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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.