Duplex stainless steel pipe is a type of steel that combines many excellent properties such as excellent corrosion resistance, high strength, and ease of manufacturing and processing. Their physical properties are between austenitic stainless steel and ferritic stainless steel, but closer to ferritic stainless steel and carbon steel. The resistance to chloride pitting and crevice corrosion of duplex stainless steel pipes is related to its chromium, molybdenum, tungsten, and nitrogen content. It can be similar to 316 stainless steel or higher than seawater stainless steel such as 6% Mo austenitic stainless steel. The ability of all duplex stainless steel pipes to resist chloride stress corrosion fracture is significantly stronger than that of 300 series austenitic stainless steel, and its strength is also much higher than austenitic stainless steel while showing good plasticity and toughness.

Duplex stainless steel pipe is called “duplex” because its metallographic microstructure is composed of two stainless steel grains, ferrite and austenite. In the picture below, the yellow austenite phase is surrounded by the blue ferrite phase. When duplex stainless steel pipe melts, it first solidifies into a complete ferrite structure when it solidifies from the liquid state. As the material cools to room temperature, about half of the ferrite grains transform into austenite grains. The result is that approximately 50% of the microstructure is the austenite phase and 50% is the ferrite phase.

Duplex stainless steel pipe has a two-phase microstructure of austenite and ferrite
Characteristics of duplex stainless steel pipe
01-High strength: The strength of duplex stainless steel pipe is approximately 2 times that of conventional austenitic stainless steel or ferritic stainless steel. This allows designers to reduce wall thickness in certain applications.

02-Good toughness and ductility: Despite the high strength of duplex stainless steel pipes, they exhibit good plasticity and toughness. The toughness and ductility of duplex stainless steel pipes are significantly better than those of ferritic stainless steel and carbon steel, and they still maintain good toughness even at very low temperatures such as -40°C/F. But it still cannot reach the level of excellence of austenitic stainless steel. Minimum mechanical property limits for duplex stainless steel pipes specified by ASTM and EN standards

03-Corrosion resistance: The corrosion resistance of stainless steel mainly depends on its chemical composition. Duplex stainless steel pipes exhibit high corrosion resistance in most applications due to their high chromium content, which is favorable in oxidizing acids, and sufficient amounts of molybdenum and nickel to withstand moderate reduction Corrosion in acid media. The ability of duplex stainless steel pipes to resist chloride ion pitting and crevice corrosion depends on their chromium, molybdenum, tungsten, and nitrogen content. The relatively high chromium, molybdenum and nitrogen contents of duplex stainless steel pipes give them good resistance to chloride pitting and crevice corrosion. They come in a range of different corrosion resistances, ranging from grades equivalent to 316 stainless steel, such as economical duplex stainless steel pipe 2101, to grades equivalent to 6% molybdenum stainless steel, such as SAF 2507. Duplex stainless steel pipes have very good stress corrosion cracking (SCC) resistance, which is “inherited” from the ferrite side. The ability of all duplex stainless steel pipes to resist chloride stress corrosion cracking is significantly better than that of 300 series austenitic stainless steel. Standard austenitic stainless steel grades such as 304 and 316 may suffer from 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 pipes are often used instead of austenitic stainless steel.

04-Physical properties: Between austenitic stainless steel and ferritic stainless steel, but closer to ferritic stainless steel and carbon steel. It is generally believed that good performance can be obtained when the ratio of ferrite phase to austenite phase in duplex stainless steel pipe is 30% to 70%. However, duplex stainless steel pipes are often considered to be roughly half ferrite and half austenite. In current commercial production, to obtain the best toughness and processing characteristics, the proportion of austenite is slightly larger. The interaction between the main alloying elements, especially chromium, molybdenum, nitrogen, and nickel, is very complex. To obtain a stable two-phase structure that is beneficial to processing and manufacturing, care must be taken to ensure that each element has an appropriate content.

In addition to phase balance, the second major concern regarding duplex stainless steel pipe and its chemical composition is the formation of harmful intermetallic phases at elevated temperatures. σ phase and χ phase are formed in high chromium and high molybdenum stainless steel and preferentially precipitate in the ferrite phase. The addition of nitrogen greatly delays the formation of these phases. It is therefore important to maintain a sufficient amount of nitrogen in the solid solution. As experience with duplex stainless steel pipe manufacturing increases, the importance of controlling narrow compositional ranges is increasingly recognized. The initially set composition range of 2205 duplex stainless steel pipe is too wide. Experience shows that to obtain the best corrosion resistance and avoid the formation of intermetallic phases, the chromium, molybdenum, and nitrogen contents of S31803 should be kept at the middle and upper limits of the content range. This led to the improved 2205 dual-phase steel UNS S32205 with a narrow composition range.