Classification and chemical composition range of cast iron

Cast iron is a general term for a series of alloys mainly composed of iron, carbon and silicon. In these alloys, the carbon content exceeds the amount that can be retained in the austenite solid solution at the eutectic temperature. The carbon content of cast iron for industrial and domestic use is usually 2.5%-4.0%.

Cast iron is melted by a reasonable combination of various metal furnace materials such as new iron, scrap iron, recycled iron, and ferroalloys. The main component of cast iron is iron, in addition to a small amount of carbon, silicon, manganese, phosphorus, sulfur, and other alloying elements as needed.

There are many ways to classify cast iron, mainly: (1) According to the fracture characteristics of cast iron, it is classified into: gray cast iron (gray cast iron), white cast iron, and hemp cast iron. (2) According to the graphite morphology of cast iron, it is classified into: gray cast iron, vermicular cast iron, ductile cast iron, and malleable cast iron. (3) According to the chemical composition of cast iron, it is classified into: ordinary cast iron and alloy cast iron. (4) According to the degree of eutecticity of cast iron, cast iron can be classified into: hypoeutectic cast iron, eutectic cast iron, hypereutectic cast iron. (5) According to the special properties of cast iron, cast iron can be classified into: wear-resistant cast iron, wear-resistant cast iron, corrosion-resistant cast iron, heat-resistant cast iron, non-magnetic cast iron, etc.

In addition, cast iron can also be classified according to the matrix structure (such as ferrite ductile cast iron, pearlite ductile cast iron, bainite ductile cast iron, etc.); according to the production process of cast iron (such as nodular cast iron, chilled cast iron, etc.); according to the alloy composition of cast iron (such as aluminum cast iron, nickel cast iron, chromium cast iron, tungsten cast iron, boron cast iron, etc.).

Gray cast iron (gray cast iron): cast iron in which carbon mainly appears in the form of flake graphite, with a gray fracture, and the matrix form is: ferrite, pearlite, pearlite plus ferrite. The chemical composition of gray cast iron is generally: C2.7%--3.8% Si1.1--2.7%, Mn0.5%--1.4%, P＜0.3%, S＜0.15%. Because gray cast iron has certain strength and good shock absorption, wear resistance, excellent cutting processability and casting processability, and is easy to produce and low in cost, it is most widely used in industrial production and civilian life.

Nodular cast iron: It still belongs to the category of gray cast iron. It is a hypoeutectic gray cast iron obtained after the molten iron is inoculated. The carbon of nodular cast iron mainly appears in the form of fine flake graphite, and the matrix form is pearlite and ferrite. The composition of the molten iron (original molten iron) before inoculation is generally selected in the pitted area or the edge of the white area on the casting structure diagram, and is usually controlled as: C2.8%--3.3%, Si0.6%--1.4%, Mn0.8%--1.4%, P＜0.15%, S＜0.12%. After inoculation, Si is often adjusted to 1.2%--1.8%, the eutectic group is significantly refined, the size and distribution of graphite are improved, and the strength is improved. Therefore, inoculated cast iron is often called high-strength gray cast iron. The tensile strength of inoculated cast iron can reach 200--400Mpa, and the bending strength can reach 450--600Mpa, but the elongation and impact toughness are still low, so it is often used for important castings with small dynamic loads and high static strength requirements, such as machine tool beds, engine cylinders, etc.

Ductile iron: It is a cast iron in which the molten iron is treated with a spheroidizing agent instead of heat treatment, so that most or all of the graphite is spherical, and sometimes a small amount is flocculent. However, the form of the matrix of ductile iron can be changed after a certain heat treatment. The matrix form of ductile iron is: ferrite, pearlite, ferrite plus pearlite, bainite, austenite plus bainite. The composition of the molten iron (original molten iron) before spheroidization is generally selected near the eutectic point, with the premise that no graphite floating occurs. It is usually hoped that it is high in carbon, low in silicon, and low in phosphorus and sulfur, that is, C3.5%--3.9%, Si1.0%--2.0%, Mn0.3--0.9%, P＜0.1%, S＜0.08%. After spheroidizing, the molten iron needs to be inoculated to eliminate the white cast iron tendency caused by the spheroidizing elements and refine the graphite balls. After spheroidizing and inoculating, Si is often adjusted to 1.8%--3.3%, the residual magnesium (Mg residue) is controlled at 0.03%--0.08%, and the residual rare earth oxide (RE residue) is controlled at 0.02%--0.04%, and none of them is too high. Ductile iron is a new type of structural material developed in the late 1940s. In addition to having good shock absorption, wear resistance, cutting processability and casting processability similar to gray cast iron, it also has much higher strength, plasticity and toughness than ordinary gray cast iron. The tensile strength can reach 1200--1450Mpa, the elongation can reach 17%, and the impact value can reach 60J/cm2. Therefore, it has been used to produce parts with complex forces and high requirements for strength, toughness, wear resistance, etc., such as crankshafts and camshafts of automobiles, tractors, internal combustion engines, etc., and medium-pressure valves of general machinery.

