Omission of Intermediate Postweld Heat Treatment (PWHT) by Utilizing Low-Temperature PWHT for Welds in Pressure Vessels

Abstract

This study was conducted in order to establish a procedure for a low-temperature postweld heat treatment (LTPWHT) to replace conventional stress relief annealing utilized as postweld heat treatment when pressure vessels are fabricated. From'the standpoint of preventing only hydrogen-induced weld cracks during fabrication before the final PWHT, the appropriate conditions of the LTPWHT for preventing the cracks in heavy-section 2 1 / 4 Cr-1Mo steel weldments were obtained by clarifying the following items: (1) the causes and characteristics of a crack, (2) the hydrogen concentration at the location of the crack initiation by applying various welding conditions, (3) the relation between the reduction of the hydrogen concentration through LTPWHT and its treating conditions. and (4) a critical hydrogen concentration below which cracking does not occur. As a result of the establishment of the LTPWHT, not only the hydrogen-induced weld crack, but also stress relief cracking can be prevented. Moreover, the deterioration of the toughness and the strength of the material caused by the repetitious stress relief annealing can be avoided, and then the condition of the final PWHT specified in pressure vessel codes can be optimized in each weld cone An additional study was completed for welds using a newly developed flux material.