Abstract: Metal heat treatment is a process in which a metal workpiece is heated to a suitable temperature in a certain medium and kept at a constant temperature for a certain period of time. Metal heat treatment is one of the important processes in mechanical manufacturing. Compared with other processing methods, heat treatment generally does not change the shape of the workpiece.
Metal heat treatment is a process in which a metal workpiece is heated to a suitable temperature in a certain medium and maintained at a constant temperature for a certain period of time.
Metal heat treatment is one of the important processes in mechanical manufacturing. Compared with other processing methods, heat treatment generally does not change the shape of the workpiece and the overall chemical composition, but changes the microstructure inside the workpiece or changes the chemical composition of the surface of the workpiece. , to give or improve the performance of the workpiece. It is characterized by improved internal quality of the workpiece, which is generally not visible to the naked eye.
In order to make the metal workpiece have the required mechanical properties, physical properties and chemical properties, in addition to the rational selection of materials and various forming processes, heat treatment processes are often essential. Steel is the most widely used material in the machinery industry. The microstructure of steel is complex and can be controlled by heat treatment. Therefore, the heat treatment of steel is the main content of metal heat treatment. In addition, aluminum, copper, magnesium, titanium, and the like can also be modified by heat treatment to obtain different mechanical properties, physical properties, and chemical properties.
During the progress from the Stone Age to the Bronze Age and the Iron Age, the role of heat treatment was gradually recognized. As early as 770 BC to 222 BC, Chinese people have discovered in production practice that the performance of copper and iron will change due to the influence of temperature and pressure deformation. The softening treatment of white cast iron is an important process for manufacturing agricultural tools.
In the sixth century BC, steel weapons were gradually adopted. In order to improve the hardness of steel, the quenching process was rapidly developed. Two swords and a scorpion unearthed in Yanxia, ​​Yixian County, Hebei Province, China, have martensite in their microstructure, indicating that they are quenched.
With the development of quenching technology, people gradually found the effect of refrigerant on the quality of quenching. The three-nation monk Pu Yuan once made 3,000 knives for Zhuge Liang in the Shaanxi Diagonal Valley. According to legend, he sent people to Chengdu to take water and quench them. This shows that in ancient China, we noticed the cooling ability of different water quality, and also paid attention to the cooling ability of oil and urine. The Western Han Dynasty (206 BC to 24 AD) unearthed in China, the sword in the tomb of King Jingshan, the carbon content of the heart is 0.15 ~ 0.4%, while the surface carbon content is more than 0.6%, indicating that the carburizing process has been applied. However, at that time, as a secret of personal "craftsmanship", it was not allowed to be rumored, so the development was very slow.
In 1863, British metallographers and geologists showed six different metallographic structures of steel under the microscope, which proved that the steel would undergo internal changes during heating and cooling, and the phase of the steel at high temperatures would change during rapid cooling. It is a harder phase. The theory of homogeneity of iron established by the Frenchman Osmond and the iron-carbon phase diagram first established by the British Austin laid the theoretical foundation for the modern heat treatment process. At the same time, people have also studied the protection of metals during the heating process of metal heat treatment to avoid oxidation and decarburization of metals during heating.
From 1850 to 1880, there were a series of patents for the protection and application of various gases (such as hydrogen, gas, carbon monoxide, etc.). From 1889 to 1890, the British Lec was patented for a variety of metallic bright heat treatments.
Since the twentieth century, the development of metal physics and the application of other new technologies have made the metal heat treatment process more developed. A remarkable development was from 1901 to 1925, in which the converter was used for gas carburizing in industrial production; in the 1930s, a dew point potentiometer appeared, which made the carbon potential of the furnace atmosphere controllable, and later developed a carbon dioxide infrared detector. , oxygen probes and other methods to further control the carbon potential in the furnace; in the 1960s, the heat treatment technology used the role of the plasma field to develop ion nitriding and carburizing processes; the application of laser and electron beam technology, and the new metal Surface heat treatment and chemical heat treatment methods.
Two metal heat treatment process
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. The first use of charcoal and coal as a heat source, and the application of liquid and gaseous fuels. Electrical applications make heating easy to control and environmentally friendly. These heat sources can be used for direct heating, or by indirect heating of molten salts or metals, or floating particles.
