The principle determined by the heat treatment process of gray cast iron is based on the original structure of the gray cast iron piece and the final microstructure, and is subjected to graphitization annealing or normalizing to reduce the hardness, improve the machinability and improve the plasticity and toughness of the casting [1]. The hardness of the casting is too high because the cooling rate of the casting during the crystallization process is large or the content of carbide elements is too high, and the silicon content is too low. If the cooling rate or composition is not properly selected, free cementite is often present in the as-cast microstructure of gray cast iron. Free cementite is a brittle hard phase that greatly reduces the mechanical properties of the casting and also affects the machinability. In order to eliminate free cementite, graphitization annealing or normalizing is required to completely graphitize the free cementite. According to the data, the critical temperature of gray cast iron is generally 677 ° C ~ 843 ° C; free cementite in the as-cast structure is allowed, the heating temperature is selected at 850 ° C ~ 900 ° C. To obtain a pearlite matrix and to make the casting have higher hardness and wear resistance, it can be normalized. When the silicon content is high, the heating temperature should be increased accordingly due to the elevated ferrite-austenite transformation temperature. To have a large amount of ferrite, it can be annealed. The holding time depends on the heating temperature, the composition of the casting, the structure, the weight, and the thickness (generally determined according to the thickness of the casting 1h/25mm). The cooling rate is different during normalizing, and the amount of precipitated ferrite (pearlite) is also different.
The larger the cooling rate, the less the amount of ferrite precipitated and the more pearlite. Therefore, the hardness of the casting can be adjusted by a method of controlling the cooling rate (air cooling, air cooling, mist cooling, or raising the temperature of the outlet).
The heat treatment process determines the metal casting cylinder castings. Because of the fast cooling rate, the white mouth tends to be large. In order to eliminate the local white mouth structure, the phosphorus eutectic morphology is improved to eliminate the internal stress caused by quenching, and the mechanical properties and metallographic structure are met. To produce qualified castings, and to stably obtain D-type graphite castings that meet the requirements, to stabilize material properties and improve cutting performance, and to perform normalizing treatment. Based on the structural characteristics of the cast iron castings of metal type, the normalizing treatment of the heat treatment temperature of 830 ° C to 950 ° C and the time of 0.5 h to 3 h is selected according to the size of the casting. In order to eliminate the internal stress generated by normalizing, tempering is performed at 400 ° C to 500 ° C.
Relationship between pearlite volume, hardness and carbon equivalent. The cast iron parts cast in metal form have high carbon and silicon content. Compared with ordinary gray cast iron and malleable cast iron, in the white mouth structure, a small amount of fine graphite precipitates, and the cementite is very small. . Even if the cementite which is finally solidified and precipitated, due to the graphitization expansion of the casting during the solidification process, the cementite is subjected to the expansion force, and a large amount of crystal defects are generated inside the cementite, so the graphitization time can be shortened compared with the general cast iron.
The hardness of the metal castings decreases with the increase of carbon and silicon (carbon equivalent) in the molten iron composition, and the as-cast hardness decreases. Due to the solid solution strengthening effect of silicon, the ferrite is solid solution strengthened. When the carbon content is constant, the hardness of the ferrite increases as the silicon content increases, and the as-cast hardness of the casting increases. As shown, the relationship between the amount of as-cast pearlite and carbon equivalent, as the carbon equivalent increases, the amount of pearlite decreases, but it is not obvious. Indicates the relationship between as-cast hardness and carbon equivalent. As can be seen from the above, the overall trend shows that the carbon equivalent increases and the hardness of the casting decreases, but the fluctuation range is large. Because the amount of graphite increases with the increase of carbon equivalent, the effect of inoculation is poor, and the structure is coarse; in addition to the influence of components (carbon equivalent), it is also affected by other factors such as the cooling rate during solidification and the cooling rate after opening. . The relationship between the as-cast hardness of the cylinder and the amount of as-cast pearlite is shown. It can be seen that the hardness increases with the increase of the amount of pearlite in the as-cast microstructure, but the hardness value varies greatly when the amount of pearlite is 60%. HRB84 ~ HRB98), it can be inferred that the hardness is also related to graphite size, distribution, the number and distribution of free cementite, the type and distribution of phosphorus eutectic, and the degree of refinement of the structure. Since D-type graphite is formed under large supercooling conditions, Mn, P, and S in the casting are segregated at the grain boundary. Due to the difference in incubation and cooling rate, the final solidified part forms phosphorus eutectic. In the case of the same amount of pearlite, the hardness of the ternary phosphorus eutectic is higher, showing the effect of cooling rate and inoculation effect.
Factors affecting the heat treatment process According to the phase diagram, as the heating temperature increases, the equilibrium carbon dissolved in the austenite increases, and the higher the tapping temperature, the more carbon is dissolved in the austenite.
