Investigation On Microstructure Characteristics And Failure Analysis Of 9%Ni Steel Weld Joint For LNG Tanks

With the increasing severity of environmental problems as well as the increasing exhaustion of natural resources such as coal,oil and so on,natural gas has caught more and more attention from all over the world as a clean energy with higher calorific value.The building technique of liquid natural gas?LNG?tanks is an important part of equipment manufacturing in natural gas industry,playing a pivotal role in the development of the industry and even the economic construction.9Ni steel,as the main material for LNG tanks,suffers a serious problem during welding that is the ductile degradation of the welded joints.This paper explored the reason of ductile degradation of the welded joints through testing equipment like optical microscope?OM?,scanning electron microscope?SEM?,transmission electron microscope?TEM?,X-Ray diffractometer?XRD?etc.The results revealed that the heataffected zone?HAZ?of the welded joints possessed favorable microstructures and properties with two typical austenite.One was film-shape reversed austenite existed along martensite,and the other one was spherical and stripe-shape residual austenite precipitated along the primal austenite boundaries.Meanwhile,a small amount of M-A was also found in HAZ.In the weld zone existed coarse columnar dendrites with a mean primary dendrite arm spacing?PDAS?of 26.70?m and a mean secondary dendrite arm spacing?SDAS?of 8.69?m.There was some precipitation of secondary phases in the FZ.The welded zone produced by Ni-Mo filler consisted of two main secondary phases which were M₆C-type?Mo-rich?carbides.These secondary phases could be further divided into two categories,stripe-shape phases precipitated along martensite,and discontinuous dot-shape phases precipitated along grain boundaries.The weld zone produced by Ni-Cr-Mo filler was composed of two secondary phases including MCtype?Nb-rich?carbides and M₂₃C₆-type?Cr-rich?carbides.There were also a small amount of ?" phase and NbC/Laves eutectic microstructures.Fracture morphology showed typical characteristics of ductile fracture.Fracture cross-section was dotted with round and oval dimples with different sizes.The investigation of microstructures of welded joints combined with the inspection of fracture cross-section proved that the main reason for failure of 9Ni welded joints in the bending test was the ductile degradation caused by coarse columnar dendrites,large-particle slag,micro-cracks and the precipitation of the secondary phases.