Drywall screws operate under a strict performance paradox: they must have a surface hard enough to pierce heavy-gauge sheet metal or hard gypsum layers without dulling the point, yet retain a sufficiently tough and ductile core to resist high shearing forces when a power driver suddenly stops. This dual requirement makes the post-forming heat treatment cycle—typically continuous gas case hardening followed by oil quenching and tempering—the most critical technical stage.

Hardness Metrics and Specifications

• Surface Hardness: The minimum standard surface hardness is 550 HV to 650 HV. This ensures the sharp twin-lead threads or self-drilling points cut cleanly through steel studs up to 1.2 mm thick without rolling or flattening.
• Core Hardness: The core hardness must be carefully maintained between 320 HV and 450 HV. A core softer than 320 HV will lead to excessive twisting and dynamic neck snapping under high torque, while a core harder than 450 HV creates extreme vulnerability to hydrogen embrittlement.
• Effective Case Depth: The hardened surface layer (case depth) should stay strictly between 0.05 mm and 0.20 mm.

Metallurgical Importance: The critical balance is achieved through controlled case hardening, where carbon atoms are diffused into the surface layer. This increases the surface hardenability, allowing the low-carbon steel casing to form uniform, hard martensite upon oil quenching, while the core remains a ductile, tempered martensite/bainite mix.