Hydrogen Induced Crack Propagation In TIG Welded Joints Of Titanium Alloy Plates

Titanium alloy weldments are widely used in key equipments manufacturing such as petrochemical,aerospace,nuclear power and other fields.Some titanium alloy welded joints had been long served in a complex hydrogen environment.Since titanium alloy has a strong affinity with hydrogen and the particularity of welded structure itself,hydrogen embrittlement of titanium alloy welded joints occurred frequently.The welded joint is the key factor to evaluate the safety and reliability of whole titanium alloy weldment.The TIG welded joints of Ti–0.3Mo–0.8Ni alloy plates was chosen as the research object.Hydrogen accumulation during long-term service in low hydrogen environment was simulated by high-temperature gas phase hydrogen charging process.Evolution of room-temperature microstructure and tensile properties,high-temperature compressive properties,fracture toughness and fatigue crack propagation behavior of the hydrogenated welded joints and hydrogen micro-mechanism were investigated.The technical support for controlling and preventing hydrogen embrittlement of titanium alloy weldments and the theoretical foundation for evaluating hydrogen embrittlement degree and the residual life prediction of titanium alloy weldments in service will be provided by hydrogen-induced crack propagation behavior and its micro-mechanism of titanium alloy welded joints.Types and precipitation mechanisms of the hydride in the hydrogenated welded joints were analyzed in detail.Indirect evidence for the redistribution of hydrogen and major alloying elements was obtained.Hydrides including large lamellar(I-?),slender plate(II-?)FCC?and long needle FCT?in the weld zone with 0.21wt%H were found.FCC?phase was one of the decomposition products of?_H containing supersaturated hydrogen.FCT?phase was I-?phase transformation product in low H concentration region.Hydrides precipitation was accompanied by the formation of H lean/rich region.Diffusion coefficients of alloying elements were changed,this resulted the redistribution of alloying elements in two regions.H rich region eventually transformed into hydride,while H lean region was characterized by the formation of white bands on either side or one side of the?and?hydride.Evolution rule of tensile strength,plasticity and fracture morphology in the hydrogenated welded joints were revealed.Tensile fracture characteristics of the welded joints with 0.21wt%H were elucidated.Tensile strength and yield strength had been increased gradually,while sectional shrinkage and elongation had been decreased significantly with hydrogen increasing.The fracture mode was changed from ductile fracture to brittle intergranular fracture and local ductile mixed fracture with hydrogen addition.Main crack propagated alternately along lamellar?grain boundary and hydride direction,and connected to final fracture of the hydrogenated welded joints.Diversion or convergence of two main propagation directions was completed by a small angle cutting mode.Evolution rule of flow stress and microstructure in large deformation zone of the hydrogenated welded joints was studied.Evolution mechanism of hydrogen-induced high-temperature properties was discussed.Steady stress had been decreased,and peak stress had been decreased to the minimum and then increased with hydrogen increasing.The hydride and lamellar?were elongated or flexed significantly with hydrogen increasing.Dynamic recrystallization of the welded zone was aggravated with hydrogen addition.Dislocation around the hydrides was more concentrated.Hindrance effect of defect and obstacle on dislocation movement was weakened by the strong interaction between the hydride and dislocation,this resulted the reduction of dislocation density in lamellar?.Evolution rule fracture toughness and fracture morphology in the hydrogenated welded joints was described.Evolution mechanism of fracture toughness caused by hydrogen and the hydride was discussed.Fracture toughness had been decreased,while the inhomogeneity of the welded joints had a little effect on fracture toughness with hydrogen increasing.Brittle fracture was more obvious with hydrogen increasing and transition from base metal zone to weld zone.Internal pressure shear component of H rich air mass that formed by stress-induced hydrogen enrichment was generated,and caused crack nucleation and propagation of welded joint under lower K_I.Local stress concentration was triggered by dislocation pile-up on hydride interface.Due to stress concentration,microcracks nucleation on hydride interface was accelerated,or high strain of hydride was induced and microcracks nucleation within hydride was promoted.Evolution rule of fatigue crack propagation rate and path,fracture morphology in the hydrogenated welded joints was characterized.Interaction model of persistent slip bands(PSB)with hydrogen atoms and the hydride during fatigue crack initiation were constructed,evolution mechanism of hydrogen induced fatigue properties was discussed.Crack propagation rate had been increased in Paris zone and instability propagation zone with hydrogen increasing.Fluctuation amplitude of crack propagation path and brittle fracture characteristics were more obvious with hydrogen addition.Dislocation movement in PSB was aggravated by hydrogen,this resulted micro-cracks initiation at PSB sharp extrusion peaks and intrusive grooves.H₂ was generated by PSB local hydrogen enrichment,micro-cracks initiation was induced by external stress and internal stress produced by H₂.Orientation distribution and morphology of the hydride were changed by interaction between hydride and PSB.Most of main crack propagation was carried out along hydride direction.The hydride in PSB was separated from the matrix or themselves fractured to formed micro-cracks under cyclic loading.