Metal heat treatment is a process in which metal workpieces are heated to a suitable temperature in a certain medium, maintained at this temperature for a certain period of time, and then cooled at different speeds.
Metal heat treatment is one of the important processes in mechanical manufacturing. Compared with other processing processes, heat treatment generally does not change the shape and overall chemical composition of the workpiece, but changes the microstructure inside the workpiece or changes the surface of the workpiece. The chemical composition gives or improves the performance of the workpiece. Its characteristic is to improve the intrinsic 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, In addition to the reasonable selection of materials and various forming processes, heat treatment processes are often essential for physical and chemical properties. Steel is the most widely used material in the machinery industry. The microstructure of steel is complex and can be controlled through heat treatment, so Heat treatment of steel is the main content of metal heat treatment. In addition, aluminum, copper, magnesium, titanium, etc. and their alloys can also change their mechanical, physical and chemical properties through heat treatment to obtain different performance properties.
In the process of progressing 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 770 BC to 222 BC, the Chinese had discovered in production practice that the properties of copper and iron It 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 developed rapidly. Two swords and a halberd unearthed from Yanxiadu, Yixian County, Hebei Province, China, all have martensite in their microstructure, indicating that they have been quenched.
With the development of quenching technology, people gradually discovered the impact of refrigerant on quenching quality. Pu Yuan, a Shu native in the Three Kingdoms, once made 3,000 knives for Zhuge Liang in Xiegu, Shaanxi today. According to legend, he sent people to Chengdu to get water for quenching. This shows that China In ancient times, attention was paid to the cooling capabilities of different water qualities, as well as the cooling capabilities of oil and urine. The sword unearthed in China from the tomb of King Jing in Zhongshan during the Western Han Dynasty (206 BC to 24 AD) had a carbon content of 0.15 in the heart. ~0.4%, but the carbon content on the surface is more than 0.6%, indicating that the carburizing process has been applied. However, as a secret of personal "craft" at that time, it was not allowed to be shared, so the development was very slow.
1863 In 2006, British metallographers and geologists demonstrated six different metallographic structures of steel under a microscope, proving that when steel is heated and cooled, internal structural changes will occur. The phase at high temperature in the steel changes to A harder phase. The allotropy theory of iron established by the Frenchman Osmond and the iron-carbon phase diagram first formulated by the British Austin laid the initial theoretical foundation for modern heat treatment technology. At the same time, people Methods for protecting metals during the heating process of metal heat treatment were also studied to avoid oxidation and decarburization of metals during the heating process.
From 1850 to 1880, for the application of various gases (such as hydrogen, There were a series of patents for protective heating using coal gas, carbon monoxide, etc. From 1889 to 1890, the British man Lake obtained patents for bright heat treatment of various metals.
Since the 20th century, the development of metal physics and other new technologies have Transplantation and application have enabled greater development of the metal heat treatment process. A significant development was the application of rotary drum furnaces for gas carburization in industrial production from 1901 to 1925. In the 1930s, the dew point potentiometer appeared, which increased the carbon potential of the atmosphere in the furnace. After achieving controllability, methods were developed to further control the carbon potential of the furnace atmosphere using carbon dioxide infrared meters, oxygen probes, etc.; in the 1960s, heat treatment technology used the role of plasma fields to develop ion nitriding and carburizing processes; laser, The application of electron beam technology has enabled metals to obtain new surface heat treatment and chemical heat treatment methods.
Bimetal heat treatment process
The heat treatment process generally includes heating, insulation and cooling. process, sometimes there are only two processes: 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 earliest ones used charcoal and coal as heat sources. , and then apply liquid and gaseous fuels. 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.
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When heating, the workpiece is exposed to the air, and oxidation and decarburization often occur (that is, the carbon content on the surface of the steel part is reduced), which has a very adverse impact on the surface properties of the parts after heat treatment. Therefore, metals should usually be placed in a controlled atmosphere or protected Heating in atmosphere, molten salt and vacuum can also be done by coating or packaging methods for protective heating.
