The Microstructure Changes of Continuous Cast iron Rods According to the Heat-Treatment Conditions

열처리 조건에 따른 주철연속주조봉의 미세조직 변화

The microstructure changes of the matrix and the graphites were observed by optical microscope and the average hardness number was investigated according to the heat-treatment conditions of the cast iron rods by the horizontal continuous casting process in 35 mm diameter. The three kinds of heat-treatments were introduced. The first treatment was performed at 900, 950, and $1000^{\circ}C$ for 2 hours and the second treatment was conducted during 5, 10, and 15 hours at $1000^{\circ}C$ respectively. The third treatments were the two-cyclic heat-treatment and the three-cyclic heat-treatment at $1000^{\circ}C$ during 2 hours. The matrix microstructure of the specimens to be treated at various temperatures for 2 hours was the ferrite to be transformed from the pearlite The hardness number of the center of the samples according to the heat treatment time at $1000^{\circ}C$ was higher than that of the surface area because of the martensite formation in the center. Also, in the cyclic heat-treatments, the hardness number of the two-cycle treatment specimens increased because of the martensite formation in the center.