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What is metal heat treatment

What is metal heat treatment?

Metal heat treatment is a process in which the metal workpiece is heated to a suitable temperature in a certain medium, maintained at this temperature for a certain 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 endows or improves the service performance of the workpiece by changing the microstructure inside the workpiece or 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, physical and chemical properties, in addition to the reasonable selection of materials and various forming processes, heat treatment process is often essential. Steel is the most widely used material in the mechanical 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 their alloys can also change their mechanical, physical and chemical properties through heat treatment to obtain different service properties.

20210909095201 21333 - What is metal heat treatment

Introduction of metal heat treatment process

From the stone age to the bronze age and iron age, the role of heat treatment has been gradually recognized. As early as 770-222 BC, Chinese people have found in production practice that the properties 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 developed rapidly. Two swords and a halberd unearthed in yanxiadu, Yi County, Hebei Province, China have martensite in their microstructure, indicating that they have been quenched.
With the development of quenching technology, people gradually find the influence of Quenchant on quenching quality. Pu Yuan, a Shu man of the Three Kingdoms, once made 3000 knives for Zhuge Liang in today’s xiegu, Shaanxi. It is said that he sent someone to Chengdu to get water for quenching. This shows that China paid attention to the cooling capacity of different water quality in ancient times, as well as the cooling capacity of oil and urine. The carbon content in the heart of the sword unearthed in the tomb of King Jing in Zhongshan in the Western Han Dynasty (206 BC ~ 24 AD) is 0.15 ~ 0.4%, but the carbon content on the surface is more than 0.6%, indicating that the carburizing process has been applied. But at that time, the secret of personal “craft” refused to be spread, so its development was very slow.
In 1863, British metallographists and geologists showed six different metallographic structures of steel under the microscope, which proved that the internal structure of steel would change during heating and cooling, and the phase at medium and high temperature would change into a harder phase during quenching. The isomorphism theory of iron established by French Osmond and the iron carbon phase diagram first formulated by British Austin have preliminarily laid a theoretical foundation for modern heat treatment technology. At the same time, people also studied the metal protection methods in the heating process of metal heat treatment to avoid metal oxidation and decarburization in the heating process.
From 1850 to 1880, there were a series of patents for the application of various gases (such as hydrogen, gas, carbon monoxide, etc.) for protective heating. From 1889 to 1890, British Lake obtained the patent for bright heat treatment of various metals.
Since the 20th century, the development of metal physics and the transplantation and application of other new technologies have led to the greater development of metal heat treatment process. A remarkable development was the application of rotary hearth furnace for Gas Carburization in industrial production from 1901 to 1925; The dew point potential difference meter appeared in the 1930s to make the carbon potential of the atmosphere in the furnace controllable. Later, the methods of further controlling the carbon potential of the atmosphere in the furnace with carbon dioxide infrared instrument and oxygen probe were developed; In the 1960s, 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 metal heat treatment

