Weld heat-affected zone liquation cracking and hot ductility in high-strength ferritic steels

Abstract

In this paper, the authors examine the correlation between hot-ductility test data obtained in simulated weld thermal cycles and weld heat-affected zone (HAZ) liquation cracking in ferritic, low-alloy steels of the En 24, SAE 4130, and ASTM A387B types. The work covered steels containing various levels of carbon, sulphur, and phosphorus whose cracking susceptibility had already been quantitatively assessed. It was shown -that hot-ductility testing under the conditions of thermal simulation reproduced the metallurgical processes occurring in the real weld HAZ and was, therefore, a valid method of investigation. Furthermore, it was demonstrated that the severity of cracking in these steels could be related quantitatively to the zero ductility range (ZDR) measured during cooling, provided that the peak temperature was equivalent to or higher than the nil strength temperature. Since the ZDR and weld-cracking susceptibility were affected in a similar way by the elements carbon, sulphur, and phosphorus, it is suggested that weld-cracking susceptibility can be estimated directly from the composition by means of the ZDR in a hot-ductility test. Metallographic and fractographic examination of the hot-ductility specimens showed that weld HAZ cracking is restricted to a region adjacent to the fusion line where the peak temperature exceeds the nil ductility temperature. This cracking occurs by fracture of liquid inclusion films at grain boundaries during cooling through a discrete zero ductility temperature range.