Gray cast iron generally refers to cast iron with high contents of carbon, silicon, and manganese, and low sulfur content. It is formed through a graphitization process when molten iron cools slowly, with graphite distributed in flake form. It gets its name from the dark gray color of its fracture surface. Gray cast iron, which contains flake graphite, primarily consists of iron, carbon, silicon, manganese, sulfur, and phosphorus. As the most widely used type of cast iron, it accounts for over 80% of the total cast iron production. Molten iron with a specific composition, after simple on-site furnace treatment, produces cast iron with flake graphite upon pouring—also known as gray cast iron.
The approximate chemical composition of gray cast iron HT150 is: carbon (3.2%–3.5%), silicon (1.9%–2.3%), manganese (0.5%–0.8%), sulfur (<0.12%), and phosphorus (<0.2%).
The typical chemical composition of gray cast iron HT200 is: carbon (3.0%–3.3%), silicon (1.4%–1.6%), manganese (0.8%–1.0%), sulfur (<0.15%), and phosphorus (<0.15%).
The general chemical composition of gray cast iron HT250 is: carbon (3.16%–3.30%), silicon (1.79%–1.93%), manganese (0.89%–1.04%), sulfur (0.094%–0.125%), and phosphorus (0.120%–0.170%).
The structure of gray cast iron consists of a metallic matrix and flake graphite. The metallic matrix mainly exists in three forms: ferrite, pearlite, and ferrite + pearlite. Based on the matrix structure, gray cast iron can be classified into the following types:
Ferrite gray cast iron: Coarse flake graphite is distributed in a ferrite metallic matrix. It has low strength and hardness but excellent castability, and is used to manufacture low-demand castings or thin-walled parts.
Ferrite-pearlite gray cast iron: Flake graphite is distributed in a metallic matrix composed of pearlite and ferrite. The flake graphite is slightly thicker and more abundant. It is easy to control during casting, has good machinability, and is widely applied.
Pearlite gray cast iron: Fine and uniform flake graphite is distributed in a pearlite matrix. It has high strength and hardness, and is mainly used to manufacture critical mechanical parts.
Gray cast iron exhibits excellent casting performance. Its chemical composition is close to the eutectic point, so molten iron has good fluidity, enabling the casting of highly complex parts. Additionally, due to the large specific volume of graphite, the shrinkage rate of castings during solidification is reduced—simplifying the process, reducing casting stress, and achieving a dense structure.
The mechanical properties of gray cast iron are relatively low. Its tensile strength is lower than that of steel, and it also has poor plasticity and toughness. The flake distribution of graphite results in a relatively small effective load-bearing area, and stress concentration tends to occur at the tips of graphite flakes. Therefore, the strength, plasticity, and toughness of gray cast iron are lower than those of other cast iron types. However, its compressive strength is relatively high, typically 3–4 times its tensile strength. Gray cast iron with a pearlite matrix has high hardness and wear resistance.
Gray cast iron has good wear resistance. This is because graphite itself has a lubricating effect, and the cavities left by fallen graphite can absorb and store lubricating oil, endowing the casting with excellent wear resistance. Furthermore, the presence of hard phosphide eutectic in the casting further enhances its wear resistance.
Gray cast iron has a certain degree of corrosion resistance. It can resist corrosion in environments such as weak acids, weak alkalis, water, and certain gases. However, compared with specialized corrosion-resistant materials, the corrosion resistance of gray cast iron is relatively limited.
Gray cast iron has good machinability due to the lubricating and chip-breaking effects of flake graphite on cutting tools. The machinability of gray cast iron itself depends on its matrix structure and hardness: the ferrite matrix offers the best machinability, followed by the pearlite matrix. The presence of free cementite drastically reduces machinability.
