Generation of compressive residual stress at the root of tube-to-tubesheet welded joints in a heat exchanger

Abstract

Stress corrosion cracking (SCC) is the main failure mode of tube-to-tubesheet welded joints in a heat exchanger, and weld residual stress can play an important role in the development of these cracks. Local post welding heat treatment (PWHT) is a feasible method to reduce weld residual stress for tube-to-tubesheet welded joints. In this paper, the residual stress evolution and relaxation mechanisms during local PWHT were discussed in depth. The effects of PWHT temperature and hold time on residual stress were clarified. A compressive stress manufacturing method based on local PWHT was proposed. The results show that the stress concentration at the root of tube-to-tubesheet welded joints is the main reason for the SCC of heat exchangers. The residual stresses in the weld and heated affected zone (HAZ) are significantly reduced during local PWHT. The residual stress relaxation of tube-to-tubesheet welded joints can be attributed to the creep strain development and deformation during local PWHT. The extrusion of the tube and tubesheet at cooling stage results in the generation of compressive stress at the root of tube-to-tubesheet welded joints. PWHT temperature has significant effect on the residual stress in the weld and HAZ in comparison with PWHT hold time. The application of the proposed compressive stress manufacturing method realizes the fabrication of compressive stress at the root of tube-to-tubesheet welded joints, and improves the economic benefit by reducing PWHT hold time. This work will provide a good guidance to reduce the SCC risk of heat exchangers.