Thermal Fatigue Cracking in Engine Valve Seat Alloys

Thermal fatigue phenomenon in IC engine valve seat insert (VSI) is discussed. Based upon the observations of VSI parts tested in dynamometer engines for which abuse thermomechanical test conditions are normally applied compared to actual field run engines, a model of thermal fatigue cracking initiation and propagation has been established. Metallography- and fractography-assisted examination on VSI thermal fatigue cracking has verified the model which can be adopted to assess the thermal fatigue cracking sensitivity in VSI alloys using materials properties and engine operational conditions.

Thermal fatigue cracking in VSI is primarily resulted from cyclic thermal exposure on seat surface of VSI together with the impulse force of valve closing impacting motion. In addition, thermal shock due to valve combustion leakage can also initiate a crack at a VSI seat surface. One important condition for thermal fatigue cracking formation in a VSI is existence of sharp temperature gradient in the valve train component during an engine test.

Several modern engine VSI alloys are analyzed in this study to obtain a better understanding of VSI materials propensity to thermal fatigue cracking. An assessment has demonstrated that physical properties of a VSI alloy, such as thermal expansion and conductivity can significantly affect VSI alloy sensitivity to thermal fatigue cracking initiation and propagation. A simple model is used to illustrate the thermal fatigue cracking formation which is verified by x-ray diffraction stress analysis results.