The green molding clay sand used in railway steel castings is mainly made of raw sand, bentonite, starch additives and water in a certain proportion. The mixed molding sand is directly molded and poured.
(1) Raw sand
When selecting raw sand, the SiO2 content of raw sand should be determined according to the type of alloy poured; Determine the mud content and other properties of raw sand according to the type of mixed materials; Determine the original sand particle size according to the size of the casting.
The melting point of cast steel is high, and the casting temperature is above 1500 ℃. Therefore, it is required that the fire resistance of sand is high, the density of cast steel is large, and the ladle leakage pouring is mostly used. The pressure and impact force of the mold are large, and the mold is required to have high strength.
Therefore, the SiO2 content should not be less than 94%, and the harmful impurities should be strictly controlled. Due to the high requirements for fire resistance and good air permeability, but the wet mold clay sand is finer than the dry mold ratio, such as 70 / 140 mesh. In actual production, two groups of grain size silica sand are often mixed according to a certain proportion, such as 50 / 100 mesh and 70 / 140 mesh. The casting size is larger, and the original sand particle size tends to be finer. The particle size distribution of raw sand should be concentrated on 3-4 sieve. High density molding wet clay sand, the original sand shape does not have to be very round. The mud content in the raw sand should preferably be less than 0.5%, generally not more than 1%, the water content of the raw sand should be controlled below 1%, and the dust should be removed. New sand is often used in the surface sand to improve the fire resistance of the mold and reduce the moisture in the molding sand.
| Raw sand | SiO2 content /% | Particle size / Mesh | Particle size distribution | Grain shape | Mud content /% | Moisture /% | Impurity dust |
| Silica sand | ≥94 | 50/100-70/140 | 3-4 sieve | Don’t cross the circle | <1 | <1 | Control removal |
Most wet clay sand foundries use natural silica sand. The basic requirements for raw sand of steel castings are shown in the table.
(2) Bentonite
Sodium bentonite has high bentonite value and excellent performance. The molding sand prepared with it has the advantages of strong sand inclusion resistance and good thermal stability. In addition, during the pouring process of liquid metal, the tendency of bentonite to become dead clay due to heating is small, that is, the durability of bentonite is good. Compared with sodium bentonite, calcium bentonite has the advantages of short sand mixing time, high wet compressive strength, good fluidity and easy sand falling after pouring. The selection of sodium based and calcium based bentonite should be determined according to the specific alloy types and process requirements. The price of sodium based bentonite is generally slightly expensive. Internationally, two or more bentonite are often used in a proper proportion to give full play to their respective advantages.
In China, the performance indicators of bentonite in the foundry industry generally include: the content of montmorillonite is more than 70%, the moisture is less than 12%, and more than 95% of the weight passes the 0.075mm standard sieve. The bentonite value, colloidal value, blue absorption and sample strength are stable.
For steel castings, natural or modified sodium bentonite should be used. In view of the lack of natural sodium bentonite in China, modified sodium bentonite or sodium calcium mixed bentonite should be used.
(3) Starch additives
In order to improve the toughness of molding sand, reduce the rebound phenomenon of molding sand and keep the moisture of molding sand in a certain range, a small amount of starch, dextrin and other additives are often added to high-density molding green clay sand.
The appearance of starch is white or gray white granular or powdery material. Starch is divided into corn starch, potato starch and sweet potato starch according to the type of raw materials. Starch is classified according to water solubility and processing process, including α Starch β There are two kinds of starch. β Starch is raw starch, untreated, insoluble or slightly soluble in water; α Starch is made by mixing raw starch and water into suspension, heating, gelatinizing, rapid cooling and grinding. because α Starch is soluble in water, so it is often used as an additive for wet clay sand. α The main functions of starch in green molding clay sand are to maintain the moisture of molding sand, avoid drying, cracking and sand loss, improve the toughness of molding sand and stabilize the strength of molding sand at the corner of steel casting mold.
Dextrin is an intermediate product of starch hydrolysis reaction under the action of acid and heat after adding water. The product formed after the reaction contains dextrin and reducing sugar. The content ratio of dextrin and reducing sugar is also different with the degree of reaction.
According to the determination, the degree of dextrinization of dextrin is generally more than 70% and the reducing sugar is less than 15%. Dextrins are mostly light yellow fine powder materials, and there are also white dextrins, but the adhesion is poor. Dextrin is an organic water-soluble binder with high wet strength and dry strength. It is easy to absorb moisture and agglomerate when exposed, and can be almost completely dissolved in cold water. Dextrin dissolves in water to form a viscous paste like sol. After drying and dehydration, it becomes a slightly elastic gel. Its hardening process is a reversible reaction. Its hygroscopicity is lower than that of pulp residue, but higher than that of syrup.
α There are obvious differences between starch and dextrin. Dextrin can significantly improve the surface strength, hot wet tensile strength and toughness of steel castings, but significantly reduce the wet compressive strength and fluidity; α Although starch has less effect on the surface strength, hot and wet tensile strength and toughness of molding sand than dextrin, it has little effect on the wet compressive strength and fluidity. Molding sand for high-pressure molding is often added with 0.5-1.0% α Starch or dextrin, especially suitable for calcium bentonite, can greatly increase the hot and wet tensile strength and significantly improve the toughness.
For the wet molding clay sand of steel castings, a certain mixing proportion should be selected α Starch and dextrin to improve the properties of molding sand.