PURPOSE: To prevent an internal hardening phenomenon from being generated in a forged product in subjecting a surface of the forged product to heat treatment due to water cooling by interrupting water cooling treatment prior to generating hardening transformation in a center part of the forged product.
CONSTITUTION: In subjecting the forged product 1 such as a crank shaft to the surface hardening heat treatment by rapidly cooling the same by injecting the mist like cooling water 3 from a spray nozzle 2, a surface temp. and a temp. of a center part are measured and, when the temp. 5 of the center part is reached to 680°C as shown by a point D, the spraying of said cooling water is interrupted for 100sec to prevent the center part from being subjected to tempering hardening due to the rapid cooling. At a time E when the temp. of the forged product is lowered, said cooling water is again sprayed to cool said forged product so as to decrease hardness C of the center part lower than hardness A, B of a surface part to prevent a reverse hardening phenomenon from being generated in the forged product.
OOTA TADAO
SUGIYAMA HITOSHI
