Application of Chromite Sand in Casting Production

Ferrochrome ore belongs to the category of special sand in molding materials. Its main mineral composition is FeO. Cr2O3, with a density of 4.0 ～ 4.8g/cm3, a Mohs hardness of 5.5 ～ 6, and a refractoriness greater than 1900 ℃. Ferrochrome ore has a good effect of resisting alkaline slag, and does not react chemically with iron oxide, etc. As the thermal conductivity of chromite sand is several times higher than that of silica sand, and solid phase sintering occurs during molten metal pouring, it is beneficial to prevent the penetration of molten metal. According to these advantages of chromite ore sand, its application in the production of various castings can solve the casting quality problems of many products. Our company has made many successful applications in many products, such as railway vehicle bolster and side frame, fixed screen grate bar of sintering machine, ball mill liner, electric shovel push gear, etc.

The bolster and side frame of the railway vehicle are the main parts of the running part of the railway vehicle, as well as the important force bearing parts during the operation of the vehicle. They have large outline dimensions, complex structure, and strict technical requirements. During the trial production, micro cracks were found at the important force bearing parts (some corners of the casting) through magnetic particle inspection, and shrinkage defects were found at the intersection of the wall and the wall at some parts through anatomical inspection. Through analysis, it is believed that these parts, compared with other parts, belong to the “hot spot” parts formed by the heat collection of molten steel. The cooling speed is different from that of the adjacent parts, and the solidification is late. When the internal stress caused by the blocked cooling solidification shrinkage is greater than the strength limit of the metal material, the casting will produce microcrack defects. When these parts cannot get the supplement of molten steel before solidification, they are prone to produce shrinkage defects. In order to eliminate these defects, the casting technicians of our company stipulated to place some sodium silicate ferrochrome ore in these parts during the process design, molding and core making. Since the thermal conductivity of ferrochrome ore is several times higher than that of silica sand, the solidification speed of this part can be balanced with that of adjacent parts, and the microcracks and shrinkage defects generated in these parts can be eliminated.

The fixed screen grate is a vulnerable spare part on the sintering machine, and its working condition is to withstand the impact wear of sinter at high temperature. Fixed screen grate bars belong to plate type structural castings. Long, deep and narrow through bar holes are evenly distributed on the castings. When the fixed screen grate bars are produced by traditional sand casting methods, the through bar holes can be formed by sand cores. However, the operation process of this process scheme is too complex, and the bar holes on the castings are easy to be blocked by sand, which makes it extremely difficult to clean up, resulting in a high scrap rate of castings. The casting technicians of our company decided to use the lost foam casting technology for production. During the process design, it was specified that the strip holes on the foam mold for fixing the screen grate bars should be filled with chromite raw sand when the box was vibrating. As the solid phase sintering of chromite ore itself occurs in the process of molten metal pouring, it is beneficial to prevent the penetration of molten metal, thus preventing the strip holes from being blocked by the sticky sand. By adopting EPC and taking advantage of chromite ore sand, our company has successfully produced fixed screen grate bars, which have smooth and flat hole walls and accurate dimensions. After being installed and used on the sintering machine, the effect is good and can meet the needs of sintering production.

The lining plate of the ball mill is the main part of the ball mill used for beneficiation production in metallurgical mines. It is made of low alloy steel. Its structure belongs to the plate casting with raised wave peaks. Two mounting bolt holes are evenly distributed on the lining plate. The hole diameter above the bolt hole is large, and the hole diameter below the bolt hole is small. Because the casting wall around the bolt hole is very thick and the molten steel solidifies slowly after pouring, making the area around the bolt hole in a high temperature state for a long time. The bolt hole sand core is made of sodium silicate sand. After pouring, the hole is often blocked by sticking sand, which is very difficult to clean up. However, after using sodium silicate chromite sand core, it can take advantage of its solid phase sintering in the process of high-temperature molten metal pouring, and can prevent the penetration of molten metal, The surface roughness and dimensional accuracy of the bolt holes of the cast lining plate meet the specified technical requirements. The installation effect on the ball mill is good, and the problem of difficult bolt hole cleaning is successfully solved.

The push gear of the electric shovel requires that the blank teeth be cast directly. The casting wall around the part between the teeth is thick. After pouring, it is surrounded by high temperature molten metal. In addition to the thermal effect of its upper riser, sodium silicate sand mold is used for casting. The root of the casting teeth is heavily bonded with sand, and the shape and size of the casting teeth cannot be guaranteed after arc gouging cleaning. In this regard, the casting technicians of our company decided to use the lost foam casting technology for production. After painting the white mold of the pressed gear and drying it, the white mold was ground with glass chromite sand between the casting teeth, and then hardened with CO2 blowing. Then the white mold with sodium silicate chromite sand was boxed. After pouring, the chromite sand fell off, the surface of the casting teeth was smooth and clean, and the shape and dimensional accuracy met the specified requirements.

Although chromite ore sand has obvious effect in eliminating micro cracks, sand adhesion and shrinkage defects of castings, improper use of chromite ore sand will also cause problems. The harmful impurity in ferrochrome ore is carbonate, which decomposes CO2 when it contacts with high temperature molten metal, resulting in porosity on the casting surface. Therefore, the chromite ore should be calcined at 900~950 ℃ to decompose the carbonate before being prepared and used, and then processed and broken to a certain size. For chromite ores that have been stored for a long time and have excessive loss on ignition, they should also be calcined at high temperature before use.

In order to ensure the quality requirements of chromite raw sand, it is required to strictly inspect the incoming chromite sand according to the regulations. First of all, the qualified technical certificates of chromite sand shall be checked when it enters the plant, and it is prohibited to enter the plant if it is unqualified or without technical certificates; Check that the appearance of chromite ore entering the plant should be kept clean, and no impurities should be mixed in; Finally, the physical and chemical indexes shall be sampled for inspection. The inspection results shall meet the following technical conditions: original sand grain size 30/50 mesh, Cr2O3 ≥ 45%, SiO2 ≤ 5%, CaO ≤ 1%, ignition loss ≤ 1%, and fire resistance 1600-1800 ℃.

In order to ensure the use effect of sodium silicate chromite ore, except that the chromite raw sand meets the specified technical conditions, the sodium silicate used must meet the following technical requirements: modulus 2.20~2.40, Baume degree 48~52 ° Be, and water insoluble matter<0.8%. Performance requirements of mixed molding sand: air permeability ≥ 250; Wet strength: 0.05-0.09MPa; Moisture ≤ 5%; Dry tensile strength is 1.2~3.5MPa.

In order to achieve the specified performance index of sodium silicate ferrochrome ore, the following technological procedures for preparation of ferrochrome ore shall be strictly implemented.

Before sanding, check whether the running mechanism of the sander is normal. The gap between the grinding wheel and the chassis should be 10~15mm. Pay attention to regular observation and adjustment; Special attention shall be paid to the water content control of the sodium silicate chromite sand mixed by the first grinding after the grinding machine is washed. The maximum water content of the grinding shall not exceed the specified 0.5%; The operator of sodium silicate chromite sand preparation must always clean the sand on the grinding wheel and keep the sand mixer clean to ensure that the sodium silicate chromite sand is evenly mixed; The mixing ratio requirements are: 100% raw sand, 2.5% ～ 3.5% sodium silicate; Mixing time: raw sand+sodium silicate mixed for 2-3.5min; The physical properties of sodium silicate chromite sand prepared each time shall be inspected, and the casting technicians shall master the inspection results so as to find problems and deal with them in a timely manner.