Study of the weldability of a high silicon bainitic steel

High silicon bainitic steels are recently developed materials that combine excellent mechanical properties of tensile strength (<1.5 GPa) and ductility (~15%) with equally important functional characteristics such as abrasion resistance. Consequently, these steels are evaluated as a possible alternative to existing proposals in numerous industrial applications including railways. However, their weldability appears critical due to the chemical composition and the formation of brittle hardening structures, with possible hot and cold cracking defects respectively. These materials therefore require an in depth study of their weldability in order to promote (evaluate) their adoption on an industrial scale. In this research activity, the weldability of a medium carbon steel (0.4 wt.%) and high silicon content (3.2 wt.%) was studied using UNI EN ISO 4063: 135 (GMAW), 111 (SMAW) and 141 (GTAW). The consumables were chosen on the basis of the mechanical properties of the base material and therefore consumables for QT steels were taken into consideration. The joints, made using the three techniques mentioned above, were subjected to a modified heat treatment known as “austempering”. The microstructural evolution and mechanical properties of the joints before and after heat treatment were characterized with optical and electronic microscopy, hardness measurements and transverse tensile tests. The residual stresses after welding were evaluated by X-ray diffraction. The results obtained highlighted good weldability and compatibility between the base material and the consumables used in all processes, not highlighting problems of hot and cold cracking. Looking ahead, these results stimulate research and development of new consumables with yield strength greater than 1 GPa, currently not covered by standards.