Section 1 Fe-C (Fe-Fe3C) Phase Diagram 1. The meaning of points, lines and surfaces in the phase diagram Because the content of carbon in iron exceeds the solubility, the remaining carbon can be in two forms, namely cementite (Fe3C) and graphite (G). Therefore, there are two kinds of phase diagrams of Fe-C alloy, namely Fe-G phase diagram and Fe-C phase diagram (phase diagram is generally the basic phase: (except liquid phase, there are three basic solid phases in the phase diagram). Ferrite-interstitial solid solution formed by carbon in α-Fe is represented by F or α. The interstitial solid solution formed by carbon in δ-Fe is called δ. Austenite-interstitial solid solution formed by carbon in γ-Fe is indicated by A, and γ is indicated in some data. Cementite-a metal compound formed by iron and carbon, denoted by Fe3C, and denoted by Cm in some data. The meaning of lines and regions (including five single-phase regions): the meaning of lines, three horizontal lines (peritectic transformation line, * * crystal transformation line and * * * precipitation transformation line) 2. Phase diagram analysis 1, peritectic transition (horizontal line HJB) is at point j, temperature is 1495℃, and carbon content is 0. This transformation only occurs in iron-carbon alloys with carbon content of 0.09 ~ 0.53%. 2.* * Crystal transformation (horizontal ECF) At the constant temperature of 1 148℃, the liquid phase with 4.3% carbon content is transformed into austenite and cementite with 2. 1 1% carbon content (carbon content is 6.69%). Its crystal product is a mixture called ledeburite (denoted by Ld). The conversion formula is: L C →A E +Fe3C. In ledeburite, cementite is a continuous phase, and austenite is distributed on cementite in granular form. Because cementite is fragile and ledeburite has poor plasticity, it has no practical value. Iron-carbon alloys with carbon content between 2. 1 1 ~ 6.69% all undergo * * crystal transformation. 3.* * * Precipitation transformation (horizontal PSK) At a constant temperature of 727℃, austenite with a carbon content of 0.77% will undergo * * precipitation transformation and transform into a mixture of ferrite and cementite. The * * * precipitated product is called pearlite, which is expressed by P, and its reaction formula is as follows: As→ FP+Fe3C pearlite is flaky, and the thicker one is F. Total carbon content >; 0.02 18% Fe-C alloy has * * * transformation. PSK line is called A 1 line. 4. Three other important solid-state transformation lines in the phase diagram of Fe-C alloy: ①GS line: the transformation line when austenite begins to precipitate ferrite during cooling, and the transformation finish line when all ferrite dissolves into austenite during heating. This line is usually called Line 3. ②ES line: solubility curve of carbon in austenite. It's called Acm line Below this temperature, Fe3C precipitated from austenite is called secondary cementite, which is denoted as Fe3C Ⅱ to distinguish primary cementite (Fe3C Ⅰ) crystallized directly from liquidus CD. ③PQ line: solubility curve of carbon in ferrite. Below this temperature, Fe3C precipitated from ferrite is called tertiary cementite, which is denoted as Fe3C Ⅲ. Section 2 Heat treatment of steel-the process of heating, insulating and cooling steel to change its structure and meet its performance. Common forms of heat treatment include: annealing, normalizing, quenching, tempering, surface quenching, etc. The reason why steel can be heat treated is that steel can undergo phase transformation in solid state. A 1, A 3 and Acm are called the critical temperature of microstructure transformation of steel during heating or cooling. Annealing of 1. Steel The heat treatment process of heating the steel to an appropriate temperature, holding it for a certain time, and then slowly cooling it to obtain a nearly balanced structure is called annealing. It includes complete annealing, incomplete annealing, spheroidizing annealing, diffusion annealing, stress relieving annealing and recrystallization annealing. The purpose of annealing is to eliminate part of internal stress, refine grains and even microstructure, so as to improve its mechanical properties and prepare for quenching. Reduce the hardness of the material to improve the processability; Eliminate the hardening phenomenon caused by cold working and restore its plasticity. 2. Normalization of steel is a heat treatment process in which the steel is heated to an appropriate temperature above Ac3 (for sub-* * * steel analysis) or Accm (for super-* * * steel analysis) for a certain period of time, so that the steel is completely transformed into austenite, and then the pearlescent structure is obtained by air cooling. Compared with complete annealing, normalizing has the same heating temperature, but the cooling rate of normalizing is faster and the phase transition temperature is lower. After normalizing, the strength, hardness and toughness of steel are higher than those of annealed steel. Third, the quenching of steel The heat treatment process of heating steel to a certain temperature above the critical point Ac3 or Ac 1, keeping the temperature for a certain time, and then cooling at a speed greater than the critical quenching speed to obtain martensite or bainite-based structure is called quenching. After quenching, the strength, hardness and wear resistance of steel are significantly improved. 