After more than ten years of development, the manufacturing process of duplex stainless steel tubes has gradually matured, making the materials from high-end applications to common applications. Considering the relative cost, more and more customers are beginning to use duplex steel instead of traditional austenitic stainless steel, especially in the fields of desalination and offshore engineering. However, compared with 304L, 316L and other austenitic stainless steel pipes, the price of duplex steel pipes is still much higher, which is mainly due to its processing technology.
Duplex stainless steel has good formability in various processes. Our duplex steel pipes are usually cold worked-cold rolled or cold drawn. Of course, for large diameter duplex steel pipes, we can also use hot forming. During cold working, a major issue is the high strength of duplex stainless steel and the power of the forming equipment. For conventional wall thickness, the outer diameter of our seamless duplex 2205 pipe can be 24 inches (DN600), and the outer diameter of the super duplex 2507 pipe can be 16 inches (DN400).
It is generally estimated that the forming force of duplex stainless steel tubes is approximately twice that of 300 series austenitic stainless steel tubes of the same thickness. Figure 01 shows the comparison of the minimum external force required for plastic deformation of several stainless steels during bending.
In addition to the large initial external force required for bending, duplex stainless steel pipes also have strong work hardening during deformation. When determining the maximum thickness of the material that can be formed on the device, the rapid increase in strength during deformation must also be considered. Even if the power of the equipment is high enough, the high strength of duplex stainless steel will lead to higher springback (see Figure 02).
Compared with austenitic stainless steel, the ductility of duplex stainless steel must also be considered. Most technical specifications require a minimum elongation rate of 15%-30% for duplex stainless steel and 40% for austenitic stainless steel. Although the actual elongation may be greater, the relationship between the minimum values is appropriate and can serve as a useful guide for cold working.
Compared with austenitic stainless steel, duplex stainless steel requires a larger bending radius, and because of its lower plasticity, intermediate annealing is required in the severe or complex forming process. In the manufacturing process of seamless duplex steel pipe, we need to carry out corresponding heat treatment for each cold forming process of duplex steel pipe. This greatly increases the manufacturing cost of duplex steel pipes.
After cold forming, the tube needs to be fully solution annealed and then quickly quenched to fully restore the mechanical properties and corrosion resistance. The dual-phase tube should be heated above the minimum solution annealing temperature, and then maintained for sufficient time to completely dissolve the intermetallic phase. A more conservative guideline is that the holding time should be equivalent to the total time the workpiece has been held within the range of 650-980°C (1200-1800°F) after the previous full annealing. The water should be quenched from the solution treatment temperature. Table 01 summarizes the minimum solution annealing temperature for duplex stainless steel tubes.
The short-term supply of 316L stainless steel seamless pipes in the Chinese market will maintain a slight downward trend
Post time: Aug-26-2020