The heat treatment process generally includes three processes of heating, heat preservation and cooling, and sometimes only two processes of heating and cooling. These processes are connected to each other and cannot be interrupted. Heating is one of the important steps in heat treatment. There are many heating methods for metal heat treatment. When the metal is heated, the workpiece is exposed to the air, and oxidation and decarburization (ie, the carbon content on the surface of the steel parts) often occur, which has a detrimental effect on the surface properties of the parts after heat treatment. Thus the metal should normally be heated in a controlled atmosphere or protective atmosphere, in molten salt and in a vacuum, or protected by coating or packaging methods. Heating temperature is one of the important process parameters of heat treatment process. Selecting and controlling heating temperature is the main problem to ensure the quality of heat treatment. The heating temperature varies depending on the metal material to be treated and the purpose of the heat treatment, but is generally heated above the phase transition temperature to obtain the desired structure. In addition, the transformation takes a certain time, so when the surface of the metal workpiece reaches the required heating temperature, it must be kept at this temperature for a certain period of time, so that the internal and external temperatures are uniform, and the microstructure is completely transformed. This period of time is called the holding time. When high-energy density heating and surface heat treatment are used, the heating rate is extremely fast, generally there is no holding time or the holding time is short, and the heat treatment time of the chemical heat treatment tends to be long. Cooling is also an indispensable step in the heat treatment process. The cooling method varies from process to process, mainly to control the cooling rate. Generally, the annealing rate is the slowest, the normalizing cooling rate is faster, and the quenching cooling rate is faster. However, there are different requirements depending on the type of steel. For example, an empty hard steel can be hardened by a normalizing cooling rate. The overall heat treatment is a metal heat treatment process that heats the workpiece as a whole and then cools it at an appropriate speed to change its overall mechanical properties. The overall heat treatment of steel has four basic processes: annealing, normalizing, quenching and tempering. Annealing→ heating the workpiece to the appropriate temperature, using different holding time according to the material and the workpiece size, and then slowly cooling (the cooling rate is the slowest, generally with the furnace cooling), the purpose is to make the internal metal structure reach or close to equilibrium, get good Process performance and performance, or preparation for further quenching. Common annealing processes are: 2 spheroidizing annealing. It is used to reduce the high hardness of tool steel and bearing steel after forging. The workpiece is heated to a temperature of 20 to 40 ° C above the temperature at which the steel begins to form austenite, and is slowly cooled after the heat preservation. During the cooling process, the lamellar cementite in the pearlite becomes spherical, thereby lowering the hardness. 3 isothermal annealing. It is used to reduce the high hardness of certain alloy structural steels with high nickel and chromium content for cutting. Generally, it is cooled to the most unstable temperature of austenite at a relatively rapid rate, and the austenite is transformed into torsite or sorbite at a suitable temperature for a suitable period of time, and the hardness can be lowered. 4 recrystallization annealing. It is used to eliminate the hardening phenomenon (hardness increase, plasticity drop) of metal wire and sheet during cold drawing and cold rolling. The heating temperature is generally 50 to 150 ° C below the temperature at which the steel begins to form austenite. Only in this way can the work hardening effect be eliminated and the metal softened. 5 graphitization annealing. It is used to make cast iron containing a large amount of cementite into a malleable cast iron with good plasticity. The process operation is to heat the casting to about 950 ° C, and after proper cooling for a certain period of time, the cementite is decomposed to form a group of flocculent graphite. 6 diffusion annealing. It is used to homogenize the chemical composition of alloy castings and improve their performance. The method is to heat the casting to the highest possible temperature without melting, and heat the steel for a long time, and the various elements in the alloy tend to be uniformly distributed and then slowly cooled. 