Lost-foam casting technology is maturely applied in aluminum, copper and iron casting, while its application in steel casting is relatively limited. Due to the high pouring temperature of molten steel, pyrolysis products of foam patterns easily react with molten steel. Poor exhaust performance will readily cause defects such as porosity, carburization and hydrogenation, among which porosity is the most common quality defect of lost-foam steel castings. To optimize production processes and improve casting quality, our company has conducted targeted analysis on porosity defects and formulated a complete set of standardized prevention and control processes.

I. Main Causes of Porosity Defects in Steel Castings

1. Invasive Porosity (Dominant Defect Type)

   (1) Excessively fast filling speed and short pouring time prevent full vaporization of foam patterns. Liquid pyrolysis products are encapsulated in molten steel and form porosity after casting solidification.

   (2) The high pouring temperature of steel leads to instant massive gas generation from foam patterns and a sharp rise in cavity pressure. The gas cannot be discharged in a timely manner, and turbulent filling of molten steel further aggravates gas entrapment.

   (3) Improper pattern bonding process causes concentrated porosity. Adhesive features high vaporization temperature and low vaporization speed, which is easily wrapped by molten steel and continuously generates gas during cooling. Mechanical connections produce no such defects.

   (4) Excessive moisture in the production system induces porosity, including incompletely dried or reabsorbed coating moisture, insufficiently dried foam patterns, and damp molding sand. Moisture vaporizes rapidly at high temperatures and forms gas pores.

2. Precipitated Porosity

   Molten steel absorbs water vapor during smelting, which decomposes into hydrogen atoms at high temperatures and dissolves in the steel. During solidification, hydrogen atoms recombine into hydrogen bubbles that fail to escape. This forms numerous tiny diffuse pores with smooth, metallic-lustered inner walls.

II. Optimized Prevention and Control Measures

1. Reduce Gas Generation and Slow Down Gas Release Rate

   Low-density foam materials (in-house standard: 0.018~0.020g/cm³) are adopted on the premise of guaranteed pattern strength. Hollow structures are applied to sprue, risers and thick casting sections. The gating system is optimized by adjusting the ingate size from 20mm×15mm to 15mm×15mm, extending the pouring time to about 20s. The reduced filling speed ensures full discharge of pyrolysis gas.

2. Optimize Bonding Process for Low Gas Generation

   Adhesive consumption is strictly controlled. Integral forming of castings and risers is prioritized. Plug-in connection, steel nail fixing or composite bonding & nail connection is adopted to eliminate gas defects caused by excessive adhesive usage.

3. Upgrade Coating Formula and Strengthen Drying Control

   High-temperature resistant coating with excellent air permeability and strength is selected, with the coating thickness stably controlled at 0.5~1mm. Uniform coating and thorough drying are mandatory, and dried patterns shall be poured timely to avoid moisture reabsorption. Our optimized formula adjusts silica sand grain size to 180 mesh, reduces latex dosage from 25kg to 20kg, and increases cellulose dosage from 4kg to 5kg. The upgraded coating features higher high-temperature strength and air permeability with no cracking.

4. Equip Exhaust Risers for Efficient Slag and Gas Removal

   Small exhaust risers are arranged at dead corners and top areas where pyrolysis products tend to accumulate. Tiny exhaust holes are drilled on the top coating of risers to accelerate vaporization and discharge of residual liquid products. This method effectively eliminates slag inclusion and porosity defects and allows convenient post-processing of risers.

5. Standardize Pouring Process for Stable Filling

   The pouring position is reasonably arranged to keep major casting surfaces vertical or inclined for stable filling. Closed bottom pouring, step pouring or side pouring systems are preferred. The pouring temperature is controlled above 1600℃, and the slow-fast-slow pouring principle is strictly followed to ensure full vaporization of foam patterns and complete gas escape.

6. Enhance Molten Steel Degassing and Strictly Control Production Environment

   Rust removal is performed on furnace materials, and smelting time at high temperature is shortened. The aluminum deoxidizer dosage is standardized at 0.04%~0.06%. Dry molding sand is adopted, and the vacuum negative pressure is stably controlled at 0.05~0.06MPa to comprehensively reduce porosity risks.

Through the implementation of the above comprehensive measures, not only has the porosity defect of cast steel parts been basically solved, but the gas shrinkage cavity phenomenon of other types of cast steel parts has also been reduced, and the yield rate has reached more than 90%.