What are the optimal ratios of each element in different types of stainless steel pipes?

The alloy element ratios for different types of stainless steel pipes need to be optimized based on their microstructure characteristics, performance requirements, and application scenarios. The following are typical element ratio ranges and key characteristic analyses for common types: 

I. Austenitic Stainless Steel (mainly composed of austenite, with good toughness and no magnetism)

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Core ratio logic:

Cr-Ni synergy: The basic ratio is 18Cr-8Ni (304), where Ni expands the austenite zone and Cr ensures the corrosion resistance of the passivation film;

The role of Mo: In the 316 series, 2-3% Mo significantly enhances the resistance to chloride ion corrosion;

Carbon content control: Low carbon (L) or adding Ti/Nb to fix the carbon content to avoid intergranular corrosion. 

II. Ferritic Stainless Steel (mainly composed of ferrite, low cost, magnetic)

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Core ratio logic:

High Cr, Low Ni: The Cr content is 16-22%, ensuring corrosion resistance, and the Ni content is extremely low to reduce cost;

Ultra-low carbon + Mo: For example, in 444, Mo enhances the resistance to pitting corrosion, suitable for environments with chloride ions;

Microstructure characteristics: The ferrite structure results in high strength but poor toughness. The Cr content needs to be controlled to avoid embrittlement. 

III. Martensitic Stainless Steel (Heat-treatable, high strength, moderate corrosion resistance)

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Core ratio logic:

Carbon-chromium balance: The higher the C content, the higher the hardness after heat treatment (e.g. 440C can reach HRC58+), but Cr must ensure basic corrosion resistance (≥11.5%);

Heat treatment: It is necessary to optimize strength and toughness through heat treatment (such as quenching + tempering), typically applied in scenarios requiring wear resistance. 

IV. Duplex Stainless Steel (Austenitic + Ferritic Dual Phase Structure, High Strength + High Corrosion Resistance)

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Core Ratio Logic:

Dual-phase balance: The synergistic effect of Cr-Ni-Mo-N forms approximately 50% austenite + 50% ferrite, with a strength (yield strength ≥ 450 MPa) that is twice that of 304;

High corrosion resistance design: High Cr + Mo + N enhances the ability to resist stress corrosion cracking (SCC), especially suitable for harsh environments containing Cl⁻ and H₂S. 

V. Precipitation Hardening Stainless Steel (can be strengthened through aging treatment, high strength + corrosion resistance)

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Core ratio logic:

Aging strengthening elements: Cu, Nb, etc. form precipitates (such as Ni₃Nb), through aging treatment (480-620℃) to enhance strength (tensile strength ≥ 1000 MPa);

Corrosion resistance foundation: Cr ≥ 15% ensures the stability of the passivation film, suitable for scenarios requiring "high strength + corrosion resistance" (such as marine engineering). 

VI. Special Types of Stainless Steel (Economical or Functional)

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Core Principles of Ratio Design

1. Corrosion Resistance Priority:

Oxidizing environment (such as air, nitric acid): Cr content ≥ 18%;

Chloride-containing environment (seawater, salt fog): Add Mo (2-4%) + high Cr (≥ 22%), such as 316L, 2205;

Intergranular Corrosion Resistance: Low carbon (C ≤ 0.03%) or add Ti/Nb to fix carbon.

2. Balance of Strength and Toughness:

Austenitic steel enhances toughness through Ni to stabilize the structure;

Duplex steel achieves high strength (yield strength ≥ 450 MPa) and fatigue resistance through duplex structure;

Martensitic steel regulates hardness through carbon content and heat treatment (for example, when C = 0.2% for 420, the hardness can reach HRC 30+).

3. Cost and Practicality Consideration:

Economical scenarios (such as decoration) can choose 200 series (high Mn, low Ni);

For high-temperature conditions, select 310S (high Cr-Ni), rather than blindly increasing expensive elements (such as Mo). 

Summary

The element ratio of stainless steel pipes does not have a "unique optimal value". It needs to be designed comprehensively based on the characteristics of the medium (acid / alkali / salt), temperature, pressure, processing method (welding / stamping), and cost. For example:

Food-grade pipes are selected as 304/316 (low C + corrosion-resistant);

Marine engineering pipes are selected as 2205/2507 (high Cr-Mo-N + resistance to SCC);

Cutting tools are selected as 420/440C (high C + Cr to ensure hardness and rust prevention).

In practical applications, the composition range in standards such as ASTM and GB can be referred to, and the microstructure can be optimized through heat treatment (such as solution treatment and aging) to achieve maximum performance.