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 part) often occur, which has a detrimental effect on the surface properties of the parts after the 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 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. Depending on the heating medium, heating temperature and cooling method, each category can be divided into several different heat treatment processes. The same metal uses different heat treatment processes to obtain different microstructures and thus different properties. Steel is the most widely used metal in the industry, and the steel microstructure is also the most complex, so there are many kinds of steel heat treatment processes.
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 is to heat the workpiece to the appropriate temperature, using different holding time according to the material and the workpiece size, and then slowly cooling, in order to make the internal structure of the metal reach or close to equilibrium, obtain good process performance and performance, or further quench Prepare for organization. Normalizing is to cool the workpiece to a suitable temperature and then cool it in the air. The effect of normalizing is similar to annealing, but the resulting structure is finer, which is often used to improve the cutting performance of materials, and sometimes used for some less demanding parts. As the final heat treatment.
Quenching is the rapid cooling of the workpiece after heating and holding it in a quenching medium such as water, oil or other inorganic salts or organic aqueous solutions. After quenching, the steel becomes hard but becomes brittle at the same time. In order to reduce the brittleness of the steel, the quenched steel is subjected to long-term heat preservation at a suitable temperature higher than room temperature and lower than 710 ° C, and then cooled. This process is called tempering. 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.
The "four fires" evolved different heat treatment processes with different heating temperatures and cooling methods. In order to obtain a certain strength and toughness, the process of combining quenching and high-temperature tempering is called quenching and tempering. After quenching some alloys to form a supersaturated solid solution, it is kept at room temperature or a slightly higher temperature for a longer period of time to increase the hardness, strength or electrical magnetic properties of the alloy. Such a heat treatment process is called aging treatment. An effective and tight combination of pressure processing deformation and heat treatment, the method of obtaining a good strength and toughness of the workpiece is called deformation heat treatment; the heat treatment in a vacuum atmosphere or vacuum is called vacuum heat treatment, which can not only make The workpiece is not oxidized, does not decarburize, keeps the surface of the workpiece smooth after treatment, improves the performance of the workpiece, and can also be subjected to chemical heat treatment by using an infiltrant.
Surface heat treatment is a metal heat treatment process that only heats the surface layer of the workpiece to change its surface mechanical properties. In order to heat only the surface layer of the workpiece without excessive heat being introduced into the interior of the workpiece, the heat source used must have a high energy density, that is, to give a large amount of heat energy to the workpiece per unit area, so that the surface layer or local portion of the workpiece can be short-time or instantaneous. Reach high temperatures. The main methods of surface heat treatment include laser heat treatment, flame quenching and induction heating heat treatment, commonly used heat sources such as aerobic acetylene or oxypropane, induction current, laser and electron beam.
Chemical heat treatment is a metal heat treatment process that changes the chemical composition, structure and properties of the surface of the workpiece. The difference between chemical heat treatment and surface heat treatment is that the latter changes the chemical composition of the surface layer of the workpiece. Chemical heat treatment is to heat the workpiece in a medium (gas, liquid, solid) containing carbon, nitrogen or other alloying elements for a long time, so that the surface of the workpiece penetrates into carbon, nitrogen, boron and chromium. After infiltration of the elements, other heat treatment processes such as quenching and tempering are sometimes performed. The main methods of chemical heat treatment include carburizing, nitriding, metal infiltration, and composite infiltration.
Heat treatment is one of the important processes in the manufacturing process of mechanical parts and tooling. In general, it can guarantee and improve various properties of the workpiece, such as wear resistance and corrosion resistance. It also improves the microstructure and stress state of the blank to facilitate various cold and hot processing.
For example, white cast iron can be obtained by long-term annealing treatment to obtain malleable cast iron, which improves plasticity. The gears adopt the correct heat treatment process, and the service life can be increased by several times or several times than that of the gears without heat treatment. In addition, the inexpensive carbon steel passes through. Some alloying elements have some high-performance alloy steel properties, which can replace some heat-resistant steels and stainless steels; almost all of the tooling tools need to be heat treated before they can be used.
