Loader Track Pad Bolts Excavator Bulldozer Accessories Bolts and Nuts 21n-32-31211

1. Raw Material Selection and Pretreatment

• Core raw materials: Alloy steel bars such as 40Cr and 45Cr are mainly used. Some high-end products adopt alloy structural steel to ensure the base material has basic strength.
• The bar stock is subjected to surface derusting and degreasing treatments. Meanwhile, the chemical composition and mechanical properties of raw materials are inspected to avoid impurities affecting subsequent processing quality.

• 2. Cold Heading/Hot Heading Forming

  • Cold heading for small-size bolts: The pretreated bar stock is extruded by a cold heading machine to form the bolt head (hexagonal head, square head, flange head, etc.) and shaft blank in one go, maintaining the continuity of metal fibers to improve strength.

• Hot heading for large-size or high-strength bolts: The bar stock is first heated to 800-1000℃, then formed by a hot heading machine, reducing processing difficulty and preventing material cracking.

3. Thread Processing

• Thread rolling process: Threads are formed by extruding the bolt shaft with rollers of a thread rolling machine. This process does not cut the thread metal fibers, resulting in higher strength and better wear resistance, making it the mainstream processing method.
• Turning thread process for special specifications: Threads are formed by cutting with a lathe, featuring high processing precision but low efficiency, suitable for customized products.

4. Heat Treatment Strengthening

• Quenching and tempering: The bolts are quenched at 850-900℃ and then tempered at 500-600℃ to achieve a good combination of strength and toughness, meeting high-strength standards such as 12.9 grade.
• Local strengthening: Stress-concentrated areas such as the transition fillet between the bolt head and shaft undergo spherical quenching or induction quenching to improve fatigue resistance.