Brief analysis of material properties and structural characteristics of cast iron and cast steel

We introduce several common cast iron and cast steel materials, as well as their material properties and structural characteristics.

1. Gray cast iron

Gray cast iron is a cast iron containing flake graphite. It is the most widely used cast iron, and its output accounts for more than 80% of the total output of cast iron. Gray cast iron material has low comprehensive mechanical properties and high compressive strength, which is 3 to 4 times its own tensile strength. The vibration damping capacity is 10 times larger than that of steel, so it is often used to make a frame that withstands vibration. The elastic modulus is low, and the wall thickness variation has a great influence on the mechanical properties.

Due to its great sensitivity to the cooling rate, gray cast iron castings often have white openings and cracks in thin sections, and often lead to looseness in thicker sections. Therefore, there is a critical value for the section thickness of gray iron castings. If this value is exceeded, the mechanical properties such as strength, vibration damping capacity and elastic modulus will not increase, but will be significantly weakened. Due to the low thermal stability of gray cast iron, it cannot be used to make parts that work over 250 degrees Celsius for a long time. Compared with cast steel materials, gray cast iron can be used to obtain castings with thinner thickness and more complex geometry, and the residual internal stress and warpage deformation in the castings are smaller. Due to the relatively uniform performance across the sections, gray cast iron is often used in castings where the requirements are high, but the sections are not necessarily thick.

2, vermicular graphite cast iron

Vermicular graphite cast iron is a worm-like cast iron. It is incorporated in the form of graphite between the flaky stone and the spherical shape, and its chemical structure is similar to that of gray cast iron. The comprehensive mechanical properties of vermicular graphite cast iron are slightly better than that of gray cast iron, but slightly lower than that of ductile iron. Its impact toughness, elongation rate compressive strength and yield strength are all in between. The influence of wall thickness change on mechanical properties is gray. Cast iron is small.

Vermicular graphite 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 temperature and large temperature gradient. Due to the high strength and compactness of the vermicular graphite cast iron material, it has low sensitivity to the notch and good process performance, and can be used to manufacture large parts with complicated geometric shapes. In order to save scrap steel and reduce the weight of castings, compacted graphite iron can also be used to replace the cast iron parts, which can also achieve the purpose of effectively improving the yield and enhancing the airtightness of the casting, and is particularly suitable for the production of hydraulic parts.

3, ductile iron

The spheroidal graphite obtained through spheroidization and inoculation treatment is called nodular cast iron. This is a kind of high-strength cast iron material developed in the middle of the last century. It has high comprehensive performance and is close to steel. It has a wide range of applications in industry. The strength, plasticity and elastic modulus of nodular cast iron are better than that of gray cast iron. The wear resistance is twice as high as that of gray cast iron. The wall thickness change has little effect on mechanical properties, but the vibration damping capacity is lower than that of gray cast iron.

Although ductile iron is sensitive to the cooling rate, it has high thermal stability and is highly resistant to high temperatures, so it can be used to manufacture parts that work in the environment of 300 to 400 degrees Celsius. Its mechanical properties are less affected by chemical composition changes, so the yield is higher than other cast iron materials. Due to the large volume shrinkage and small line shrinkage, it is easy to form shrinkage cavities and shrinkage. Ductile iron castings are usually designed to have a uniform thickness and avoid thick sections as much as possible.

4, cast steel

Cast steel is an iron-carbon alloy material with a carbon content of 2.11% or less. Carbonaceous steel with a carbon content of less than 0.2% belongs to low carbon steel, and between 0.2% and 0.5% belongs to medium carbon steel. Above 0.5% is high. Carbon steel. In addition, there are types such as low alloy steel and high manganese steel. Cast steel materials have high comprehensive mechanical properties, compressive strength and tensile strength are basically the same, wall thickness changes have little effect on mechanical properties, and vibration damping capacity is low.

Because its wall thickness changes have little effect on mechanical properties, it is suitable for the manufacture of large-thickness parts. However, due to its internal stress and large warpage, it is not suitable for manufacturing parts with complicated geometric structures. Gas saturation tends to be large, and the surface of the material is prone to flag bubbles, so the machining allowance is more than that of cast iron. Although the cast steel material has high thermal stability, it is easy to crack due to the large heat shrinkage rate, so the design should emphasize the uniform wall thickness and the rounded corner.