Mechanical Properties and Working of Metals and Alloys by Amit Bhaduri

Author:Amit Bhaduri
Language: eng
Format: epub, pdf
Publisher: Springer Singapore, Singapore

8.14.2 High Temperature

As the temperature is increased above room temperature, the fatigue strength of metals generally decreases. A diagram of plastic strain amplitude , , versus number of cycles to failure, N, at a constant frequency of cycling in air, showing the effect of temperature on fatigue life, is schematically presented in Fig. 8.27. The effect of frequency of cycling on fatigue at a constant high temperature has already been discussed in Sect. 8.13. Figure 8.27 shows that except at very high strain amplitude that corresponds to low number of cycles to failure, fatigue life of a material generally decreases with increasing temperature when frequency of cycling is constant. Mild steel is an exception, where a maximum in the fatigue limit is exhibited in the temperature range of around 230–350 °C (503–623 K). A maximum in the rate of strain ageing in this temperature region is the cause for the existence of a maximum in the tensile strength as well as in the fatigue strength of mild steel. In this temperature range, the pinning of dislocations by solute atmosphere, such as carbon and nitrogen atoms, would be the greatest, whereas at lower temperatures, dislocations would be free of solute atoms and precipitate. At higher temperatures, the increased diffusivity of carbon and nitrogen atoms would lead to a greater mobility of dislocations.

Fig. 8.27Schematic graph of plastic strain amplitude, , against number of cycles to failure, N, at a constant frequency of cycling in air, showing the effect of temperature on fatigue life

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