Before quenching, it is very beneficial to reduce quenching deformation by preparatory heat treatment to eliminate stress and improve the structure of the workpiece. Pretreatment generally includes spheroidizing annealing, and stress relief annealing, and some also use quenching and tempering or normalizing treatment.
① Stress relief annealing: During the machining process, under the influence of the processing method, back-cutting amount, cutting speed, etc., the surface of the workpiece will produce a certain amount of residual stress. Due to its uneven distribution, the workpiece will be produced during quenching. out of shape. In order to eliminate the influence of these stresses, it is necessary to perform stress-relief annealing on the workpiece before quenching. The temperature of stress relief annealing is generally 500-700 ℃. When heating in the air medium, in order to prevent oxidation and decarburization of the workpiece, it can be annealed at 500-550 ℃, and the holding time is generally 2-3h. When the workpiece is loaded into the furnace, attention should be paid to the possible deformation caused by its own weight, and other operations are the same as general annealing operations.
② Preheating treatment for the purpose of improving the structure: this pretreatment includes spheroidizing annealing, quenching and tempering, and normalizing.
—Spheroidizing annealing: Spheroidizing ball annealing is an essential process in the heat treatment process of carbon tool steel and alloy tool steel. The structure obtained after spheroidizing annealing has a great influence on the quenching deformation trend. Therefore, the regular quenching deformation of some workpieces can be reduced by adjusting the structure after annealing.
—Other pretreatments: There are many pretreatment methods used to reduce quenching deformation, such as quenching and tempering treatment, normalizing treatment, etc. Aiming at the reasons for the quenching deformation of the workpiece and the materials used in the workpiece, it is effective to reduce the quenching deformation by rationally selecting pretreatments such as normalizing and quenching and tempering. However, attention should be paid to the adverse effects of residual stress and hardness increase on machining caused by normalizing. At the same time, quenching and tempering treatment can reduce expansion during quenching for steels such as W and Mn, but it has no effect on reducing the deformation of steels such as GCr15. big.
In actual production, it is necessary to distinguish the cause of quenching deformation, that is, to distinguish whether quenching deformation is caused by residual stress or poor organization. Only in this way can symptomatic treatment be performed. If the quenching deformation is caused by residual stress, stress relief annealing should be carried out without pretreatment such as quenching and tempering to change the structure, and vice versa. Only in this way can the purpose of reducing quenching deformation be achieved, the cost can be reduced and the quality can be guaranteed.
The specific operations of the above various preprocessing are the same as other corresponding operations, and will not be repeated here.
2. Quenching and heating operation
①Quenching temperature: The quenching temperature has a great influence on the quenching deformation of the workpiece. The general law that affects the quenching deformation trend is shown in the figure. According to the influence law of the curve shown in the figure on the quenching distortion, we can reduce the deformation by adjusting the quenching temperature, or setting aside the machining allowance at the same quenching temperature to achieve the purpose of reducing the deformation, or setting aside the machining allowance The allowance is reasonably selected and used after the heat treatment test at the same quenching temperature, 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.
②Quenching holding time: In addition to ensuring that the workpiece is thoroughly burned and quenched to achieve the required hardness or other mechanical properties, the selection of holding time should also consider its influence on quenching deformation. Prolonging the quenching and holding time actually increases the quenching temperature accordingly. Especially for high-carbon high-chromium steel, this effect is particularly prominent.
③ Furnace loading method: If the workpiece is placed in an unreasonable form during heating, deformation will occur due to the weight of the workpiece or deformation caused by the mutual extrusion between the workpieces, or deformation caused by uneven heating and cooling due to excessive stacking of workpieces.
For example, a spring part was heated and kept in a protective atmosphere heating furnace at 860±10°C for 30 minutes by hanging vertically during quenching. After keeping warm, it was taken out of the furnace and quenched vertically into the cooling oil. After quenching, the total length of the spring was shortened. 27mm, and the deformation of the upper and lower pitches is different due to the time difference of quenching into the quenching medium. After that, the spring is used to cover the mandrel, and the furnace is hung horizontally. The other operations are the same as before. After quenching, the deformation is greatly improved, the pitch is uniform, and the total length shrinkage is small.
