During the heat treatment of aluminum and aluminum alloys, various issues are commonly encountered, such as:
-Improper part placement: This can lead to part deformation, often due to insufficient heat removal by the quenching medium at a fast enough rate to achieve the desired mechanical properties.
-Rapid heating: This can result in thermal deformation; proper part placement helps ensure even heating.
-Overheating: This can lead to partial melting or eutectic melting.
-Surface scaling/high-temperature oxidation.
-Excessive or insufficient aging treatment, both of which can result in loss of mechanical properties.
-Fluctuations in time/temperature/quenching parameters that can cause deviations in mechanical and/or physical properties between parts and batches.
-Additionally, poor temperature uniformity, insufficient insulation time, and inadequate cooling during solution heat treatment can all contribute to inadequate results.
Heat treatment is a crucial thermal process in the aluminum industry, let’s delve into more related knowledge.
1.Pre-treatment
Pre-treatment processes that improve the structure and relieve stress before quenching are beneficial for reducing distortion. Pre-treatment typically involves processes like spheroidizing annealing and stress relief annealing, and some also adopt quenching and tempering or normalizing treatment.
Stress Relief Annealing: During machining, residual stresses can develop due to factors like machining methods, tool engagement, and cutting speeds. Uneven distribution of these stresses can lead to distortion during quenching. To mitigate these effects, stress relief annealing before quenching is necessary. The temperature for stress relief annealing is generally 500-700°C. When heating in an air medium, a temperature of 500-550°C with a holding time of 2-3 hours is used to prevent oxidation and decarburization. Part distortion due to self-weight should be considered during loading, and other procedures are similar to standard annealing.
Preheat Treatment for Structure Improvement: This includes spheroidizing annealing, quenching and tempering, normalizing treatment.
-Spheroidizing Annealing: Essential for carbon tool steel and alloy tool steel during heat treatment, the structure obtained after spheroidizing annealing significantly affects the distortion trend during quenching. By adjusting the post-annealing structure, one can reduce regular distortion during quenching.
-Other Pre-treatment Methods: Various methods can be employed to reduce quenching distortion, such as quenching and tempering, normalizing treatment. Selecting suitable pre-treatments like quenching and tempering, normalizing treatment based on the cause of distortion and the material of the part can effectively reduce distortion. However, caution is necessary for residual stresses and hardness increases after tempering, especially the quenching and tempering treatment can reduce the expansion during quenching for steels containing W and Mn, but has little effect on reducing deformation for steels such as GCr15.
In practical production, identifying the cause of quenching distortion, whether it’s due to residual stresses or poor structure, is essential for effective treatment. Stress relief annealing should be conducted for distortion caused by residual stresses, while treatments like tempering that alter the structure are not necessary, and vice versa. Only then can the goal of reducing quenching distortion be achieved to lower costs and ensure quality.
2.Quenching Heating Operation
Quenching Temperature: The quenching temperature significantly affects distortion. We can achieve the purpose of reducing deformation by adjusting the quenching temperature, or the reserved machining allowance is the same as the quenching temperature to achieve the purpose of reducing deformation, or reasonably selected and reserved the machining allowance and the quenching temperature after heat treatment tests, so as to reduce the subsequent machining allowance. The effect of quenching temperature on quenching deformation is not only related to the material used in the workpiece, but also related to the size and shape of the workpiece. When the shape and size of the workpiece are very different, although the material of the workpiece is the same, the quenching deformation trend is quite different, and the operator should pay attention to this situation in actual production.
Quenching Holding Time: The selection of holding time not only ensures thorough heating and attaining the desired hardness or mechanical properties after quenching but also considers its effect on distortion. Extending quenching holding time essentially increases the quenching temperature, especially pronounced for high carbon and high chromium steel.
Loading Methods: If the workpiece is placed in an unreasonable form during heating, it will cause deformation due to the weight of the workpiece or deformation due to mutual extrusion between the workpieces, or deformation due to uneven heating and cooling due to excessive stacking of the workpieces.
Heating Method: For complex-shaped and varying thickness workpieces, especially those with high carbon and alloy elements, a slow and uniform heating process is crucial. Utilizing preheating is often necessary, sometimes requiring multiple preheating cycles. For larger workpieces not effectively treated through preheating, using box resistance furnace with controlled heating can reduce distortion caused by rapid heating.
3. Cooling Operation
Quenching deformation primarily results from the cooling process. Proper quenching medium selection, skillful operation, and each step of the cooling process directly influence quenching deformation.
Quenching Medium Selection: While ensuring the desired hardness post-quenching, milder quenching media should be preferred to minimize distortion. Using heated bath mediums for cooling (to facilitate straightening while the part is still hot) or even air cooling is recommended. Mediums with cooling rates between water and oil can also replace water-oil dual mediums.
