Systematic Solutions for Gas Porosity in 304 Stainless Steel Castings Using Sodium Silicate Sand
To eliminate gas porosity, the approach must be systematic, addressing both gas generation and gas evacuation, while optimizing molten steel quality.
2.1 Primary and Mandatory Measure: Reduce Sodium Silicate Content
1. Reduce sodium silicate addition from 8% to 5.0–5.8%
This is the most effective and fundamental solution to reduce gas generation at the source.
The minimum addition level should be determined through trials while still meeting stripping and handling strength requirements.
2. Use modified sodium silicate or additives
Adopt low-modulus sodium silicate (M = 2.1–2.4), which has lower gas evolution.
Add 0.5–1.0% Fe₂O₃ powder during sand mixing.
Iron oxide reacts with sodium silicate, reducing gas generation and significantly improving collapsibility.
3. Ensure dry base sand
Use clean base sand with a moisture content below 0.3%.
2.2 Strengthen Venting: "Guiding Gas Out Is Better Than Blocking It"
1. Mold venting
Install dense φ8–12 mm vent holes at the highest points and dead zones of the cope, extending to the outside.
Cut vent grooves along the parting line.
2. Core venting (Critical)
All sand cores must include effective vent channels.
Use vent ropes, drilled vent holes, or embedded wax wires (which form channels after burnout).
Ensure core prints are not sealed and gas can escape freely.
Large cores should be designed as hollow, or loosely filled with permeable materials such as coke or slag, with external vent outlets.
3. Vent risers
Set open risers at the highest or last-solidifying areas to serve both feeding and venting functions.
2.3 Optimize Pouring Practice: Stable Filling, Minimal Air Entrainment
1. Use an open gating system
Design the sprue, runner, and ingates with progressively increasing cross-sectional areas to promote smooth flow and reduce turbulence.
2. Control pouring temperature and speed
Pouring temperature should not be too low.
Slightly higher temperatures (e.g. 1540–1560 °C) improve fluidity and facilitate bubble flotation.
Pouring speed should be stable, continuous, and relatively fast.
Avoid interruptions. A "fast-first, slow-later" approach helps expel cavity gas early.
3. Ignition-assisted venting
At the start of pouring, ignite vent holes or open risers to enhance gas evacuation.
2.4 Strict Control of Coating and Drying
Use low-gas, fast-drying alcohol-based zircon coatings.
Thoroughly dry coatings using flame drying or oven baking until completely moisture-free.
Before closing the mold, blow out loose sand with compressed air and check for coating flaking or sand fall.
2.5 Refine Molten Steel to Reduce Internal Gas
Dry all charge materials-no damp scrap or alloys allowed.
Strengthen deoxidation, combining diffusion and precipitation deoxidation.
Avoid using aluminum alone for final deoxidation; prefer Si–Ca alloys to reduce Al₂O₃ inclusions acting as gas nucleation sites.
Argon protection and stirring during tapping or ladle treatment helps remove hydrogen and inclusions.
2.6 Process Design and Management
Design slag-collecting and venting risers at the end of the gating system or mold flow turns.
Minimize time between mold closing and pouring; ideally less than 4 hours to prevent moisture pickup.
Record and analyze porosity location, morphology (smooth vs. oxidized), size, and correlate with pouring temperature, time, and sand properties.
3. Emergency Troubleshooting and Temporary Measures
If immediate production is required:
Check core venting
Submerge a core in water and blow air through one end; uniform bubbling confirms clear vent paths.
Re-dry molds and cores thoroughly.
Add vent holes at the highest points and blind core areas.
Increase pouring temperature and speed temporarily and observe porosity reduction.
Final Summary
For 304 stainless steel castings weighing 135 kg produced with 8% sodium silicate sand, gas porosity is a system-level problem caused by excessive gas generation combined with difficult gas evacuation.
Fundamental corrective actions:
Immediately reduce sodium silicate addition to ~5.5%
Completely redesign and improve core venting systems
Supporting measures:
Stable and fast pouring
Fully dried low-gas coatings
Enhanced molten steel deoxidation
Priority should be given to reducing sodium silicate content and reworking core venting-these two actions alone can eliminate over 80% of the problem.
Adjustments to melting or pouring alone will have very limited effect under such an excessively high sodium silicate level.
