What is Grade 6.8 8.8 10.9 Hex bolts H.D.G mechanical quality request？

In short, Grade (or Property Class) 6.8, 8.8, 10.9, and 12.9 are international standards (ISO 898-1) that define the mechanical properties of steel bolts. They indicate the bolt's strength, hardness, and material quality.

The numbers are not arbitrary; they have a specific meaning.

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Decoding the Numbers: The "X.Y" System

· The first number (e.g., 8 in 8.8): Represents the minimum tensile strength in megapascals (MPa) divided by 100.
  · For an 8.8 bolt: 8 x 100 = 800 MPa minimum tensile strength.
· The second number (e.g., the .8 in 8.8): Represents the yield strength ratio. Multiply the first number by the second number to get the yield strength as a percentage of the tensile strength.
  · For an 8.8 bolt: 8 x 8 = 64. This means the yield strength is 64% of the tensile strength.
  · Calculation: 800 MPa x 0.64 = ~640 MPa yield strength.

Yield Strength: The stress at which the bolt begins to permanently deform (stretch). This is the critical point for most design applications.
Tensile Strength: The maximum stress the bolt can withstand before fracturing.

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Mechanical Quality & Property Requirements by Grade

Here is a breakdown of each common grade, its mechanical properties, typical applications, and material/processing notes.

Property Class 6.8 8.8 10.9 12.9
Min. Tensile Strength 600 MPa 800 MPa 1000 MPa 1200 MPa
Yield Strength (min.) 480 MPa 640 MPa 900 MPa 1080 MPa
Core Hardness (HV) ~170-240 ~250-320 ~320-380 ~385-435
Typical Material Low/Medium Carbon Steel Medium Carbon Steel (Quenched & Tempered) Medium Carbon Steel, Low-Alloy Steel (Q&T) Alloy Steel (Q&T)
Key Mechanical Request General purpose, mild strength. More ductile than higher grades. The standard for high-strength applications. Excellent balance of strength, toughness, and cost. High strength. Used for critical, high-stress connections. Less ductile than 8.8. Very high strength. Used in high-precision, high-stress applications (e.g., engines). Can be brittle and susceptible to hydrogen embrittlement.
Common Applications Furniture, light fixtures, non-critical assemblies. Automotive frames, machinery, structural steel connections (often with a nut of class 8 or higher), general engineering. Most common industrial grade. Critical automotive components (connecting rods, suspension), high-pressure flanges, heavy machinery. Aerospace, high-performance engines, precision machinery. Requires careful installation and often specific lubrication.
Marking on Head Usually marked "6.8" Marked "8.8" with radial lines Marked "10.9" with radial lines Marked "12.9" with radial lines

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Critical "Mechanical Quality" Requirements (Common to All)

Beyond the numbers, the grade specifies a set of mandatory quality controls:

1. Material Chemistry: Limits on carbon, manganese, phosphorus, sulfur, and sometimes alloying elements like chromium or boron.
2. Heat Treatment:
   · 8.8, 10.9, and 12.9 bolts MUST be quenched and tempered. This process (heating, rapid cooling, reheating) is what gives them their high strength and controlled toughness.
   · 6.8 bolts are typically not quenched and tempered (may be strain-hardened or case-hardened). They are supplied in the as-rolled or drawn condition.
3. Mechanical Testing: Bolts from each production batch must pass tests for:
   · Tensile Strength: Measured on the threaded portion.
   · Hardness: Tested on the head or unthreaded shank.
   · Proof Load: A stress level applied without causing permanent deformation. (Essentially a functional test of the yield strength).
   · Ductility: Measured by the wedge test (tensile test with a wedge under the head to check for brittleness) and minimum elongation after fracture.
4. Surface Integrity: Controls on decarburization (loss of carbon from the surface, which weakens threads) and surface defects.

Important Selection & Usage Notes

· Matching Nuts: Always use a nut of equal or higher property class. A 10.9 bolt requires a class 10 nut. Using a weaker nut is the weak link.
· Torque Settings: Higher grade = higher allowable clamping force = higher torque. Always follow torque specifications based on the bolt grade, size, and lubrication. Overtorquing a high-grade bolt can lead to thread stripping or bolt failure.
· Brittleness: As strength increases, ductility generally decreases. 12.9 bolts are very strong but can be brittle and are sensitive to notches and corrosion.
· Corrosion: These grades are for carbon steel. They will rust unless coated (e.g., zinc plating, galvanizing, Dacromet). Important: Electroplating (like zinc plating) of high-strength bolts (10.9 and above) requires careful baking to avoid hydrogen embrittlement, which can cause sudden, delayed failure.

Summary

· Grade 6.8: Mild Strength. Good for general, non-critical use.
· Grade 8.8: High Strength (Standard). The workhorse of industry. Quenched & Tempered. Used where reliable strength is needed.
· Grade 10.9: Higher Strength. For critical, high-stress, dynamic loads.
· Grade 12.9: Highest Strength. For precision, high-clamp force applications. Requires careful handling and installation.

Choosing the correct grade is fundamental to ensuring the safety, reliability, and performance of any bolted joint.