significant change in the microstructure was not observed in the present steels revealed that high temperature played an important role in degradation of the practical steels.