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Stainless Steel Fastener Grades: Compare strength & corrosion resistance for accurate fastener specification.
Stainless Steel Fastener Grades
Selecting the correct stainless steel fastener grade is critical for structural integrity, corrosion performance, and long-term reliability.
The most common grades of stainless steels for UK fasteners are: A1 (303), A2 (304), A4 (316) & C1 (410), with Duplex, A5 and A8 being used in more specialised environments.
For high-strength stainless steel bolts, engineered solutions such as Bumax® are also widely specified in structural, offshore and critical industrial sectors.
This technical guide compares mechanical strength, corrosion resistance, chloride performance, stress corrosion cracking (SCC) and magnetic behaviour to support accurate specification of stainless steel fasteners.
Quick selection
- General Outdoor Construction: A2-70
- Costal & marine: A4-70 / A4-80
- High Load + Corrosion: Duplex 2205 / Bumax 88 / Bumax 109
- Self-Drilling Screws: 410 (Hardened)
Which Stainless Steel Grade Do I Need?
Match your environment to the right grade using the guide below.
A2-70 Stainless Steel
The most widely used grade for general indoor fastening. Excellent corrosion resistance in dry and low-humidity environments. Suitable for manufacturing, furniture, joinery and light structural work.
- Good general corrosion resistance
- Non-magnetic in annealed condition
- Min. tensile strength 700 N/mm²
- Not suitable for marine or chloride-rich environments
A4-70 Stainless Steel
The standard choice for outdoor and weather-exposed applications. Superior resistance to chlorides compared to A2, making it suitable for external construction, cladding and outdoor structures.
- Contains molybdenum for enhanced corrosion resistance
- Suitable for outdoor and damp environments
- Min. tensile strength 700 N/mm²
- Recommended for UK outdoor construction as standard
Common A4-70 Products
A4-80 Stainless Steel
Specified for coastal, tidal and marine environments where salt spray and chloride exposure demand maximum grade performance. Higher strength class than A4-70 with the same corrosion-resistant alloy.
- 316 alloy with higher proof load than A4-70
- Min. tensile strength 800 N/mm²
- Recommended for sea walls, marinas and coastal facades
- Resists pitting corrosion in chloride-rich conditions
Common A4-80 Products
Duplex 2205
A dual-phase austenitic-ferritic alloy offering roughly twice the yield strength of standard austenitic grades. Ideal where high mechanical load and aggressive corrosion resistance must be met simultaneously.
- Min. yield strength ~450 N/mm²
- Excellent SCC (stress corrosion cracking) resistance
- Used in offshore, chemical processing and structural bridges
- Good resistance to pitting and crevice corrosion
Bumax® 88 / 109
The highest performance stainless fastener available. Bumax® delivers extraordinary tensile strength combined with superior corrosion resistance — engineered for critical structural, military and offshore applications where standard grades are insufficient.
- Bumax 88: 880 N/mm² tensile strength
- Bumax 109: 1090 N/mm² tensile strength
- PREN value of 40+ for extreme chloride resistance
- Specified in nuclear, defence and critical infrastructure
How to Understand ISO (Stainless Steel) Fastener Classifications
Under ISO 3506:
- A = Austenitic stainless steel.
- C = Martensitic stainless steel.
- D = Duplex stainless steel.
- F = Ferritic stainless steel.
Strength class is shown as:
- 35 → 350 MPa minimum tensile strength
- 50 → 500 MPa minimum tensile strength
- 70 → 700 MPa minimum tensile strength
- 80 → 800 MPa minimum tensile strength
- 100 → 1000 MPa minimum tensile strength
Example: A4-80 stainless steel bolts = 316-grade with ≥800 MPa tensile strength.
Links for related reading
To understand more about Tensile Strength and what this means for different size fasteners, please reference our Tightening and Breaking Torques article.
For resistance against specific chemicals, check out our table on Stainless Steel Chemical Resistance.
