| With the rapid development of high-end equipment manufacturing industry in energy conservation and environmental protection,marine engineering,petrochemical and other key areas,the urgent demand for high-performance,long-life,low-cost key materials is increasing with each passing day.As the spire of stainless steel,super austenitic stainless steel is favored for its super corrosion resistance,excellent comprehensive mechanical properties and relatively low cost advantages.It has become one of the most urgently needed key materials in high-end equipment manufacturing industry.However,super austenitic stainless steel is one type of stainless steels with the highest technical requirements and manufacturing difficulties.At present,only a few international enterprises master the core manufacturing technology and keep it strictly confidential.The research and development level of super austenitic stainless steel in China is backward.High-grade products have not yet been localized and rely heavily on imports.Their high prices and long delivery cycles have become a "bottleneck" problem restricting the transformation and upgrading of China's high-end equipment manufacturing industry.This paper focuses on the high cleanliness smelting,hot deformation behavior,aging precipitation and intergranular corrosion behavior,corrosion behavior in typical service environment and friction stir welding process of super austenitic stainless steel S32654.The aim is to provide support for the development of high performance super austenitic stainless steel in China and further satisfy the urgent need of materials for transformation and upgrading of high-end equipment manufacturing industry.A high-cleanliness smelting process with Al+Mg+Ce composite treatment has been developed.The oxygen and sulfur contents have been controlled at ultra-low level and the inclusions have been treated to be harmless.The addition of Mg modifies Al2O3 to MgAl2O4.Then,the MgAl2O4 is modified to CeAlO3 and MgO by adding a small amount of Ce.With the increase of Ce content,oxide and sulfide inclusions are completely modified to Ce2O3 and CeS,respectively.According to the effect of Ce on the oxygen and sulfur contents and the inclusion characteristics,the Ce content should be controlled at 0.008%?0.024%.With the increase of deformation temperature and rate,the deformation resistance gradually decreases and increases,respectively.With the N content increasing,the dislocation motion is blocked.As a result,the deformation resistance increases,and the hot working process become more difficult.Based on the Zener-Hollomon parameter and Arrhenius hyperbolic sine function,the flow stress constitutive equation of S32654 has been established.It is found that increasing the N content results in a significant increase in the activation energy of hot deformation(L-N steel:767.54 kJ·mol-1 and H-N steel:834.22 kJ·mol-1).The dynamic recrystallization mechanism in the hot deformation process is the"bumping" nucleation of the grain boundary.As the deformation temperature increased,the dislocation density decreases,and the dynamic recrystallization degree increases.With the deformation rate increasing,the adiabatic heating effect is enhanced.Thus,the dynamic recrystallization degree first decreases and then increases.Under different deformation conditions,N significantly inhibits the dislocation motion and the transitions from the low-angle grain boundary to the ?3 or ?9.Meanwhile,N also promotes the intergranular precipitation,enhancing the pinning effect of precipitates on the grain boundary.The joint effects of N in these three aspects significantly inhibit the occurrence of dynamic recrystallization,narrowing the optimal hot working window(L-N steel:1150?1200?,0.5?4.5 s-1 and 1200?1250?,0.2?4.5 s-1;H-N steel:1200?1250?,0.5?4.5 s-1).At 700?1100?,the precipitation of ? phase with the increase of temperature follows a sequence of intergranular,cellular and intragranular.The cellular ? phase was detected in S32654 for the first time,and its growth process is successively controlled by intergranular diffusion and then by bulk diffusion of Cr and Mo.Both Cr and Mo bulk diffusion and large needle-like ? phase hinder the growth of cellular ? phase.After long term aging,cellular ?phase could induce the precipitation of Cr2N ahead of cell boundary,between lamellar ?phases and at the ?/? interfaces.Local parts of needle-like ? phase gradually transform into Cr2N and R phase through Cr and Mo diffusion.At 1000?,? phase,Cr2N and ? phase nucleate in succession and grow up gradually.An efficient.reliable and quantitative DL-EPR method for evaluating the intergranular corrosion susceptibility of S32654 has been established.The precipitation of secondary phases results in the formation of obvious Cr and Mo depletion zones around them,causing intergranular corrosion.With the increase of Cr content,the driving force for ? phase and the Cr activity increase,and the driving force for Cr2N and the Mo activity decrease,which promote the precipitation of ? phase and inhibit the formation of Cr2N,R and ? phase.With the increase of Mo content,the driving force for ? phase and the Mo activity increase,and the driving force for Cr2N and the Cr activity decrease,which significantly accelerate the precipitation of ? phase and suppress the formation of Cr2N and ? phase.Mo has stronger promotion effect on the formation of ? phase than Cr.The promotion effect of Cr and Mo on precipitation accelerates the formation of massive Cr-and Mo-depleted zones and increases intergranular corrosion susceptibility.With the increase of N content,the driving force for Cr2N and the Mo activity increase,and the driving force for ? phase and the Cr activity decrease,which promote the precipitation of Cr2N and ? phase and inhibit the formation of ? phase.Affected by total amounts of precipitates,the intergranular corrosion susceptibility first decreases and then increases with the N content increasing.In simulated waste incineration environment,S32654 did not suffer hot corrosion at 400 and 500?,and its corrosion resistance is equivalent to that of S31254 and Inconel 625.At 600?,S32654 suffers catastrophic corrosion under the deterioration mechanisms of"electrochemical corrosion+chlorine activation corrosion" of molten chloride and "alkaline flux corrosion" of molten sulfate.At 800?,the corrosion of molten chloride and sulfate is accelerated significantly.The formation of precipitates accelerates the Cr and Mo depletion of the matrix,reduces the formation ability of protective oxide layer and promotes the external diffusion of elements.Under the combined action of molten salts and precipitates,S32654 suffers catastrophic corrosion.S32654 has a comparable hot corrosion resistance with Inconel 625 below 600?.It could be an ideal candidate material for waste incineration equipment.In simulated flue gas desulfurization environment,the corrosion resistance of S32654 are significantly better than that of nickel-based alloy C-276.The pitting corrosion of both materials originates at the inclusion/matrix interface and extends to the inclusion.The immersion corrosion of S32654 and C-276 is mainly slight pitting corrosion and severe intergranular corrosion,respectively.The corrosion degree increases gradually with time and temperature.High Cr and N contents are beneficial to the formation of Cr2O3,NH3 and CrN in passivation film,which promotes the formation of more compact,stable and protective passivation film in S32654.Therefore,the corrosion resistance of S32654 is better than that of C-276.The mechanical properties of S32654 are also superior to those of C-276,and it is expected to be an alternative material and used in flue gas desulfurization equipment.S32654 has been successfully welded by friction stir welding.The sound joints with no defects and nitrogen loss,good surface and internal quality have been successfully produced.Under the combined action of thermal cycling and strong mechanical stirring,apparent continuous dynamic recrystallization occurs in the weld stir zone,resulting in high grain refinement.Moreover,the strain rate plays a dominant role in the grain refinement of S32654.The combination of grain refinement,high density dislocations,and substructures improves the hardness and strength,but significantly reduces the plasticity of the welded joints.The more suitable welding parameters are determined as 300 rpm and 100 mm·min-1 for S32654. |
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