오스템퍼드 구상흑연주철의 기계적 특성에 미치는 주조방법 및 오스템퍼링 열처리의 영향

Ductile cast irons were cast in two methods; permanent mold casting and sand caseins, with same chemical compositions. In case of permanent mold casting, the microstructure was consisted of the matrix with pearlite, ferrite and cementie and fine spheroidal graphite, which nodule count was about 1000 ea/mm^2. However, in case of sand casting, the microstructure was consisted of the matrix with pearlite and ferrite and coarsen spheroidal graphite, which nodule count was about 200 ea/mm^2. As an austempering temperature increased from 340 to 400℃, the microstructure was changed from lower ausferrite, which was consisted of fine acicular ferrite and retained austenite with high carbon content, to upper ausferrite, which was consisted of coarsen ferrite-like ferrite and retained austenite with high carbon content. As austempering temperature was increased, the tensile strength and hardness decreased, but the elongation increased. At the same heat treatment conditions, the specimen fabricated by permanent mold casting showed higher tensile strength, hardness and finer ferrite than that by sand casttng.