Classification of Sangang
Steel is an alloy containing iron and carbon as its main components, and its carbon content is generally less than 2.11%. Steel is an extremely important metal material in economic construction. Steel is divided into two categories according to its chemical composition: carbon steel (carbon steel for short) and alloy steel. Carbon steel is an alloy obtained from the smelting of pig iron. In addition to iron and carbon as its main components, it also contains a small amount of impurities such as manganese, silicon, sulfur and phosphorus. Carbon steel has certain mechanical properties, good process performance and low price. Therefore, carbon steel has been widely used. However, with the rapid development of modern industry and science and technology, the performance of carbon steel can not fully meet the needs, so people have developed a variety of alloy steel. Alloy steel is a multi-component alloy obtained by preferentially adding certain elements (called alloying elements) on the basis of carbon steel. Compared with carbon steel, the performance of alloy steel is significantly improved, so it is increasingly used.
Due to the wide variety of steel products, in order to facilitate production, storage, selection and research, steel must be classified. Steel can be divided into many categories according to the use, chemical composition and quality of steel:
(One). Classified by purpose
According to the use of steel, it can be divided into three categories: structural steel, tool steel and special performance steel.
1. Structural steel:
(1). Steel used as various machine parts. It includes carburized steel, quenched and tempered steel, spring steel and rolling bearing steel.
(2). Used as steel for engineering structures. It includes A, B, special steel and ordinary low alloy steel in carbon steel.
2. Tool steel: Steel used to make various tools. According to the use of tools, it can be divided into cutting tool steel, die steel and measuring steel.
3. Special performance steel: It is a steel with special physical and chemical properties. Can be divided into stainless steel, heat-resistant steel, wear-resistant steel, magnetic steel and so on.
(two). Classified by chemical composition
According to the chemical composition of steel, it can be divided into two categories: carbon steel and alloy steel.
Carbon steel: According to the carbon content, it can be divided into low carbon steel (carbon content ≤0.25%); medium carbon steel (0.25% <carbon content <0.6%); high carbon steel (carbon content ≥0.6%) .
Alloy steel: According to the alloying element content, it can be divided into low alloy steel (total content of alloying elements ≤ 5%); medium alloy steel (total content of alloying elements = 5% -10%); high alloy steel (total content of alloying elements > 10%). In addition, depending on the type of main alloying elements contained in the steel, it can also be classified into manganese steel, chrome steel, chrome-nickel steel, chromium-manganese-titanium steel, and the like.
(three). Classified by quality
According to the content of harmful impurities such as phosphorus and sulfur in steel, it can be divided into ordinary steel (phosphorus content ≤0.045%, sulfur content ≤0.055%; or phosphorus and sulfur content ≤0.050%); high quality steel (phosphorus and sulfur content sulphur) The amount is ≤0.030%).
In addition, according to the type of smelting furnace, the steel is divided into open hearth steel (acid flat furnace, alkaline open hearth furnace), air converter steel (acid converter, alkaline converter, oxygen top-blown converter steel) and electric furnace steel. According to the degree of deoxidation during smelting, the steel is divided into boiling steel (incomplete deoxidation), killed steel (complete deoxidation) and semi-killed steel.
When steel mills name steel products, they often combine the three classification methods of use, composition and quality. For example, steel is called ordinary carbon structural steel, high-quality carbon structural steel, carbon tool steel, high-quality carbon tool steel, alloy structural steel, alloy tool steel and so on. ≤0.040%); high-quality steel (phosphorus content ≤0.035%,
Mechanical properties of four metal materials
The properties of metallic materials are generally classified into two types: process performance and performance. The so-called process performance refers to the performance of metal materials under the specified cold and hot processing conditions during the manufacturing process. The technical performance of metal materials determines its adaptability to the forming process during the manufacturing process. Due to different processing conditions, the required process properties are different, such as casting properties, weldability, forgeability, heat treatment properties, and machinability. The so-called performance refers to the performance of metal parts under the conditions of use, including mechanical properties, physical properties, chemical properties, and the like. The performance of metal materials determines its range of use and service life.
In the mechanical manufacturing industry, general mechanical parts are used in normal temperature, normal pressure and non-strong corrosive media, and each mechanical part will be subjected to different loads during use. The resistance of metal materials to damage under load is called mechanical properties (or mechanical properties). The mechanical properties of metal materials are the main basis for the design and selection of parts. Different applied loads (such as tensile, compression, torsion, impact, cyclic loading, etc.), the mechanical properties required for metallic materials will also be different. Common mechanical properties include: strength, plasticity, hardness, toughness, multiple impact resistance and fatigue limit. The various mechanical properties are discussed separately below.
