Damage and failure processes in structural 347H SS materials

The relationship between microstructure and high temperature strength of 347H SS welded joints was investigated. The materials studied correspond to two preheated furnaces of the petrochemical industry, working at 700 C and 45 atm (called furnaces A and B). Furnace A presented failures, hot fissure in the Heated Affected Zone, after 60 h operation, whereas furnace B operated for 70000 h. The microstructure research was made by a non conventional replica technique, which allowed more accuracy in the nature, quantity, morphology and distribution of the different phases. The probable cause of the remarkable different behaviour of the two materials, although both fulfil the A213/TB/347 H, ASTM standard requirements, has been discussed taking into account the chemical composition of the steels, matrix characteristics, nature of the inclusions, carbides, intermetallic particles and N/Mn, Al/N, C/Nb relationships. (orig.)