A new surface hardening technology for cast irons was developed by applying the Friction Stir Processing (FSP) in which the matrix can be controlled in the solid state. Flake graphite cast iron (FC300) and spheroidal graphite cast iron (FCD700) were used to investigate the validity of this new method. As a result, it has been clarified that a Vickers hardness of about 700 HV is obtained for both the flake graphite cast iron and the spheroidal graphite cast iron, and that the hardness depends on the size and the density of the martensite phase. Moreover, the hardness can be controlled down to about 500 HV by changing the stirring degree. For previous hardening methods, post surface-processing was required because a large distortion was generated. However, with this new method, many advantages, such as a higher surface hardness and unnecessary post surface-processing, could be obtained.
The authors clarified that various cast irons can be hardened by friction stir processing (FSP) in a previous study. However, the high applied tool load was indispensable for obtaining the hardened layer. In this study, the effect of the tool geometry on the applied tool load was investigated for forming the hardened layer by the FSP with a lower applied tool load. As a result, the applied tool load could be reduced by 36% using a concave tool.
A ferrite-based spheroidal graphite cast iron (FCD450) is difficult to harden using a conventional surface hardening method, because the carbon content in the matrix is very low. In order to solve this problem, the friction stir processing (FSP) was used in this study as a new hardening method for cast irons. The authors have clarified in a previous study that the pearlite-based cast iron, such as FC300 and FCD700, can be hardened using the friction stir processing and that there are several advantages, such as a higher hardness and no required post surface machining. In this study, it was clarified that a Vickers hardness of about 700 HV is obtained due to the formation of fine martensite even in the ferrite-based spheroidal graphite cast irons, although the optimal process range is much narrower than that of the pearlite-based cast iron due to the requirement of both the heat input for diffusion of the carbon into the matrix and the high cooling rate for the martensitic transformation.
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