The current history of duplex stainless steel has reached the third generation. The first generation represented the 3RE60 stainless steel in the mid-1960s, which posed the problem of HAZ. The second generation was developed in the 1970s , Is characterized by ultra-low carbon and nitrogen, representing the grade 2205, the pitting equivalence index PRE is 32 to 39; the third generation is super-low carbon, high molybdenum, high nitrogen content developed in the late 1980s Duplex stainless steel, on behalf of grades 2507,32750, etc., their PRE value must be greater than 40.
Now, with the use of stainless steel higher and higher requirements, duplex stainless steel has developed a new generation of higher strength, better corrosion resistance of duplex stainless steel – super-duplex stainless steel, on behalf of the steel is 32,707, 33207,2707HD.
The new generation of super duplex stainless steel product features can be elaborated from three angles, the first is the chemical composition and microstructure, the design of such stainless steel materials is high alloying, not only to further increase the chromium, molybdenum, nitrogen content To improve the strength and corrosion resistance, but also a reasonable adjustment of the ratio of other elements, special super duplex stainless steel PRE value close to 50.
In addition, the proportion of metallurgical phase is also controlled. When the super duplex stainless steel is solid solution at lower temperature, there is a certain amount of harmful phase σ phase in the microstructure, and the phase can basically dissolve when it is above 1100 ℃. The austenite phase and the austenite phase each account for approximately half, and to avoid the weaker phase being preferentially corroded, the respective pitting resistance index (PRE) design of both austenite and ferrite phases in the steel is equivalent to 32707 , The average PRE value of each phase is less than 1 on average.
Second is the corrosion resistance, 32707 and 33207 theoretical equivalent pitting resistance at about 50, the critical pitting temperature of both are higher than 90 ℃, and 32750 is about 80 ℃. Ultra-duplex stainless steels can even replace high-alloy austenitic stainless steels and nickel-base alloys even in the most corrosive environments where standard austenitic stainless steels are used.
The third is the mechanical properties, the super duplex stainless steel yield strength and pitting resistance and its almost equal to 2 times the performance of austenitic stainless steel. The heat exchanger tube using 32707 material yield strength is usually 800Mpa, tensile strength is 1000Mpa, quenching heat treatment state elongation greater than 25%. The 33207 stainless steel has better tensile strength, quenching heat treatment, the tensile strength is higher than the super dual-phase steel 32750 about 20%. In addition, the impact toughness of 32707 is also very good, its brittle transition temperature below -50 ℃.
Of course, the domestic mainstream now also 2205 types of traditional duplex stainless steel, super duplex stainless steel production is still relatively small, and special super duplex stainless steel has just started, still in the research and development stage, the need to speed up the domestic special super duplex stainless steel Development and production.
Source: China Stainless Steel Pipe Fittings Manufacturer – Yaang Pipe Industry Co., Limited (www.yaang.com)