The Effect Of Heat Treatment On Microstructure Evolution And Local Corrosion Behavior Of Duplex Stainless Steels And Their Weld Joints

Duplex stainless steels have gradually become an important structure material due to their attractive combination of the better atributions of austenite and ferrite. On the other hand, duplex stainless steels are widely used in many fields, such as petrochemical industries, metallurgy, papermaking, nuclear power and so on due to their lower contents of nickel and molybdenum compared with austenite stainless steels which have been widely used in the industry. However, like other stainless steels, although duplex stainless steels have good corrosion ressistence, to some grade, it also has a limite application in media condition ranges, such as temperature, pressure, media content, pH and so on. Thus, the right duplex stailess steel grade must be cautiously chosen to meet the needs of service environment in applications. Some investigations indicated that it is essential for these alloys to maintain a ferrite/austenite ratio approximate to 1:1 and an absence of other precipitates for insuring a combination of good corrosion resistance and high strength. However, this phase balance has been disturbed and some undesirable intermetallic phases have formed easily during hot working, welding or serving at high temperature. These have severe impacte on the corrosion ressistence of duplex stainless steel and their applications. Therefore, investigating the effect of heat treatment on the mircstructure evolution and the corrosion behavior of duplex stainless steels and their weld joints is an important measurement in benefiting the nation and people. This work not only has importent significance in establishing a safe environment for duplex stainless steels and their manufacturing equipments to run with a long period and a good effectiveness, but also guid for developing a new alloy with high properties. On the other hand, on the influence of the fluctuating nickel and molybdenum prices, the lean duplex stainless steels are predicted to be promising for their low contents of nickel and molybdenum. However, during hot work or welding, their phase balance is disturbed and some undesirable phases are prone to form. This makes the alloys prone to producing local corrosion in corrosive media.Based on the background mentioned above, the thesis focuses on the effect of the heat treatment on microstructure evolution and local corrosion behavior of UNS S32304 and UNS S82441 lean duplex stainless steels and the UNS S31803 duplex stainless steel weld joints welded by TIG welding process by using morphological observation and electrochemical detection. The effect of the heat treatment on microstructure evolution and local corrosion behavior of UNS S32304 and UNS S82441 lean duplex stainless steels and the UNS S31803 duplex stainless steel weld joints welded by TIG welding process was discussed in detail. Under heat treatment, the mechanism of local corrosion of UNS S32304 and UNS S82441 lean duplex stainless steels and the UNS S31803 duplex stainless steel weld joints welded by TIG welding process was discussed and analysised both theoretically and experimentally. The detailed research contents are as follows:(1) The effect of annealing temperature on the pitting corrosion behavior of UNS S32304 duplex stainless steel was investigated using morphological observation, the potentiodynamic polarization and potentiostatic critical pitting temperature techniques. The results demonstrated that the volume fraction of ferrite in UNS S32304 duplex stainless steel is enhanced continuously as the annealing temperature increased from 1000 to 1200 ?. The pitting corrosion ressitence of single phase in duplex stainless steels can be improved by adding appropriate amount of alloying elements, such as Cr, Mo, N and so on. Chromium and molybdenum are concentrated in ferrite phase while nickel and nitrogen are enriched in austenite phase. As the annealing temperature increases, the chromium and molybdenum content decreases and the nickel content increases in ferrite phase, while the chromium and molybdenum concentration in austenite phase increases slightly. The pitting corrosion resistance of UNS S32304 duplex stainless steel is determined by the PREN value of the weaker phase, which has been proved by both the pit morphologies and the electrochemical test results. When the specimens were annealed at the temperatures below 1080 ?, austenite is the weak phase, and ferrite is the weak phase when the annealing temperatures increase over 1080 ?. The best pitting corrosion resistance for this steel was found when annealed at approximately 1080 ?.(2) The effect of tungsten inert gas welding and subsequent post-weld heat treatment on the microstructure evolution and pitting corrosion behavior of UNS S31803 duplex stainless steel was investigated using morphological observation, the potentiodynamic polarization and potentiostatic critical pitting temperature techniques. The results demonstrated that the phase balance and the steady state distribution of alloying elements in both phases of UNS S31803 duplex stainless steel welds have been disturbed due to the high peak temperature and fast cooling rate associated with welding process.Cr2N precipitated in the welds during rapid cooling from high temperature. Ferrite in the WM of as-welded specimen is easier to suffer attack of pitting corrosion than austenite because of the approximate average distribution of chromium and molybdenum in both phase and the low nitrogen content in ferrite. After PWHT, precipitations of nitrides are dissolved into the matrix again and microstructure evolution in the WM and HAZ is controlled mainly by incomplete diffusion of alloying elements until an appearance of the equilibrium state. The volume fraction of ferrite both in the WM and HAZ decreases with the increase of PWHT temperature up to 1080? and then increases evidently. The equilibrium state of the welded zone is achieved at 1080?. The volume fraction of ferrite in the WM decreases with a more rapid velocity than in the HAZ after PWHT at 1020 ?. An approximate calculation of PREN value for the annealed duplex stainless steel welds has been seted up in this work. A good agreement between the weaker phase predicted by PREN value and experimental observations has been found in all the analyzed specimens. The best online annealing temperature for UNS S31803 duplex stainless steel welds was 1080 ?.(3) The microstructure evolution in UNS S82441 duplex stainless steel during isothermal ageing at 700 ? for various times was investigated using morphological observation,such as SEM, TEM, EDS and so on. The relationship between ageing time and microstructure evolution of this alloy has been discussed in detail. The grain growth process of a phases has been verified theoretically. The results demonstrated that Cr2N, M23C6 and a and ? phases nucleate simultaneously in the UNS S82441 duplex stainless steel specimens aged at 700 ? and the growth of the intermetallic phases lags behind that of Cr2N and M23C6. Although some Cr2N and M23C6 of small size nucleated at the ?/? interfaces, these phases of large size precipitated heavily at the ?/?interfaces With longer ageing time, the chromium-depleted zones near the ?/? interfaces transform to 2 and advance toward ?, while the chromium-depleted zones near the ?/? interfaces are replenished with chromium from the matrix. Cr2N and M23C6 precipitated at the ?/? interfaces are dissolved and ? phase transforms to a phase with prolonged ageing. In the initial ageing stages, the granular a and ? phases grow and coalesce according to Ostwald ripening. With a treatment time exceeding 4h, intermetallic grains having a size larger than those of the adjacent ? phase grow following a normal grain growth law, while those small in size keep ripening.(4) The effect of isothermal heat treatment at 700 ? for various times on intergranular corrosion behavior of UNS S82441 duplex stainless steel was investigated using the double loop-electrochemical potentiodynamic reactivation and electrochemical impedance spectroscopy techniques. The relationships among ageing time, microstructures evolution and intergranular corrosion behavior of this alloy were discussed in detail. Time to self-healing was determined with an approximate calculation to explain the desensitization occurred in ?. The results demonstrated that a local self-healing process in intergranular corrosion near the ferrite-ferrite interface ocuured after ageing for 1h. Although a local healing process in ? presents at the specimens aged at 700? for 1 h, the intergranular corrosion resistance decreases with ageing time. The decrease of the ratio Ir/Ia does not mean an occurrence of a self-healing process, but a severer intergranular corrosion attack on the specimens aged for 100 h. The test results of electrochemical impedance spectroscopy technique reflected more directly and precisely the effct of ageing time on intergranular corrosion behavior of UNS S82441 duplex stainless steel compared with the double loop-electrochemical potentiodynamic reactivation.