Dual phase seamless steel pipe

The main characteristic of duplex stainless steel seamless pipe is that austenite and ferrite each account for a certain proportion in its internal structure, and the proportion of the two phases is usually close to equal. The content of at least one phase should be greater than 15%. This structure enables seamless duplex steel pipes to possess both the good strength, wear resistance, and corrosion resistance of austenitic stainless steel, as well as the high plasticity and toughness of ferritic stainless steel.
1、Excellent corrosion resistance: Dual phase stainless steel seamless pipes have excellent resistance to pitting and stress corrosion, and their ability to resist pitting corrosion far exceeds traditional austenitic stainless steels such as 304L and 316L. This characteristic makes it perform well in corrosive environments such as seawater and chloride.
2、High strength and high wear resistance: Compared with austenitic stainless steel, duplex stainless steel seamless pipes have higher yield strength and wear resistance, which can meet more demanding working conditions.
Good welding performance and processability: The welding performance and processability of duplex stainless steel seamless pipes are superior, and welding and bending can be carried out without annealing or special treatment, reducing manufacturing costs and difficulties.
Excellent low-temperature toughness: In low-temperature environments, duplex stainless steel seamless pipes still maintain good toughness and strength, making them suitable for applications in low-temperature environments.
3、High thermal conductivity and low coefficient of linear expansion: The thermal conductivity of duplex stainless steel is higher than that of austenitic stainless steel, and the coefficient of linear expansion is lower, which helps to reduce the thermal stress generated during the temperature rise and fall process of equipment and heat exchangers, and improve structural stability.

Classification：
According to the different content of alloy elements and corrosion resistance, duplex stainless steel seamless pipes can be divided into low-alloy type, medium alloy type, high alloy type, and super duplex stainless steel type. Among them, medium alloy types such as S31803 (or S32205) and super duplex stainless steel types such as S32750 are the most widely used varieties in the market.
Low alloy type: such as UNS S32304, the main feature is good stress corrosion resistance, but it does not contain molybdenum element.
Medium alloy type: such as UNS S31803 (or S32205), with corrosion resistance between AISI 316L and 6% Mo+N austenitic stainless steel, widely used in oil and gas pipelines and other fields.
High alloy type: such as UNS S32550, containing high levels of chromium, molybdenum, and nitrogen elements, with higher corrosion resistance.
Super duplex stainless steel type: such as UNS S32750, containing high molybdenum and nitrogen, suitable for harsh medium conditions, with extremely high corrosion resistance and mechanical comprehensive performance.

Application Fields：
Due to its excellent performance, duplex stainless steel seamless pipes have been widely used in multiple industrial fields
Marine engineering: In seawater systems, seawater pumps, and valves, duplex stainless steel seamless pipes can effectively resist seawater corrosion and extend equipment service life.
Petrochemical industry: In equipment such as chemical process vessels, pipelines, and heat exchangers, duplex stainless steel seamless pipes can withstand the corrosion of corrosive media such as acids, alkalis, and salts.
Food processing: In food processing equipment, duplex stainless steel seamless pipes can ensure hygiene and safety during the food production process, while also having good corrosion resistance.
In other fields, duplex stainless steel seamless pipes are widely used in various fields such as FGD washer systems, pulp mill digesters, bleach washers, and oil field pipelines.

Manufacturing process：
The manufacturing process of duplex stainless steel seamless pipes includes multiple steps such as billet processing, hot perforation, cold rolling (or cold expansion), heat treatment, acid pickling and passivation. Among them, hot piercing and cold rolling are key process steps that require strict control of parameters such as temperature and deformation to ensure that the microstructure and properties of the pipe meet the requirements.
During the heat treatment process, a rapid cooling method is required to avoid prolonged residence in the temperature ranges of 850~950 ℃ and 350~525 ℃, in order to prevent the precipitation of harmful phases such as σ and reduce the corrosion resistance and toughness of the material.
In summary, duplex stainless steel seamless pipes have been widely used in various industrial fields due to their excellent corrosion resistance, high strength, high wear resistance, and good welding performance. With the continuous improvement of manufacturing technology and further reduction of costs, the market prospect of duplex stainless steel seamless pipes will be even broader.