Super Duplex Stainless Steel
The development of duplex stainless steel has gone through 3 generations: the first generation is represented by the 3RE60 steel in the mid-1960s, which has the problem of welding heat affected zone; the second generation is after the 1970s, the development of ultra-low carbon and Nitrogen-containing characteristics, the representative grade is 2205, and the pitting equivalent index PRE is 32-39; the third generation is the ultra-low carbon, high molybdenum, high nitrogen content super duplex stainless steel developed in the late 1980s, and the representative grade is 2507, 32750, etc., the PRE value is greater than 40.
In recent years, with the increasing requirements of people and the development of duplex stainless steel, a new generation of duplex stainless steel material with higher strength and better corrosion resistance-super duplex stainless steel came into being, with 32707, 33207, 2707HD as they represent.
Its performance characteristics are
1. Chemical composition design and organizational characteristics
1) Ingredients
High alloying is the design idea of super duplex stainless steel. While further increasing the content of chromium, molybdenum, and nitrogen in the steel to improve strength and corrosion resistance, it also adjusts the reasonable composition ratio of other elements and the PRE value of super duplex stainless steel. Close to 50.
2) Control of phase ratio
The super duplex stainless steel is solid-solved at a lower temperature, and there is a certain amount of harmful phase σ phase in the structure. This phase can be basically solid-solved above 1100°C. At this time, the ferrite phase and the austenite phase each account for about half. In addition, in order to prevent the weaker phase from being preferentially corroded, the pitting resistance index (PRE) of the austenite and ferrite phases of the steel is designed to be equivalent. Taking 32707 as an example, the average difference between the PRE values of the two phases is less than 1.
2, corrosion resistance
32707 and 33207 have a theoretical pitting corrosion equivalent of about 50, the critical pitting temperature of both is higher than 90℃, while 32750 is about 80℃. In the use environment where standard austenitic stainless steels corrode quickly, super duplex stainless steels can even well replace high-alloy austenitic stainless steels and nickel-based alloys.
3. Mechanical properties
The yield strength of special super duplex stainless steel is twice that of austenitic stainless steel which is equivalent to its pitting corrosion resistance. The yield strength of 32707 material for heat exchanger tubes is generally 800MPa, the tensile strength is 1000MPa, and the elongation rate in the quenched heat treatment state is greater than 25%. 33207 has a higher tensile strength. In the quenched heat treatment state, the tensile strength is 20% higher than that of super duplex steel 32750. In addition, 32707 has good impact toughness, and its brittle transition temperature is lower than -50°C.
At present, the mainstream grades in my country are still 2205 traditional duplex stainless steel. Super duplex stainless steel has a small amount of production, while super duplex stainless steel has just started and is in the research and development stage. The research and development of domestic super duplex stainless steel should be actively promoted.