Study On The Microstructure And Corrosion Behavior Of 2507 Duplex Stainless Steel

2507 duplex stainless steel was developed in the 1980 s. It has two kinds of microstructures(ferrite phase and austenite phase), the volume ratio of ferrite phase and austenite phase is about 1:1 generally, and the minimum content of the two phases is also required up to 30% at least. The Pitting Resistance Equivalent Number(PREN) of 2507 duplex stainless steel is greater than 40, so it achieves the considerably high resistance performance to pitting corrosion. 2507 duplex stainless steel is highly alloyed and it has many excellent properties such as good resistance to chloride stress corrosion at high temperatures, high strength, good welding performance and toughness, so it is widely used in the oil and natural gas. With the rapid development of modern industry, the requirements of use duplex stainless steel become more and more strict. Therefore, doing research on microstructure and properties of 2507 duplex stainless steel has some guiding significance for practical production.In this paper, phase structure of DSS2507 after solution treatment at different temperatures was studied by quantitative metallography, the effect of solution treatment temperature, solution temperature, sulfide ion and molybdate ion on corrosion resistance properties were researched by electrochemical test, slow strain rate test and hydrogen permeation experiment.The results of quantitative metallography indicated that the content of ferrite phase increased but austenite phase decreased with the increase of solution treatment temperature, at 1050℃ the proportion of ferrite phase and austenite phase was about 1:1, at 1000℃ there was some sigma phase(σ) precipitating in the ferrite/austenite interfaces. The hardness of DSS2507 decreased firstly and then increased with the increase of solution treatment temperature, when the solution treatment temperature was at the range of 1000℃ to1050℃,the hardness decreased, with the solution treatment temperature increased from 1050 ℃to 1200℃, the hardness showed a tendency to increase.The results of electrochemical test indicated that corrosion pits mainly exists in the ferrite phase, the capacity of resist corrosion for austenitic phase is better than ferrite phase, moreover, the capacity of resist uniform corrosion and pitting corrosion increased firstly and then decreased with the solution treatment temperature increase from 1000 ℃to 1200℃, and at 1050 ℃the electrochemical corrosion resistance of DSS2507 was the best.The capacity of resist electrochemical corrosion decreased with higher the concentration of sulfide ion and the solution temperature, but increased with higher the concentration of molybdate ion. Sulfide ion increase the corrosion pits and when the concentration of molybdate ion was higher than 0.005 M, the effect of inhibition on corrosion was more obvious.With the increase of solution treatment temperature from 1000℃ to 1200℃, stress corrosion cracks spread in the ferritic phase priority and terminate in austenite phase, the capacity of resist stress corrosion cracking increased firstly and then decreased, at 1050 ℃the capacity of resist stress corrosion cracking was the strongest, showing as typical ductile fracture. The stress corrosion cracking sensitivity coefficient increased with higher the concentration of sulfide ion, but decreased with the addition of molybdate ion, so sulfide ion decreased the capacity of resist stress corrosion cracking, but molybdate ion increased the capacity.It can be obtained from hydrogen permeation experiment that hydrogen permeation coefficient increased with the higher solution treatment temperature and the concentration of sulfide ion, so the hydrogen embrittlement sensitivity of DSS2507 become strong, the resistance to hydrogen embrittlement performance become weak. However, the addition of molybdate ion to solution inhibited the diffusion of hydrogen in steel, decreased the hydrogen embrittlement sensitivity and increased the capacity of resist hydrogen embrittlement.