Creep Crack-tip Mechanics Field And Damage Analysis Of P92Steel Welded Joints

In this paper, a finite element model of CT specimen of P92steel welded joint was established, which took the weld metal, base metal, corase-grain HAZ and fine-grain HAZ materials into consideration. Based on the creep damage model of ductility exhaustion, the influence of crack location, mismatch of material’s creep properties and welded joint’s structure size on the creep fracture mechanic parameter C(t)/C*, the distribution of stress/strain, damage field and CCI were studied. The main results obtained are as follows:(1) For multi-material welded joint CT specimen, under the same load, the stress field paremater C*is basically the same when the pre-crack is located in the different positions of the welded joints, but the C(t) has some difference. And due to the constraint induced by different material creep properties ahead of crack tip. the damage and stress/strain fields for the cracks with different locations exist significant difference. When the creep develops from un-steady to steady state, the creep damage of HAZ material is always the highest, especially in the fine-grain HAZ, the triaxial stress and creep strain are the highest, this will lead to the fastest creep damage accumulation, and make the crack initiation and growth in fine-grain HAZ. this result is consistent with the experimental observations in the literature. The damage and fracture of the fine-grain HAZ are related to it’s lower creep deformation resistance,(creep-soft materials) and constraint induced by the surrounding creep-hard material.(2) For the crack located at the interface between fine-grain HAZ and base metal, under the same C*. the greater the degree of mateial mismatch in front of crack tip is. the higher the constraint level is,and higher creep deformation resistance of crack-tip material will also lead to elevation of constraint level, and then higher crack-tip constraint can make the princial stress, triaxial stress, creep strain and damage ahead of crack tip increase, this will lead to a shorter CCI time. As for the material (weld metal and coarse-grain HAZ material) which is far away from the crack tip. they also can affect the stress/strain field, constraint and damage ahead of crack tip.(3) When the crack was located at the interface between fine-grain HAZ and base metal. under the same C*,increasing the width of fine-grain HAZ can effectively reduce the constraint level at crack tip, thereby slowing crack initiation time. And increasing the width of weld zone and coarse-grain HAZ can also increase the triaxial stress and damage ahead of crack tip.(4) For the crack located at the center of weld, under the same load, with increasing the width of weld, the C*. the stress level and creep strain all will decrease. And under the same C*. with increasing the weld width, the principal stress, triaxial stress and creep strain will increase, and the damage will also increase too. this will lead to the decrease of the CCI time.(5) The analysis method based on creep damage model can predict the creep damage and fracture behavior of welded joint. The analyses in this paper show that by designing welded joint structure, selecting welding materials and controlling welding parameters, the sizes of weld and HAZ and the creep behavior of welded joint can achieve a optimum matching. This will decrease the principal stress, triaxial stress (constraint), creep strain and damage accumulation ahead of crack tip, and hence, the CCI initiation time and the CCG rate can be decreased, this will increase the creep life of the welded joints. This needs to be further studied.