Flame hardening is a localized heat-treatment technique. An oxy-fuel flame is used to rapidly heat a specific area of the steel tube—usually the surface or the ends—above the upper critical temperature. This is followed immediately by a "quench" (water or air spray). This transforms the surface structure into martensite, which is extremely hard and wear-resistant. The advantage of flame hardening in tube manufacturing is that it is selective. For example, the ends of a drive shaft tube can be hardened to resist wear at the connection points, while the main body of the tube remains ductile to absorb torque and vibration without fracturing.