3. Carbon in cast iron and cast steel

Carbon also has a very wonderful feature, that is, it is insoluble in most acidic or alkaline solutions, but has a relatively high solubility in molten liquid iron. At the same time, the solid solubility of carbon in solid iron at normal temperature. Very small. Therefore, when the molten iron is solidified and cooled, the carbon dissolved therein may be precipitated in different forms depending on the specific conditions: it may be a graphite having a soft texture; or a carbide having a high hardness. Graphite and carbide in steel and iron can be in many different forms and in different quantities. In this way, "carbon" can make the performance of cast iron and cast steel ever-changing, it is not too much to say. There are many factors that affect the precipitation of carbon from iron and solid iron, such as carbon content, other alloying elements, cooling conditions, various special methods of iron treatment and heat treatment of castings. An important task for foundry workers is to create appropriate conditions in all aspects to control the precipitation of carbon and to provide the required properties for cast iron or cast steel.

Throughout the ages, many people with lofty ideals in the foundry industry have dedicated their lives to understanding and controlling “carbon” and have studied and explored it with various latest scientific and technological achievements. Looking back at the achievements they have made is really fruitful. Because of this, the long-established foundry industry can continue to absorb new technologies and keep up with the times.

1. Carbon in cast steel

Compared with cast iron, the form of carbon in cast steel is relatively simple. Except for a special type of "graphite steel", it is basically precipitated in the form of carbide, but not in the form of graphite. Graphite steel is a hypereutectoid steel with a relatively high carbon content. After proper heat treatment, a part of the carbon contained is precipitated in the form of graphite, thereby having the properties of cast steel and cast iron. Because the structure contains free graphite, it is a wear-resistant structural material used in the manufacture of crankshafts, stamping dies and other components. In the past 30 years, graphite steel has been used in applications due to advances in the production process and performance of ductile iron and compacted graphite cast iron.

The carbide in the iron-carbon alloy is iron carbide (Fe ₃ C), commonly referred to as "cementite", which is a gap compound with a complex crystal structure and a hardness of about 950 to 1050 HV. The crystal structure of the cementite is shown in Fig. 4. The carbon atoms form an orthorhombic lattice. The angle between the three axes is 90°, and the three lattice constants a = 45. 235 nm, b = 50. 888 nm, c = 67. 431 nm. There are 12 iron atoms and 4 carbon atoms in each unit cell. There are six iron atoms around each carbon atom that make up the octahedron, and each iron atom is shared by two carbon atoms. The axes of the octahedrons are inclined at an angle to each other.

In addition to carbon, cast steel usually contains other alloying elements and unintentionally added elements. Therefore, in addition to Fe3C, steel contains carbides of other elements due to different compositions, such as Mn3C, Cr 3C, Cr 7C3. , Cr 23 C6, Mo2C, MoC, WC, W2C, VC, V4C3, Ti C, NbC, Nb4C3, Zr C, etc.; it is also possible to form various composite carbides such as Fe Mo2C6, Fe4 W2C, Fe21 W2C6, 3C 4 Mo2 C, 3C (Ni,Co) 4C, (Mo,W) 2C pure iron has a melting point of 1538 °C, and solid iron has three homomorphic crystals: from low temperature to 910 °C, it is a body-centered cubic lattice. It is called α-iron; between 910 and 1400 °C, it is a face-centered cubic lattice, called γ-iron; above 1400 °C, it is a body-centered cubic lattice, called δ-iron. The solubility of carbon in body-centered cubic lattices is extremely limited. 01% (质量质量) The solubility in the α-iron is at most 0. 0218% (mass fraction), the solid solution with a low carbon content is called ferrite; the solubility in the δ-iron is at most 0. 09% (mass fraction) This solid solution is called high temperature ferrite.

In iron-carbon alloys, the solid solution of carbon in γ-iron is called austenite, and its solubility varies with temperature. In the eutectic temperature, the maximum value of the solubility is 2.11% (mass fraction), and the carbon content is usually used as the boundary between the cast iron and the cast steel, and the carbon content is 2.11% or more. The content below this value is cast steel. In fact, there are very few cast irons with low carbon content close to this value, and few cast steels that are close to this value. Free cementite can only be seen in the structure of hypereutectoid steel, and the jade steel is usually used in the manufacture of steel castings for engineering and structure. Therefore, it is impossible to have free two in the cast steel structure. Secondary cementite. An important task for foundry workers is to control the shape of the carbon by optimizing the chemical composition and using a suitable heat treatment process to ensure the properties of the steel. The carbon-containing matrix structure has the following three forms.

(1) When the pearlite hypoeutectoid steel cools from the austenite region, the pro-eutectoid ferrite is precipitated from the austenite, and the carbon content of the pearlite is increased, and the carbon content in the austenite is close to the total. After the components are separated, an eutectoid transformation occurs, and a pearlite structure is formed by diffusion of iron atoms and carbon atoms. Under the condition that the austenite composition is relatively uniform, the pearlite obtained by cooling decomposition is usually in the form of a sheet, which is composed of alternating ferrite sheets and cementite sheets. Granular pearlite can also be obtained by appropriate heat treatment. Sheet-like pearlites can be divided into three categories according to their interlayer spacing:

The first type: slow cooling, the coarse pearlite formed by transformation of austenite at a higher temperature (700 ~ 650 °C), the average lamellar spacing > 0.3 μm, usually called pearlite. The second type: the fine pearlite which is formed by the rapid cooling and transformation of austenite at a lower temperature (650 to 600 °C), and the average interlamellar spacing is between 0.1 and 0.3 μm, which can be distinguished under a high power optical microscope. Layer, also known as sorbite. The third category: rapid cooling, ultra-fine pearlite transformed by austenite at a lower temperature (600 ~ 550 ° C), the average sheet spacing <0.1 um, even under high power optical microscope can not distinguish the film In the layer, only the electron microscopy can be used to observe the characteristics of the sheet, also known as the tortite (troostite).

All of the above three belong to the lamellar pearlite structure, and there is no essential difference between the thickness and the thickness. The boundaries between them are also relative. The interlayer spacing is reduced, the tensile strength and hardness of the pearlite are significantly improved, and the elongation is not changed much.

(2) When the austenitic steel of martensite is cooled to a low temperature at a rapid rate, the diffusion of various elements is extremely difficult, and thus local changes in chemical composition do not occur during the transformation. The iron atoms do not diffuse, only the lattice reconstruction of iron occurs, and it is impossible for carbon atoms to precipitate in the form of cementite by diffusion. Thus, a solid solution in which carbon is supersaturated in ferrite is formed, which is generally called martensite, and its main feature is high hardness.

(3) Bainite is a transitional structure between a diffuse pearlite and a non-diffuse martensite. When bainite is formed, the iron atoms do not diffuse, and only the lattice reconstruction of iron occurs. Since the transition temperature is slightly higher, the carbon atoms have a certain diffusion ability, and carbide formation precipitates. The bainite in the steel can be classified into three types: upper bainite, lower bainite, and granular bainite.

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