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

Abstract

From the point of welding procedure evaluation, it is important to analyse welds for their impact toughness and transition temperature for three main zones: weld metal, heat-affected zone and base metal. This paper covers butt welds of two QT steel grades, 690 and 890, with thicknesses of 30 mm and 20 mm, respectively. They are interesting regarding yield strength and characteristic weld zones. Basic details of GMAW process used in experiment are provided, with temperatures for impact toughness tests of weld zones 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, i.e. hyperbolic tangent function. Acquired transition temperatures (T_T) 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 (T_T-30J and T_T-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 T_T-30J values than minimal required by standard (T_T-30J=-40 °C) for QT structural steels.