Research On Creep Property Of Heat Resistant Steel Welding Joint In Ultra Super-critical Boilers

Based on the good high temperature mechanical properties, T91 steel has been widely used in superheaters, reheaters and main steam-pipes of the boilers in the thermal power stations. Along with the import of T91 steel, its welding technology and welding quality have been intensively studied. The microstructures and properties of the weld metal are different from that of the parent metal, which is prone to be problem areas. Therefore, evaluating the welding quality by studying the mechanical properties of the welded joints is really significant for engineering.In this paper, the microstructures and mechanical properties of T91 steel welded joints have been studied by OM, micro-hardness test, high-temperature tensile tests and small punch tests. The creep properties of T91 steel welded joints were studied by conventional creep tests and small punch creep tests. The main results are as follows.The microstructure of T91 steel is lath martensite. with a large number of dislocations. The grains of the welded joints and heat affected zone were more coarse than that of the base metal.The hardness of the welded zone was higher than that of T91 steel. The hardness of the fusion lines was the highest. The small punch tests mechanical properties of the welded joints near the fusion line were quite consistent with the results of the micro-hardness tests.The results of small punch creep tests showed that the deflection-time curve by small punch creep tests has the characteristic of typical three stages with deceleration, constant rate and acceleration. Under the same temperature and the tested load, the creep properties of coarse grain heat affected zone and welded zone was better than that of fine grain heat affected zone.The Larson-Miller law about L-M parameters and load based on the small punch creep tests was established:P(F)=-0.035(1gF)3+2.002(1gF)2-39.168(1gF)+256.125,The relationship between load and stress respectively in small punch creep tests and conventional creep tests have been analyzed by Larson-Miller law, With the same fracture time, the relationship between the load value in small punch creep tests and the stress value in conventional creep tests is F=2.62σ.The Monkman-Grant equation about minimum deflection rate and fracture time based on the small punch creep tests was established:1gtF+1.01921g δM=-0.4081.With the same fracture time, the relationship between the center deflection value in small punch tests and the strain value in conventional creep tests is εM=1.539δM1.011.The results of SEM observation on small punch creep tests fracture of T91 steel welding joint showed that T91 steel welded joints on small punch creep fracture types are microporous gathered fracture.