Stainless steel seamless pipe usage

The new type of martensitic stainless steel seamless pipe KL-HP12CR for pipelines has excellent weldability, mechanical properties, and corrosion resistance. The weldability was improved by reducing the carbon and nitrogen content. Reducing carbon content also significantly improved carbon dioxide corrosion resistance, with a corrosion rate lower than 0.127mm/a in carbon dioxide environments with temperatures up to 160 ℃ and 2.0MPa. Due to the addition of molybdenum, the resistance to sulfide stress corrosion cracking (SSC) has been improved. This new type of steel pipe can be used in environments with pH values of 4.0 and 0.001 MPa. This type of steel pipe has a strength of X80 grade and has sufficient low-temperature toughness when actually used in pipelines. Postweld heat treatment for several minutes, reducing carbon content, and adding cnc machining titanium can effectively prevent intergranular stress corrosion cracking (IGSCC) in the heat affected zone. This type of steel pipe is expected to be further used for transporting liquids containing corrosive gases, such as carbon dioxide, which is an economical material with a high lifespan and low cost.
People’s attention to the reduction of oil resources is increasing. Currently, the temperature and pressure of oil and gas wells being exploited have reached unprecedented heights, and the extracted liquids generally contain carbon dioxide, which causes more corrosion. Therefore, it is extremely important to prevent the pipelines of the flow and collection lines from being corroded by carbon dioxide when transporting liquids before removing corrosive substances and water. In addition, these liquids typically contain trace amounts, so it is also necessary to prevent chloride stress cracking. In such a corrosive environment, for pipelines made of carbon steel, the traditional anti-corrosion method is to inject rust inhibitor into the liquid and use rust inhibitor to prevent corrosion. However, this increases production costs, especially for offshore pipelines, resulting in less use of rust inhibitors, especially considering life cycle costs. Another reason for not using rust inhibitors is to worry about pollution caused by leakage accidents. Therefore, there is a need for an economical material that does not require the use of rust inhibitors. The existing pipelines use corrosion-resistant alloys, including duplex stainless steel, but the disadvantage is the high material cost. Compared to this, martensitic stainless steel typically has poor weldability and requires preheating and prolonged post weld heat treatment. Therefore, considering the efficiency of pipeline laying, martensitic stainless steel is rarely used in pipelines. However, martensitic stainless steel has appropriate carbon dioxide corrosion resistance and is cheaper than duplex stainless steel.
Therefore, a Japanese steel company has taken a large number of steelmaking technical measures, such as reducing the content of carbon and nitrogen, controlling and adding alloy elements to improve the weldability of martensitic stainless steel, and developing seamless martensitic stainless steel pipes for pipelines with good weldability and corrosion resistance.
1. Development process
1.1 Target characteristics
The development objectives are as follows:
(1) Weldability: Welding does not require preheating;
(2) Maximum hardness of heat affected zone: HV350 or lower;
(3) Carbon dioxide corrosion resistance: resistant to 5% NaCl, carbon dioxide partial pressure of 3.0MPa, and corrosive environment at 150 ℃;
(4) Sulfide stress corrosion resistance (SSC): 5% NaCl resistance, 0.001MPaH2S resistance, pH4.0;
(5) Strength: X80 grade (550MPa or higher yield strength);
(6) Low temperature toughness: 100J or higher Charpy impact toughness absorption energy at -40 ℃.
1.2 Chemical composition design
The chemical composition design of steel pipes should consider the influence of alloy elements on the weldability, corrosion resistance, hot workability, and other characteristics of martensitic stainless steel. Especially the research on weldability is based on the chemical composition of KO-13Cr (0.20C-13Cr-0.03N) for petroleum pipes used in carbon dioxide environments, while maintaining equal corrosion resistance in the matrix material. According to the research results on the influence of chemical composition on hot workability and other characteristics in Table 1, the chemical composition of this steel was ultimately determined to be 12Cr-5Ni-2Mo-0.01N, 0.015C or lower.
