In the processing of metal materials, heat treatment technology is widely used. The technology through heating, holding and cooling steps, change the internal organization of metal materials, and then improve their mechanical properties and stability.
However, the deformation and cracking problems generated in the heat treatment process have always been a difficult problem for the metal processing industry, which not only affect the machining accuracy and surface quality of metal parts, but also may lead to the failure and scrap of parts, which bring great losses to the production.
Therefore, it is of great significance to deeply analyze the deformation and cracking problems in the heat treatment of metal materials and seek effective measures to solve them in order to improve the processing quality and production efficiency of metal materials.
Overview and classification of heat treatment of metal materials
Overview of heat treatment
Heat treatment of metal materials is a process method to change the internal organization of metal materials through heating, holding and cooling steps, so as to improve their mechanical properties and stability, the operation of heating and cooling in practice, as shown in Figure 1 and Figure 2.
This process is widely used in automobile manufacturing, aerospace, machinery manufacturing and other fields, aimed at adjusting the hardness, strength, toughness and other performance indicators of metal materials to meet the demand for use under different working conditions.
However, the heat treatment process also exists deformation and cracking and other problems, mainly caused by the heating temperature, cooling rate, material composition and organizational structure and other factors, need to strictly control the process parameters to avoid these problems.
Heat treatment classification
The heat treatment process of metal materials mainly contains quenching, annealing and tempering three categories.
First, the quenching process, the process consists of heating the steel to a critical temperature above, so that its complete austenitization, so as to obtain a high hardness martensitic organization.
This process is mainly used to increase the hardness and strength of steel, but the quenched martensite usually requires tempering to reduce brittleness.
-The key lies in the selection of suitable cooling medium and cooling rate, as well as the control of heating temperature and holding time.
The second is annealing process, the metal material is heated to a certain temperature and then slowly cooled, used to eliminate or reduce the internal stress of the material, refine the grain, reduce the hardness, and improve the plasticity.
Based on the differences in heating temperature, holding time and cooling strategy, the annealing process can be subdivided into complete annealing, isothermal annealing, spheroidal annealing and stress relief annealing and other forms to adapt to the different types of metal materials and differences in application requirements.
Thirdly, the tempering process involves heating the quenched metal material to a certain temperature and holding it for a certain period of time, followed by slow cooling, which is used to eliminate or reduce quenching stresses, reduce brittleness and improve plasticity and toughness.
Selecting the appropriate tempering temperature and holding time is the key to the tempering process, and different parameters will have different effects on the organization and properties of metal materials.
Metal materials heat treatment causes deformation and cracking
Improper cooling method
Different cooling media and cooling rate will have different effects on the organization and properties of metal materials.
In the quenching process, if the cooling rate is too fast, the metal material will accumulate significant thermal stress and organizational stress, which in turn induces material deformation and even cracking.
Especially when there is a large internal stress in the metal material, it is easier to cause cracking phenomenon.
On the contrary, if the cooling rate is too slow, it will lead to the transformation of austenite to pearlite in the cooling process of the metal material, resulting in large volume changes, which will lead to deformation problems.
In addition, the type and temperature of the cooling medium will also have an effect on the deformation and cracking of the metal material.
For example, when water is used as the cooling medium, it is easy to cause rapid cooling and cracking on the surface of the metal material due to its strong cooling ability;
While the use of oil as a cooling medium, due to the weak cooling capacity of oil, can slow down the cooling rate of metal materials and reduce the risk of cracking.
Inaccurate temperature control
The accuracy of temperature control directly affects the organization and properties of metal materials, as well as the occurrence of deformation and cracking problems.
In the heat treatment process, if the heating temperature is too high or the holding time is too long, it will lead to problems such as coarse grains and uneven organization within the metal material, which in turn exacerbates the risk of deformation and cracking.
Conversely, if the heating temperature is low or the holding time is insufficient, it may lead to insufficient austenitization of the metal material or incomplete transformation of the tissue structure, which will also adversely affect the material properties and increase the probability of deformation and cracking.
In addition, the temperature control in the cooling process is also critical to avoid excessive cooling speed or uneven cooling occurs, otherwise it will lead to greater thermal stress and organizational stress within the metal material, thus triggering deformation and cracking.
Working environment
The temperature, humidity, gas composition and other factors in the working environment will have an impact on the organization and properties of metal materials.
When heat treatment is carried out in a high-temperature environment, metal materials are prone to oxidation and decarburization and other reactions, leading to a decline in the surface quality of the material and increasing the risk of deformation and cracking.
