Because seawater contains electrolyte chloride ions, it is highly corrosive. Therefore, super duplex steel pipes used in marine environments must have strong corrosion resistance.
Offshore oil platform risers must not only be immersed in seawater for a long time but also can withstand the impact and distortion of waves. Therefore, materials with good corrosion resistance and high strength are required, and super duplex steel is echoed.
Super duplex steel has austenite and ferrite dual-phase structure, high pitting corrosion resistance, good mechanical properties, and chloride ion corrosion resistance, and is an ideal material for risers.
However, super duplex steel is more sensitive to the precipitation of the third phase such as intermetallic compounds. How control the process in the welding and heat treatment process is a difficult point in the production of this product.
The Mature Super Duplex Steel S32750 Welded Pipe Manufacturing Process Is:
1. Process Flow
Specifically: raw material inspection-steel plate ultrasonic flaw detection-edge planing-forming-pre-welding-full circle-cutting-welding-sizing + heat treatment-pipe end plus-welding seam radiographic inspection- Pipe end penetration inspection-water pressure test-pickling passivation-finished product inspection marking the packaging warehouse.
2. Forming
In the production of longitudinally welded pipes, the production difficulty lies in the guarantee of effective application of bending moment to the edge of the steel strip, which is difficult to solve by using traditional forming technology; while the yield strength of duplex stainless steel is about twice that of ordinary austenitic stainless steel, and its forming It’s more difficult.
As a new type of forming technology, FFX (flexible forming) cleverly uses the tension effect when the coil is bent to maximize the bending moment to the edge of the steel strip; at the same time, it takes advantage of the advantages of the non-polar dual use of the rolls, which can be based on the strength of the steel strip. And thickness, freely adjust the degree of forming when bending to achieve the best welding conditions.
3. Welding
To this end, a combination of plasma arc welding (PAW) and argon tungsten arc welding (GIAW) is used. First, plasma arc welding is used for penetration fusion to achieve the effect of single-sided welding and double-sided forming; after that, tungsten argon arc welding is used for filling and covering, and welding is protected by pure argon gas.
In order to reduce the tendency of harmful phases to separate out, use lower heat input as much as possible in the welding process, and the heat input should preferably not exceed 16kJ/cm. In addition, the interlayer temperature is very important for the welding of dual-phase steel and must be controlled below 100°C. The too-high interlayer temperature will increase the tendency of harmful phases to precipitate and ultimately affect product performance.
4. Heat Treatment
The heat treatment after welding uses an induction furnace to perform solution treatment on the whole welded pipe. The heat treatment temperature is 1050-1120°C, and the water spray is cooled to below 40°C.
Only by controlling the processing technology of super duplex steel, can the super duplex steel maintain its original state under the corrosion and impact environment of seawater, and the corresponding work can be better completed.