Common raw materials for stainless steel pipe fittings are mainly classified according to their crystal structure (austenitic, ferritic, martensitic, duplex) and chemical composition, to meet different requirements for corrosion resistance, high-temperature resistance and mechanical strength. Specific classifications are as follows:
1. Austenitic Stainless Steel
This is the most widely used type of stainless steel for pipe fittings, featuring excellent corrosion resistance, heat resistance, low-temperature toughness, and non‑magnetic properties (weak magnetism may appear after cold working).
- 304 / 304L: The most general‑purpose standard stainless steel.304 is suitable for general corrosive environments; 304L is the low‑carbon version, mainly used to prevent intergranular corrosion during welding. It is widely used in pipe fittings for chemical, food, pharmaceutical and civil construction applications.
- 316 / 316L: Molybdenum (Mo) is added on the basis of 304. It provides excellent corrosion resistance to chloride ions (e.g., seawater, brine) and reducing media such as sulfuric acid. 316L is the low‑carbon grade, widely used in pipe fittings for marine engineering, seawater desalination, high‑end chemical and paper industries.
- 321: Stabilized with titanium (Ti), which combines with carbon to prevent chromium carbide precipitation. It has excellent high‑temperature creep resistance and intergranular corrosion resistance, commonly used in pipe fittings for high‑temperature heating furnaces, heat exchangers and aviation exhaust systems.
- 310S: High chromium‑nickel alloy (25Cr‑20Ni) with exceptional high‑temperature oxidation resistance, capable of long‑term service above 1000℃. Often used in pipe fittings for boiler burners, high‑temperature furnace tubes and heat treatment equipment.
2. Duplex Stainless Steel (DSS)
The microstructure consists of approximately 50% austenite and 50% ferrite, combining the advantages of both phases: twice the strength of conventional austenitic stainless steel and excellent resistance to chloride stress corrosion cracking.
- 2205 (S32205): The most commonly used duplex stainless steel. It offers good corrosion resistance and moderate strength at a relatively low cost, widely used in pipe fittings for oil and gas pipelines (especially sour gas fields), seawater cooling systems and the paper industry.
- 2507 (S32705): Super duplex stainless steel. With higher molybdenum and nitrogen content, it provides extremely high corrosion resistance and strength, suitable for severely corrosive environments such as highly acidic oil wells, high‑pressure seawater desalination pumps and extreme chemical process conditions.
3. Ferritic Stainless Steel
Contains relatively high chromium (11%–30%) with no or very low nickel, and has a body‑centered cubic crystal structure. It features high thermal conductivity, low thermal expansion coefficient and good resistance to chloride stress corrosion, but has relatively poor low‑temperature toughness.
- 430: The most common ferritic grade, with good corrosion resistance and thermal conductivity at a low cost, but formability inferior to 304. Often used in interior decoration, appliance panels and some industrial pipe fittings with low corrosion requirements.
- 409: Titanium‑containing ferritic steel, mainly used in manifolds and tailpipe fittings for automotive exhaust systems due to its good high‑temperature oxidation resistance and thermal fatigue resistance.
4. Martensitic Stainless Steel
Mechanical properties can be adjusted through heat treatment (quenching and tempering). It features high strength, high hardness and good wear resistance, but relatively lower corrosion resistance.
- 2Cr13 / 3Cr13: High strength and hardness with average corrosion resistance. Commonly used to manufacture wear‑resistant or high‑load valve components, pump shafts and mechanical structural parts, rather than general‑purpose pipeline fittings.
- 1Cr17Ni2: A high‑strength martensitic grade with improved corrosion resistance and mechanical strength, often used in aerospace components and high‑load precision pipe fittings.
Summary
- General corrosion resistance and moderate high temperature: 304 / 321
- Seawater resistance and strong corrosion service: 316L
- High pressure and stress corrosion resistance: 2205 duplex stainless steel
- High‑temperature oxidation resistance (>1000℃): 310S
- Low cost, decorative or heat‑resistant use: 430 / 409
