Heat Treatment and Surface Finishing in Sand Casting

By /

Introduction

Sand casting is one of the most widely used metal casting processes, primarily due to its versatility and cost-effectiveness. This process involves creating a mold from a sand mixture and pouring molten metal into the cavity to form a desired shape. However, the raw castings produced from sand casting often require further treatment to enhance their mechanical properties and surface quality. Heat treatment and surface finishing are critical post-casting processes that significantly influence the final properties and performance of cast components.

Heat Treatment in Sand Casting

Heat treatment is a thermal process applied to metal castings to alter their physical and mechanical properties without changing their shapes. The primary objectives of heat treatment in sand casting are to improve hardness, strength, ductility, and wear resistance. The common heat treatment processes include annealing, normalizing, quenching, and tempering.

  1. AnnealingAnnealing is a process that involves heating the casting to a specific temperature, holding it at that temperature for a certain period, and then slowly cooling it. The purpose of annealing is to reduce hardness, relieve internal stresses, and improve machinability. This process makes the material more ductile and less brittle, which is essential for further machining operations.StepTemperature (°C)Holding Time (hours)Cooling Method1600-7002-4Furnace cooling2700-8001-3Air cooling
  2. NormalizingNormalizing involves heating the casting above its critical temperature and then allowing it to cool in air. This process refines the grain structure, enhances mechanical properties, and improves toughness. Normalizing is particularly useful for steel castings, ensuring uniformity in their microstructure.StepTemperature (°C)Holding Time (hours)Cooling Method1800-9501-2Air cooling
  3. QuenchingQuenching is the process of rapidly cooling a heated casting by immersing it in a quenching medium such as water, oil, or air. This process increases the hardness and strength of the material but can also make it more brittle. The selection of the quenching medium and cooling rate depends on the type of material and the desired properties.Quenching MediumTemperature (°C)Cooling RateWater20-30FastestOil60-80ModerateAirAmbientSlowest
  4. TemperingTempering follows quenching to reduce brittleness while retaining the increased hardness and strength. The casting is reheated to a lower temperature and held for a specific time before cooling. Tempering improves toughness and ductility, making the material suitable for practical applications.StepTemperature (°C)Holding Time (hours)Cooling Method1150-3001-3Air cooling

Surface Finishing in Sand Casting

Surface finishing involves various techniques to enhance the surface quality of cast components. These processes remove surface imperfections, improve aesthetic appeal, and enhance the performance of the casting. Common surface finishing techniques in sand casting include sandblasting, grinding, polishing, and coating.

  1. SandblastingSandblasting uses high-speed abrasive particles to clean and smooth the casting surface. This process removes surface oxides, scale, and other contaminants, resulting in a clean and uniform surface. Sandblasting is often used as a preliminary step before other finishing processes.Abrasive MaterialParticle Size (µm)Pressure (psi)Silica sand200-40050-100Aluminum oxide50-20060-120Glass beads100-40040-80
  2. GrindingGrinding involves the use of abrasive wheels to remove excess material and smoothen the surface. This process is essential for achieving the desired dimensional accuracy and surface finish. Different types of grinding wheels and techniques are used depending on the material and the required finish.Grinding Wheel TypeGrit SizeSpeed (rpm)Silicon carbide60-1203000-5000Aluminum oxide80-1503500-5500
  3. PolishingPolishing is a finer finishing process that provides a high-gloss surface finish. It involves using polishing compounds and buffing wheels to achieve a smooth and shiny surface. Polishing improves the aesthetic appeal and can also enhance corrosion resistance.Polishing CompoundAbrasive TypeApplication MethodRougeIron oxideBuffing wheelTripoliSilicaBuffing wheel
  4. CoatingCoating involves applying a protective layer on the casting surface to prevent corrosion, wear, and improve appearance. Various coatings such as paint, powder coating, and electroplating are used based on the application requirements. Coating enhances the durability and lifespan of the casting.Coating TypeApplication MethodDrying Time (hours)PaintSpraying2-4Powder coatingElectrostatic spray1-2ElectroplatingElectrochemical3-5

Conclusion

Heat treatment and surface finishing are integral to the sand casting process, ensuring the cast components meet the required mechanical properties and surface quality standards. Heat treatment processes like annealing, normalizing, quenching, and tempering enhance the material’s strength, hardness, and ductility. Surface finishing techniques such as sandblasting, grinding, polishing, and coating improve the aesthetic appeal and functional performance of the castings. By carefully selecting and applying these post-casting processes, manufacturers can produce high-quality, reliable components for various applications.

Scroll to Top