Performance characteristics of duplex stainless steel

Duplex stainless steel is a member of the stainless steel family. Its structure is composed of austenite and ferrite, and it has the characteristics of both ferrite and austenitic stainless steel. Compared with austenitic stainless steel, it has high strength and good corrosion resistance, especially the chloride ion corrosion resistance has been significantly improved.In environments with high chloride ion content such as neutral chloride environment and seawater environment, the commonly used corrosion-resistant alloy austenitic stainless steel will experience pitting and stress corrosion cracking and cannot be effectively used. At this time, duplex stainless steel has attracted widespread attention for its excellent chloride ion corrosion resistance, and is used in design conditions where austenitic stainless steel cannot be used, and gradually replaces it. However, to ensure the performance of duplex stainless steel and enable it to serve in engineering projects for a long time and effectively, it is necessary to understand the performance characteristics of duplex stainless steel, so as to ensure its various performances through necessary measures.

At present, the commonly used duplex stainless steels at home and abroad are S31803, S32205, S32750, S32760, etc. This chapter will take the S31803 pipe in the American standard system as an example to introduce the characteristics of the metallographic structure, chemical composition, heat treatment, mechanical properties, corrosion resistance and welding performance of duplex stainless steel.

Metallographic structure

The mechanical properties and corrosion resistance of duplex stainless steel pipe are closely related to the equilibrium ratio (phase ratio) of its main phase composition-ferrite and austenite, and the phase ratio depends to a large extent on the chemical composition and heating temperature of the steel.

Chemical composition

The main alloying elements in duplex stainless steel have a great influence on its mechanical properties and corrosion resistance. Among them, Cr is a ferrite-forming element. A higher Cr content can increase the yield strength, increase the passivation current of steel, and reduce the pitting rate. However, too high Cr content causes the precipitation of intermetallic phase (σ phase), which reduces the toughness and plasticity of duplex stainless steel, significantly increases the brittle transition temperature, and reduces the corrosion resistance. Ni is an austenite-forming element, which can promote the formation and expansion of austenite phase, but too high Ni content causes Cr and Mo elements to be enriched in ferrite to form σ phase. Mo is a ferrite-forming element, which can effectively improve the chloride ion corrosion resistance of duplex stainless steel, but too high Mo content makes the steel brittle. N is an austenite-forming element. N can replace metal Ni and play a role in saving Ni. N can also improve the strength of steel, inhibit the precipitation of σ phase, and improve its local corrosion resistance, creep resistance, and fatigue wear resistance.

Heat treatment

S31803 requires solution annealing and quenching. The heat treatment temperature of S31803 is 1020~1100℃. The structure of duplex stainless steel after solution treatment depends on the solution treatment temperature. Luo Sheji et al. studied the effect of heat treatment process on the microstructure and brittle precipitation phase morphology of 22Cr duplex stainless steel plate. The study showed that only when the heat treatment temperature is within the specified range, the structure after solution treatment will show α+γ phase interphase distribution. Rapid cooling is required after solution treatment to avoid the precipitation of brittle phase.

Mechanical properties

Duplex stainless steel has high strength and good toughness, and its yield strength is almost more than twice that of austenitic stainless steels such as SS304L and SS316L.

Corrosion resistance

The corrosion resistance of duplex stainless steel includes local corrosion resistance and uniform corrosion resistance. Among them, local corrosion includes pitting, intergranular corrosion, stress corrosion cracking, galvanic corrosion, selective corrosion, etc. The following will focus on the more common pitting resistance, intergranular corrosion resistance and stress corrosion cracking resistance of duplex stainless steel.

Pitting corrosion resistance

The so-called pitting corrosion is the corrosion morphology of pits or holes in the form of pits, so pitting corrosion is also called pitting corrosion, which is extremely harmful to metals. Compared with austenitic stainless steel, duplex stainless steel has a higher pitting equivalent and better pitting corrosion resistance. However, pitting corrosion is the most important form of corrosion in duplex stainless steel, and pitting corrosion resistance is one of the important properties to measure duplex stainless steel.

Intergranular corrosion resistance

Intergranular corrosion refers to corrosion that occurs between grain boundaries. Due to the presence of two phases in duplex stainless steel and the high Cr content, Cr23C6 is easily formed at the junction of austenite and ferrite, making the surrounding area a chromium-poor area. The chromium-poor area and austenite become the anode and cathode of the electrochemical reaction, respectively, accelerating the corrosion at the grain boundary.

Stress corrosion cracking resistance

Stress corrosion cracking refers to cracking caused by the combined action of stress and corrosive environment, including stress corrosion cracking SCC under static stress and corrosion fatigue CF under cyclic stress. The SCC critical temperature of duplex stainless steel is 175℃, which is much higher than the SSC critical temperature of austenitic stainless steel. This makes duplex stainless steel have better stress corrosion resistance than austenitic stainless steel. From the perspective of organizational structure, duplex stainless steel has high yield strength, is not prone to coarse slip, and is not prone to stress corrosion cracking; the presence of the second phase acts as a barrier to the expansion of stress corrosion cracking; ferrite plays an electrochemical protective role on austenite. Therefore, duplex stainless steel has better stress corrosion resistance than austenitic stainless steel.

Welding performance

The carbon content of duplex stainless steel is generally less than 0.03%, which is ultra-low carbon, with few impurities. It is not easy to produce a low-melting point liquid film during welding. At the same time, it has a higher thermal conductivity and a lower expansion coefficient than austenitic stainless steel, and is not prone to thermal cracking after welding. Even so, improper welding process or improper selection of welding materials may cause imbalance of welded joint organization and the generation of harmful phases, thereby affecting the performance of welded joints.https://www.hengyangpiping.com/