Microstructural features and erosion wear resistance of friction stir surface hardened spheroidal graphite cast iron

A ferritic spheroidal graphite cast iron (also named as SG cast iron, ductile cast iron, ductile iron) was treated with friction stir process (FSP) to harden the surface layer owing to a unique microstructure into which the ferritic structure transforms after high temperature deformation and subsequent direct cooling. When the friction stirred surface experiences thermomechanical cycle during FSP (here named friction stir surface hardening, FSSH), a non-traditional bainite structure can be obtained through subsequent cooling process. The bainite structure primarily consists of iron carbide (Fe ₃C), acicular ferrite and martensite with retained austenite aggregates. It is evident that the FSSH structure caused by deformation at austenite temperature has resulted in a significant increase in the microhardness of about 1000HVyielding a primarily martensitic accompanying bainitic phase transformation. The experimental results also show that the process has resulted in significant improvement in erosion resistance at low angle impingement than that of ferritic specimens. In addition, the maximum erosion rate of ferritic specimens occurs at 2025° of impact while the peak of the FSSH specimen shifts to higher angle resulting from the formation of continuously cooled martensitic and bainitic structure.