Heat treatment process generally includes three processes: heating, heat preservation and cooling. Sometimes there are only two processes: heating and cooling. These processes are interconnected and uninterrupted.
Heating is one of the important processes of heat treatment. There are many heating methods for metal heat treatment. Charcoal and coal were first used as heat sources, and then liquid and gas fuels were used. The application of electricity makes heating easy to control and 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 the air, and oxidation and decarburization often occur (that is, the carbon content on the surface of steel parts is reduced), which has a very adverse impact 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, or protected by coating or packaging.
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 with the treated metal materials and the purpose of heat treatment, but it is generally heated above the phase transformation temperature to obtain high-temperature microstructure. In addition, the transformation takes a certain time. Therefore, when the metal workpiece surface reaches the required heating temperature, it must be maintained at this temperature for a certain time to make the internal and external temperatures consistent and complete the transformation of microstructure. This period of time is called holding time. When high-energy density heating and surface heat treatment are adopted, the heating speed is very fast, and generally there is no holding time, while the holding time of chemical heat treatment is often longer.
Cooling is also an indispensable step in the process of heat treatment. The cooling method varies with different processes, mainly controlling the cooling rate. Generally, the cooling speed 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 types. For example, air hardening steel can be hardened at the same cooling rate as normalizing.
Metal heat treatment process can be divided into three categories: overall heat treatment, surface heat treatment and chemical heat treatment. According to the different heating medium, heating temperature and cooling method, each category can be divided into several different heat treatment processes. Different structures can be obtained by different heat treatment processes for the same metal, so it has different properties. Steel is the most widely used metal in industry, and the microstructure of steel is also the most complex, so there are many kinds of steel heat treatment 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. There are four basic processes for overall heat treatment of iron and steel: annealing, normalizing, quenching and tempering.
Annealing is to heat the workpiece to an appropriate temperature, adopt different holding time according to the material and workpiece size, and then cool it slowly. The purpose is to make the internal structure of the metal reach or close to the equilibrium state, obtain good process performance and service performance, 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 annealing, but the obtained structure is finer. It is often used to improve the cutting performance of materials, and sometimes used for some parts with low requirements as the final heat treatment.
Quenching is the rapid cooling of the workpiece in water, oil or other inorganic salts, organic aqueous solutions and other quenching media after heating and insulation. After quenching, the steel becomes hard but brittle at the same time. In order to reduce the brittleness of steel parts, the quenched steel parts are kept warm for a long time at an appropriate temperature higher than room temperature and lower than 650 ℃, and then cooled. 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 often used together.
“Four fires” evolve different heat treatment processes with different heating temperatures and cooling methods. In order to obtain 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 supersaturated solid solutions, they are kept at room temperature or slightly higher appropriate temperature for a long time to improve the hardness, strength or electrical magnetism of the alloys. Such heat treatment process is called aging treatment.
The method of effectively and closely combining pressure machining deformation and heat treatment to make the workpiece obtain a good combination of strength and toughness is called thermomechanical treatment; The heat treatment in negative pressure atmosphere or vacuum is called vacuum heat treatment. It can not only prevent the workpiece from oxidation and decarburization, maintain the surface of the workpiece after treatment and improve the performance of the workpiece, but also carry out chemical heat treatment with infiltrating agent.
Surface heat treatment is a metal heat treatment process that only heats the surface of the workpiece to change its mechanical properties. In order to heat only the surface layer of the workpiece and not transfer too much heat 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 instantaneously. The main methods of surface heat treatment include flame quenching and induction heating heat treatment. The common heat sources include oxyacetylene or oxypropane flame, induced current, laser and electron beam.
Chemical heat treatment is a metal heat treatment process by changing the surface chemical composition, microstructure and properties 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 heat the workpiece in the medium (gas, liquid and solid) containing carbon, nitrogen or other alloy elements for a long time, so as to infiltrate carbon, nitrogen, boron, chromium and other elements into the surface of the workpiece. After infiltration of elements, other heat treatment processes, such as quenching and tempering, are sometimes carried out. The main methods of chemical heat treatment are carburizing, nitriding and metalizing.
Heat treatment is one of the important processes in the manufacturing process of mechanical parts and tools and dies. Generally speaking, it can ensure and improve various properties of the workpiece, such as wear resistance, corrosion resistance and so on. It can also improve the microstructure and stress state of the blank, so as to facilitate all kinds of cold and hot processing.
For example, malleable cast iron can be obtained by annealing white cast iron for a long time to improve plasticity; With the correct heat treatment process, the service life of the gear can be doubled or dozens of times longer than that of the gear without heat treatment; In addition, cheap carbon steel has some expensive alloy steel properties by infiltrating some alloy elements, which can replace some heat-resistant steel and stainless steel; Almost all tools and dies need heat treatment before they can be used.

What is steel

Steel is a general term for iron carbon alloys with carbon content ranging from 0.02% to 2.11%. The chemical composition of steel can vary greatly. Steel containing only carbon element is called carbon steel (carbon steel) or ordinary steel; In actual production, steel often contains different alloy elements according to different uses, such as manganese, nickel, vanadium and so on. The application and research of steel has a long history, but until the invention of bainite steelmaking method in the 19th century, the preparation of steel was a work with high cost and low efficiency. Nowadays, with its low price and reliable performance, steel has become one of the most used materials in the world. It is an indispensable component in construction, manufacturing and people’s daily life. It can be said that steel is the material basis of modern society.

Classification of steel

Steel is an alloy with iron and carbon as the main components, and its carbon content is generally less than 2.11%. Steel is a very important metal material in economic construction.
According to chemical composition, steel is divided into carbon steel (carbon steel for short) and alloy steel. Carbon steel is an alloy obtained from pig iron smelting. 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, good process properties and low price. Therefore, carbon steel has been widely used. However, with the rapid development of modern industry, 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 alloy elements) on the basis of carbon steel. Compared with carbon steel, the properties of alloy steel are significantly improved, so it is widely used.
Due to the wide variety of steel, in order to facilitate production, storage, selection and research, steel must be classified. Steel can be divided into many categories according to the purpose, chemical composition and quality of steel:

Classification by use

According to the purpose of steel, it can be divided into three categories: structural steel, tool steel and special performance steel.
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 ordinary low alloy steel in carbon steel.
  • Tool steel: steel used to make various tools. According to different uses of tools, they can be divided into cutting tool steel, die steel and measuring tool steel.

Special performance steel: it is a steel with special physical and chemical properties. It can be divided into stainless steel, heat-resistant steel, wear-resistant steel, magnetic steel, etc.

Classification by chemical composition

According to the chemical composition of steel, it can be divided into carbon steel and alloy steel.
Carbon steel: according to 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 content of alloy elements, it can be divided into low alloy steel (total content of alloy elements ≤ 5%); Medium alloy steel (total alloy element content = 5% – 10%); High alloy steel (total alloy element content > 10%). In addition, according to the different types of main alloy elements contained in steel, it can also be divided into manganese steel, chromium steel, chromium nickel steel, chromium manganese titanium steel, etc.

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 ≤ 0.050%); High quality steel (phosphorus and sulfur content ≤ 0.040%); High quality steel (phosphorus content ≤ 0.035%, sulfur content ≤ 0.030%).
In addition, according to the type of smelting furnace, the steel is 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, the steel is divided into boiling steel (incomplete deoxidation), killed steel (relatively 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, etc.

