Heat treatment is a process that changes the internal structure of the metal by controlling the heating temperature, holding time, cooling rate and atmosphere in the furnace without changing the shape of the part, thereby obtaining the required hardness, strength, toughness or wear resistance. The four basic processes are annealing, normalizing, quenching, and tempering. when the surface needs to be wear-resistant and the core remains tough, chemical heat treatments such as carburizing and nitriding can be added.
- The three elements of heat treatment: temperature, time, and cooling rate; controlled atmosphere heat treatment plus the fourth element "atmosphere"
- Annealing softens steel, normalizing refines its microstructure, quenching hardens it, and tempering improves toughness.
- Chemical heat treatment changes the surface composition by infiltrating carbon, nitrogen and other elements into the surface. The representative processes are carburizing and nitriding.
- Quality stability depends on process control capabilities, not just the equipment itself
How temperature and time change the microstructure
Steel will undergo phase transformation (for example, ferrite + pearlite transforms into austenite) when heated to a certain temperature. When cooled, it transforms into different structures according to the cooling rate: pearlite is obtained by slow cooling, and martensite is obtained by rapid cooling. Different microstructures produce different properties. Heat treatment is the conscious use of this law.
The three basic parameters that determine the results of heat treatment are:
- Heating temperature: Determine whether to complete phase transformation, austenite grain size and carbide dissolution degree;
- Holding time: Determines whether the temperature penetrates evenly and whether the ingredients are fully diffused;
- cooling rate: Decide which microstructure you end up with.
For processes heated in a furnace gas environment, there is a fourth parameter -Furnace atmosphere. When the atmosphere is not controlled, the surface of steel parts will be oxidized, decarburized or carburized, directly affecting surface hardness and fatigue life. This is why controlled atmosphere heat treatment and process control systems exist.
Four basic processes: annealing, normalizing, quenching, and tempering
| Craftsmanship | Typical approach | Main purpose | Common applications |
|---|---|---|---|
| Annealing | After heating, slowly cool in the furnace | Reduce hardness, eliminate stress, improve cutting and cold forming performance | Cold heading steel wire spheroidizing annealing, stamping parts stress relief |
| Normalizing | Air cooling after heating | Refine grains and uniform structure | Forging pretreatment |
| Quenching | Heating, austenitization followed by rapid cooling (oil, water, polymer or gas) | Obtain martensite and increase hardness and strength | Gears, shafts, bearings |
| Tempering | Reheat to lower temperature after quenching | Reduce brittleness, stabilize size, and adjust the balance of hardness and toughness | Almost all quenched parts |
The combination of quenching and high temperature tempering is often called Conditioning, used for structural parts that require good comprehensive mechanical properties.
Surface heat treatment: hard surface and tough core
Many parts require "surface wear resistance and core impact resistance", such as gearbox gears. It is difficult to satisfy both needs by overall quenching alone, so two types of surface strengthening methods have been developed:
- Surface hardening: Only the surface layer is heated (induction heating, laser, etc.), and the composition remains unchanged;
- Chemical heat treatment: Infiltrate elements into the surface in a controlled atmosphere to change the surface composition. The most common is Carburizing(Quenching after increasing the carbon content of the surface layer) and Nitriding(Nitrogen is infiltrated at a lower temperature to form a nitride layer with small deformation).
Chemical heat treatment relies most on atmosphere control: carburization must be controlled Carbon potential, nitriding should be controlled nitriding potential (Kn). These two types of controls are the core of the subsequent columns in this knowledge base.
Why is it said that “heat treatment quality is controlled”
Heat treatment is a "special process" - the results cannot be fully verified by simple inspection of the finished product, and deep hardened layers and structural defects often require destructive testing. Therefore, systems such as CQI-9 in the automotive industry and AMS2750 in aerospace require continuous monitoring and recording of temperature, atmosphere, and time. Reliable generators, sensors, control systems and data software are the basis for repeatable and traceable heat treatment results.
Frequently asked questions
Will heat treatment change the shape and size of the part?
The design does not change the shape, but phase change and thermal stress will bring about certain dimensional changes and deformation. Nitriding temperature is low, there is no phase change, and the deformation is usually minimal; carburizing and quenching deformation is relatively large, and it needs to be controlled by the furnace loading method, quenching medium and process parameters.
How to choose between carburizing and nitriding?
Carburizing is suitable for parts that require a deeper hardened layer and withstand high contact stress (such as heavy-duty gears); nitriding is suitable for parts that require small deformation, wear and corrosion resistance, and shallow hardened layers (such as crankshafts, molds, precision gears). The specifics need to be combined with material, layer depth requirements and cost assessment.
What kind of heat treatment requires a controlled atmosphere?
Any process where contact with air at high temperatures will cause oxidative decarburization and have requirements for surface quality is suitable for the use of protective atmosphere or controlled atmosphere, including bright annealing, heating before quenching, carburizing, carbonitriding and nitriding, etc.