2205双相不锈钢焊接接头微观组织调控及变形矫正技术研究

发布时间：2018-10-29 09:56

【摘要】：双相不锈钢具有优良的力学性能和腐蚀性能,被广泛应用于石油化工、海洋结构和船舶行业。双相不锈钢的优异性能是建立在合理的双相比基础上,然而焊接后接头双相比例失衡严重,造成接头性能显著降低。在实际应用中手工焊条电弧焊(SMAW)的使用最为普及,所以研究手工焊条电弧焊焊后接头组织及性能很有必要。本文采用三层三道焊,使用两种焊条(焊条E2209-16、焊条309MoL)对6mm厚2205双相不锈钢进行平板对接试验,每种焊条分别进行不同焊接位置、不同热输入的焊接试验。重点研究了不同焊接位置、不同热输入对接头微观组织、力学性能的影响,以及不同工艺参数对接头残余应力的影响。研究表明,焊后接头没有出现宏观缺陷,焊缝成形良好。接头焊缝区铁奥两相含量差距明显,焊缝区奥氏体含量最高,热影响区奥氏体含量最低,由于焊条309MoL中Ni元素含量远远高于母材和焊条E2209-16,使焊后获得接头焊缝区奥氏体含量比焊条E2209-16获得接头高。XRD分析表明所有接头焊缝区没有出现有害次生相。扫描电镜和能谱分析表明接头合金元素在热作用下发生了扩散迁移,使得冷却时铁素体晶界优先析出奥氏体,后续的热循环使铁素体晶内析出大量细小奥氏体。三层三道焊得到的接头每层焊道之间的奥氏体组织形态和含量存在差异。接头力学性能测试表明,焊条309MoL焊接试样抗拉强度低于焊条E2209-16焊接试样,且试样全部断裂在焊缝。平焊刚性固定试样延伸率最小,这与接头较大。弯曲试验中所有接头面弯和背弯都没有出现裂纹,表明接头塑韧性良好。冲击测试中热影响区冲击值高于焊缝,但都低于母材,焊缝冲击断口断口存在大量韧窝且全部为韧性断裂,而热影响区断口存在较大范围的平齐的解理面或准解理面以及少量韧窝,所以断裂方式为脆性为主的韧脆混合断裂机制。接头硬度值和所在区域的奥氏体含量相关,热影响区硬度值最高,焊缝中盖面层的硬度值最小。利用ANSYS有限元软件对焊接过程进行温度场和应力场模拟计算,结果表明三层三道焊中第二层焊道焊接热循环曲线的最高温度高于其他两层焊道,达到了2800°C,对接头冷却时间产生很大影响,从而影响接头组织析出含量。接头下层热影响区受到三次热循环作用,每次热循环最高温度都超过母材相变温度。应力场测试表明接头焊缝区纵向和横向残余应力相反,横向表现为拉应力,纵向表现为压应力,横向残余应力在远离焊缝的区域拉应力逐渐过渡为压应力最后趋为零,小孔法测试应力与模拟值较吻合。对变形严重的板材进行机械矫正,拉伸测试结果显示矫正后板材抗拉强度略升高,应力应变曲线中屈服阶段不明显,且试样断裂于焊缝,断口存在细小的韧窝,韧性降低。
[Abstract]:Duplex stainless steel has been widely used in petrochemical industry, marine structure and ship industry because of its excellent mechanical and corrosion properties. The excellent properties of duplex stainless steel are based on reasonable double comparison. However, the imbalance of the two phase ratio of welded joints is serious, which results in a significant decrease of joint properties. The use of (SMAW) in manual electrode arc welding is the most popular in practical application, so it is necessary to study the microstructure and properties of welding joint after manual electrode arc welding. In this paper, three layers and three pass welding are used, and two kinds of electrode (electrode E2209-16, 309MoL) are used to test 6mm thick 2205 duplex stainless steel. Each electrode has different welding position and different heat input. The effects of different welding positions and heat input on the microstructure and mechanical properties of the joints and the effects of different process parameters on the residual stress of the joints were studied. The results show that there is no macroscopic defect in the welded joint and the weld is well formed. The difference of iron and oxygen content in weld zone is obvious. The content of austenite in weld zone is the highest, and the content of austenite in heat affected zone is the lowest. The content of Ni element in 309MoL of welding rod is much higher than that of base metal and electrode E2209-16. The austenite content of weld zone obtained after welding is higher than that obtained by electrode E2209-16. XRD analysis shows that no harmful secondary phase appears in all weld areas. SEM and EDS analysis show that the alloy elements in the joints are diffused and migrated under thermal action, which makes the ferrite grain boundary precipitate first during cooling, and the subsequent thermal cycling results in the precipitation of a large amount of fine austenite in the ferrite crystal. The microstructure and content of austenite are different between three layers and three pass welded joints. The mechanical properties of the joint show that the tensile strength of the 309MoL welding specimen is lower than that of the electrode E2209-16 welding specimen, and all the specimens are broken in the weld. The elongation of flat welded rigid fixed specimen is the smallest, which is larger than that of joint. No cracks were found in the bending and back bending of the joints in the bending test, which indicated that the joints had good ductility and toughness. In the impact test, the impact value of the HAZ is higher than that of the weld, but lower than that of the base metal. There are a lot of dimples on the impact fracture surface of the weld and all of them are ductile fracture. However, there are a wide range of flat or quasi cleavage surfaces and a few dimples on the fracture surface of the HAZ, so the fracture mode is ductile-brittle mixed fracture mechanism. The hardness value of the joint is related to the content of austenite in the zone, the hardness value of the heat affected zone is the highest, and the hardness value of the cover layer in the weld is the smallest. The temperature field and stress field of the welding process were simulated by ANSYS finite element software. The results show that the maximum temperature of the thermal cycle curve of the second layer welding pass is 2800 掳C higher than that of the other two layers. It has a great influence on the cooling time of the joint, thus affecting the precipitation content of the joint structure. The heat affected zone of the lower layer of the joint is subjected to three thermal cycles, and the maximum temperature of each thermal cycle is higher than the phase transition temperature of the base metal. The stress field test shows that the longitudinal and transverse residual stresses in the weld zone of the joint are opposite, the transverse stress is tensile stress, the longitudinal stress is compressive stress, the transverse residual stress is gradually transferred to zero compressive stress in the area far away from the weld seam. The stress measured by the small hole method is in good agreement with the simulated value. The tensile test results show that the tensile strength of the plate is slightly increased, the yield stage of the stress-strain curve is not obvious, and the specimen is broken in the weld, there is a small dimple on the fracture surface, and the toughness decreases.
【学位授予单位】：江苏科技大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TG457.11

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