On the Fatigue Strength of Nodular Cast Iron and Ni-Zr Cast Steel at Low Temperature

Abstract

Regarding the mechanical properties of metals at low temperature the investigations have hitherto been mostly confined to the impact and tensile properties, and there have been but few works^1)∼3)on the fatigue. In this paper, the fatigue strength of the nodular cast iron and Ni-Zr cast steel for the unnotched and notched specimens at +20°C, -16°C and -78°C, were measured with the rotating cantilever Wöher-type fatigue machine at the revolution speed of 4500rpm.
The experimental results are summarized as follows:-
(1) The tensile strength of nodular cast iron decreases with lowering testing temperature from room temperature so that 61kg/mm² at room temperature becomes 58.5kg/mm² at-78°C.
(2) The fatigue endurance limit of nodular cast iron and Ni-Zr cast steel increased at lower testing temperature than that in room temperature.
In the case of unnotched specimen of nodular cast iron, the value of 22kg/mm² at room temperature increased to 25kg/mm² at -78°C. Similarly, the value of Ni-Zr cast steel rose to 29.5kg/mm² from 25kg/mm².
(3) The fatigue limit of 10kg/mm² at room temperature for the notched Ni-Zr cast steel specimen increased to 13kg/mm² at -78°C. For the notched nodular cast iron, 14kg/mm² rose to 18kg/mm².
(4) The value of fatigue notch factor β_k of the Ni-Zr cast steel decreased from about 2.45 to 2.25 as the test temperature was lowered from +20°C to -78°C. Similarly, the value of nodular cast iron decreased from 1.6 to 1.4.
(5) The value of notch sensitivity η_k decreased as the test temperature was lowered. The value of η_k of Ni-Zr cast steel keeps twice as high as that of nodular cast iron, that is, for the Ni-Zr cast steel the range from 1.5 to 1.2 at the test temperature +20°C and -78°C respectively, corresponded to 0.7 to 0.4 of the nodular cast iron.