Effect Of Microalloyed Element Niobium On The Microstructure And Properties Of Austenite-Based Weld Metals

Austenite-based welding consumables are widely applied to nuclear power plants,aerospace,petrochemical and other fields due to their comprehensive mechanical properties,excellent corrosion resistance and good oxidation resistance,especially for Ni-based alloy and austenitic stainless steel welding materials.With the service environments becoming more severe and the prolonged service life requirement,the performance optimization of the austenite-based welding consumables has been a significant research interest.Microalloying generally is an important and effective mean of improving the performance of austenite-based welding consumables.As a common microalloyed element,Nb is often added to materials for multiple purposes to enhance their service performance.However,the elemental niobium could enlarge the solidification temperature region of the austenite-based weld metals and promote the precipitation of the low melting eutectic precipitates,which are extremely disadvantageous for the control of hot cracks in the austenite-based weld metals.Therefore,systematically studying the role of microalloyed element Nb in austenite-based weld metal has an important theoretical guiding significance for the development of high-performance austenite-based welding consumables.In present thesis,we take the nickel-based welding consumables Ni-30Cr-9Fe and the austenitic stainless steel welding material Fe-25Cr-20Ni as research objects,and aim to solve issues of the welding defects and inadequate high temperature service performance.Nine kinds of austenite-based welding consumables with different Nb contents are designed and fabricated to systematically investigate the influence of Nb on the microstructure,mechanical properties,corrosion resistance,elevated temperature deformation behaviors and high temperature oxidation behaviors of the austenite-based weld metals.The main research contents and conclusions of the thesis are summarized as follows:The microstructure,mechanical properties and corrosion resistance were studied for the Nb-bearing Ni-30Cr-9Fe weld metals prepared by multiple semi-automatic gas tungsten arc welding.The existence forms and roles of elemental Nb in the weld metals are analyzed.The experimental results indicated that the strength of the weld metals is enhanced by the solid solution strengthening effect of the elemental Nb.However,the high Nb content could promote the precipitation of the large Laves phase in the interdendritic regions of the weld metals.The large Laves phases induce the nucleation and propagation of the cracks during the tensile testing,resulting in a decrease in the plasticity of the weld metals.In addition,the electrochemical difference between the Laves phase and the austenitic matrix results in the formation of the corrosive primary battery in the oxidizing medium,which causes the rapid dissolution of the Laves phase,and then weld metals have a severe pitting corrosion sensitivity.Further,elemental Nb has an important impact on the diffusion and redistribution of the elements such as Cr and Ni in the weld metals during the post weld heat treatment at 620 ℃.The Cr depletion would occur in the interdendritic region of the weld metals,increasing the interdendritic corrosion susceptibility of the Ni-30Cr-9Fe weld metals.Additionally,the ductility-dip cracking sensitivity of the weld metals is well controlled after the inhibition of the grain boundary precipitation of M₂₃C₆ by the stabilization effect of the elemental Nb.The elevated temperature deformation behaviors of Fe-25Cr-20Ni weld metals were studied by the high temperature uniaxial tensile test method.The influence of elemental Nb and temperature on the strength and plasticity of Fe-25Cr-20Ni weld metals was analyzed.The results show that the ultimate tensile strength and yield strength of Fe-25Cr-20Ni weld metals decrease monotonously with the increase of temperature.However,the elongation of the weld metal decreases first,then increases and finally decreases with the increasing temperature.The formation of the deformation twins results in high strength and plasticity of the Fe-25Cr-20Ni weld metal at the room temperature.The addition of elemental Nb significantly improved the high temperature strength of the Fe-25Cr-20Ni weld metals.The ultimate tensile strength of the weld metals is enhanced by about 57%at 1000 ℃.In addition,the precipitation of the Nb（C,N）in the solidification of the weld metal could prevent the motion of the grain boundaries and promote the formation of the curved grain boundaries,which leads to the fracture mode changing from the intergranular brittle fracture to the intragranular ductile fracture at the elevated temperature.Correspondingly,the elevated temperature elongations of the weld metals are greatly improved.However,Nb（C,N）would fracture in the room temperature tensile process,which accelerates the nucleation and propagation of the cracks and results in a decrease in the room temperature plasticity of the Fe-25Cr-20Ni weld metals.The relationship between the microstructure and mechanical properties of the Fe-25Cr-20Ni weld metal during aging treatment at 700 ℃ is analyzed and the role of the elemental Nb on the evolution of the microstructure and mechanical properties is demonstrated.The results suggest that Nb promotes the precipitation of the a-Cr phase with a bcc structure and the Z-CrNbN phase with a tetragonal structure in the aging treated Fe-25Cr-20Ni weld metals.The rapid precipitation and coarsening of the a-Cr phase in the interdendritic region and the M₂₃C₆ at the grain boundaries deteriorate the plasticity and toughness of the weld metals.In addition,the precipitation of the second phase during the aging treatment reduces the solid solution content of the elemental Nb in the weld metal matrix,decreasing the tensile strength of the weld metal at 700 ℃.Since the Z-CrNbN phase precipitates by consuming the primary Nb（C,N）,the strengthening effect of the Z-CrNbN phase on the weld metals can be negligible.The isothermal oxidation behaviors of the Fe-25Cr-20Ni weld metals in ambient air at 1100 ℃ are investigated,and the influence of elemental Nb on the formation of the oxide scales on the surface of the weld metals is revealed.The results show that the oxide scales forming on the surface of the weld metals after exposure to 1100 ℃ are a bilayer structure consisted of the inner chromia and the outer Fe-rich and Mn-rich spinel oxide.At elevated temperature,Nb could inhibit the diffusion of the Cr and then result in a severe Cr depletion region in the inner oxidation zone of the weld metals,which cause the weakness of the compactness of the inner chromia layer.The outward diffusion of the metal ions through the chromia layer will become easy in the Nb-containing weld metal.With the addition of the elemental Nb,the oxidation rate of the weld metals increase greatly.Meanwhile,the number and size of the oxidation pores in inner oxidation zone increase due to the improvement of the outward diffusion of the metal ions by Nb,decreasing the cohesion between the oxide scale and the matrix.The Nb-bearing weld metals take place a catastrophic spallation oxidation behavior after exposure to 1100 ℃ for long term.In addition,the oxidation of the elemental Nb at the chromia/matrix interface can generate a large stress,which also lead to the occurrence of the spallation of the oxide scales.Therefore,Nb has a harmful effect on the elevated temperature oxidation resistance of the Fe-25Cr-20Ni weld metals by reducing the compactness and intactness of the oxide scales.