【作者】 张旭；

【导师】 陆善平； 李依依；

【作者基本信息】 中国科学技术大学 ， 材料加工工程， 2019， 博士

【摘要】 由于具有良好的综合力学性能、优异的耐腐蚀性能以及出色的抗氧化性能,奥氏体基焊材广泛地应用于核电、航天、化工等领域,其中以镍基合金与奥氏体不锈钢应用最为广泛。鉴于苛刻的服役条件和长寿命使用要求,所以奥氏体基焊材的性能优化一直是一个重要的研究方向。微合金化是奥氏体基焊材性能改进的重要手段。铌作为一种重要的微合金化元素,常被添加到材料中以改善合金的使役性能。但是铌元素会扩大奥氏体焊缝金属的凝固区间,促进低熔点共晶相的析出,不利于奥氏体基焊缝金属中热裂纹的控制。因此,系统研究微合金元素铌在奥氏体基焊缝金属中的作用,对研制高性能奥氏体基焊材具有重要的指导意义。本文以核用镍基焊丝Ni-30Cr-9Fe与奥氏体不锈钢焊丝Fe-25Cr-20Ni为研究对象,针对焊缝金属在核电装备应用中出现的失塑裂纹缺陷以及高温服役性能不足等问题,系统研究了铌元素对奥氏体基焊缝金属微观组织、力学性能、耐腐蚀性能、高温变形行为以及高温氧化行为的影响机制。论文的主要研究内容及结论包括:分析了多层多道钨极氩弧焊制备的含铌Ni-30Cr-9Fe焊缝金属的微观组织、力学性能以及耐腐蚀性能,讨论了铌在焊缝金属中的存在形式与作用机制。实验结果表明,由于铌元素的固溶强化作用,提高了焊缝金属的强度。但较高含量铌元素反倒会促进Laves相在焊缝金属枝晶间析出。我们发现,在拉伸过程中大尺寸的Laves相促进了裂纹的形核与扩展,导致焊缝金属塑性下降。由于Laves相与奥氏体基之间的接触电位差,在氧化性介质中构成了腐蚀原电池,导致Laves相快速溶解,焊缝金属出现了严重的点蚀敏感性。尽管在620℃焊后热处理过程中,焊缝金属中的铌影响铬、镍等元素的扩散再分配,焊缝金属枝晶间出现严重的贫铬现象,增加了Ni-30Cr-9Fe焊缝金属的枝晶间腐蚀敏感性,然而,在铌元素稳定化作用下,Ni-30Cr-9Fe焊缝金属晶界上M₂₃C₆被抑制,失塑裂纹得到了很好的控制。利用高温下单轴拉伸实验方法对Fe-25Cr-20Ni焊缝金属的高温变形行为进行了研究,分析了铌元素与温度对Fe-25Cr-20Ni焊缝金属强度与塑性的影响机制。结果表明,随着温度的升高,Fe-25Cr-20Ni焊缝金属的抗拉强度与屈服强度均单调下降,而其延伸率出现了先降低后升高再降低的趋势。室温变形过程中,Fe-25Cr-20Ni焊缝金属中变形孪晶产生,是造成焊缝金属具有较高的强度与塑性的主要因素。铌元素的添加显著提高了Fe-25Cr-20Ni焊缝金属的高温强度,使其1000℃时抗拉强度提升了 57%。另外,碳氮化铌在焊缝金属凝固过程中钉扎晶界,促进了弯曲晶界的形成,使焊缝金属在高温阶段断裂由沿晶脆断转变为穿晶韧断,大大提高了其高温塑性。但在室温拉伸过程中,碳氮化铌会发生破裂,促进裂纹的萌生与扩展,从而造成焊缝金属室温塑性降低。分析了 Fe-25Cr-20Ni焊缝金属在700℃时效过程中微观组织与力学性能之间的关系,阐明了铌元素在焊缝金属微观组织与力学性能演变过程中的作用机制。结果表明,铌元素促进了焊缝金属时效过程中体心立方结构的α-Cr相和四方结构的Z-CrNbN相的析出。枝晶间α-Cr相与晶界上M₂₃C₆的快速析出与粗化,恶化了焊缝金属的塑性与韧性。此外,时效过程中第二相的析出,也降低了焊缝金属基体中铌元素的固溶含量,导致焊缝金属700℃抗拉强度出现下降。Z-CrNbN相的析出会消耗焊缝金属中一次碳氮化铌,因而其对焊缝金属强度提升效果不明显。探索了Fe-25Cr-20Ni焊缝金属1100℃干燥空气中的静态氧化行为,揭示了铌元素对焊缝金属表面氧化膜形成的影响机制。研究结果表明,Fe-25Cr-20Ni焊缝金属在1100℃氧化过程中表面形成的氧化膜为双层结构:内层为铬的氧化物,外层为富铁、锰的尖晶石型氧化物。铌元素在高温下抑制了铬元素的扩散,焊缝金属近表面出现严重的贫铬区,导致表面形成的Cr₂O₃氧化膜致密性变差,这使合金元素在氧化膜中的扩散变得容易,大大增加了焊缝金属的氧化速率。同时,由于焊缝金属基体中合金元素的大量外扩散,基体中氧化孔洞增加,氧化膜与基体之间的结合力变差,高温氧化过程中出现了严重的氧化膜剥落现象。此外,铌的氧化物在氧化膜与基体界面处生成,在界面处产生较大的内应力,进一步增加了焊缝金属表面氧化膜的剥落。因此,Fe-25Cr-20Ni焊缝金属在高温氧化过程中,铌元素的添加降低了表面氧化膜的致密性与完整性,对其抗氧化性能不利。更多还原

【Abstract】 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.更多还原