The essential difference between cast iron and cast steel lies in the difference in chemical composition. In engineering, it is generally considered that the carbon content is higher than 2% as iron, and the lower value is steel. Due to the different components, the microstructure and properties are also different. Generally speaking, the plasticity and toughness of cast steel are better, the elongation, section shrinkage, and impact toughness are good, and the mechanical properties of cast iron are hard and brittle.
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Cast iron
①Grey cast iron. The carbon content is high (2.7% to 4.0%), the carbon mainly exists in the form of flake graphite, and the fracture is gray, which is referred to as gray iron. Low melting point (1145-1250℃), small shrinkage during solidification, compressive strength and hardness close to carbon steel, and good shock absorption. It is used to manufacture structural parts such as machine tool beds, cylinders, and boxes.
② White cast iron. The content of carbon and silicon is low, carbon mainly exists in the form of cementite, and the fracture is silvery white. It shrinks greatly during solidification and is prone to shrinkage cavities and cracks. High hardness and brittleness, can not withstand impact loads. It is mostly used as a blank for malleable iron and for making wear-resistant parts.
③ Malleable cast iron. It is obtained after annealing treatment of white cast iron, and the graphite is distributed in a flocculent shape, referred to as ductile iron. Its organizational properties are uniform, wear-resistant, and have good plasticity and toughness. It is used to manufacture parts with complex shapes and can withstand strong dynamic loads.
④ Ductile iron. The gray cast iron molten iron is obtained by spheroidizing, and the precipitated graphite is spherical, referred to as ductile iron. Compared with ordinary gray cast iron, it has higher strength, better toughness, and plasticity. Used in the manufacture of internal combustion engines, auto parts, and agricultural machinery.
⑤ Vermicular graphite cast iron. The gray cast iron molten iron is obtained by vermicularization, and the precipitated graphite is in the form of worms. The mechanical properties are similar to ductile iron, and the casting performance is between gray cast iron and ductile iron. Parts are used in the manufacture of automobiles.
⑥ Alloy cast iron. Ordinary cast iron is obtained by adding appropriate alloying elements (such as silicon, manganese, phosphorus, nickel, chromium, molybdenum, copper, aluminum, boron, vanadium, tin, etc.). Alloying elements change the matrix structure of cast iron, so that it has corresponding heat resistance, wear resistance, corrosion resistance, low-temperature resistance, or non-magnetic properties. Used in the manufacture of parts for mining, chemical machinery, instruments, meters, etc.
Cast steel
A type of cast alloy. Cast steel is divided into three categories: cast carbon steel, cast low alloy steel, and cast special steel.
①Cast carbon steel.
Cast steel with carbon as the main alloying element and small amounts of other elements. The carbon content is less than 0.2% in cast low carbon steel, the carbon content is 0.2% to 0.5% in cast medium carbon steel, and the carbon content is more than 0.5% in cast high carbon steel. With the increase of carbon content, the strength and hardness of cast carbon steel increase. Cast carbon steel has high strength, plasticity, toughness, and low cost. It is used in heavy machinery to manufacture parts that bear heavy loads, such as rolling mill stands, hydraulic press bases, etc. Parts that are also impacted such as bolsters, side frames, wheels, and couplers.
②Cast low alloy steel.
Cast steel containing alloying elements such as manganese, chromium, and copper. The total amount of alloying elements is generally less than 5%, which has greater impact toughness and better mechanical properties through heat treatment. Casting low alloy steel has better performance than carbon steel, which can reduce the quality of parts and improve the service life.
③ Cast special steel.
Alloy cast steel made to meet special needs has a wide variety, usually containing one or more high-alloying elements to obtain certain special properties.
For example, high manganese steel containing 11% to 14% manganese is resistant to impact wear and is mostly used for wear-resistant parts of mining machinery and construction machinery; various stainless steels with chromium or chromium-nickel as the main alloying elements are used in corrosion or 650 Parts that work under high-temperature conditions above ℃, such as chemical valve bodies, pumps, containers or steam turbine casings of large-capacity power plants.
Although the two are both iron-carbon alloys, due to the different percentages of chemical elements such as carbon, silicon, manganese, phosphorus, and sulfur, they have different metallographic structures after crystallization and show many differences in mechanical properties and process properties.
For example, in the as-cast state, the elongation, section shrinkage, and impact toughness of cast iron are lower than those of cast steel; the compressive strength and shock absorption performance of cast iron is better than those of cast steel; It is suitable for casting thin-walled castings with complex structures; in the bending test, the cast iron is a brittle fracture, and the cast steel is bending deformation. and many more. Therefore, they are respectively suitable for casting parts with different requirements.
How to distinguish between cast iron and cast steel in actual production and life:
- Brightness. The cast steel is bright, the cast iron is dark and gray, and the gray iron in the cast iron is different from the ductile iron, and the ductile iron is brighter than the gray iron.
- Particles. Cast steel is very dense and particles are generally invisible to the naked eye. Both gray iron and ductile iron can see particles, and the gray iron particles are larger.
- Sound. The collision of steel castings is “just”, which is not the same as the sound of iron castings.
- Gas cutting. The surface of the steel casting is rough, and the riser and gate area is large, which must be cleaned by gas cutting. Nodular cast iron gas cutting continuously.
- Toughness. The toughness of cast steel is close to that of steel plate, while the toughness of ductile iron is slightly inferior. Thin-walled parts can reach 20-30 degrees of bending, and gray has no toughness.
- The difference between malleable steel and ductile iron. The hardness, wear-resistance, and tensile strength of ductile iron are far greater than those of malleable steel, and the tensile strength can reach 1000MPa. Ductile iron can be used as various high-strength structural parts such as engine crankshafts and gears. The sound of malleable iron can be distinguished from ductile iron by listening to the sound. The sound of malleable iron is very sharp and short; the sound of ductile iron is loud and the echo is long. Although the two are both iron-carbon alloys, due to the different percentages of chemical elements such as carbon, silicon, manganese, phosphorus, and sulfur, they have different metallographic structures after crystallization, and show many differences in mechanical properties and process properties. . For example, in the casting state, the elongation, reduction of area and impact toughness of cast iron are lower than those of cast steel, and the compressive strength and shock absorption performance of cast iron are better than those of cast steel. The fluidity of gray cast iron is better than that of cast steel, and it is more suitable for casting thin-walled castings with complex structures. In the bending test, the cast iron is a brittle fracture, and the cast steel is bending deformation. and many more. Therefore, they are suitable for casting parts with different requirements.
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