Evaluation of Stress Relaxation Cracking on 800HT Pigtails of the Steam Reformer

Abstract

Stress relief cracking occurs when susceptible alloys are subjected to thermal stress after welding to reduce residual stresses and improve toughness. Cracks were observed on the external surface of the outlet pigtails of a steam reformer. The failures (cracks) occurred at the toe of the socket weld connecting the pigtail to the catalyst tube. Design parameters are 900 °C and 22 barg; the actual operating conditions are 875 °C and 20 barg. The material of construction is Alloy 800HT, which is an iron–nickel–chromium alloy. The outlet pigtail was used to transfer the process fluid of the steam reformer which is connected to a manifold header. It was requested to conduct a failure analysis on damage in the outlet pigtail tubes that occurred in the form of cracks. The failure analysis study is described in detail in this paper, and the mechanism of the crack is identified and proper recommendation is given to avoid such issues in future operations.