Core Process: Forging
High-quality marine shackles are almost exclusively manufactured using the drop-forging process.
Purpose: To apply immense pressure to a metal billet within a die, thereby breaking down internal casting defects in the raw material and aligning the metal grain flow continuously along the load bearing shape of the shackle—particularly in the U-shaped bow section.
Effect: This significantly enhances the material’s toughness, impact resistance, and fatigue life, enabling it to withstand dynamic and alternating loads in marine environments. Cast shackles, due to their porous internal structure, have been phased out in critical applications.
Performance Assurance: Heat Treatment
After forging, shackles must undergo stringent heat treatment to adjust their mechanical properties.
Process: Typically includes quenching followed by high temperature tempering, i.e., quenching and tempering (Q&T) treatment.
Objective: To achieve a uniform and stable sorbitic microstructure. This provides the material with both high tensile strength and sufficient ductility and toughness, preventing brittle fracture under low-temperature or impact conditions.
Indicator: Different strength grades are essentially achieved through variations in heat treatment parameters.
Conclusion: From an industrial perspective, the value proposition of a reliable marine shackle lies in its optimized metal grain flow (imparted by forging) and balanced microstructure (imparted by heat treatment). This explains why products that appear similar can differ fundamentally in safe working load (SWL), durability, and price.