In the casting alloy family, the development of various casting alloys depends on the development of their advantages and the suppression of their disadvantages. Compared with cast steel and non-ferrous alloys, cast iron has better casting properties. Therefore, the utilization rate of molten iron is high and the product has a wide application range.
In the competition for materials, it is very important to improve the strength of cast iron, increase toughness, and eliminate inherent defects to reduce the weight of castings, extend service life, and save materials. In order to ensure the reliability and stability of cast iron performance, good molten iron quality is first necessary. Therefore, be sure to do a good job of pre-furnace treatment!
Desulfurization of molten iron can reduce the amount of nodularizing agent and the amount of sulfide slag inclusions in castings. Almost all ductile iron production in developed countries adopts the desulfurization process, which requires S ≤ 0.01% after desulfurization. Under China’s conditions, the target is S ≤ 0.02%, and Xichai and Changchai can reach S ≤ 0.015%.
The Na2CO3 flushing method at the bottom of the bag is simple, but the desulfurization rate is low and the effect is unstable. During the treatment, the smoke pollutes the environment and is irritating to the throat. In large-scale ductile iron workshops, it is advisable to place the desulfurized on the liquid surface and use mechanical stirring, shaking bags, or pneumatic stirring for desulfurization. Among them, the pneumatic method is relatively simple and consumes less power, so it is the most popular. The pneumatic desulfurization device can be installed upstream of the front furnace for continuous desulfurization, or it can be used for intermittent desulfurization in a single package in front of the furnace. The pneumatic source is mostly N2.
Desulfurizers are divided into two categories: CaC2 series and CaO. CaC2 has a good desulfurization effect and can reduce sulfur from 0.04~0.06% to less than 0.01%. The scum is granular and easy to remove. However, it is expensive, has a high melting point, and hinders transportation and storage, so it is limited to near-point supply. The CaO system is more commonly used. No matter which series of desulfurizers it is, it should be moisture-resistant and explosion-proof. A composite desulfurizer made of active CaO that has been surface film-formed and added with additives is not subject to moisture and has high desulfurization efficiency. It can be desulfurized by the pneumatic method and has a desulfurization rate of 60 to 90%. For factories with small output, low-melting point composite desulfurizer should be used for single-pack flushing desulfurization, with a desulfurization rate of 30 to 50%. At this time, the desulfurization slag is in a molten state.
After pneumatic desulfurization, the molten iron enters the induction furnace to raise the temperature.
2. Slag removal
The slag removal process can be carried out in a bag or in an induction furnace. Sprinkle the slag-removing agent (slag-gathering agent ) on the liquid surface to form a molten layer immediately, which plays the role of covering and slag-gathering. The slag removal agent expands when heated, creating many small holes in the molten layer, which plays a good role in heat preservation.
The slag removal agent is divided into three levels: low, medium, and high. Products made from raw ore that have been simply crushed and screened are low-grade and are produced in all provinces. It is no longer favored by foundries. Mid-range slag removers are selective in raw ore and add processes such as water washing. They can meet the basic requirements of castings and are accepted by most factories. Imported high-grade slag remover, selected from high-quality mineral sources, and the processing procedures are different from ordinary ones. This product spreads quickly when sprinkled on the liquid surface, covering the entire liquid surface. It has excellent slag removal ability with a small dosage. It does not stick to the furnace lining and ladle. The entire molten slag layer can be lifted up with a stick. The slag removal is convenient and thorough. In the past three years, In recent years, it has begun to attract the attention of the domestic industry.
The ballization treatment is still mainly based on the in-bag punching method. The selection of nodularizing agent depends on the furnace, tapping temperature, desulfurization or not, and the type of ductile iron. Generally speaking, cupola furnace molten iron should use Mg7~9%, RE3~7% spheroidizing agent, and induction furnace molten iron should use low Mg (5~6%), low RE (1.5~2.5%) spheroidizing agent. Some factories still use the Mg8RE3 spheroidizing agent. JB/T9228-1999 standard stipulates that the composition range deviation of Mg and RE is ±1%. Some factory standards have been reduced to ±0.5%. There are also clear regulations for Ca, Al, etc. In the quality of the spheroidizing agent, attention should be paid to the MgO content, the degree of segregation of the components, and the concentration of the particle size. Any spheroidizing agent with uneven particle size, lots of powder, and dark color should not be used.
In principle, as long as the chemical composition and incubation process of the molten iron is well understood, the above-mentioned nodularizing agent can be used to produce cast ductile iron. However, there are also factories that use nodularizing agents for cast ductile iron. Generally, the as-cast ferrite spheroidizing agent contains Ba and Bi, and the as-cast pearlite spheroidizing agent contains Ba and Sb. However, for important crankshaft castings above QT700-2, an Sb-containing spheroidizing agent is not recommended. agent, at this time the matrix should be controlled by adjusting the content of Cu, Mo, etc. It must be pointed out that regardless of whether a cast spheroidizing agent is used, incubation is always an important link that cannot be ignored.
Heavy rare earth magnesium-containing spheroidizing agents are used for thick-walled ductile iron parts. In order to prevent spheroidization recession and graphite distortion, the Si content should also be reduced, and technological means should be adopted to increase the number of graphite spheres and refine the diameter of the graphite spheres.
