Development of UN S31803/ UN S32205 pipe for KANGLONG

by kanglong
2020-04-02
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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 (%)

Furnace No.:	SIZE	C	Mn	P	S	Si	Cr	Ni	Mo	N
Round Bar		≤0.03%	≤2	≤0.030	≤0.020	≤1.00	22~23	4.5~6.5	3~3.5	0.14~0.20
FK1804-171	Φ75	0.014	1.283	0.028	0.001	0.503	22.57	4.80	3.127	0.1823
FK1807-138	Φ60	0.012	1.319	0.026	0.001	0.331	22.58	4.80	3.099	0.1779
FK1903-122	Φ100	0.015	1.236	0.027	0.001	0.384	22.71	4.88	3.112	0.1748

Mechanical Properties

Tensile Strength（Mpa）	Yield Strength (Mpa）	HRC	Elongation（%）
≥620	≥450	≤28	≥25

D．Innovative Heat Treatment technology:
In the state of solid solution, the ferritic content ranges from 60% to 50%. With the increase of temperature, the ferritic content increases gradually. It has a better plasticity in the range of 1000~1100℃, and the plasticity decreases and the strength increases when the temperature is higher than 1150℃. In order to avoid heat treatment cracks, high solid solution temperature is used in the heat treatment process. The annealing temperature of the product is controlled at 1080℃~1100℃ for semi-finished products, and the annealing temperature of the product is controlled at 1050℃~1100℃ for finished products. When the annealing temperature of semi-finished products is a little higher, the carbide can be fully dissolved into the solid solution; The annealing temperature of the finished product is a little lower to avoid too much oxidation skin and excessive bending of finished products, meanwhile, improve the surface perfection.
E．Innovative Cold processing technology
The cold deformation resistance of Cr 23 Ni 6 ferrite - austenitic stainless steel is large due to the existence of two phase - ferrite and austenitic. Therefore, during the cold drawing/cold rolling process, the cold deformation is controlled to be less than 50%. Although the trace number of cold drawing/ cold-rolling is increased, the waste products of cold deformation are greatly reduced, and the finish and precision of cold deformation are improved.
Considering that the resistance of cold deformation is large, the proportion of grease in lubricant is increased appropriately. The cold drawing lubricant adopts 3 # industrial grease and lime which the ratio is 1.5:10. Rub off the lubricant after drying each time, and then carefully apply lime, then drying. The grease have to apply evenly!
After each drawing, before the heat treatment the pipes should be clean, so that the surface of the steel pipe does not be a water droplets, in order to prevent carburization affecting the corrosion performance.
F．Innovative Pickling technology
The usage of higher annealing temperature will increase the thickness of the oxide skin. Appropriately increase the acid concentration of the pickling and use steam to heat the pickling solution. The steel tube was pickled by the solution with 6%~8%HF, 12%~15%HNO₃ and water. The acid solution is heated to 40℃~60℃ with steam. The steel pipe should be suspended frequently to check the pickling condition until the oxide skin is cleaned.

We welcome your enquiry on Duplex Stainless Steel Pipe! Thank you!