What is the difference between 2205 and 2507 duplex steel?

What is the difference between 2205 and 2507 duplex steel?

Duplex stainless steel is called "duplex" because its metallurgical microstructure is composed of two stainless steel grains, ferritic and austenitic. In the image below, the yellow austenitic phase appears as an "island" surrounded by a blue ferritic ocean. When duplex stainless steel is melted, it first solidifies into a complete ferritic structure from liquid solidification, and as the material cools to room temperature, about half of the ferritic grains transform into austenitic grains (" islands "). As a result, the microstructure is approximately 50% austenitic and 50% ferritic.

The strength of duplex stainless steel is about twice that of conventional austenitic stainless steel or ferritic stainless steel. Therefore, designers can reduce the wall thickness in some applications. The following figure compares the yield strength of several duplex stainless steels and 316L austenitic stainless steels at temperatures ranging from room temperature to 300 ° C.

Despite their high strength, duplex stainless steels exhibit good plasticity and toughness. The toughness and ductility of duplex stainless steel are significantly better than that of ferritic stainless steel and carbon steel, and maintain good toughness even at very low temperatures such as -40 ° C /F. But not up to the excellent degree of austenitic stainless steel.

The corrosion resistance of stainless steel mainly depends on its chemical composition. In most application environments, duplex stainless steels show high corrosion resistance due to their high chromium content, which is favorable in oxidizing acids, and contain sufficient amounts of molybdenum and nickel to resist corrosion in medium reducing acid media.

The ability of duplex stainless steel to resist chloride ion pitting and crevice corrosion depends on its chromium, molybdenum, tungsten and nitrogen content. The relatively high chromium, molybdenum and nitrogen content of duplex stainless steels gives them good resistance to chloride pitting and crevices. They are available in a range of different corrosion resistance grades, both equivalent to 316 stainless steel corrosion resistance, such as economic duplex stainless steel 2101©, and equivalent to 6% molybdenum stainless steel corrosion resistance grades, such as SAF 2507©.

Duplex stainless steel has very good stress corrosion cracking (SCC) resistance, which is "inherited" from the ferrite side. The resistance of all duplex stainless steels to chloride stress corrosion cracking is significantly better than that of 300 series austenitic stainless steels. Standard austenitic stainless steel grades, such as 304 and 316, may suffer stress corrosion cracking in the presence of chloride ions, humid air and elevated temperatures. Therefore, in many applications in the chemical industry where there is a greater risk of stress corrosion, duplex stainless steel is often used to replace the use of austenitic stainless steel.