Duplex stainless steel has gradually become the preferred material for seawater pipeline systems in nuclear power plants because its corrosion resistance to seawater is better than austenitic stainless steel and its cost is better than super austenitic stainless steel. Nowadays, with the increasing installed capacity of nuclear power plants, pipeline systems are developing towards large diameter and lightweight.
 
3. Test results
3.1 Metallographic examination
Take 2 full-thickness samples from the head and tail ends of the large-diameter S32205 duplex stainless steel seamless pipe (marked as sample 1, sample 2, sample 3, and sample 4), and measure them in accordance with GB/T 13298-2015 " Microstructure Examination Method for Metal ", measuring the ferrite content and austenite content by a microscope with a magnification of 400 times and observing the metallographic phase by an optical microscope with a magnification of 500 and a scanning electron microscope with a magnification of 1000 to know whether there are harmful phases in the sample. The ferrite content of the four samples was 49.62%, 50.60%, 48.11%, and 48.53% after inspection, harmful precipitates such as σ Phase and X phase were found for them, meeting the requirements of the technical agreement.
 
Table 3 Normal temperature mechanical property test results of large-diameter S32205 duplex stainless steel seamless pipes

 
Table 4  Instantaneous high-temperature tensile test results of S32205 large-diameter seamless pipes

 
3.2 Mechanical properties
The large-diameter S32205 duplex stainless steel seamless pipe specimens were subjected to room temperature tensile tests, room temperature impact tests, hardness tests, and flattening tests. The test results are shown in Table 3, which meet the requirements of GB/T 21833.2 and the technical agreement. An instantaneous high-temperature tensile test was conducted for the steel pipe sample according to GB/T 228.2-2015 "Tensile Tests of Metal Materials Part 2: High-temperature Test Methods". The test results are shown in Table 4, which meet the requirements of the GB/T21833.2 standard and technical agreement.
 
3.3 Corrosion resistance tests
The pitting corrosion test was carried out according to GB/T 17897 standard A method. The test temperature was (22±1)°C. The corrosion rates of the two samples were 0.162 mg/(dm2·d) and 0.187 mg/(dm2·d) after immerging in 6% ferric chloride solution for 72 hours, which is much less than the 10 mg/(dm2.d) required by GB/T 21833.2 and the technical agreement, and is far less than the 10 mg/(dm2·d) required by GB/T 21833.2 and the technical agreement. Carry out intergranular corrosion tests in accordance with NB/T 20004 standard, and sensitize the sample according to method A. The sample is heated to (675±10)°C within 4 to 5 minutes, kept warm for 10 minutes and then immersed in water to cool. Then the sample is immersed in boiling copper sulfate-16% sulfuric acid solution with copper scraps added for 24 hours, and then bent at an angle of 90. It was found that there were no intergranular corrosion cracks on the surface of the sample after observing by a magnifying glass with a magnification of 10. Carry out chloride stress corrosion tests according to YB/T 5362 standard. The sample was immersed in a boiling magnesium chloride solution with a boiling point of (155±1)°C, and a tensile stress of 80% Rp0.2 was applied to the sample. After 96 hours of testing, the surface of the sample was visually inspected and there were no cracks.
 
4. Conclusion
A manufacturing method for large-diameter duplex stainless steel seamless pipes was developed. To ensure the appropriate proportion of ferrite phase and austenite phase in the steel pipe, the required mechanical properties and corrosion resistance, avoid harmful phase precipitation during billet smelting, forging billet opening, hot processing and heat treatment of steel pipes, and avoid surface damage during hot processing. Through experimental research, the reasonable heating system and cooling rate for each process were determined. The impact toughness required for seawater pipelines in low temperatures is effectively improved through the forging process. In particular, by selecting the appropriate thermal expansion temperature and the forced air cooling or spray cooling after thermal expansion, the medium-frequency thermal expansion process has been successfully applied to the manufacturing of large-diameter duplex stainless steel seamless pipes, which can simultaneously complete large-diameter expansion and wall reduction processing. large-diameter thin-walled S32205 duplex stainless steel seamless pipes for nuclear power plant seawater piping systems that meet the requirements were developed.