What is Heat Treatment Processing?
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What is Heat Treatment Processing?

Regardless of traditional machinery manufacturing, or high-tech semiconductors, electric vehicles, and drones, most key components require heat treatment.
Published: Jun 10, 2022
What is Heat Treatment Processing?

With the gradual development of 3C products toward the trend of light, thin, short, and small, related components are becoming more and more precise and miniaturized. For example, carbon steel screws in mobile phones, small and medium motor shafts, or pinions in fans, etc. are all common micro components. After the stamping process, it will enter the heat treatment stage. Heat treatment is one of the important processes in the manufacture of mechanical parts and tools. 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.

What is Heat Treatment Technology?

Heat treatment (heat treating) refers to a metal thermal processing process in which the chemical composition and structure of the surface or interior of the material are changed using heating, heat preservation, and cooling in a solid state to obtain the required properties. Heat treatment is one of the important processes in the manufacture of mechanical parts and tools. The heat treatment process generally includes three procedures: heating, heat preservation, and cooling.

What are the Characteristics of Heat Treatment Processing?

Metal heat treatment is one of the important processes in machinery manufacturing. Compared with other processing technologies, 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 chemical composition of the workpiece surface. Endows or improves the performance of the workpiece, which is characterized by improving the intrinsic quality of the workpiece.

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, the heat treatment process is also a commonly used processing procedure. Steel is the most widely used material in the machinery industry. The microstructure of steel is complex and can be controlled by heat treatment, which is also the main content of metal heat treatment. In addition, aluminum, copper, magnesium, titanium, etc., and their alloys can change their mechanical, physical, and chemical properties through heat treatment to obtain different performance.

What is the Processing Procedure for Thermal Processing?

The heat treatment process generally includes three processes heating, heat preservation, and cooling, and sometimes there are only two processes of heating and cooling. These processes are interconnected and uninterrupted.

Heating is one of the important processes of heat treatment. There are many heating methods for metal heat treatment. The earliest ones used charcoal and coal as heat sources, and more recently, liquid and gas fuels were used. The use of electricity makes the heating easy to control and has no environmental pollution. These heat sources can be used for direct heating or indirect heating through molten salts or metals, as well as floating particles. When the metal is heated, the workpiece is exposed to the air, and oxidation and decarburization often occur, which has a 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, in molten salt and a vacuum, and can be protected by coating or packaging methods.

The heating temperature is one of the important process parameters of the heat treatment process. The selection and control of the heating temperature are the issues to ensure the quality of the heat treatment. The heating temperature varies with the metal material to be processed and the purpose of the heat treatment, but generally it is heated above the phase transition temperature to obtain a high-temperature structure. 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 be maintained at this temperature for a certain period to make the internal and external temperatures consistent and the microstructure changes completely. This period 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, while the holding time of chemical heat treatment is often longer.

Cooling is an indispensable step in the heat treatment process. The cooling method varies with different processes, mainly controlling the cooling rate. Generally, the cooling rate of annealing is the slowest, the cooling rate of normalizing is faster, and the cooling rate of quenching is faster. But there are also different requirements due to different steel grades.

What are the Process Classifications of Heat Treatment?

The metal heat treatment process can be roughly divided into three categories: overall heat treatment, surface heat treatment, and chemical heat treatment. According to the different heating mediums, heating temperature, and cooling methods, each category can be divided into different heat treatment processes. The same metal adopts different heat treatment processes to obtain different structures and thus has different properties. Steel is the most widely used metal in industry, and the microstructure of steel is the most complex, so there are many kinds of steel heat treatment processes.

