metal heat treatment is one of the important processes in mechanical manufacturing. Compared with other processing technologies, heat treatment generally does not change the shape and overall chemical composition of the workpiece, but endows or improves the working performance of the workpiece by changing the microstructure inside the workpiece or changing the chemical composition on the surface of the workpiece. Its characteristic is to improve the 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 reasonable selection of materials and various forming processes, heat treatment process is often essential. Steel is the most widely used material in machinery industry, and its microstructure is complex and can be controlled by heat treatment, so the heat treatment of steel is the main content of metal heat treatment. In addition, aluminum, copper, magnesium, titanium and their alloys can also change their mechanical, physical and chemical properties through heat treatment to obtain different performance.
In the process from the Stone Age to the Bronze Age and the Iron Age, the role of heat treatment was gradually recognized by people. As early as 77 BC ~ 222 BC, China people had discovered in their production practice that the properties of copper and iron would change due to the influence of temperature and pressure deformation. The softening treatment of white cast iron is an important process to manufacture farm tools.
in the 6th century BC, steel weapons were gradually adopted. In order to improve the hardness of steel, the quenching process was developed rapidly. Two swords and a halberd unearthed in Yanxiadu, Yixian County, Hebei Province, China, have martensite in their microstructures, indicating that they have been quenched.
with the development of quenching technology, people have gradually discovered the influence of coolant on quenching quality. Pu Yuan, a Shu man from the Three Kingdoms, once made 3, knives for Zhuge Liang in this oblique valley in Shaanxi. According to legend, he sent people to Chengdu to fetch water for quenching. This shows that China paid attention to the cooling capacity of different water quality in ancient times, and also paid attention to the cooling capacity of oil and urine. The sword in zhongshan jing wang's tomb of the Western Han Dynasty (26 BC-24 AD) unearthed in China contains .15-.4% carbon in the heart, but more than .6% carbon in the surface, indicating that carburizing technology has been applied. But at that time, as a personal "craft" secret, it refused to spread it, so it developed slowly.
in p>1863, British metallographers and geologists showed six different metallographic structures of steel under a microscope, which proved that when steel is heated and cooled, the internal structure will change, and the phase at high temperature in steel will change into a harder phase when it is rapidly cooled. The theory of iron isomerism established by Frenchman Osmond and the iron-carbon phase diagram first formulated by Englishman Austin laid a theoretical foundation for modern heat treatment technology. At the same time, people have also studied the protection methods of metal during the heating process of metal heat treatment to avoid oxidation and decarbonization of metal during the heating process.
from 185 to 188, there were a series of patents on the application of various gases (such as hydrogen, gas, carbon monoxide, etc.) for protective heating. From 1889 to 189, Lake, an Englishman, obtained patents for bright heat treatment of various metals.
since the 2th century, with the development of metal physics and the transplantation and application of other new technologies, the metal heat treatment technology has been greatly developed. A remarkable progress was that from 191 to 1925, rotary hearth furnace was used for gas carburization in industrial production; Dew point potentiometer appeared in 193s, which made the carbon potential in the furnace atmosphere controllable. Later, methods to further control the carbon potential in the furnace atmosphere were developed by using carbon dioxide infrared instrument and oxygen probe. In 196s, plasma field was used in heat treatment technology, and ion nitriding and carburizing processes were developed. With the application of laser and electron beam technology, new surface heat treatment and chemical heat treatment methods have been obtained for metals.
process of bimetal heat treatment
generally, the heat treatment process includes three processes of heating, heat preservation and cooling, and sometimes there are only two processes of heating and cooling. These processes are interconnected and uninterrupted.
heating is one of the important steps of heat treatment. There are many heating methods for metal heat treatment. At the earliest, charcoal and coal were used as heat sources, and then liquid and gas fuels were applied. The application of electricity makes heating easy to control and has no environmental pollution. These heat sources can be used for direct heating or indirect heating through molten salt or metal or even floating particles.
when the metal is heated, the workpiece is exposed to air, which often leads to oxidation and decarbonization (that is, the carbon content on the surface of steel parts is reduced), which has a very adverse effect on the surface properties of parts after heat treatment. Therefore, metals should usually be heated in controlled atmosphere or protective atmosphere, molten salt and vacuum, and can also be protected by coating or packaging.
