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 an 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. Based on their uses, carbon steels can be classified into three categories: carbon structural steel, carbon tool steel, and free-cutting structural steel. Carbon structural steel is further divided into two types: construction structural steel and machine manufacturing structural steel. According to the carbon content, carbon steel can be divided into low-carbon steel (WC ≤ 0.25%), medium-carbon steel (WC 0.25%–0.6%), and high-carbon steel (WC > 0.6%). By phosphorus and sulfur content, carbon steel can be categorized into common carbon steel (higher phosphorus and sulfur), high-quality carbon steel (lower phosphorus and sulfur), and superior quality steel (lowest phosphorus and sulfur). Generally, in carbon steel, higher carbon content results in higher hardness and strength but lower plasticity.
Cast Iron: An iron-carbon alloy with a carbon content above 2%. Industrial cast iron typically has a carbon content ranging from 2% to 4%. The carbon in cast iron mostly exists in the form of graphite, sometimes as cementite. Besides carbon, cast iron also contains 1%–3% silicon, as well as elements such as manganese, phosphorus, and sulfur. Alloy cast iron may contain additional elements like nickel, chromium, molybdenum, aluminum, copper, boron, vanadium, etc. Carbon and silicon are the primary elements that influence the microstructure and properties of cast iron. Cast iron can be classified as follows:
1. Grey cast iron: Contains a higher carbon content (2.7%–4.0%), where carbon mainly exists in the form of flake graphite, resulting in a grey fracture surface, often abbreviated as grey iron. It has a low melting point (1145–1250℃), minimal contraction during solidification, compressive strength and hardness close to that of carbon steel, and good vibration damping properties. Used for making structural components such as machine beds, cylinders, and boxes.
2. White cast iron: Contains lower carbon and silicon, where carbon mainly exists in the form of cementite, resulting in a silvery-white fracture surface. It contracts significantly during solidification, prone to shrinkage cavities and cracks. It has high hardness and brittleness, unable to withstand impact loads. Often used as raw material for malleable cast iron or for wear-resistant parts.
3. Malleable cast iron: Obtained by annealing white cast iron, where graphite forms in a nodular distribution, often referred to as ductile iron. Its structure and properties are uniform, wear-resistant, with good plasticity and toughness. Used for manufacturing complex-shaped parts that can withstand strong dynamic loads.
4. Ductile iron: Obtained by treating grey cast iron with a spheroidizing agent, where graphite precipitates in a spherical form, often abbreviated as ductile iron. Compared to ordinary grey cast iron, it has higher strength, better toughness, and plasticity. Used for manufacturing components such as internal combustion engines, automobile parts, and agricultural machinery.
5. Compacted graphite iron: Obtained by treating grey cast iron with a compacting agent, where graphite precipitates in a worm-like form. Its mechanical properties are similar to ductile iron, while its casting properties lie between grey cast iron and ductile iron. Used for manufacturing automotive components.
6. Alloy cast iron: Obtained by adding appropriate alloying elements (such as silicon, manganese, phosphorus, nickel, chromium, molybdenum, copper, aluminum, boron, vanadium, tin, etc.) to ordinary cast iron. These alloying elements alter the matrix structure of the cast iron, providing corresponding characteristics such as heat resistance, wear resistance, corrosion resistance, low-temperature resistance, or non-magnetic properties. Used for manufacturing components in mining, chemical machinery, instruments, and meters.