Bolt grades and markings refer to a standardized system used to indicate the mechanical properties of bolts—such as strength, hardness, and toughness—as well as their material type. Through grade classification and head markings, engineers can quickly identify whether a bolt is suitable for specific load, structural, or environmental requirements.
SAE J429 Bolt Grades and Markings (Imperial Bolts)
In U.S. standards, the SAE J429 specification defines a series of bolt grades, such as Grade 5 and Grade 8, including their variants. These grades are clearly identified by radial lines or alphanumeric symbols on the bolt head, indicating their tensile and yield strength. Understanding these grades and their corresponding properties helps ensure that the correct bolts are selected for applications requiring specific strength and safety levels. The following section provides details on common grades such as Grade 5, 5.1, 5.2, 8, 8-BD, and 8.2, including their definitions, performance characteristics, and head marking styles—useful for engineers and procurement teams.
| Grade | Material Type | Typical Applications | Head Marking |
| Grade 5 | Medium carbon steel, heat-treated | Commonly used in automotive and machinery assembly | 3 radial lines (evenly spaced at 120°) |
| Grade 5.1 | Variant of Grade 5 | Slight design variation | Same as Grade 5 |
| Grade 5.2 | Improved Grade 5 | For higher consistency and performance | Same as Grade 5 |
| Grade 8 | Alloy steel, heat-treated | Heavy-duty structures, automotive suspensions | 6 radial lines (evenly spaced at 60°) |
| Grade 8-BD | Low-alloy steel, heat-treated | Special applications with “BD” marking | BD mark or other symbol + 6 lines |
| Grade 8.2 | High-strength carbon steel | Enhanced durability or environmental resistance | Similar to Grade 8 |
| Grade | Nominal Diameter Range (inch) | Tensile Strength(ksi / MPa) | Yield Strength(ksi / MPa) | Min. Shear Strength(ksi / MPa) | Min. Elongation(%) | Hardness Range(Rockwell) |
| Grade 5 | 1/4″ – 1″ | 120 ksi / 827 MPa | 92 ksi / 635 MPa | ~75 ksi / 517 MPa | 12% (for <1″) | C25 – C34 |
| Grade 5.1 | Same as above | Same as Grade 5 | Same as Grade 5 | Same as Grade 5 | Same as above | Same as above |
| Grade 5.2 | Same as above | Same as Grade 5 | ≥ Grade 5 | Slightly higher than Grade 5 | ≥12% | C25 – C35 |
| Grade 8 | 1/4″ – 1.5″ | 150 ksi / 1034 MPa | 130 ksi / 896 MPa | ~90 ksi / 620 MPa | 14% (for <1″) | C33 – C39 |
| Grade 8-BD | Same as above | Same as Grade 8 | Same as Grade 8 | Same as Grade 8 | Same as above | C33 – C39 (with “BD” mark) |
| Grade 8.2 | 1/4″ – 1.5″ | 150 ksi / 1034 MPa | 120 ksi / 827 MPa | ~88 ksi / 607 MPa | ≥12% | C33 – C38 |
Head Marking Explanation
- No marking: Typically indicates Grade 2 (low strength bolts)
- Three radial lines: Grade 5 (medium strength)
- Six radial lines: Grade 8 (high strength)
- Additional markings (e.g., BD): Indicate manufacturer-specific or special-use variants
Reference Standards
- SAE J429 – Mechanical and material property requirements for inch-series bolts
- ASME B18.2.1 – Dimensional standards for bolts and nuts
ASTM A307 Bolt Grades and Markings
ASTM A307 bolts are mainly classified into two grades: Grade A and Grade B, differing in mechanical properties and head markings as detailed below:
| Grade | Material Type | Minimum Tensile Strength | Head Marking | Typical Applications |
| Grade A | Carbon steel, annealed or softened | Approximately 60 ksi (415 MPa) | No marking | Low-strength general use, repairs, light structural applications |
| Grade B | Carbon steel, quenched and tempered | Approximately 75 ksi (520 MPa) | Four radial lines | Medium-strength structural applications |
Head Marking Explanation
- Grade A: Usually no head marking.
- Grade B: Head stamped with four radial lines for easy identification.
Notes
- ASTM A307 Grade B bolts have higher strength than Grade A, suitable for higher load-bearing structural connections.
- This standard covers low- to medium-strength carbon steel bolts widely used in construction and mechanical assembly.
