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Material Properties and Structural Characteristics of Cast Iron and Castings

Nov. 25, 2020

1.Grey cast iron

Gray cast iron, which contains flaky graphite, is the most widely used type of cast iron, accounting for more than 80% of total cast iron production. Gray cast iron material has low overall mechanical properties and high compressive strength, which is 3 to 4 times its tensile strength. The vibration damping capacity is 10 times greater than that of steel, so it is often used to manufacture machine seats to withstand vibration. It has a low modulus of elasticity, and its wall thickness variation has a large impact on its mechanical properties.

Due to its great sensitivity to cooling rate, gray iron castings are often white-cracked and cracked in thin cross-sections, while thicker cross-sections often result in a loose condition. Therefore, there is a critical value for the cross-sectional thickness of gray cast iron parts, beyond which, as the wall thickness increases, the strength, vibration damping capacity, elastic modulus and other mechanical properties not only does not increase, but also significantly reduces. Due to its low thermal stability, gray cast iron cannot be used for parts that are subjected to prolonged service in environments exceeding 250 degrees Celsius. Compared to cast steel, the use of gray cast iron results in thinner castings with more complex geometries and less residual internal stresses and warping in the castings. Gray cast iron is often used to produce castings with high requirements, but not necessarily thicker cross-sections, due to its more uniform properties across all sections.

Cast Iron

2.Vermicular graphite cast iron

Peristaltic ductile iron is a worm-like cast iron, which is mixed with graphite in a form between flake and ductile, and has a chemical structure similar to gray cast iron. Its impact toughness, elongation, compressive strength, and yield strength are all in between, and wall thickness variations have less effect on mechanical properties than gray cast iron.

Peristaltic cast iron is much less sensitive to cooling rate than gray cast iron, and has good thermal conductivity, so it is often used to manufacture parts with harsh working environment and large temperature gradients. Due to the high strength and density of the material, it is less susceptible to notches and has good processability, and can be used to make large parts with complex geometries. In order to save scrap steel and reduce the weight of castings, vermicular graphite cast iron can also be used to replace nodular cast iron parts, which can also be used to effectively improve the yield and enhance the gas tightness of castings, especially suitable for the production of hydraulic parts.

Cast Iron

3.Nodular graphite cast iron

The spheroidal graphite obtained through spheroidization and incubation treatment is called ductile iron, which is a high-strength cast iron material developed in the middle of the last century with high overall performance, close to steel, and has a wide range of industrial applications. Ductile cast iron has better strength, plasticity and modulus of elasticity than gray cast iron, twice the abrasion resistance, and less influence on mechanical properties by wall thickness variation, but lower vibration damping ability than gray cast iron.

Although ductile cast iron is more sensitive to cooling rate, it can be used for parts operating at 300 to 400 degrees Celsius because of its high thermal stability and high temperature tolerance. Its mechanical properties are less affected by changes in chemical composition, resulting in higher yields compared to other cast iron materials. Due to the large volume shrinkage and small line shrinkage, it is easy to form shrinkage and loosening, so ductile iron castings are usually designed to have a uniform thickness structure, trying to avoid thick and large cross-sections.

4.Steel Casting

Cast steel is an iron-carbon alloy material with a carbon content of 2.11% or less, with a carbon content of less than 0.2% belonging to low carbon steel, between 0.2% and 0.5% belonging to medium carbon steel, and above 0.5% belonging to high carbon steel. In addition, there are types of low alloy steels and high manganese steels. The comprehensive mechanical properties of the cast steel material is high, compressive strength and tensile strength is basically equal, wall thickness variation on the mechanical properties of the small impact, vibration damping ability is low.

Because of its wall thickness changes on the mechanical properties of small effect, it is suitable for manufacturing large thickness parts, but due to the characteristics of internal stress and warping larger, not suitable for manufacturing complex geometry parts. The tendency of gas saturation is greater, and the surface of the material is prone to produce flag bubbles, so the processing allowance is more than cast iron materials. Although cast steel has high thermal stability, it is susceptible to cracking during temperature changes due to large thermal shrinkage, so the design should emphasize uniform wall thickness and rounded corners.

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