Gray cast iron generally refers to cast iron with high carbon, silicon, and manganese content but low sulfur content. It is formed through the graphitization process when liquid iron is slowly cooled. The graphite is distributed in flakes. It is named after its fracture is dark gray. Gray cast iron refers to cast iron with flake graphite. Its main components are iron, carbon, silicon, manganese, sulfur, and phosphorus. It is the most widely used cast iron, and its output accounts for more than 80% of the total cast iron output. The molten iron with a certain composition is subjected to simple pre-furnace treatment, and after pouring, cast iron with flake graphite is obtained, also known as gray cast iron.
Composition, Structure and Performance Characteristics of Gray Cast Iron
Chemical composition of gray cast iron
The chemical composition of gray iron HT150 is roughly: 3.2% to 3.5% carbon, 1.9% to 2.3% silicon, 0.5% to 0.8% manganese, less than 0.12% sulfur and less than 0.2% phosphorus.
The chemical composition of gray iron HT200 is typically between 3.0% and 3.3% carbon, 1.4% to 1.6% silicon, 0.8% to 1.0% manganese, less than 0.15% sulfur and less than 0.15% phosphorus.
The chemical composition of gray iron HT250 is generally: carbon content is 3.16% to 3.30%, silicon content is 1.79% to 1.93%, manganese content is 0.89% to 1.04%, sulfur content is in the range of 0.094% to 0.125%, and phosphorus content is in the range Between 0.120% and 0.170%.
Gray cast iron structure
The structure of gray cast iron consists of a metal matrix and flake graphite. There are three main forms of metal matrix: ferrite, pearlite, and ferrite + pearlite. According to different matrix structures, gray cast iron can be divided into the following types:
Ferritic gray cast iron: Coarse flake graphite is distributed on the ferrite metal matrix. It has low strength and hardness and good casting performance, and can be used to manufacture castings or thin parts with low requirements.
Ferrite-pearlitic gray cast iron: Flake graphite is distributed on the metal matrix composed of pearlite and ferrite. The graphite flakes are slightly thicker and more numerous. They are easy to control during casting, have good cutting performance, and are widely used. .
Pearlite gray cast iron: Fine and uniform flake graphite is distributed on the pearlite matrix. It has high strength and hardness and is mainly used to manufacture important mechanical parts.
Performance characteristics of gray cast iron
Casting performance characteristics of gray cast iron
Gray iron has excellent casting properties. Its chemical composition is close to the eutectic point, the molten iron has good fluidity, and it can cast very complex parts. Moreover, due to the large specific volume of graphite, the shrinkage of the casting during solidification is reduced, which can simplify the process, reduce the stress of the casting and obtain a dense structure.
Mechanical properties of gray cast iron
The mechanical properties of gray iron are relatively low. Its tensile strength is lower than steel, and its plasticity and toughness are also poor. Graphite is distributed in flakes, the effective bearing area is relatively small, and the graphite tip is prone to stress concentration, so the strength, plasticity and toughness of gray cast iron are lower than other cast irons. However, the compressive strength of gray iron is relatively high, generally 3-4 times its tensile strength. Gray iron with pearlite matrix has high hardness and wear resistance.
Wear resistance characteristics of gray cast iron
Gray iron has better wear resistance. This is because graphite itself has a lubricating effect, and the cavities after the graphite falls can absorb and store lubricating oil, making the castings have good wear resistance. Moreover, the casting contains phosphorus eutectic with high hardness, which can further improve the wear resistance.
Corrosion resistance characteristics of gray cast iron
Gray iron is corrosion resistant to a certain extent. Gray iron can resist corrosion in some weak acid, weak alkali, water and gas environments. However, gray iron’s corrosion resistance is relatively limited compared to some specialized corrosion-resistant materials.
Cutting performance characteristics of gray cast iron
Gray iron has good cutting performance due to the lubrication and chip breaking effects of flake graphite on the tool. The cutting performance of gray iron itself depends on the matrix structure and hardness. Ferrite matrix is the best, followed by pearlite matrix. When free cementite exists, the cutting performance drops sharply.
