What is the optimal ratio of each element in duplex stainless steel pipes?

The performance of duplex stainless steel pipes is determined by their unique austenite and ferrite two-phase structure, and the ratio of each element needs to balance the two-phase ratio (usually 40%~60% each), while optimizing corrosion resistance, strength, and processability. The following is the optimal elemental ratio and functional analysis of typical duplex stainless steel pipes (such as 2205, 2507, etc.):

1、 Element ratio of typical duplex stainless steel pipes (taking common grades as examples)

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2、 The logic and impact of the ratio of core elements

1. Chromium (Cr) and nickel (Ni): the cornerstone of two-phase equilibrium

Cr: As a ferrite forming element, excessive content can lead to excessive ferrite, making the steel brittle; If the content is insufficient, sufficient passivation film cannot be formed, and the corrosion resistance decreases.

Ni: As an austenite forming element, it needs to be combined with Cr to ensure that the ratio of the two phases is close to 1:1. For example, in 2205, Ni ≈ 5.5% and Cr ≈ 22.5%, the equilibrium can be calculated using the "Ni equivalent" and "Cr equivalent" formulas:

Cr Equivalent (CE)=Cr+Mo+1.5Si+0.5Nb (%)

Ni equivalent (NE)=Ni+30C+30N+0.5Mn (%)

In an ideal situation, CE/NE≈1.0~1.2， To ensure equilibrium between the two phases.

2. Molybdenum (Mo) and Nitrogen (N): The Key to Corrosion Resistance and Strength

Mo: In media containing Cl ⁻ (such as seawater, chemical solutions), Mo can inhibit the initiation of pitting corrosion. 2507 has better corrosion resistance than 2205 due to its higher Mo content (4%~5%).

N: In solid solution form, it not only strengthens austenite, but also synergistically improves pitting corrosion resistance with Mo (pitting index PREN=Cr+3.3Mo+16N, the higher the PREN, the stronger the corrosion resistance). For example:

PREN of 2205 ≈ 40~45, suitable for moderate corrosive environments;

The PREN of 2507 is approximately 45~50, suitable for highly corrosive environments such as marine engineering and petroleum refining.

3. Carbon (C): The lower the better

The carbon content of duplex steel is strictly controlled at ≤ 0.03% to avoid intergranular corrosion caused by Cr ₂ ∝ C ₆ precipitation at high temperatures, which is also an important difference from austenitic stainless steel (such as 304).

4. Manganese (Mn): an economical austenite stabilizing element

The austenite forming ability of Mn is about 1/20 of that of Ni, but the cost is lower. It is commonly used to adjust the two-phase ratio and improve the cold working performance of steel.

3、 Proportion adjustment in different application scenarios

Corrosion resistance priority (such as in marine engineering and chemical equipment): Increase the Mo (4%~5%) and N (0.25%~0.3%) content, such as in 2507 duplex steel.

Strength priority (such as structural components and high-pressure pipelines): increase the N content (about 0.2%), utilize its solid solution strengthening effect, and ensure the two-phase ratio.

Weldability requirements: Reduce the content of C and Si, control Mn below 1.5%, and avoid excessive growth of ferrite in the heat affected zone (HAZ) leading to embrittlement.

4、 Summary: The core principle of optimal ratio

The element ratio of duplex stainless steel pipes needs to meet the optimization of "two-phase balance, corrosion resistance, and strength synergy":

Chromium nickel equilibrium: Control the two-phase ratio by Cr equivalent/Ni equivalent ≈ 1.0~1.2;

Molybdenum nitrogen synergy: Adjust Mo (2.5%~5%) and N (0.15%~0.3%) according to the corrosive environment to enhance the resistance to pitting corrosion;

Low carbon control: C ≤ 0.03% to avoid intergranular corrosion;

Impurity limitation: The lower the content of P and S, the better the toughness and weldability.

The ratio of specific grades needs to be refined based on standards (such as ASTM A240, GB/T 21832) and actual working conditions to achieve the optimal balance between performance and cost.

Tags: The logic and impact of the ratio of core elements, Proportion adjustment in different application scenarios, Synergistic optimization of two-phase equilibrium, corrosion resistance, and strength