IMPROVEMENT OF RESIDUAL STRESS BY CONTROLLING WELD HEAT INPUT

摘要

PURPOSE:To improve the residual stress on the inside surface side in a weld zone and to prevent generation of a stress corrosion crack by heating the outside surface side in the weld zone of a butt joint of metallic pipes under a specified condition with a welding machine and utilizing the temp. difference between the inside and outside surfaces of the pipes. CONSTITUTION:Residual tensile stress generates on the inside surface side of the weld zone and heat-affected zone thereof and leads to a stress corrosion crack in the case of forming a joint part by butt welding the end parts of metallic pipes to each other. The joint part is subjected to a heating treatment under control of heat input by using a welding machine such as a TIG welding machine, MIG welding machine, plasma arc welding machine or the like in order to prevent such crack. More specifically, said part is so heated that g/t2 attains approximately =0.01 with the quantity of heat input defined as (q) and the wall thickness of the welding pipe as (t), and that alphat/lambda as relative heat transfer rate (where alpha is the coefft. of heat transfer between the inside surface of the pipe and a refrigerant and lambda is the coefft. of heat conduction of the pipe) attains approximately =2. The residual tensile stress in the weld zone and heat-affected zone is considerably decreased or made into compressive stress, by which the generation of a stress corrosion crack is obviated.