In addition, when working in a high-temperature environment, operators need to pay attention to preventing safety accidents such as burns and fires.
When heat treatment is carried out in a humid environment, metal materials are prone to corrosion and rust, which will also affect the performance of the materials and increase the possibility of deformation and cracking.
Influence of raw material organization and properties
The organization and properties of raw materials also affect the deformation and cracking of metal materials during heat treatment.
If there are more inclusions, porosity and other defects in the raw material, it will cause the material to be more prone to deformation and cracking during the heat treatment process.
In addition, the chemical composition and microstructure of the raw material will also have an impact on the heat treatment effect and deformation and cracking behavior of the material.
For example, high-strength materials such as high-carbon steel and alloy steel are more prone to deformation and cracking during heat treatment.
This is because these materials have more carbides and alloying elements and other hard phases, resulting in the material in the heat treatment process is more likely to produce stress and cracks.
Therefore, in the selection of raw materials need to fully consider its organization and performance characteristics, as well as heat treatment requirements and other factors .
Deformation and cracking measures to solve the problem
Optimize the cooling method
Selection of suitable cooling medium
According to the type, shape and size of the metal material and other factors to choose the appropriate cooling medium. For high hardness, high brittleness of the metal material, you can choose the weak cooling capacity of the oil as a cooling medium; for low hardness, low brittleness of the metal material, you can choose the cooling capacity of the water as a cooling medium.
Control cooling speed
Control the cooling rate by adjusting the temperature and flow rate of the cooling medium. In the quenching process, you can take section quenching or graded quenching, etc., to slow down the cooling rate and reduce the stress generated; in the tempering process, you need to take a slow cooling method to eliminate or reduce the quenching stress.
Use of quenching medium additives
Adding the right amount of additives to the quenching medium can improve its cooling performance and lubricating properties, thereby reducing the risk of deformation and cracking of metal materials in the quenching process.
For example, adding inorganic salts such as table salt or alkali to water can improve the cooling capacity and stability of water;
Adding organic substances such as antioxidants or anti-corrosion agents to the oil can improve the lubricating properties and antioxidant properties of the oil.
Enhance temperature control
Calibrate heat treatment equipment
Regularly calibrate and maintain the heat treatment equipment to ensure that its temperature measurement system and control system can accurately measure and control the temperature, and the aging equipment needs to be replaced or upgraded in time.
Optimize heat treatment process parameters
Optimize the heat treatment process parameters according to the type, shape and size of the metal material, including heating temperature, holding time and cooling speed.
Determine the optimal heat treatment process parameters through testing and simulation and other methods to reduce the risk of deformation and cracking.
Adopt advanced temperature measurement technology
Adopt advanced temperature measurement technology, such as infrared thermometer, thermocouple, etc., to monitor the temperature change of metal materials in the heat treatment process in real time, so as to adjust the process parameters in time and reduce the risk of deformation and cracking.
Scientific selection of quenching medium
Cooling capacity
The cooling capacity of the quenching medium directly affects the cooling rate and thermal stress of the metal material. Therefore, in the selection of quenching media need to fully consider its cooling capacity to meet the heat treatment requirements.
Stability
Quenching medium in the use of the process needs to maintain stability and uniformity, in order to avoid changes in the composition of the medium due to changes in the performance of metal materials or deformation caused by cracking and other issues.
Environmental protection
With the growing awareness of environmental protection, the choice of environmentally friendly quenching medium has become an important trend in the metal heat treatment industry.
Traditional quenching media, such as oil, water, in some cases may cause pollution to the environment, therefore, the development and use of non-toxic, harmless, biodegradable quenching media for the protection of the environment and sustainable development is of great significance.
For example, some new water-soluble quenching agent, polymer quenching agent, etc., not only has good cooling performance and stability, but also environmentally friendly, in line with the green production requirements of modern industry.
Improve the working environment
Control temperature and humidity
Install temperature and humidity control equipment in the heat treatment workshop to ensure that the temperature and humidity in the working environment are kept within a suitable range.
This helps to reduce problems such as oxidation, decarburization and corrosion of metal materials during heat treatment, thereby reducing the risk of distortion and cracking.
Optimizing gas composition
The rate of oxidation and decarburization of metal materials during heat treatment can be reduced by controlling the composition of the furnace atmosphere.
For example, the use of inert gas protection or vacuum heat treatment and other methods can effectively reduce the contact between metal materials and harmful substances such as oxygen and moisture in the air, thus reducing the possibility of deformation and cracking.
