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Selection of a duplex steel flange material grade suitable for a particular application scenario requires comprehensive consideration of media properties (corrosivity, composition), operating conditions (temperature, pressure), mechanical property requirements, cost budget, and other factors. The following is a systematic selection procedure and key rationale:
I. Define the key parameters of the application scenario
1. Evaluation of medium corrosivity
Chloride ion (Cl-) concentration:
Low Cl- environment (<1000ppm): economic duplex steel (e.g. 2304) is available.
Medium Cl- environment (1000~5000ppm): Standard duplex steel (e.g. 2205) is suitable.
High Cl-environment (>5000ppm) or containing hydrogen sulfide (H₂S), seawater, salt solution: Super duplex steel (e.g. 2507, 2553) is required.
pH and type of media:
Acidic / alkaline media: focus on Cr and Mo content, Cr for oxidation resistance, Mo for reduction resistance.
Fluorinated (F-) or strong oxidizing agents (e.g. concentrated nitric acid): higher Cr content is required (e.g. 2507 = 25% Cr). 2.
2. Working temperature and pressure
Temperature range:
Low temperature (-50 ~ 150 ℃): ordinary duplex steel (2205, 2304) can meet the toughness requirements.
Medium-high temperature (150 ~ 300 ℃): need to consider the thermal stability of the material, super duplex steel (2507) in the high temperature strength retention rate is higher.
High temperature (> 300 ℃): may need nickel-based alloys, duplex steel is generally not recommended for long-term use above 300 ℃.
Pressure level:
High-pressure scenarios (e.g. oil pipeline PN ≥ 16): give preference to high-strength duplex steels (e.g. 2507 yield strength ≥ 550MPa). 3.
3. Mechanical performance requirements
Strength requirements:
High load or vibration environment: super duplex steel (e.g. 2507) has a yield strength 1.3 times that of 2205 and better fatigue resistance.
Processing and weldability:
need on-site welding or complex molding: 2205 welding process is mature, the risk of thermal cracking is lower than 2507; super duplex steel (such as 2507) welding need to strictly control the heat input and welding consumables to match (such as ER2594 wire).
Second, duplex steel material grade classification and application scenarios
Grade type Typical grade Key components (%) Corrosion resistance (PREN value) Typical application scenarios
Economy 2304 (S32304) Cr = 23, Ni = 4, Mo = 0.5, N ≈ 0.05 ~ 25 Fresh water treatment, atmospheric corrosive environments, low Cl-content chemical pipelines (such as pH neutral sulfate solutions) Standard sulfate solution)
Standard 2205 (S32205) Cr=22, Ni=5.5, Mo=3, N=0.14~0.22 ~32 Seawater desalination, medium Cl-concentration chemical media (e.g., paper bleaching solution, Cl-containing oil well injection),
III. Selection Process and Decision Points
1. Preliminary Screening : Based on the corrosive medium
Step 1: Determine the concentration of Cl- medium and whether it contains H₂S, F- and other strong corrosive components.
Example: seawater (Cl- ≈ 20000ppm) → directly exclude 2304, priority 2507.
Step 2: Evaluation of the acidity and alkalinity of the medium, to determine the need for high Cr or high Mo grade.
Example: Acidic medium containing Cl- (e.g. hydrochloric acid) → high Mo (e.g. 2205, 2507) is needed; alkaline medium → high Cr (e.g. 2507) is better.
2. Review mechanical properties and temperature
Strength matching: according to the pressure level selection of grades, such as high-pressure pipeline (PN = 25) need to 2507 (higher strength), low-pressure 2205 can be used.
Temperature correction:
low temperature (<-40 ℃): need to confirm the material of low-temperature impact toughness (eg, 2205 at - 40 ℃ impact work ≥ 27J).
High temperature (> 250 ℃): 2507 oxidation resistance is better than 2205, more suitable for long-term high-temperature conditions.