Heating temperature is one of the important process parameters of the heat treatment process. Selecting and controlling the heating temperature is the key to ensuring the quality of heat treatment. The main problem is that the heating temperature varies with the metal material being processed and the purpose of the heat treatment, but it is generally heated above the phase transformation temperature to obtain the required structure. In addition, the transformation takes a certain amount of time, so when the surface of the metal workpiece reaches When the required heating temperature is reached, it must be maintained at this temperature for a certain period of time to ensure that the internal and external temperatures are consistent and the microstructure transformation is complete. This period of time is called the holding time. When high-energy density heating and surface heat treatment are used, the heating speed is extremely fast. Generally, There is no holding time or the holding time is very short, and the holding time of chemical heat treatment is often longer.
Cooling is also an indispensable step in the heat treatment process. The cooling method differs from process to process, mainly to control cooling. Speed. Generally, the cooling speed of annealing is the slowest, the cooling speed of normalizing is faster, and the cooling speed of quenching is faster. However, there are different requirements due to different steel types. For example, hollow hard steel can use the same cooling speed as normalizing. Hardening.
Metal heat treatment processes can be roughly divided into overall heat treatment, surface heat treatment, local heat treatment and chemical heat treatment. Each category can be distinguished according to the difference in heating medium, heating temperature and cooling method. It is a number of different heat treatment processes. Using different heat treatment processes for the same metal can obtain different structures and thus have different properties. Steel is the most widely used metal in industry, and the microstructure of steel is also the most complex, so steel heat treatment There are many types of processes.
Integral 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 generally includes annealing, normalizing, quenching and tempering. There are four basic processes of fire.
Annealing is to heat the workpiece to an appropriate temperature, adopt different holding times according to the material and workpiece size, and then slowly cool it. The purpose is to make the internal structure of the metal reach or approach an equilibrium state. Obtain good process performance and usability, or prepare the structure for further quenching. Normalizing is to heat the workpiece to a suitable temperature and then cool it in the air. The effect of normalizing is similar to that of annealing, except that the obtained structure is finer. It is often used for To improve the cutting performance of materials, it is also sometimes used as the final heat treatment for some parts with low requirements.
Quenching is to heat and preserve the workpiece and then quench it in water, oil or other inorganic salts, organic aqueous solutions, etc. Rapid cooling in the medium. After quenching, the steel parts become hard, but at the same time become brittle. In order to reduce the brittleness of the steel parts, the quenched steel parts are kept for a long time at an appropriate temperature higher than room temperature and lower than 710°C. Then cooling, this process is called tempering. Annealing, normalizing, quenching, and tempering are the "four fires" in the overall heat treatment. Among them, quenching and tempering are closely related and are often used together. One is indispensable. < /p>
The "Four Fires" have evolved into 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 some alloys are quenched to form a supersaturated solid solution, they are kept at room temperature or a slightly higher temperature for a longer period of time to improve the hardness, strength or electromagnetic properties of the alloy. This heat treatment process is called aging. Treatment. The method of effectively and closely combining pressure processing deformation and heat treatment to obtain good strength and toughness of the workpiece is called deformation heat treatment; heat treatment in a negative pressure atmosphere or vacuum is called vacuum heat treatment, which not only It can prevent the workpiece from oxidation and decarburization, keep the surface of the workpiece smooth and clean after treatment, and improve the performance of the workpiece. It can also pass in a penetrating agent for chemical heat treatment.
Surface heat treatment only heats the surface of the workpiece to change its surface. Metal heat treatment process based on the mechanical properties of the surface layer. In order to only heat the surface layer of the workpiece without transferring too much heat into the interior of the workpiece, the heat source used must have a high energy density, that is, a large amount of heat energy is given to the workpiece per unit area, making the workpiece The surface or local area can reach high temperatures for a short time or instant. The main methods of surface heat treatment include laser heat treatment, flame quenching and induction heating heat treatment. Commonly used heat sources include oxygen acetylene or oxygen propane and other flames, induced current, laser and electron beam, etc.
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 of the workpiece. Chemical heat treatment is to put the workpiece Heating in a medium (gas, liquid, solid) containing carbon, nitrogen or other alloy elements, and holding it for a long time, so that the surface of the workpiece can be infiltrated with elements such as carbon, nitrogen, boron and chromium. After the elements are infiltrated, other processes are sometimes performed. Heat treatment processes such as quenching and tempering. The main methods of chemical heat treatment include carburizing, nitriding, metallurgy, composite carburizing, etc.
