Which of the following is NOT a method of surface hardening?

Surface hardening is a process used to increase the hardness of a material's surface while maintaining a softer and tougher core. This technique is essential in applications where wear resistance is needed without compromising ductility.

Case hardening involves infusing a hardening element into the surface of steel, typically carbon or nitrogen, to produce a hardened layer. This process enhances wear resistance and fatigue strength.

Nitriding, similarly, is a method that introduces nitrogen into the surface layers of steel, resulting in a hard, wear-resistant case. This technique is particularly effective for high-strength steels.

Flame hardening employs a high-temperature flame to heat the surface of the material followed by rapid cooling, which hardens the area affected by the heat. This method is effective for localized hardening of gears and similar components.

Carbonizing, on the other hand, refers to the process of converting carbon-containing materials into carbon-rich materials, typically through pyrolysis, rather than a method of hardening the surface of metals directly. It does not describe a surface hardening technique in the traditional sense, which generally involves principles meant to increase hardness and performance characteristics of metallic surfaces. Hence, it does not align with the intended methods of surface hardening like the other options do.