DECREASING MECHANICAL PROPERTIES OF THE SUPERHEATER STEEL GRADE P22 HEATED AT ELEVATED TEMPERATURE UNDER CONSTANT STRESS

This paper investigated change of the mechanical properties of the superheater pipe steel grade P22 (ASTM A335) under constant stress of 9.68 MPa and various temperatures of 500-700 oC. The steel specimens were prepared from the new steel pipe taken from a coal-fired power plant in Vietnam, then heated to the given temperature and held for the predetermined time as 24, 48 and 72 hours in the atmosphere. The results showed that the mechanical properties of the steel P22 decreased with the increased temperature and time. This deterioration of the steel strength was caused by the redistribution of the carbide in the steel subjected under the elevated temperature and the constant stress. Although change in the microstructure including phase and grain size was not observed, the SEM micrographs showed that the carbide of the heated steel accumulated on the grain boundaries and the triple regions when the temperature and holding time increased. This phenomenon was attributed to reduce number of the carbide in the steel, resulting in decrease of the mechanical properties. The obtained results indicated that decrease of the mechanical properties of the steel pipe P22 can occur in the early period of working time in the practice.