| Duplex stainless steels (DSS) are highly important engineering materials, due to their generally high corrosion resistance combined with high strength. They are widely used in various industrial sectors, such as chemical and petrochemical industry (pipelines and storage tanks), desalination (evaporators and pumps), pulp and paper (digester and bleaching reactors) industry process. These excellent comprehensive properties can be possessed by solution treatment. However, properties of DSS weld joint will be weakened because of residual stress and strain, changing of two-phase ratio, segregation and redistribution of alloy elements, and precipitations caused by welding process. In this paper, a cracked phosgenation reactor made of DSS was investigated firstly. Then, based on the root cause of failure, the effect of elastic stress combined with aging treatment and plastic strain on corrosion behavior of DSS was carried out. thirdly, the sensitivity of hydrogen embrittlement and stress corrosion cracking (SCC) resistance in boiling 42wt% MgCl2 were studied. Finally the validity of Normalization method for gaining fracture toughness of DSS was analyzed. The main work and conclusion were as follows:(1) The root causes that lead to failure of welded nozzle joints of phosgenation reactor are solidification cracks and stress corrosion cracks. Solidification cracks took place during welding and then propagated at room temperature with transgranular and cleavage morphology. Unbalanced dual phase ratio with 80-90% ferrite in weld fusion and severity of constraint induced solidification cracks because of unreasonable welding process. Stress corrosion cracks originated from corrosion pits, where large amount of σ and secondary austenite precipitates existed in weld fusion and heat-affected zone. Cr-, Mo-enriched intermetallic σ phase and lean alloyed secondary austenite lowered local corrosion resistance, which resulted in pitting corrosion and stress corrosion cracking under high concentrated hydrochloric acid.(2) There are positive and negative two aspects of the effect of elastic stress on corrosion behavior of DSS. One is decreasing pitting corrosion potential and making passive film more unstable, the other is improving uniform corrosion resistance. For SAF2205, pitting corrosion resistance deceases, but only to a small extent and anodic current density increased significantly with the increasing of plastic strain. However plastic strain has no effect on corrosion properties for high alloyed SAF2507 with a more stable passivation film than that of SAF2205, which can remain stable even if SAF2507 experienced severe plastic deformation.(3) Stress not only accelerates precipitation rate, but also changes the volume, shape and distribution of the precipitates during the aging process of DSS. Stress accelerates alloy elements diffusion,reduces the content of alloy elements in intermetallic σ and χ phases, and increases the content of alloy elements in the secondary austenite, which delays the decline of the corrosion resistance and transforms the corrosion pattern from severe localized corrosion to uniform corrosionfor aged DSS without stress. Chromium depleted theory can explain pitting and intergranular corrosion of duplex stainless steel reasonably.(4) The HE susceptibility of duplex stainless welded joint mainly depends on dual phase ratio. The HAZ of welded joint shows the highest susceptibility. Small punch testing (SPT) is a very useful means of distinguishing HE susceptibility for base metal, HAZ and weld fusion considering sampling with very narrow heat affected zone.(5) The slow strain rate testing indicates that stress corrosion cracking resistance of duplex stainless steel welded joints is lower than that of base metal and fractured positions mostly locates in HAZ. The stress corrosion sensitivity of the welded joints along the rolling direction is significantly less than that of the normal to the rolling direction. The microstructures and mechanical properties of HAZ of the welded joints are obviously changed by the thermal cycling, but they still retains some of base metal properties.(6) J-△a resistance curves obtained by the Normalization method and the unloading compliance method have very coincidence degree when the length crack propagation is less than 0.5mm, which makes known that fracture toughness of DSS can be gained accurately by the Normalization method. But with the increase of crack growth, the difference between the two curves is more remarkable. The DSS welded joints belong to high toughness material with high hardening and the yield strain, a higher slope of passivation line should be choosen to combined with J-Aa resistance curves. The error of J0.2 by using the 3.75Rm passivation line slope is below 10%, compared with unloading compliance method, which shows that the Normalization method is suitable for obtaining fracture toughness of DSS.(7) During fracture toughness testing, the splitting phenomenon often occurs on the fractography. The splitting cracks cleave before steady ductile tearing extension. The existence of the splitting phenomenon leads to loss of constrains and makes ductile tearing extension take place, which improves fracture toughness. So the fracture toughness of the poorest HAZ decreases little compared with base and weld metal. The splitting behavior, microstructure and mechanical properties of the samples have significant effect on severity of constrain of the specimen, which changes fatigue crack growth behavior during pre-cracking and ductile tearing behavior during fracture toughness testing and affects the results of fracture toughness and the test accuracy. |
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