How to prevent sand sticking defects in the production of heavy castings with Chromite Foundry sand

The production of thick-walled large and medium-sized steel castings often produces heavy skin and sand sticking defects, sometimes referred to as “sintering, glaze layer or elephant skin” defects. It is a mixture of foundry grade chromite sand, enamel slag and metal, which is magnetic. The highest incidence occurs in areas exposed to heat radiation for a long time during pouring. After heat treatment and shot blasting, the surface of the castings can sometimes be cleaned, and sometimes adhered to the castings, which can only be removed by chiseling, grinding or arc gouging, which seriously increases the workload of cleaning and finishing.

The main chemical composition of chromite sand is Cr2O3, which is a natural Fe-Cr oxide mineral with a little 　Mg and Al. It usually exists in igneous rocks and is mainly produced in South Africa. In terms of quality, South African chrome ore has high purity and the best quality. The foundry grade chromite sand grains have a very regular cubic structure with a complex interior, consisting of eight basic structures. Al and Mg can replace Cr and Fe atoms in the structure respectively, so they are not regarded as impurities. Silicate gangue such as serpentine (magnesium silicate, aluminum silicate) is an impurity component. Ideal grade chromite sand containing Cr2O3>46%, Fe content <30%, is black, specific gravity 4.0～4.8g/cm3, bulk density 　2.5～3.0, four-sieve AFS fineness of 50-90. Its heat transfer ability is better than other raw sand, its resistance to infiltration is excellent, and its linear thermal expansion is lower than silica sand, olivine sand, and close to zircon sand.

In the production process of foundry grade chromite sand, in addition to washing to remove impurities in the raw ore, the main processing procedure is magnetic separation, the purpose of which is to remove iron and silicon in the raw sand as much as possible.

In actual work, even if high-quality chromite sand used, wrinkles and sticky sand defects still occur, which need to be improved by adjusting the process.

Analyzing the causes of wrinkled skin defects, we will find the following:

Shelling and easy to remove, and can leave the wrinkles on the surface of smooth castings: the weight of the same volume of heavy leather and compact chrome ore sand is roughly equal. It shows that there is no infiltration. The metallization of the wrinkles is due to the increase in the volume of the sand on the surface of the chrome ore in the reducing atmosphere when the temperature of the sand rises above 1250 ℃, and the iron droplets adhere to each other. When cooling, the resin in the sand has been burned out, and air enters the interlayer of the interface to cause iron oxidation, bonding the entire surface of the sand into a layer of wrinkle defects.

It is difficult to remove the wrinkle defect firmly adhered to the surface of the casting: the heavy leather sample with the same volume is obviously heavier than the molding sand, indicating that there is serious infiltration. It is calculated that the voids in the molding sand are about 25%, but the weighing shows that it is higher than 25%. It is obvious that the sand has been eroded and the voids have increased. According to analysis, this is because the iron droplets reduced from the chrome ore are subsequently oxidized, and combine with the silicate impurities in the chrome ore under an oxidizing atmosphere to form low-melting iron olivine, which then absorbs SiO2, softens and erodes the chromium Ore sand particles increase the voids and are easily infiltrated by molten steel. The surface layer of the mold may also form a sandwich-like interlayer: the humid air in the mold forms an oxidizing atmosphere; the resin binder on the surface of the mold contains water, and the water vapor migrates to the inside and condenses during pouring. Oxidizing atmosphere; the surface layer is a reducing atmosphere due to incomplete combustion of the resin. What is interesting is that the zircon powder paint itself contains 34% SiO2, and there is a small amount of free SiO2 impurities. Under oxidizing atmosphere conditions, iron oxide will attack the SiO2 in the paint and form enamel slag, causing SiO2. The etched part is in contact with the liquid phase of the steel, and there can be 34% of the contact area, so that the heavy skin adheres to the surface of the casting, and the uneroded ZrO2 still plays the role of the parting interface. After the heavy skin layer is removed, the white metal color of the adhesion points can explain this phenomenon, and the iron-rich heavy skin layer will cause serious corrosion.

For the above phenomena, the comprehensive measures to effectively prevent are:

1. The turbidity of chrome ore should be <150ppm. The content of SiO2 should be <1.5%. Zircon powder coating should contain Fe less than 0.5%, and free SiO2 less than 2.0%.
2. For castings with a modulus of more than 5cm, the pouring temperature should be controlled within the range of 30-50°C above the liquidus. The pouring time is calculated by the formula: GZ (pouring time) = 5.60×G (piece weight, kg) 0.286. If the piece weight is 5000kg, the pouring time is 37.8 seconds. Pouring flow rate: sand gate and inner gate ≤1.0m/s; refractory brick pipe gate ≤3.5m/s.
3. The thickness of the surface sand should be 50-300mm. Should prevent mixing with the back sand of the SiO2 system. The amount of resin bonding is reduced, which is similar to 30-50% for silica sand to meet the requirements.
4. The oxygen content of molten steel should be kept <10ppm.
5. Water-based zircon powder coating should be dried thoroughly. When applying double-layer coatings, the second layer should be applied after the first layer is properly dried. Use a dryer to dry thoroughly. When conditions permit, use a hot air dryer to continue baking after closing the box, until the ladle is in place and then remove it before pouring, and further reduce the content of the admixture in the adhesive to avoid mold Condensation of water vapor occurs.

After the above comprehensive measures are taken, heavy-skin defects are rarely produced, and the advantages of using chromite foundry sand for large steel castings can also be demonstrated.