Mechanical properties of metal materials

The properties of metal materials are generally divided into process properties and service properties. The so-called process performance refers to the performance of metal materials under the set cold and hot working conditions in the processing and manufacturing process of mechanical parts. The process performance of metal materials determines its adaptability to processing and forming in the manufacturing process. Due to different processing conditions, the required process properties are also different, such as casting properties, weldability, malleability, heat treatment properties, machinability and so on. The so-called service performance refers to the performance of metal materials under the service conditions of mechanical parts, including mechanical properties, physical properties, chemical properties and so on. The service performance of metal materials determines its service range and service life.
In the machinery 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 in the process of use. The resistance of metal materials to failure under load 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 mechanical properties of metal materials will be different with different properties of applied load (such as tension, compression, torsion, impact, cyclic load, etc.). Common mechanical properties include strength, plasticity, hardness, impact toughness, multiple impact resistance and fatigue limit. Various mechanical properties will be discussed below.
1. Strength
Strength refers to the resistance of metal materials to failure (excessive plastic deformation or fracture) under static load. Since the action modes of load include tension, compression, bending and shear, the strength is also divided into tensile strength, compressive strength, flexural strength and shear strength. There is often a certain connection between various strengths, and tensile strength is generally used as the most basic strength index.
2. Plasticity
Plasticity refers to the ability of metal materials to produce plastic deformation (permanent deformation) without damage under load.
3. Hardness
Hardness is an index to measure the hardness and softness of metal materials. At present, the most commonly used method for measuring hardness in production is the indentation hardness method. It uses an indenter with a certain geometry to press into the surface of the tested metal material under a certain load, and determines its hardness value according to the degree of indentation.
The commonly used methods include Brinell hardness (HB), Rockwell hardness (HRA, HRB, HRC) and Vickers hardness (HV).
4. Fatigue
The strength, plasticity and hardness discussed above are the mechanical properties of metals under static load. In fact, many machine parts work under cyclic load, under which fatigue will occur.
5. Impact toughness
The load acting on the part at a great speed is called impact load, and the ability of metal to resist failure under impact load is called impact toughness.

Annealing – quenching – tempering

Type of annealing

1. Complete annealing and isothermal annealing
Complete annealing and weighing crystallization annealing are generally referred to as annealing. This annealing is mainly used for casting, forging and hot rolled profiles of various carbon and alloy steels with sub eutectoid composition, and sometimes for welded structures. Generally, it is often used as the final heat treatment of some non heavy parts, or as the pre heat treatment of some workpieces.
2. Spheroidizing annealing
Spheroidizing annealing is mainly used for hypereutectoid carbon steel and alloy tool steel (such as steel used for manufacturing cutting tools, measuring tools and molds). Its main purpose is to reduce hardness, improve machinability and prepare for future quenching.
3. Stress relief annealing
Stress relief annealing, also known as low temperature annealing (or high temperature tempering), is mainly used to eliminate the residual stress of castings, forgings, weldments, hot rolled parts, cold drawn parts, etc. If these stresses are not eliminated, deformation or cracks will occur in the steel after a certain time or in the subsequent machining process.

During quenching

The most commonly used cooling media are brine, water and oil. The workpiece quenched by brine is easy to obtain high hardness and smooth surface, and it is not easy to produce soft spots that cannot be hardened, but it is easy to cause serious deformation and even cracking of the workpiece. Using oil as quenching medium is only suitable for quenching of some alloy steel or small-size carbon steel workpieces with high stability of undercooled austenite.

Purpose of steel tempering

  • 1. Reduce brittleness and eliminate or reduce internal stress. Steel parts have great internal stress and brittleness after quenching. If they are not tempered in time, they will often deform or even crack.
  • 2. Obtain the mechanical properties required by the workpiece. After quenching, the workpiece has high hardness and high brittleness. In order to meet the requirements of different properties of various workpieces, the hardness can be adjusted through appropriate tempering to reduce brittleness and obtain the required toughness and plasticity.
  • 3. Stabilize workpiece size
  • 4. For some alloy steels that are difficult to soften after annealing, high temperature tempering is often used after quenching (or normalizing) to properly aggregate carbides in the steel and reduce the hardness for cutting.

Temper brittleness

When tempering quenched steel, with the increase of tempering temperature, its strength and hardness usually decrease, while its plasticity and toughness increase. However, when tempering in some temperature range, the impact toughness of steel is not improved, but significantly reduced. This embrittlement phenomenon is called tempering embrittlement.
Therefore, tempering is generally not carried out at 250-350 degrees, which is because tempering brittleness occurs when quenching steel is tempered within this temperature range. This temper brittleness is called low temperature temper brittleness or the first type temper brittleness.
The cause of low temperature temper brittleness is not very clear at present. It is generally believed that the carbide precipitates along the interface of martensite sheet or martensite strip in the form of intermittent flakes. The combination between the hard and brittle sheet carbide and martensite is weak, which reduces the strength at the martensite grain boundary, so the impact toughness decreases.

