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

•The cracking failure of Incoloy 800H welded joints in hydrogen reformer outlet pigtail tubes is systematically studied.•Stress relaxation cracking in the welded joints is the root cause of the failure.•The intergranular microcracks are associated with intergranular oxidation, which induces the embrittlement.