Thermomechanical treatment is a metal heat treatment process that combines pressure processing with heat treatment to effectively utilize deformation strengthening and phase transformation strengthening in metal materials.
By combining pressure processing with heat treatment operations, this process unifies the forming process with obtaining final properties.
Now, let's explore the categories and applications of thermomechanical treatment.
1. Review the iron-carbon phase diagram
| Symbol | Meaning |
| TO 1 | Temperature at which austenite, ferrite, cementite or carbide coexist in equilibrium |
| A3 | The highest coexistence temperature of austenite and ferrite in hypoeutectoid steel at equilibrium |
| Ac1 | Temperature at which austenite begins to form when steel is heated |
| Ac3 | The temperature at which all ferrite is transformed to austenite when hypoeutectoid steel is heated. |
| Ar1 | The temperature at which austenite decomposes into ferrite and pearlite during austenitization and high-temperature cooling of steel. |
| Ar3 | The temperature at which cooling austenite begins to precipitate free ferrite. |
| Acm | The highest temperature at which austenite and cementite or carbide coexist in hypereutectoid steel in an equilibrium state is the upper critical point of hypereutectoid steel. |
| Accum | The final temperature at which all secondary cementite dissolves into austenite during heating. |
| Bow | The temperature at which austenite begins to precipitate secondary cementite during cooling. |
Thermomechanical treatment
This is a comprehensive strengthening process that combines deformation strengthening and phase transformation strengthening. It involves two processes, namely plastic deformation and solid phase transformation of metallic materials, which are integrated to influence the phase transformation process and the products. By utilizing the changes in the structure of metallic materials during deformation, this process aims to obtain the desired structure and properties.
2. High temperature strain hardening
Process characteristics
Deformation is carried out above the Ar3 transformation temperature of the steel or between the temperature range Ar1 and Ar3. Alternatively, it can be carried out above the solution heat treatment temperature of the alloys. After deformation, the material is quenched and tempered.
Effect and application
Eliminating reheating and quenching processes can increase steel strength by 10-30%, improve its toughness and fatigue resistance, and decrease the risk of quenching brittleness. This technique is also effective in improving the strength and plasticity of non-ferrous alloys.
The method is commonly used in the production of sheets, strips, tubes, wires and bars of carbon steel, low and medium alloy steel, as well as simple shaped mechanical parts.
3. Controlled rolling
Process characteristics
The steel is deformed above Ar3 or between Ar1 and Ar3. Then it is cooled with air or water until it reaches above 550℃, and further air-cooled to obtain the ferrite-pearlite or bainite structure.
Effect and application
By increasing the yield strength, excellent toughness at low temperatures can be achieved, making it suitable for the production of a range of products including low carbon steel, weldable steel plate, strip, wire rod and other items containing Nb , V and Ti. . These products are not quenched or tempered.
4. Low temperature strain hardening
Process characteristics
The steel is deformed in the stable zone of subcooled austenite (500 ~ 600 ℃) and then quenched and tempered.
Effect and application
Under the condition of ensuring the plasticity of steel, its strength can be significantly increased. This is applicable to components made of medium-alloy high-strength steel that require high-strength and small-section high-strength steel wires, as well as high-alloy steel molds, high-speed steel tools and so on.
5. Isothermal deformation heat treatment
Process characteristics
(a) Deformation occurs before and during the temperature range of pearlite transformation from steel.
(b) Deformation can also occur after pearlite transformation.
Effect and application
(a) Fine subgrains of ferrite and spherical carbides can improve the impact strength of steel by several times and are suitable for the production of small structural alloy steel parts.
(b) This process can significantly reduce the spheroidization time, lower the spheroidization temperature and improve the spheroidization structure. It is commonly used in the manufacture of tool steel and bearing steel.
6. Deformation heat treatment inducing martensitic transformation
Process characteristics
Deformation is carried out in the temperature range Ms ～ Md of the steel.
Effect and application
Improve strength under the condition of ensuring plasticity.
It is applicable to austenitic stainless steel and steel with transformation-induced plasticity (TRIP steel).
7. Supersaturated solid solution deformation aging treatment
Process characteristics
After solution treatment, the steel or alloy must be cold or hot worked before aging.
Effect and application
The strength is significantly improved and the required plasticity can still be guaranteed.
Used for types of steel or alloys that require reinforcement, such as austenitic steel, maraging steel, nickel-based superalloy, aluminum alloy, copper alloy, etc.
8. Thermomechanical pre-treatment
Process characteristics
The process begins with cold deformation at room temperature, followed by intermediate tempering. After that, rapid secondary heating and quenching are carried out, followed by final tempering.
Effect and application
It can still retain the deformation strengthening effect, making it suitable for producing cold-rolled steel pipes, cold-drawn high-strength steel wires or small parts with simple shapes that can be cold formed.
