In order to improve the overall performance of cast iron parts, white-mouth annealing is often used, ductile iron annealing to improve toughness, normalizing and quenching to increase the strength of ductile iron.
1. Elimination of white-mouth annealing the surface or thin wall of ordinary gray cast iron or ductile iron castings appear white in the casting process due to the excessive cooling speed, and the iron castings cannot be cut. In order to eliminate the whiteness and reduce the hardness, such cast iron parts are often reheated to above the eutectoid temperature (usually 880~900℃), and kept for 1~2h (if the cast iron has high Si content, the time can be short) for annealing, and the cementite is decomposed It is graphite, and then the iron castings are slowly cooled to 400℃-500℃ and air cooled. At a temperature of 700-780°C, that is, near the eutectoid temperature, the cooling rate should not be too slow, so that too much cementite can be converted into graphite, which reduces the strength of cast iron.
2. Annealing of ductile iron with improved toughness. In the casting process, this ordinary gray cast iron has a large tendency to white mouth and large internal stress. It is difficult to obtain pure ferrite or pearlite matrix for iron castings. In order to improve iron castings For high ductility or toughness, iron castings are often reheated to 900-950°C and kept for sufficient time for high-temperature annealing, and then cooled to 600°C and then released from the furnace. During the process, the cementite in the matrix decomposes graphite, and graphite is precipitated from austenite. These graphites are gathered around the original spherical graphite, and the matrix is ??completely converted to ferrite. If the as-cast structure is composed of (ferrite + pearlite) matrix and spheroidal graphite, in order to improve the toughness, it is only necessary to decompose the cementite in the pearlite into ferrite and spheroidal graphite, and then reheat the cast iron After the temperature reaches the eutectoid temperature of 700-760℃, the furnace is cooled to 600℃, and then it becomes cold.