【作者】 杨莉；

【导师】 强颖怀；

【作者基本信息】 中国矿业大学 ， 矿物材料工程， 2013， 博士

【摘要】 2205双相不锈钢是一种兼具良好的力学性能和耐蚀性能的钢种，目前，2205双相不锈钢与Q345R高强度低合金钢的焊接尚未成熟，尤其是对接头的组织和性能变化机理还缺乏深入研究。本文采用焊条电弧焊和钨极氩弧焊两种焊接方法，2209、309和309Mo三种焊接材料合金，共六种焊接工艺对8mm厚2205双相不锈钢板与Q345R高强度低合金钢板进行了对接焊接试验。在此基础上对接头的显微组织、力学性能、耐蚀性能和微纳米力学性能进行了研究，以期为双相不锈钢和其他钢种之间的异种钢焊接提供科学依据。采用ANSYS有限元分析软件研究了焊接过程中特征区域的热循环过程，观察了焊后接头各区域的显微组织，测试了接头的拉伸性能、弯曲性能、显微硬度和耐蚀性，同时研究了时效过程中接头各区域的显微组织和显微硬度的变化，测试了接头微区的微纳米力学性能。通过本文的试验研究和理论分析，得出了以下主要结论：有限元分析表明，打底层焊缝的Q345R侧熔合区250℃以上的加热时间最长，为1225s，盖面层最短，为1066s，中间层焊缝的熔合区加热停留时间为1148s。同时，钨极氩弧焊打底焊Q345R侧熔合区700℃以上停留时间为311s，高于焊条电弧焊盖面层Q345R侧熔合区700℃以上的停留时间211s。2205双相不锈钢母材、2205双相不锈钢侧热影响区和接头焊缝区均为奥氏体相和铁素体相组成的双相组织，组织比例和形貌受热循环和合金元素的共同影响，焊缝区-Q345R母材的界面存在“碳迁移”区域，“碳迁移”程度也受到热循环和合金元素的共同影响。焊缝区铁素体含量受到铬当量与镍当量比值（[Creq]/[Nieq]）的制约，铁素体含量与[Creq]/[Nieq]成正比关系，由于焊条电弧焊700℃以上高温停留时间低于钨极氩弧焊，故焊条电弧焊焊缝区的铁素体含量高于钨极氩弧焊焊缝的铁素体含量；焊条电弧焊时，由于焊材E2209比E309和E309Mo含有更多的Ni元素，E2209焊材焊接时Q345R侧熔合区的组织更为细小；“碳迁移”程度与焊材中Mo元素的含量成反比关系，与加热时间成正比关系。焊接接头的拉伸、弯曲性能均能满足使用要求。接头拥有高于Q345R母材的抗拉强度，断裂位置在Q345R母材，断口特征为韧窝。同时，接头的面弯和背弯均未出现裂纹，表明接头拥有良好的塑性。接头从Q345R母材-焊缝-2205双相不锈钢母材，硬度呈现逐渐上升的趋势，在Q345R母材-焊缝区的界面，接头显微硬度存在“降低-上升-降低”的波动，且加热时间越长，波动越大。E2209焊缝的耐蚀性接近2205双相不锈钢母材。在浓度为3.5%的NaCl溶液中，E2209焊缝金属的自腐蚀电位为-0.519V，高于2205双相不锈钢母材-0.521V，同时E2209焊缝的晶间腐蚀速率为0.4082g.m-2.h-1，略高于2205双相不锈钢母材的晶间腐蚀速率0.4006g.m-2.h-1。晶间腐蚀产物研究结果表明，腐蚀剩余组织为铁素体组织。对接接头高温时效过程中，2205双相不锈钢母材与焊缝中的铁素体相不断向奥氏体相转变，还伴随着σ相的析出，接头的显微硬度发生了显著变化，σ相的析出受到时效温度、时效时间和合金元素的共同制约。在700℃时效时，铁素体随时间延长逐渐转变为奥氏体相；在900℃时效时，组织为三相共存状态，即奥氏体相、铁素体相和σ相，时效后焊缝区的σ相与焊缝金属的Mo元素含量成正比关系，且σ相是Fe和Cr的金属间化合物，其中Cr的质量分数约为33.5%；900℃时效后焊缝区和2205双相不锈钢母材区的显微硬度的提高幅度大于700℃时效；900℃时效时，同种焊材下，焊条电弧焊焊缝的显微硬度提高幅度大于钨极氩弧焊焊缝，同种焊接方法下，E2209焊缝硬度提高幅度大于E309和E309Mo；Q345R母材的显微硬度低于焊缝区，随着时效时间的延长，呈现先下降后上升的趋势。有限元模拟结果表明材料的Meyer指数与其加工硬化指数存在正比关系；钨极氩弧焊焊缝的平均Meyer指数最小，其次是焊条电弧焊焊缝，最后为2205双相不锈钢母材，即钨极氩弧焊焊缝塑性最好，2205双相不锈钢母材最差，且同一种焊接方法中，309焊材施焊的焊缝的Meyer指数最小，可见309焊缝拥有最佳的塑性；铁素体的硬度为346.83HV，高于奥氏体的硬度261.62HV，铁素体的稳态应力敏感指数为82.79，远大于奥氏体的36.26，表明铁素体比奥氏体拥有更好的蠕变抗力；σ相的硬度为1764.8HV，为一种脆硬相，远高于铁素体和奥氏体的显微硬度。更多还原

