PURPOSE: To prevent the cracking and peeling in the surface hadened layers of calibers by injecting a coolant to the caliber surface of a roughly formed roll die material which is austenitized at prescribed temps., thereby hardening the surface.

CONSTITUTION: A roughly formed roll die material 5 consisting of low alloy tool or die steel is heated to 780W830°C, by which it is austenitized. Such material 5 is immersed in the coolant in a quencging tank 7, and the coolant is injected from injection nozzles 9 toward the caliber surface of the material 5. While the caliber surface is hardened uniformly, the coolant in the tank 7 is agitated and circulated to harden the entire part of the material 5. Thus the cracking or peeling in the surface hardened layer of the caliber surface are prevented.