heating temperature is one of the important process parameters of heat treatment process, and the selection and control of heating temperature is the main problem to ensure the quality of heat treatment. The heating temperature varies with the metal material to be treated and the purpose of heat treatment, but it is generally heated above the phase transition temperature to obtain the required 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 time, so that the internal and external temperatures are consistent and the microstructure is completely transformed. This time is called heat preservation time. When high-energy density heating and surface heat treatment are used, the heating speed is extremely fast, and there is generally no or short heat preservation time, while the heat preservation time of chemical heat treatment is often longer.
cooling is also an indispensable step in the heat treatment process, and the cooling method varies from process to process, mainly controlling the cooling speed. Generally, the cooling rate of annealing is the slowest, that of normalizing is faster, and that of quenching is faster. However, there are different requirements due to different steel grades. For example, air-hardened steel can be hardened at the same cooling rate as normalizing.
metal heat treatment processes can be generally divided into overall heat treatment, surface heat treatment, local heat treatment and chemical heat treatment. According to the difference of heating medium, heating temperature and cooling method, each category can be divided into several different heat treatment processes. The same metal can obtain different structures by different heat treatment processes, thus having different properties. Steel is the most widely used metal in industry, and its microstructure is also the most complex, so there are many kinds of heat treatment processes for steel.
integral heat treatment is a metal heat treatment process that heats the whole workpiece and then cools it at an appropriate speed to change its overall mechanical properties. Overall heat treatment of iron and steel has four basic processes: annealing, normalizing, quenching and tempering.
annealing is to heat the workpiece to an appropriate temperature, adopt different holding times according to the material and the size of the workpiece, and then slowly cool it, so as to make the internal structure of the metal reach or approach the equilibrium state, obtain good technological performance and service performance, or prepare the structure for further quenching. Normalization is to heat the workpiece to a suitable temperature and then cool it in the air. The effect of normalization is similar to that of annealing, except that the obtained structure is finer, which is often used to improve the cutting performance of materials and sometimes used as the final heat treatment for some parts with low requirements.
quenching means that the workpiece is heated and insulated, and then rapidly cooled in quenching media such as water, oil or other inorganic salts and organic water solution. After quenching, the steel becomes hard, but at the same time it becomes brittle. In order to reduce the brittleness of steel parts, the quenched steel parts are kept at an appropriate temperature higher than room temperature but lower than 71℃ for a long time, and then cooled. This process is called tempering. Annealing, normalizing, quenching and tempering are the "four fires" in the whole heat treatment, among which quenching and tempering are closely related, and they are often used together and are indispensable.
with the different heating temperature and cooling mode, the "four fires" have evolved different heat treatment processes. In order to obtain certain strength and toughness, the process of combining quenching with high temperature tempering is called quenching and tempering. After some alloys are quenched to form supersaturated solid solution, they are kept at room temperature or a slightly higher temperature for a long time to improve the hardness, strength or electrical magnetism of the alloys. Such a heat treatment process is called aging treatment. The method of combining pressure processing deformation with heat treatment effectively and closely to obtain good strength and toughness of the workpiece is called thermomechanical treatment; The heat treatment in negative pressure atmosphere or vacuum is called vacuum heat treatment, which can not only make the workpiece not oxidized and decarbonized, but also keep the surface of the treated workpiece clean and improve the performance of the workpiece, and can also introduce infiltration agent for chemical heat treatment.
surface heat treatment is a metal heat treatment process that only heats the surface layer of the workpiece to change its mechanical properties. In order to heat only the surface layer of the workpiece without excessive heat transfer into the interior of the workpiece, the heat source used must have high energy density, that is, large heat energy is given to the workpiece per unit area, so that the surface layer or part of the workpiece can reach high temperature in a short time or instantly. The main methods of surface heat treatment include laser heat treatment, flame quenching and induction heating heat treatment, and the commonly used heat sources are oxyacetylene or oxypropane flame, induced current, laser and electron beam.
chemical heat treatment is a metal heat treatment process by changing the chemical composition, microstructure and properties of the surface layer 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 and solid) containing carbon, nitrogen or other alloying elements, and keep the temperature for a long time, so that the surface of the workpiece is infiltrated with carbon, nitrogen, boron and chromium. After the elements are infiltrated, other heat treatment processes such as quenching and tempering are sometimes carried out. The main methods of chemical heat treatment are carburizing, nitriding, metallizing, compound carburizing, etc.
heat treatment is one of the important processes in the manufacturing process of mechanical parts and dies. Generally speaking, it can ensure and improve various properties of the workpiece, such as wear resistance and corrosion resistance. It can also improve the structure and stress state of the blank, so as to facilitate various cold and hot machining.
