Study On The Low Cycle Fatigue Behavior And Microstructure Of 9Ni Steel Joint For LNG Ship

Low cycle fatigue failure is one of the main failure forms in LNG ship service,and its fatigue performance is very important.The low cycle fatigue property test shows that under the normal temperature(23 ℃),the relationship between strain amplitude and life of 9%Ni steel manual arc welding joint shows a downward trend of power function,and the higher the total strain amplitude is,the lower the fatigue life is.Under the condition of the constant strain of 0.60%,the average fatigue life of the specimen increases at low temperature(-180℃).When the strain is high,the specimen shows cyclic softening characteristics at room temperature and shows obvious cyclic hardening characteristics at low temperature.Compared to room temperature,the difficulty of crack initiation increases at low temperature.The fatigue source region of fatigue fracture under different temperature basically manifestes with the multi-source feature.The area of fatigue crack propagation in the fatigue fracture is the largest,and the unstable fracture zone in low temperature environment has shallow and less dimples than in room temperature environment,and the toughness is worse.Whether at room temperature or low temperature,the crack initiation and the final fracture surface are mainly in the welded area.In low temperature environment,the brittleness of welded metal increases,and the fracture cross-section is almost perpendicular to the workpiece.Microstructure analysis shows that the parent organization is mainly tempered martensite and has optimal comprehensive performance,and the coarse grain zone is relatively weak in the heat affected zone because of coarse strip martensite.Compared to base material,its hardness increases,but the toughness decreases.The microstructure of weld is austenite dendrite.Compared to the parent material and heat affected zone,grain boundary precipitates and carbides form dislocation nucleations easily,and phase transition and migration at low temperature are more difficult,leading to crack initiation source more difficult,so the low temperature fatigue life increases.