The development and application of duplex stainless steel have experienced the first, second, and third generations of the development process of duplex stainless steel. 7 for acid oil and gas Wells with the requirements of oil well pipe and line pipe, Sweden has developed the second generation of SAF2205 duplex stainless steel. It is in a neutral chloride solution and H2S stress corrosion-resistant performance is better than 304 l, and 306 l austenitic stainless steel. In addition, due to the nitrogen, hole corrosion resistance is also very good, but also has good strength and toughness, can be cold, hot working, and good weldability, therefore is one of most applications of all the duplex stainless steel material.
About SAF 2507 Stainless Steel
SAF2507 after SAF2205, Sweden and developed the third generation of the super duplex stainless steel, used in harsh medium containing chloride. The steel grade PREN (pitting equivalent) = 43, ferrite and austenite phase 50% of each steel Gao Ming, high alumina and high nitrogen balance in the composition design, the steel has a high resistance to stress corrosion cracking, pitting corrosion and crevice corrosion resistance performance. The steel grade of the submarine pipeline was used in the north sea.
Duplex Austenite and Ferrite Stainless Steel
Duplex austenite and ferrite stainless steel to a certain extent with the characteristics of austenitic stainless steel and ferritic stainless steel, duplex stainless steel in the ideal organization is ferrite and austenite of 50% each with proper control of chemical composition and heat treatment process, can be the good toughness and weldability of austenitic stainless steel and ferritic stainless steel of high strength and resistance to chloride stress corrosion performance together.
Duplex stainless steel ferritic/austenitic best proportion of the problem is the key to its corrosion resistance. As Cr, Mo in high solid solubility of ferrite, and the tendency to M and N Yu Zaiao solid solution in the austenite, so relative to the best proportion of ferritic/austenitic, duplex stainless steel in the austenite phase increase will reduce the alloy elements such as Cr, Mo, Ni, the overall content of the solid solution and reduce the corrosion resistance of stainless steel; In addition, Cr, Mo content of ferrite is easy to precipitate σ and χ phase, reduced material toughness, stress corrosion sensitivity increase.
Duplex Stainless Steel Properties
Increasing the proportion of ferrite is equal to the reduced content of Cr, Mo in ferrite, also can reduce corrosion resistance; Austenite phase reduction on the one hand can reduce the impact toughness of duplex stainless steel, on the other hand, will cause the nitride precipitation. As a result, the organization of the duplex stainless steel is not only related to the composition but also related to thermal and processing technology, control is not good to make the material’s mechanical properties and corrosion-resistant performance, to a certain extent, also affect the use of duplex stainless steel.
Duplex stainless steel stress corrosion of the main influence factor for the Cl ion concentration, temperature, H2S partial pressure, pH value, and stress level. Its mechanism is the passivation membrane and ultimately affects the behavior of the crack. From the Angle of material, the influence factors include austenitic/ferrite and component level, and the proportion of cold deformation. Ferrite austenite compared with high transverse crack sensitivity, brittleness of sigma phase will significantly increase the brittleness of duplex stainless steel; Big grain crack sensitivity is higher than the sketch.
H2S will increase significantly dual phase steel on the absorption of hydrogen atoms, on the other hand, will increase the dissolution of ferrite in the active zone, and the austenite activation/passivation. Once the ferrite on the surface of the passivation film is destroyed, it will be hard to repair, eventually leading to local corrosion and crack. There is, of course, Cl – catalysis, when no Cl – in the solution, won’t produce local corrosion or cracks.
NACE standard MR0175 / IS015156-3 in the United States the use of duplex stainless steel more stringent environmental restrictions: H2S partial pressure ≤ 20 kpa, temperature, pH value in any combination. The current regulations are controversial. Many experiments prove that: to a certain temperature, salinity, and Cl – concentration range, duplex stainless steel in a 0.1 MPa H2S environment doesn’t craze. Reports confirm the duplex stainless steel can be used for the lMPa H2S environment.
Duplex stainless steel in the thermoplastic is poorer, the reason is that it is hot working when the deformation behavior of austenite and ferrite phases is different. Due to two different phases of the softening process, hot working in two-phase produces a non-uniform distribution of stress and strain, which leads to interfacial crack nucleation and propagation. Therefore, the traditional hot extrusion process commonly used duplex stainless steel hot working.
Kawasaki steel company adopts Mannesmann piercing and successfully produced KLC – 22 cr duplex stainless steel (SAF2205). Reducing the S content in the steel, improve the deformation capacity of steel; By adding Ca, will work with soluble S fixed in the steel, so as to further improve the deformation capacity of steel. At the same time, according to the duplex stainless steel in the tandem rolling mill and the automatic pipe mill rolling groove roll gap in the pipe wall thickness is easy to pull thin even with perforation problems, to formulate the appropriate rolling schedule.