1, the workpiece with quenching stress often deforms or cracks during quenching, which is mainly due to the existence of quenching stress. Quenching stress can be divided into thermal stress and organizational stress. (1) When a thermal stress workpiece is heated or cooled, the stress caused by inconsistent thermal expansion and contraction due to temperature differences in different parts is called thermal stress. After the workpiece is quenched, the residual stress caused by thermal stress is generally the compressive stress of the surface layer and the tensile stress of the center. Because the thermal stress is caused by the temperature difference on the cross section of the workpiece during rapid cooling, the greater the cooling speed, the greater the temperature difference and the greater the thermal stress. In addition, high quenching temperature, large workpiece size, poor thermal conductivity of materials and large linear expansion coefficient will also increase thermal stress. (2) Organizational Stress When the workpiece is cooled, the internal stress caused by the difference in organizational transformation in different parts due to temperature difference is called organizational stress. The residual stress caused by tissue stress is generally manifested as surface tensile stress and central compressive stress. During quenching, austenite with the smallest specific volume is transformed into martensite with the largest specific volume. In the cooling process, because the surface temperature is lower than the core temperature, martensite transformation occurs first, which leads to compressive stress on the surface. With the decrease of temperature, martensitic transformation also occurs on the secondary surface, resulting in tensile stress on the surface and compressive stress in the center. Microstructure stress is related to the cooling rate, workpiece size, thermal conductivity, yield strength, carbon content and hardenability of steel in the martensite transformation temperature range. 2. The selection of quenching heating temperature should be based on the principle of obtaining uniform and fine austenite grains, so as to obtain fine martensite structure after quenching. Generally, it is 30 ~ 50℃ above the critical point Ac3 (sub-* * * steel precipitation) or Ac 1 (super * * * steel precipitation) of steel. If the heating temperature is too low, more martensite cannot be obtained, but if the heating temperature is too high, austenite grains will be coarse, and coarse martensite will be obtained after quenching, which will reduce the toughness of steel. In order to complete the structural transformation of each part of the workpiece, it is necessary to keep the temperature at quenching heating temperature for a certain period of time. The main factors affecting heating time are heating medium, steel composition, heating temperature, shape and size of workpiece, feeding method and feeding amount. 3. Quenching cooling In order to obtain martensite structure in steel, the quenching cooling rate must be greater than the critical cooling rate. However, if the cooling rate is too high, the internal stress of the workpiece will increase, and the trend of deformation or cracking of the workpiece will become greater. Therefore, it is necessary to reasonably determine the quenching cooling rate in order to obtain martensite structure and reduce the tendency of deformation and cracking. In order to achieve this goal, it is very important to choose a suitable quenching medium. Quenching media include water, saline or alkaline water with different concentrations and various mineral oils. 4. Hardenability of steel The hardenability of steel refers to the ability of steel to obtain martensite when quenching, and its size is expressed by the depth of hardened layer obtained by quenching steel under certain conditions. The deeper the hardened layer, the better its hardenability. Fourth, tempering of steel Most steels are tempered after quenching. Tempering is a heat treatment process in which the quenched workpiece is heated to a temperature below the critical point of phase transformation, kept for a certain time, and then cooled to room temperature. Tempering is essentially a process of decomposition of quenched martensite and precipitation, aggregation and growth of carbides. 1, the necessity of tempering ① obtain the required structure of the workpiece to improve the performance. The quenched structure is M and a small amount of residual A, with high strength and hardness and poor plastic toughness. The performance requirements of actual parts are different. ② Stabilize the workpiece size. The structure after quenching is M and a small amount of residual A, which are not stable structures, and will spontaneously transform into stable structures, thus affecting the shape and size of the workpiece. Tempering can transform the quenched structure into a stable structure, thus ensuring that the shape and size of the workpiece will not change again during use. ③ Eliminate quenching internal stress. If the internal stress is not eliminated by tempering in time, the workpiece will be further deformed and even cracked. 2. The essence of tempering is the decomposition of quenching M and the precipitation, aggregation and growth of carbides. 3. Mechanical properties after tempering With the increase of tempering temperature, the strength and hardness of the material decrease, while the plasticity and toughness increase. 4. Types and applications of tempering ① Microstructure obtained by tempering at low temperature (150 ~ 250℃): M-shaped properties: high strength, high hardness, good wear resistance and certain toughness (reducing quenching internal stress and brittleness) Uses: various high-carbon tools, measuring tools, cold stamping dies, rolling bearings and carburized parts. Hardness after tempering: 58 ~ 64 HRC ② Microstructure obtained by tempering at medium temperature (350 ~ 500℃): T cycle (tempered troostite) Performance characteristics: high yield strength, elastic limit and high toughness. Uses: Heat treatment of various springs and dies. Hardness after tempering: 35 ~ 50 HRC ③ Microstructure obtained by tempering at high temperature (500 ~ 650℃): S-cycle (tempered sorbite) Properties: Good comprehensive mechanical properties Uses: Various mechanical structural parts made by tempering medium carbon steel. Important structural parts of automobiles, tractors and machine tools, such as connecting rods, bolts, shafts and gears. Hardness after tempering: 200 ~ 330 HBW V. Surface Quenching Surface quenching: It is a local quenching method that hardens the surface layer of the workpiece to a certain depth, while the center remains in an unquenched state. Commonly used quenching methods are: flame surface quenching and induction heating surface quenching. 6. Chemical heat treatment Chemical heat treatment: it is a heat treatment process in which the workpiece is heated and insulated in a chemical medium to change the chemical composition and structure of the surface layer, thus changing the performance of the surface layer. At present, carburizing, nitriding and carbonitriding are commonly used.

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