Effect of Dynamic Strain Aging on Isothermal (473 K) Low Cycle Fatigue of Ferritic Ductile Cast Iron

Abstract

The ferritic ductile iron (FCD400) is widely used as industrial material. As regards to its high temperature application, fatigue at elevated temperature, and advanced mechanical properties has been investigated and clarified. In low cycle fatigue (LCF), the S-N curve is presented by the relation of plastic strain to the number of cycles as fatigue life, which can be predicted by the Manson-Coffin model. In this model, the ductile index is appointed as a material constant of 0.5 and the ductile coefficient is related directly to the so-called plastic deformation capacity of the material. Namely, the low cycle fatigue life shall be dominated by the elongation rate. Concerned with it, one of authors reported that the elongation-to-fracture is 20% at room temperature, but reduces to about 9% at 473 K. This temperature is just within the range of dynamic strain ageing (DSA), which is the phenomenon of fluctuating stress due to mobile atoms in solid solution. So, in this study, strain controlled high temperature LCF-tests were carried out on ferritic cast iron at 473 K in air and the result is compared with those performed at room temperature, aiming to clarify the effect of elongation or DSA on the fatigue life. Particularly, this paper mentions about DSA effect to cyclic stress and cyclic plastic strain that causes contradiction of Manson–Coffin’s model in ‘‘the plastic strain range versus number of cycles’’ and ‘‘the stress range versus number of cycles’’ between room temperature (293 K) and 473 K. The fatigue life time at 473 K is 175 cycles lower than that at 293 K for all stress levels. However, DSA phenomena did not occur at high strain in 473 K cyclic fatigue test although it cannot be observed in tensile tests at 473 K. Based on the Manson-Coffin rule, the parameters (Cp ¼ 0:09 and 421, n ¼ 0:65 and 0.89 at T ¼ 293 and 473 K, respectively) for the fatigue life time prediction have been determined. [doi:10.2320/matertrans.MRA2008482]