For example, white cast iron can be annealed for a long time to obtain malleable cast iron and improve its plasticity; Using the correct heat treatment process, the service life of gears can be doubled or dozens of times longer than that of gears without heat treatment; In addition, cheap carbon steel has some properties of expensive alloy steel by infiltrating some alloying elements, which can replace some heat-resistant steels and stainless steels; Almost all tools and dies need heat treatment before they can be used.
Classification of Three Steels
Steel is an alloy with iron and carbon as the 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 carbon steel and alloy steel according to chemical composition. Carbon steel is an alloy obtained by smelting pig iron. Besides iron and carbon, it also contains a small amount of impurities such as manganese, silicon, sulfur and phosphorus. Carbon steel has certain mechanical properties, good technological properties and low price. Therefore, carbon steel has been widely used. However, with the rapid development of modern industry and science and technology, the properties of carbon steel can not fully meet the needs, so people have developed various alloy steels. Alloy steel is a multi-element alloy obtained by purposefully adding some elements (called alloying elements) on the basis of carbon steel. Compared with carbon steel, the properties of alloy steel have been significantly improved, so it is widely used.
due to the variety of steel products, in order to facilitate production, storage, selection and research, steel products must be classified. According to the different uses, chemical composition and quality of steel, steel can be divided into many categories:
(1). Classification by use
Steel 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). Steel used as engineering structure. It includes A, B, special steel and common low alloy steel among carbon steels.
2. Tool steel: steel used to manufacture various tools. According to the different uses of tools, it can be divided into cutting tool steel, die steel and measuring tool steel.
3. steel with special properties: steel with special physical and chemical properties. Can be divided into stainless steel, heat-resistant steel, wear-resistant steel, magnetic steel and so on.
(2). Classification 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 carbon content, it can be divided into low-carbon steel (carbon content ≤ .25%); Medium carbon steel (.25% < carbon content < .6%); High carbon steel (carbon content ≥.6%).
alloy steel: according to the content of alloying elements, it can be divided into low alloy steel (the total content of alloying elements is ≤ 5%); Medium alloy steel (total content of alloying elements = 5%-1%); High alloy steel (total content of alloying elements > 1%). In addition, according to the different types of main alloying elements contained in steel, it can also be divided into manganese steel, chromium steel, chromium-nickel steel, chromium-manganese-titanium steel and so on.
(3). Classification by quality
According to the content of harmful impurities phosphorus and sulfur in steel, it can be divided into ordinary steel (phosphorus content ≤.45%, sulfur content ≤ .55%; Or both phosphorus and sulfur content are ≤ .5%); High quality steel (phosphorus and sulfur content ≤.3%).
in addition, according to the types of smelting furnaces, steel can be divided into open hearth steel (acid open hearth furnace and alkaline open hearth furnace), air converter steel (acid converter, alkaline converter and oxygen top-blown converter steel) and electric furnace steel. According to the degree of deoxidation during smelting, steel is divided into boiling steel (incomplete deoxidation), killed steel (complete deoxidation) and semi-killed steel.
when naming steel products, steel mills 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, advanced high-quality carbon tool steel, alloy structural steel and alloy tool steel. ≤ .4%); High-quality steel (phosphorus content ≤.35%,
mechanical properties of metal materials
The properties of metal materials are generally divided into two categories: process performance and service performance. The so-called process performance refers to the performance of metal materials under the given cold and hot working conditions during the processing and manufacturing of mechanical parts. The technological performance of metal material determines its adaptability in the manufacturing process. Due to different processing conditions, the required technological properties are different, such as casting performance, weldability, malleability, heat treatment performance, machinability and so on. The so-called serviceability refers to the performance of metal materials under the use conditions of mechanical parts, which includes mechanical properties, physical properties, chemical properties and so on. The performance of metal material determines its application range and service life.
in the mechanical manufacturing industry, general mechanical parts are used in normal temperature, normal pressure and non-strongly corrosive media, and each mechanical part will bear 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 properties of external loads (such as tension, compression, torsion, impact, cyclic load, etc.) will require different mechanical properties of metal materials. Commonly used mechanical properties include: strength, plasticity, hardness, toughness, multiple impact resistance and fatigue limit. Various mechanical properties will be discussed separately below.
1. strength
strength refers to the resistance of metal materials to failure (excessive plastic deformation or fracture) under static load. Strength is also divided into tensile strength because the load acts in the forms of tension, compression, bending and shearing.