What size of chromite sand is used for stainless steel casting?

The most commonly used size for stainless steel casting is AFS 45-55 GFN (Grain Fineness Number).

Here’s a detailed breakdown of what this means and why it’s used:

The Standard: AFS 45-55 GFN

This is a medium-fine sand grade. The AFS Grain Fineness Number is an index of the average size of the sand grains. A higher number means finer sand.

AFS 45-55offers the best balance for most stainless steel applications.
Why this range?
- Fine Enoughto produce a smooth mold surface, which translates directly to a superior casting finish with minimal veining or penetration.
- Coarse Enoughto maintain good permeability, allowing gases from the mold and binder system to escape during the pour. Poor permeability can lead to gas defects (pinholes, blows) in the casting.

Grain Size Distribution: The Key to Packing

The actual sand is not a single-sized grain but a mixture of sizes within a specific range. A typical grading for AFS 50 chromite sand might be:

Primary Screen:Retained on 70 mesh (212 microns)
Secondary Screens:Retained on 100 mesh (150 microns) and 140 mesh (106 microns)
Pan:A small amount in the pan (fines)

This distribution ensures the grains pack together tightly, minimizing the spaces between them and creating a dense, strong mold surface that resists metal penetration.

Why Grain Size Matters So Much

Surface Finish:This is the primary reason for using chromite. Finer sand (higher AFS number) packs more tightly, creating a smoother mold cavity surface that is replicated onto the casting. This reduces cleaning and machining costs.
Permeability:Coarser sand (lower AFS number) has higher permeability, which is good for gas escape but bad for surface finish. The AFS 45-55 range provides adequate permeability while still delivering an excellent finish for stainless steel.
Mold Strength:A well-graded sand with a variety of interlocking grains creates a stronger mold, which is less likely to erode when the high-density molten stainless steel flows through it.

How the Choice is Made: Application-Specific Selection

The exact size can be adjusted based on the specific casting:

Application / Factor	Recommended AFS GFN	Rationale
General Stainless Steel Castings(Valves, pumps, impellers)	45 – 55	The standard workhorse range. Ideal balance of finish and permeability.
Large, Heavy-Section Castings	40 – 45(Slightly coarser)	Higher permeability is prioritized to ensure gases from the larger volume of binder can escape from the deep mold.
Thin-Wall, Precision Castingsrequiring very smooth surfaces	55 – 65(Finer)	Surface finish is the top priority. The molds are often smaller with better gas venting options.
Casting Process:Furan No-Bake	45 – 55	Standard range.
Casting Process:Silicate-CO₂	May go slightly coarser	This process can generate more gas, so a slight adjustment for permeability might be needed.

Practical Takeaway

When ordering chromite sand for stainless steel, you would typically specify:

Type:South African Chromite(as discussed in the previous answer).
Size/Grade:AFS 45-55 GFNor the equivalent commercial designation from your supplier (e.g., “Medium Grade”).

The foundry will then perform a sand system audit to confirm the grain distribution and adjust their binder ratios accordingly to achieve the perfect mold for a clean, high-integrity stainless steel casting.

What size of chromite sand is used for stainless steel casting?