Failure analysis of cracking in the welded joints of hydrogen reformer outlet pigtail tubes

Abstract

Cracking failure of Incoloy 800H welded joints used for about eight months occurred in hydrogen reformer outlet pigtail tubes. Visual inspection, microscopic examination, chemical composition and microhardness testing were performed to find out the failure mechanism. The results indicated that microcracks were discovered in the fusion zone and heat affected zone (HAZ). The crack propagation mode was intergranular and irregular. The cracked regions were occupied with some oxides in the fusion zone. The Cr content at the grain boundaries was abnormal and intergranular oxidation was found in the HAZ. The microcracks in the outlet pigtail tubes were caused by stress relaxation, and oxidation at grain boundaries. The intergranular oxidation caused the embrittlement at the grain boundaries during long-term service. From the investigations, it was believed that the failure of welded joints in the outlet pigtail tubes originated from stress relaxation cracking during high temperature service.