Research On Hydrogen Induced Cracking Of Precipitation Strengthened Fe-Ni Based Austenitic Alloy Weldments

Precipitation-strengthened Fe-Ni-based austenitic alloys are widely used in turbochargers,jet engines and hydrogen storage due to their excellent mechanical properties,good corrosion resistance,wider service temperature range and lower hydrogen embrittlement sensitivity tank.In practical engineering applications,welding technology is often used to connect the components.However,the welding process will inevitably cause differences in the microstructure of the base metal and the weld,which will result in a decrease in the mechanical properties and hydrogen embrittlement resistance of the weld.In the process of evaluating the service life of precipitation-strengthened Fe-Ni-based austenitic alloy weldments,this paper found that the magnitude of the applied tensile stress affects the hydrogen-induced delayed fracture mode of the weld.A detailed study was carried out on this basis.Above,further study the surface hydrogen-induced cracking behavior of weldments during electrochemical hydrogen charging under no applied stress,explore the hydrogen-induced cracking mechanism of weldments,and provide the necessary theoretical basis for optimizing the hydrogen embrittlement resistance of this type of alloy weldments.A constant-load dynamic hydrogen charging tensile test was carried out on the weldments after a solution treatment at 980? for 1 hour and an effect treatment at 740? for 8 hours,combined with a scanning electron microscope(SEM)to study the effect of different stress states on the hydrogen-induced fracture mode of the weld Influence,and observe the crack growth process through the transmission electron microscope(TEM)in-situ tensile experiment.The results show that the hydrogen-induced fracture mode of the weld is related to the magnitude of the applied stress.When the applied stress far exceeds the yield strength of the weld,the hydrogen-induced fracture mode sequentially changes from transcrystalline brittle fracture,quasi-cleavage fracture,and toughness from the surface of the sample to the inside.Dimple zone transition;when the applied stress is close to the yield strength of the weld,the hydrogen-induced fracture mode transitions from the surface of the weld to the interior of the weld,from intergranular brittle fracture,quasi-cleavage fracture,and dimple zone.The reason why the different applied stress can cause the different hydrogen-induced fracture mode of the weld is related to the proliferation and movement of dislocations.When the applied stress far exceeds the yield strength of the weld,the plastic deformation of the weld is serious and a large number of dislocations are generated.Dislocations act as hydrogen traps to capture a large amount of hydrogen.Hydrogen quickly migrates with the dislocations and is concentrated on the slip surface,reducing the bonding force of the slip surface,causing transcrystalline brittle fracture on the weld surface.When the applied stress is close to the yield strength of the weld,there is no obvious plastic deformation of the weld,that is,there will be no generation and movement of a large number of dislocations.The grain boundary will act as a strong hydrogen trap to enrich a large amount of hydrogen and reduce the grain boundary bonding.Force,causing a brittle fracture on the surface of the weld.In order to further explore the hydrogen-induced cracking behavior of the weldment without plastic deformation,a more extreme special case is selected,that is,when the weldment is completely unloaded,electrochemical hydrogenation is performed to prevent the hydrogen-induced cracking on the surface of the weldment.research.Hydrogen-induced cracking appeared on the surface of weldments after solid solution treatment at 980? for 1 hour + 740? for 8 hours,and the degree of hydrogen-induced cracking of the welds was more serious than that of the base metal.Electrochemical hydrogen charging of the base metal was continued for different times,and it was found that grain boundary cracking and twin grain boundary cracking occurred first in the base metal,which is consistent with the intergranular fracture on the outer side of the weld observed under low stress.As the hydrogen charging time increases,slip bands appear on the surface of the weldment,and some of the slip bands crack.Focused ion beam(FIB)was used to cut out the base material grain boundary cracks and twin grain boundary cracks charged with hydrogen for 25 hours,and characterized them by transmission electron microscopy.The nucleation and propagation mechanism of hydrogen induced cracks were analyzed,and it was found that the grain boundaries were depleted in Ni,Ti,Al elements,and Ni,Ti,Al elements are rich in twin boundaries.Whether the occurrence of hydrogen-induced cracking at the grain boundary and twin boundary is related to element segregation remains to be further studied.