The relationship between heat treatment processes, operations, and deformation

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I. Pretreatment: Performing preliminary heat treatment on the workpiece prior to quenching to relieve stress and improve its microstructure is very beneficial for reducing quenching deformation. Preprocessing generally includes spheroidizing annealing and stress-relief annealing; some processes also involve quenching and tempering or normalizing. ①Stress relief annealing: During the machining process, the surface layer of the workpiece develops certain residual stresses as a result of factors such as the machining method, depth of cut, and cutting speed. Due to the uneven distribution of these stresses, the workpiece deforms during quenching. To eliminate the effects of these stresses, it is necessary to perform a stress-relief annealing on the workpiece before quenching. The temperature for stress-relief annealing is generally 500–700 °C. When heating in an air atmosphere, an annealing temperature of 500–550 °C can be used to prevent oxidation and decarburization of the workpiece, with a holding time of typically 2–3 hours. When loading the workpiece into the furnace, attention should be paid to deformation that may occur due to its own weight; the other operations are the same as those for regular annealing. ②Preheating treatments aimed at improving the structure: Such treatments include spheroidizing annealing, quenching and tempering, and normalizing. ——Spheroidizing annealing: Spheroidizing annealing is an essential process in the heat treatment of carbon tool steel and alloy tool steel; the microstructure obtained after this annealing has a significant impact on the tendency of deformation during quenching. Therefore, the regular quenching deformation of certain workpieces can be reduced by adjusting the microstructure after annealing. ——Other pre-treatments: There are many pre-treatment methods used to reduce quenching distortion, such as quenching and tempering, normalizing, etc. Considering the causes of quenching deformation in the workpiece as well as the material used for it, selecting appropriate pre-treatments such as normalizing or quenching and tempering is effective in reducing quenching deformation. However, attention should be paid to the adverse effects of the residual stresses and increased hardness resulting from normalizing on machining. Meanwhile, quenching and tempering treatment can reduce swelling during quenching in steels containing W and Mn, but has little effect on reducing deformation in steels such as GCr15. In actual production, it is important to determine the cause of quenching deformation, that is, to figure out whether it is caused by residual stresses or by poor microstructure; only then can appropriate measures be taken. If the quenching deformation is caused by residual stresses, stress-relief annealing should be carried out rather than pretreatment methods such as quenching and tempering that alter the microstructure, and vice versa. Only in this way can the goal of reducing quenching deformation be achieved, costs can be lowered, and quality can be ensured. The specific procedures for the various aforementioned preprocessing steps are the same as those for other corresponding operations; they will not be elaborated upon here. II. Quenching heating procedures ① Quenching temperature: The quenching temperature has a significant impact on the deformation of the workpiece during quenching. The general rule governing the effect on the quenching deformation trend is shown in the figure. Based on the trend shown by the curve in the figure regarding the effect of quenching distortion, we can reduce deformation by adjusting the quenching temperature, or by determining an appropriate value for the mechanical machining allowance in conjunction with the quenching temperature; through thermal treatment tests, these values can be selected and used appropriately in order to reduce the amount of machining required later on. The effect of quenching temperature on quenching deformation depends not only on the material used for the workpiece but also on its size and shape. When the shapes and sizes of workpieces vary significantly, even though they are made of the same material, their tendencies toward quenching deformation differ greatly. Operators should pay attention to this in actual production. ②Quenching holding time: When determining the holding time, it is necessary not only to ensure that the workpiece is thoroughly heated and attains the required hardness or other mechanical properties after quenching, but also to consider its effect on quenching distortion. Increasing the quenching holding time actually corresponds to raising the quenching temperature. This effect is particularly pronounced in high-carbon, high-chromium steels. ③Loading method: If the workpieces are not arranged properly during heating, deformation can occur due to the weight of the workpieces themselves, or as a result of the pressure exerted between the workpieces, or encore because of overcrowding which leads to uneven heating and cooling. For example, in the case of a certain spring component, during quenching it was vertically suspended and