Manufacturing Process of Duplex Pipe - Why Duplex Pipe is Expensive
After more than ten years of development, the manufacturing process of duplex stainless steel pipes has gradually matured, it made the material from high-end to ordinary applications. Considering the relative cost, more and more customers have begun to use duplex steel instead of traditional austenitic stainless steel, especially in areas such as seawater 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 caused by its processing technology.
Cold Forming for Duplex Pipe
Duplex stainless steels have a 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 may also use hot-forming. During the cold working, a major concerning is that the high strength of duplex stainless steel and the power of forming equipment. For conventional wall thickness, the outer diameter of our seamless duplex 2205 pipe can be 24 "(DN600), and super duplex 2507 pipe can be 16"(DN400) the outer diameter.
A common estimate is that the forming force for duplex pipe is about twice of the same thickness forming of 300 series austenitic stainless steel pipe. The comparison of the minimum external force required for the plastic deformation of several stainless steels during the bending process is shown in the Figure 01.
Figure 01 Minimum External Force of plastic deformation for Stainless Steel & Duple Steel 2304,2205 (specimen: 50mm, thickness 2mm)
In addition to the large initial external force required for bending, duplex stainless steel pipe also has strong work hardening during deformation. When determining the maximum thickness of the material that can be formed on the equipment, the rapid increase in strength during deformation must also be considered. Even if the power of the equipment is high enough, there must be a margin for the higher rebound caused by the high strength of duplex stainless steel (See Figure 02).
It is also necessary to take into account the lower ductility of duplex stainless steels compared to austenitic stainless steels. Most technical specifications require a minimum elongation of 15%-30% for duplex stainless steels and a minimum elongation of 40% for austenitic stainless steel. Although the actual elongation may be larger, the relationship between the lowest values is appropriate and can be a useful guide for cold working operations.
Compared with austenitic stainless steels, duplex stainless steels require larger bending radius, and because of their lower plasticity, intermediate annealing is required in severe or complex forming. During the manufacturing of seamless duplex pipe, we need to do a corresponding heat treatment for each cold forming process of duplex steel pipes. It greatly increases the manufacturing cost of duplex steel pipes.
Heat Treatment of Duplex Pipe
After cold forming, a complete solution annealing is required for the pipe, then rapid quenching to fully restore mechanical properties and corrosion resistance. The duplex pipe should be heated above the minimum solution annealing temperature, and then maintained for a sufficient time to completely dissolve the intermetallic phase. A more conservative guideline is that the holding time should be comparable to the total time of the workpiece stayed in the 650-980 ° C (1200-1800 ° F) range after the previous full annealing. The pipe should be water quenched from the solution treatment temperature. Table 01 summarizes the minimum solution annealing temperatures for duplex stainless steel pipe.
Table 01 Min. Solution Annealing Temperature of Duplex Steel Pipe (ASTM A 790)