Gray cast iron has many grades, with common ones including HT100, HT150, HT200, HT250, HT300, and HT350.
| Grade | Applications |
|---|---|
| HT100 | Used to manufacture simple castings that only bear light loads, such as covers, outer covers, pallets, oil pans, handwheels, hand racks, base plates, handles, ingot molds, blast furnace counterweights in metallurgical equipment, and steelmaking furnace counterweights. |
| HT150 | Capable of withstanding moderate bending stress, it is used to manufacture components such as gearboxes, exhaust pipes, intake pipes of agricultural vehicles, and workbenches—parts that involve relative movement and wear. |
| HT200 | Used to manufacture castings that bear moderate bending stress and have a pressure between friction surfaces greater than 500MPa, such as machine tool workbenches, sliders, bases, automobile gearboxes, intake/exhaust pipes, pump bodies, valve bodies, and valve covers. |
| HT250 | Suitable for manufacturing cast iron platforms, cast iron flat plates, marking flat plates, cast iron workbenches, assembly flat plates, welding flat plates, gears, levels, machine tool beds, deflectometers, and large castings. |
| HT300 | Suitable for manufacturing castings with high bending stress and strict airtightness requirements, such as 3D flexible welding platforms, heavy machine tool beds, gears, cams, crankshafts of large engines, cylinder blocks, high-pressure cylinders, and rolling mill bases. |
| HT350 | Used to manufacture heavy machinery that bears high stress, such as lathes and punch presses. Examples include machine tool bases, steel rolling slide rails, rolls, coking columns, ring gears of cylindrical mixers, and support wheel seats. |
Gray cast iron can undergo various heat treatment processes, mainly including the following:
To eliminate residual stress in castings, stabilize their geometric dimensions, and reduce or eliminate deformation caused by machining, stress relief annealing is required. When determining the stress relief annealing temperature, the chemical composition of the cast iron must be considered. The stress relief annealing temperature for ordinary gray cast iron is usually 550°C, 600°C for low-alloy gray cast iron, and can be increased to 650°C for high-alloy gray cast iron. The heating rate is generally 60–120°C/h. The holding time depends on the heating temperature, casting size, structural complexity, and the required degree of stress relief. The cooling rate for stress relief annealing of castings must be slow, generally controlled at 20–40°C/h. When the temperature drops to below 150–200°C, the castings can be cooled in air and removed from the furnace.
If there is no eutectic cementite in the casting or its content is low, low-temperature graphitization annealing can be performed; if the content of eutectic cementite in the casting is high, high-temperature graphitization annealing is necessary. During low-temperature graphitization annealing, graphitization and granulation of eutectic cementite occur in the cast iron, thereby reducing its hardness and increasing its plasticity. The high-temperature graphitization annealing process involves heating the casting to a temperature above the upper limit of Ac1, decomposing the free cementite in the cast iron into austenite and graphite. After holding for a certain period, the casting is cooled in different ways according to the desired matrix structure.
The purpose of normalizing gray cast iron is to improve the strength, hardness, and wear resistance of castings, or to serve as a preliminary heat treatment for surface quenching to optimize the matrix structure. Generally, normalizing involves heating the casting to 30–50°C above the upper limit of Ac1, transforming the original structure into austenite, holding it at this temperature for a period, and then removing it from the furnace for air cooling. For castings with complex shapes or critical applications, normalizing is performed before residual stress relief.
The quenching process involves heating the casting to 30–50°C above the upper limit of Ac1 (usually 850–900°C) to transform its structure into austenite, holding it at this temperature, and then quenching (usually oil quenching). For tempering, the temperature is generally kept below 550°C to avoid graphitization.
To improve the surface hardness, wear resistance, and fatigue strength of certain castings, surface quenching can be adopted. Both gray cast iron and ductile iron castings can undergo surface quenching. Common methods include high (medium)-frequency induction heating surface quenching and electrical contact surface quenching.
This article explores and analyzes the various characteristics, production processes, and application fields of gray cast iron castings in depth. Through the study of the composition of gray cast iron castings, we have clarified that the contents of elements such as carbon, silicon, and manganese have a significant impact on their performance. For example, appropriately increasing the silicon content can effectively improve the strength and toughness of gray cast iron castings, but excessive silicon will lead to a decrease in casting hardness. In terms of applications, gray cast iron castings are widely used in automobile manufacturing, machinery industry, and other fields due to their good wear resistance, vibration damping properties, and cost advantages.
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