Investigation On Corrosion Behaviors And Microstructure Evolution Of Duplex Stainless Steels Welded Joint

Due to the composition of ferrite (a) and austenite (y) phase in microstructure, duplex stainless steels (DSS) have combined the merits of ferritic stainless steel and austenitic stainless steel. Their outstanding properties strongly rely on the a balanced two-phase microstructure with approximately equal ferrite phase and austenite phase and no or less formation of harmful secondary phases such as Cr2N, sigma(σ) and chi(χ). However, welding process modifies the ideal microstructure of duplex stainless steel and deteriorates the perfect properties especially the pitting corrosion resistance in high chlorine ion environment. Besides the ferrite and austenite phase, there are a lot of other secondary harmful phases such as σ,%, M23C6, Cr2N,γ2, α’in temperature range of400℃～1300℃. In the high temperature heat treatment of duplex stainless steels (1000℃～1250℃), the emphasis is focused on a/y volume ratio and distribution of major alloying elements in single phase. While in the middle heat process (400℃～1000℃), the key point is the precipitation of the secondary harmful phases and their influence on corrosion behaviors. Since the welding process covers the temperature from room temperature to the melting point of the steels (above1500℃) including the high temperature section and middle temperature section, there exist problems induced by the unbalanced microstructure and the precipitation of harmful secondary phase. Besides, the difference between welding process and normal heat treatment is listed as follows:a. the fusion zone the welded join experienced the process of solidification; b. the thermal cycle of the heat affected zone is a process with fast heating and cooling rate; c. the thermal cycle is not only influenced by welding parameters but also determined by position. Therefore, it is very difficult to investigate the welding of duplex stainless because of the complexity of welding process and microstructure welding joints. In this thesis, the microstructure evolution including a/y ratio, distribution of alloying elements and precipitation of secondary harmful phases has been investigated for the specimens treated by Gleeble thermal simulated and made by industrially welding process. A series novel electrochemical evaluating technique on local corrosion of duplex stainless steel complex welded joints including potentiostatic critical pitting temperature (CPT), potential pulse technique, metastable pit controlling current fluctuation technique and double loop electrochemical potentiodynamical reaction (DLEPR) to reveal the weakest phase in the weakest zone in the multi-phase welding microstructure. The inner relationship between corrosion behaviors, microstructure evolution and welding procedure has been clarified. All these investigations are of great significance both in scientific investigation and industrial application, which serve as the scientific evidence and data supporting for the industrial materials design and procedure optimization such as optimization of welding process, alloying composition optimization, microstructure controlling, post weld heat treatment optimization.(1) In the first part of our investigation, the microstructure evolution and corrosion behaviors of2507super duplex stainless steel isothermal treatment including high temperature solution process (1000℃～1200℃) and middle temperature aging treatment (900℃). The relationship between pit corrosion behavior and alloying element distribution in single phase and a/y ratio has been clarified. The best solution-treated temperature has been confirmed at1080℃. The optimized DLEPR for evaluating the intergranular corrosion sensitization of super DSS2507has been established and applied to investigate the relationship between intergranular corrosion sensitization and precipitation of intermetallic phase such as σ,χ in the middle temperature aging process. The results shown that as time increase, the intergranular corrosion susceptibility Ra increased firstly then decreased. When the aging time is4h, the Ra value reached the maximum17%. The cause of increase of Ra is Cr-depleted γ2increased with σ phase as the time prolonged, while the decrease of Ra is due to the self-healing effect by Cr diffusion from the adjacent γ to γ2.(2) The different thermal cycle of heat-affected zone (HAZ) has been performed on lean duplex stainless steel2304and super duplex stainless steel2507through Gleeble thermal-mechanical simulator. The corresponding microstructure and pitting corrosion behaviors have been studied. The influence of heat input or cooling rate between1350and800℃on lean DSS2304and super DSS2507on microstructure and pitting corrosion resistance have been systematically investigated. The multipass HAZ of two kinds of2304has been investigated too and the influence of welding pass, alloying composition on microstructure and pitting corrosion resistance has been analyzed deeply. An investigation on the effect of alloying composition on the microstructure and pitting corrosion resistance of the2304HAZ was carried out to clarify the inner relationship between welding properties and Creq/Nieq value. The optimized alloying composition has been determined for welding of DSS2304. (3) Microstructure and pitting corrosion resistance of the complex welded joints by three kinds of different welding process including plasma arc welding (PAW), laser beam welding (LBW) and tungsten inert gas welding (TIG) has been investigated. Ferrite phase in the high temperature HAZ has been revealed as the weakest phase in the weakest zone. The degradation of pitting corrosion in the welded joints is due to the lower chromium and molybdenum content in ferrite phase of the high HAZ resulted from the insufficient diffusion of alloying elements in the non-equilibrium welding process, compared with that in the base metal. The difference of three kind welded joints has been compared and analyzed. Shot time post weld heat treatment (PWHT) has been carried out to restore the balanced microstructure and pitting corrosion resistance. The influence of PWHT on microstructure and pitting corrosion resistance has been studied and the optimized PWHT system has been determined. Cr2N precipitation in the welded joint has been studied by SEM and TEM.