What is the hardness test of bolts?

The hardness testing of bolts is a critical quality control process to ensure they meet specified mechanical properties, such as strength and durability. Here’s an organized overview:

 1. Purpose of Hardness Testing

•  Material Integrity: Verifies the bolt’s resistance to deformation and wear.
•  Correlation with Strength: Hardness values correlate with tensile strength, ensuring bolts meet grade specifications (e.g., Grade 5, 8.8, 10.9).
•  Heat Treatment Validation: Confirms proper quenching, tempering, or case hardening processes.

2. Common Hardness Testing Methods

Rockwell Hardness (HR):

•    Method: Uses a diamond cone or steel ball indenter. Depth of penetration is measured.
•    Advantages: Fast, widely used for bolts (e.g., Rockwell C scale for highstrength steel).
•    Application: Suitable for flat surfaces like bolt heads; may require fixtures for curved shanks.

Brinell Hardness (HB):

•    Method: Uses a tungsten carbide ball. Measures indentation diameter.
•    Advantages: Effective for rough or curved surfaces but leaves larger marks.
•    Application: Often used for larger bolts or raw materials.

 3. Testing Considerations

•  Surface Preparation: Testing areas (e.g., head, shank end) may be ground/polished for accuracy.
•  NonDestructive Testing (NDT): Eddy current or ultrasonic methods for critical applications where indentation is unacceptable.
•  Sampling vs. Batch Testing: Typically, samples are tested unless highstakes applications (e.g., aerospace) require full inspection.

4. Standards and Procedures

•  ASTM Standards: ASTM F606 covers mechanical testing, including hardness.
•  ISO/DIN Standards: Specify methods for metric bolt classes (e.g., ISO 8981).
•  Conversion Tables: Relate hardness values (e.g., HRC) to tensile strength for grade compliance.

5. Challenges

•  Curved Surfaces: Fixtures or specialized testers (e.g., portable Rockwell) accommodate cylindrical shanks.
•  Case Hardening: Microhardness tests assess the hardened layer depth and core toughness.

 6. Importance of Hardness Testing

•  Prevents failures due to brittleness (overhardening) or weakness (underhardening).
•  Ensures consistency in mass production and compliance with international standards.