When the D-type graphite cast iron is solidified, the molten iron has a large degree of subcooling, the dendrites are developed, and the crystal grains are fine. To dissolve the graphite between the dendrites, it takes a short time for the carbon to reach the equilibrium carbon amount between the austenite graphites. In addition, at the time of cooling, since the dendrites are developed, the diffusion distance of carbon from austenite to graphite is short, and ferrite is easily obtained, so that the amount of pearlite is difficult to control. The as-cast hardness of D-type graphite cast iron increases as the amount of pearlite increases. Test results and discussion of the relationship between the amount of as-cast pearlite and carbon equivalent. The relationship between the as-cast hardness and the carbon equivalent. The relationship between the amount of as-cast pearlite and the as-cast hardness is such that the amount of pearlite is high and the hardness is high. The number of precipitated graphite decreases, and the amount of matrix increases. Under the same carbon equivalent, the cooling rate is large, and the number of precipitated dendrites increases, and the number of graphite decreases. Under the rapid cooling condition after opening, a small amount of carbon in austenite is desolventized. On the eutectic graphite, ferrite is formed around the graphite, and most of the austenite is transformed from hypereutectoid to pseudoeutectoid. The pearlite content is low and the hardness is low because the carbon equivalent is high or the cooling rate is small during solidification, the number of dendrites is reduced, and the number of graphite is increased. Under the same unpacking conditions, the pearlite content is small and the ferrite content is large.
The castings with the same difference in pearlite content and hardness are different in the number of dendrites and the amount of graphite, the number of dendrites is large, the amount of graphite is low and the hardness is low, and the opposite is low; on the other hand, due to the cooling rate during solidification and the influence of gestation Free cementite exists in castings with high hardness; in the case of small cooling rate during solidification, the number of dendrites decreases, the amount of graphite increases, and there is a tendency to change toward A-type; the same amount of pearlite, pearlite layer The spacing is different, the amount of cementite is different, the spacing of the plies is small, the amount of cementite is high, the hardness is high, the spacing of the plies is large, the amount of cementite is small, and the hardness is low. In the case where the primary crystal structure is substantially the same, the casting speed is different after the casting is unpacked and the stacking method is different or the ambient temperature is different.
The cooling rate is fast, the pearlite content is high, and the pearlite content is low. If the pearlite content is different, the hardness is different and the hardness increases as the pearlite content increases. When austenitizing is heated, the change of the original structure of the cast iron is different. The rate of graphitization is mainly determined by the distribution and quantity of carbides under the same conditions of silicon. The reaction occurs according to the original structure: pearlite + graphite → Austenite + graphite (1) ferrite + graphite → austenite + graphite (2) data indicate that the reaction (1) is faster than the reaction (2), that is, the carbide in the pearlite exceeds the total temperature at the heating temperature. After the temperature zone is analyzed, it can be completely dissolved, the pearlite content is high, the carbon in the austenite is easy to reach the equilibrium carbon amount, the pearlite content is low, and the carbon in the austenite diffuses into the dendrites through the dissolution of graphite to reach the equilibrium carbon. the amount. When the hardness of D-type graphite cast iron is high, the dendrite is developed, the pearlite content is much, the graphite is less, and the austenitization is mainly carried out by the formula (1). The pearlite dissolves to form austenite and reaches a certain balance with graphite. The higher the titanization temperature, the higher the equilibrium carbon amount. When cooling, due to the development of dendrites, the diffusion distance of carbon is long, and the faster the cooling rate, the more the amount of pearlite obtained, the higher the hardness. Therefore, the amount of pearlite is large and the hardness is high, and the performance is not satisfactory. Therefore, low austenitizing temperature and small cooling rate are adopted, and for uniformity, long-term heat preservation should be adopted. When the hardness of D-type graphite cast iron is low, the amount of pearlite is small, the amount of ferrite and graphite is large, the number of dendrites is small, and the number of pseudo-eutectic crystals formed is small. When cooling, the carbon in austenite is de-dissolved onto graphite, so In the case of the austenitization heating, the formation of austenitization is mainly carried out in the manner of the formula (2). During the heating and holding process, the carbon in the austenite is lower than the equilibrium carbon, and the carbon is dissolved and diffused into the austenite to reach equilibrium. When cooling, due to the large amount of graphite, carbon easily diffuses to form graphite, and the amount of ferrite and pearlite decreases.
Therefore, the amount of pearlite and the hardness of the casting are ensured by increasing the amount of carbon in the austenite and increasing the cooling rate. At the same time, in order not to grow the grain, a shorter holding time can be used.
In short, the heating and cooling of D-type graphite cast iron is mainly affected by the amount of pearlite in the matrix and the amount of cementite in the pearlite. The diffusion distance of carbon is affected by the dendrite size, and the carbon diffusion rate is affected by the holding temperature. It is also affected by the nucleation of pearlite. Therefore, normalizing uses different heating temperatures and holding times to ensure the body structure and mechanical properties are met.
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