Fretting fatigue tests at elevated temperature were carried out using shot-peened specimens to improve the high temperature fretting fatigue strength of 12Cr-Mo-W-V steam turbine steel. Shot peening improved fretting fatigue strengths by a factor of 1.8 at an elevated temperature (773 K) as well as at room temperature. Residual compressive stresses first drop rapidly by a factor of three and then level off. Residual compressive stresses of 200 MPa were measured after a 30 000 h exposure at 773 K. Shot peening is thus shown to be effective over long periods at 773 K.

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Fatigue tests and fretting fatigue tests of two steam turbine steels at room temperature and 773 K were carried out. The reduction of fatigue life and strength in the fretting test were significant at 773 K as well as at room temperature. The values of the friction coefficient at 773 K was almost equal to those at room temperature. The geometry of the fretting fatigue crack was flat in the early stage of fatigue life where a significant effect of fretting was observed. With increasing crack length and with a reducing effect of fretting, the fatigue crack shape changed to a semi-circular form. The fretting fatigue lives predicted on the basis of elastic-plastic fracture mechanics analysis, with the frictional force between the fretting pad and the specimen taken into consideration, agreed well with experimental results at both temperatures.

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The fretting fatigue tests under various levels of contact pressure were carried out at both room and elevated temperatures using 12Cr-MoW -V steam turbine steel in order to investigate the effect of contact pressure on fretting fatigue properties such as fatigue life and fatigue strength. Two step tests, where fretting action is removed at certain cycles, were also carried out at elevated temperature to investigate the effects of contact pressure on fretting fatigue behavior and propagation rate of fretting fatigue crack. Although the fretting fatigue strength decreased with increasing contact pressure, it attained a constant value when the contact pressure was higher than 100 MPa. The fretting fatigue crack initiated at the early stage of life and its growth rate was accelerated by fretting action. This acceleration of fatigue crack was significant in the case of the higher contact pressure of 300MPa. The effect of fretting on fatigue life seemed to appear within 30% or 40% of fretting fatigue life regardless the contact pressure level and test temperature. This result was explained by comparing two fatigue crack growth curves with and without fretting.