Heat treatment is one of the important processes in the manufacturing process of mechanical parts and tools and molds. Generally speaking, it can ensure and improve various properties of the workpiece, such as wear resistance, corrosion resistance, etc. It can also improve the structure and stress state of the blank to facilitate various cold and hot processing.
For example, after long-term annealing treatment of white cast iron, malleable cast iron can be obtained to improve the plasticity; by adopting the correct heat treatment process for gears, the service life can be doubled or dozens of times longer than that of gears without heat treatment; in addition, cheap carbon steel can be infiltrated into Certain alloy elements have certain properties of expensive alloy steel and can replace some heat-resistant steel and stainless steel; almost all tools and molds need to be heat treated before they can be used.
Classification of Three Steels
p>Steel is an alloy with iron and carbon as its main components. 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 (referred to as carbon steel) according to its chemical composition. Steel) and alloy steel. Carbon steel is an alloy obtained by smelting pig iron. In addition to iron and carbon as its main components, it also contains a small amount of manganese, silicon, sulfur, phosphorus and other impurities. Carbon steel has certain mechanical properties , and has 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 no longer fully meet the needs, so people have developed various Alloy steel. Alloy steel is a multi-component alloy obtained by adding certain elements (called alloying elements) on the basis of carbon steel. Compared with carbon steel, the performance of alloy steel has been significantly improved, so it is increasingly used.
Due to the wide variety of steel materials, steel materials must be classified to facilitate production, storage, selection and research. According to the use, chemical composition and quality of steel, steel can be divided into many categories:
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(1). Classification by use
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 for various machine parts. It includes carburized steel, quenched and tempered steel, spring steel and rolling bearing steel.
(2). Steel used 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 tool It can be divided into cutting tool steel, mold steel and measuring tool steel for different uses.
3. Special performance steel: It is steel with special physical and chemical properties. It can be divided into stainless steel, heat-resistant steel, wear-resistant steel, magnetic steel and so on. Steel, etc.
(2). Classification according to 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 alloy elements The content 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, According to the main alloying elements contained in steel, it can also be divided into manganese steel, chromium steel, chromium-nickel steel, chromium-manganese-titanium steel, etc.
(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 ≤0.045%, sulfur content ≤0.055%; or phosphorus and sulfur content both ≤0.050%) ;High-quality steel (phosphorus and sulfur content, sulfur content ≤0.030%).
In addition, according to the type of smelting furnace, steel is divided into open-hearth steel (acid open-hearth, alkaline open-hearth), air converter Steel (acid converter, alkaline converter, 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 (deoxidation is relatively complete) and semi-killed steel. < /p>
Steel factory
When naming steel products, the three classification methods of use, composition, and quality are often combined. For example, steel is called ordinary carbon structural steel, high-quality carbon structural steel, carbon tool steel, and advanced high-quality carbon tools. Steel, alloy structural steel, alloy tool steel, etc., all ≤0.040%); high-grade high-quality steel (phosphorus content ≤0.035%,
Mechanical properties of four metal materials
Metal materials The performance of metal materials is generally divided into two categories: process performance and use performance. The so-called process performance refers to the performance of metal materials under specified cold and hot processing conditions during the processing and manufacturing of mechanical parts. The quality of the process performance of metal materials, It determines its adaptability to processing and forming during the manufacturing process. Due to different processing conditions, the required process properties are also different, such as casting performance, weldability, forgeability, heat treatment performance, cutting processability, etc. The so-called use performance is It refers to the performance of metal materials under the conditions of use of mechanical parts, which includes mechanical properties, physical properties, chemical properties, etc. The performance of metal materials determines its range of use and service life.
< p>In the machinery manufacturing industry, general mechanical parts are used in normal temperature, normal pressure and non-strongly corrosive media, and during use, each mechanical part will bear different loads. Metal materials are under load The ability to resist damage is called mechanical properties (or mechanical properties). The mechanical properties of metal materials are the main basis for the design and material selection of parts. The nature of the applied load is different (such as tension, compression, torsion, impact, cyclic load etc.), the mechanical properties required for metal materials will also be different. Commonly used mechanical properties include: strength, plasticity, hardness, toughness, multiple impact resistance and fatigue limit, etc. Various mechanical properties will be discussed below.1. Strength
Strength refers to the resistance of metal materials to damage (excessive plastic deformation or fracture) under static load. Due to the action of load, there are tension, compression, bending and shearing. and other forms, so strength is also divided into tensile strength, compressive strength, flexural strength, shear strength, etc. There is often a certain relationship between various strengths. In use, 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 destruction under load.
