Theoretical Analysis And Experimental Research On Stressfacilitated Nucleation Of Hydrogen Induced Cracking In Welded Joint

Hydrogen induced cracking is a common welding defect of high strength steel structure using security impact. At present, the main problem is the three main influencing factors of stress, hardening organization, and the diffusion of hydrogen. The relationship between stress, hardening organization, and the diffusion of hydrogen is not clear. The formation mechanism is not unified. Based on principle of the crack nucleation, energy change of hydrogen induced cracking nucleation was investigated. Hydrogen induced cracking nucleation energy criterion and critical mechanics condition is established. The critical radius of hydrogen induced cracking nucleation is derived. Formation Mechanism of stress–facilitated nucleation of cleavage is proposed. The relationship of stress the diffusion hydrogen and hydrogen induced cracking aresystematically studiedand this mechanism was verified by experiments. A new method for low stress brittle fracture of the welded joint and hydrogen induced cracking serviced in hydrogen environment of H2-presence which is based on hydrogen stretching method evaluation of weld hydrogen induced cracking sensitivity is proposed.The contents of this dissertation include the following aspects. Diffusion hydrogen is rich in high stress area by stress induction which enter the microscopic cavity produced internal hydrogen pressure. The hydrogen pressure is proportional to the diffusion of hydrogen concentration. When the hydrogen pressure in local position and tensile stress by additional load with internal stress superposition is equal to atomic binding force grain boundarythe second phase matrix binding forcehydrogen induced cracking formed nucleation. The critical hydrogen pressure of hydrogen induced cracking nucleation decreas as tensile stress increasesthus advancing the success of nucleation. A method and device is designed, which researched the relations of stress and diffusible hydrogen and hydrogen damage, and studied the escape and distribution law of sample diffusion hydrogen with the action of gradient tensile stress. The results show that diffusible hydrogen is easily gathered them together in high stress area and proved to diffusion hydrogen induced enrichment in high stress area by stress induction.Hydrogen blisters evolution rule of tensile sample with a circular central hole indicated that high tensile stress area appeared first hydrogen blisters. Distribution law in hydrogen blisters formation process is consistent with distribution rule of stress. Gradient tensile samples show that the density of hydrogen blisters is proportional to the tensile stress. It is provided direct evidence for the development of hydrogen induced cracking.Through plastic deformation introduced internal stress, the experiment of electrochemical hydrogen charging indicate that sensitive rate and hydrogen-induced ductility loss of prestrain specimens were higher than unstrained specimen under condition of hydrogeniog. TEM found that increase of dislocation density did not change the position of crack nucleation. The main contributions of dislocation by prestrain were the hydrogen diffusion aggregation in the process of crack nucleation.The analysis of the tensile specimens fracture surface showed that the percentage of characteristic of intergranular brittle ruptures increased with cathodic current density. Experimental results confirmed that the stress of hydrogen induced cracking formation play an important role to promote. A calculation model of hydrogen-induced cracking the critical hydrogen concentration has been established. Concept and acquired methods of dissolve the apparent activation energy was proposed. The internal hydrogen pressure could be estimated using the parameter by xihua specific law.Production mechanism of welding joint hydrogen induced cracking is studied. The weld zone of low carbon steel TIG welding joint was coarse grains widmanstatten structure. The density of grain boundaries was lower than the parent metal area. Diffusible hydrogen was easily gathered in the parent metal. Hydrogen apparent diffusion coefficient of the parent metal was smaller than the weld area. Internal stress field caused by crystal lattice aberrance in the grain boundary was the main place to hydrogen diffusion and aggregation, which critical induced for diffusion hydrogen. Hydrogen damage characteristics existed great differences in weld joint different areas. Density of hydrogen blisters on parent metal surface was higher, but fusion zone and the welded zone surface were less within their organization which had more hydrogen induced cracking.The weld zone and completely quenching zone microstructure of high-strength steel weld joint was of lath martensitemicrostructure of parent metal was of tempered sorbite. Diffusible hydrogen caused by martenitic transformation produced internal stress was easily gathered in the weld zone and heat affected zone, sub-grain boundary was a preference walkway for the diffusion hydrogen diffusion and aggregation.Hydrogen induced cracking grows along the direction parallel to the board face, showing the step-shape distribution. Most of the cracks expand along the direction perpendicular to the plane in the weld area due to the existence of columnar grain. Cracks are nucleated preferentially between carbide and substrate interface.Hydrogen induced cracking in the weld zone are nucleated mainly in the grain boundary, such as boundary of martensite lath, the lath boundaries of lath martensites, nucleated preferentially in region of martensitic high dislo-cation density boundary.Pre-filled hydrogen tensile experiment method and evaluation index of welded joint is proposed.The analyzed result is verified by experiments using 30 Cr Mn Si Ni2 steel welded joint tensile sample. With the experimental results, the influence of pre-filled hydrogen current on mechanical properties of welded joint was analyzed. The results show that fracture location of the hydrogen pre-filled tensile samples is transformed into HAZ coarse grain area from without filling hydrogen of the parent metal, which is weak links serviced in environment of H2-presence. The experimental results are consistent with traditional bolt experiment. With increasing pre-filled hydrogen currentstrength and plasticity plastic of welded joint decrease.The analysis of fracture surface shows that with hydrogen current density increase,fracture mode is transformed into quasi-cleavage fracture form quasi-cleavage and dimple mixed fracture, but the fracture position has not changed. This model can determine weak links of welded joint serviced in environment of H2-presence accurately, fast and conveniently.