What Is Graphite Cast Iron?
Graphite cast iron is a type of gray cast iron in which the graphite is shaped like a cluster of flakes.
Graphite cast iron has an excellent ability to absorb vibration and has a high damping capacity. In addition, graphite has a lubricant effect and good thermal conductivity, making it easy to dissipate heat from friction. In addition, it has a high vibration absorption capacity and is resistant to thermal shock.
Uses of Graphite Cast Iron
1. Automobile Parts
Engine blocks, cylinder heads, etc.
2. Parts for Construction Machinery
Cylinder blocks, crankcases, etc.
3. Piping parts
Water pipes, valves, etc.
4. Parts for Electrical Equipment
Motor housings, transformer boxes, etc.
A motor housing is one of the exterior parts of an electric motor and protects the internal structure of the motor. A transformer box is one of the parts of an electric transformer (English: Power Transformer). It is a box-shaped part with a built-in coil for transforming electric power and a terminal block for connecting the coil in the transformer.
5. Gas Turbine and Turbocharger Parts
Turbine blades, compressor blades, and impellers are examples.
Compressor blades are impellers used in gas turbine engines, compressors, and other turbomachinery to accelerate and compress fluids. An impeller is a rotating impeller used in turbomachinery, a type of fluid machinery, and pumps.
6. Parts of Driving Units and Bogies of Rolling Stock
These include crankshafts, wheels, etc.
Principles of Graphite Cast Iron
Graphite cast iron is formed by rapid cooling in the mold during casting. During the rapid cooling process, the carbon contained in the cast iron precipitates in the form of graphite and is arranged in a graphite cast iron strip.
When cast iron is injected into a mold during casting, it exists in a liquid state at high temperatures. The temperature inside the mold is very high and cools rapidly, causing the cast iron to solidify. During this process, the carbon contained in the cast iron forms graphite crystal nuclei, which gradually grows.
Normally, there are two microstructures in cast iron, ferrite and pearlite, with the carbon being present in the pearlite. However, during casting, rapid cooling inside the mold inhibits the crystal growth of ferrite and pearlite, resulting in the precipitation of carbon in the form of graphite, which is characterized by its fragmented arrangement.
Ferrite is a material composed mainly of iron and some non-metallic elements (mainly carbon and small amounts of silicon, manganese, sulfur, etc.) and is a weakly magnetic alloy.
Pearlite, a type of steel, is composed of microscopic layers of ferrite and cementite (carbon iron). This layered structure makes pearlite very hard, strong, and wear resistant. It is the most common microstructure in steel and is typically formed by heat treatment.
Properties of Graphite Cast Iron
1. High Temperature Resistance
There are several reasons why graphite cast iron has high strength and durability against deformation at high temperatures. First, the flaky arrangement of graphite improves durability against loads and thermal stresses. Also, the crystalline structure increases strength, which means that strength can be maintained at high temperatures, and the high carbon content means that the iron does not soften at high temperatures, which means that strength can be maintained.
2. Thermal Conductivity
The reason graphite cast iron has excellent thermal conductivity is due to its fine crystal structure. This crystalline structure is able to efficiently transfer heat due to its uniform arrangement, and the weak bond between iron and graphite makes it highly resistant to thermal stress. Furthermore, it contains additives that make it suitable for use in high-temperature environments, thus enabling it to exhibit high heat resistance. Therefore, graphite cast iron is a suitable material for use in high-temperature environments and when thermal conductivity is required.
3. Corrosion and Wear Resistance
The reason for the high corrosion and wear resistance of graphite cast iron is its fine microstructure. This microstructure is a specific crystalline structure formed by the cooling rate during casting and contains graphite in which iron and carbon are crystallized.
The specific crystalline structure is the “fine iron and graphite crystalline structure (ferrite + graphite structure)” formed by the rapid cooling of graphite cast iron during casting.
This crystalline structure forms an oxide film on the surface of the castings and prevents chemical reactions. In addition, graphite cast iron is extremely hard, dense, and chemically stable, making it highly resistant to wear and tear and less prone to deformation and damage even after long-term use.
4. Vibration Damping Properties
Due to the shape of the graphite arrangement, graphite cast iron has a high vibration absorption capacity and is sometimes used to suppress vibration in machine parts and machine tools. The flake-like arrangement of graphite allows stresses within the material to propagate efficiently, absorbing and dispersing vibration energy, making graphite cast iron a highly vibration-resistant material.
Due to the excellent acoustic properties of graphite cast iron, it is sometimes used in musical instruments and speakers. However, depending on the vibration conditions of the actual machine parts or machine tools used, the vibration absorption capacity of graphite cast iron may not be sufficient, so a comprehensive judgment must be made when selecting a material.
Other Information on Graphite Cast Iron
Durability of Graphite Cast Iron
The crystalline structure of graphite cast iron affects its strength and durability because the graphite is arranged in flakes during casting. However, when rapid cooling occurs during casting, the iron and carbon can crystallize to form white cementite. Note that cementite is harder and more brittle than graphite, which can reduce the strength and wear resistance of castings.
Cementite is a hard, brittle compound composed of iron and carbon, with the chemical formula Fe3C. Cementite can be found in the microstructure of cast iron and steel, where iron and carbon form an eutectic structure. The percentage of cementite tends to increase when the cooling rate of cast iron or steel is fast or in high-carbon steels.