Will High-Strength Extended Screws Have Brittle Fracture Risk？

Release time：2020-01-16 18:44 Browse：

1. Material Factors

• Brittleness Tendency of High-Strength Steel
  High-strength screws (e.g., Grade 8.8, 10.9, 12.9) are typically quenched and tempered to enhance strength, but this may reduce toughness. Insufficient tempering or improper alloy composition (e.g., high sulfur/phosphorus content) increases brittle fracture risk.
  Recommendation: Use materials compliant with standards (e.g., ISO 898-1, ASTM A490) and ensure proper heat treatment.

• Hydrogen Embrittlement Risk
  Electroplating (e.g., zinc coating) or acid pickling may introduce hydrogen atoms into the steel, leading to delayed brittle fracture (especially in stress-concentrated areas).
  Recommendation: Perform hydrogen baking (200°C for 4+ hours) after plating or use hydrogen-free coatings (e.g., Dacromet).

2. Geometric Design Impact

• Stress Concentration in Extended Screws
  Screws with a high length-to-diameter ratio are prone to lateral vibration or bending, causing stress concentration at thread roots or transition zones.
  Recommendation:

• Optimize thread run-out (e.g., tapered threads).

• Avoid abrupt diameter changes; increase fillet radius.

• Use rolled threads (better fatigue resistance than cut threads).

3. Manufacturing Defects

• Improper Cold Working or Heat Treatment
  Micro-cracks from cold forging or uneven heat treatment (e.g., localized overheating) can significantly reduce toughness.
  Recommendation: Verify quality via metallography and hardness testing to ensure uniform microstructure (e.g., tempered martensite).

4. Usage Condition Risks

• Dynamic Loads and Fatigue
  Extended screws under cyclic loads (e.g., vibration, impact) are more susceptible to fatigue crack propagation.
  Recommendation:

• Evaluate fatigue strength; use high-fatigue-resistance alloys (e.g., 42CrMo).

• Apply anti-loosening measures (e.g., thread locker, double nuts).

• Low-Temperature Environments
  Steel toughness decreases in cold conditions (ductile-to-brittle transition effect), risking sudden fracture.
  Recommendation: Use low-temperature-grade materials (e.g., AISI 316L stainless steel or nickel alloys).

5. Preventive Measures

• Testing & Monitoring:

• Ultrasonic or magnetic particle inspection for internal defects.

• Regularly check preload tension (avoid over-tightening or loosening).