Vermicular cast iron: It is a cast iron in which the molten iron has been vermicularized and most of the graphite is vermicular graphite. The matrix forms of vermicular cast iron are: ferrite, pearlite, ferrite plus pearlite. The selection of the composition of the molten iron (original molten iron) before vermicularization is similar to that of the original molten iron of ductile iron. The carbon equivalent is generally controlled at 4.3%--4.6%. It is also hoped to be high carbon, low silicon, and low phosphorus and sulfur, that is, C3.5%--3.9%, Si1.1%--2.0%, Mn0.4%--0.8%, P＜0.1%, S＜0.1%. The molten iron after vermicularization also needs to be inoculated, mainly to eliminate the white cast structure caused by the vermicular elements, and to refine the graphite to increase the number of eutectic groups. After vermicular cast iron inoculated with vermicular inoculation, Si is often adjusted to 2.0%--3.0%. When adding magnesium-titanium vermicular inoculation agent, the residual magnesium (Mg residual) is controlled at 0.015%--0.03%, and the residual titanium (Ti residual) is controlled at 0.08%--0.1%; when adding rare earth magnesium-titanium vermicular inoculation agent, the residual rare earth oxide (RE residual) is controlled at 0.001%--0.002%, the residual magnesium is controlled at 0.015%--0.035%, and the residual titanium (Ti residual) is controlled at 0.06%---0.13%. Vermicular cast iron is a new type of cast iron material developed in the 1960s. Its tensile strength can reach 500Mpa, which is higher than that of gray cast iron and has better casting performance than ductile iron. In addition, it has good thermal conductivity, etc. Therefore, it has been used in the production of diesel engine cylinder heads, motor housings, drive box bodies, brake drums, hydraulic valve bodies, metallurgical ingot molds, etc. Malleable cast iron (also known as malleable iron, malleable cast iron, ductile iron): is a cast iron with higher toughness obtained by graphitizing or oxidative decarburization forgeable treatment of white cast iron to change its metallographic structure or composition. Malleable cast iron can be divided into blackheart malleable cast iron and whiteheart malleable cast iron due to different treatment processes. Blackheart malleable cast iron is a malleable cast iron with a normal black velvety fracture and a gray outer ring after heat treatment of white cast iron in a neutral atmosphere to decompose iron carbide into flocculent graphite and ferrite. Among them: blackheart malleable cast iron with a matrix mainly composed of pearlite is called malleable cast iron, and blackheart malleable cast iron with a matrix mainly composed of ferrite is called ferritic malleable cast iron; whiteheart malleable cast iron is a malleable cast iron that is almost completely decarburized after annealing white cast iron in an oxidizing atmosphere. The composition of the original molten iron (original molten iron) of malleable cast iron is generally selected within the hypoeutectic range to ensure that the ingot obtains a pure white structure without precipitation of primary graphite and has good casting performance. When selecting, the matching relationship between the carbon silicon content and the manganese and sulfur content should be comprehensively considered. It is usually hoped that: C+Si=3.6%--4.6%, Mn= (2.5-3.5)S, for blackheart malleable cast iron: C2.3%--2.8%, Si1.1%--2.2%, Mn0.4%--1.2%, P≤0.20%, S≤0.25%; for whiteheart malleable cast iron: C2.8%--3.4%, Si0.3%--1.0%, Mn0.2%--0.8%, P≤0.20%, S≤0.25%. The original iron liquid of malleable cast iron usually needs to be inoculated. Commonly used inoculants are: aluminum, bismuth, aluminum bismuth, boron bismuth, boron bismuth aluminum, etc. When inoculated with pure aluminum, add 0.005%-0.012% aluminum; when inoculated with pure bismuth, add 0.006%-0.01% bismuth; when inoculated with aluminum-bismuth composite, add 0.005%-0.0015% aluminum and 0.01%-0.02% bismuth; when inoculated with boron-bismuth composite, add 0.0015%-0.0025% boron and 0.01%-0.02% bismuth; when inoculated with boron-bismuth aluminum composite, add 0.001%-0.0025% boron, add 0.006%-0.01% bismuth, and 0.008%-0.012% aluminum. After heat treatment, the tensile strength of malleable cast iron can reach 300--700Mpa, and the elongation can reach 2%---16%. The strength is higher than that of gray cast iron, the toughness is close to that of cast steel, and the casting performance is better than that of cast steel. Therefore, it has been widely used in automobiles, tractors, agricultural machinery and railway parts, as well as power line tools, pipe connectors, hardware tools and household appliances.

