Common cast iron grades and uses
Cast iron, an alloy primarily composed of iron, carbon, and silicon, offers excellent casting, machinability, and vibration damping properties, making it widely used in machinery manufacturing, automotive, metallurgy, and other fields. Depending on the form of carbon and graphite present in the cast iron, common cast iron can be classified as gray cast iron, ductile iron, malleable iron, and vermicular graphite iron. Different grades of cast iron exhibit significant variations in mechanical properties, processing performance, and usability, aligning with their suitability for different working conditions and parts. Understanding the meaning and uses of common cast iron grades is crucial for optimal material selection and product quality assurance.
Gray cast iron is the most widely used type of cast iron. Its designation begins with “HT,” followed by a number indicating the minimum tensile strength (MPa), such as HT150, HT200, and HT300. The carbon in gray cast iron exists primarily in the form of flake graphite. Due to the fractured nature of graphite, its tensile strength is relatively low (100-350 MPa), and its plasticity and toughness are almost zero. However, it exhibits high compressive strength and excellent casting properties (good fluidity and low shrinkage), machinability (easy chip breakage), and shock absorption. HT150, with its lower strength, is commonly used in the manufacture of parts subject to medium loads, such as machine tool bases, worktables, and housings. HT200, with its moderate strength, is used in the manufacture of parts subject to higher loads, such as engine blocks, gearboxes, and hydraulic pump housings. HT300, with its higher strength, is suitable for the manufacture of parts subject to high loads, such as lathe spindles and press bodies. Gray cast iron has low cost and simple production process. It accounts for more than 80% of the total cast iron output and is an indispensable basic material in the machinery industry.
Ductile iron is produced by adding nodulizing agents (such as magnesium and cerium) and inoculants to molten iron, causing graphite to precipitate in a spherical form. Ductile iron grades begin with “QT,” followed by two numbers representing the minimum tensile strength (MPa) and elongation (%), respectively. Examples include QT400-15, QT500-7, and QT600-3. Ductile iron offers significantly superior mechanical properties to gray cast iron, with tensile strengths ranging from 400 to 1200 MPa. It also possesses considerable plasticity and toughness, while retaining the excellent casting and machinability of cast iron. QT400-15 offers excellent toughness and is commonly used in parts subject to impact loads, such as automotive rear axle housings and tractor chassis. QT500-7 offers moderate strength and toughness, making it suitable for manufacturing transmission components such as gears, crankshafts, and camshafts. QT600-3 offers higher strength and is suitable for parts requiring high strength but low toughness, such as machine tool spindles and hydraulic cylinder blocks. The performance of ductile iron is close to that of steel, and its cost is lower than that of forged steel. It can replace steel in many fields to manufacture structural parts, especially in the automotive industry, where it is widely used, such as engine crankshafts, connecting rods and other key parts.
Malleable cast iron is obtained by graphitizing and annealing white cast iron. Its graphite is in the form of flocculent clusters, hence the name “ductile cast iron.” Grades begin with “KT” and are divided into blackheart malleable cast iron (KTH) and pearlitic malleable cast iron (KTZ). The following two numbers indicate the minimum tensile strength (MPa) and elongation (%), such as KTH300-06 and KTZ450-06. Malleable cast iron has higher tensile strength (300-700 MPa) and toughness than gray cast iron, but lower than ductile iron. It offers good corrosion resistance and machinability, but poor casting properties, making it suitable only for small, complex, and thin-walled parts. KTH300-06 offers excellent toughness and is commonly used in plumbing fittings, low-pressure valves, radiators, and other applications. KTZ450-06, with its higher strength, is used in automotive rear axle housings, universal joint housings, and machine tool wrenches. The production cycle of malleable cast iron is long (annealing takes several days) and the cost is high, so its application scope is gradually being replaced by ductile iron, but it still has irreplaceable advantages in some areas of complex-shaped thin-walled small parts.
Compacted graphite iron (CGI) is a new type of cast iron whose graphite is vermicular, somewhere between flake and spherical. Grades begin with “RuT,” followed by a number indicating the minimum tensile strength (MPa), such as RuT420, RuT380, and RuT340. CGI combines the excellent casting properties of gray cast iron with the high strength of ductile iron, offering high tensile strength (300-500 MPa), excellent heat resistance, wear resistance, and vibration damping, and superior thermal conductivity to ductile iron. RuT420’s higher strength makes it suitable for manufacturing parts subject to high loads and temperatures, such as diesel engine cylinder heads and exhaust pipes; RuT380 can be used in hydraulic valve blocks and gearboxes; and RuT340 is suitable for machine tool beds and flywheels. CGI’s production process demands precise control of its chemical composition and processing. Currently, it is primarily used in high- performance applications such as engines and hydraulic systems. With advances in manufacturing technology, its application range is expanding.