—Air-cooling quenching: Air-cooling quenching is effective for reducing the quenching deformation of high-speed steel, chromium mold steel and air-cooling micro-deformation steel. For the 3Cr2W8V steel that does not require high hardness after quenching, air quenching can also be used to reduce deformation by properly adjusting the quenching temperature.
—Oil cooling and quenching: oil is a quenching medium with a much lower cooling rate than water, but for those workpieces with high hardenability, small size, complex shape and large deformation tendency, the cooling rate of oil is too high, but for workpieces with small size but poor hardenability, the cooling rate of oil is insufficient. In order to solve the above contradictions and make full use of oil quenching to reduce the quenching deformation of workpieces, people have adopted methods of adjusting oil temperature and increasing quenching temperature to expand the utilization of oil.
—Changing the temperature of quenching oil: using the same oil temperature for quenching to reduce quenching deformation still has the following problems, that is, when the oil temperature is low, the quenching deformation is still large, and when the oil temperature is high, it is difficult to ensure that the workpiece after quenching hardness. Under the combined effect of shape and material of some workpieces, increasing the temperature of quenching oil may also increase its deformation. Therefore, it is very necessary to determine the oil temperature of the quenching oil after passing the test according to the actual conditions of the workpiece material, cross-sectional size and shape.
When using hot oil for quenching, in order to avoid fire caused by high oil temperature caused by quenching and cooling, necessary fire-fighting equipment should be equipped near the oil tank. In addition, the quality index of quenching oil should be tested regularly, and new oil should be replenished or replaced in time.
—Increase the quenching temperature: This method is suitable for small cross-section carbon steel workpieces and slightly larger alloy steel workpieces that cannot meet the hardness requirements after heating and heat preservation at normal quenching temperatures and oil quenching. By appropriately increasing the quenching temperature and then oil quenching, the effect of hardening and reducing deformation can be achieved. When using this method to quench, care should be taken to prevent problems such as grain coarsening, reduction of mechanical properties and service life of the workpiece due to increased quenching temperature.
—Classification and austempering: When the quenching hardness can meet the design requirements, the classification and austempering of the hot bath medium should be fully utilized to achieve the purpose of reducing quenching deformation. This method is also effective for low-hardenability, small-section carbon structural steel and tool steel, especially chromium-containing die steel and high-speed steel workpieces with high hardenability. The classification of hot bath medium and the cooling method of austempering are the basic quenching methods for this kind of steel. Similarly, it is also effective for those carbon steels and low-alloy structural steels that do not require high quenching hardness.
When quenching with a hot bath, the following issues should be paid attention to:
First, when oil bath is used for grading and isothermal quenching, the oil temperature should be strictly controlled to prevent the occurrence of fire.
Second, when quenching with nitrate salt grades, the nitrate salt tank should be equipped with necessary instruments and water cooling devices. For other precautions, please refer to the relevant information, and will not repeat them here.
Third, the isothermal temperature should be strictly controlled during isothermal quenching. High or low temperature is not conducive to reducing quenching deformation. In addition, during austempering, the hanging method of the workpiece should be selected to prevent deformation caused by the weight of the workpiece.
Fourth, when using isothermal or graded quenching to correct the shape of the workpiece while it is hot, the tooling and fixtures should be fully equipped, and the action should be rapid during operation. Prevent adverse effects on the quenching quality of the workpiece.
Cooling Operation: Skillful operation during the cooling process has a significant impact on quenching deformation, especially when water or oil quenching mediums are used.
-Correct Direction of Quenching Medium Entry: Typically, symmetrically balanced or elongated rod-like workpieces should be vertically quenched into the medium. Asymmetric parts can be quenched at an angle. The correct direction aims to ensure uniform cooling across all parts, with slower cooling areas entering the medium first, followed by faster cooling sections. Consideration of the workpiece’s shape and its influence on cooling speed is vital in practice.
-Movement of Workpieces in Quenching Medium: Slow cooling parts should face the quenching medium. Symmetrically shaped workpieces should follow a balanced and uniform path in the medium, maintaining a small amplitude and quick movement. For thin and elongated workpieces, stability during quenching is crucial. Avoid swinging and consider using clamps instead of wire binding for better control.
-Speed of Quenching: Workpieces should be quenched rapidly. Particularly for thin, rod-like workpieces, slower quenching speeds can lead to increased bending deformation and differences in deformation between sections quenched at different times.
-Controlled Cooling: For workpieces with significant differences in cross-section size, protect faster-cooling sections with materials like asbestos rope or metal sheets to reduce their cooling rate and achieve uniform cooling.
-Cooling Time in Water: For workpieces mainly experiencing deformation due to structural stress, shorten their cooling time in water. For workpieces primarily undergoing deformation due to thermal stress, extend their cooling time in water to reduce quenching deformation.
Edited by May Jiang from MAT Aluminum
Post time: Feb-21-2024
