Failure analysis of a welded outlet manifold pipe in a primary steam reformer by improper selection of materials

A welded manifold pipe in a primary steam reformer used to transport hydrogen gas at about 873 °C developed leakage in the weld fusion zone after about 22,000 h of operation. Detailed microstructural characterization using various electron-optical techniques and mechanical property evaluation were employed to determine the cause of failure. Selection of pipe material and filler metal with relatively high Si contents in the presence of Nb was found to be the cause of failure. It is shown that this combination had promoted intergranular precipitation of the embrittling Ni3Nb2Si Laves phase in the weld fusion zone leading to loss of rupture ductility and associated cracking.