【作者】 王志强；

【导师】 郑振太；

【作者基本信息】 河北工业大学 ， 材料加工工程， 2016， 硕士

【摘要】 Monel400镍基合金在苛刻的腐蚀环境中有较高的机械性能和良好的耐蚀性能,主要用于制造在各类复杂腐蚀工况下的设备与构件。焊接常常作为结构和设备生产的重要连接工艺之一,然而Monel400镍基合金的导热性较差,在高温加热时,易使熔池金属温度过高,增大了接头晶粒粗化的倾向,从而影响焊接接头的力学性能,并且焊接接头的微观组织和腐蚀性能紧密相关。焊接冶金是一个熔池体积小、高温停留时间短、熔合区温度梯度大、各个区域冷却凝固结晶速度不同的过程。而且Monel400镍基合金中的碳硅含量低、铜镍含量高,熔体粘度大,液态金属的流动性差,往往在焊接热循环的作用下,会使焊接接头各个区域的微观组织存在差异化和化学成分分布的不均匀化,降低了焊接接头的力学性能和腐蚀性能。因此,使得在腐蚀环境中使用的焊接接头常常成为腐蚀失效最薄弱环节。本文研究了两组脉冲频率与两种不同填充材料的焊接工艺下微观组织及腐蚀性能。首先通过采用脉冲TIG焊对Monel400镍基合金的板材进行焊接试验,模拟在HF水溶液腐蚀环境中,对焊接接头试样进行全浸泡腐蚀试验和电化学测试试验。然后借助金相显微镜(OM)、电化学测量设备、扫描电镜(SEM)、能谱仪(EDS)及X射线衍射等分别对Monel400镍基合金的母材和焊接接头的微观组织及腐蚀形貌进行了观察与分析,并通过维氏硬度的测量,分析焊接接头不同区域的硬度。得到的研究结果如下:(1)Monel400镍基合金的脉冲TIG焊接接头的热影响区(HAZ)为粗大的奥氏体,焊缝(WM)为树枝状枝晶γ基体相+枝晶间的析出β相,母材为单相的奥氏体,组织均匀,晶粒细小,呈等轴晶均匀分布。随着TIG的脉冲频率的增大,相同填充材料的焊接接头的晶粒也随之变得略微细小。同时在相同脉冲频率下,填充焊丝的焊缝组织也比填充母材裁条的焊缝组织细小。(2)维氏硬度的测定表明,Monel400镍基合金母材区域显微硬度比焊缝略高,而热影响区的显微硬度最高。同时HAZ中的硬度波动也最大,其次是焊缝,而最稳定的是母材。(3)经静态失重腐蚀试验,以及失重公式计算母材与不同焊接工艺下获得的焊接接头的腐蚀速率并进行对比,得出Monel400镍基合金的基体组织更耐腐蚀,其次是焊丝填充的焊接接头,腐蚀性最差的是母材裁条填充的焊接接头。因此,为了保证接头腐蚀性能,不宜使用母材裁条代替焊丝进行焊接。(4)组织表面腐蚀形貌分析表明,随着析出相被腐蚀脱落,组织表面形成不同程度的腐蚀形貌,填充母材裁条的焊接接头腐蚀形貌的最严重,耐蚀性最差,其次是焊丝的焊接接头,耐蚀性最好的是母材。(5)经电化学腐蚀试验结果表明,不同工艺的焊接接头与母材的极化曲线变化规律都很相似,均存在一定的钝化现象。但母材的自腐蚀电位更高,耐腐蚀性能更好,其次是填充焊丝的焊接接头,填充母材裁条的焊接接头最差。相同填充材料、不同脉冲频率的工艺下的焊接接头腐蚀差异性较小。更多还原

【Abstract】 Monel400 nickel base alloy has high mechanical properties and good corrosion resistance in the harsh environment.It is mainly used in the manufacture of equipment and components under various complex corrosion conditions.Welding is often used as structure and equipment for the production of the important connection technology.However,poor thermal conductivity of nickel base alloy Monel 400,when heated to a high temperature,easy to make metal in the molten pool temperature too high,increases the joint grain coarsening tendency,thus affecting the mechanical properties of welded joints,welding joint microstructure and corrosion performance are closely related.Welding metallurgy is a process which has the advantages of small size,short residence time,high temperature gradient in the fusion zone,different cooling and solidification rate in each region.And nickel base alloy Monel 400 silicon carbon content is low,high contents of Cu and Ni,melt viscosity,liquid metal liquidity is poor,often under the effect of welding thermal cycle,welding joints in various regions of the microstructure exists difference and chemical composition distribution is not uniform and reduce the welding joint mechanical properties and corrosion resistance.Therefore,the welded joints used in the corrosion environment often become the weakest link of corrosion failure.In this paper,the microstructures and corrosion properties of two groups of pulse frequency and two different filling materials were studied.First,by using the pulse TIG welding on the Monel400 nickel base alloy welding test,simulated in the HF water solution corrosion environment,the welding joint test specimens for full immersion corrosion test and electrochemical test.And with the help of optical microscope(OM),electrochemical measurement equipment,scanning electron microscopy(SEM)(SEM),spectrum(EDS)and X-ray diffraction,respectively of MONEL400 nickel base alloy base metal and the microstructure of welded joints and corrosion morphology observation and analysis,and through Vickers hardness measurement and analysis of different regions of the hardness of welded joints.The results are as follows:(1)MONEL400 nickel base alloy pulse TIG welding joint of heat affected zone(HAZ)for the coarse austenite,weld metal(WM)dendritic dendritic gamma matrix phase + interdendritic precipitation of beta phase,parent material for single-phase Austenitic,homogeneous microstructure,fine grains was equiaxed and uniform distribution of the.With the increase of pulse frequency of TIG,the same grain filling material welded joints have become fine small.At the same time in the same frequency,filler wire weld microstructure than the parent metal strip filling weld microstructure.(2)the determination of hardness of Vivtorinox showed that Monel400 Ni base alloy parent metal zone hardness is slightly higher than the weld heat affected zone,and the highest microhardness.At the same time the HAZ hardness fluctuation is largest,followed by the weld,and the most stable is the parent material.(3)the static weight loss corrosion test,and weight loss formula calculation base material with different welding process for welded joint corrosion rate and compared,and comes to the conclusion that the matrix microstructure of Ni base alloy Monel 400 more resistant to corrosion,followed by the filler wire welding joints,the worst corrosion is parent material cut a filling of the welded joints.Therefore,in order to ensure the corrosion performance of the joints,should not use the parent material strip instead of wire welding.(4)surface corrosion morphology analysis showed that with precipitates by exfoliation corrosion,surface microstructure formed different degrees of corrosion morphology,filled with base metal cutting welding joints corrosion morphology of the most serious,corrosion resistance was the worst,followed by the welded joints of welding wire,the best corrosion resistance is the parent material.(5)the electrochemical corrosion test results showed that the different process of welded joints and base metal polarization curves are very similar,there are some passivation phenomenon.But the parent metal corrosion potential of high corrosion resistance,followed by welding filler wire and base metal strip filling welding joint is the worst.The same process of filling materials,different pulse frequency of the welding joint corrosion of small differences.更多还原