Abstract
Automotive engine valves have been applied of surface-hardening treatments such as tufftriding or chromium plating to prevent sticking and to ensure wear-resistance in contact with valve guides during their reciprocating motion at high speed. A new gas-nitriding surface treatment of engine valves has lately been developed with the feature that it is less costly and non-polluting. In the new nitriding process, the valve surface is activated with NF3 gas as a pre-processing. Since this treatment is available for Ni-based heat-resisting alloys while conventional nitriding treatment is hardly applicable. This paper describes the development of the new low-temperature gas nitriding process from such aspects as selection of the optimum condition, analysis of nitriding layers in heat-resisting steels/alloys and hardware diagnosis on mechanical properties. Then it verifies that the new processing is comparable or superior to conventional tufftriding and chromium plating. As a result, this nitriding process has come to be adopted to the exhaust valves of diesels and both intake and exhaust valves of gasoline engines installed on volume production vehicles. Furthermore, the authors foresee a prominent future, where the newly developed nitriding process will have a wider application in the production of automotive engine valves.