【Abstract】 2205duplex stainless steel is a kind of steel which has good mechanical property andcorrosion resistance property. Until now, there are still some problems in the weldingprocessing between2205duplex stainless steel and Q345R high strength low alloy steel,especially mechanism in the change of microstructure and properties during welding. In thisstudy, two kinds of welding methods, including shielded metal arc welding and gas tungstenarc welding were utilized. Also three kinds of welding alloy, including2209,309and309Mo,were employed to joint2205duplex stainless steel and Q345R steel. After welding, themicrostructure, mechanical properties and corrosion resistance properties of welded jointswere investigated. All of these were done to supply scientific basis for dissimilar steelswelding between2205duplex stainless steel and other steels.Thermal cycle of special location during welding was studied by ANSYS finite elementanalysis software. The microstructure of welded joints was observed. Tensile property, bendproperty, micro hardness and corrosion resistance property were tested. Moreover, the changeof microstructure and micro hardness during aging treatment was investigated. The micro&nano mechanical properties of micro location in welded joints were also studied.Following conclusions can be drawn from tests and analysis in this study.Results of finite element analysis show that there is a longest time of1225s in thebottom of fusion zone from weld metal to Q345R base metal when the temperature is below250℃. However, the time of top fusion zone from weld metal to Q345R base metal is only1066s and is the shortest. The place located in the middle layer of fusion zone from weldmetal to Q345R has a medium time of1148s when the temperature is below250℃. Thetime below700℃of gas tungsten arc welding metal is311s longer than that of top shieldedmetal arc welding metal which is211s.The base metal, heat affect zone of2205duplex stainless steel and weld metals are allconstituted by duplex microstructure of austenite phase and ferrite phase. The ratio andmorphology of phase are all influenced by thermal cycle and alloying element. The carbonmigration area, which located at the interface from weld metal to Q345R steel base metal, isaffected by thermal cycle and alloying element. Ferrite phase fraction is influenced by ratio ofChrome equivalent and Nickel equivalent and is positive proportional to the ratio of Chromeequivalent and Nickel equivalent. Because of the heating time higher than700℃of shieldedmetal arc welding is shorter, ferrite phase fraction of shielded metal arc welding is higher thanthat of gas tungsten arc welding. When the shielded metal arc welding is applied, massfraction of Nickel in E2209weld metal is higher than that in E309and E309Mo. Because of the refining effect of Nickel element, the microstructure of E2209fusion zone is finer thanthat of E309and E309Mo. Carbon migration degree is negative proportional to mass fractionof Mo and is positive proportional to heat time.The tensile and bend properties of welded joints can all meet the operating requirement.The tensile strengths of all joints are higher than that of Q345R steel base metal. The tensilefracture is dimple and locates at Q345R steel base metal. Moreover, there is no crack duringface bending and reserve bending. All of these show that there is good plasticity of all joints.The hardness from Q345R steel based metal to weld metal, and to2205duplex stainless steelis nearly increased. There is a fluctuation in micro hardness which is decreased firstly, thenincreased and decreased again, at the interface from Q345R steel to weld metal. The degree offluctuation is affected by heating time, and is positive proportional to the heating time.Corrosion resistance property of E2209weld metal is comparable to that of2205duplexstainless steel. Corrosion potential tested in3.5wt.%NaCl solution of E2209weld metal is-0.519V higher than that of2205duplex stainless steel which is-0.521V. Moreover,intergranular corrosion rate of E2209weld metal is0.4082g.m-2.h-1higher than that of2205duplex stainless steel which is0.4006g.m-2.h-1. The investigate results show that theintergranular corrosion products are ferrite.When butt joints are aged at high temperature, ferrite phase in2205duplex stainless steeland weld metal transformed into austenite phase continually, and accompany with theprecipitation of σ phase. The hardness of joints is changed greatly. Precipitation of σ phase isinfluenced by aging temperature, aging time and alloying element. Ferrite phase transformsinto austenite phase gradually at700℃. Ferrite phase, austenite phase and σ phase coexist at900℃. Fraction of σ phase in weld metal after aging is positive proportional to mass fractionof Molybdenum. σ phase is intermetallic compound of Iron and Chrome, and the massfraction of Chrome is33.5%. Improvement in hardness of2205duplex stainless steel andweld metal aged at900℃is greater than that aged at700℃. When using same weldingalloy, hardness of shielded metal arc weld metal aged at900℃is higher than that aged at700℃. When using same welding method, hardness of E2209weld metal is higher than thatof E309and E309Mo. Hardness of Q345R is lower than weld metal, which decreased firstlyand increased lastly with aging time extended.Results of finite element analysis show that Meyer exponent increases when the workinghardening exponent increases. Average Meyer exponent of gas tungsten arc weld metal isminimum. Meyer exponent of2205duplex stainless steel base metal is maximum, whichhigher than that of gas tungsten arc weld metal and shielded metal arc weld metal. Thisindicates that plasticity of tungsten arc weld metal is the best, then is shielded metal arc weld metal, and the2205duplex stainless steel is the worst. When only one kind of weldingmethod is employed, Meyer exponent of309weld metal is the minimum. This illustrates that309weld metal owes the best plasticity. Hardness of ferrite phase is346.83HV, higher thanthat of austenite phase which is261.62HV. Steady creep stress exponent of ferrite phase is82.79much higher than that of austenite phase which is36.26. This indicates that creepresistance of ferrite phase is greater than that of austenite phase. The hardness of σ phase is1764.8HV much higher than that of ferrite and austenite phase. This illustrates that σ phase isa kind of hard and brittle phase.更多还原