Hydrogen-Embrittlement Cracking in a Large Alloy Steel Vessel

A vessel made of ASTM A204, grade C, molybdenum alloy steel and used as a hydrogen reformer was found to have cracked in the weld between the shell and the lower head. Six samples from different sections were investigated. The crack was found to be initiated at the edge of the weld in the coarsegrain portion of the HAZ. The microstructure was found to be severely embrittled and severely gassed in an area around the crack. The microstructure of the metal in the head was revealed to be banded and contained spheroidal carbides. The lower head was established by hardness values and microscopic examination to have been overheated for a sufficiently long time to reduce the tensile strength below the minimum required for the steel. It was interpreted that the wide difference in tensile strength between head and weld metal (including HAZ) formed a metallurgical notch that enhanced the diffusion of hydrogen into the metal in the cracked region. The resultant embrittlement and associated fissuring was established to have caused the failure. The hydrogen was diffused out by wrapping the vessel in asbestos and heating followed by cooling as prescribed by ASME code.