Normalizing

The steel is heated above the critical point (AC3, ACCM), fully austenitized, and then cooled in air. This heat treatment process is called normalizing.
(1) Normalizing process
The heating temperature of normalizing is 50-100 ℃ above the positive chemical composition AC3; The heating temperature of hypereutectoid steel is 30-50 ℃ above ACCM. The holding time mainly depends on the effective thickness of the workpiece and the type of heating furnace. For example, when heating in a box furnace, it can be calculated by holding for one minute per mm of effective thickness. Cooling after heat preservation can generally be cooled in the air, but sometimes some large workpieces or in high temperatures in summer are also cooled by blowing or spray cooling.
(2) Microstructure and properties after normalizing
Normalizing is essentially a special case of annealing. The main difference between the two is that the cooling rate is faster and the undercooling degree is faster, so the pseudo eutectoid transformation occurs, which increases the amount of pearlescent in the tissue and reduces the lamellar spacing of pearlescent. It should be pointed out that some high alloy steels can obtain bainite or martensite structure after air cooling, because the supercooled austenite of high alloy steel is very stable and the C curve is very stable.
Due to the structural characteristics after normalizing, the strength, hardness and toughness after normalizing are higher than those after annealing, and the plasticity is not reduced.

Application of normalizing

Compared with annealing, normalizing steel has high mechanical properties, simple price increase, short production cycle and less energy consumption. Therefore, normalizing treatment should be given priority under possible conditions. Current applications are as follows:

  • 1. Final heat treatment as common structural parts
  • 2. Improve the machinability of low carbon steel and low carbon alloy steel
  • 3. Pre heat treatment of important parts made of medium carbon structural steel.
  • 4. Eliminate the apoplectic secondary cementite of hypereutectoid steel and prepare for spheroidizing annealing.
  • 5. For some large or complex parts, quenching may have the risk of cracking, and normalizing often replaces quenching and tempering as the final heat treatment of such parts. Very right. At this time, it can not be called normalizing, but air quenching. In order to increase the hardness of low carbon steel, the normalizing temperature can be appropriately increased.

Selection of common furnace types

The furnace type shall be determined according to different process requirements and the type of workpiece.

  • 1. For those that cannot be produced in batch, with unequal workpiece sizes and many types, and require universality and versatility in technology, box furnace can be selected.
  • 2. When heating long shafts, long screw rods, pipes and other workpieces, deep well electric furnace can be selected.
  • 3. Well type gas carburizing furnace can be selected for small quantities of carburized parts.
  • 4. For the production of large quantities of automobile, tractor gear and other parts, continuous carburizing production line or box type multipurpose furnace can be selected.
  • 5. Rolling furnace and roller hearth furnace are the best choice for the heating of sheet blank of stamping parts in mass production.
  • 6. For batch shaped parts, push rod type or conveyor belt type resistance furnace (push rod furnace or casting belt furnace) can be selected in production
  • 7. Vibrating bottom furnace or mesh belt furnace can be selected for small mechanical parts such as screws and nuts.
  • 8. Internal spiral rotary tube furnace can be used for heat treatment of steel ball and roller.
  • 9. Push rod furnace can be used for mass production of nonferrous metal ingots, while air circulation heating furnace can be used for small nonferrous metal parts and materials.

Heating defects and control

Overheating phenomenon

We know that overheating during heat treatment is most likely to lead to the coarseness of austenite grains and the decline of mechanical properties of parts.

  • 1. General overheating: overheating occurs when the heating temperature is too high or the holding time at high temperature is too long, resulting in austenite grain coarsening. The coarse austenite grain will reduce the strength and toughness of the steel, increase the brittle transition temperature and increase the tendency of deformation and cracking during quenching. The cause of overheating is that the furnace temperature instrument is out of control or mixed (often due to ignorance of the process). After annealing, normalizing or multiple high temperature tempering, the superheated structure can be re austenitized under normal conditions to refine the grain.
  • 2. Fracture inheritance: for steel with overheated structure, although austenite grain can be refined after reheating and quenching, sometimes coarse granular fracture still appears. There are many disputes about the theory of fracture inheritance. It is generally believed that impurities such as MNS have been dissolved into austenite and enriched at the crystal interface due to too high heating temperature, and these inclusions will precipitate along the crystal interface during cooling, which is easy to fracture along the coarse austenite grain boundary when impacted.
  • 3. Heredity of coarse structure: when the steel parts with coarse martensite, bainite and widmanstatten structure are re austenitized, the austenite grains are still coarse when they are slowly heated to the conventional quenching temperature or even lower. This phenomenon is called structure heredity. To eliminate the heredity of coarse structure, intermediate annealing or multiple high temperature tempering can be used.

Over burning phenomenon

When the heating temperature is too high, not only the austenite grain is coarse, but also the grain boundary is oxidized or melted locally, resulting in the weakening of the grain boundary, which is called overburning. The properties of steel deteriorate seriously after overburning, and cracks are formed during quenching. The burned tissue cannot be recovered and can only be scrapped. Therefore, over burning should be avoided in work.

