Chromium ore powder (mainly composed of chromite, containing Cr₂O₃·FeO) has become a key material in precision casting coatings due to its high refractoriness, excellent corrosion resistance and thermochemical stability. It can significantly improve the surface quality of castings and reduce the defect rate. The following is a systematic description of the application principle, process key points, formula design and actual cases:
🔬 1. Core application principle and advantages
Anti-sand sticking and anti-penetration mechanism
When chromium ore powder comes into contact with molten metal at high temperature (>1900℃), the FeO it contains will be preferentially oxidized to form a dense spinel phase (such as FeCr₂O₄), blocking the molten metal from penetrating into the pores of the sand mold, thereby eliminating mechanical sticking14. At the same time, Cr₂O₃ can promote the sintering and peeling of the coating, and the coating will automatically fall off in pieces after pouring, reducing the cleaning cost.
Thermal stability and shrinkage optimization
The high melting point (1890~2100℃) and low thermal expansion coefficient make chromium ore powder dimensionally stable at high temperatures, especially suitable for thick and large parts of castings. Its heat storage coefficient is higher than that of quartz sand, which prolongs the solidification time of molten metal, enhances the shrinkage effect, and reduces shrinkage/shrinkage defects.
Chemical inertness and shielding effect
In nitrogen-containing resin sand (such as furan resin), chromium ore coating can block the diffusion of N and C elements into castings, avoid carbon increase and subcutaneous pores in low-carbon steel castings, and improve corrosion resistance.
⚙️ 2. Application process and technical points
Particle size selection and graded application
Coarse particles (80~280 mesh): as refractory aggregate, enhance the ability to resist molten metal scouring, and are used in hot spots of large castings.
Ultrafine powder (>320 mesh, accounting for 15~20%): fills the pores of the coating, improves the density, and is used for steel castings with high shielding requirements.
Paint carrier adaptation
Water-based paint: suspension rate up to 98% (24h), gas emission ≤16ml/g (1000℃), suitable for scenes requiring low-temperature drying such as lost foam, and the drying temperature needs to be controlled at 40~60℃ to prevent foam deformation.
Alcohol-based paint: suspension rate ≥95% (8h), dries immediately after ignition, suitable for fast-paced production, but the ethanol volatilization rate needs to be controlled to prevent cracking.
Formula synergistic optimization
Composite fillers: combined with diatomaceous earth, vermiculite, etc. (such as patented formula: chromite 65~85% + diatomaceous earth 15~20%), improve air permeability and reduce sintering temperature.
Binder system: composite bonding (such as ethyl cellulose + phenolic resin) is used to take into account both low-temperature strength and high-temperature ceramicization (sodium tripolyphosphate promotes sintering).
Functional additives: Add Fe₂O₃ powder (1~3%) to promote the formation of oxide film and enhance peelability; stearic acid (0.1~0.15%) to improve leveling.
📊 3. Formula design and performance optimization
The following is a typical chrome ore powder coating formula and performance comparison:
Table: Performance comparison of water-based and alcohol-based chrome ore powder coatings
Component/performance Water-based coating Alcohol-based coating Function
Chromium ore powder ratio 50~70% (coarser than 280 mesh) 60~80% (280~325 mesh) Refractory skeleton, anti-penetration
Suspending agent Lithium-based bentonite (3~6%) Organic bentonite (2~4%) Anti-settling, maintain uniformity
Binder Silica sol (2~3%) Phenolic resin (3~5%) Improve coating strength
Suspension rate (24h) ≥98% ≥95% (8h) Storage stability
Gas generation (1000℃) ≤16 ml/g ≤19 ml/g Reduce pore defects
Crack resistance (1200℃) Rapid heating for 2min without cracks Same as left Prevent metal liquid from penetrating into cracks
🧪 IV. Key points of quality control
Suspension stability control: Prevent precipitation through thickeners (such as CMC 0.15~0.5%) and thixotropic agents (such as nano-SiO₂), and the thixotropic rate needs to be >30% to ensure uniformity of coating24.
Sintering peeling adjustment: Add flux (lithium carbonate 0.3~0.5%) to make the coating moderately melt at the pouring temperature to form a glassy and easy-to-peel layer.
Environmental protection and safety: Ventilation and dust removal equipment is required (chromium ore dust is harmful), and wastewater containing Cr needs to be neutralized.
🏭 V. Typical application scenarios and effects
Large steel castings (supports, sprockets)
After using chromium ore powder composite coating, the sand adhesion rate is reduced to <3%, the surface roughness after shot blasting is Ra≤12.5μm, and the handwriting is clear and has no missing meat (compared with uncoated parts Ra≥25μm).
High chromium cast iron (lining plate, hammer head)
Coating with nano FeCr₂O₄ composite powder (15~20%) can increase the surface hardness of castings by nearly 3 times (HV 800→2200) and extend the wear life by 2 times.
Precision alloy steel parts
After application in furan resin sand mold, the subcutaneous porosity is reduced from 8% to 0.5%, and the carbon increase of low-carbon stainless steel is controlled below 0.02%
