Characteristics of microstructure in ultrahigh carbon steel produced during friction stir welding

Ultrahigh carbon steels containing 1 to 2wt% C have very poor fusion-weldability, but exhibit high strength-ductility balance, good wear resistance and superplasticity at high temperatures, when a microduplex structure consisting of a fine ferrite grain structure with a uniform distribution of spheroidized cementite is formed. Thus, ultrahigh carbon steels with the microduplex structure show promise for use in construction. In the present study, two kinds of initial microstructure, a fully pearlitic structure and a microduplex structure, were produced in an ultrahigh carbon steel with 1 wt% C by different thermal treatments, this steel then being subjected to friction stir welding (FSW) at different rotational speeds using a polycrystalline cubic boron nitride tool. The feasibility of friction stir welding for ultrahigh carbon steel and the effect of welding parameters and initial microstructure on the residual microstructure characteristics of the weld were systematically examined. FSW successfully yielded defect-free welds in the ultrahigh carbon steel at all welding parameters employed. All welds exhibited very high hardness in the weld center due to the martensitic transformation. The difference in characteristics of the martensitic structure among the welds was negligible, but the austenite substructure was affected by the rotational speed. Microstructural development of various regions in the welds was reasonably accounted for by the friction stir welding process, the thermal stability of the initial microstructure and the solid-state transformation during cooling cycle of FSW.