Wear-resistant cast iron: It is cast iron that is not easy to wear under lubrication conditions. It is mainly formed by adding certain alloy elements to form a certain amount of hardening phase in cast iron. Usually: wear-resistant cast iron used for machine tool parts and general parts includes: phosphorus wear-resistant cast iron, vanadium-titanium wear-resistant cast iron, chromium-molybdenum-copper wear-resistant cast iron, rare earth wear-resistant cast iron, antimony wear-resistant cast iron, boron wear-resistant cast iron, etc. Wear-resistant cast iron used for piston rings includes: tungsten wear-resistant cast iron, molybdenum-copper wear-resistant cast iron, nickel-chromium wear-resistant cast iron, phosphorus wear-resistant cast iron, etc. Wear-resistant cast irons used for cylinder liners include: phosphorus wear-resistant cast iron, chromium-molybdenum-copper wear-resistant cast iron, etc.

Wear-resistant cast iron: cast iron that is not easy to wear under friction conditions. Usually: ordinary white cast iron, alloy white cast iron, medium manganese ductile iron, etc. Ordinary white cast iron is mainly used for agricultural plowshares, crusher hammers, mill grinding discs, etc. Alloy white cast iron is mainly used for impurity pump impellers and pump bodies, shot blasting machine slides and blades. Medium manganese ductile iron is mainly used for ball mill grinding balls, agricultural machinery rakes and plowshares, etc.

Chilled cast iron (chilled cast iron): It is a cast iron in which some parts of the casting solidify quickly through chilling, and all or most of the carbon is in a chemical state. The structure of chilled cast iron is composed of the white structure of the outer layer and the gray structure (flaky graphite or spherical graphite) of the core. The preparation of chilled cast iron, on the one hand, is to control the chemical composition to cause greater chilling, and on the other hand, to take chilling measures in the process. Chilled cast iron is often used for rollers, especially metallurgical rollers. In addition, it is also used for diesel engine tappets, tractor pulleys, sand mill wheels, etc.

Heat-resistant cast iron: It is cast iron whose oxidation or deformation at high temperature meets the use requirements. Certain alloying elements can be added to cast iron to improve the oxidation resistance and growth resistance of cast iron at high temperatures. Generally, heat-resistant cast iron can be divided into three categories according to the addition of different alloying elements: silicon-containing heat-resistant cast iron, aluminum-containing heat-resistant cast iron, and chromium-containing heat-resistant cast iron. It is mainly used for sintering machine trolleys, petroleum refining furnace tube plates, electric furnace doors, boiler grates, flue dampers, heat exchangers, etc.

Corrosion-resistant cast iron: It is cast iron with a certain corrosion resistance. Such as high-silicon acid-resistant cast iron, high-silicon-molybdenum chlorine-resistant cast iron, aluminum cast iron, high-chromium cast iron, nickel cast iron, alkali-resistant cast iron, etc., are mainly used for many parts in equipment such as petroleum, chemical industry, and fertilizer.

Non-magnetic cast iron: It is cast iron with low magnetic permeability. The matrix structure is austenite, often called austenitic cast iron, which can be divided into flake graphite austenitic cast iron and spheroidal graphite austenitic cast iron. It is mainly used for oil switch covers, transformer tail boxes, motor clamps, magnet brackets, submarine parts, etc.