Heat treatment of surface treatment

What is heat treatment?

Heat treatment refers to a metal thermal processing process in which the material is heated, insulated and cooled in a solid state to obtain the expected organization and properties. Metal heat treatment process can be roughly divided into integral heat treatment, surface heat treatment and chemical heat treatment three categories. The same metal using different heat treatment processes, can obtain different structures, and thus have different properties.

Process features

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 of the workpiece and the overall chemical composition, but changes the chemical composition of the workpiece surface, improve the performance of the workpiece. It is characterized by improving 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, physical properties 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 machinery industry, the microstructure of steel is complex, 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.

Process steps

• Normalizing: Normalizing is the workpiece heated to the appropriate temperature after cooling in the air, the normalizing effect is similar to annealing, but the obtained organization is finer, often used to improve the cutting performance of the material, and sometimes used for some parts with low requirements as the final heat treatment.

• Quenching: quenching is to heat and hold the workpiece, in water, oil or other inorganic salts, organic water solution and other quenching medium rapid cooling. After quenching, the steel becomes hard, but at the same time becomes brittle, in order to eliminate brittleness in time, it is generally necessary to temper in time.

• Tempering: In order to reduce the brittleness of the steel, the quenchable steel is kept warm for a long time at an appropriate temperature above room temperature and below 650 ° C, and then cooled, this process is called tempering.

• Tempering: refers to the composite heat treatment process of quenching and tempering steel or steel parts. Steels used for tempering are called tempering steels. It generally refers to medium carbon structural steel and medium carbon alloy structural steel.

• Chemical heat treatment: refers to the metal or alloy workpiece placed in a certain temperature of the active medium heat preservation, so that one or several elements into its surface to change its chemical composition, organization and performance of the heat treatment process.

• Common chemical heat treatment processes are: carburizing, nitriding, carbonitriding, aluminizing, boronizing and so on. The purpose of chemical heat treatment is to improve the hardness, wear resistance, corrosion resistance, fatigue strength and oxidation resistance of the steel surface.

• Solid solution treatment: refers to the heat treatment process of heating the alloy to a high temperature single-phase area to maintain a constant temperature, so that the excess phase is fully dissolved into the solid solution and then cooled quickly to obtain the supersaturated solid solution. The purpose of solution treatment is mainly to improve the plasticity and toughness of steel and alloy, and prepare for precipitation hardening treatment.

• precipitation hardening (precipitation strengthening): refers to the metal in the supersaturated solid solution solute atomic segregation zone and (or) by the dissolution of the particles dispersed in the matrix and lead to hardening of a heat treatment process. Such as austenitic precipitation stainless steel after solution treatment or after cold processing, at 400~500℃ or 700~800℃ for precipitation hardening treatment, can obtain a high strength.

• Aging treatment: refers to the alloy workpiece after solution treatment, cold plastic deformation or casting, forging, placed at a higher temperature or room temperature maintenance, its performance, shape, size change with time heat treatment process.

• If the workpiece is heated to a higher temperature and the aging process is used for a longer time, it is called artificial aging treatment. If the workpiece is placed at room temperature or under natural conditions for a long time, the aging phenomenon is called natural aging treatment. The purpose of aging treatment is to eliminate the internal stress of the workpiece, stabilize the structure and size, and improve the mechanical properties.

• Critical diameter (critical penetration diameter): The critical diameter refers to the maximum diameter of the steel after quenching in a certain medium, when the heart gets all martensitic or 50% martensitic structure, and the critical diameter of some steel can generally be obtained by the hardenability test in oil or water.

• Secondary hardening: Some iron-carbon alloys (such as high-speed steel) must be tempered several times before further improving their hardness. This hardening phenomenon, called secondary hardening, is due to the precipitation of special carbides and/or due to participation in the transformation of austenite into martensite or bainite.

• Tempering brittleness: refers to the embrittlement phenomenon of hardened steel tempered in certain temperature intervals or slowly cooled from the tempering temperature through the temperature interval. Temper brittleness can be divided into the first type of temper brittleness and the second type of temper brittleness.The first type of tempering brittleness is also known as irreversible tempering brittleness, mainly occurs when the tempering temperature is 250~400℃, after the reheating brittleness disappears, repeated tempering in this interval, no longer occurs brittleness, the second type of tempering brittleness is also known as reversible tempering brittleness, The temperature is 400~650℃, when the brittleness disappears after reheating, it should be cooled rapidly, and it can not stay in the range of 400~650℃ for a long time or slow cooling otherwise the catalytic phenomenon will occur again. The occurrence of temper brittleness is related to the alloying elements contained in the steel, such as manganese, chromium, silicon, nickel will produce temper brittleness tendency, and molybdenum, tungsten will weaken the temper brittleness tendency.

