Metal shaft having longitudinally varying hardness, golf shaft using the metal shaft, golf club using the metal golf shaft, method of manufacturing the metal shaft, and tempering apparatus
A metal shaft includes a hollow tube made of metal, capable of having hardness varying in a longitudinal direction of the metal shaft while preventing increase in the number of variations on a shape of the metal shaft. The tube has a hardness pattern exhibited by hardness of the tube varying in a longitudinal direction of the tube according to Young's modulus or tensile strength of the metal varying in the longitudinal direction.
