A repair of a refractory lined product gas transfer line at a hydrogen plant required welding of aged 21Cr 33Ni 1Nb castings. The repair weld procedure that was developed was the result of past experiences of industry experts as well as trial and error in the field. Subsequent to the field work, laboratory investigations were performed in an attempt to understand metallurgical issues associated with weld cracking in this alloy. Key words: liquation cracking, niobium carbides, high temperature HAZ cracking, reformer outlet, high temperature castings