Technological process

Heat treatment process generally includes heating, insulation, cooling three processes, sometimes only heating and cooling two processes. These processes are interconnected and uninterruptible.
Heating is one of the important jobs of heat treatment. There are many heating methods for metal heat treatment, the earliest is the use of charcoal and coal as heat sources, and the recent application of liquid and gas fuel. 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 salts or metals, and even floating particles.When the metal is heated, the workpiece is exposed to the air, and oxidation and decarbonization often occur (that is, the surface carbon content of steel parts is reduced), which has a very adverse effect on the surface properties of the parts after heat treatment. Therefore, the metal should usually be heated in a controlled atmosphere or protective atmosphere, molten salt and vacuum, and can also be protected by coating or packaging methods.The heating temperature is one of the important parameters of the heat treatment process, and the selection and control of the heating temperature is the main problem to ensure the quality of the heat treatment. The heating temperature varies with the metal material being treated and the purpose of the heat treatment, but it is generally heated above the phase change temperature to obtain high temperature tissue. In addition, the transformation takes a certain amount of time, so when the surface of the metal workpiece reaches the required heating temperature, it must also be maintained at this temperature for a certain time, so that the internal and external temperature is consistent, so that the microstructure is completely transformed, this period of time is called the holding time. When high energy density heating and surface heat treatment are used, the heating speed is extremely fast, and there is generally no holding time, and the holding time of chemical heat treatment is often longer.
Cooling is also an indispensable step in the heat treatment process, and the cooling method varies according to the process, mainly to control the cooling speed. Generally, the cooling rate of annealing is the slowest, the cooling rate of normalizing is faster, and the cooling rate of fire is faster. However, there are different requirements due to different types of steel, for example, air-hard steel can be hardened with the same cooling rate as normalizing

Process classification

Metal heat treatment process can be roughly divided into integral heat treatment, surface heat treatment and chemical heat treatment three categories. According to the heating temperature of the heating medium and the different cooling methods, each category can be divided into several different heat treatment processes. The same metal using different heat treatment processes, 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 there are many kinds of steel heat treatment process, the overall heat treatment is the overall heating of the workpiece, and then cooling at the appropriate speed, to obtain the required metallographic structure, to change its overall mechanical properties of the metal heat treatment process. The overall heat treatment of steel has four basic processes: annealing, normalizing, fire and tempering.

Types of annealing

Annealing is a heat treatment process in which the workpiece is heated to an appropriate temperature, held for a certain time, and then slowly cooled
There are many kinds of annealing processes for steel, which can be divided into two categories according to the heating temperature: one is the annealing above the critical temperature (Ac1 or Ac3), also known as phase change recrystallization annealing, including complete annealing, incomplete annealing, spheroidizing annealing and diffusion annealing (homogenizing annealing); The other is the annealing below the critical temperature, including recrystallization annealing and stress relief annealing. According to the cooling method, annealing can be divided into isothermal annealing and continuous cooling annealing.

•Complete annealing and isothermal annealing

Complete annealing and weighing crystallization annealing, generally referred to as annealing, it is the steel or steel heated to Ac3 above 20~30 ° C for a long enough time, so that the organization is completely austenitizing after slow cooling, in order to obtain a heat treatment process close to the balance of the organization. This annealing is mainly used for the casting of various carbon steels and alloy steels with hypoeutectoid composition, forgings and hot-rolled profiles, and sometimes for joint structures. It is usually used as the final heat treatment of some non-heavy parts, or as the pre-heat treatment of some workpieces.

Spheroidizing annealing is mainly used for over-eutectoid carbon steel and alloy tool steel (such as steel used in manufacturing cutting tools, measuring tools and molds). Its main purpose is to reduce the hardness, improve the machinability, and prepare for later quenching.

•Spheroidizing annealing

When quenching, the most commonly used cooling media are salt water, water and oil.
Brine quenching workpiece, easy to get high hardness and smooth surface, not easy to produce soft points that are not hard, but it is easy to cause serious deformation of the workpiece, and even crack. Oil as a quenching medium is only suitable for quenching of some alloy steels or small-size carbon steel workpieces with relatively large stability of supercooled austenite.

Purpose of steel tempering

• reduce brittleness, eliminate or reduce internal stress, steel quenching after there is a lot of internal stress and brittleness, such as not timely tempering often make steel deformation or even cracking.

• to obtain the required mechanical properties of the workpiece, the workpiece after quenching hardness is high and brittle, in order to meet the requirements of different performance of various workpieces, you can adjust the hardness through the appropriate tempering coordination, reduce brittleness, get the required toughness, plasticity.

• stable workpiece size

• for some alloy steel that is difficult to soften annealing, high temperature tempering is often used after quenching (or normalizing), so that the carbides in the steel are properly gathered, and the hardness is reduced to facilitate cutting.

Deformation prevention

• Reasonable selection of materials. For sophisticated and complex molds, micro-deformed die steel with good material (such as air-quenched steel) should be selected, die steel with serious carbide segregation should be reasonably forged and tempered heat treatment, and solid solution double refinement heat treatment can be carried out for large and unforgeable die steel.

• The mold structure design should be reasonable, the thickness should not be too wide, the shape should be symmetrical, the deformation law should be mastered for the large deformation mold, the processing allowance should be reserved, and the combination structure can be used for the large, precise and complex mold.

• Precision complex mold counties should be pre-heat treated to eliminate the residual stress generated during machining.

• Reasonable selection of heating temperature, control of heating speed, slow heating, preheating and other balanced heating methods can be adopted for precision and complex molds to reduce mold heat treatment deformation.

• Under the premise of ensuring the hardness of the mold, try to use pre-cooling, fractional cooling quenching or warm quenching process

• For precision and complex molds, if conditions permit, try to use vacuum heating quenching and cryogenic treatment after quenching.

• For some precision and complex molds, pre-heat treatment, aging heat treatment, tempering and ammoniating heat treatment can be used to control the accuracy of the mold.