Bolts play a critical role in mechanical assemblies, structural systems, and industrial applications. However, not all bolts are created equal. They come in various strength grades—such as 4.6, 8.8, 10.9, and 12.9 for metric bolts, or Grade 2, 5, and 8 in imperial standards—each designed for specific performance requirements. Understanding these grades and their mechanical properties helps ensure safety, efficiency, and cost-effectiveness in engineering and construction projects.
Common Carbon Steel Bolt Grades (According to ISO / DIN / GB / SAE Standards)
1. ISO / DIN / GB (Metric System) – Strength Classes
In the metric system (used in ISO, DIN, and GB standards), carbon steel bolts are graded using a two-number format like 4.6, 8.8, 10.9, and 12.9. These numbers represent the bolt’s tensile strength and yield strength.
Example: What does Grade 8.8 mean?
- First number (8): Tensile strength = 800 MPa
- Second number (8): Yield strength = 80% of tensile = 640 MPa
This means Grade 8.8 bolts are high-strength and suitable for structural or mechanical use.
Grade 4.6 Bolts
- Tensile Strength: 400 MPa
- Yield Strength: 240 MPa (400 × 0.6)
- Material: Low carbon steel
- Marking: Head marked with “4.6”
- Features: Low strength, for non-structural use
- Applications: Furniture, light-duty structures, no shock or vibration
Grade 5.8 Bolts
- Tensile Strength: 500 MPa
- Yield Strength: 400 MPa
- Material: Medium carbon steel, sometimes tempered
- Marking: “5.8” on the head
- Features: Moderate strength
- Applications: Machinery structures, general industrial use
Grade 8.8 Bolts (Start of high-strength bolts)
- Tensile Strength: 800 MPa
- Yield Strength: 640 MPa
- Material: Medium carbon or alloy steel, quenched and tempered
- Marking: “8.8”
- Features: Widely used high-strength bolt
- Applications: Structural steel, automotive, bridges, mechanical assemblies
Grade 10.9 Bolts
- Tensile Strength: 1000 MPa
- Yield Strength: 900 MPa
- Material: Alloy steel (e.g., SCM435), heat treated
- Marking: “10.9”
- Features: Higher strength, good fatigue resistance
- Applications: Heavy-duty machinery, construction equipment, pressure vessels
Grade 12.9 Bolts (Ultra-high strength)
- Tensile Strength: 1200 MPa
- Yield Strength: 1100 MPa
- Material: High alloy steel, precision heat treated
- Marking: “12.9”
- Features: Very high strength, needs precise torque control
- Applications: Precision molds, aerospace, racing, high-stress clamping
2.Imperial Bolt Grades (SAE Standard)
Grade 2 (≈ ISO 4.6)
- Tensile Strength: ~400 MPa
- Material: Low carbon steel
- Marking: No marks or one dot on head
- Applications: Light-duty fastening, home projects, wood structures
Grade 5 (≈ ISO 8.8)
- Tensile Strength: ~830 MPa
- Marking: 3 radial lines on bolt head
- Applications: Automotive, general machinery, structural components
Grade 8 (≈ ISO 10.9/12.9)
- Tensile Strength: ~1240 MPa
- Marking: 6 radial lines on head
- Applications: Heavy equipment, structural joints, high-impact areas
3. Stainless. Steel Bolt Grades (ISO 3506 Standard)
A2-70 (304 Stainless Steel)
- Tensile Strength: 700 MPa
- Yield Strength: ≥450 MPa
- Features: Good corrosion resistance, general-purpose use
- Applications: Food equipment, outdoor fixtures, construction
A4-80 (316 Stainless Steel)
- Tensile Strength: 800 MPa
- Yield Strength: ≥600 MPa
- Features: Excellent corrosion resistance in acids and chlorides
- Applications: Marine, chemical plants, pharmaceutical equipment
Summary Table of Bolts Grades
| Grade | Tensile Strength (MPa) | Material | Features | Typical Applications |
| 4.6 | 400 | Low carbon steel | Low strength | Home, light-duty use |
| 5.8 | 500 | Medium carbon steel | Moderate strength | Machinery, general use |
| 8.8 | 800 | Alloy steel | High strength | Construction, vehicles |
| 10.9 | 1000 | Alloy steel | Higher strength | Heavy machinery |
| 12.9 | 1200 | High alloy steel | Ultra strength | Aerospace, racing |
| A2-70 | 700 | 304 SS | Corrosion resistant | Outdoors, food-grade use |
| A4-80 | 800 | 316 SS | Acid/chemical resistant | Marine, chemical |
Common Bolt Grades & Specifications for High-Performance Alloys
| Material | Common Grades / UNS Numbers | Standards / Grades | Key Features | Typical Applications |
| Hastelloy | C-276 (UNS N10276), C-22 (N06022) | ASTM B574 (bar), ASTM B564 (forgings) | Excellent resistance to acids, pitting, and crevice corrosion | Chemical processing, pickling equipment, harsh corrosive environments |
| Monel | 400 (UNS N04400), K500 (N05500) | ASTM B164 (bar), ASTM B865 (K500) | Seawater resistant, high-temperature strength, some magnetic properties | Marine engineering, heat exchangers, pump and valve systems |
| Inconel | 600 (N06600), 625 (N06625), 718 (N07718) | ASTM B166, B446, B637 | High-temperature oxidation resistance, excellent creep strength | Aerospace, turbines, high-pressure vessels |
| Incoloy | 800 (N08800), 825 (N08825) | ASTM B408, B564 | Resistant to sulfuric, phosphoric, and chloride-containing acids | Petrochemical, heat-treating furnaces, acid environments |
| Nickel | Nickel 200 (N02200), Nickel 201 | ASTM B160 | High-purity nickel, good ductility and conductivity | Batteries, chemical tanks, electronic components |
| Duplex Stainless Steel | 2205 (S32205), 2507 (S32750) | ASTM A479, ASTM A182 | High strength + corrosion resistance (duplex microstructure) | Offshore, desalination plants, FGD systems |
Additional Notes:
- These bolts are typically identified by alloy grade (e.g., Inconel 625, Hastelloy C276), rather than traditional strength classes like 8.8 or Grade 5.
- Mechanical strength depends on the alloy and its heat treatment condition. For example, Inconel 718 bolts can be age-hardened to HRC 35–45.
- Mechanical properties and usage are defined in standards such as ASTM, ASME, DIN, and ISO.
Examples (Material + Grade + Standard)
Inconel 718 Bolt
- Grade: Aged (HRC 36–44)
- Standard: ASTM B637
- Yield Strength: ~1035 MPa
- Application: Aerospace fasteners, high-stress connections
Monel 400 Hex Bolt
- Grade: Annealed
- Standard: ASTM B164
- Tensile Strength: ~550–620 MPa
- Application: Marine flanges, seawater systems
FAQs About Bolts Grades
It is not recommended because different grades have different strengths and mechanical properties, risking failure or unsafe assemblies.
Select based on the required load capacity, environmental conditions (corrosion), and whether the joint needs high fatigue or impact resistance.
No, stainless steel bolts are graded differently (like A2-70, A4-80) and focus more on corrosion resistance along with strength.
The bolt may deform or fail under load, leading to joint loosening or structural failure.
Contact MT.BOLTS
MT.BOLTS specializes in the manufacturing and supply of Steel Fasteners, including screws, bolts, nuts, and washers. Our products conform to international standards (ANSI / ASTM / DIN / ISO / JIS) and are available in various materials, dimensions, and surface finishes.
Contact us today to learn more about fasteners and to procure high-quality products. Our technical team is ready to provide expert support and custom selection solutions.
