Bolt grade (also known as “strength grade” or “performance grade”) is a standard used to indicate the mechanical properties of the bolt material and after heat treatment. It mainly includes tensile strength, yield strength, and hardness.
In high-load environments such as industry, infrastructure, machinery, and aerospace, selecting the wrong grade or using mismatched grades of bolts may cause insufficient preload, bolt yielding, breakage, or structural loosening.
In other words, the bolt grade tells us whether the bolt can withstand a certain load and whether it is suitable for harsh conditions such as heavy load, high vibration, high temperature, and corrosion.
Therefore, the bolt grade must be understood and verified during the selection, design, procurement, and acceptance processes.
In carbon steel and alloy steel bolts, there are three common grading systems: imperial, metric, and American standard.
SAE Inch Bolt Grades
The Society of Automotive Engineers (SAE) J429 standard for inch bolts. Common grades include: Grade 1, Grade 2, Grade 5, Grade 8, Grade 12, and others.
| SAE Grades | Tensile strength (ksi / MPa) | Yield strength (ksi / MPa) | Hardness (HRC) | Material |
| SAE Grade 1 | 60 ksi / 414 MPa | 36 ksi / 248 MPa | ≤ HRB 100 | Mild steel |
| SAE Grade 2 | 74 ksi / 510 MPa | 57 ksi / 393 MPa | ≤ HRB 100 | Mild steel |
| SAE Grade 5 | 120 ksi / 828 MPa | 92 ksi / 634 MPa | HRC 26–36 | Medium carbon steel |
| SAE Grade 5.2 | 120 ksi / 828 MPa | 92 ksi / 634 MPa | HRC 26–36 | Low-carbon martensitic steel |
| SAE Grade 7 | 133 ksi / 917 MPa | 115 ksi / 793 MPa | HRC 32–38 | Medium carbon alloy steel |
| SAE Grade 8 | 150 ksi / 1034 MPa | 130 ksi / 896 MPa | HRC 35–40 | Alloy steel |
| SAE Grade 12 | 180 ksi / 1241 MPa | 160 ksi / 1103 MPa | HRC 39–44 | Ultra-high-strength alloy steel |
ISO Metric Bolt Grades
The International Organization for Standardization (ISO) 898-1 standard (Property Class) applies to metric bolts.
Common grades (or “performance levels”) are: 3.6, 4.6, 4.8, 5.6, 5.8, 6.8, 8.8, 9.8, 10.9, 12.9, etc.
In grade “8.8”, ‘8’ means that the nominal tensile strength is 8×100 = 800 MPa; “.8” means that the yield strength is 0.8 times the tensile strength. “.8” means that the yield strength is 0.8 times the tensile strength.
For this reason, it is common to mark the grade of metric bolts with a number such as “10.9”, “12.9”, etc. on the head of the bolt.
| Material | Grade | Metric tensile strength (MPa) | Yield strength（MPa） | Application scenarios |
| Carbon steel | Class 3.6 | 300 | 180 | Lightweight brackets, temporary fixings |
| Carbon steel | Class 4.6 | 400 | 240 | General mechanical assembly, furniture, low load connections |
| Carbon steel | Class 4.8 | 400 | 320 | Household appliances, small equipment |
| Carbon steel | Class 5.6 | 500 | 300 | Automobile interior parts, agricultural machinery accessories |
| Carbon steel | Class 5.8 | 500 | 400 | Medium load connection |
| Carbon steel | Class 6.8 | 600 | 480 | Industrial equipment, small power transmission components |
| Low-carbon alloy steel | Class 8.8 | 800 | 640 | Automobile chassis, building steel structure, heavy machine bolts |
| Low-carbon alloy steel | Class 9.8 | 900 | 720 | Wind power bolts and key connections of construction machinery |
| Alloy steel & Medium carbon steel | Class 10.9 | 1000 | 900 | Bridge anchors, engine crankshaft bolts, and excavator hinges |
| Alloy steel | Class12.9 | 1200 | 1080 | Aerospace fasteners, precision hydraulic equipment |
ASTM Bolt Grades
ASTM International (formerly “American Society for Testing and Materials”) standard in English, widely used in structural bolts (e.g. infrastructure steelwork).
Typical specifications are: ASTM A307 (for general joining), ASTM A325 (high strength bolts for structural use), ASTM A490 (higher strength structural bolts).
