PURPOSE: To improve the working efficiency of a build-up welding device and to prevent the crack of a root pass by executing a preheating prior to welding of a roll and a heat treatment after the welding in a heating vessel which is divided to openable and closable upper and lower vessels and is disposed with electric resistance heating elements to permit temp. control.

CONSTITUTION: The cylindrical heating vessel 1 having the inside surface lined with a refractory heat insulating material is divided openably and closably to the upper and lower vessels 4, 5 along the plane parallel with the axial center. The roll W to be treated is imposed on a stand (b) in the vessel 5 from which the vessel 4 is removed and the vessels 4, 5 are fixed. Electricity is conducted in this state to may pieces of nichrome wires 6 disposed approximately at equal angular distances to preheat the roll W. The temps. at respective points in the vessel 1 are detected by plural temp. detectors 7 in this state and the electric power supply to the respective wires 6 is so controlled by a temp. control device that the respective detected temps. attain prescribed temps. The vessel 4 is removed and the roll W is removed when the roll W is heated to the prescribed temp. The roll is then subjected to welding by the build-up welding device. The roll W is housed into the vessel 1 after the end of welding and is heated at the prescribed heat pattern, by which the heat treatment is executed.