Study On Microstructure And Property Of Welding Join Of Service Different Years T91Steel Tub

For its good endurance strength, better thermostability and excellent high temperature creep resistance, T91steel has been widely used in superheater, reheater and main steam-pipe of the boiler in the thermal power station. Though it has been more than ten years for T91steel to be introduced from abroad, studied on localization and put into service, the welding procedure and welding quality have been the focus problems of the thermal power plant for a long time. Therefore the systematical research on the structure and mechanical properties of T91steel welded joints after different year service will provide not only the theoretical basis for improvement of welding technology and post-welding heat treatment but also the technical support for the safe and stable operation of the power station.Based on the characteristics of T91steel welding joint, the microstructures, micro-hardness, conventional tensile properties and SPT (small punch test) properties of T91steel welded joints after different year service have been fully investigated by OM, Vickers microhardness test, conventional tensile test, and small punch test. The corresponding tensile fracture and SPT fracture have been observed with SEM. The results obtained are as follows.The microstructure of T91steel was analyzed in this aper, which was made of lath martensite, acicular martensite and a small amount of ferrite. The grain size of the welded joints, heat affected zone and base metal were researched and compared. The grains of the welded joints and heat affected zone were more coarse than that of the base metal. When TIG was respectively adopted, the welded joint of the former was mainly isometric austenitic crystal, and the latter was basically columnar crystal. The microstructure of T91welding joint at the normal service temperature had little change with increase of service years, but the martensitic laths became wide on the excessive temperature.The yield strength and tensile strength of T91steel wielding joint and matrix decreased with increase of service years and the deterioration degree of the welding joint was higher than that of the matrix. The hardening gradient curves showed the shape of saddle and the SPT properties hardening gradient curves shared the same shape with the hardening gradient curves and they also decreased with the increasing of the service time.The results of SEM observation on the tensile fracture and SPT fracture of T91steel welding joint showed that there were a lot of big and deep dimples in the extended zone of the pre-service tensile fracture and the dimples became small and shadow with increment of the service time. The SPT fracture morphology of each zone in T91steel welding joint transferred from ductile fracture with elongated dimple to quasi-cleavage fracture with elongated dimples and tear ridges with the service years.