Technical Comparison of Stainless Steel Grades
Structural Overview
| Grade | Microstructure | Magnetic | Heat Treatable |
|---|---|---|---|
| A1 (303) | Austenitic | No | No |
| A2 (304) | Austenitic | No | No |
| A4 (316) | Austenitic | No | No |
| A5 (316Ti) | Austenitic | No | No |
| A8 | Austenitic | Slight | No |
| C1 (410) | Martensitic | Yes | Yes |
| D6 (Duplex 2205) | Duplex | Yes | No |
| D8 (Super Duplex) | Duplex | Yes | No |
| Bumax (Various) | Austenitic / Duplex | Varies | No |
Composition Overview
| Grade | EN / ISO | C (%) | Cr (%) | Ni (%) | Mo (%) | N (%) | Ti (%) |
|---|---|---|---|---|---|---|---|
| A1 | (303) 1.4305 | 0.15 | 17–19 | 8–10 | 0–0.6 | 0 | — |
| A2 | (304) 1.4301 | 0.07 | 17–19 | 8–10 | 0–0.75 | 0.03 | — |
| A4 | (316) 1.4401 | 0.08 | 16–18 | 10–14 | 2–3 | 0.1 | — |
| A5 | (316Ti) 1.4571 | 0.08 | 16–18 | 10–14 | 2–3 | 0.1 | 0.5 |
| A8 | (254 SMO) 1.4529 | 0.02 | 20 | 18 | 6 | 0.2 | — |
| C1 | (410) 1.4006 | 0.08–0.15 | 11.5–13.5 | 0–0.75 | 0–0.75 | 0 | — |
| D6 | (Duplex 2205) 1.4462 | 0.03 | 22 | 5–6 | 3 | 0.14 | — |
| D8 | (Super Duplex) 1.4410 | 0.03 | 25–26 | 6–7 | 3–4 | 0.2–0.3 | — |
| Bumax | (88/108) N/A | 0.03–0.08 | 23–25 | 12–20 | 4–6 | 0.2–0.3 | NS |
Mechanical Strength Comparison
| Grade | Tensile Strength (MPa) | Yield Strength (MPa) |
|---|---|---|
| A2-70 | ≥700 | ~450 |
| A4-70 | ≥700 | ~450 |
| A2-80 | ≥800 | ~600 |
| A4-80 | ≥800 | ≥600 |
| C1 (410) (Heat Treated) | 800–1000+ | 700-800 |
| D6 (Duplex 2205) | 800–1000 | 550-700 |
| Bumax 88 | ~800 | High |
| Bumax 109 | ~1000 | Very High |
| Bumax 129 | ~1200 | Extremely |
Corrosion Resistance Ranking (Low → High)
| Grade | Corrosion Resistance |
|---|---|
| 410 | ★★★★★★★ |
| A1 | ★★★★★★★ |
| A2 | ★★★★★★★ |
| A4 | ★★★★★★★ |
| A5 | ★★★★★★★ |
| A8 | ★★★★★★★ |
| Duplex | ★★★★★★★ |
| Bumax | ★★★★★★★ |
Stress Corrosion Cracking Resistance
| Grade | SCC Resistance |
|---|---|
| A1 to A5 | Susceptible in chloride environments |
| 410 | Moderate |
| A8 | Excellent |
| Duplex | Excellent |
| Bumax | Excellent |
For offshore or warm saline conditions, Duplex-based solutions or Bumax provide superior protection compared to standard A2 or A4 stainless.
CRC Classed by Fastener Grade
| Grade | Corrosion Resistance Class (CRC) | Notes |
|---|---|---|
| C1 | CRC 1 / Low | Indoor dry; avoid water & salt. |
| A1 | CRC 1 / Low | Basic stainless; indoor only. |
| A2 | CRC 2 / Moderate | General-purpose, outdoor, not coastal |
| A4 | CRC 3 / High | Marine/Coastal Outdoor |
| A5 | CRC 3–4 / High–Very High | Chloride environments |
| D6 | CRC 4 / Very High | Sea Water + Strength, SCC Resistant |
| A8 | CRC 5 / Very High | Hot Salty/Chloride Environments |
| D8 | CRC 5 / Extreme | Extreme Chloride |
| Bumax | CRC 5 / Extreme | Ultra-resistant to pitting, crevice corrosion, and SCC |
What are CRC Values?
CRC stands for Corrosion Resistance Classifications and are a way to rank Stainless Steel’s corrosion resistance from 1 to 5 which are relative rankings and not formal standardised numbers.
PREN Numbers for Common Fastener Grades
| Grade | PREN |
|---|---|
| C1 | ~12 |
| A1 | ~18 |
| A2 | ~18 |
| A4 | ~24 |
| A5 | ~24 |
| D6 | ~35 |
| A8 | ~40 |
| D8 | ~40 |
| Bumax | 40–45 |
What is PREN?
