Study On Corrosion Resistance Of Super Duplex Stainless Steel By SAF2507 Welding Thermal Simulation

Duplex stainless steel, as an important branch of the stainless steel family, which combines excellent corrosion resistance with high mechanical performance, plays an important role in the field which demands high strength and high corrosion resistance, such as shipbuilding industry, chemical industry, construction industry, desalination industry, oil and gas industrial. Especially in recent years, duplex stainless steels(DSS) are widely used as alternatives to austenitic stainless steels and nickel-based alloys because of their cost-effective and high performance. The good performance of duplex stainless steel benefits from the correct composition design, equilibrium phase ratio and no other harmful phase precipitation. But when duplex stainless steel is welding, the process of rapid heating and cooling always leads to the phase ratio deviating from the equilibrium position in heat affected zone and improper heat input can also cause some harmful intermetallic compounds precipitation such as Cr2 N, sigma, χ, M23C6. Thus it reduces the corrosion resistance in the heat affected zone, which becomes the source of stress corrosion cracking. At the same time, for the HTHAZ which is likely to produce problem of duplex stainless steel joint, the rapid temperature change in the process of welding and the different distance between the heat centre have an effect on the transition of the microstructure and morphology in the high temperature heat affected zone. The thesis, by doing simulation on high temperature heat affected zone of SAF2507 using thermal simulation test machine Gleeble3800, studies the changing behavior of microstructure and the changing process of ferrite and austenite phase ratio and the law of precipitation by changing the heat input and welding thermal cycle times. The thesis clarifies the relationship between the changing microstructure and the corrosion resistance in the welding process by using PREN value calculation, dynamic potentio polarization curves of cyclic voltammetry and electrochemical impedance spectroscopy, which provides adequate scientific basis and necessary support to make duplex stainless steel welding technology system and optimize welding parameters.The thesis does the simulation on high temperature heat affected zone by using thermal simulation. It does study on the relationship between the changing microstructure and the corrosion resistance when the heat input is 0.80kJ/mm, 1.14kJ/mm, 1.60kJ/mm, 1.98kJ/mm, 2.53 kJ/mm, 3.60kJ/mm respectively by using high temperature simulation test machine Gleeble 3800. The results indicate that with the increasing of heat input, three kinds of morphologies austenite precipitate from the ferrite grain boundary and grain interior. When the heat input is 3.60kJ/mm, at the junction of the Ferrite and austenite, the two phases will form granular χ phase. The corrosion resistance test results show that austenite is higher corrosion resistance than ferrite. Along with the increasing of heat input, more transformed austenite can improve the corrosion resistance of the total heat affected zone, but χ phase which forms when the heat input is 3.60kJ/mm will decrease the corrosion resistance.The thesis uses three kinds of heat input namely 0.80 kJ/mm、1.60 kJ/mm、3.60 kJ/mm to simulate the single pass welding, double pass welding and triple pass welding. The thesis studies the change of morphology and size of the microstructure caused by welding thermal cycles when three kinds of heat are inputted and the content of austenite and the influence of precipitated phase. Using the electrochemical method, it tests the relationship between number of welding thermal cycle and the corrosion resistance. The research results show that under the condition of three kinds of heat input, the content of austenite obviously rises with the increasing of thermal cycles. When heat input is 0.80kJ/mm, after three thermal cycles, HAZ austenite content roughly amounts to 50%, while when the heat input is 1.60kJ/mm only two heat cycles can be achieved. When the heat input is 3.60kJ/mm, the austenite content is more than 50% after only one thermal cycle. Under the condition of 3.60 kJ/mm heat input, the content of precipitated phase is increasing with the increasing of the number of thermal cycles. The electrochemical test results show that when the heat input is 0.80kJ/mm and 1.60 kJ /mm, increasing the number of welding thermal cycle can increase the corrosion resistance. However, when the heat input is 3.6 kJ/mm, the corrosion resistance is reducing with the number of thermal cycles is increasing.