Decarburization and oxidation

When the steel is heated, the surface carbon reacts with oxygen, hydrogen, carbon dioxide and water vapor in the medium (or atmosphere), reducing the surface carbon concentration, which is called decarburization. After quenching, the surface hardness, fatigue strength and wear resistance of decarburized steel are reduced, and residual tensile stress is formed on the surface, which is easy to form surface network cracks.
When heated, the iron and alloy on the steel surface react with elements and oxygen, carbon dioxide and steam in the medium (or atmosphere) to form oxide film, which is called oxidation. The dimensional accuracy and surface brightness of high temperature (generally above 570 ℃) workpiece deteriorate after oxidation, and the steel parts with poor hardenability of oxide film are prone to quenching soft spots.
Measures to prevent oxidation and reduce decarburization include: coating on workpiece surface, sealing and heating with stainless steel foil, heating with salt bath furnace, heating with protective atmosphere (such as purified inert gas, controlling carbon potential in furnace), flame combustion furnace (reducing furnace gas)

Hydrogen embrittlement

Hydrogen embrittlement is the phenomenon that the plasticity and toughness of high strength steel decrease when heated in hydrogen rich atmosphere. The workpiece with hydrogen embrittlement can also eliminate hydrogen embrittlement through hydrogen removal treatment (such as tempering and aging). Hydrogen embrittlement can be avoided by heating in vacuum, low hydrogen atmosphere or inert atmosphere.
Of course, in practical work, some people use this phenomenon to serve people (such as alloy crushing)

Several common heat treatment concepts

  • 1. Normalizing: heat the steel or steel parts to the appropriate temperature above the critical point AC3 or ACM for a certain time and then cool them in the air to obtain the heat treatment process of pearlite structure.
  • 2. Annealing: heat the hypoeutectoid steel workpiece to 20-40 degrees above AC3. After holding for a period of time, slowly cool it with the furnace (or bury it in sand or lime) to less than 500 degrees and cool it in air
  • 3. Solution heat treatment: heat the alloy to the high-temperature single-phase zone, keep the constant temperature, fully dissolve the excess phase into the solid solution, and then cool quickly to obtain the heat treatment process of supersaturated solid solution
  • 4. Aging: after solution heat treatment or cold plastic deformation, when the alloy is placed at or slightly higher than room temperature, its properties change with time.
  • 5. Solution treatment: fully dissolve various phases in the alloy, strengthen the solid solution, improve the toughness and corrosion resistance, eliminate stress and softening, so as to continue processing and forming
  • 6. Aging treatment: heat and keep warm at the precipitation temperature of the strengthening phase to precipitate the strengthening phase, harden and improve the strength
  • 7. Quenching: the heat treatment process of austenitizing the steel and cooling it at an appropriate cooling rate to make the workpiece undergo martensite and other unstable structural transformation in the cross section or within a certain range
  • 8. Tempering: heat the quenched workpiece to an appropriate temperature below the critical point AC1 for a certain period of time, and then cool it with a satisfactory method to obtain the required microstructure and properties
  • 9. Carbonitriding of steel: carbonitriding is the process of infiltrating carbon and nitrogen into the surface layer of steel at the same time. Traditionally, carbonitriding is also called cyanidation. At present, medium temperature gas carbonitriding and low temperature gas carbonitriding (i.e. gas soft nitriding) are widely used. The main purpose of medium temperature gas carbonitriding is to improve the hardness, wear resistance and fatigue strength of steel. Low temperature gas carbonitriding is mainly nitriding, and its main purpose is to improve the wear resistance and bite resistance of steel.
  • 10. Quenching and tempering: Generally speaking, the heat treatment combining quenching and high temperature tempering is called quenching and tempering treatment. Quenching and tempering treatment is widely used in various important structural parts, especially those connecting rods, bolts, gears and shafts working under alternating load. Tempered sorbite was obtained after quenching and tempering, and its mechanical properties were better than normalized sorbite with the same hardness. Its hardness depends on the high temperature tempering temperature and is related to the tempering stability of steel and the cross-section size of workpiece, generally between hb200-350.
  • 11. Brazing: a heat treatment process in which two kinds of workpieces are bonded together with solder.

Types and application of tempering

According to different performance requirements of workpiece and tempering temperature, tempering can be divided into the following types:
(1) Low temperature tempering (150-250 ℃)
The microstructure obtained by low temperature tempering is tempered martensite. The purpose is to reduce the quenched internal stress and brittleness on the premise of maintaining the high hardness and wear resistance of quenched steel, so as to avoid cracking or premature damage during use. It is mainly used for various high carbon cutting tools, measuring tools, cold stamping molds, rolling bearings and carburized parts. The hardness after tempering is generally HRC58-64.
(2) Medium temperature tempering (350-500 ℃)
The microstructure obtained by medium temperature tempering is tempered troostite. The purpose is to obtain high yield strength, elastic limit and high toughness. Therefore, it is mainly used for the treatment of various springs and hot working dies. The hardness after tempering is generally HRC35-50.
(3) High temperature tempering (500-650 ℃)
The microstructure obtained by high temperature tempering is tempered sorbite. Traditionally, the heat treatment combining quenching and high temperature tempering is called quenching and tempering treatment. Its purpose is to obtain comprehensive mechanical properties with good strength, hardness, plasticity and toughness. Therefore, it is widely used in important structural parts of automobiles, tractors and machine tools, such as connecting rods, bolts, gears and shafts. The hardness after tempering is generally HB200-330.