PREN stands for Pitting Resistance Equivalent Number which is a calculated value used to estimate the resistance of stainless steel to localized corrosion (pitting and crevice corrosion), especially in chloride-rich environments. A higher PREN indicates better reisstance to pitting and crevice corrosion. PREN is only used to compare stainless steel grades in aggressive environments.
PREN Formula:
PREN = %Cr + 3.3 x %Mo + 16 x %N
PREN Link
British Stainless Steel Association
Application-Based Specification
| Application-Based Specification | |
|---|---|
| General Outdoor Construction | A2-70 |
| Costal & marine | A4-70 / A4-80 |
| High Load + Corrosion | Duplex 2205 / Bumax 88 / Bumax 109 |
| Self-Drilling Screws | 410 (Hardened) |
What is Duplex Stainless Steel?
Duplex stainless steel contains a mixed microstructure of approximately 50% austenite and 50% ferrite. This hybrid structure combines:
- Higher strength than standard austenitic grades
- Improved resistance to chloride pitting
- Excellent resistance to stress corrosion cracking
- Good fatigue performance
The most common grade used in UK fasteners is Duplex 2205 (1.4462). For applications requiring even higher corrosion resistance, particularly in aggressive chloride environments, Super Duplex (1.4410) may be used.
Key Characteristics of Duplex 2205
- Tensile strength typically 800–1000 MPa
- Yield strength approximately double that of 316
- PREN ~35
- Magnetic (due to ferrite content)
Duplex is commonly specified for:
- Offshore wind structures
- Marine infrastructure
- Chemical processing
- High-load structural bolting
What is Bumax?
BUMAX is a premium manufacturer of high-strength stainless steel fasteners. Unlike standard ISO 3506 grades, Bumax fasteners are produced using proprietary stainless steel alloys and advanced cold-working processes to achieve significantly higher strength levels.
Bumax bolts are often used where:
- Standard A4-80 is insufficient
- High preload and clamp force are required
- Corrosion resistance must be maintained at elevated strength
Why Specify Bumax?
- High tensile strength (up to ~1600 MPa depending on grade)
- Maintains corrosion resistance at elevated strength
- Approved for offshore, energy and structural applications
- Reduced bolt size possible due to higher load capacity
Bumax effectively bridges the gap between carbon steel high-tensile bolts and corrosion-resistant stainless steel fasteners.
What are the Common Bumax Grades?
| Bumax Grade | Approx Strength Class | Corrosion Level | Typical Application |
|---|---|---|---|
| Bumax 88 | Comparable to 8.8 carbon steel | High | Structural |
| Bumax 109 | Comparable to 10.9 | High | Heavy mechanical |
| Bumax 129 | Comparable to 12.9 | High | Critical Bolting |
| Bumax Ultra | Very high strength | Very High | Offshore / Subsea |
| Bumax DX | Duplex-based | Excellent | Marine Structural |
Technical Summary
When specifying stainless steel fasteners in the UK, the correct grade depends on:
- Required tensile strength
- Chloride exposure
- Structural preload requirements
- Risk of stress corrosion cracking
- Budget and availability
For most external UK environments, A2 stainless steel fixings are sufficient.
For marine conditions, A4 (316 stainless steel) is the minimum recommendation.
For structural marine or offshore applications, Duplex 2205 stainless steel bolts or engineered solutions such as Bumax stainless fasteners offer superior mechanical and corrosion performance.
Vital Parts supplies a wide range of stainless steel fasteners including A2, A4 and specialist grades suitable for industrial and structural applications. Our technical team can assist with specification guidance to ensure optimal long-term performance.
Stainless Steel Fastener FAQs
A2 vs A4 Stainless Steel Fasteners
| Feature | A2 | A4 |
|---|---|---|
| Outdoor Use | ✓ | ✓ |
| Marine Use | ✗ | ✓ |
| Contains Molybdenum | ✗ | ✓ |
| Corrosion Resistance | Good | Excellent |
| Chloride Resistance | Moderate | High |
| Cost | Lower | Higher |
| Strength (70 class) | Similar | Similar |
| Typical Standard | 304 / 1.4301 | 316 / 1.4401 |
| Magnetic | Generally no | Generally no |
Related Products
A2 Stainless Steel
A4 Stainless Steel
Need help specifying a grade
If you are unsure between A2 A4 Duplex or high strength stainless options our team can help you match grade and strength class to your environment and load requirements.
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