A series of tensile tests, Charpy impact tests, optical microscopy observations, and field emission-scanning electron microscopy examinations, were carded out to investigate the mechanical properties and microstructural evolution of 20Cr32NilNb steel. Experimental results indicate that the as-cast microstructure of the steel typically consists of a supersaturated solid solution of austenite matrix with a network of interdendritic primary carbides （NbC and M23C6）. In the ex-service samples, large amounts of secondary carbides precipitate within austenite matrix. Besides the growth and coarsening of NbC and M23C6 carbides during service condition, the Ni-Nb silicides known as G-phase （Nil6Nb6Si7） are formed at the interdendritic boundaries. The microstructural evolution results in the degradation of the mechanical properties of the ex-service steel. In addition, the precipitate rate of G-phase, depending in part on Si content, varies greatly for the 20Cr32NilNb steel, which plays a key role in the long-term microstructural stability of the steel. Based on the X-ray diffraction data, time-temperature-transformation curve for the steel is obtained from the aged specimens.
Xiao-Feng GuoYing-Ying NiJian-Ming GongLu-Yang GengJian-Qun TangYong JiangXian-Kai JiaXin-Yu Yang
Cr25Ni35Nb and Cr35Ni45Nb alloys are usually used in the ethylene cracking furnaces. However, premature failure of furnace tubes often occurs ahead of design life due to elevated temperature exposure conditions （1050-1100 ℃） and aggressive service environment. Effects of exposure temperature and time on microstructure and mechanical properties of Cr25Ni35Nb and Cr35Ni45Nb steel at aging temperature （1200 ℃） with various exposure time were simulated different service times at 1050 ℃. Change of mechanical properties at room temperature and elevated temperature （900 ℃） of the aged Cr25Ni35Nb and Cr35Ni45Nb steel were investigated. Under exposure at 1200 ℃, ultimate tensile and yield strength, elongation of Cr35Ni45Nb steel increase initially and then decrease, however, strength and ductility of Cr25Ni35Nb steel decrease with aging time increasing. Large amount of fine secondary carbide particles precipitated and dispersed in matrix of Cr35Ni45Nb steel, which increased strength and ductility for dispersion strengthening. However, the effect of the dispersion strengthening is weakened by needle-like secondary carbides. Strength and ductility decreased with fine secondary carbide particles growing. For Cr25Ni35Nb steel, few fine secondary carbide particles precipitated and dispersed in the matrix, and needle-like secondary carbides generated in the matrix, which causes strength and ductility decreased with aging time increasing.
