Research On Microscopic Mechanism Of Burn-through During In-service Welding

In-service welding has been gaining considerable attentions due to its significant economic benefits.At high temperature,several technical difficulties exist in repair process and the burn-through has been one of the critical issues.To reveal the essence of burn-through,finite element simulation and molecular dynamics simulation were combined to investigate the micro dynamic performance of different micro regions in welded joint and the micro-dynamic behavior during failure process under service conditions.Thus the dynamic evolution process of burn-through instability was clarified.Meanwhile,the effects of micro-defects on the micro-mechanical properties of pipelines and the dynamic evolution process of the nucleation and expansion of micro-defects under service conditions were studied.It provids a scientific theoretical support for the micro-level research of in-service welding repair technology.Molecular dynamics method was used to investigate the effects of carbon content,vacancy concentration,temperature and crack size on the micro-mechanical performance of pipeline steel.The results indicate that,the yield strength gradually decreases as the carbon content rises and dislocation slip bands are more likely to occur in the regions where C atoms abound.Vacancies can significantly destroys the stability of the structure.The fracture strength decreases and dislocations would be prone to occur with the increase of vacancy concentration.Temperature is a key factor affecting the micro-mechanical properties of materials.The yield strength and fracture strength of the material significantly decrease as the temperature rises.Subjected to high temperature,the atomic structures are unstable and more micro-defects such as dislocations are easily to germinate,which accelerates the failure of the system.Furthermore,microcracks significantly reduce the strength of materials and larger size of the crack which is perpendicular to tensile force can cause greater decrease of the yield strength and it is more likely to fail.Finite element method was used to simulate the in-service welding process,the temperature and strain fields of the area around the molten pool were obtained.Then the instantaneous temperature and axial strain rate of the nodes obtained by finite element simulation were taken as the initial calculation conditions to perform the molecular dynamics simulation of the micro failure process of material by LAMMPS software.The results show that the strength near the fusion line is low and its mechanical properties are poor,thus it is easier to fail under service conditions.Therefore,it cracks along the fusion line when a burn-through occurs during in-service welding.During the solidification process,the micro-mechanical properties of the welded joints are significantly reduced.Pressure in pipeline also decreases the micro-mechanical properties of welded joints.From the dynamic failure process of welded joints under service conditions,it can be seen that the micro essence of burn-through is the process of nucleation,growth,accumulation,connection and expansion of a large number of micro-defects,which ultimately leads to the failure.Models with cracks and vacancies were established and the LAMMPS software was used to simulate the failure process of materials under service condition.The micro process of burn-through is investigated and it reveals the effect of crack and vacancy on the micro mechanical properties of welded joints at the atomic level.In-service welding experiences the crack initiation and propagation,which is characterized by the nucleation,accumulation and propagation of a large number of micro-defects at the micro level.Under service conditions,a large number of atomic structures are destroyed,and the whole failure process experiences the formation and propagation of micro-holes and micro-cracks.The existence of micro-defects significantly reduces the stability of the structure.Subjected to stress concentration,the system is prone to crack along the defects,which leads to the failure of welded joints and increases the possibility of burn through.