1. What is Chromite Flour?

Chromite flour is a finely ground powder derived from chromite ore (a mixed oxide of chromium and iron: FeO·Cr₂O₃). It inherits the ore’s exceptional properties but in a form suitable for use in slurries and coatings.

Key Properties:

• Very High Refractoriness: >1900°C.

• High Thermal Conductivity: Promotes rapid chilling of the casting.

• Excellent Thermal & Chemical Stability: Highly resistant to basic slags and metallic oxides (e.g., FeO, MnO).

• Low Thermal Expansion: Minimal linear expansion prevents shell cracking.

• High Bulk Density (~4.5 g/cm³): Contributes to dense, impermeable coatings.

• Neutral Behavior: Chemically inert with most molten alloys.

2. Primary Applications in Foundry

A. Investment Casting (Primary Application)

In investment casting, chromite flour is used as a functional refractory filler in slurries, typically for the primary (face) coat or a secondary coat.

• Purpose:

  • Prevent Chemical Burn-on/Reaction: This is its most critical role. For reactive superalloys (especially those high in Al, Ti – e.g., Ni-based, Co-based) and for austenitic manganese steels, chromite provides a chemically inert barrier that prevents a severe interfacial reaction between the molten metal and the silica-based binder system, which can cause a “glassy” and tenacious sand fusion defect.

  • Provide Localized Chilling: Its high thermal conductivity helps directional solidification at hot spots, reducing shrinkage porosity and refining grain structure.

  • Improve Surface Finish: Produces a dense, smooth shell surface that replicates the wax pattern well.

• How It’s Used:

  • Slurry Formulation: Chromite flour is blended with a binder (colloidal silica or ethyl silicate) and wetting/suspension agents to create the primary slurry. It may fully replace or be blended with zircon flour (e.g., 30-70% mix) depending on the alloy and cost considerations.

  • Application: The ceramic shell is built by repeatedly dipping the wax cluster into this slurry, stuccoing with a coarse refractory grain (often fused silica or alumino-silicate for backup layers), and drying.

B. Sand Casting (Large Steel & Non-Ferrous Castings)

Used as the primary refractory aggregate in mold/core washes (paints).

• Purpose:

  • Prevent Metal Penetration & Burn-on: Its high density and chemical inertness create an excellent barrier against molten metal, especially for heavy-section steel castings and copper-based alloys where high metallostatic pressure is an issue.

  • Improve Casting Peel: Facilitates easy removal of sand from the casting surface, minimizing cleaning costs.

  • Achieve Smoother As-Cast Finish.

• How It’s Used:

  • Wash/Paint Formulation: Chromite flour is milled with a carrier (water or alcohol), binders (e.g., resins, clays), and suspension agents to form a stable paint.

  • Application: The wash is applied by spraying, brushing, or dipping onto the surface of resin-bonded or clay-bonded sand molds/cores. It is then dried (water-based) or flashed-off (alcohol-based).

C. Specialty Applications

• Chill Paste/Washes: Applied selectively to mold areas opposite hot spots to act as an external chill.

• Additive to Self-Setting Sands: Added in small percentages to acid-cured (furan) or alkaline (phenolic) resin sands when casting highly alloyed steels to improve resistance to burn-on.

3. Comparison with Zircon Flour

Selection Rule of Thumb:

• Use Zircon Flour for most stainless steels, alloy steels, and applications where supreme surface finish is the top priority.

• Use Chromite Flour when casting reactive superalloys (high Al, Ti) or austenitic manganese steel, where preventing chemical reaction is the critical challenge.