Effect of normalising heat treatment on microstructure and properties of nickel alloyed C-Mn weld metals

Abstract

Five different basic manual metal arc welding electrodes, containing varying amounts of nickel (from 0 to 3.5%) were deposited in an all weld metal joint. Mechanical testing and microstructure examination was performed in the as deposited and heat treated conditions. The heat treatment was carried out at three different temperatures (930, 980, and 1030° C) for 20 min. The tensile strength was decreased by the heat treatment, but the magnitude of the decrease varied between the weld metals. The impact properties were also affected by the heat treatment. For impact properties, however, a decrease was found at low testing temperatures, whereas an increase was observed at higher testing temperatures. The decrease in tensile strength after normalisation, compared with the as deposited condition, is due to an increasing grain size and a decreasing dislocation content. The strength achieved by the different weld metals in the normalised condition can be explained by the variation in solid solution hardening resulting from differences in the alloying content.Two factors seemed to be especially important in determining the variations in impact properties between weld metals in the as deposited condition. The nitrogen content of the weld metals decreased the impact toughness, whereas increasing nickel content was associated with improved impact toughness. In the normalised condition, reduced at lower testing temperatures, because cleavage fracture started readily in the resulting coarser grains. Furthermore, traces of segregated bands of microphases probably acted as initiation sites for cleavage cracks. At higher testing temperatures, higher impact toughness was obtained, owing to the lower strength of the weld metals. One of the electrodes showed superior impact toughness values to the other electrodes, in both the as deposited and heat treated conditions. The main reason for the high toughness in the as deposited condition was the ability of this electrode to refine previously deposited beads to a high degree. The reason for the high toughness after normalising is still not certain, but it was noted that this weld metal had a very low oxygen content and also a comparatively low volume fraction of segregated microphases. These factors might be important in achieving the very high impact toughness observed.