The most comprehensive guide to common hex bolt problems and their solutions

Causes of  hex head screw Loosening

Causes:

Under-torqued bolts;

Failure to use anti-loosening measures (such as flange hex bolts) in a vibrating environment.

How to fix loosening din931 bolts?

Re-tighten to the correct torque;

Install disc spring washers or nylon lock nuts (preferred for vibration environments).

2. Seized Threads (Cannot Be Removed)
Cause: Stainless steel/alloy steel bolts heat up due to friction and become stuck.
Solution:

Apply a loosening agent (WD-40) and tap and vibrate the bolts;

Heat the nuts to 300°C (quickly cool them before unscrewing).

hex Bolt Fracture

Cause Analysis:

Fracture Type	Cause
Head fracture	Excessive torque or wrench slippage
Thread fracture	Hydrogen embrittlement (installation in a humid environment)
Uniform shank fracture	Overload or fatigue failure

Preventative Measures for hex screws Fracture

Use grade 12.9 hex bolt instead of grade 10.9 hex bolt in environments with hydrogen embrittlement risk (more resistant to hydrogen embrittlement);
Increase the number of bolts in heavy-duty structures to avoid overloading a single bolt.

How to Address Corrosion and Rusting of High-Strength hex Bolts Scenario

Scenario:

Galvanized hex bolts rust in chemical/seaside environments;
Stainless steel hex bolts galvanically corrode in contact with carbon steel.

Solution:
Use Dacromet-coated bolts (rust protection >1000 hours) in chemical areas;
Insulate between dissimilar metals with insulating spacers (e.g., nylon, rubber).

How to Maintain hex head bolt?

Regular Inspection:
Vibration equipment should be re-tightened every three months (torque should be adjusted if the torque drops by 10%). Corrosion Inspection:
Clean rust with a wire brush and apply anti-rust grease to protect the threads.
Scrap Criteria:
Thread damage > 1/3 turn, visible cracks on the shaft, elongation exceeding 5% (replacement required).

Considerations for Choosing Hexagon Bolts

Incorrect Operations	Consequences	Correct Alternatives
Using Standard Nuts with Grade 12.9 Bolts	Nut Thread Collapse, Bolt Falling Out	Use Special Grade 12 Nuts
Carbon Steel Washers with Stainless Steel Bolts	Electrochemical Corrosion Acceleration	Washers and Bolts Made of the Same Material
Reusing Stretched Bolts	Fatigue Fracture Risk Increased by 300%	Single-Use (Critical Structure)