Release time：2020-01-16 18:44 Browse：
The surface hardness and core hardness of Grade 8.8 long screws are typically not the same.
This is because Grade 8.8 screws usually undergo surface treatments (such as quenching and tempering + black oxide, phosphating, zinc plating, or Dacromet coating), and the core properties mainly rely on the quenching and tempering process that defines the screw’s mechanical strength. The surface may differ slightly due to post-treatment or localized hardening.
Core Hardness: Determines the screw’s overall tensile and shear strength. For Grade 8.8, it is typically HRc 22–30 (approximately 240–320 HV).
Surface Hardness: If the screw undergoes surface hardening or carburizing, the surface becomes harder than the core to enhance wear resistance, corrosion protection, or self-tapping performance.
In most cases, without surface hardening, the difference is small. With carburizing or surface hardening, the surface is significantly harder than the core.
For standard quenching and tempering processes, the core hardness is dominant, and surface hardness may slightly differ. In cases with additional surface treatments, the hardness difference between the surface and core can be significant.
