Stress Rupture Performance And Microstructure Variation For Welded Joint Of F91Steel Made By Hot Wire NF-TIG

F91steel used as high-temperature components in power plant underhigh temperature was developed by the United States in the1970s, such asthe boiler steam pipeline, collection hot box, reheater, steam pipe,etc.F91steel with high allowable stress, stress-rupture strength, fatiguestrength, creep and corrosion resistance has been widely used in coal-firedpower plants in the world for good economic benefit.NG-TIG welding method with hot wire was employed to weld P91steel for high welding efficient and quality to investigate serviceperformance of weld joint, especially for stress-rupture performance at600℃service condition. Microstructure for different parts of weld jointsincluding weld seam, heat effected zone and base metal was observed bySEM and TEM to investigate the microstructure variation afterstress-rupture test at high temperature, such as second phase microstructureand composition.The research results show that the stress-rupture strength is about122MPa at1000hours for600℃, which is higher than that of same steelmade in China. The amount of second phase participating on grainboundaries for weld seam is less than that in base metal analyzed by SEM,so as metal elements achieved by EDAX. The second phase is M23C6verified by TEM electron diffraction patterns, solution strengthen andgrain boundary strengthen caused by precipitation decreased due to theaggregation and growth of second phase. And also the decreasing degree isobvious for base metal comparison with other positions, which makes the fracture occurring on base metal. The requirement performance of weldjoint for F91steel made by NG-TIG with hot wire was proved bystress-rupture test and microstructure observations, which could providethe possibility for security in future service.