The main difference between cast iron and steel valves is the carbon content, which directly affects the strength and plasticity of the steel.
Carbon steel, also known as plain carbon steel, is a type of iron-carbon alloy with a carbon content (WC) less than 2%. In addition to carbon, carbon steel generally contains small amounts of silicon, manganese, sulfur, and phosphorus. According to their uses, carbon steels can be divided into three categories: carbon structural steel, carbon tool steel, and free-cutting structural steel. Carbon structural steel can further be divided into construction structural steel and machine manufacturing structural steel. Based on carbon content, carbon steel can be classified as low-carbon steel (WC ≤ 0.25%), medium-carbon steel (WC 0.25%-0.6%), and high-carbon steel (WC > 0.6%). Based on phosphorus and sulfur content, carbon steel can be categorized into common carbon steel (higher phosphorus and sulfur content), high-quality carbon steel (lower phosphorus and sulfur content), and superior quality carbon steel (even lower phosphorus and sulfur content). Generally, in carbon steel, higher carbon content leads to higher hardness and strength but lower plasticity.
Cast Iron: An iron-carbon alloy with a carbon content exceeding 2%. Industrial cast iron usually has a carbon content ranging from 2% to 4%. Carbon in cast iron often exists in the form of graphite, sometimes as cementite. Besides carbon, cast iron also contains 1%-3% silicon, as well as manganese, phosphorus, and sulfur. Alloy cast iron may contain additional elements such as nickel, chromium, molybdenum, aluminum, copper, boron, and vanadium. Carbon and silicon are the primary elements that influence the microstructure and properties of cast iron. Cast iron can be divided into:
1. Grey cast iron. It has a higher carbon content (2.7%-4.0%) where carbon mainly exists in the form of flake graphite, giving it a grey fracture surface, commonly referred to as grey iron. It has a low melting point (1145-1250°C), minimal shrinkage during solidification, compressive strength and hardness close to carbon steel, and good vibration damping properties. It is used for making structural components like machine beds, cylinders, and housings.
2. White cast iron. It has lower carbon and silicon content, where carbon mainly exists in the form of cementite, resulting in a silvery white fracture surface. It contracts significantly during solidification, easily leading to shrinkage cavities and cracks. It is hard and brittle, unable to withstand impact loads. It is mostly used as blanks for malleable cast iron and for making wear-resistant parts.
3. Malleable cast iron. It is obtained by annealing white cast iron, where graphite is distributed in a nodular or flocculent form, commonly referred to as ductile iron. It has uniform microstructure and properties, good wear resistance, excellent plasticity, and toughness. It is used for manufacturing complex-shaped parts that can endure heavy dynamic loads.
4. Ductile iron. It is produced by treating molten grey cast iron to achieve spheroidal graphite, commonly referred to as ductile iron. Compared with ordinary grey cast iron, it has higher strength, better toughness, and plasticity. It is used for manufacturing components like internal combustion engines, automobile parts, and agricultural machinery.
5. Compacted graphite iron. It is obtained by treating molten grey cast iron, where graphite takes on a vermicular shape. Its mechanical properties are similar to ductile iron, and its casting properties lie between those of grey cast iron and ductile iron. It is used for manufacturing automobile components.
6. Alloy cast iron. It is produced by adding appropriate alloying elements (such as silicon, manganese, phosphorus, nickel, chromium, molybdenum, copper, aluminum, boron, vanadium, tin, etc.) to ordinary cast iron. The alloying elements change the matrix structure of the cast iron, thereby imparting corresponding characteristics such as heat resistance, wear resistance, corrosion resistance, low-temperature resistance, or non-magnetic properties. It is used for manufacturing components of mining, chemical machinery, instruments, and meters.