Fastener Strength Grades
This article thoroughly explains all the knowledge about strength grades in one go. From the numerical meaning, material differences, selection principles, to methods for identifying authenticity and avoiding pitfalls, after reading this article, you will never make a wrong choice again.
I. What exactly do the numbers in strength grades mean?
Core Formula
Strength Grade = Tensile Strength (in MPa) × Yield Ratio
This is a formula every engineer must memorize—yet 90% of people interpret it incorrectly.
| Strength Grade | Tensile Strength ≥ (MPa) | Yield-to-Tensile Ratio ≥ | Yield Strength ≥ (MPa) | Proof Stress ≥ (MPa) |
| Grade 3.6 | 300 | 0.6 | 180 | 150 |
| Grade 4.6 | 400 | 0.6 | 240 | 210 |
| Grade 4.8 | 400 | 0.8 | 320 | 290 |
| Grade 5.8 | 500 | 0.8 | 400 | 360 |
| Grade 8.8 | 800 | 0.8 | 640 | 580 |
| Grade 9.8 | 900 | 0.8 | 720 | 650 |
| Grade 10.9 | 1000 | 0.9 | 900 | 830 |
| Grade 12.9 | 1200 | 0.9 | 1080 | 970 |
Key Points:
• The digit before the decimal point: tensile strength divided by 100. For example, grade 8.8 means tensile strength ≥ 800 MPa.
• The digit after the decimal point: yield ratio (yield strength / tensile strength). For example, grade 8.8 has a yield ratio of 0.8, so yield strength = 800 × 0.8 = 640 MPa.
• Yield strength is the actual usable strength. Beyond this value, the screw will undergo permanent deformation and cannot recover.
A clear comparison:
• One M12 grade 4.8 bolt: theoretical maximum tensile load ≈ 3.6 tons
• One M12 grade 8.8 bolt: theoretical maximum tensile load ≈ 7.2 tons
• One M12 grade 12.9 bolt: theoretical maximum tensile load ≈ 12.2 tons
For bolts of the same size, the load capacity of a grade 12.9 bolt is more than three times that of a grade 4.8 bolt.
II. Different strength grades vary significantly in material and manufacturing process
Many people assume that different strength grades merely differ by numbers, but in reality, they involve entirely different materials and heat treatment processes, resulting in cost differences of 5 to 10 times.
| Strength Grade | Common Materials | Heat Treatment Process | Core Characteristics | Cost Comparison |
| Grade 3.6 / 4.6 / 4.8 | Q235, 10# steel, 15# steel (low carbon steel) | None; formed by cold heading only | Good plasticity and toughness, low cost, low strength, not quenchable | 1 |
| Grade 5.8 / 6.8 | 25# steel, 35# steel (medium carbon steel) | Normalizing | Medium strength, excellent machinability | 1.5–2 |
| Grade 8.8 / 9.8 | 35# steel, 45# steel, 40Cr (medium carbon steel / low alloy steel) | Quenching & high-temperature tempering (quenching and tempering) | Balanced strength and toughness, most widely used | 3–4 |
| Grade 10.9 | 40Cr, 35CrMo, 42CrMo (alloy steel) | Quenching & high-temperature tempering (quenching and tempering) | High strength, good toughness, high impact resistance | 5–6 |
| Grade 12.9 | 35CrMo, 42CrMo, SCM435 (premium alloy steel) | Quenching & medium-temperature tempering | Ultra-high strength, high brittleness, strong notch sensitivity | 8–10 |
Lesson: I once encountered a supplier who used 45# steel quenched to impersonate 12.9-grade screws made from 42CrMo. Although the hardness appeared similar, the toughness was drastically inferior. When used in vibrating equipment, all the bolts fractured within less than a month.
III. Core: How to Choose the Right Strength Grade?
Remember one principle: Use what’s sufficient—don’t blindly pursue high strength. High-strength screws aren’t a universal solution; using them incorrectly can actually be more dangerous.
1. Grade 3.6 / 4.6: Never use in load-bearing areas
• Suitable for: Connections under no or only slight stress
• Equipment nameplates, protective covers, panels
• Wire clamps, plastic component assembly
• Furniture, office supplies
• Strictly prohibited: Any structural connections, any parts subjected to tensile or shear forces
2. Grade 4.8: The most commonly misused grade
• Suitable for: Secondary load-bearing connections, primarily static loads
• Internal supports and partitions in equipment
• Small components not subject to impact or vibration
• Hand tools, standard hardware
• Strictly prohibited: Equipment bases, transmission components, rotating parts, any vibrating environments
Key point: Many unscrupulous manufacturers cut costs by using Grade 4.8 screws throughout entire machines. If all the screws in your purchased equipment are Grade 4.8, quality is likely poor.