For example: A325 bolts have mechanical properties equivalent to metric grade 8.8.
Stainless Steel Bolt Grades
For stainless steel bolts (usually austenitic, e.g. 304/316, etc.), grades are marked differently than for carbon steel. Common markings are A2-70, A2-80, A4-70, A4-80 and so on.
- “A2” usually stands for Type 304 (or equivalent) and “A4” usually stands for Type 316 (or equivalent) for greater corrosion resistance.
- The following numbers 70, 80 indicate the minimum tensile strength of the bolt (approx. 700 MPa or 800 MPa level).
Uses: Due to excellent corrosion resistance, it is widely used in marine, chemical, food, pharmaceutical, petrochemical equipment, coastal structures and other environments. For example, A4-80 (316 material, high-strength grade) is commonly used in seawater, salt spray, and highly corrosive environments.
| Material | Category | Performance Class | Thread diameter | Tensile strength(Mpa) |
| Austenitic stainless steel | A1; A2; A3; A4; A5 | 50 | ≤M39 | 500 |
| 70 | ≤M24 | 700 | ||
| 80 | ≤M24 | 800 |
Titanium Bolt Grades
Titanium and titanium alloy bolts are more and more commonly used in aerospace, marine, chemical and other high-end fields due to their light weight, high strength and high corrosion resistance characteristics.
Common grades include: Grade 1 (Pure Titanium), Grade 2 (Pure Titanium Medium Strength), Grade 5 (Ti-6Al-4V High Strength), Grade 16 (Ti-0.04-0.08Pd Ultra Corrosion Resistant), Grade 23 (Ti-6Al-4V ELI Bio-compatible), and others.
| Grade | Material composition | Tensile strength（MPa） | Advantage | Uses |
| Grade 1 | Pure titanium（CP Ti） | ≥240 | Low strength, good plasticity, and high corrosion resistance | Chemical equipment, seawater pipeline |
| Grade 2 | Pure titanium（CP Ti） | ≥345 | General-purpose | Marine, heat exchanger, general industrial fasteners |
| Grade 5 | Ti-6Al-4V | ≥895 | High Intensity, Aviation/Medical | Aircraft Structure, Engine Bolts |
| Grade 16 | Ti-0.04-0.08Pd | ≥345 | Super corrosion resistance | Strong acid/alkali environment |
| Grade 23 | Ti-6Al-4V ELI | ≥828 | High toughness, biocompatibility | Orthopedic implant screws, medical fixation |
Aluminum Bolt Material Grades
The grade of aluminum alloy bolts is usually not marked with “8.8, 10.9” or the like as steel bolts, but is indicated by the material type + heat treatment status (e.g. 2024-T4, 6061-T6, 7075-T6, etc.).
- 6061-T6: Tensile strength of approximately 310-380 MPa, equivalent to a steel bolt grade of roughly Grade 5.8.
- 2024-T4: Tensile strength about 400-450 MPa, equivalent to Grade 6.8.
- 5052-H32: Tensile strength about 210-260 Mpa, equivalent to Grade 4.8.
- 7075-T6: Tensile strength about 520-570 MPa, equivalent to Grade 8.8.
Uses: Aluminum bolts due to the specific gravity is significantly lower than steel, suitable for aviation, rail transportation, ship lightweight, electronic equipment, instrumentation, etc. But need to pay attention to the strength of its threaded joints, abrasion resistance, fatigue resistance is poorer than steel; and the state of heat treatment, machining process, surface treatment is particularly important.
| Material | Heat treatment state | Tensile strength (MPa) | Steel bolt grade | Application scenarios |
| 6061 | T6 | 310-380 | Grade 5.8 | General machinery, automobile |
| 2024 | T4 | 400-450 | Grade 6.8 | Aircraft accessories |
| 7075 | T6 | 520-570 | Grade 8.8 | Aerospace and heavy industry |
| 5052 | H32 | 210-260 | Grade 4.8 | Shipbuilding, chemical industry |
Brass Bolt Grades
Brass (copper-zinc alloy) bolts are commonly used for electrically conductive, corrosion-resistant, decorative and light-duty connections. Their strength is much less than that of high-strength steel, but they have the advantage of good electrical/thermal conductivity and processability.
- C26000 (ASTM): Tensile strength approx. 300-400 MPa, yield approx. 100-200 MPa.