Gray cast iron grades and applications
There are many grades of gray cast iron, the common ones are HT100, HT150, HT200, HT250, HT300, HT350, etc.
| HT100 | HT100 is used to manufacture simple castings that only bear light loads, such as covers, outer covers, pallets, oil pans, handwheels, hand frames, bottom plates, handles, and steel ingot molds, blast furnace counterweights in metallurgical equipment, steelmaking furnace weights, etc. |
| HT150 | HT150 can withstand moderate bending stress and is used to manufacture gearboxes, exhaust pipes, air intake pipes in agricultural vehicles, as well as parts subject to relative movement and wear, such as workbenches. |
| HT200 | HT200 is used to manufacture castings that bear moderate bending stress and have a pressure between friction surfaces higher than 500MPa, such as machine tool worktables, slides, bases, automobile gearboxes, intake and exhaust pipes, pump bodies, valve bodies, valve covers, etc. |
| HT250 | HT250 is 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, deflection meters, large castings, etc. |
| HT300 | HT300 is suitable for manufacturing castings that are subject to high bending stress and require high air tightness, such as three-dimensional flexible welding platforms, heavy machine tool beds, gears, cams, large engine crankshafts, cylinder blocks, high-pressure cylinders, rolling mill bases, etc. |
| HT350 | HT350 is used in the manufacture of lathes, punch presses and other heavy machinery that are subject to greater stress, such as machine bases, steel rolling slides, rollers, coking columns, cylindrical mixer ring gears, supporting wheel seats, etc. |
Heat treatment of gray cast iron
There are various heat treatment methods for gray cast iron, mainly including the following:
Stress relief annealing
In order to eliminate the residual stress of the casting, stabilize its geometric dimensions, and reduce or eliminate the distortion caused by cutting, the casting needs to be stress-relieved annealed. The chemical composition of the cast iron must be considered when determining the stress relief annealing temperature. The stress relief annealing temperature for ordinary gray cast iron is usually 550°C, for low alloy gray cast iron it is 600°C, and for high alloy gray cast iron it can be increased to 650°C. The heating rate is generally 60°C. ~120℃/h. The holding time depends on the heating temperature, the size and structural complexity of the casting, and the degree of stress relief required. The cooling rate of stress relief annealing for castings must be slow, generally controlled at 20 to 40°C/h, and when cooled to below 200 to 150°C, it can be air-cooled out of the furnace.
Graphitization annealing
If there is no eutectic cementite in the casting or its quantity is small, low-temperature graphitization annealing can be performed; when the quantity of eutectic cementite in the casting is large, high-temperature graphitization annealing must be performed. During low-temperature graphitization annealing, the cast iron will undergo graphitization and granulation of eutectoid cementite, which will reduce the hardness of the cast iron and increase its plasticity. The high-temperature graphitization annealing process is to heat the casting to a temperature higher than the upper limit of Ac1, so that the free cementite in the cast iron decomposes into austenite and graphite. After being kept warm for a period of time, it is cooled in different ways according to the required matrix structure.
Normalizing
The purpose of normalizing gray cast iron is to improve the strength, hardness and wear resistance of castings, or as a preparatory heat treatment for surface quenching to improve the matrix structure. General normalizing is to heat the casting to the upper limit of Ac1 30 ~ 50℃, so that the original structure is transformed into austenite, and then it is released from the furnace and air-cooled after being kept for a period of time; castings with complex shapes or important ones are normalized before internal stress is eliminated. annealing.
Quenching and tempering
The quenching process is to heat the casting to a temperature of Ac1 upper limit + 30 ~ 50℃, usually 850 ~ 900℃, so that the structure is transformed into austenite, and is kept at this temperature, and then quenched, usually using Oil quenching. Tempering In order to avoid graphitization, the tempering temperature should generally be lower than 550°C.
Surface quenching: In order to improve the surface hardness, wear resistance and fatigue strength of certain castings, surface quenching can be used. Both gray cast iron and ductile iron castings can be surface quenched. Generally, high (medium) frequency induction heating surface quenching and electrical contact surface quenching are used.
This article provides an in-depth discussion and analysis of the various characteristics, production processes and application areas of gray iron castings. Through research on the composition of gray iron castings, we have clarified that the content of carbon, silicon, manganese and other elements has a significant impact on its performance. For example, appropriately increasing the silicon content can effectively improve the strength and toughness of gray iron castings, but excessive silicon will lead to a decrease in the hardness of the castings. In the field of application, gray iron castings are widely used in automobile manufacturing, machinery industry and other fields due to their good wear resistance, shock absorption and cost advantages.
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