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Understanding the wear mechanisms of valve seat inserts (VSIs) is an important aspect in the developments of new alloys for VSI applications. The microhardness of worn VSI is much higher than its original hardness and amount of work-hardening seems to relate to testing conditions and work-hardening coefficient of the material. Worn surfaces of common iron, nickel, and cobalt base alloy VSIs are examined under a scanning electron microscope (SEM). The appearances of worn surfaces show several distinctive characteristics depending on valve seat and VSI materials as well as engine testing conditions. In many intake and dry fuel exhaust applications, worn surfaces exhibit pitting type failure that is associated with crack formation and propagation underneath the surface. In many diesel fuel exhaust applications, oxidation wear mechanism is observed. The wear mechanisms of common VSI materials are also discussed.

Wear testing methods are the key factor in the development of new materials and surface treatment processes for valve seat insert applications. A new wear tester was developed to simulate the wear process of valve seat/insert based on the analysis of valve seat insert wear mechanisms. Normal contact and sliding motions were generated by two motors to synthesize the complex actions of the valve on the insert in an internal combustion engine. Initial wear test at 260°C (500°F) indicates there is a good agreement with the field engine test results of intake applications. Because of the short testing time and simple geometry shape of the specimens, it is possible to conduct massive wear tests for new valve seat insert alloy developments. This new tester also provides a useful tool to the study of wear mechanisms under complex mechanical actions.