Heat treatment analysis and solution of ductile cast iron

In addition to the annealing of malleable iron ductile iron to decompose the carbonizing body into flocculent graphite, the purpose of heat treatment of cast iron lies in two aspects: one is to change the matrix organization, improve the performance of cast iron, the other is to eliminate the casting stress. It is worth noting that the heat treatment of the casting can not change the original graphite shape and distribution of the casting, that is, the original graphite heat treatment of sheet or spherical is still sheet or spherical, at the same time, its size will not change, the distribution will not change.
1. Ductile cast iron aging
In the process of casting, the cooling speed of cast iron is different from the surface to the inside, forming the internal stress of casting. If it is not eliminated, it will make the parts deform or even crack in the process of cutting and use. To release the stress, artificial aging and natural aging are often used. Heat the casting to about 500~560℃ for a certain time, and then remove the casting with the furnace cold. This aging is artificial aging. The natural aging is to store the cast iron outdoors for 6~18 months to release the stress naturally, which can release the stress part, but it is not used due to the long time and low efficiency.
2. Improve the overall performance of the cast iron parts for the purpose of heat treatment
In order to improve the overall performance of cast iron, often eliminate white mouth annealing, improve the toughness of ductile iron annealing, improve the strength of ductile iron ignition, quenching, etc.
(1). Improve the strength of ductile iron by positive ignition
The purpose of ductile iron ignition is to convert matrix tissue into fine pearlite tissue. The process is to reheat the ductile iron casting whose matrix is ferrite and pearliite to a temperature of 850-900℃, the original ferrite and pearlizite are converted into austenite, and some spherical graphite is dissolved in austenite. After insulation, the air-cooled austenite is transformed into fine beads, so the strength of the casting is increased.
(2). Improve the toughness of the ductile iron annealing
In the casting process, the ordinary gray cast iron is more white, and it is also more internal stress. The cast iron is difficult to get pure iron body or pearletic matrix. In order to improve the ductility or toughness of the iron cast, the cast iron is often reheated to 900-950℃ and insulated for enough time for high temperature annealing, and then the furnace is cooled to 600℃ for cooling. In the process, the carburite in the matrix decomposes the graphite, and the graphite emerges from the austenite. These graphite gathers around the original spherical graphite, and the matrix is completely converted into ferrite.
If the cast tissue is composed of (ferrite + pearlite) matrix and spherical graphite, in order to improve the toughness, we only need to decompose the carburized body in the pearlite and convert it to ferrite and spherical graphite. For this reason, the iron casting is reheated to the coprecipitation temperature of 700-760℃ and the furnace is cooled to 600℃ and cooled.
(3). Isothermal quenching treatment of ductile cast iron
The purpose of isothermal quenching treatment of ductile iron is to convert the matrix tissue of iron cast into strong lower bainite tissue, the strength limit can exceed 1100MPa, and the impact toughness AK 32J. The treatment process is: after heating the ductile iron casting to 830-870℃ temperature insulation substrate austenite, put 280-350℃ molten salt for insulation, so that the austenite part is transformed into lower beskite, and the original spherical graphite remains unchanged. Obtain a high-strength ductile cast iron.
The above heat treatment of cast iron shows that the heat treatment of cast iron can only change the matrix organization, not the shape of graphite