Welding temperature fields

Abstract

The origin of welding residual stresses and welding distortion is the concentrated heat input, both locally and in terms of time, by which a fusion zone is produced at the welding spot (fusion welding). With the combined action of pressure producing local plastic deformations, heating to just below the melting temperature is also sufficient (pressure welding). Only in exceptional cases is welding performed solely under local pressure (cold welding). The high heat concentration is necessary because metallic materials rapidly diffuse the heat. The temperature fields during welding are consequently extremely inhomogeneous and transient. The basic temperature of the component in the most unfavourable case is −40 °C (severe frost); the local maximum temperature in the weld pool is at the evaporation temperature of the metal (approx. 3000 °C in the case of steel). In this temperature range, base metal and filler metal fuse, metallurgical processes proceed in the weld pool, regions of the metal solidify and recrystallize, and microstructural transformations take place during heating and cooling. The temperature field, therefore, determines the welding residual stresses not only directly through the thermal strains, but also indirectly through the transformation strains which accompany the changes in state and microstructure (Fig. 3). The temperature field should be assessed in both respects. In addition, it is of interest in connection with questions of material and process engineering.