2000 Volume 18 Issue 3 Pages 496-506
The effects of both microstructure and oxygen content on the toughness of weld metals of a 950 MPa high strength steel have been investigated. The weld metals were prepared by SMAW (shielded metal arc welding), SAW (submerged arc welding), GMAW (gas metal arc welding) and GTAW (gas tungsten arc welding) processes. The microstructures of the weld metals were characterized with optical microscopy, SEM (scanning electron microscopy) and TEM (transmission electron microscopy) as well as continuous cooling transformation (CCT) diagram. The oxygen content was measured with a non-dispersive infrared absorption analyzer.
The microstructures of all weld metals in as-welded state prepared by SMAW, SAW, GMAW and GTAW processes were martensitic structures, although slight differences in their morphologies were observed. The microstructures of the reheated weld metals in multi-pass weldment depended on both reheating temperatures TP and welding process. When TP≥Ac3, the reheated microstructures of SAW and GMAW weld metals consisted of fine martensitic structures, while that of SMAW was martensite with coarsened laths. The microstructure of the reheated weld metal of GTAW was lath martensite with a large amount of carbide precipitates. When TP≥Ac3, especially when Tp-Ac1, a large amount of precipitation were observed in the reheated weld metals of SAW, GMAW and GTAW, while the precipitation in that of SMAW was only slight. The toughness of the weld metals was closely related to their microstructures and oxygen contents. In particular, superior toughness could be associated with the formation of fine martensitic structures. Furthermore, the toughness of the weld metal was increased with the decrease in oxygen content, when the microstructure consisted primarily of martensite.