3. Hardness
Hardness is a measure of the softness and hardness of metal materials. At present, the most commonly used method for measuring hardness in production is the indentation hardness method, which uses an indenter of a certain geometric shape to press into the material under a certain load. The surface of the tested metal material is measured according to the degree of indentation.
Commonly used methods include Brinell hardness (HB), Rockwell hardness (HRA, HRB, HRC) and Vickers hardness ( HV) and other methods.
4. Fatigue
The strength, plasticity, and hardness discussed above are all mechanical performance indicators of metal under static load. In fact, many machine parts They all work under cyclic loads, and parts will fatigue under such conditions.
5. Impact toughness
The load acting on the machine part at a very high speed is called impact load, and the ability of the metal to resist damage under the impact load is called impact toughness.
Five annealing-- Quenching--Tempering
(1). Types of annealing
1. Complete annealing and isothermal annealing
Complete annealing is also called recrystallization annealing, generally referred to as annealing. This kind of annealing is mainly used for castings, forgings and various carbon steels and alloy steels with sub-element analysis components. Hot-rolled profiles are sometimes used for welded structures. They are generally used as the final heat treatment of some unimportant workpieces, or as the pre-heat treatment of some workpieces.
2. Spheroidizing annealing
Spheroidizing annealing is mainly used for carbon steel and alloy tool steels that have been analyzed (such as steel types used in the manufacture of cutting tools, measuring tools, and molds). Its main purpose is to reduce the hardness and improve Machinability and preparation for subsequent quenching.
3. Stress relief annealing
Stress relief annealing is also called low-temperature annealing (or high-temperature tempering). This kind of annealing is mainly used to eliminate residual stress in castings, forgings, welded parts, hot-rolled parts, cold-drawn parts, etc. If these stresses are not eliminated, it will cause the steel parts to deform or crack after a certain period of time or during the subsequent cutting process.
(2). Quenching
The main methods used to improve hardness are heating, heat preservation, and rapid cooling. The most commonly used cooling media are brine, water and oil. It is easy to obtain high hardness and smooth surfaces for workpieces quenched in salt water, which is not easy. It produces soft spots that are not hardened by quenching, but it can easily cause serious deformation of the workpiece and even cracking. The use of oil as quenching medium is only suitable for some alloy steels or small-sized carbon steel workpieces where the stability of supercooled austenite is relatively large. Quenching.
(3). Tempering
1. Reduce brittleness and eliminate or reduce internal stress. After quenching, steel parts will have great internal stress and brittleness. If they are not tempered in time, the steel parts will often deform or even crack.
2. Obtain the required mechanical properties of the workpiece. After quenching, the workpiece has high hardness and high brittleness. In order to meet the different performance requirements of various workpieces, the hardness can be adjusted through appropriate tempering, reducing the brittleness and obtaining the required toughness. Plasticity.
3. Stable 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 gather carbides in the steel and reduce the hardness to facilitate cutting.
Six commonly used methods Selection of furnace type
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, have unequal sizes of workpieces, and have many types, and require versatility and versatility in the process, box-type furnaces can be used.
2. When heating long shafts, long lead screws, pipes and other workpieces, deep well electric furnaces can be used.
3. For small batches of carburized parts, a well-type gas carburizing furnace can be used.
4. For the production of large quantities of automobile, tractor gear and other parts, a continuous carburizing production line or a box-type multi-purpose furnace can be selected.
5. When heating stamping plate blanks for mass production, it is best to use rolling furnaces and roller hearth furnaces.
6. For batches of finalized parts, push rod or conveyor belt resistance furnaces (push rod furnaces or cast belt furnaces) can be used for production
7. For small mechanical parts such as screws, nuts, etc., vibrating bottom furnaces or mesh belt furnaces can be used.
8. The internal spiral rotary tube furnace can be used for heat treatment of steel balls and rollers.
9. Push rod furnaces can be used for mass production of non-ferrous metal ingots, while air circulation heating furnaces can be used for small non-ferrous metal parts and materials.