Strengthen safety management
In the heat treatment process, safety management needs to be strengthened to ensure the safety and health of the operators.
For example, regular maintenance and inspection of heat treatment equipment to ensure its normal operation;
Provide necessary protective equipment and training for operators to improve safety awareness and operation skills;
Formulating emergency plans and measures to deal with possible safety accidents.
Optimize raw material selection and handling
Selection of high-quality raw materials
When purchasing raw materials, attention should be paid to their quality and performance. Selecting those high-quality raw materials with few inclusions, low porosity, and uniform chemical composition and microstructure will help reduce the risk of deformation and cracking during heat treatment.
Pre-treatment of raw materials
Pre-treatment of raw materials before heat treatment, such as forging, rolling, annealing, etc., can improve their organization and properties and reduce the tendency of deformation and cracking in the heat treatment process.
For example, forging can refine the grain of the raw material and eliminate its internal defects; annealing can reduce the hardness and brittleness of the raw material and improve its plasticity.
Case Study
Case background
A machinery manufacturing enterprises manufactured a number of specifications for the outer diameter φ140mm, inner diameter φ100mm, length 320mm of No. 45 steel bushing. In order to enhance the mechanical properties of the bushing, the manufacturer decided to adopt the tempering heat treatment program.
The program includes: firstly, the bushing is heated to 840℃ for quenching, and cooled by water cooling; then it is tempered at 550℃ with a holding time of 2h, and finally air-cooled.
The expected hardness of the bushing after this process should be between 25 and 30 HRC in order to meet the established specifications.
However, out of the first 12 bushings treated, 2 developed cracks. This problem seriously affected the quality of the product and the production schedule, so the cause of the cracks needed to be analyzed in depth.
Cause analysis
First, the cooling is not uniform. Since the bushing is hollow, during the quenching and cooling process, there is a significant difference between the cooling rate of the inner and outer walls.
The external cooling is faster, while the inner wall is slower, and this uneven cooling rate leads to uneven stress distribution, which in turn induces cracks.
Secondly, residual stress. The bushing may have generated internal stresses during the rolling process and failed to eliminate these stresses well before heat treatment.
Therefore, in the heat treatment process, the rolling residual stress and quenching stress are superimposed on each other, and when the superimposed stress level exceeds the tensile strength of the steel, the bushing will fracture.
Process optimization strategy
For the above causes, the following two key improvement measures were implemented:
First, stress relief annealing.
Before quenching, the first sleeve for stress relief annealing, the specific operation is to heat the sleeve to 700 ℃, and maintain a 1h insulation, and then out of the air-cooled, this step effectively reduces the residual stress in the sleeve, for the subsequent quenching process to lay a good foundation.
Secondly, the quenching and cooling method innovation.
In the quenching and cooling stage, the use of vertical water method, and so that the bushing in the quenching liquid continues to move up and down, which accelerates the cooling speed of the inner wall of the bushing, effectively alleviate the inconsistency of the cooling rate of the inner and outer walls, thereby reducing the accumulation of cooling stress.
Evaluation of Improvement Effectiveness
After the implementation of the above process improvement measures, the bushing in the heat treatment process did not appear cracks, this significant improvement not only improves the quality of the product, but also ensures that the bushing production is carried out smoothly.
Therefore, in the heat treatment of metal materials, it is necessary to give full consideration to the inherent characteristics of the material and the reasonable selection of the heat treatment process, in order to avoid the occurrence of deformation and cracking problems.
Conclusion and Prospect
To sum up, the deformation and cracking problem in the heat treatment process of metal materials is a complex and important subject.
This article deeply analyzes the reasons behind these deformation and cracking issues. It also puts forward targeted solution measures to address them.
The risk of deformation and cracking in the heat treatment process of metal materials can be effectively reduced through several methods.
These include optimizing the cooling method and strengthening temperature control.
It is also important to scientifically select the quenching medium and improve the working environment.
Additionally, optimizing the selection and treatment of raw materials plays a crucial role.
In the future, with the progress of science and technology and industrial development, metal heat treatment technology will be continuously improved and innovated.
For example, the use of advanced computer simulation technology can predict the deformation and cracking behavior of metal materials during the heat treatment process.
Additionally, the development of new types of heat treatment equipment and processes can further improve the performance and quality of metal materials.
These advancements provide new opportunities and challenges for the development of the metal heat treatment industry.