Atmosphere reaction

Chemical reaction between atmosphere and steel

1. Oxidation

  • 2Fe+02→2Fe0
  • Fe+H20→Fe0+H2
  • FeC+C02→Fe+2C0

2. Restore

  • Fe0+H2→Fe+H20Fe0+C0→Fe+02

3. Carburizing

  • 2C0→[C]+C02
  • Fe+[C]→FeC
  • CH4→[C]+2H2

4. Nitriding

  • 2NH3→2[N]+3H2
  • Fe+[N]→FeN

Effects of various atmospheres on metals

Nitrogen: it will react with Cr, C0 and al. Ti at ≥ 1000 ℃.
Hydrogen: can reduce copper, nickel, iron and tungsten. When the water content in hydrogen reaches 0.2-0.3%, the steel will decarburize.
Water: ≥ 800 ℃, iron and steel will be oxidized and decarburized, and will not react with copper.
Carbon monoxide: its reducibility is similar to that of hydrogen and can carburize steel.

Influence of various atmospheres on resistance components

Nickel chromium wire, iron chromium aluminum: sulfur atmosphere is harmful to resistance wire.

Heat treatment of brass

Ordinary brass is a copper zinc alloy. According to its structure, it can be divided into simple brass (also known as а Brass), ω (Cu) is 100% – 62.4%, and two-phase brass( а+β Brass), ω (Cu) is 56.5% – 62.4%. The solid solubility of Zn in Cu increases with the decrease of humidity, so there is no heat treatment strengthening effect. Annealing treatment is often used to improve the cold working properties of brass. The mechanical properties and cold deformation properties of finished brass after annealing mainly depend on the grain size. The annealing temperature of brass during cold working is shown in the table below:

Material grade

Thickness (δ)>5mm

Thickness (δ)=1-5mm

Thickness (δ)=0.5-1mm

Thickness (δ)<0.5mm

H96

560-600

540-580

500-540

450-550

H90HS700-1

650-720

620-780

560-620

450-560

H80

650-700

580-650

540-600

500-560

H68

580-650

540-600

500-560

440-500

H62H59

650-700

600-660

520-600

460-530

HFe59-1-1

600-650

520-620

450-550

420-480

HMn58-2

600-660

580-640

550-600

500-550

HSn70-1

600-650

560-620

470-560

450-500

HSn62-1

600-650

550-630

520-580

500-550

HPb63-3

600-650

540-620

520-600

480-540

HPb59-1

600-650

580-630

550-600

480-550

Nitriding and carbonitriding of steel

Nitriding of steel (gas nitriding)

Concept: nitriding is the process of infiltrating nitrogen atoms into the surface layer of steel. Its purpose is to improve surface hardness and wear resistance, fatigue strength and corrosion resistance.
It uses ammonia to decompose active nitrogen atoms during heating, forms a nitride layer on its surface after being absorbed by steel, and diffuses to the heart at the same time.
Nitriding is usually carried out by special equipment or well carburizing furnace. It is suitable for various high-speed transmission precision gears, machine tool spindles (such as boring bar and grinder spindles), high-speed diesel engine crankshafts, valves, etc.
Nitriding workpiece process route: forging annealing rough machining quenching and tempering finish machining stress removal rough grinding nitriding finish grinding or grinding.
Because the nitrided layer is thin and brittle, it is required to have a high-strength core structure, so it is necessary to carry out quenching and tempering heat treatment first to obtain tempered sorbite and improve the mechanical properties of the core and the quality of the nitrided layer.
After nitriding, steel has high surface hardness (greater than HV850) and wear resistance without quenching.
The nitriding temperature is low and the deformation is very small. Compared with carburizing and induction surface quenching, the deformation is much smaller

Carbonitriding of steel: carbonitriding is the process of infiltrating carbon and nitrogen into the surface of steel at the same time. Traditionally, carbonitriding is also called cyanidation. At present, medium temperature gas carbonitriding and low temperature gas carbonitriding (i.e. gas soft nitriding) are widely used. The main purpose of medium temperature gas carbonitriding is to improve the hardness, wear resistance and fatigue strength of steel. Low temperature gas carbonitriding is mainly nitriding, and its main purpose is to improve the wear resistance and bite resistance of steel.

Chemical heat treatment of steel oxygen Nitrocarburizing

When some oxygen-containing medium is introduced into the steel while nitriding, the oxygen nitrocarburizing treatment can be realized. The workpiece after treatment has the common advantages of steam treatment and nitriding treatment.

  • 1. Characteristics of oxynitriding: after oxynitriding, the nitriding layer can be divided into three areas: surface oxide film, subsurface oxide area and nitriding. The thickness of the surface oxide film is similar to that of the sub surface oxide zone, generally 2-4 μ m. The porous Fe3O4 layer formed after oxygen nitriding has good friction reduction, heat dissipation and adhesion resistance.
  • 2. Oxynitriding medium: ammonia with different concentrations is generally used during oxynitriding. Nitrogen atoms diffuse inward to form a nitrided layer, water decomposes to form oxygen atoms diffuse inward to form an oxide layer, and a black oxide film is formed on the surface of the workpiece.
  • 3. Main uses of oxynitriding: oxynitriding is mainly used for surface treatment of high speed steel cutting tools. The temperature during CO infiltration is generally 540-590 ℃, and the time is usually 1-2 hours. The ammonia concentration should be 25% – 30%. When the exhaust gas is heated up, the amount of ammonia should be larger to facilitate the rapid emptying of the air in the furnace. The amount of ammonia should be moderate during CO infiltration, and the amount of ammonia should be reduced during cooling and diffusion. The heat treatment furnace can be replaced by well nitriding furnace with furnace tank made of 1Cr18Ni9Ti stainless steel. The furnace tank shall be protected against sealing (vacuum water-cooled rubber sealing is preferred). There shall be a sealed circulating fan on the furnace top. Maintain a positive pressure of 300-1000pa in the furnace.