7 stress relief annealing. Used to eliminate internal stresses in steel castings and weldments. After the steel product is heated, the austenite is formed at a temperature of 100 to 200 ° C below, and after cooling, it is cooled in the air to eliminate the internal stress. Annealing In order to eliminate the internal stress of a plastic article or to control the crystallization process, the article is heated to an appropriate temperature for a certain period of time and then slowly cooled. Normalizing→ The workpiece is heated to a suitable temperature and then cooled in the air. The effect of normalizing is similar to annealing, except that the obtained structure is finer and is often used to improve the cutting performance of the material. The main application areas of normalizing are: The difference between normalizing and annealing Normalizing is mainly used for steel workpieces. Normal steel normalizing is similar to annealing, but the cooling rate is slightly larger and the structure is finer. Some steels with a small critical cooling rate (see quenching) can be transformed into martensite by cooling in air. This treatment is not a normalizing property, but is called air quenching. In contrast, some large-section workpieces made of steel with a large critical cooling rate cannot obtain martensite even if quenched in water, and the quenching effect is close to normalizing. The hardness of the steel after normalizing is higher than that of annealing. In normalizing, it is not necessary to cool the workpiece with the furnace like annealing, occupying the furnace for a short time and high production efficiency, so in normal production, normalizing is used instead of annealing. For low carbon steels with a carbon content of less than 0.25%, the hardness achieved after normalizing is moderate, and it is easier to cut than annealing. Generally, normalizing is used for cutting and working. For medium carbon steel with a carbon content of 0.25 to 0.5%, it can meet the requirements of cutting after normalizing. For light-loaded parts made of this type of steel, normalizing can also be used as the final heat treatment. High carbon tool steels and bearing steels are normalized to eliminate the reticulated carbides in the structure and prepare the tissue for spheroidizing annealing. The final heat treatment of ordinary structural parts, because the workpiece has better comprehensive mechanical properties than the annealed state after normalizing, for some ordinary structural parts with little force and low performance requirements, normalizing can be used as the final heat treatment to reduce the process, Save energy and improve production efficiency. In addition, for some large or more complex parts, when quenching has the risk of cracking, normalizing can often replace quenching and tempering as the final heat treatment. Quenching→ Steel quenching is to heat the steel to a temperature above the critical temperature Ac3 (hypoeutectoid steel) or Ac1 (hypereutectoid steel), hold it for a period of time, make it all or part austenitized, and then exceed the critical cooling rate. The cold speed is rapidly cooled to below the Ms (or isothermal near Ms) for a heat treatment process of martensite (or bainite) transformation. The solution treatment of materials such as aluminum alloys, copper alloys, titanium alloys, tempered glass, or heat treatment processes with rapid cooling processes is also referred to as quenching. The hardness of the quenched workpiece affects the quenching effect. The quenched workpiece is generally measured for its HRC value using a Rockwell hardness tester. The HRA value can be determined by the quenched thin hard steel plate and the surface hardened workpiece, and the quenched steel sheet having a thickness of less than 0.8 mm, the shallow surface hardened workpiece and the quenched steel rod having a diameter of less than 5 mm can be changed to the HRC value by a surface Rockwell hardness tester. Quenching is one of the basic means of strengthening steel materials. Martensite in steel is the hardest phase in iron-based solid solution structure, so steel parts can be quenched to obtain high hardness and high strength. However, the brittleness of martensite is very large, and after quenching, there is a large quenching internal stress inside the steel piece, so it is not suitable for direct application and must be tempered. Tempering→ is a heat treatment process in which the workpiece is hardened and heated to a temperature below AC1 (the starting temperature at which the pearlite is transformed to austenite during heating), kept for a certain period of time, and then cooled to room temperature. Tempering is generally followed by quenching, the purpose of which is: According to the tempering temperature range, tempering can be divided into low temperature tempering, medium temperature tempering and high temperature tempering. Tempering of medium temperature tempered workpieces between 350 and 500 °C. Tempering of high temperature tempered workpieces above 500~650 °C. Annealing, normalizing, quenching and tempering are the "four fires" in the overall heat treatment. Among them, the quenching and tempering are closely related, and often used together, they are indispensable. 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The metal heat treatment process can be roughly divided into an overall heat treatment, a surface heat treatment, a local heat treatment, and a chemical heat treatment.
