Volumetric Examination After PWHT on a Carbon Steel Pressure Vessel

Abstract

Carbon steel pressure vessels may require post weld heat treatment (PWHT) for stress relief in accordance with global pressure vessel safety codes. When stress relieved, most of these vessels receive a full volumetric exam, such as radiography or ultrasound. Many supplementary design specifications require the volumetric exam take place after PWHT; however, when a volumetric exam after PWHT is required, vessel fabricators commonly perform an additional examination before PWHT to mitigate the risk of a second PWHT cycle. The increased cost associated with performing two full volumetric exams is passed on to the end-user of the pressure vessel. This investigation examines the need for a volumetric exam after PWHT on a carbon steel pressure vessel. Following a literature review and after analysis of radiography results from vessel fabricators in the United States, there is little evidence supporting a volumetric examination requirement after PWHT. PWHT does not cause sub-surface flaw development or sub-surface flaw propagation. A surface examination after PWHT is still recommended to confirm flaws have not developed near the surface during PWHT; however, a full volumetric examination on a carbon steel vessel after PWHT may be unnecessary.