3. Grade 8.8: The “universal currency” of mechanical engineering
• Suitable for: About 90% of standard equipment connections
• Equipment bases, frames, steel structures
• Standard transmission components, flange connections
• Most general mechanical structures without special requirements
• Recommendation: Unless higher strength is explicitly required, always choose Grade 8.8 first. It offers the best cost-performance ratio, is widely available, and provides stable, reliable quality.
4. Grade 10.9: First choice for heavy-duty and vibrating applications
• Suitable for: Connections under heavy loads, impact, or vibration
• Automotive chassis, construction machinery, mining equipment
• Cranes, elevators, conveyor lines
• High-speed rotating components
• Features: 25% stronger than Grade 8.8, with sufficient toughness to resist brittle fracture
5. Grade 12.9: Use only when absolutely necessary
• Suitable for: Extreme heavy loads, high precision, space-constrained applications
• Molds, fixtures, hydraulic equipment
• Aerospace, military products
• Applications requiring reduced screw size while maintaining strength
• Critical drawback: Highly brittle, extremely sensitive to notches, scratches, and stress concentrations. Even minor impacts or over-tightening can cause immediate brittle failure—without warning signs beforehand.
Selection guideline: “
Use 4.8 for cover plate nameplates, 8.8 for standard structures, 10.9 for heavy-duty and vibrating applications, and only resort to 12.9 when absolutely necessary.
IV: Learn 3 Tips to Instantly Spot Genuine High-Strength Screws
Counterfeit products are rampant in today’s market—many screws stamped with “12.9” actually fail to meet even the 8.8 grade standard. The following three methods allow you to identify 90% of fake products without any specialized equipment.
1. Check the Head Marking (Most Basic and Crucial)
• Genuine high-strength screws from reputable manufacturers always feature a clear strength grade number plus a manufacturer code on the head.
• If only the number is present without a manufacturer code, there’s a 90% chance it’s counterfeit.
• Never buy any screw that has no marking at all, regardless of how cheap it seems.
• Fake markings are usually blurry, uneven in depth, and can be easily scratched off with a fingernail.
2. Test Hardness (Simplest and Most Effective)
• Take a scrap 8.8-grade screw and use its head to scratch the screw you want to test.
• If a mark appears, the hardness is below 8.8 grade—it’s definitely fake.
• If the test screw doesn’t scratch but gets scratched instead, the hardness is equal to or greater than 8.8 grade.
• Similarly, use a 10.9-grade screw to verify the authenticity of a 12.9-grade one.
Note: Hardness should only serve as a reference, not the sole criterion. Some counterfeit screws may have surface hardening to mimic higher hardness, but their core material remains weak in toughness and durability.
3. Examine Price and Weight
• A 12.9-grade screw should cost at least twice as much as an 8.8-grade one. If prices are similar, it’s likely fake.
• For the same size, high-strength screws are slightly heavier due to denser materials—you can feel the difference by hand.
Final Advice: Always purchase screws for critical applications through official channels. Don’t save a few dollars and risk losing hundreds of thousands later.
V. Five Deadly Misconceptions About High-Strength Screws
1. Misconception 1: High-strength screws can be reused
Wrong! After being tightened, high-strength screws undergo plastic deformation. Screws that have been subjected to force—especially those exposed to impact or vibration—experience a significant reduction in strength and are prone to breakage upon reuse.
Correct practice: For critical connections, all high-strength screws must be replaced after removal. Examples include engine cylinder head bolts, transmission bolts, and equipment base mounting bolts.
2. Misconception 2: The tighter, the safer
Wrong! Every screw has a maximum allowable torque. Exceeding this value will stretch the screw or even cause it to snap immediately.
Correct practice: For all critical joints, use a torque wrench and tighten according to the specified torque value. I’ve seen too many people using extension bars to over-tighten, resulting in broken screws stuck inside threaded holes—requiring time-consuming and labor-intensive drilling to remove.
3. Misconception 3: High-strength screws don’t rust
Wrong! Most high-strength screws are made of carbon steel, which is more prone to rust than standard 4.8-grade screws. Due to more surface imperfections, high-strength screws corrode faster through electrochemical reactions.
Correct practice: Choose appropriate surface treatments based on the operating environment. Use Dacromet or hot-dip galvanizing in humid conditions, and stainless steel or titanium alloys in marine environments.
4. Misconception 4: High-strength screws can be welded
Wrong! The high temperatures generated during welding damage the heat-treated microstructure of high-strength screws, drastically reducing their strength and toughness, turning them into brittle “fragile screws” that break easily.
Correct practice: Never weld high-strength screws. If additional fixation is needed, use nuts or welded connection plates instead.
5. Misconception 5: Higher strength grade is always better
Wrong! Although 12.9-grade screws offer higher strength, they are also more brittle. In many applications, 10.9-grade screws provide better overall performance and greater safety.
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