Development of cold-formable iron-based superalloy for exhaust valves of motorcycles

摘要

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 toreduce 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 newalloys 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 thisstudy, to develop a cold formable newγ' 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. Tosatisfy 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 wasfound 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+Nband showed the highest at Ti/Al=0.8. Growth ofγ' precipitates andη phases were least at Ti/Al=0.8 in over aged condition after cold work. Consequently, NCF2415C (Fe-24Ni-15Cr- l.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.