엔진 밸브 융착부의 잔류 응력 최적화 공법에 대한 연구

Many gaseous fuel (LPG and CNG) engines are being developed due to economic efficiency compared with gasoline engines. Wear-resistant material development of engine valves and valve seat insert (VSI) is important for the gaseous engine development. The use of gaseous fuels often result in significant increases valve seat insert (VSI) wear and valve seat wear due to the lack of the lubricity of the fuel. The hardened materials are adapted for valve seat insert (VSI) and welded valve seat material. The heavy wear and thermal/mechanical impact can make cracks in welded valve in the last of engine endurance test. The brittleness of hardening material and residual stress made by welding are also reasons of valve cracks. Developments of valve welding material and valve seat insert material (VSI), welding quality and heat treatment process after welding are required for increased gaseous engine life. In this study, the measurement method of residual hoop stress was developed for welded valve head. The optimized control factors in post weld heat treatment are defined without any negative side effect using DFSS process. The selected control factors have to be more robust factors for noise. After finding the effects of control factors on optimized heat treatment using DFSS process, Negative side effects are investigated. Finally the improved fatigue characteristics of welded valve are validated with real engine dynamometer endurance test.