  • Overall heat treatment:
    It is a metal heat treatment process that heats the workpiece as a whole and then cools it at an appropriate rate to obtain the required metallographic structure to change its overall mechanical properties. The overall heat treatment of steel generally has four basic processes: annealing, normalizing, quenching, and tempering. The method of combining pressure deformation and heat treatment effectively and closely to make the workpiece obtain good strength and toughness is called deformation heat treatment; heat treatment in a negative pressure atmosphere or vacuum is called vacuum heat treatment, which not only makes the workpiece is not oxidized or decarburized, the surface of the workpiece after treatment is kept smooth, and the performance of the workpiece is improved.
  • Surface heat treatment:
    It is a metal heat treatment process that only heats the surface of the workpiece to change the mechanical properties of the surface. To only heat the surface layer of the workpiece without allowing too much heat to pass into the interior of the workpiece, the heat source used must have a high energy density, that is, a larger amount of heat energy is given to the workpiece per unit area so that the surface layer or local energy of the workpiece can be reached in a short time or instantaneously. high temperature. The main methods of surface heat treatment are flame quenching and induction heating heat treatment. Commonly used heat sources are flames such as oxyacetylene or oxy propane, induced current, laser, and electron beam.
  • Chemical heat treatment:
    It is a metal heat treatment process by changing the chemical composition, structure, and properties of the workpiece surface. The difference between chemical heat treatment and surface heat treatment is that the former changes the chemical composition of the surface of the workpiece. Chemical heat treatment is to heat the workpiece in a medium (gas, liquid, solid) containing carbon, salt, or other alloying elements, and keep it for a long time so that the surface layer of the workpiece is infiltrated with elements such as carbon, nitrogen, boron, and chromium. After the elements are infiltrated, other heat treatment processes such as quenching and tempering are sometimes carried out. The main methods of chemical heat treatment are carburizing, nitriding, and metalizing.

Different Types of Heat Treatment Process:

  1. Annealing:
    It is to heat the workpiece to an appropriate temperature, adopt different holding times according to the material and the size of the workpiece, and then slowly cool it, the purpose is to make the internal structure of the metal reach or close to the equilibrium state, obtain good process performance and performance, or for further quenching organizing preparations. It refers to the heat treatment process in which the metal material is heated to an appropriate temperature, kept for a certain period, and then slowly cooled. Common annealing processes include recrystallization annealing, stress relief annealing, spheroidizing annealing, and complete annealing. The purpose of annealing is mainly to reduce the hardness of metal materials, improve plasticity, facilitate cutting or pressure processing, reduce residual stress, improve the homogenization of structure and composition, prepare the structure for subsequent heat treatment, etc.
  2. Normalizing:
    It is to heat the workpiece to a suitable temperature and then cool it in the air. The effect of normalizing is like that of annealing, but the obtained structure is finer. It is often used to improve the cutting performance of materials and is sometimes used for some requirements tall parts as final heat treatment. It refers to the heat treatment process of heating steel or steel parts to a temperature above the critical point, keeping it at 30 to 50 °C for an appropriate time, and cooling it in still air. The purpose of normalizing is mainly to improve the mechanical properties of low carbon steel, improve machinability, refine grains, eliminate microstructure defects, and prepare microstructure for subsequent heat treatment.
  3. Quenching:
    After the workpiece is heated and kept warm, it is rapidly cooled in a quenching medium such as water, oil, or other inorganic salts and organic aqueous solutions. After quenching, the steel becomes hard, but at the same time becomes brittle. To eliminate the brittleness in time, it is generally necessary to temper in time. Refers to the heat treatment process of heating the steel to a temperature above the lower critical point temperature of the steel, maintaining it for a certain time, and then obtaining a martensite (or bainite) structure at an appropriate cooling rate. Common quenching processes include single-medium quenching, double-medium quenching, martensite graded quenching, bainite isothermal quenching, surface quenching, and local quenching. The purpose of quenching: is to obtain the required martensitic structure of the steel, to improve the hardness, strength, and wear resistance of the workpiece, to prepare the structure for the subsequent heat treatment, etc.
  4. Tempering:
    To reduce the brittleness of steel parts, the quenched steel parts are kept at an appropriate temperature higher than room temperature but lower than 650 ° C for a long time and then cooled. This process is called tempering. It refers to the heat treatment process in which the steel parts are hardened, then heated to a temperature below Ac1, held for a certain time, and then cooled to room temperature. Common tempering processes are low-temperature tempering, medium-temperature tempering, high-temperature tempering, and multiple tempering.

