I. Pretreatment
Before hardening, stress relief and preparatory heat treatment to improve the structure of the part are very beneficial to reduce hardening deformation.
Pretreatment often includes spheroidization annealing and stress relief annealing, and some also utilize quenching or normalizing treatments.
1. Stress relief annealing:
During the mechanical machining process, the workpiece surface will generate residual stress under the influence of machining methods, rear tool consumption, cutting speed, etc.
Due to its uneven distribution, it causes the workpiece to deform during hardening. To eliminate the influence of these stresses, it is necessary to anneal the part once before hardening to relieve the stresses. The temperature of stress relieving annealing is generally 500-700℃.
When heated in air, to prevent oxidation and decarburization of the workpiece, annealing can be done at 500-550 ℃, and the holding time is generally 2-3 hours.
Care must be taken when loading the furnace to avoid deformation caused by its own weight, and other operations are the same as general annealing operations.
2. Pre-treatment with the aim of improving the structure:
This type of pretreatment includes spheroidizing annealing, quenching and normalizing, etc.
(1) Spheroidization Annealing:
Spheroidization annealing is an indispensable process in the heat treatment of carbon tool steel and alloy tool steel. The structure obtained after spheroid annealing greatly influences the hardening deformation tendency.
Therefore, the structure after annealing can be adjusted to reduce the regular hardening deformation of some workpieces.
(2) Other pretreatments:
There are many pretreatment methods used to reduce hardening deformation, such as quenching treatment, normalizing treatment, etc.
Choosing the right normalizing, tempering and other pretreatments according to the reasons of hardening deformation and the materials used in the workpiece is effective in reducing hardening deformation.
Attention should be paid to the adverse effects of residual stress and hardness increase after normalizing in machining, and at the same time, tempering treatment can reduce the expansion during hardening of steel containing W and Mn, but does not significantly reduce the deformation of steel such as GCr15.
In actual production, it is important to distinguish the causes of hardening deformation, that is, to clarify whether the hardening deformation is caused by residual stress or poor structure.
Only then can treatment be targeted. If hardening deformation is caused by residual stress, stress relieving annealing should be carried out instead of pretreatments such as tempering that change the structure and vice versa.
Only in this way can the objective of reducing hardening deformation be achieved, and the cost can be reduced and quality guaranteed.
The specific operations of the above various pretreatments are the same as those of other corresponding operations and are not elaborated here.
II. Quenching heat treatment operation
1. Quenching Temperature:
The quenching temperature greatly influences the distortion of the part during quenching. The general trend of its impact on extinction distortion is shown in the figure.
Based on the curve shown in the figure, we can reduce the distortion by adjusting the quenching temperature or judiciously selecting and using the machining tolerance in conjunction with the quenching temperature after heat treatment tests, thereby reducing the subsequent machining tolerance.
The impact of quenching temperature on quenching distortion is related not only to the material used in the part, but also to the size and shape of the part.
Even when parts are made from the same material, their distortion tendencies can differ greatly when their shapes and sizes vary significantly. Operators must pay attention to this in actual production.
2. Quench retention time:
In addition to ensuring that the part is completely heated and reaches the required hardness or other mechanical properties after quenching, retention time selection must also consider its impact on quench distortion.
Prolonging the quenching holding time actually correspondingly increases the quenching temperature. This impact is particularly prominent for high carbon and high chromium steels.
3. Furnace loading method:
If the workpiece is positioned incorrectly during heating, it may deform due to its own weight, deformation caused by compression between workpieces, or uneven heating and cooling due to dense stacking.
For example, a spring component was hung vertically and heated in a furnace with a protective atmosphere at 860±10°C for 30 minutes. After fixing, the part was quenched vertically in cooling oil.
After quenching, the total length of the spring was reduced by 27 mm, and the pitch at the top and bottom was deformed differently due to the difference in time at the inlet of the quenching medium.
By changing the method to hang the spring horizontally on a central axis in the furnace and keeping all other processes the same, the distortion was greatly improved after quenching – the pitch was uniform and the total length shrinkage was less.
Especially for slender parts, not only should they not be stacked densely sideways in the furnace, but the possibility of deformation caused by the rolling of the heating medium in the salt bath furnace must also be considered.
When loading slender, lightweight rod-like workpieces into the salt bath furnace, the furnace is first raised to a temperature slightly above the quenching temperature, power is cut off, and then the workpiece is loaded into the furnace of salt bath. The furnace load must be constant and the workpiece must be heated without power to reduce its quenching distortion.
4. Heating method:
For parts with complex shapes and significant variations in thickness, especially when their materials have a high content of carbon and alloy elements, the heating process must be slow and uniform, taking full advantage of the preheating process.
If preheating is not sufficient, use secondary or tertiary preheating. For larger parts that still deform from preheating, the box shield can be used for heating in a box-type resistance furnace.
In addition to limiting the rate of temperature increase during heating, the isothermal process can be increased to reduce quenching distortion caused by heating too quickly.
III. Cooling operation
Quenching deformation results mainly from the cooling process. Appropriate choice of quenching medium, proficient operating skills and each step of the cooling process directly influence the deformation of the quenching workpiece.
1. Appropriate Selection of Quenching Medium:
To ensure that the hardness of the part meets the design requirements after quenching, a softer quenching medium should be used as much as possible during quenching.
