| Long-term use of boiler steam pipes When temperatures and pressures are high,creep damage is more likely to occur.Moreover,residual stress from pipe welding will cause creep deformation to rise and will hasten the onset of creep fractures.As a result,the influence of residual stress cannot be disregarded when studying creep life at high temperatures and pressures.In order to characterize the damage behavior of the pipe,this thesis applies a strainbased damage model and ductile depletion theory.It also establishes the real-world operating conditions for the steam pipe creep crack sprouting and expansion of the numerical simulation model,solves the pipe creep crack sprouting time,compares the three-dimensional spatial crack expansion rate,and identifies the causes of the various expansion rates.The findings indicate that the root cause of creep damage in steam pipe welded joints is the fine grain area’s poor creep resistance,which causes the fine grain area’s stress triaxiality to be at its peak.This stress triaxiality encourages the nucleation and growth of creep holes,which weakens the fine grain area’s bearing capacity and eventually leads to creep cracks.The fracture forms at the intersection of the coarse and fine grain zones and spreads to the parent material zone.It extends along the annulus at a rate that is significantly higher than the axial and radial expansion rates.The results of numerical simulation of the welding process of pipeline welded joints,analysis of the evolution of the welding temperature field and stress field,and use of temperature field clouds to determine the heat-affected zone of the welded joint show that the residual stress field after welding is primarily distributed around the weld,presenting residual compressive stress in the outer wall area of the weld and residual tensile stress in the inner wall area of the weld.The study of creep damage behavior under the working load and analysis of the effect of residual stress on its creep life are based on the ductile depletion theory of microscopic creep hole growth mechanism,the high temperature and high pressure conditions in which the steam pipe is coupled with the welding residual stress.The results show that: the steam pipe service process will be creep stress relaxation phenomenon,the residual stress in the first 10 h rapid relaxation,of which the axial residual tensile stress relaxation rate is the fastest,down about 91%,followed by relaxation rate tends to stabilize;stress relaxation state,the residual stress will partially offset the material’s own stress,in the base material,coarse grain area and weld area slightly reduces the creep strain,does not change the pipe creep Strain trend,but the residual tensile stress increases the creep strain in the fine grain zone;stress relaxation process creep damage accumulates rapidly,making the creep crack emergence time of steam pipes accelerated by about 15%,the peak of the first residual tensile stress in the fine grain zone,and the residual tensile stress shifts the location of fracture emergence from the intersection of the fine grain zone and the coarse grain zone. |
