Fitting curves and impact toughness transition temperature of quenched and tempered steel welds

Welded products of quenched and tempered (QT) structural steels are used from ambient to moderately low design/service temperatures. Therefore, besides base metal, a weld also must possess required minimal design impact toughness (KV) at temperatures lower than transition temperature (TT), where the transition from ductile to brittle fracture occurs. A common way to determine transition temperature is by use of appropriate fitting curves, in accordance with specified standardised criterion. From the point of welding procedure evaluation, it is important to analyse welds for its impact toughness and transition temperature, particularly for three main zones: weld metal, heat-affected zone (HAZ) and base metal. This paper covers welds of two QT steel grades, 690 and 890, which are interesting regarding their yield strength and characteristic weld zones. Basic details of gas metal arc welding (GMAW, process used in this experiment) are provided, while temperatures for impact toughness tests of weld zones were varied from +20 °C down to -60 °C. Based on acquired experimental results of impact toughness, fitting curves were developed by use of Oldfield model, e.g. hyperbolic tangent function. Acquired transition temperatures (TT) from fitting curves show mostly allowable values for all three weld zones. As expected, lower strength grade 690 possess higher impact toughness, in comparison to higher strength grade 890. The standardized criteria of minimal absorbed energy of 30 J (KV) and 50 % of shear fracture (SF) show different transition temperatures (TT-30J and TT-50%SF), while general dependence of impact toughness to shear fracture (KV vs. SF) shows a reasonable trend. Finally, used GMAW procedures may be considered as acceptable, since for both steel grades (690 and 890) all three weld zones show better TT-30J values than minimal required by standard (TT-30J=-40 °C) for QT structural steels.