1. strength
Strength refers to the property of a metal material against damage (excessive plastic deformation or fracture) under static load. Since the load acts in the form of stretching, compression, bending, shearing, etc., the strength is also divided into tensile strength, compressive strength, flexural strength, shear strength and the like. There is often a certain relationship between various strengths, and tensile strength is generally used as the most basic strength index.
2. Plasticity
Plasticity refers to the ability of a metal material to undergo plastic deformation (permanent deformation) without damage under load.
3. hardness
Hardness is an indicator of the hardness of a metal material. At present, the most commonly used method for measuring hardness in production is the indentation hardness method, which presses the surface of the metal material to be tested under a certain load with a certain geometric indenter, and determines the hardness value according to the degree of being pressed.
Common methods include Brinell hardness (HB), Rockwell hardness (HRA, HRB, HRC) and Vickers hardness (HV).
4. fatigue
The strength, plasticity, and hardness discussed above are all mechanical properties of the metal under static load. In fact, many machine parts work under cyclic loading, under which conditions parts can fatigue.
5. Impact toughness
The load acting on the machine at a large speed is called the impact load, and the ability of the metal to resist damage under the impact load is called impact toughness.
Five annealing - quenching - tempering
(One). Type of annealing
1. Full annealing and isothermal annealing
Complete annealing, also known as recrystallization annealing, is generally referred to as annealing. This annealing is mainly used for casting, forging and hot-rolled profiles of various carbon steels and alloy steels of hypoeutectoid composition, and sometimes for welded structures. It is often used as a final heat treatment for some unimportant workpieces or as a pre-heat treatment for certain workpieces.
2. Spheroidizing annealing
Spheroidizing annealing is mainly used for hypereutectoid carbon steel and alloy tool steel (such as steel cutting tools, measuring tools, and steels used in molds). Its main purpose is to reduce hardness, improve machinability, and prepare for later quenching.
3. Stress relief annealing
Stress relief annealing, also known as low temperature annealing (or high temperature tempering), is mainly used to eliminate residual stresses in castings, forgings, welded parts, hot rolled parts, cold drawn parts, and the like. If these stresses are not eliminated, it will cause deformation or cracking of the steel after a certain period of time or during subsequent cutting operations.
(two). Quenching
In order to improve the hardness, the main form is through heating, heat preservation and rapid cooling. The most common cooling media are brine, water and oil. Brine-quenched workpieces are easy to obtain high hardness and smooth surface, and it is not easy to produce hard spots that are hard to quench, but it is easy to deform the workpiece seriously and even crack. The use of oil as a quenching medium is only suitable for the quenching of some alloy steels or small-sized carbon steel workpieces with relatively high stability of supercooled austenite.
(three). Temper
1. Reduce brittleness, eliminate or reduce internal stress, there is great internal stress and brittleness after quenching steel. If it is not tempered in time, it will cause deformation or even cracking of steel.
2. Obtaining the mechanical properties required for the workpiece, the workpiece has high hardness and high brittleness after quenching. In order to meet the different performance requirements of various workpieces, the hardness can be adjusted by appropriate tempering to reduce the brittleness and obtain the required toughness. Plasticity.
3. Stabilize workpiece size
4. For some alloy steels that are difficult to soften by annealing, high temperature tempering is often used after quenching (or normalizing) to properly aggregate the carbides in the steel and reduce the hardness to facilitate cutting.
Selection of six common furnace types
The furnace type should be determined according to different process requirements and the type of workpiece.
1. For those that cannot be produced in batches, the workpieces are not equal in size, and there are many types, which require versatility in the process.
Multi-purpose, optional box furnace.
2. When heating long shafts and long screws, pipes and other workpieces, deep well type electric furnaces can be used.
3. For small batches of carburized parts, a well gas carburizing furnace can be used.
4. For the production of large quantities of automobiles, tractor gears and other parts, continuous carburizing production line or box type multi-purpose furnace can be selected.
5. For the mass production of stamping sheet blanks, it is best to use a rolling furnace and a roller hearth furnace.
6. For batches of shaped parts, push rod or belt type resistance furnaces (push furnace or cast strip furnace) are available for production.
7. For small mechanical parts such as screws and nuts, a vibrating bottom furnace or a mesh belt furnace can be used.
8. Steel ball and roller heat treatment can be selected from the inner spiral rotary tube furnace.
9. Non-ferrous metal ingots can be used in push-type furnaces in mass production, and air-circulating furnaces can be used for small parts and materials of non-ferrous metals.
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