Heat treatment of steel — soft nitriding

In order to shorten the nitriding cycle and make the nitriding process not limited by steel grade, two new nitriding processes, soft nitriding and ion nitriding, have been developed on the basis of the original nitriding process in recent one or two decades.
Soft nitriding is essentially a low-temperature carbonitriding mainly based on nitriding. At the same time, a small amount of carbon atoms infiltrate into the steel. Compared with the above-mentioned general gas nitrogen, the hardness and brittleness of the nitriding layer are lower, so it is called soft nitriding.
1. Soft nitriding methods. Soft nitriding methods are divided into gas soft nitriding and liquid soft nitriding. At present, gas soft nitriding is the most widely used in domestic production. <, Br > gas soft nitriding is low-temperature carbonitriding and nitrocarburizing in an atmosphere containing activated carbon and nitrogen atoms. The commonly used CO nitriding media are urea, formamide and triethanolamine. They undergo thermal decomposition reaction at the soft nitriding temperature to produce activated carbon and nitrogen atoms.
Activated carbon and nitrogen atoms are absorbed by the surface of the workpiece and penetrate into the surface of the workpiece through diffusion, so as to obtain a carbonitriding layer dominated by nitrogen.
Gas soft nitriding temperature is usually 560-570 ℃, because the hardness value of nitriding layer is the highest at this temperature. The nitriding time is usually 2-3 hours, because over 2.5 hours, the nitriding layer depth increases slowly with time.
2. Soft nitriding layer structure and soft nitriding characteristics: after soft nitriding, the outermost layer of the steel surface can obtain a white layer of several microns to tens of microns, which is made of ε Phase γ` Phase and nitrogenous cementite Fe3 (C, n), and the sublayer is 0. 3-0。 4 mm diffusion layer, which is mainly composed of γ` Phase harmony ε Phase composition.
Soft nitriding has the following characteristics:

  • (1) Low processing temperature, short processing time and small workpiece deformation.
  • (2) Without the limitation of steel grade, carbon steel, low alloy steel, tool and die steel, stainless steel, cast iron and iron-based powder non metallurgical materials can be subject to soft nitriding treatment. The surface hardness of the workpiece after soft nitriding is related to the nitriding process and materials.

3. It can significantly improve the fatigue limit, wear resistance and corrosion resistance of the workpiece. It also has anti scratch and anti bite properties under dry friction conditions.
4. There is no brittleness in the soft nitrided layer ξ Therefore, the nitrided layer is hard and has certain toughness and is not easy to peel off.
Therefore, soft nitriding has been widely used in the treatment of wear-resistant workpieces such as molds, measuring tools, high-speed steel cutting tools, crankshafts, gears, cylinder liners and so on.
It should be noted that the current problem of gas soft nitriding is that the thickness of iron nitride compound layer on the surface is thin (0.01-0.02MM) and the hardness gradient of nitriding layer is steep, so it is not suitable to work under heavy load conditions. In addition, in the nitriding process, HCN, a toxic gas, will be produced in the furnace. Therefore, pay attention to the sealing of the equipment in production to prevent the furnace gas from leaking out and polluting the environment.

Heat treatment of beryllium bronze

Beryllium bronze is a widely used precipitation hardening alloy. After solid solution and aging treatment, the strength can reach 1250-1500mpa (1250-1500kg). The heat treatment is characterized by good plasticity after solution treatment and cold working deformation. However, after aging treatment, it has excellent elastic limit, and the hardness and strength are also improved.
(1) Solution treatment of beryllium bronze
Generally, the heating temperature of solution treatment is 780-820 ℃. For materials used as elastic elements, 760-780 ℃ is adopted to prevent coarse grains from affecting the strength. The temperature uniformity of solution treatment furnace shall be strictly controlled at ± 5 ℃. The holding time can generally be calculated as 1 hour / 25mm. When beryllium bronze is heated by solid solution in air or oxidizing atmosphere, an oxide film will be formed on the surface. Although it has little effect on the mechanical properties after aging strengthening, it will affect the service life of tools and dies during cold working. In order to avoid oxidation, it shall be heated in vacuum furnace or ammonia decomposition, inert gas and reducing atmosphere (such as hydrogen, carbon monoxide, etc.), so as to obtain bright heat treatment effect. In addition, pay attention to shorten the transfer time as far as possible (during water quenching), otherwise it will affect the mechanical properties after aging. Thin materials shall not exceed 3 seconds, and general parts shall not exceed 5 seconds. Water is generally used as quenching medium (without heating requirements). Of course, oil can also be used for complex parts to avoid deformation.
(2) Aging treatment of beryllium bronze
The aging temperature of beryllium bronze is related to the content of be. Alloys with be less than 2.1% should be aged. For alloys with be greater than 1.7%, the optimum aging temperature is 300-330 ℃, and the holding time is 1-3 hours (according to the shape and thickness of parts). For the high conductivity electrode alloy with be less than 0.5%, due to the increase of melting point, the optimal aging temperature is 450-480 ℃, and the holding time is 1-3 hours. In recent years, two-stage and multi-stage aging have also been developed, that is, first aging at high temperature for a short time, and then aging at low temperature for a long time. The advantage of this is that the performance is improved but the deformation is reduced. In order to improve the dimensional accuracy of beryllium bronze after aging, fixture clamping can be used for aging, and sometimes two-stage separate aging treatment can be used.
(3) Stress relief treatment of beryllium bronze
The stress relief annealing temperature of beryllium bronze is 150-200 ℃, and the holding time is 1-1.5 hours. It can be used to eliminate the residual stress caused by metal cutting, straightening and cold forming, and stabilize the shape and dimensional accuracy of parts in long-term use.
Determination of quenching and aging temperature of partially deformed aluminum alloy

