Duplex stainless steel is a special stainless steel material in which ferrite phase (α phase) and austenite phase (γ phase) each account for a certain proportion in the solid solution structure, and the minimum phase content is usually 30%. Because it combines the advantages of ferrite stainless steel and austenite stainless steel, such as high strength, good plasticity and toughness, excellent stress corrosion resistance, etc., duplex stainless steel has been widely used in industrial fields such as petrochemical equipment, seawater and wastewater treatment equipment, oil and gas pipelines, and papermaking machinery. However, the performance of duplex stainless steel depends largely on the control of its phase ratio, especially the mechanical properties.

The mechanical properties of duplex stainless steel, including tensile strength, yield strength, impact toughness, etc., are directly affected by the phase ratio. Specifically, ferrite phase and austenite phase show different characteristics in mechanical behavior:

Ferrite phase: mainly affects the strength properties of duplex stainless steel. With the increase of ferrite, the yield strength and tensile strength of steel usually increase, but the plasticity and toughness decrease. In addition, the increase of ferrite phase will also lead to an increase in the brittle transition temperature of steel, making the material more susceptible to brittle fracture at low temperatures.
Austenite phase: mainly affects the plasticity and toughness of duplex stainless steel. As the amount of austenite increases, the elongation and impact toughness of the steel will increase, thereby enhancing the material's resistance to deformation and impact. The presence of austenite phase also helps to reduce the brittle transition temperature of the material and improve low-temperature toughness.

Specific manifestations of phase ratio control on mechanical properties
Tensile strength and yield strength:
Within a certain range, as the proportion of ferrite phase increases, the tensile strength and yield strength of duplex stainless steel will increase accordingly. However, when the proportion of ferrite phase is too high, although the strength may continue to increase, the plasticity and toughness will decrease significantly, resulting in the deterioration of the overall performance of the material.
The increase of austenite phase will lead to a decrease in yield strength, but the tensile strength may reach the highest value within a certain austenite phase ratio range. This is because the austenite phase has good plasticity and toughness and can absorb more energy without breaking.
Impact toughness:
Impact toughness is an important indicator of a material's ability to resist impact loads. As the proportion of austenite increases, the impact toughness of duplex stainless steel increases significantly. This is because the austenite phase has a lower yield strength and higher plastic deformation capacity, and can absorb more energy under impact loads.
In contrast, an increase in the proportion of ferrite will lead to a decrease in impact toughness. When the proportion of ferrite is too high, the material is more likely to undergo brittle fracture under impact loads.
Plasticity and toughness:
Plasticity refers to the ability of a material to undergo plastic deformation without destruction after being subjected to force. The increase in austenite phase will increase the plasticity of duplex stainless steel because the austenite phase has better plasticity and deformation capacity.
Toughness refers to the ability of a material to absorb a large amount of energy without brittle fracture after being subjected to force. Similarly, the increase in austenite phase will increase the toughness of duplex stainless steel.

In order to ensure that duplex stainless steel has optimal mechanical properties, the phase ratio needs to be precisely controlled. Here are some optimization strategies:
Chemical composition adjustment:
By adjusting the content of alloying elements, such as chromium, nickel, molybdenum, etc., the ratio of ferrite phase and austenite phase in duplex stainless steel can be changed. For example, increasing the nickel content can promote the formation of austenite phase, while increasing the chromium content helps stabilize the ferrite phase.
Heat treatment process optimization:
Heat treatment is an important means to adjust the phase ratio of duplex stainless steel. By controlling the solution treatment temperature and cooling rate, the phase ratio can be optimized to obtain the desired mechanical properties. For example, rapid cooling can inhibit the precipitation of brittle phases and improve the toughness and local corrosion resistance of the material.
Welding process control:
During the welding process, the phase ratio of duplex stainless steel may change due to the presence of heat-affected zone. Therefore, it is necessary to strictly control the welding process parameters, such as welding current, voltage, welding speed, etc., to ensure that the welded joint has the same performance as the base material. At the same time, it is also necessary to pay attention to the cooling rate of the welded joint to avoid the precipitation of brittle phases.
Phase ratio detection and evaluation:
During the production process, the phase ratio of duplex stainless steel pipe needs to be detected and evaluated regularly. This can be achieved through metallographic analysis, X-ray diffraction and other methods. By timely detecting and evaluating the changes in phase ratios, the production process and parameters can be adjusted in time to ensure that the product has stable mechanical properties.

In summary, the phase ratio control of duplex stainless steel has an important influence on its mechanical properties. By precisely controlling the ratio of ferrite phase and austenite phase, the mechanical properties of duplex stainless steel, such as tensile strength, yield strength, impact toughness, plasticity and toughness, can be optimized. To achieve this goal, strategies such as chemical composition adjustment, heat treatment process optimization, welding process control and phase ratio detection and evaluation are needed. The implementation of these strategies will help improve the comprehensive performance of duplex stainless steel and meet its application needs in different industrial fields.