The first generation of duplex stainless steel is 3re60 in the 1960s, which has the problem of welding thermal zone. The second generation was developed in the late 1970s with ultra-low carbon and nitrogen bearing characteristics, the typical number was 2205, and the steel’s pitting equivalent index was 32-39. The third generation is ultra-low carbon, high molybdenum and high nitrogen content, which were developed in the late 1980s. The typical steel is 2507, 32750, etc., and the etching equivalent index is greater than 40.
Until now, along with the rapid development of duplex stainless steel production technology, strength and more excellent corrosion resistance of a new generation of duplex stainless steel, super duplex stainless steel has appeared, on behalf of the steel grade is 32707, 33207, 2707, hd.
Their performance characteristics are mainly embodied in chemical composition, metallographic structure, corrosion resistance and mechanical properties.
First for the chemical composition, design of super duplex stainless steel is a high alloying, in addition to increased again chromium, molybdenum and nitrogen content in stainless steel to enhance its strength and corrosion resistance, also need to regulate other elements of the allocation, which will improve to nearly 50 pitting equivalent index.
Followed by the phase ratio regulation, super duplex stainless steel in low temperature solid solution, there is a certain amount of harmful phase in the microstructure of sigma phase, the phase at above 1100 ℃ can basic solution, the ferrite and austenite facies can be roughly half, in addition to avoid the weaker phase first from corrosion, including each of the two phase austenite and ferrite of pitting corrosion resistance of the index design were similar, 32707, for example, the pre values in which each of the two phase of an average difference of less than 1.
The third is the corrosion resistance performance, pitting the theory of equivalent to 32707 and 33207 about 50, the critical pitting temperature should be higher than 90 ℃, and 32750 is about 80 ℃. In the standard austenitic stainless steel corrosion fast use environment, super duplex stainless steel can also replace the high alloy austenitic stainless steel and nickel base alloy.
In the end, the characteristics of mechanical properties, the yield strength of super duplex stainless steel is twice as high as that of austenitic stainless steel. The yield strength of the heat exchanger for 32707 stainless steel is usually 800mpa, the tensile strength reaches 1000mpa, and the elongation rate of quenching heat treatment state is higher than 25%. And 33207 has better tensile strength. Under the condition of quenching heat treatment, the tensile strength is 20% higher than that of super duplex stainless steel 32750. In addition, the 32707 has the very good impact toughness, its brittle transition temperature less than – 50 ℃.
Although the latest “super duplex stainless steel has so many advantages, but at present domestic application or a maximum of2205 duplex stainless steel such as traditional second generation, super duplex stainless steel output is less, and special research and development of super duplex stainless steel is in early stage.
Source: China Stainless Steel Pipe Fittings Manufacturer – Yaang Pipe Industry Co., Limited (www.yaang.com)