Effect Of Residual Stress Field On Fatigue Crack Propagation Behavior Of Welded Joints Of Large Marine Gas Valves

As a major mode of transportation,large ships are developing rapidly in logistics transportation,military,fishing and other industries.As the main transmission mechanism components of marine diesel engines,the requirements for its product quality and service life have also been improved accordingly.Once the valve malfunction,it will threaten the reliable operation of the whole engine set,and even affect the life safety of the crew.To use more technology at present,the valve sealing surface is powder plasma surfacing,unreasonable welding process parameters of the engine cylinders and complex conditions will have an effect on the service life of the valve,but most of the existing research only for the effects of welding process on valve failure,lack of the entire life cycle of the control valve.In view of the structural failure phenomena such as falling blocks in the service process of large Marine air valves,this thesis adopts the research idea of "finding problems,analyzing problems and solving problems" and uses the finite element software ABAQUS to calculate and analyze the welding process of air valves and the crack propagation of air valves under different stress states under the welding residual stress state.And through the relevant experiments to verify the accuracy of the calculation model,the following conclusions are drawn:(1)Based on the actual failure valve welding of impellers for failure analysis,by means of macro examination,microscopic analysis and hardness test method,find the valve failure reasons of impellers,the results show that the valve the grain size of raw materials is bad,there is no obvious welding defects such as porosity,slag inclusion,but the fusion line is not clear,prone to local stress concentration;The valve disc bottom works in the bad cylinder for a long time,resulting in more corrosion pits.(2)According to the conclusions of failure analysis,based on the UMATHT material thermal constitutive subroutine,the finite element software ABAQUS was used to calculate and analyze the welding process of marine air valve,and compared with the conventional calculation method,the results showed that: UMATHT subroutine can effectively improve the stress gradient at the weld joint,thus reducing the risk of cracking caused by stress concentration.After hardness testing,metallographic analysis and X-ray testing,the results calculated by UMATHT subroutine can all meet the actual process requirements,and the error is less than 10%.(3)Based on the sub-model technology,the crack growth was calculated by loading the welding residual stress with the predefined temperature field method,and the laws of crack growth under different residual stress sizes were compared and analyzed.The results show that: The energy at the crack tip can drive the crack to expand,and the residual stress is proportional to the stress at the crack tip,which has a great influence on the service life of the gas valve.(4)Based on the control variable method,the crack propagation law of the sub-model of the air valve under the influence of different temperature loads and external force loads was simply verified,and the reasons for the falling block failure of the air valve were explored.The results showed that the crack propagation of the air valve was less affected by temperature;The external force has a great influence on the crack propagation of the valve,and with the increase of the external force,the crack propagation phenomenon is more obvious,which is the key factor causing the crack propagation.