2205双相不锈钢激光复合焊接接头组织与耐蚀性研究

发布时间：2018-04-06 22:34

本文选题：2205双相不锈钢　切入点：激光复合焊接　出处：《江苏科技大学》2017年硕士论文

【摘要】：2205双相不锈钢是一种具有优良性能的不锈钢,其强度高、耐蚀性好,被广泛应用在使用条件苛刻的环境中。然而材料在经过焊接后,焊接接头的组织与母材会产生很大的差异,因此研究接头组织与耐蚀性的关系具有重要的意义。本课题通过调节激光与熔化极惰性气体保护焊(MIG)的焊接参数,双热源同时作用复合焊接2205双相不锈钢,得到不同组织的焊接接头,并对焊接接头进行力学性能和耐蚀性能测试。通过对焊接接头进行金相观察发现:焊缝组织中,在厚度方向上,奥氏体相逐渐减少;并且在焊缝中下部相界和铁素体基体以及热影响区会析出有害相;此外,热输入的增加会适当提高焊缝中奥氏体的含量,但是接头中奥氏体整体含量一般会低于50%。拉伸测试结果表明,焊接接头的强度较母材略有下降,同时对断口进行分析发现,在焊缝中,部分铁素体相受晶内析出相的影响发生准解理断裂。硬度测试结果表明:焊缝金属硬度有所降低,热影响区硬度稍微增加;出现这种情况是因为在焊缝中,焊丝会填入一定量的Ni,从而降低了硬度,同时接头中的N由于高温作用会有一定的烧损,降低固溶强化作用;在热影响区(HAZ)由于晶粒的长大,导致硬度增加。动电位扫描测试结果表明,焊接接头的耐蚀性比母材低,而接头耐蚀性薄弱区域在热影响区位置;此外,焊接接头的点蚀电位与母材相近而保护电位明显低于母材,这说明接头点蚀再钝化能力较差。临界点蚀温度和动电位扫描测试结果表明,焊接接头的临界点温度比母材要低,并且接头中组织均匀性越差,临界点蚀温度越低。通过扫描电镜(SEM)观察发现:接头中点蚀发生的部位在各区域有明显差别,在没有析出相的部位,点蚀主要发生在奥氏体相上,并且会向铁素体相中扩展;在含有大量析出相的部位,点蚀主要在析出相部位产生。不同温度下的Mott-Schottky(M-S)测试结果表明,焊接接头在3.5%NaCl溶液中表面钝化膜完整性与母材相近;在不同的外加电位下,钝化膜会表现出不同的半导体特性;同时,在不同焊接接头的相似部位,奥氏体含量越高,钝化膜中载流子浓度越大,点蚀越易发生。此外,适当的温度可以促进钝化膜的生长,提高耐蚀性,并且温度的增加对钝化膜内载流子浓度没有影响,但是会提高钝化膜的平带电位,使得耐点蚀性能下降。
[Abstract]:2205 duplex stainless steel is a kind of stainless steel with excellent properties, its strength is high, corrosion resistance is good, and it is widely used in harsh environment.However, after welding, the microstructure and base metal of welded joints will be very different, so it is of great significance to study the relationship between microstructure and corrosion resistance of welded joints.In this paper, by adjusting the welding parameters of laser and MIG, the double heat source is used to weld 2205 duplex stainless steel at the same time. The welded joints with different structures are obtained, and the mechanical properties and corrosion resistance of the welded joints are tested.Through metallographic observation of welded joints, it is found that the austenitic phase decreases gradually in the thickness direction of the weld, and the harmful phase is precipitated in the middle and lower part of the weld, the ferrite matrix and the heat-affected zone.The increase of heat input will increase the content of austenite in weld properly, but the total content of austenite in joint is generally lower than 50.The tensile test results show that the strength of the welded joint is slightly lower than that of the base metal. At the same time, by analyzing the fracture surface, it is found that the quasi-cleavage fracture occurs in some ferrite phases affected by the precipitation phase in the weld.The hardness test results show that the hardness of the weld metal decreases and the hardness of the HAZ increases slightly, which is due to the fact that the wire will fill in a certain amount of Niin in the weld, thus reducing the hardness.At the same time, the N in the joints will be burned due to the effect of high temperature, and the solution strengthening will be reduced, and the hardness will increase due to the growth of the grains in the HAZ.The results of potentiodynamic scanning showed that the corrosion resistance of the welded joint was lower than that of the base metal, while the weak zone of the joint corrosion resistance was located in the heat-affected zone, in addition, the pitting corrosion potential of the welded joint was close to that of the base metal and the protective potential was obviously lower than that of the base metal.This indicates that the pitting and repassivation ability of the joints is poor.The results of critical pitting temperature and potentiodynamic scanning show that the critical point temperature of welded joint is lower than that of base metal, and the worse the microstructure uniformity is, the lower the critical pitting temperature is.Scanning electron microscopy (SEM) shows that the pitting sites in the joints are different in different regions. In the regions without precipitated phases, the pitting corrosion occurs mainly in austenitic phase and extends to ferrite phase.The pitting corrosion occurs mainly in the precipitated phase where there is a large amount of precipitated phase.The results of Mott-Schottkyy M-S test at different temperatures show that the integrity of the passivation film on the surface of the welded joint in 3.5%NaCl solution is similar to that of the base metal, and that the passivation film exhibits different semiconductor properties at different applied potentials.The higher the austenite content is, the higher the carrier concentration in the passivation film is, and the pitting is easy to occur.In addition, the proper temperature can promote the growth of passivation film and improve the corrosion resistance, and the increase of temperature has no effect on the carrier concentration in the passivated film, but it will increase the flat band potential of the passivated film and decrease the pitting corrosion resistance.
【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】：TG407

【参考文献】