| Duplex stainless steel has been widely used in petroleum, chemical industry, military and other fields due to its special characteristics of two-phase microstructure. Duplex stainless steel exhibits complex precipitation transformation process in the long-term thermal insulation at 300-1000℃ due to high content of alloy elements, which results in the decline of mechanical and corrosion resistance performance in use process. Ni as a kind of noble metal element which is heavily used in the duplex stainless steel, make the price of steel is still high. Mn is the element that can expand the austenite phase region, and it can be used to replace the Ni in duplex stainless steel to reduce the cost. In this paper, by comparing the type, morphology and distribution of precipitation phase, as well as the influence of precipitation on the impact toughness, pitting corrosion resistance properties and intergranular corrosion sensitivity of three kinds of different Mn content of 19% Cr Ni-free duplex stainless after solution treatment at 800℃,700℃,600℃ and 500℃ for varies times, the effect of Mn on the aging behavior of 19% Cr Ni-saving duplex stainless steel were studied.The effect of solution treatment temperature on microstructure was studied. The results show that the tested steels exhibit typical two-phase microstructures after solution treatment at 1040-1220℃, and no other precipitates exist. Besides, the two-phase ratio is close to 1:1 when the solution treatment is applied at 1120℃. Steels have a better overall property.High temperature aging treatment research results show that σ phase precipitated in the stainless steel when aged at 800℃. After aging treatment for 54 hours, when Mn content is high, the precipitation speed of σ phase is fast and the size is large. Finally, a lamellar σ phase covered in ferrite base; when the Mn content is low, fine σ phase particles are dispersed in the phase boundary of austenite and ferrite. σ phase precipitation accelerates the decline of impact energy. After aging treatment for 54 hours, the impact energy of stainless steel with high Mn content is significantly lower than that of a low Mn and a medium Mn content. The pitting corrosion equivalent decreases rapidly with the precipitation of σ phase. The intergranular corrosion sensitivity are obviously influenced by σ phase and Cr23C6, and intergranular corrosion sensitivity increases rapidly when a large number of precipitated phases precipitate.Research results of intermediate temperature aging treatment show that rectangular R phase and Cr23C6 precipitated in the ferrite at the early stage of 600℃ aging treatment, and with prolonging of aging time, R phase transforms into the σ phase in the late period of aging. The impact energy of low and middle Mn content stainless steels decreases slowly during aging process. The impact energy of high Mn content after aging treatment at 600℃ for 108 hours decreases to 8 J, which is only the 5% of impact energy aging for 11.5 hours. The pitting corrosion potential of low and middle Mn content steels decreases slowly during aging time expanding. The pitting corrosion potential of high Mn content steels decrease rapidly after 24 hours and then remain at a low level. The intergranular corrosion sensitivity of three kinds of stainless steels all increase slowly during aging time expanding.Under the condition of low temperature aging treatment, a large number of dislocation lines appear in the ferrite of duplex stainless steel aged at 500℃. The appearance of dislocation lines show that ferrite tends to spinodal decomposition, also R phase and Cr2N particles precipitate in the ferrite.χ phase precipitation in the high Mn content stainless steel reduces the trend of spinodal decomposition that occurs in ferrite, and the number of dislocation lines in the matrix decreases significantly. After aging treatment for 144 hours, the impact energy of three kinds of tested stainless steel are still higher than 107 J, shows that tested steels has a good impact toughness after aging at 500℃. Meanwhile, it can be stated that there is no α’ phase precipitation happens.The increase of Mn content plays a significant role in promoting the precipitation of aging precipitates. With the increase of Mn content, the precipitation rate is accelerated, the phase transition time is advanced and the size of the precipitates increases. One of the reasons is that the increasing Mn content contributes to the increase of austenite content in the duplex stainless steel, which leads to more ferrite forming element Cr enriched in the ferrite and provides the conditions for the formation of the precipitates. The other is Mn element is directly involved in the formation of σ phase and χ phase, and the segregation of Mn element in two phases are found. More importantly, Mn content is higher in the χ phase.
|
PDF Full Text Request