Classification And Analysis Of Stainless Steel Bolt Materials
Mar 27, 2025
In engineering applications, users often inquire about the "grade" of stainless steel bolts. It must be clarified that strength grades such as 8.8 or 4.8 apply only to carbon steel bolts, while stainless steel bolts follow entirely different performance standards. The industry typically uses material designations like SUS304, SUS316, and specialized grade systems such as A2-70 and C1-110 under the ISO 3506 standard.
1. Strength Standard System for Stainless Steel Bolts
The strength of stainless steel bolts is denoted by specific codes:
A2-70: Austenitic stainless steel (e.g., 304), minimum tensile strength 700 MPa, yield strength ≥450 MPa (ISO 3506).
A4-80: Austenitic stainless steel (e.g., 316), minimum tensile strength 800 MPa, yield strength ≥600 MPa.
C1-110: Martensitic stainless steel (e.g., 12Cr13 quenched and tempered), tensile strength 1100–1400 MPa, yield strength ≥820 MPa.
[Note: The strength classification system for stainless steel is distinct from carbon steel grades. Engineering selection must align with these specialized codes.]
2. Classification of Stainless Steel Materials
2.1 Austenitic Stainless Steel (Most Widely Used)
Typical Grades: 304 (06Cr19Ni10), 316 (022Cr17Ni12Mo2).
Properties: Non-magnetic, excellent corrosion resistance.
Applications: General fasteners (hex bolts, nuts, etc.).
2.2 Duplex Stainless Steel
Typical Grades: 2205 (UNS S31803/S32205/022Cr23Ni5Mo3N).
Properties: Tensile strength **≥640 MPa** (ISO 3506-1:2020), long-term service temperature ≤250°C (to avoid σ-phase embrittlement), stress corrosion resistance.
Applications: Chemical equipment, offshore platforms.
Standard Reference: ISO 3506-1:2020.
2.3 Martensitic Stainless Steel
Typical Grades: 410 (12Cr13), 420 (20Cr13).
Properties: Heat-treatable, magnetic. Surface treatments (e.g., passivation or epoxy coating) must be selected based on environment; galvanizing/chromium plating may reduce corrosion resistance.
Applications: Turbine blades, high-strength shafts.
3. Key Material Performance Factors
3.1 Nickel Content
304: 8–10% Ni; 316: 10–14% Ni (Chinese standard 022Cr17Ni12Mo2 specifies 10.0–14.0% Ni).
Molybdenum (2–3% in 316) significantly enhances chloride ion resistance.
3.2 Carbon Content
Austenitic steel: ≤0.08% (ultra-low carbon grades like 304L: ≤0.03%).
Martensitic steel: 0.15–0.40%.
3.3 Chromium Content
All stainless steels require ≥12% Cr to form a passivation layer.
4. Clarification of Common Misconceptions
4.1 Strength vs. Corrosion Resistance
316 (A4-80) has higher strength (800 MPa) than 304 (A2-70, 700 MPa) and superior chloride resistance due to molybdenum (2–3%).
4.2 Welding Practices
Use low-carbon welding wire (e.g., ER308L with ≤0.03% C); carbon steel wire (0.06–0.15% C) is prohibited.
4.3 Surface Treatments
For marine environments: passivation or chromium-free Dacromet (zinc-aluminum coating). Traditional Dacromet (containing chromates) is being phased out for environmental reasons.
Harsh environments: Extend neutral salt spray (NSS) testing to 2000 hours (ISO 9227:2017), not CASS (copper-accelerated salt spray).
5. Quality Control Requirements
5.1 Chemical Composition Testing
Core elements: Cr (≥12%), Ni (316≥10%), Mo (316≥2%), C (austenitic ≤0.08%).
5.2 Mechanical Performance Testing
Austenitic steel: Yield strength ≥210 MPa (GB/T 3098.6).
Hardness range: A2-70 corresponds to HV 210–300.
5.3 Corrosion Resistance Validation
Salt spray test: ≥480 hours (ISO 9227 NSS standard), extended to 2000 hours for harsh environments.
Intergranular corrosion test: ASTM A262.
6. Typical Grade Cross-Reference (Updated per GB/T 20878-2022)
| Category | U.S. Standard | Chinese Standard | Tensile Strength (MPa) | Standard Reference |
|---|---|---|---|---|
| Austenitic | 304 | 06Cr19Ni10 | 700 | ISO 3506-1:2020 |
| Austenitic | 316 | 022Cr17Ni12Mo2 | 800 | ISO 3506-1:2020 |
| Duplex | 2205 | 022Cr23Ni5Mo3N | **≥640** | ISO 3506-1:2020 |
| Martensitic | 410 | 12Cr13 | ≥440 | GB/T 1220-2022 |
| Martensitic (QT) | - | 12Cr13 Quenched & Tempered | 1100–1400 | ISO 3506-1:2020 |