heated in a protective-atmosphere furnace at 860±10°C for 30 minutes. After that, it was removed from the furnace and vertically quenched into cooling oil. Following quenching, the total length of the spring decreased by 27 mm; moreover, due to differences in the time spent in the quenching medium, the deformation amounts of its upper and lower pitches were also different. Later, a spring was used to wrap around the mandrel, and the part was hung horizontally in the furnace; the other operation steps remained the same. After quenching, its deformation improved significantly, with uniform pitch and minimal reduction in total length. In particular, for elongated workpieces, not only cannot they be loaded into the furnace in a closely packed horizontal arrangement, but when heated in a salt bath furnace, the possibility of deformation due to the movement of the heating medium must also be taken into account. When loading slender and lightweight rod-shaped workpieces into the furnace, the salt bath furnace is usually heated first to a temperature slightly above the quenching temperature; after turning off the power, the workpieces are placed into the furnace. The loading process must be done carefully, and the workpieces are heated without electricity being applied, thereby reducing their deformation during quenching. ④Heating method: When the workpiece has a complex shape and significant variations in thickness—especially when its material contains high levels of carbon and alloying elements—the heating process must be slow and uniform. The preheating process should be utilized to the fullest extent; if one preheating cycle is insufficient, two or three preheating cycles should be employed. For larger workpieces for which deformation cannot be resolved even through preheating, box protection can be employed along with heating in a box-type resistance furnace. In addition to controlling the heating rate, an isothermal phase can also be included to reduce quenching deformation caused by too rapid heating. III. Cooling process The deformation that occurs during quenching is mainly due to the cooling process. A suitable quenching medium and skilled operational techniques – every stage of the cooling process – directly affect the quenching deformation of the workpiece. 1. Select the quenching medium appropriately: While ensuring that the hardness of the workpiece meets the design requirements after quenching, it is advisable to use a milder quenching medium as much as possible during this process. Cooling can be achieved using a heating bath medium (when cooling with a heating bath medium, the workpiece can be straightened while it is still hot). Where possible, use air cooling for quenching, and replace water and oil quenching media with quenching media that have a cooling rate between that of water and oil. ——Air cooling quenching: Air cooling quenching is effective in reducing the quenching distortion of high-speed steels, chromium-type tool steels, and air-cooling low-distortion steels. For 3Cr2W8V steel, which does not have high hardness requirements after quenching, air quenching can also be used to reduce deformation by appropriately adjusting the quenching temperature. ——Oil quenching: Oil is a quenching medium with a much slower cooling rate compared to water. However, for workpieces that have high hardenability and are small in size with complex shapes and a tendency to deform, oil’s cooling rate is still considered too fast. On the other hand, for workpieces that are not very large but have poor hardenability, oil’s cooling rate proves to be insufficient. To resolve the aforementioned contradiction and make full use of oil quenching to reduce the quenching deformation of workpieces, people have adopted methods such as adjusting the oil temperature and raising the quenching temperature to expand the application of oil. ————Changing the temperature of the quenching oil: Using the same oil temperature for quenching to reduce quenching deformation presents the following problem: when the oil temperature is low, quenching deformation remains high, while when the oil temperature is high, it is difficult to ensure the hardness of the workpiece after quenching. For certain workpieces, under the combined effects of shape and material, raising the temperature of the quenching oil may actually increase their deformation. Therefore, it is highly necessary to determine the oil temperature for quenching through experiments, based on actual conditions such as the workpiece material, cross-sectional dimensions, and shape. When using hot oil quenching, to prevent fires caused by excessively high oil temperatures resulting from quenching cooling, necessary fire-fighting equipment should be installed near the oil tank. In addition, the quality parameters of the quenching oil should be regularly checked, and new oil should be added or replaced in a timely manner. ————Increase the quenching temperature: This method is suitable for carbon steel workpieces with small cross-sections, as well as alloy steel workpieces of slightly larger size, for which the required hardness cannot be achieved through oil quenching after heating and holding at normal quenching temperatures. By appropriately raising the