Brief Analysis of Material Properties and Structural Characteristics of Cast Iron and Steel

1. Grey cast iron

Gray cast iron is cast iron containing flake graphite and is the most widely used cast iron, accounting for more than 80% of the total output of cast iron. Gray cast iron has low comprehensive mechanical properties and high compressive strength, which is 3 to 4 times its own tensile strength. The vibration-absorbing capacity is 10 times greater than that of steel, so it is often used to manufacture bases that withstand vibration. The elastic modulus is low, and the change of its wall thickness has a great influence on the mechanical properties.

Due to its great sensitivity to cooling rate, gray cast iron castings often have white mouths and cracks on thin sections, and often lead to glaze on thicker sections. Therefore, there is a critical value for the section thickness of gray iron castings. If this value is exceeded, as the wall thickness increases, the mechanical properties such as strength, vibration damping ability, and elastic modulus will not be enhanced, but will be significantly weakened. Due to the low thermal stability of gray cast iron, it cannot be used to manufacture parts that work in an environment exceeding 250 degrees Celsius for a long time. Compared with cast steel materials, gray cast iron can 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 on each section, gray cast iron is often used for castings with high manufacturing requirements but not necessarily thicker sections.

2. Vermicular graphite cast iron

     Vermicular graphite cast iron is worm-like cast iron. The graphite it incorporates is between flake and spherical, 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 inferior to that of ductile iron. Its impact toughness, elongation compressive strength, and yield strength are all between the two. Cast iron small.

     Compacted graphite 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 gradient. Due to the high strength and compactness of the vermicular graphite cast iron material, the sensitivity to notches is small, and it has good process performance, which can be used to manufacture large parts with complex geometric shapes. In order to save scrap steel and reduce the weight of castings, vermicular graphite iron can also be used to replace iron castings, which can effectively improve the yield and enhance the air tightness of castings, especially suitable for the production of hydraulic parts.

       3. Ductile iron

    Nodular graphite obtained by spheroidization and inoculation treatment is called ductile iron, which is a high-strength cast iron material developed in the middle of the last century. The strength, plasticity and elastic modulus of ductile iron are better than those of gray cast iron, and its wear resistance is twice as high as that of gray cast iron.

   Although ductile iron is more sensitive to cooling rates, due to its high thermal stability and high tolerance to high temperatures, it can be used to manufacture parts that operate in environments ranging from 300 to 400 degrees Celsius. 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 linear shrinkage, shrinkage cavities and shrinkage porosity are easy to form. Ductile iron castings are usually designed to have a uniform thickness structure, and try to avoid thick and large sections.

       4. Cast steel

   Cast steel is an iron-carbon alloy material with a carbon content of less than 2.11%, low carbon steel with a carbon content of less than 0.2%, medium carbon steel with a carbon content of 0.2% to 0.5%, and high carbon steel with a carbon content of more than 0.5%. carbon steel. In addition, there are types of low alloy steel and high manganese steel. The comprehensive mechanical properties of cast steel materials are high, the compressive strength and tensile strength are basically equal, the change of wall thickness has little effect on the mechanical properties, and the vibration damping ability is low.

   Because the change of wall thickness has little effect on the mechanical properties, it is suitable for the manufacture of parts with large thickness, but due to the characteristics of large internal stress and warpage, it is not suitable for the manufacture of parts with complex geometric structures. The gas saturation tends to be large, and flag bubbles are easily generated on the surface of the material, so the machining allowance is more than that of cast iron materials. Although the cast steel material has high thermal stability, due to the large thermal shrinkage rate, cracking is easy to occur when the temperature changes, so the design should emphasize uniform wall thickness and smooth corners.