Alloy grade

  Partially Prepared Products

Quench

Prescription

Minimum temperature ℃

Optimum temperature 

Danger temperature of over burning ℃

Aging temperature ℃

Aging time/H

LY 12

Plates, extrusions

485 ~ 490

495 ~ 503

505

185 ~ 195

6 ~ 12

LY16

Various

520 ~ 525

530 ~ 542

545

160 ~ 175

200 ~ 220

10 ~ 16

8 ~ 12

LY17

Various

515

520 ~ 530

180 ~ 195

12 ~ 16

LY2

Various

490

495 ~ 508

512

165 ~ 175

10 ~ 16

LD2

Various

510

525 ± 5

596

150 ~ 165

6 ~ 15

LD5, LD6

Various

 

500

515 ± 5

545

150 ~ 165

6 ~ 15

LD7

Various

520

535 ±5

545

180 ~ 195

8 ~ 12

LD8

Various

510

525 ~ 535

545

165 ~ 180

8 ~ 14

LD9

Extrusion

510

510 ~ 530

135 ~ 150

2 ~ 4

LD10

Various

490

500 ± 5

515

175 ~ 185

5 ~ 8

 

 

LC4

Aluminum clad plate

 

450

 

455 ~ 480

 

525

120 ~ 125

24

No aluminum plate

135 ~ 145

16

Structural section

120 ± 5

3

160 ± 3

3

 

LC6

Die forging

 

 

 

100 ± 5

5

 

 

 

155 ~ 160

8 ~ 9

450

455 ~ 473

 

145 ± 5

16

 

LC9

Extrusion

450

455 ~ 480

520 ~ 530

140 ± 5

16

Die forging

110 ± 5

6 ~ 8

117 ± 5

6 ~ 10

General technical requirements for common quenching media

Quenching medium

General technical requirements

Scope of application

Water and aqueous solution

Water

Cleaning, flowing (or circulating, stirring)
Water temperature 20-40 ℃

Carbon structural steel
Carbon tool steel
Alloy structural steel
aluminium alloy
titanium alloy

Inorganic aqueous solution

Select the concentration as required
Common concentration (mass fraction) (5% – 15%)
High concentration (mass dispersion) (≥ 20%, saturation concentration)
Liquid temperature 20-45 ℃
Circulation or stirring
PH 6.5-8.5

Carbon structural steel
Alloy structure
Carbon tool steel

Inorganic aqueous solution

Select the concentration according to the technical conditions and requirements of special products
Low concentration, medium concentration, high concentration (depending on the medium)
Liquid temperature 20-50 ℃
Stirring or thermal cycling
PH 6.5-8.5 (or as specified)

Carbon structural steel
Alloy structural steel
Bearing steel
Spring steel
Carbon tool steel
Alloy tool steel
aluminium alloy

Quenching oil

Total loss system oil

According to gb443 technical conditions
Conventional oil temperature 20-80 ℃
Hot oil temperature > 100 ℃
Circulation or stirring

Carbon tool steel (cross section ≤ 6mm)
Alloy structural steel
Alloy tool steel
Bearing steel
Spring steel
high speed steel

Special quenching oil

Different quenching oils (fast, bright, isothermal, vacuum and other quenching oils) shall be selected according to the process requirements
The technical conditions shall be in accordance with the provisions of special oil products
The oil temperature shall be 80-100 ℃ lower than the flash point
Stirring or thermal cycling

Hot bath

Salt bath

Allowable fluctuation range of service temperature ± 20 ℃
Select the formula according to the required bath temperature
Chloride ion in nitrate bath ≤ 0.3% (mass dispersion)
Sulfate ≤ 0.5%
PH 6.5-8.5 (mass dispersion)

ω (C) ≥ 0.45% carbon structural steel
Carbon tool steel
Alloy structural steel
Alloy tool steel
high speed steel

Alkali bath

Allowable fluctuation range of service temperature ± 10 ℃
Select the formula as required
Carbonate ≤ 4%

Source: Network Arrangement – China Flanges Manufacturer – Yaang Pipe Industry (www.epowermetals.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

If you want to have more information about the article or you want to share your opinion with us, contact us at sales@epowermetals.com

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