quenching temperature and then oil-quenching, it is possible to achieve both hardening and reduced distortion. When quenching using this method, care must be taken to prevent issues such as grain coarsening that may arise from increasing the quenching temperature, as well as a decrease in the mechanical properties and service life of the workpiece. ——Gradual and isothermal quenching: When the hardness achieved through quenching meets the design requirements, it is advisable to make full use of gradual and isothermal quenching with a thermal bath medium in order to reduce quenching deformation. This method is also effective for carbon structural steels and tool steels with low hardenability and small cross-sections. In particular, for chromium-containing die steels and high-speed steel workpieces with higher hardenability, the heat bath medium grading and isothermal quenching cooling methods are the basic quenching techniques for such steels. Similarly, it is also effective for carbon steels and low-alloy structural steels whose requirements regarding quenched hardness are not very high. When using hot bath quenching, the following points should be noted: First, when performing step quenching or isothermal quenching in an oil bath, the oil temperature must be strictly controlled to prevent fires. Second, when using nitrate grading quenching, the nitrate tank should be equipped with the necessary instruments and water cooling systems; other precautions are detailed in relevant materials and need not be repeated here. Third, during isothermal quenching, the isothermal temperature must be strictly controlled; either too high or too low a temperature is not conducive to reducing quenching deformation. Additionally, when performing isothermal quenching, the hanging method for the workpieces must be chosen appropriately to prevent deformation caused by their own weight. Fourth, when correcting the shape of workpieces while they are still hot using isothermal or step quenching, all necessary fixtures and tools must be readily available, and operations must be carried out swiftly. Prevent adverse effects on the quenching quality of the workpiece. 2. Cooling operation: The proficiency in performing the cooling process has a significant impact on quenching deformation; this is especially true when using quenching media such as water or oil, where the level of skill in operating such media is even more crucial. ——Correct direction of quenching medium application: Generally, for workpieces with symmetric cross-sections and slender rods, the quenching medium should be applied vertically. For workpieces with asymmetric cross-sections, the quenching medium can be applied at an angle. In fact, the correct direction for immersing the workpiece in the quenching medium is one that allows uniform cooling of all parts of the workpiece. The parts that cool more slowly are quenched in the quenching medium first, while the parts that cool faster are quenched in it later. In actual production, special attention must be paid to the effect of the workpiece’s shape on the cooling rate. A larger surface area of the workpiece does not necessarily mean a faster cooling rate; especially when the shape of that area is complex, uneven cooling can result in a cooling rate that is slower than that of areas with a smaller surface area. Therefore, how to choose the appropriate quenching medium should be determined based on the shape of the workpiece. An example of the effect of the direction in which the workpiece is immersed into the quenching medium on quenching deformation is shown in the figure — movement of the workpiece in the quenching medium: the parts where the cooling rate is low should move in the direction opposite to the flow of water. For workpieces with relatively symmetrical shapes, their movement path in water should be symmetric and uniform; the amplitude of motion should be small while the speed should be high. When quenching long and thin workpieces in a quenching medium, it is necessary to ensure stability; the workpieces should not sway within the quenching medium. For such workpieces, it is better to clamp them instead of tying them with wire for quenching. ——Speed at which the workpiece is immersed in the quenching medium: The workpiece should be immersed in the quenching medium at a fast speed. In particular, for slender cylindrical workpieces, a slow quenching speed into the quenching medium can lead to increased bending deformation, as well as a greater difference in the degree of deformation between the portion quenched first and the portion quenched later. ——Protected cooling is employed: For workpieces with significant differences in cross-sectional dimensions, the areas with a faster cooling rate should be protected using materials such as asbestos rope or iron sheeting, in order to reduce their cooling rate and thus ensure uniform cooling across all parts of the workpiece. ——Cooling time of the workpiece in water: For workpieces deformed primarily due to residual stresses, the cooling time in water can be reduced ； For workpieces deformed primarily due to thermal stress, the cooling time in water can be appropriately extended, thereby reducing the quenching deformation of the workpieces.