1.2.1 Weldability
Due to the tendency of martensitic stainless steel to produce welding cracks during welding, preheating is necessary in practical applications to prevent cracks from occurring. Welding cracks are caused by hydrogen dissolved into the welding metal and the welding heat affected zone, as well as hardening and residual stress induced by martensitic transformation in the heat affected zone. Therefore, an effective way to prevent welding cracks in materials is to reduce the content of carbon and nitrogen to suppress the hardening induced by martensitic transformation. Table 1 shows the results of Y-shaped groove welding crack resistance test for low C+N martensitic stainless steel. The steel used for crack resistance test contains 0.03% carbon or nitrogen, and the carbon and nitrogen in the steel are reduced to 0.01%. No crack resistance test is conducted, and preheating is conducted at 30 ℃. The results indicate that if the carbon and nitrogen content decreases to 0.01%, welding without preheating is possible. The existing steelmaking technology can reduce the content of carbon and nitrogen to such low levels.
No cracks
Plate thickness: 15mm
Welding material: 410HSMAW, 4 Ф (Diffusible hydrogen; 4.28cm3/100g)
Welding conditions: Current: 160A
Voltage: 24-26V
Speed: 150mm/min
Test conditions: Room temperature: 30 ℃,
Humidity: 60% RH
1.2.2 Carbon dioxide corrosion resistance
Reducing carbon content can also improve the carbon dioxide corrosion resistance of steel. The experiment shows that martensitic stainless steel with different chemical compositions has different carbon dioxide corrosion resistance, and the relationship between corrosion rate and carbon dioxide index is determined by Cr 10C+2Ni. Increasing the content of chromium and nickel and reducing the carbon content can improve the carbon dioxide corrosion resistance of steel. This is probably because reducing the carbon content reduces the content of chromium carbide, thereby increasing the solubility of chromium and effectively preventing corrosion.
1.2.3 Sulfide stress corrosion resistance
Due to the fact that sulfide stress corrosion cracking of martensitic stainless steel originates from pitting corrosion, improving the resistance to pitting corrosion can improve the resistance to sulfide stress corrosion cracking. It is known that the alloying element molybdenum can improve the pitting corrosion resistance of steel. The experiment showed that there was no difference in the results of increasing the nickel content from 4% to 5%, while increasing the molybdenum content from 1% to 2% led to the occurrence of sulfide stress corrosion cracking tending towards low pH values, high partial pressures, or even more harsh environments. This phenomenon indicates that adding 1% molybdenum can fully ensure the resistance to sulfide stress corrosion cracking in an environment of 5% NaCl, 0.001MPaH2S, and pH4.0, which is the goal of developing this type of steel. However, due to the fact that the pitting resistance of the heat affected zone may be lower than that of the base metal, adding 2% molybdenum can ensure stable pitting resistance.
2. Characteristics of new steel pipes
The characteristics of the newly developed steel pipe were tested, and the sample was a seamless pipe with an outer diameter of 273mm and a wall thickness of 12.7mm. Its chemical composition is listed in Table 2, and quenching and tempering treatment were carried out to obtain X80 grade products. Using this product and 25Cr duplex stainless steel as welding material, the first pass is gas shielded tungsten arc welding (W), and the second pass is gas shielded metal arc welding (GMAW) for circumferential weld welding. The chemical composition of the welding material is shown in Table 2, and the welding conditions are shown in Table 3, without preheating or post weld heat treatment.
2.1 Mechanical properties
Table 4 shows the tensile test results, with the strength set at X80 level. The welded joint fractures in the base metal, indicating high performance. The cross-sectional distribution of the welded joint indicates that the maximum hardness of the heat affected zone is approximately HV330, which meets the design goal of HV350 or lower. The results of the Charpy impact test on welded joints indicate that the absorption energy is about 200J at -80 ℃ and -40 ℃, proving that the newly developed steel has excellent low-temperature toughness.
2.2 Carbon dioxide corrosion resistance
Immersion tests are conducted in environments with high temperature and high partial pressure of carbon dioxide to evaluate the carbon dioxide corrosion resistance of steel by measuring weight loss. Assuming an acceptable corrosion rate of 0.127mm/a as the standard, the newly developed material is suitable for 160 ℃, 2.0MPa carbon dioxide partial pressure.
2.3 Sulfide stress corrosion resistance
To evaluate the resistance of welded joints to sulfide stress corrosion cracking using a balanced load tensile sulfide stress corrosion test. Mix 5% or 10% NaCl in the aqueous solution, add 0.5% CH3COOH, and adjust the pH value from 3.5 to 5.0 when using CH3COONa. The partial pressure of the test gas mixture is between 0.001 and 0.007MPa. The applied stress is 567MPa, and its yield strength is equivalent to 90% of the base metal. The experimental results indicate that although sulfide stress cracking occurred in the heat affected zone at a pH value of 3.5, sulfide stress cracking did not occur in the environment with a pH value of 4.0 and a partial pressure of 0.001 MPa.
