Sand casting is a metal forming process in which a mold is first formed from a three-dimensional pattern of sand, and molten metal is poured into the mold cavity for solidification. As part of the process, castings may undergo a series of heating and cooling processes to ensure they meet the hardness, metallographic, and mechanical specifications established by the customer.

Typical products made through sand casting include engine blocks, cylinder heads, cylinder pistons, frames, brackets, manifolds for high-horsepower engines, impellers, housings, gearbox parts of transmission systems for construction, and agriculture equipment. Heat treatment improves the mechanical and physical properties of the castings, to increase the tensile strength and reduce the ductility, or stretch, of the metal. This article looks at the types of heat treatment for sand castings.

What are the Stages of Heat Treatment for Sand Castings?

Heat treatment involves controlled heating and cooling of sand castings. The purpose is to use high temperatures to alter the microstructure of the material in order to achieve desired changes in the casting’s mechanical properties. The three main stages of heat treatment include:

• Heating: The metal is slowly heated to a certain point, so that it retains its shape without cracking. It is critical to ensure that the metal heats evenly during this stage to avoid cracking or distortion. The heating rate is based on the type of material, the metal’s heat conductivity, and the size of the casting.
• Soaking (holding): The metal is held at the desired temperature for a prescribed period of time. The purpose of this step is to keep the material at the appointed temperature until the necessary internal structural changes are accomplished. The length of the soaking stage depends on the type of material, as well as the size and complexity of the workpiece. Some castings only require structural changes to the metal’s surface, while others might need to achieve more uniform properties throughout the piece.
• Cooling: The casting is then cooled back to room temperature in a controlled manner in order to attain the desired mechanical properties. Cooling may be accomplished in different ways, depending on the type of material. It may be air-cooled, cooled with a cooling medium, or quickly quenched in a solution.

What are the Types of Heat Treatment for Sand Castings?

Although the heating, soaking and cooling steps are the same, there are variations in the heat treatment process that will affect the final outcome. These include the temperature to which the material is heated, the length of time it is held at that temperature, and the rate at which it is cooled. Types of heat treatment include:

• Annealing: The material is heated beyond its upper critical temperature, and then cooled slowly. Annealing is used to improve the properties of the metal to be more durable and workable. It also reduces residual stress, which can affect further processes. This provides flexibility in the casting process, as there is less risk of fracturing. Annealing will increase the ability to perform additional machining on the workpiece.
• Normalizing: Normalizing increases the hardness of the workpiece. In normalizing, the temperature of the material is increased above the annealing range to a point where its crystal structure changes. It is then cooled in room temperature air. This heat treatment leads to workpieces with less stress and more machinability.
• Hardening: Hardening makes the metal stronger, but it may also make it more brittle. The material may then need to be tempered to reduce this brittleness. Instead of a slow cooling process, the material is rapidly cooled, or quenched, by being plunged into a water, brine or oil solution. In some cases, only the surface material is hardened.
• Tempering: In tempering, the material is heated to a temperature below its hardening point, held at that temperature, and air-cooled. Tempering reduces brittleness, but may also soften the material.

Choosing the appropriate heat treatment method is related to the type of metal involved and the desired end properties.

Benefits of Heat Treatment for Sand Castings

There are various benefits to be achieved through heat treatment, depending on the type of material, the utilization of the casting, and the customer’s specifications. Typical benefits of heat treatment might include:

• Achieve a desired softness or hardness for the material.
• Affect electrical and heat conductivity.
• Change magnetic qualities.
• Relieve stress from earlier processes.
• Add desirable chemical properties.

Learn More About Heat Treatment for Sand Castings

Impro is a global, integrated manufacturer of high-precision, high-complexity and mission-critical components for diversified markets. We use state-of-the-art process technologies and equipment throughout our manufacturing operations. If you are interested in finding more information about sand casting and our process capabilities, please contact our team of experts today.