Stainless steel is a corrosion-resistant iron-based alloy primarily containing chromium (Cr, at least 10.5%), often along with other elements such as nickel (Ni), molybdenum (Mo), manganese (Mn), etc. It can be classified in various ways, with the most fundamental being classification by microstructure (metallurgical phase). Here is a detailed breakdown:
(A). Classification by Microstructure (The most essential and widely used method)
The properties (strength, toughness, corrosion resistance, weldability, etc.) of stainless steel are primarily determined by its microstructure.
1. Austenitic Stainless Steel
· Series Designations: 200 series, 300 series.
· Common Grades: 304 (AISI 304 / UNS S30400), 316 (AISI 316 / UNS S31600), 321, 309S, etc.
· Key Characteristics:
· High nickel (Ni) and chromium (Cr) content, typically non-magnetic (may become slightly magnetic after cold working).
· Excellent corrosion resistance, particularly against acids and oxidation.
· Superior ductility and toughness, easy to form via cold working.
· Excellent weldability.
· Relatively low strength, cannot be strengthened by heat treatment.
· Primary Applications: The most widely used type, accounting for over 70% of stainless steel consumption. Used in cookware, cutlery, appliances, architectural decoration, chemical processing equipment, medical devices, etc.
2. Ferritic Stainless Steel
· Series Designations: Part of the 400 series.
· Common Grades: 430 (AISI 430 / UNS S43000), 409, 443, etc.
· Key Characteristics:
· Chromium is the main alloying element (typically 12%-30%), contains little to no nickel, magnetic.
· Moderate corrosion resistance (better resistance to chloride stress corrosion cracking than austenitic grades), good oxidation resistance.
· Higher strength than austenitic grades, but lower ductility and poorer weldability.
· Lower cost (due to the absence of nickel).
· Primary Applications: Architectural trim, automotive exhaust systems, appliance exteriors (e.g., washing machine drums), kitchen equipment.
3. Martensitic Stainless Steel
· Series Designations: Another part of the 400 series.
· Common Grades: 410 (AISI 410 / UNS S41000), 420 (AISI 420 / "Cutlery Grade"), 440C (high carbon and chromium, high hardness).
· Key Characteristics:
· Higher carbon content, magnetic.
· Can be significantly hardened and strengthened via heat treatment (quenching and tempering).
· Relatively lower corrosion resistance (compared to austenitic and ferritic grades).
· Poor ductility and weldability.
· Primary Applications: Cutting tools, blades, surgical instruments, shafts, valves, turbine blades, and other components requiring high hardness and strength.
4. Duplex Stainless Steel
· Common Grades: 2205 (UNS S32205), 2507 (UNS S32750).
· Key Characteristics:
· Microstructure is roughly a 50/50 mix of austenite and ferrite, combining characteristics of both.
· Very high strength (about twice that of standard austenitic grades).
· Excellent corrosion resistance, particularly against chloride stress corrosion cracking and pitting.
· Good weldability, but requires controlled procedures.
· Magnetic.
· Primary Applications: Demanding environments such as offshore engineering, oil & gas industry, seawater handling, desalination plants.
5. Precipitation-Hardening (PH) Stainless Steel
· Common Grades: 17-4PH (UNS S17400), 15-5PH (UNS S15500).
· Key Characteristics:
· Achieve very high strength through the precipitation of hardening phases (via additions of Cu, Al, Ti, etc.) after heat treatment.
· Maintain good toughness and reasonable corrosion resistance.
· Fair machinability.
· Primary Applications: Aerospace, nuclear industry, high-strength fasteners, precision machinery—high-tech fields requiring exceptional strength.
(B). Classification by Chemical Composition (Closely related to microstructure)
1. Chromium Series: Primarily ferritic and martensitic grades (e.g., 430, 410).
2. Chromium-Nickel Series: Primarily austenitic grades (e.g., 304, 316).
3. Chromium-Nickel-Molybdenum Series: Molybdenum-added for improved pitting resistance (e.g., 316).
4. Chromium-Manganese-Nickel Series: Austenitic grades where manganese and nitrogen partially replace nickel (e.g., 201, 202).
(C). Classification by Function or Property
· Acid-Resistant Stainless Steel: e.g., 316, with strong resistance to acid and alkali corrosion.
· Heat-Resistant Stainless Steel: e.g., 310S, with good oxidation resistance and high-temperature strength.
· Non-Magnetic Stainless Steel: Primarily austenitic grades, used in precision instruments, MRI equipment, etc.
· Free-Machining Stainless Steel: Additions of sulfur or selenium (e.g., 303) to improve machinability.
· Low Carbon / "L" Grades: Very low carbon content (e.g., 304L, 316L, C ≤ 0.03%), excellent resistance to intergranular corrosion, suitable for welded components that cannot be heat-treated afterward.
(D). Common Grade Cross-Reference
Type Common Chinese Grade (GB) Common U.S. Grade (AISI/UNS) Key Feature Summary
Austenitic 06Cr19Ni10 304 / S30400 General purpose, food grade, non-magnetic
Austenitic 06Cr17Ni12Mo2 316 / S31600 Molybdenum-added, better acid resistance
Ferritic 10Cr17 430 / S43000 Magnetic, cost-effective, good heat resistance
Martensitic 20Cr13 420 / S42000 Hardenable by heat treatment, used for cutlery
Duplex 022Cr23Ni5Mo3N 2205 / S32205 High strength, high corrosion resistance
Precipitation Hardening 0Cr17Ni4Cu4Nb 17-4PH / S17400 Ultra-high strength
(E). Selection Guidelines
1. Corrosion Resistance Needs: General environment: 304; Chloride-containing environments (coastal, saline) or acidic conditions: 316; Very demanding: Duplex.
2. Strength & Hardness Needs: High-strength structural parts: Duplex or PH grades; High hardness required (cutting tools): Martensitic grades.
3. Magnetic Properties: Non-magnetic required: Austenitic; Magnetic acceptable or needed: Ferritic/Martensitic.
4. Forming & Welding Needs: Extensive deep drawing/cold working: Austenitic; For welding, consider low-carbon grades (e.g., 304L).
5. Cost Considerations: Ferritic grades (e.g., 430) are generally lower cost than austenitic grades (e.g., 304).
In summary, stainless steel is not a single material but a diverse family. The key to correct selection lies in understanding the service environment (corrosive media, temperature, mechanical stress) and fabrication requirements (welding, forming) to identify the appropriate microstructure type and specific grade.