In particular, slender workpieces cannot be loaded in the furnace in a close-packed and horizontal manner. When heating in a salt bath furnace, the possibility of deformation of the workpiece due to the tumbling of the heating medium should also be considered. When slender and lightweight rod workpieces are loaded into the furnace, the salt bath furnace is generally raised to a temperature slightly higher than the quenching temperature, and then the workpiece is loaded into the salt bath furnace after the power is turned off. The workpiece is heated without electricity, so as to reduce the quenching deformation of the workpiece.
④ Heating method: When the shape of the workpiece is complex and the thickness disparity is large, especially when the material contains high carbon and alloy elements, the heating process must be slow and uniform, and the preheating process should be fully utilized. One-time preheating is not enough, using two or three preheating. For larger workpieces that cannot be deformed by preheating, they can also be heated in a box-type resistance furnace with box protection. During heating, in addition to speed-limited heating, an isothermal process can also be added to reduce damage caused by excessive heating speed. resulting in quenching deformation.
3. Cooling operation
Quenching deformation mainly comes from the cooling process. Reasonable quenching medium, proficient operation skills, and every link in the cooling process directly affects the quenching deformation of the workpiece.
1. Reasonable selection of quenching medium: In the case of ensuring that the hardness of the workpiece after quenching meets the design requirements, a milder quenching medium should be selected as much as possible when quenching the workpiece. For example, use the heating bath medium to cool (the workpiece can be straightened while it is hot when the heating bath medium is used for cooling). Use air-cooled quenching as much as possible, and replace water-oil dual-medium quenching with a quenching medium between water and oil cooling rates.
—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, no doubt people will still It is considered that 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.
—Change 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 Hardness after quenching. 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. It is the basic quenching method for this type 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 an 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 the 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.
2. Cooling operation: The proficiency of the operation in the cooling process has a great influence on the quenching deformation, especially when using quenching media such as water and oil, the proficiency of the operation is more important.
—Correct direction of quenching medium: Generally speaking, workpieces with symmetrical cross-sections and slender rods should be quenched into the quenching medium vertically, and workpieces with asymmetrical cross-sections can be quenched into the quenching medium obliquely. In fact, the correct direction of quenching into the quenching medium is the direction in which all parts of the workpiece can be cooled uniformly. The slow-cooling part is first quenched into the quenching medium, and the fast-cooling part is then quenched into the quenching medium. In actual production, special attention should be paid to the influence of the shape of the workpiece on the cooling rate. The part with a large surface area of the workpiece does not mean that its cooling rate is large, especially when the shape of the part is relatively complicated, due to uneven cooling, it is likely to The resulting cooling rate is slower than that of parts with a small surface area. Therefore, how to choose the direction of entering the quenching medium should be mastered according to the shape of the workpiece. An example of the influence of the direction of the workpiece quenching into the quenching medium on the quenching deformation is shown in the figure
—The movement of the workpiece in the quenching medium: the parts with slow cooling speed should move against the water. Workpieces with symmetrical shapes should move symmetrically and evenly in water, with small movement range and fast speed. For slender and thin workpieces, they should be stable when quenched into the quenching medium, and the workpiece should not swing in the quenching medium. It is better to clamp and quench such workpieces without iron wire binding and quenching.
—The speed at which the workpiece is quenched into the quenching medium: the speed at which the workpiece is quenched into the quenching medium should be fast. Especially for slender and cylindrical workpieces, if the speed of quenching into the quenching medium is slow, it will lead to an increase in bending deformation and increase the difference in deformation between the part of the tube that is first quenched into the quenching medium and the part that is quenched into the quenching medium. big.
—Cooling with protection: For workpieces with large differences in cross-sectional size, the part with fast cooling speed should be bound and protected with asbestos rope, iron sheet, etc., so as to reduce the cooling speed of this part, so that all parts of the workpiece can be cooled evenly.
—Cooling time of workpieces in water: For workpieces whose deformation is mainly caused by tissue stress, the cooling time in water can be shortened; for workpieces whose deformation is mainly caused by thermal stress, the cooling time in water can be extended appropriately, So as to achieve the purpose of reducing the quenching deformation of the workpiece.
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