The cover bag treatment method greatly reduces smoke and light pollution during treatment. It is a simple process that saves spheroidizing agents ensures spheroidizing quality, and is worthy of promotion. Inspired by the molten steel wire-feeding technology, wire-feeding spheroidization has been successfully used in enterprises such as Xinxing Casting Pipe Company after trials. Wire-feeding spheroidization is a process in which the wire-feeding machine continuously feeds the alloy core-clad wire to the bottom of the ladle for spheroidization. This process can accurately control the amount of residual magnesium and has the characteristics of high spheroidization quality, low slag volume, low-temperature drop, and low pollution. The outer skin of the cored wire is 0.3mm thick cold-rolled steel skin. The core material generally contains Mg25~30% and has composite components such as Ca, Ba, (RE), etc. The cost of the wire feeding method is 20~40% lower than that of the punching method, and it is very attractive to mass-produced pipe casting factories, automobile foundries, etc. At present, there are many domestic units that can provide complete sets of wire feeding machines and cored wires. Cored wires are divided into multiple varieties such as spheroidization, creeping, inoculation, desulfurization, carburization, and alloying.
In-mold spheroidization is to set up a reaction chamber in the runner, and the spheroidization reaction occurs while the molten iron flows through. The in-mold spheroidizing working conditions are good, the oxidation loss is small, and the Mg absorption rate is as high as 80%, which overcomes the spheroidization recession and incubation recession and improves the performance of ductile iron. In-mold spheroidization has strict requirements on the sulfur content of molten iron, and the process yield of castings is low. It was used in China in the 1970s but is no longer in production.
4. Give birth to
Incubation is an important step in improving the quality of cast iron. After more than ten years of effort, my country’s inoculants have basically achieved serialization and commercialization. At present, domestic inoculants are mainly silicon-based and carbon-silicon-based.
Silicon-based inoculants have low melting points and are suitable for various molten irons with high carbon and low carbon, high sulfur, and low sulfur, and are the most widely used. The special ferrosilicon has a strong inoculation effect and a small dosage, causing small fluctuations in the chemical composition of the molten iron and stable cast iron quality. The first choice for large coverage is FeSi-Ba-Ca, which significantly increases the number of eutectic clusters, inhibits D and E-type graphite in gray iron and promotes the formation of A-type graphite, improves the spheroidization level, and Ba, especially Ba, and Ca, coexist time, good recession resistance. FeSi-Sr is an inoculant suitable for thin-walled engine parts. It has excellent anti-whitening ability without significantly increasing the number of eutectic clusters, so it can eliminate the disadvantages of leakage caused by shrinkage and porosity. In the production of ductile iron, the inoculation effect of Sr is weakened due to the presence of Ce. Zr-containing ferrosilicon has a similar effect to Ba-containing ferrosilicon, but is weaker. However, Zr has a degassing function and micro-alloying effect, which stabilizes the mechanical properties of cast iron. Ferrosilicon containing Zr has a high melting point, so Zr-Mn often coexists to lower its melting point. High-Ca inoculants, such as CaSi, have the functions of desulfurization and deoxidation, whitening prevention, and increasing graphite nuclei. They were mostly used in the 1950s and 1960s. Later, due to the improvement of the quality of coke and molten iron, CaSi was Rarely used alone, occasionally used in combination with FeSi or FeSiRE. The carbon (graphite) in the carbon-silicon inoculant gives this inoculant excellent ability to prevent whitening. It is more suitable for producing gray iron parts with many corners and thin wall thicknesses from high-temperature molten iron. Hydraulic thin-walled parts can also be used as an option when leakage prevention is required. In the production of ductile iron, carbon-silicon inoculant should not be used.
large-dose inoculation in the tap trough by various forms of post-inoculation is a general trend in the development of inoculation methods. Post-inoculation should pay attention to the particle size and particle size uniformity of the inoculant. Especially when the bag mouth is inoculated with the flow and the pouring stream is sprayed, the particle size is too large, and poor melting will produce defects such as hard spots and intergranular inclusions in the casting. When using high-efficiency inoculant, avoid excessive inoculation to prevent the occurrence of shrinkage cavities, pores, and graphite segregation.
There are many methods for in-mold inoculation. Currently, most of them use block inoculant placed at the bottom of the sprue for overflow inoculation. This method captures the nucleation peak of incubation and can theoretically obtain the best incubation. Effect.
Commonly used alloying elements in cast iron include Si, Mn, Cu, Cr, Mo, Ni, Sn, Sb, V, Ti, P, etc. Among them, Si, Mn, Cr, Mo, and P are added in the form of iron alloy, Cu, Ni, Sb, and Sn are added in the form of metal, and V and Ti are added in the form of an iron alloy or VTi pig iron. The timing of adding is behind the furnace (cupola furnace), inside the furnace (induction furnace), or in front of the furnace, depending on their melting point, oxidability, and density. In order to facilitate the melting and absorption of high melting point iron alloys, in addition to controlling the particle size, it is an ideal measure to use a heating alloy agent or wire feeding method.