Chromite powder (FeCr₂O₄) is a widely used refractory material in foundry coatings, particularly in investment casting (lost-wax casting) and sand casting, due to its excellent thermal stability, resistance to metal penetration, and compatibility with molten alloys. Below are its key applications, benefits, and technical considerations in foundry coatings.

1. Key Roles of Chromite Powder in Foundry Coatings

(1) Primary Refractory Filler

• High-Temperature Resistance (melting point ~2180°C): Maintains structural integrity during metal pouring, even for high-melting-point alloys (e.g., steel, stainless steel, nickel-based alloys).

• Low Thermal Expansion: Reduces cracking risk in the coating, preventing metal penetration and sand burning defects.

• Chemical Inertness: Minimizes reactions with molten metals (especially Fe, Cr, and Ni alloys), reducing surface defects like slag inclusions or gas porosity.

(2) Coating Performance Enhancer

• Improved Suspension Stability: Chromite’s particle size distribution (typically 200-400 mesh) helps maintain uniform slurry dispersion.

• Controlled Permeability: Balances gas escape during casting while blocking metal penetration.

• Erosion Resistance: Withstands the mechanical impact of molten metal flow.

2. Applications in Foundry Coatings

(1) Investment Casting (Lost-Wax)

• Face Coat (Primary Layer)

  • Used for direct contact with molten metal. Often blended with zircon flour (ZrSiO₄) for ultra-high-temperature alloys.

  • Typical Formula: Chromite powder (30–50%) + silica sol/ethyl silicate binder + suspending agents (e.g., bentonite).

• Backup Coat (Secondary Layer)

  • Cost-effective alternative to zircon; mixed with alumina or mullite for strength.

(2) Sand Casting (Mold/ Core Coatings)

• Applied as a spray or brush-on coating to sand molds/cores to improve surface finish.

• Formula Example: Chromite powder + refractory clay + water-based/alcoolic binder (e.g., sodium silicate).

(3) Special Alloy Castings

• Stainless Steel/High-Chromium Alloys: Chromite’s Fe-Cr composition reduces interfacial reactions.

• Titanium Alloys: Requires high-purity chromite (low SiO₂, CaO) to avoid contamination.

3. Advantages Over Alternatives