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In the petrochemical industry, the performance differences between duplex steel tees and tees made of other materials primarily manifest in terms of strength, corrosion resistance, high-temperature resistance, machinability, and cost. The following is a detailed comparative analysis:
1. Strength and pressure resistance: Duplex steel significantly outperforms austenitic stainless steel and carbon steel
The yield strength of duplex steel tees (e.g., 2205, 2507) can reach 450–700 MPa, which is 2–3 times that of 316L austenitic stainless steel (170–210 MPa) and 1.5–2 times that of carbon steel (200–300 MPa). This property enables it to reduce equipment weight by decreasing wall thickness in high-pressure applications (such as hydrocracking units operating at 10-15 MPa), while avoiding deformation caused by stress concentration. Carbon steel, with its lower strength, requires thicker walls at high pressures and is prone to fatigue cracking; ferritic stainless steel, though stronger than austenitic steel, has poor toughness and weak impact resistance.
II. Corrosion Resistance: Duplex Steel Excels in Chloride and Hydrogen Sulfide Environments
Resistance to pitting corrosion and stress corrosion: The corrosion resistance of duplex steel is ensured by its “ferritic + austenitic” dual-phase microstructure. Its pitting resistance equivalent (PREN) value is ≥34. In seawater and crude oil processing environments containing chloride ions, the critical pitting temperature (CPT) reaches 30–40°C, far exceeding the 15°C of 316L stainless steel. For example, in seawater injection pipelines on offshore platforms, 2205 duplex steel tees can resist seawater corrosion, with wall thickness reduction of less than 0.1 mm after five years of operation. Additionally, in acidic environments containing H₂S (such as natural gas processing facilities), duplex steel has a higher stress corrosion cracking (SCC) threshold, preventing “hydrogen embrittlement” fractures. while austenitic stainless steel is prone to stress corrosion cracking in chloride ion or hydrogen sulfide environments.
Differences from nickel-based alloys: Nickel-based alloys (such as Inconel 625) offer the best corrosion resistance in strong acidic media like concentrated hydrochloric acid or hydrofluoric acid, but their cost is 5–10 times that of duplex steel, making them suitable only for extreme corrosion scenarios; duplex steel offers better cost-effectiveness in moderate corrosion environments.
3. High-temperature resistance: Austenitic stainless steel is more suitable for high temperatures, while duplex steel is limited to medium temperatures.
Austenitic stainless steel (such as 310S) can be used long-term at temperatures between 600-1100°C, making it suitable for high-temperature applications like furnace tubes; however, the long-term operating temperature for duplex steel typically does not exceed 300°C (2205) or 400°C (2507), as temperatures above these thresholds can lead to σ phase precipitation, resulting in reduced toughness. Carbon steel experiences a significant decrease in strength above 350°C, and ferritic stainless steel is prone to creep above 450°C, making both materials suitable only for low-temperature applications.
4. Processing and Cost: Duplex steel offers better overall cost-effectiveness than high-end materials.
Welding and Formability: Duplex steel has a low risk of heat-affected zone embrittlement and can be welded using conventional welding processes, with lower difficulty than nickel-based alloys; during cold forming, deformation must be controlled, but its hot formability is superior to ferritic steel. Austenitic stainless steel has excellent cold formability but tends to stick to tools during hot processing; nickel-based alloys exhibit significant work hardening, requiring specialized processes for both forming and welding.
Cost Comparison: The material cost of duplex steel is higher than that of 316L austenitic stainless steel but lower than that of nickel-based alloys. In high-pressure corrosive environments, the service life of duplex steel fittings is 2–3 times that of 316L, resulting in lower overall costs. For example, after a refinery replaced its fittings with duplex steel, the maintenance cycle was extended from 2 years to 5 years. While carbon steel has lower initial costs, it requires frequent corrosion protection maintenance (e.g., annual sandblasting for rust removal), which may result in higher long-term operational costs.
5. Application Scenario Selection Recommendations
Prioritize duplex steel: seawater pipelines containing chloride ions, crude oil processing systems, high-pressure hydrogenation units (pressure > 8 MPa), H₂S-containing acidic gas pipelines, and offshore platforms where both strength and corrosion resistance are required in compact applications.
Select other materials: 316L austenitic stainless steel for ambient temperature dilute acid environments; ferritic stainless steel for high-temperature non-corrosive flue gas; carbon steel for non-corrosive low-pressure pipelines; and nickel-based alloys for concentrated acid, high-temperature, high-pressure fully immersed corrosion environments.
Summary
Duplex steel tees, with their comprehensive advantages of “high strength, moderate to high corrosion resistance, and cost-effectiveness,” have become the core choice for high-pressure corrosive environments in the petrochemical industry. Other materials, however, play an irreplaceable role under specific temperature conditions or extreme corrosion conditions. When selecting materials, it is essential to conduct a comprehensive assessment considering medium composition, operating parameters, and lifecycle costs to achieve the optimal balance between performance and cost.