Fatigue strength of ferritic ductile cast iron hardened by super rapid induction heating and quenching

To clarify the effects of Super Rapid Induction Heating and Quenching (SRIHQ) on fatigue properties of Ferrite Ductile Cast Iron (FDI), rotational bending fatigue tests were carried out on specimens treated with four types of heating cycle. Results showed that; (i) the SRIHQ process generated a thin dark gray area around the graphite. This dark area was composed of a martensite structure (ringed martensite). (ii) The ringed martensite generated a compressive residual stress field at the surface hardened layer. Two types of compressive residual stress generative mechanisms were observed. One was a microscopic residual stress generative process due to the formation of ringed martensite and the other was a macroscopic residual stress generative process due to the expansion of the surface hardened layer. (iii) The fatigue strength of SRIHQ treated FDI specimen was higher than that of the untreated one. This was because the compressive residual stress field generated by the ringed martensite suppressed initiation and propagation of fatigue cracks.