Heat treatment is the most critical step in screw manufacturing.
While wire processing, cold heading, thread rolling, and electroplating all significantly impact the final product quality, heat treatment directly determines the screw's mechanical properties and service safety. It is the core process ensuring that high-strength screws (such as grade 8.8 and above) possess sufficient hardness, toughness, tensile strength, and fatigue resistance. If heat treatment goes awry, even with perfect initial processes, the finished product may still break, fail, or even cause safety accidents during use.
Comparison of the roles and criticalities of each process:
Wire Processing: Provides qualified raw materials for subsequent processing, focusing on removing oxide scale and improving machinability. Improper processing can lead to mold wear or cracking, but can be remedied through rework.
Cold Heading: Determines the geometric accuracy of the screw head and shank, affecting material utilization and initial strength. Its advantage lies in work hardening to increase strength, but its impact on the final overall performance is limited.
Thread rolling (thread rolling) forms threads through plastic deformation, increasing thread strength by 10%-30% and extending fatigue life by over 50%. While extremely important, it still depends on the properties of the base material.
Heat treatment (most critical)
Purpose: To balance hardness and toughness, preventing brittle fracture.
Core process: Quenching + Tempering
Influencing factors: Raw material composition, heating temperature, cooling rate, residual stress control
Risk consequences: Defects such as overheating, decarburization, and quenching cracks will directly lead to batch scrapping or service failure.
Electroplating primarily provides corrosion protection and aesthetic enhancement. While it affects durability, it does not change the screw's load-bearing capacity.