1 fully annealed. It is used to refine the coarse superheated structure of medium and low carbon steel which has poor mechanical properties after casting, forging and welding. The workpiece is heated to a temperature of 30 to 50 ° C above the temperature at which all of the ferrite is transformed into austenite, and is kept for a period of time, and then slowly cooled with the furnace, and the austenite is again transformed during the cooling process, so that the microstructure of the steel is thinned. .
1 For low carbon steel, after normalizing, the hardness is slightly higher than annealing, and the toughness is also good, which can be used as a pretreatment for cutting.
2 For medium carbon steel, it can be used as the final heat treatment instead of quenching and tempering treatment (quenching + high temperature tempering), or as a preliminary treatment before surface hardening by induction heating.
3 Used in tool steel, bearing steel, carburized steel, etc., can reduce or inhibit the formation of reticulated carbides, thus obtaining a good structure required for spheroidizing annealing.
4 For steel castings, it can refine the as-cast structure and improve the cutting performance.
5 For large forgings, it can be used as the final heat treatment to avoid a large tendency to crack during quenching.
6 Used in ductile iron, which improves hardness, strength and wear resistance, such as crankshafts and connecting rods used in the manufacture of automobiles, tractors and diesel engines.
7 Normalizing a pre-eutectoid steel before spheroidizing annealing can eliminate the reticulated secondary cementite to ensure that the cementite is completely granulated during spheroidizing annealing.
After normalizing, the sub-aluminal steel is F+S, the eutectoid steel is S, and the hypereutectoid steel is S+ secondary cementite, which is discontinuous. 
When welding medium carbon steel and some alloy steels, quenching may occur in the heat-affected zone and become hard, which is easy to form cold cracks, which is to be prevented during the welding process.
Since the metal is hard and brittle after quenching, the residual surface stress will cause cold cracking, and tempering can be used as one of the means to eliminate cold cracks without affecting the hardness.
Quenching is suitable for parts with small thickness and diameter. For oversized parts, the quenching depth is not enough, and carburizing has the same problem. In this case, alloys such as chromium should be added to the steel to increase the strength.
(a) Eliminating residual stress generated during quenching of the workpiece to prevent deformation and cracking;
(b) Adjust the hardness, strength, ductility and toughness of the workpiece to meet the performance requirements;
(c) Stabilize the organization and size to ensure accuracy;
(d) Improve and improve processability. Therefore, tempering is the last important step in obtaining the required performance of the workpiece. The desired mechanical properties can be obtained by the combination of quenching and tempering.
Tempering of low temperature tempered workpieces at 150~250 °C.
The purpose is to maintain the high hardness and wear resistance of the quenched workpiece, reduce the quench residual stress and obtain the tempered martensite after brittle tempering, which refers to the structure obtained when the quenched martensite is tempered at low temperature. Mechanical properties: 58 ~ 64HRC, high hardness and wear resistance.
Applications: Mainly used in various types of high carbon steel tools, cutting tools, measuring tools, molds, rolling bearings, carburizing and surface hardened parts.
The aim is to obtain a higher elasticity and yield point with appropriate toughness. After tempering, a tempered troostite is obtained, which means that the ferrite matrix formed during the tempering of the martensite is distributed with a fine phase structure of extremely fine spherical carbides (or cementite).
Mechanical properties: 35 ~ 50HRC, high elastic limit, yield point and certain toughness.
Applications: Mainly used for springs, springs, forging dies, impact tools, etc.
The aim is to obtain comprehensive mechanical properties with good strength, ductility and toughness.
After tempering, tempered sorbite is obtained, which refers to a multiphase structure in which fine spheroidal carbides (including cementite) are distributed in the ferrite matrix formed during martensite tempering.
Mechanical properties: 25 ~ 35HRC, better comprehensive mechanical properties.
Applications: Widely used in a variety of more important structural members such as connecting rods, bolts, gears and shaft parts.