For example, using a heated bath medium for cooling (straightening the part while it is still hot during cooling using a heated bath medium) can be beneficial. As much as possible, air-cooled quenching or a quenching medium with a cooling rate between water and oil should be used instead of a double quenching medium with water and oil.
(1) Air-cooled quenching:
Air-cooled quenching is effective in reducing quenching deformation of high-speed steel, chrome mold steel, and air-cooled microdeformed steel.
For 3Cr2W8V steel, which does not require high hardness after quenching, air quenching can also be used to reduce deformation by properly adjusting the quenching temperature.
(2) Oil cooling quenching:
Oil is a quenching medium with a much slower cooling rate than water. However, for parts with high quenching permeability and small size or complex shape, the oil cooling speed can still be considered too high.
For larger parts with low quenching permeability, the oil cooling speed may not be sufficient. To resolve these contradictions and fully utilize oil quenching to reduce the quenching deformation of the workpiece, measures such as adjusting the oil temperature and increasing the quenching temperature have been adopted.
(3) Quenching oil temperature change:
There are several problems with using quenching oil to reduce quenching deformation. When the oil temperature is too low, the quenching deformation is still high, and when the oil temperature is too high, it is difficult to guarantee the hardness of the workpiece after quenching.
For some workpieces, increasing quenching oil temperature can actually increase deformation due to the combined effects of shape and material. Therefore, it is essential to determine the quenching oil temperature based on the actual conditions of the workpiece material, cross-sectional size and shape through experimentation.
During hot oil quenching, to avoid fire caused by high oil temperatures due to quenching cooling, necessary fire-fighting equipment should be provided near the oil tank.
In addition, the quality of quenching oil must be checked regularly and must be refilled or replaced in a timely manner.
(4) Increase in quenching temperature:
This method is suitable for small cross-section carbon steel parts and slightly larger alloy steel parts that cannot reach the required hardness after quenching in oil at normal quenching temperatures.
By appropriately increasing the quenching temperature and then oil quenching, both hardening and deformation reduction can be achieved. Pay attention to avoid potential problems such as grain coarsening, reduced mechanical properties, and decreased part life caused by increased quenching temperature when using this quenching method.
(5) Graduated Quenching, Isothermal:
When the hardness can meet the design requirements, the graduated isothermal quenching of the heated bath medium should be fully utilized to reduce the quenching deformation.
This method is equally effective for low-permeability and small cross-section carbon structural steel and tool steel, especially for high-permeability chrome mold steel and high-speed steel parts.
The graduated isothermal quenching cooling method is the basic quenching method for these types of steel. Likewise, this method is also effective for carbon steel and low-alloy structural steel with lower hardness requirements after quenching.
In the process of using tempering in a hot bath, the following points must be taken into account:
- First, during the oil bath and isothermal quenching phase, the oil temperature must be strictly controlled to prevent fires from occurring.
- Secondly, when using graded nitrate quenching, the nitrate tank must be equipped with the necessary instruments and water cooling devices. For other precautions, please refer to the relevant materials and will not be detailed here.
- Third, during isothermal quenching, it is crucial to strictly control the isothermal temperature. Very high or low deviations do not lead to a reduction in quenching deformation. Furthermore, the method of suspending the part during isothermal quenching must be carefully selected to avoid deformation caused by the part's own weight.
- Fourth, when correcting the shape of the part using isothermal or graduated quenching, the device must be fully equipped and the movements must be fast during operation to avoid any adverse effects on the quenching quality of the part.
2. Cooling operation:
Operational proficiency during the cooling process greatly impacts the distortion resulting from quenching, especially when using quenching media such as water or oil, where operational proficiency is even more crucial.
(1) Correct Direction for Immersion in Quenching Medium:
Generally, transversely symmetrical and elongated rod-shaped workpieces are immersed vertically in the quenching medium, while asymmetrical workpieces can be immersed diagonally.
The correct immersion direction is one that allows uniform cooling of all parts of the part. The slower cooling parts should be immersed in the quenching medium first, followed by the faster cooling parts.
In actual production, it is important to pay attention to the impact of part shape on cooling speed. A larger surface area of the part does not necessarily mean faster cooling, especially if the shape of that part is complex.
Uneven cooling can lead to slower cooling speeds than parts with smaller surface areas. Therefore, the direction of entry into the quenching medium must be determined based on the specific shape of the part.
(2) Movement of the part in the quenching medium:
Slower cooling parts should move against the flow of water. Symmetrical pieces must move symmetrically and uniformly in the water, with a small range of motion and high speed.
Thin and elongated workpieces should be stable when immersed in the quenching medium and should not wobble. These types of workpieces are best tempered with pliers rather than strung with wire.
(3) Speed of immersion of the part in the quenching medium:
The immersion speed of the part in the quenching medium must be fast. Particularly for elongated tubular parts, slow immersion speeds can lead to increased bending and distortion, and cause a greater difference in distortion between the parts of the tubular part that are immersed first and last.
(4) Cooling with additional protection:
Workpieces with significant differences in cross-sectional dimensions should have the faster-cooling parts tied and protected with materials such as asbestos rope or sheet metal to reduce the rate of cooling of these parts, thus ensuring uniform cooling of all parts of the workpiece. work piece.
(5) Cooling time of the part in water:
For workpieces that are distorted mainly due to internal stresses, the water cooling time can be reduced. On the other hand, for parts distorted mainly due to thermal stress, the water cooling time can be appropriately extended to reduce distortion after quenching.
