Inspection Requirements for Grade 10.9 Hexagon Head Bolts

The inspection of Grade 10.9 hexagon head bolts is a rigorous process governed by international standards such as ISO 898-1. The "10.9" designation indicates a minimum tensile strength of 1000 MPa and a yield strength of 900 MPa (calculated as 1000 x 0.9). To ensure these high mechanical properties and reliable performance in critical applications, a comprehensive set of inspections must be performed.

1. Dimensional and Dimensional Verification:
This is the first step,ensuring the bolt can be assembled correctly. Key dimensions checked include the nominal diameter and pitch of the thread, the across-flats width of the hexagon head, the head height, the shank diameter, the length under the head, and the thread length. Gauges are used to verify thread acceptability, including a Go gauge (must screw on freely) and a No-Go gauge (must not screw on beyond two turns). The thread geometry is also inspected for correct form, pitch, and flank angles.

2. Mechanical Properties Testing:
This is the core of the inspection to confirm the 10.9 grade performance.

· Wedge Tensile Test: A bolt is tension-tested with a hardened wedge placed under its head. This test simulates the bending stresses encountered in service. The bolt must fracture in the shank or threaded portion, not at the head-shank junction, and must achieve a minimum tensile load as specified for its size and grade.
· Hardness Test: The bolt's hardness is measured on the unthreaded shank or on a transverse section of the head. For Grade 10.9, the core hardness must fall within a specified range, typically 31 HRC to 39 HRC (Rockwell C scale). This ensures the material has the necessary strength and resistance to deformation.
· Proof Load Test: The bolt is subjected to a tensile load greater than its service load but less than its yield point. For a short period, a stress corresponding to the proof load (typically 830 MPa for 10.9) is applied. The bolt must not exhibit permanent elongation or failure, proving its ability to withstand preload without yielding.

3. Surface Discontinuity and Surface Integrity Checks:
High-strength bolts are susceptible to hydrogen embrittlement,making surface quality critical.

· Surface Defects: The surface is inspected for seams, folds, cracks, and other imperfections that could act as stress concentrators and lead to premature failure.
· Decarburization Check: The bolt is sectioned and examined microscopically to measure decarburization. The absence of decarburization on the surface (or its limitation to a very thin layer) is mandatory to ensure the surface hardness and fatigue strength are not compromised.

4. Material and Chemical Composition:
While not always checked on every batch,the material must conform to standards specifying medium carbon steel, alloy steel, or boron steel. The chemical composition is verified to ensure it can achieve the required hardenability and mechanical properties through heat treatment (quenching and tempering).

5. Head Marking Inspection:
Each bolt must be clearly and permanently marked on the head with the manufacturer's identification mark and the property class"10.9". This provides traceability and confirms the manufacturer's certification of the grade.

6. Torque-Tension Test (Optional but Critical for Assembly Validation):
While not a standard material inspection,this test is often performed to verify the bolt's behavior under applied torque. It ensures the bolt can achieve the required clamp force (preload) with a given torque and has a consistent friction coefficient on the bearing surface and threads.

In summary, a Grade 10.9 hexagon head bolt is only considered合格 after passing this multi-faceted inspection regime, which verifies its dimensions, mechanical strength, material integrity, and surface condition to guarantee it will perform safely and reliably under high-stress conditions.