ASTM A354 Bolt Grades and Markings
ASTM A354 covers quenched and tempered alloy steel bolts, studs, and threaded rods, typically used in high-strength applications such as heavy machinery and structural components. The specification defines several Grades with varying mechanical properties:
| Grade | Material Type | Minimum Tensile Strength | Minimum Yield Strength | Head Marking | Typical Applications |
| Grade BC | Alloy steel, quenched and tempered | 125 ksi (860 MPa) | 105 ksi (725 MPa) | “BC” or manufacturer’s mark | High-strength machinery and structural bolts |
| Grade BD | Alloy steel, quenched and tempered | 150 ksi (1035 MPa) | 130 ksi (895 MPa) | “BD” or manufacturer’s mark | Very high strength heavy-duty bolts |
| Grade BE | Alloy steel, quenched and tempered | 175 ksi (1205 MPa) | 150 ksi (1035 MPa) | “BE” or manufacturer’s mark | Ultra-high strength applications |
Head Marking Explanation
- ASTM A354 bolts typically carry a letter grade marking on the bolt head (e.g., BC, BD, BE), often accompanied by the manufacturer’s identification.
- Unlike SAE grades with radial lines, ASTM A354 uses these letter codes to denote the grade.
Notes
- ASTM A354 bolts are used where high tensile and yield strength are critical, such as in heavy equipment, automotive, and construction machinery.
- They must be quenched and tempered to meet the mechanical property requirements.
- These bolts are often paired with compatible nuts meeting ASTM A194 or A563 standards.
ASTM A325 Bolt Grades and Markings
ASTM A325 is a standard for high-strength structural bolts, widely used in steel connections for bridges, buildings, and other structures. The standard classifies bolts into the following types based on material and strength:
| Grade / Type | Material Type | Minimum Tensile Strength | Minimum Yield Strength | Head Marking | Typical Applications |
| Type 1 | Medium carbon steel, heat treated (quenched & tempered) | 105 ksi (724 MPa) | 90 ksi (620 MPa) | 6 radial lines | General high-strength structural bolts |
| Type 2 | Weathering steel (less common) | Same as Type 1 | Same as Type 1 | 6 radial lines + “W” | Corrosion-resistant structural bolts |
| Type 3 | Alloy steel, heat treated | 120 ksi (827 MPa) | 105 ksi (724 MPa) | 8 radial lines | Higher strength structural bolts |
| Type 4 | Weathering alloy steel (less common) | Same as Type 3 | Same as Type 3 | 8 radial lines + “W” | Higher strength corrosion-resistant bolts |
Head Marking Explanation
- Type 1 and Type 2: Bolt heads are marked with 6 radial lines; Type 2 additionally has a “W” to indicate weathering steel.
- Type 3 and Type 4: Bolt heads are marked with 8 radial lines; Type 4 additionally has a “W” for weathering alloy steel.
Notes
- Type 1 is the most commonly used grade for general high-strength structural bolts.
- Type 3 provides higher strength for more demanding load conditions.
- Types 2 and 4 are weathering steel bolts with improved corrosion resistance.
- ASTM A325 bolts are typically used with ASTM A563 nuts to form complete high-strength bolted connections.
ASTM A490 Bolt Grades and Markings
| Grade | Head Marking | Type | Coating / Finish | Notes |
| A490 Type 1 | “A490” + Manufacturer’s ID | Alloy Steel | Uncoated or Zinc-coated (mechanical only, no hot-dip galvanizing) | Standard high-strength structural bolts |
| A490 Type 2 | [Withdrawn] | Withdrawn in 2002 | N/A | Was based on low carbon martensitic steel; no longer in use |
| A490 Type 3 | “A490” + Manufacturer’s ID + “3” | Weathering Steel (Corrosion-resistant) | Uncoated | For use in atmospheric corrosion-resistant applications |
| A490M | “A490M” + Manufacturer’s ID | Metric Equivalent | Same as A490 | Specified in metric units; mechanical properties match imperial types |
ISO (Metric) Bolt Grades and Markings
ISO 898-1 is an international standard defining the mechanical properties of metric bolts, widely used in mechanical manufacturing and structural engineering. The standard uses a two-part numeric grade marking to indicate the mechanical performance of bolts:
- The first digit: Represents the minimum tensile strength in hundreds of MPa.