- C36000 (ASTM): Tensile strength approx. 350-550 MPa, yield approx. 150-300 MPa.
- C48500 (ASTM): High strength brass, tensile up to 450-700 MPa.
Uses: Electrical connections, instrumentation, seawater-resistant parts of ships, decorative fasteners, low to medium loaded mechanical structures. Due to the different corrosive environments, the mechanical load is lower than that of steel bolts, so they should be used with caution in highly loaded structures.
| Material number（ASTM） | Tensile strength（MPa） | Yield strength（MPa） | Advantage | Application scenarios |
| C26000 (ASTM) | 300～400 | 100～200 | Good electrical/thermal conductivity, excellent cold workability | Electrical connections, trim parts |
| C36000 (ASTM) | 350～550 | 150～300 | Free cutting, medium strength | General machinery, valves |
| C46400 (ASTM) | 380～600 | 180～350 | Seawater corrosion resistance | Ship, Marine Engineering |
| C48500 (ASTM) | 450～700 | 200～400 | High-strength brass | Load-bearing assembly |
How to Check the Bolt Grade?
Confirming the bolt grade is an essential step in the procurement, acceptance and installation process. The following are commonly used inspection methods:
- Header Logo
SAE inch bolts: grade determined by head marking lines (e.g. three lines = Grade 5, six lines = Grade 8)
ISO metric bolts: Head marking numbers such as “8.8”, “10.9”, “12.9” indicate the performance class.
Stainless steel bolts: Head markings such as “A2-70”, “A4-80” identify the material + grade.
- Material Certificate
Check the material statement (Mill test certificate), heat treatment report, UT/hardness test report, etc. provided by the supplier.
- Size and Thread Matching
Confirm the use of imperial or metric bolts and that the thread size, length, and head type match the design requirements. Mixing imperial and metric or using the wrong grade may result in connection failure.
Mechanical Performance Test
If necessary, samples may be sampled for tensile strength, yield strength and hardness testing to ensure that the actual performance of the bolt meets the grade regulations.
Typical Application Scenarios
The following are typical application suggestions based on different levels to help procurement or technical personnel make selection references in industries such as infrastructure, machinery, electronics, automotive, aerospace, railway, petrochemical, and shipbuilding.
| Grades | Features | Recommended Scenarios |
|---|---|---|
| SAE Grade 5 或 ISO Class 8.8 | Medium strength | Automobile chassis, general mechanical connections, structural parts of small equipment. |
| SAE Grade 8 或 ISO Class 10.9 | High strength | Building steel structure, bridge anchor bolts, heavy machinery, wind turbine key connections. |
| ISO Class 12.9 | Ultra-high strength | Aerospace, high precision hydraulic equipment, extreme load components. |
| Stainless steel A4-80 | High corrosion resistance and high strength | Marine platforms, chemical equipment, pharmaceutical/food equipment. |
| Titanium Grade 5 | Very high strength-to-weight ratio, high corrosion resistance | Aircraft structures, engine components, and deep-sea equipment. |
| Aluminum 7075-T6 | Lightweight and high strength | Aircraft structures, lightweight components for rail transportation, and electronic housing structures. |
| Brass C36000／C48500 | Conductive, corrosion-resistant, decorative | Electrical equipment connections, light-load ship structures, and decorative fasteners. |
In the actual selection, but also consider: bolt preload, bolt length, thread fit, tolerances, environment (temperature, corrosion, vibration), surface treatment (zinc plating, hot-dip galvanizing, chrome plating, anodizing, etc.) and other factors. Improper selection of high-grade bolts may also lead to problems such as “high strength but brittle” and “insufficient preload design”.
Summarize
Although the bolt grade is only a number or identification, what it represents behind is the material, heat treatment, mechanical properties, reliability and safety. Correct understanding and selection of appropriate grade of bolts is the basis for ensuring long-term, safe and reliable operation of the connection structure.
For infrastructure, machinery, electronics, automotive, aerospace, railroad, petrochemical, shipping and many other industries, the quality requirements are higher, the working environment is more demanding, so in the selection of bolt grade, acceptance, installation of all aspects must be standardized and rigorous.
Recommendations: Specify the required design grade at the time of purchase, check the head marking and performance report at the time of acceptance, apply the appropriate torque/preload force at the time of installation, and avoid mixing imperial/metric and different material grades.