Advanced Heat Treatment Technology Methods:

  • Controlled atmosphere heat treatment:
    Controlled atmosphere heat treatment is mainly to prevent oxidation and decarburization, and to achieve precise control of carburizing and nitriding. In the late 1980s, it began to be used in industrial production, and it has developed into a very wide range of applications. It can not only meet carburizing and carbonitriding, but also realize various heat treatment processes such as bright quenching and bright annealing. Can realize computer-aided design, production management, logistics management, on-site control, quality management, process management, and other series of work. The whole process of engineering is relatively simple and lively. Suitable for mass production, such as in the automotive industry. Most of them can optimize process parameters, predict, and precisely control the distribution of carbon concentration, obtain ideal concentration distribution and layer structure, realize computer management, and friendly man-machine interface.
  • Vacuum heat treatment:
    Vacuum heat treatment is a new heat treatment technology that combines vacuum technology and heat treatment technology. The vacuum environment in which vacuum heat treatment is located refers to an atmosphere environment below one atmospheric pressure, including low vacuum, medium vacuum, high vacuum, and ultra-high vacuum. The actual vacuum heat treatment belongs to atmosphere-controlled heat treatment. The application and development of vacuum heat treatment technology have been further improved and promoted. It has the characteristics of no oxidation, no decarburization, clean and bright workpiece surface after quenching, high wear resistance, no pollution, and a high degree of automation. Heat treatment technologies such as vacuum annealing, vacuum degassing, vacuum oil quenching, vacuum water quenching, vacuum air quenching, vacuum tempering, and vacuum carburizing are widely used in industrial production. Vacuum heat treatment will become one of the most popular technologies in heat treatment workshops.
  • Induction heat treatment and ion nitriding heat treatment technology:
    Induction heat treatment is widely used in the automotive industry, construction machinery, petrochemical, and other industries with the advantages of high efficiency, energy-saving, cleanliness, and flexibility. Nearly 40% of auto parts can use induction heat treatment, such as crankshafts, gears, universal joints, half shafts, etc. Using induction heating, many products can be processed into fully automatic or semi-automatic production lines, which can improve the stability of product quality, reduce labor intensity, and beautify the working environment. Nowadays, with the development of technology, microcomputer-controlled transistors are used to adjust the power supply, which is stable and convenient, with high precision, and greatly reduces the interference to the harmonics of the power grid. The solid-state high-frequency heating device developed in recent years has obvious advantages. It no longer uses expensive, fragile, and energy-consuming electronic tubes and the anode booster transformers, anode water jackets, and filament voltage regulators that are used with them. Etc., replaced by MOSFET power electronics: it saves a third of electricity and a half of water.
    Ion nitriding technology the remarkable feature is that the surface of the parts after treatment is clean, anti-corrosion, has small deformation and has high wear resistance. Compared with gas nitriding, it has the advantages of a short cycle, high efficiency, and less pollution. In recent years, ion nitriding has developed rapidly, especially with the advent of pulse power supply for ion nitriding furnaces, which separates the physical parameters of discharge (voltage, current, air pressure) and temperature control parameters (pulse width). The adjustability of the process is increased, and the selection and precise control of process parameters are easy to achieve.
  • Quenching medium and cooling technology:
    In recent years, the quenching medium has developed very rapidly. The performance of quenching oil has been improved, and the research and application of organic polymer quenching medium have achieved unprecedented development. The ideal quenching medium should have the following characteristics:
    • Quality type: fast cooling in the high-temperature stage and slow cooling in the low-temperature stage. That is to eliminate cracks and reduce quenching deformation.
    • Environmental protection type: no toxic, harmful smoke, or gas volatilization, not easy to burn, easy to clean after the workpiece is quenched, no corrosion to equipment, no irritation to burn skin.
    • Stable type: The performance of the workpiece is uniform and stable after quenching, and the performance of the medium itself is stable within a certain time.
    • Economic type: high quality and high price, low energy consumption.
  • Adopt new surface strengthening technology and promote heat treatment in a nitrogen-based atmosphere:
    With the development and evolution of technology, QPQ salt bath composite treatment technology can stably improve tool life, simple equipment, and low cost, especially suitable for drying common tools. PVD titanium oxide physical coating technology can increase tool life by 3 to 5 times and is suitable for drying various precision and precious gear tools.
    The nitrogen-based atmosphere is used for protective heat treatment and chemical heat treatment, which can realize non-oxidative decarburization heat treatment, and can avoid nitrogen embrittlement in heat treatment. Chemical heat treatment in a nitrogen-based atmosphere can reduce defects such as internal oxidation and improve the quality of chemical heat treatment.
Published by Jun 10, 2022 Source :easyatm

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