- The second digit: Represents the ratio of yield strength to tensile strength (typically less than 1), reflecting the bolt’s plastic deformation capability.
For example, a grade “8.8” bolt means:
- Minimum tensile strength of 8 × 100 = 800 MPa
- Minimum yield strength of 8 × 0.8 × 100 = 640 MPa
This marking system is intuitive and easy to identify, helping engineers and purchasers quickly determine if the bolt meets design requirements.
| Grade | Minimum Tensile Strength (MPa) | Minimum Yield Strength (MPa) | Minimum Elongation (%) | Typical Applications |
| 4.6 | 400 | 240 | ≥ 23 | Low-strength applications, general mechanical connections |
| 4.8 | 400 | 320 | ≥ 20 | Low strength, slightly higher load than 4.6 |
| 5.6 | 500 | 280 | ≥ 17 | Medium-low strength mechanical connections |
| 8.8 | 800 | 640 | ≥ 12 | Common high-strength structural bolts |
| 10.9 | 1000 | 900 | ≥ 10 | High-strength industrial machinery and automotive bolts |
| 12.9 | 1200 | 1080 | ≥ 9 | Ultra-high strength, used for high-load and safety-critical parts |
Additional Notes:
- Elongation: Indicates the bolt’s toughness and ability to undergo plastic deformation; higher elongation means less brittleness.
- Application Scope: Higher grades are suited for greater loads, typically used in critical load-bearing structures and high-strength machinery.
- Head Marking: The grade number is directly stamped on the bolt head for quick on-site identification.
Stainless Steel Bolt Grades and Markings
| Grade | Material Standard | Common Marking | Description |
| 18-8 (A2-70) | ASTM F593 / ISO 3506 | A2-70 | Austenitic SS, ~18% Cr, 8% Ni, good corrosion resistance, common general grade. |
| A2-80 | ISO 3506 | A2-80 | Similar to A2-70 but with higher tensile strength. |
| A4-70 | ASTM F593 / ISO 3506 | A4-70 | Austenitic SS with Mo (~2%), better corrosion resistance, marine grade. |
| A4-80 | ISO 3506 | A4-80 | Higher strength version of A4-70. |
| 304 (UNS S30400) | ASTM A193 / ASTM F593 | Usually unmarked | General-purpose austenitic stainless steel. |
| 316 (UNS S31600) | ASTM A193 / ASTM F593 | Usually unmarked | Contains Mo for improved pitting resistance. |
| B8 (304 SS) | ASTM A193 | B8 | Bolting grade 304 SS, high temp and pressure service. |
| B8M (316 SS) | ASTM A193 | B8M | Bolting grade 316 SS, improved corrosion resistance. |
| B8 Class 1 | ASTM A193 | B8 | Solution annealed only. |
| B8 Class 2 | ASTM A193 | B8C or B8^2 | Solution annealed and strain hardened for higher strength. |
| B8M Class 1 | ASTM A193 | B8M | Solution annealed 316 SS. |
| B8M Class 2 | ASTM A193 | B8MC or B8M^2 | Solution annealed and strain hardened 316 SS. |
Grades and Typical Markings for Monel, Hastelloy, Inconel, Incoloy, and Nickel Alloy bolts
| Alloy Type | UNS Number | Common Grade / Standard | Typical Bolt Marking |
| Monel 400 | N04400 | ASTM F467, ASTM B164 | “M400” or material batch marking |
| Monel K500 | N05500 | ASTM F467, ASTM B865 | “K500” or alloy code |
| Hastelloy C22 | N06022 | ASTM B574, ASME SB574 | “C22” or “HC22” |
| Hastelloy C276 | N10276 | ASTM B574, ASME SB574 | “C276” or “HC276” |
| Inconel 600 | N06600 | ASTM B166 | “600” or “IN600” |
| Inconel 625 | N06625 | ASTM B446 | “625” or “IN625” |
| Inconel 718 | N07718 | ASTM B637 | “718” or “IN718” |
| Incoloy 800 | N08800 | ASTM B408 | “800” or “IC800” |
| Incoloy 825 | N08825 | ASTM B425 | “825” or “IC825” |
| Nickel 200 | N02200 | ASTM B160, ASTM F2281 | “200” or “Ni200” |
| Nickel 201 | N02201 | ASTM B160 | “201” or “Ni201” |
Contact MT.BOLTS
MT.BOLTS specializes in the manufacturing and supply of industrial 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.
