The application of South African chromite sand in steel casting is primarily due to its high melting point, excellent thermal stability, and resistance to metal penetration. The following is an analysis of its specific applications and advantages:
1. As a molding material
Refractory: South African chromite sand has a melting point of approximately 1900°C, far higher than the steel casting temperature (typically 1500-1600°C). This effectively withstands high temperatures in molten steel and reduces the risk of sintering.
Low Thermal Expansion: Its low coefficient of expansion at high temperatures reduces cracking or deformation in castings caused by thermal stress, making it particularly suitable for large steel castings.
Metal Penetration Resistance: Its dense surface prevents molten steel from penetrating the sand mold, reducing surface sand adhesion defects and improving surface finish.
Applications: Commonly used for face sand (the surface layer that contacts the molten steel) in steel castings, especially for high-alloy steel and stainless steel castings that require high surface quality.
2. As a coating or additive
Chromium ore powder coating: Chromium ore sand is ground into a powder and mixed with water or alcohol to create a refractory coating. This coating is then sprayed onto the surface of sand molds or cores to enhance corrosion resistance.
Mixed use: Mixed with materials such as silica sand to improve overall refractory performance (for example, silica sand is low-cost but lacks refractory properties; mixing can balance performance and cost).
3. Applications in investment casting
Mold shell reinforcement: In precision investment casting, chromium ore sand can be used to form a mold shell reinforcement layer, improving high-temperature strength and reducing deformation.
4. Special process requirements
Thick and large-section castings: For thick-walled steel castings, chromium ore sand has a high heat storage coefficient, which promotes directional solidification and reduces shrinkage defects.
Active alloys such as high-manganese steel: Chromium ore sand is chemically inert, reducing reactions with reactive elements like manganese and preventing oxide inclusions on the casting surface.
Advantages of South African chromite sand
High chromium iron content (Cr₂O₃ ≥ 45%): Provides excellent refractory and slag resistance.
Low impurities (e.g., low SiO₂ and Fe₂O₃): Reduces chemical reactions with molten steel and reduces the risk of porosity.
Rich resources: South Africa is a major global producer of chromium ore, with a stable supply. However, attention should be paid to the impact of environmental protection policies on exports.
Precautions:
High cost: Typically used locally (e.g., face sand) to control costs.
Dust control: chromite sand contains trace amounts of trivalent chromium, requiring dust control measures during processing (although low in toxicity, long-term inhalation is harmful).
Recycling: Recyclable sand can be reused through regeneration, reducing waste sand emissions.
Typical Process Example:
Molding: Face sand uses 100% chromium ore sand, and back sand uses silica sand or reclaimed sand.
Coating: Spray chromite powder alcohol coating, dry, and then cast.
Cleaning: After the casting cools, the chromite sand is easily peeled off, reducing cleaning time.
The rational use of South African chromite sand can significantly improve the yield, surface quality and dimensional accuracy of steel castings, especially in the field of high-end steel casting.