Intergranular stress corrosion cracking in welding of 3 circular welds
According to reports, laboratory research has found that intergranular stress corrosion cracks generated on specimens welded to circumferential welds in high-temperature carbon dioxide environments have chemical composition similar to newly developed steel. In addition, there have been reports that a molybdenum free material with a chemical composition similar to newly developed steel used in actual pipelines has experienced gas leakage due to intergranular stress corrosion cracking.
3.1 Mechanism of producing intergranular stress corrosion cracks
In order to verify the influence of welding conditions on sensitization behavior, the stress corrosion cracking test used a sample that underwent two simulated welding thermal cycles. In order to conduct tests under harsh conditions, a corrosive environment with a pH value of 2.0 was used, and the U-bend test method was used to apply greater strain. The test results indicate that some specimens undergo a second thermal cycle and crack. The sample that only passed through the first pass did not produce cracks.
These results indicate that the causes of intergranular stress corrosion fracture are as follows: during the high-temperature heating cycle, carbon is dissolved, and in the subsequent thermal cycle, it becomes carbides at the grain boundaries of the original austenite and precipitates, forming a chromium depleted zone near the carbides at the grain boundaries, thereby sensitizing the material.
3.2 Methods for preventing intergranular stress corrosion cracking
Due to the fact that intergranular stress corrosion cracking is likely caused by chromium depletion zones, possible methods to prevent intergranular stress corrosion cracking include post weld heat treatment to restore chromium diffusion, reducing carbon content to very low levels, and adding titanium to suppress the precipitation of chromium carbide.
To determine the impact of post weld heat treatment, materials containing 100ppm carbon were sensitized through two heating cycles, followed by a third heating cycle under different conditions. Use a U-shaped bending stress corrosion crack test similar to the above to evaluate the prepared specimens. The test results show that the sensitized sample has no cracks when heated for several minutes within the range of 550~700 ℃. This effect may be due to heat treatment increasing the diffusion of chromium, thereby reducing the chromium depletion zone. By using a short period of post weld heat treatment (several minutes), intergranular stress corrosion cracking can be prevented, which does not affect the actual laying efficiency of the pipeline.
In order to determine the effect of reducing carbon content and adding titanium, materials with different carbon and cnc milling titanium contents were evaluated. The sample is subjected to a heating cycle of 450 ℃ and 1000 seconds, which is prone to sensitization. Similar U-shaped bending stress corrosion cracking tests are conducted. As the test conditions change, notches are generated in the U-shaped bending section of the specimen. The experimental results indicate that reducing carbon content and adding titanium can suppress the generation of cracks. This is probably because during welding, the dissolution of carbon is inhibited and converted into titanium carbide, which inhibits the precipitation of chromium carbide that can cause chromium depletion. Therefore, reducing carbon content and adding titanium are effective methods to improve the resistance of materials to intergranular stress corrosion cracking.
The new martensitic stainless steel seamless pipe has improved its weldability by reducing the content of carbon and nitrogen, and has excellent corrosion resistance and mechanical properties by optimizing other alloy elements. The main characteristics of this new type of steel pipe are as follows: (1) The new type of steel pipe has excellent weldability and no welding cracks even without preheating. (2) The strength of the new steel pipe is X80 grade, and the absorption energy of the Charpy impact test at -40 ℃ is about 200J or higher. (3) This steel has excellent carbon dioxide corrosion resistance, with a corrosion rate of 0.127mm/a or lower in 160 ℃ and 2.0MPaCO2 environments. (4) This steel grade has excellent resistance to sulfide stress corrosion cracking in environments with pH 4.0 and 0.001 MPa partial pressure. (5) After a short period (several minutes) of post weld heat treatment, intergranular stress corrosion cracking can be prevented. Reducing carbon content and adding titanium can effectively improve the resistance to intergranular stress corrosion cracking of materials. Due to its excellent weldability, mechanical properties, and corrosion resistance, this new type of steel pipe can be used to transport liquid pipelines containing corrosive gases, such as carbon dioxide, making it an economical material with low life cycle cost.