Related properties of duplex stainless steel

Duplex stainless steel is a kind of steel with excellent corrosion resistance, high strength and easy manufacturing. Their physical properties are between austenitic stainless steel and ferritic stainless steel, but more similar to ferritic stainless steel and carbon steel. The resistance of duplex stainless steel to chloride pitting and crevices is related to its chromium, molybdenum, tungsten and nitrogen contents, and can be similar to 316 stainless steel or higher than that of seawater stainless steel such as 6%Mo austenitic stainless steel. The resistance of all duplex stainless steels to chloride stress corrosion fracture is obviously stronger than that of 300 series austenitic stainless steels, and its strength is also much higher than that of austenitic stainless steels, while showing good plasticity and toughness.

Biphasic stainless steel is called "biphasic" because its microstructure is composed of ferritic and austenitic stainless steel grains. In the image below, the yellow austenitic phase is surrounded by the blue ferritic phase. When biphasic stainless steel is melted, it first solidifies into a complete ferrite structure from its liquid setting. As the material cools to room temperature, about half of the ferrite grains change to austenitic grains. The result is a microstructure that is approximately 50% austenitic and 50% ferritic.

Duplex stainless steel has austenitic and ferritic two-phase microstructure

Characteristics of duplex stainless steel

01- High Strength

Duplex stainless steel is about twice as strong as conventional austenitic or ferritic stainless steel. So designers can reduce the thickness of the wall in some applications. The following figure compares the yield strength of several duplex stainless steels with that of 316L austenitic stainless steels at temperatures ranging from room temperature to 300℃.

02- Good toughness and ductility

Despite their high strength, duplex stainless steels exhibit good ductility and toughness. The toughness and ductility of duplex stainless steel is significantly superior to that of ferritic stainless steel and carbon steel, maintaining good toughness even at very low temperatures such as -40℃/F. But not as good as austenitic stainless steel.

03- Corrosion resistance

Corrosion resistance of stainless steel depends mainly on its chemical composition. In most applications, duplex stainless steels show high corrosion resistance due to their high chromium content, favorable in oxidizing acids, and adequate amounts of molybdenum and nickel to resist corrosion in medium reducing acid media.

The resistance of duplex stainless steel to chlorine ion pitting and crevices depends on its chromium, molybdenum, tungsten and nitrogen contents. The relatively high chromium, molybdenum and nitrogen content of duplex stainless steel makes them have good resistance to chloride pitting and crevices corrosion. They are available in a range of different corrosion resistance grades, both equivalent to 316 stainless steel corrosion resistance, such as economic duplex 2101, and to 6% molybdenum stainless steel corrosion resistance, 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 obviously 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, duplex stainless steel is often used to replace austenitic stainless steel in many applications in the chemical industry where the risk of stress corrosion is greater.

04- Physical properties

Between austenitic stainless steel and ferritic stainless steel, but closer to ferritic stainless steel and carbon steel.

Chemical composition of duplex stainless steel

It is generally believed that good properties can be obtained when the ratio of ferrite phase to austenitic phase in duplex stainless steel is 30% ~ 70%. But duplex stainless steel is often considered to be roughly 50-50 ferrite and austenite, and in current commercial production, in order to obtain the best toughness and processing characteristics, the proportion of austenite tends to be slightly larger. The interactions between the main alloying elements, especially chromium, molybdenum, nitrogen and nickel, are very complex. In order to obtain a stable biphasic structure conducive to processing and manufacturing care must be taken to have the appropriate amount of each element.

In addition to phase equilibrium, the second major problem concerning duplex stainless steel and its chemical composition is the formation of harmful intermetallic phases at elevated temperatures. σphase and χ-phase form in high chromium and high molybdenum stainless steel, and are preferently precipitated in ferrite phase. The addition of nitrogen greatly delayed the formation of these phases. So it's important to keep enough nitrogen in the solid solution. With the increase of experience in the manufacture of duplex stainless steel, the importance of controlling a narrow range of components is becoming more and more recognized. 2205 duplex stainless steel (UNS S31803, Table 1) was initially set with an excessively wide composition range. Experience has shown that for optimal corrosion resistance and to avoid the formation of intermetallic phases, the chromium, molybdenum, and nitrogen contents of S31803 should be kept in the upper middle of the content range. Thus, an improved 2205 two-phase steel UNS S32205 with a narrow composition range was derived (Table 1).