Study On The Microstructure And Properties Of 2205 Duplex Stainless Steel Welded Joint By High-Efficient Welding

Duplex stainless steels (DSS) refer to a class of stainless steel whose microstructure is composed of ferrite (δ) and austenitic (y) phase. With excellent physical, mechanical properties and corrosion resistance, they are widely used in environment required strong corrosion resistence and high strength. In the pulp industry, the filter plates of press rolls in the replacement, press, twin-roll pulp washer are fabricated by the 2205 DSS sheets in 4.0 mm thickness. The filter plates are uasually punched, bent and then welded together. Currently, the filter plates were welded by ordinary TIG welding process, but there were serious problems such as the presence of cracks and short life during operation. Based on the investigation of the microstructure and properties of TIG welding joint, this paper studied the effects of high-efficient welding process such as A-TIG welding, plasma arc welding and laser beam welding on the microstructure and properties of the welding joint.Firstly, the TIG welded joint was divided into four different zones according to weld metal zone (WMZ), high temperature heat affected zone (HT-HAZ), low-temperature heat affected zone (LT-HAZ) and base metal (BM), and the chemical composition, microstructure, hardness and corrosion resistance in different zones were studied. It can be found that the microhardness of both austenite phase and ferrite phase are firstly increased and then decreased from WMZ to BM. Considering the corrosion potential, the average passive current density and pitting potential, the corrosion resistance of the HT-HAZ was the worst, followed by WMZ. In contrast, BM and LT-HAZ had better corrosion resistance.Secondly, the single-pass A-TIG welding process was carried out without groove. Then, the effect of heat input on the weld formability and microstructure was investigated. In addition, the influence of different filler material (ER2209 and ER316L) on the microstructure, hardness and corrosion resistance of weld metal was studied. It could be found that A-TIG welding process can significantly increase the weld penetration. Also, this paper derived the welding quality dilution rate ψ with filler wire and calculated the weight percentage of specified alloying elements in weld metal. Through this method, we can find that the Cr and Mo contents decreased and Ni content increased in the weld metal as the filler metals were None, ER2209 and ER316L respectively. As a result, the decrease of Cr/Ni equivalent ratio could improve the critical transition temperature of ferrite to austenite phase which is beneficial to the generation austenite phase and improving the balance of austenite and ferrite phase, and thus prevent the generation of precipitates. In terms of microstructure and the results of electrochemical test and immersion test, it was expected that corrosion resistance of weld metal was the best as the filler metal was ER316L.Thirdly, plasma arc welding (PAW) technology was employed to weld 4.0 mm thick 2205 DSS without groove and the effects of nickel content on the microstructure and properties of weld metal were investigated. As the nickel content was 5.71 wt.%, the microstructure of weld metal was typical as-welded microstructure. When Ni content increased to 7.66 wt.%, the content of austenite phase was significantly increased, while the ferrite content decreased, with thefine ferrite phase separated by austenite phase. The microstructure of weld metal was lath austenite+ferrite and solidification mode altered from FA mode to AF mode when the Ni content came to 8.89 wt.%. Nickel content does not directly influence the corrosion resistance of the weld metal, but it can indirectly affect the corrosion resistance through controlling the phase ratio and precipitation of nitrides. According to the microstructure and the results of electrochemical test and immersion test, it can be considered that the weld metal had the best corrosion resistance when the Ni content was 7.66 wt.%.Finally, a preliminary study of laser welding (LBW) process on 2205 DSS was conducted. Results showed that the HAZ was very narrow with balanced phase ratio and no coarse ferrite grains. However, the weld metal was almost single-phase ferrite microstructure with coarse ferrite grains, in which there were a large number of precipitates. Results of immersion tests indicated that the corrosion resistance was extremely poor.