What is Heat Treatment Processing?

With the gradual trend of 3C products becoming lighter, thinner, shorter, and smaller, 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, components will enter the heat treatment stage. This is an important process in the manufacturing of mechanical parts and tools. It can improve wear and corrosion resistance, and can improve the structural strength.

What is Heat Treatment Technology?

Heat treatment refers to a metal thermal processing procedure in which the chemical composition and structure of the surface or interior of the material are changed using heat. The heat treatment process generally includes three procedures: heating, heat preservation, and cooling.

What are the Characteristics of Heat Treatment Processing?

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. Heat treatment will improve the performance and intrinsic quality of the workpiece.

In addition to the proper selection of materials and forming processes to produce an item, choosing the proper heat treatment will give the metal workpiece the required mechanical, physical, and chemical properties, Steel is the most widely used material in the machinery industry. Its microstructure is complex and its physical properties can be controlled by heat treatment. Heat treatment can also be used for aluminum, copper, magnesium, titanium, and other alloys as well.

What are the Procedures for Thermal Processing?

Heat treatment generally includes three processes; heating, heat preservation, and cooling, although sometimes it only involves heating and cooling. These processes are interconnected and are performed uninterrupted.

There are many different heating methods used for metal heat treatment. The earliest ones used charcoal and coal as heat sources, and more recently, liquid and gas fuels have been used. The use of electricity makes the heating easy to control and has low environmental pollution. When the metal is heated, if the workpiece is exposed to the air, oxidation and decarburization can occur. To prevent these adverse effects on the surface properties of the parts, the metal should usually be heated in a controlled or protective atmosphere, such as in molten salt and a vacuum.

The heating temperature is one of the important process parameters of the heat treatment process. The selection and control of the heating temperature will determine the quality of the heat treatment. Heating temperature selection will vary with the metal material to be processed and the purpose of the heat treatment. Generally, it is heated to above the phase transition temperature of the metal to obtain a high-temperature state. It is maintained at this temperature for a certain period to make the internal and external temperatures consistent and the microstructure changes complete. 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. To maintain the proper cooling rate, various cooling methods will be used. Generally, the cooling rate of annealing is the slowest, the cooling rate of normalizing is faster, and the cooling rate of quenching is faster.

What are the Various 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 temperatures, and cooling methods, a single type of metal can be treated to have different internal structures and properties.

• Overall heat treatment:
  This heat treatment process 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 prevents the workpiece from oxidizing or decarburizing, but assures the surface of the workpiece remains smooth after treatment.
• Surface heat treatment:
  This metal heat treatment process 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 period. 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:
  This metal heat treatment process changes the chemical composition, structure, and properties of the workpiece surface. Chemical heat treatment heats the workpiece in a medium (gas, liquid, or solid) containing carbon, salt, or other alloying elements, and maintains the temperature for a long enough time to allow the surface layer of the workpiece to be 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:
   Annealing is to heat the workpiece to an appropriate temperature, adopt different holding times according to the material and the size of the workpiece, and then slowly cooling it. The purpose is to make the internal structure of the metal reach a nearly equilibrium state. 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, and prepare the structure for subsequent heat treatment.
2. Normalizing:
   A workpiece is normalized by heating it to a suitable temperature and then cooling 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 mechanical properties of low carbon steel, improve machinability, refine grains, eliminate microstructure defects, and prepare microstructure for subsequent heat treatment.
3. Quenching:
   The workpiece is heated above the lower critical point temperature of the steel, maintained at this temperature for a certain time, and then 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. 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 improve the hardness, strength, and wear resistance of the workpiece, and to prepare the structure for subsequent heat treatment.
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. 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 used 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 a very wide range of applications. It can not only meet carburizing and carbonitriding requirements, but also be used during various heat treatment processes such as bright quenching and bright annealing. The whole process of engineering is relatively simple and suitable for mass production, such as in the automotive industry. Process parameters can be optimized to precisely control the distribution of carbon concentration, and obtain ideal concentration, distribution and layer structure.
• 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 performed refers to an environment below one atmospheric pressure. It can include low vacuum, medium vacuum, high vacuum, or ultra-high vacuum. The application and development of vacuum heat treatment technology has been highly developed 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. This can assure stability, convenience, and high precision, and greatly reduce interference to the harmonics of the power grid. Solid-state, high-frequency heating devices developed in recent years have obvious advantages. They no longer use expensive, fragile, and energy-consuming electronic tubes and the anode booster transformers, anode water jackets, and filament voltage regulators that are used with them. Replacing electronic tubes with MOSFET power electronics saves a third of the electricity and a half of water.
  Ion nitriding technology assures that the surface of the parts after treatment is clean, anti-corrosion, low deformation, and 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, there has been rapid development of quenching mediums used for heat treatment. The performance of quenching oil has been improved, and the research and application of organic polymer quenching mediums 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 has been used to improve tool life and lower costs. 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.
  A nitrogen-based atmosphere is used for protective heat treatment and chemical heat treatment, to realize non-oxidative decarburization. Chemical heat treatment in a nitrogen-based atmosphere can reduce defects such as internal oxidation, and improve the quality of chemical heat treatment.