Characterization And Regulation Of Precipitated Phase Of Precipitation Strengthened Fe-Ni-Cr Alloy Weldment

Fe-Ni-Cr precipitation-strengthening austenitic alloys,with high strength and sound corrosion resistance,are developed by adding Ti and Al elements on the basis of single-phase austenitic alloy.Such alloys are strengthened mainly by the coherent and orderedγ′[Ni₃（Al,Ti）]phase precipitated during aging treatment.Compared with single-phase austenitic alloy,the plasticity of precipitation-strengthening austenitic alloys with higher hydrogen embrittlement sensitivity is impeded hugely in the critical hydrogenated environment.There are some main factors causing plasticity deterioration,including theηphase distributed in the interior of grain and along grain boundaries,the size and distribution ofγ′precipitated phase,and the coherence ofγ′/γinterface structure,etc.Based on the previous research,OM,SEM,TEM,EDS and other means were utilized in this study to characterize the size,the distribution,the structure of theγ′precipitated phase and theγ′/γinterface structure of both the base metal and the weld zone metal.The mechanism divergence of the dislocation and theγ′precipitated phase interaction（during the deformation under in-situ tensile test）in the base metal zone and the weld zone was observed by TEM.Therefore,different heat treatment processes were used to adjust the microstructure of the weldment to optimize the overall mechanical properties of the weldment and then to improve its resistance to hydrogen embrittlement.Regarding the weldment subjected to isothermal holding at 740℃ for 8 hours,the average size ofγ′precipitate phase in the base metal zone is about 10 nm andγ′precipitated particles are uniformly and finely distributed on the austenite matrix.Different from the base metal,the average size of theγ′precipitated phase in the weld zone is about 35 nm,and the distribution is sparse,resulting in a decrease in the weld zone strengthening effect of theγ′phase,which in turn leads to a lower hardness of the weld zone than the base metal zone.It is further shown by HRTEM results that there is no transition zone between theγ′precipitated phase and the matrixγin the base metal,and the interface between the two has a good coherence;However,there is a transition zone between theγ′precipitated phase and the matrixγin the weld zone with a width of approximately 5 nm.The interface between theγ′precipitated phase and matrixγbecomes disordered,and the coherence is destroyed causing a semi-coherent interface,Moreover,there is a large stress in the transition zone betweenγ′precipitated phase and matrixγin the weld zone,that is a strong hydrogen trap.On resorting to in-situ tensile experiments in TEM,it is found that the dislocations of motion cut through theγ′precipitated particles during the plasticity deformation of the base metal region.When the weld zone is in under plasticity deformation andγ′precipitated particles are encountered by the dislocations of motion,the dislocations are entangled with theγ′precipitated particles and form a orowan dislocation loop around theγ′particles.The newly formed dislocation loop will exclude other dislocations on the same slip surface and block the further slip of dislocations by cutting steps and twisting with dislocations from other slip surfaces,thus causing dislocation entanglement.After various aging treatments,the microstructure and mechanical properties of the weldment are different.In terms of the specimens held at the same temperature for 8 hours,γ′particles are evenly distributed in the columnar grain zone and the equiaxed grain zone of the weld when the aging temperature is 665℃,and no precipitation ofηphase can be found at the outside and inside grain boundaries;With the increase of aging temperature,the flakyηphase gradually appears at the grain boundary and in the interior of grains.A large amount of flakyηphase is precipitated at the grain boundary when the holding temperature reaches740℃.The grain boundaries are weakened by a large number ofηphases at the grain boundaries,causing cracking along the grain boundaries of the weldment during the deformation process.Meanwhile,the number ofγ′particles decreases as theηphase precipitates,and the precipitation ofηphase is at the expense of theγ′phase.Therefore,the incubation temperature of the aging treatment should be lower than the precipitation temperature of theηphase.It is illustrated by the microhardness test results that the microhardness of the weldment after aging treatment has been significantly improved.