二輪車冷鍛排気バルブ用Ｆｅ基超合金の開発

Abstract

Conventional exhaust engine valves of motorcycles made of heat resistant steels of SUH35 or SUH38 have been produced through hot forming, heat treatment and hard-faced weld of Stellite #6 or 12. In order to reduce valve cost, it is necessary to reduce processing cost. One of the promising processes is cold forming and direct aging without solution heat treatment after cold forming and hard-faced weld. However, cold formability of SUH35 and SUH38 are too poor to cold-form valves. To develop new alloys having superior cold-formability, high temperature strength and good wear resistant is a key to realize the process. γ' strengthened iron-based superalloy with good cold formability is a candidate alloy for cold-forming valve material. In this study, to develop a cold formable new γ' strengthened iron-based superalloy for exhaust valves of motorcycle engines, Fe-24Ni-15Cr-Cu-Al-Ti-Nb-C-B alloy system was selected. To enhance cold formability of the alloy, effect of Cu addition was examined. To satisfy high temperature strength and phase stability required for engine valves, optimization of the amount of γ' former elements (Al+Ti+Nb) and the ratio of Ti/Al was carried out in direct aged and over aged conditions after cold-forming. It was found that addition of 2mass%Cu was effective for cold-formability. Moreover, lower amount of Al+Ti+Nb and lower ratio of Ti/Al improved cold-formability. High temperature fatigue strength of the alloys satisfied target strength over 5.4mol% of Al+Ti+Nb and showed the highest at Ti/Al=0.8. Growth of γ' precipitates and η phases were least at Ti/Al=0.8 in over aged condition after cold work. Consequently, NCF2415C (Fe-24Ni-15Cr-1.5Al-2.2Ti-0.5Nb-0.03C-0.003B) showed best combination of cold-formability, high temperature strength and phase stability. NCF2415C was successfully cold-formed to exhaust valves.