A. The purpose and significance of DUPLEX STAINLESS STEEL PIPE development
 
Austenitic-ferritic Duplex stainless steel roughly has the same proportion both austenite and ferrite phase. Austenitic-ferritic duplex stainless steel has the characteristics of both ferritic and austenitic stainless steel, the mechanical properties with high strength, and good toughness and weldability. Duplex stainless steel resist stress corrosion, pitting corrosion which is applicable to dilute sulphuric acid, dilute hydrochloric acid and phosphoric acid and corrosion of various chloride aqueous solution.
 
When touched with Chloride solution, stainless steel is easy to be pitting corrosive. Commonly used stainless steel has an extremely thin film of chromium oxide, called a passivation film. The film can prevent metal from further oxidation, so stainless steel has a strong corrosion resistance. However, the passivation film on the surface of stainless steel is more fragile because of the defects, impurities and inhomogeneity of solute in the steel. In a specific corrosive solution is easy to be destroyed, the damaged part will become the activated anode, the surrounding area is the cathode area. The anode area is very small.  The anode current density is very large caused the active dissolution speed, then become a lot of needle-like holes, become the "hole corrosion". The formation of the hole is the result of competitive adsorption by O₂ and Cl-. Strong Cl- adsorption and rapid dissolution of metal will occur near the hole corrosion point. When the O₂ adsorption point on the metal surface is replaced by Cl-, the corrosion will happen. Due to the reversible competitive adsorption of O₂ and Cl- to the Cl- producing complex ions and the destruction of the passivation film, the complex ions and Cl- were generated after hydrolysis. The metal ions dissolved locally at the anode, such as Fe²⁺, Cr³⁺, Ni²⁺, react with water to form H+, which reduces the pH value of the solution in the etching hole and also causes the accelerated dissolution of the metal ions. In order to maintain electrical neutrality in the etched hole, Cl- is added into the etched hole and combined with metal ions. The resulting high concentration of metal chloride increases the activity coefficient of H+ and prevents the re-passivation of the anode area of the etched hole.
 
In addition, under the action of tensile stress, dislocation of commonly used stainless steel moves along the sliding surface to the metal surface, resulting in local failure of the surface passivation film. A microcell is formed between the metals with and without passivation film. The bare metal as the anode will dissolve and occur "stress corrosion damage" . The 304L and 316L austenitic stainless steels commonly used in neutral chloride solutions above 60℃ are prone to stress corrosion damage, while the stress corrosion tendency of duplex stainless steels is small.
 
With the development of oil and gas exploitation industry and chemical industry, some corrosion-resistant pipes are urgently needed. For example, China discovered a world-class gas field in the east of sichuan province, but the natural gas contained a large amount of H₂S, CO₂, SO₂, Cl₂ and other gases, and the corrosion of steel was very serious. There has been a number of deaths due to corrosion of steel pipe leakage accidents. Because 304L (00Cr19Ni9), 316L (00Cr18Ni14Mo2) and other stainless steel cannot withstand the corrosion of H₂S, CO₂, SO₂, Cl₂ gas, it is urgent to develop a high corrosion resistant dual-phase stainless steel seamless steel tube, in order to meet the development needs of oil and gas exploitation industry and chemical industry.
 
With the development of offshore oil, more and more pipelines are laid in seawater. Since seawater contains a large amount of Cl-ions, it is highly corrosive to steel tubes. 304L, 316L and other stainless steels cannot withstand Cl-ion corrosion. Therefore, it is necessary to develop a highly corrosion-resistant dual-phase seamless stainless steel tube to meet the needs of offshore oil exploitation.
 
There are two phases in the two-phase steel, one of which prevents the development of cracks in the other phase. For example, the cracks expand in the ferrite phase and bypass the island austenitic phase, which acts as a barrier. Duplex stainless steel has high yield strength and good uniform corrosion resistance, so it also has high corrosion fatigue resistance.
 
DUPLEX STEEL PIPE used in the manufacture of chemical, petroleum, petrochemical and power industries such as water, groundwater and seawater cooling medium heat exchange equipment, also suitable for manufacturing warships, nuclear submarines, landing craft, large Marine heat exchangers, favored by the oil industry, chemical industry, papermaking industry, salt industry, natural gas, energy, environmental protection industry, light industry machinery, food industry, the Marine environment in the areas of concern.





B. Technical route of product development
Technical process
Piercing Bar → Cutting Head → Pickling → Grinding → Welding Head → Lubrication → Baking → Drawing → Pickling → Heat treatment → Straightening → Pickling → Inspection → Inner grinding and Outer casting → Rolling → Pickling → Recycling → Cold-Drawing for finished products → Pickling→ Heat treatment → Sampling → Physical and Chemical Test → Straightening → Pickling → Surface Inspection → Soaking in Boiled Water→ NDT Test → Printing Storage.
Piercing Bar → Cutting Head → Pickling → Grinding → Lubrication → Baking → Enlarge→Pickling → Heat treatment →Pickling  → Inspection → Inner grinding and Outer casting →Enlarged to the size → Pickling → Grinding  → Heat treatment → Straightening→ Pickling→Welding Head→Lubrication → Baking →Cold-drawing → Recycling → Cold-Drawing for finished products →Cutting Head →Pickling→Heat-treatment→Sampling→ Physical and Chemical Test → Straightening → Pickling → Surface Inspection → Soaking in Boiled Water→ NDT Test → Printing Storage.
 
 
C. Improve corrosion resistance by controlling chemical composition and structure. 
 
The performance of stainless steel against stress corrosion, pitting corrosion and crevice corrosion can be improved by increasing the content of chromium in stainless steel. 
The performance of stainless steel against stress corrosion, pitting corrosion and crevice corrosion can be improved by increasing the content of chromium in stainless steel.
 
 
Chemical Composition (%)