Jig unit and method for heat treatment of vessel-like workpiece

A method of heat treating a metal workpiece with a depression which provides a concave inner surface and an open end, said method comprising the steps of:

• providing a jig unit which comprises a transferable support means and a constraining templet member which has an outer convex surface shaped correspondingly to the inner concave surface of the workpiece, and means securing said templet to said support means, the material of said templet member having an expansion coefficient greater than the expansion coefficient of the material of the workpiece;
• securing the workpiece near the open end to said support means such that, prior to heat treatment, there is space between said outer surface of said templet member and the inner surface of the workpiece and when the workpiece and said templet member are heat treated, because the expansion coefficient of said templet is greater than that of the workpiece, said outer surface of said templet comes into contact with the inner surface of the workpiece;
• heating the workpiece together with said jig unit at said predetermined temperature for a predetermined time while introducing an inactive gas into the space between the inner surface of the workpiece and said outer surface of said templet member, whereby said templet member comes into contact with the inner surface of the workpiece due to thermal expansion and said templet member constrains the workpiece;
• cooling the heated workpiece to a predetermined temperature together with said templet member while said outer surface of said templet member is still in contact with the inner surface of the workpiece.