The causes and control methods of heat treatment deformation

 

Date：[2024/1/25]
 

1、 Temperature is a key factor in deformation
There are many forms of heat treatment processes applied in industry, but their basic processes are all thermal processes, which are composed of three stages: heating, insulation, and cooling. The entire process can be described by several parameters such as heating rate, heating temperature, insulation time, cooling rate, and heat treatment cycle.
In the heat treatment process, various heating furnaces are used, and metal heat treatment is carried out in these heating furnaces, such as annealing, quenching, tempering in basic heat treatment, carburizing, nitriding, aluminizing in chemical heat treatment, gas-phase multi-component composite co infiltration, chromizing or dehydrogenation, and so on
Therefore, temperature measurement inside the heating furnace has become an important process parameter measurement for heat treatment. In every heat treatment process specification, temperature is a very important content. If the temperature measurement is not accurate, the heat treatment process specifications cannot be executed correctly, leading to a decrease in product quality or even scrapping. The measurement and control of temperature is crucial for heat treatment processes and also a key factor affecting deformation.
After the process temperature decreases, the high-temperature strength loss of the workpiece is relatively reduced, and the plastic resistance is enhanced. In this way, the comprehensive ability of the workpiece to resist stress deformation, quenching deformation, and high-temperature creep is enhanced, and deformation will be reduced.
After the process temperature decreases, the temperature range for heating and cooling the workpiece decreases, resulting in a decrease in temperature inconsistency in various parts. As a result, the thermal stress and tissue stress are relatively reduced, leading to a reduction in deformation;
If the process temperature decreases and the heat treatment process time is shortened, the high-temperature creep time of the workpiece will decrease, and the deformation will also decrease.
2、 Other influencing factors of deformation and measures to reduce them
1. Preparation for heat treatment
Excessive normalizing hardness, mixed crystals, a large amount of martensite or Weinstein structure can increase the deformation of the inner hole, so temperature controlled normalizing or isothermal annealing should be used to treat forgings. The normalization, annealing, and quenching and tempering of metals all have a certain impact on the final deformation of the metal, directly affecting the changes in the metal's microstructure. Practice has shown that isothermal (graded) quenching during normalizing can effectively homogenize the metal structure and reduce its deformation.
2. Use reasonable cooling methods
The cooling process after metal quenching is also an important cause of deformation. In the case of quenching, the deformation of hot oil quenching is smaller than that of cold oil quenching, generally controlled at 100 ± 20 ℃. The cooling ability of oil is also crucial for deformation. The stirring method and speed of quenching both affect deformation.
The faster the cooling rate of metal heat treatment, the more uneven the cooling, the greater the stress generated, and the greater the deformation of the mold. Pre cooling can be used as much as possible while ensuring the hardness requirements of the mold; The use of graded cooling quenching can significantly reduce the thermal and structural stresses generated during metal quenching, and is an effective method to reduce the deformation of some complex shaped workpieces; For some particularly complex or precision demanding workpieces, isothermal (or graded) quenching can significantly reduce deformation.
3. The structure of the parts should be reasonable
During the cooling process after metal heat treatment, the thin parts always cool faster and the thick parts cool slower. In order to meet the actual production needs, the thickness difference of the workpiece should be minimized as much as possible, and the cross-section of the parts should be made uniform to reduce the distortion and cracking tendency caused by stress concentration in the transition zone; The workpiece should maintain symmetry between its structure, material composition, and microstructure as much as possible to reduce distortion caused by uneven cooling; Workpieces should avoid sharp edges, grooves, etc. as much as possible, and there should be rounded transitions at the junction of thickness and steps of the workpiece; Minimize the asymmetry of hole and groove structures on the workpiece as much as possible; The method of reserving machining quantity is used for parts with uneven thickness.
4. Adopt reasonable clamping methods and fixtures
The purpose is to evenly heat and cool the workpiece, reduce uneven thermal stress and tissue stress, and reduce deformation. The clamping method can be changed, with disc parts perpendicular to the oil surface and shaft parts installed vertically. Compensation washers, support washers, overlay washers, etc. can be used, and carburized mandrels can be used for keyhole parts.
5. Mechanical processing
When heat treatment is the final step in the workpiece processing, the allowable value of heat treatment distortion should meet the workpiece size specified on the drawing, and the distortion variable should be determined based on the machining size of the previous step. Therefore, according to the distortion pattern of the workpiece, size pre correction should be carried out before heat treatment to ensure that the heat treatment distortion is within the qualified range. When heat treatment is an intermediate process, the machining allowance before heat treatment should be considered as the sum of machining allowance and heat treatment deformation. Usually, the machining allowance is easy to determine, while heat treatment is more complex due to multiple influencing factors. Therefore, sufficient machining allowance is left for machining, and the rest can be used as allowable deformation variables for heat treatment. After heat treatment and further processing, according to the deformation pattern of the workpiece, apply anti deformation and pre expansion holes at the contraction end to improve the qualified deformation rate after quenching.
6. Use appropriate media
On the premise of ensuring the same hardness requirements, oily media should be used as much as possible. Experiments and practices have shown that, under no other conditions, the cooling speed of oily media is slower, while that of water-based media is relatively faster. Moreover, compared with oil-based media, changes in water temperature have a greater impact on the cooling characteristics of water-based media. Under the same heat treatment conditions, the deformation of oil-based media after quenching is relatively small and stable compared to water-based media.
conclusion
Heat treatment can improve the mechanical properties of parts, increase their strength and hardness, and meet various performance needs, but the deformation caused by it is inevitable. When selecting specific methods to prevent heat treatment deformation, specific methods should be developed based on the specific situation. Many methods are derived from practice and require repeated experimentation to explore patterns.