Five common conceptual differences in heat treatment

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I. Pre-treatment: Performing preliminary heat treatment on the workpiece to relieve stress and improve its microstructure before quenching is highly beneficial for reducing quenching deformation. Pre-treatment generally includes spheroidizing annealing and stress-relief annealing; some processes also employ quenching and tempering or normalizing. ①Stress relief annealing: During the machining process, the surface layer of the workpiece develops certain residual stresses due to factors such as the machining method, depth of cut, and cutting speed. The uneven distribution of these stresses causes the workpiece to deform 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 deformation, 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 instead of pre-treatments 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 pattern of its influence on the quenching deformation trend is shown in the figure. Based on the trend shown by the curves in the graph regarding the effect of quenching distortion, we can reduce deformation by adjusting the quenching temperature, or by setting an appropriate amount of machining allowance in conjunction with the quenching temperature; alternatively, after conducting heat treatment tests, we can choose and apply these parameters 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 there are significant differences in the shape and size of workpieces, even if they are made of the same material, their tendency to deform during quenching varies greatly; operators should be aware of this in actual production. ②Quenching holding time: The selection of the holding time takes into account not only ensuring complete heating of the workpiece and achieving the desired hardness or other mechanical properties after quenching, but also its impact on quenching deformation. 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 pressure exerted between the workpieces, or because the workpieces are stacked too densely, leading to uneven heating and cooling and thus deformation. In the case of a certain spring component, during quenching it was heated in a protective atmosphere furnace at 860±10°C by vertical hanging for 30 minutes. After that, it was taken out of the furnace and vertically plunged into cooling oil. As a result of quenching, the total length of the spring decreased by 27 mm, and the pitch between the upper and lower parts also changed due to differences in the time spent in the quenching medium. Later, a spring was used to wrap around the mandrel, and the part was hung horizontally in the furnace; the other operating procedures remained the same. After quenching, its deformation improved significantly, with a 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 first heated 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 shape of the workpiece is complex and its thickness varies significantly, especially when the material contains high levels of carbon and alloying elements, the heating process must be slow and uniform. The preheating step should be utilized to the fullest extent; if a single preheating session is not sufficient, secondary or tertiary preheatings can be used. For larger workpieces for which deformation cannot be resolved even with 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 added 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. An appropriate quenching medium and skilled operating techniques—every step in the cooling process directly affects the quenching deformation of the workpiece. 1. Select a suitable quenching medium: 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 heat 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 speed between that of water and oil. ——Air cooling quenching: Air cooling quenching is effective in reducing the quenching deformation of high-speed steels, chromium-based die steels, and air-cooled micro-deformed steels. For 3Cr2W8V steel, for which high hardness after quenching is not required, 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. Conversely, for workpieces that are not very large but have poor hardenability, oil’s cooling rate turns out to be insufficient. To resolve the aforementioned contradictions 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 in order to make better use of the oil. ——Changing the temperature of the quenching oil: Although using a consistent oil temperature during quenching helps to reduce quenching distortion, there are still several problems. Specifically, when the oil temperature is too low, the quenching distortion remains significant; whereas when the oil temperature is too high, it becomes difficult to ensure that the workpiece attains the desired hardness after quenching. For certain workpieces, due to the combined effects of their shape and material, increasing the temperature of the quenching oil may further 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 excessive oil temperature 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. ——Increasing the quenching temperature: This method is applicable to carbon steel workpieces with smaller cross-sections and slightly larger alloy steel workpieces, which fail to meet the required hardness level after being heated and held at normal quenching temperatures followed by oil quenching. By appropriately increasing the quenching temperature followed by oil quenching, it is possible to achieve both hardening and reduced deformation. When quenching using this method, care must be taken to prevent issues such as grain coarsening, which may result 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 obtained 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 that do not require high hardness after quenching. 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, the hanging method for the workpiece during isothermal quenching must be chosen appropriately to prevent deformation caused by the workpiece’s own weight. Fourth, when using isothermal or staged quenching to correct the shape of the workpiece while it is still hot, the tooling and fixtures must be fully equipped, and the operations must be carried out quickly. To 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 immersion in the quenching medium: Generally, for workpieces with symmetrical cross-sections and elongated shapes, immersion perpendicular to the quenching medium is used; for workpieces with asymmetrical cross-sections, immersion at an angle can be employed. 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 direction for entering the 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 a relatively symmetrical shape, their path of movement in water should be symmetrical and even, with a small amplitude of movement and a high speed. When quenching elongated 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. Especially for elongated, tubular workpieces, if the speed at which they are immersed in the quenching medium is slow, it will lead to increased bending deformation, as well as a greater difference in the degree of deformation between the parts of the tubular component that are immersed in the quenching medium first and those that are immersed later. ——Protected cooling is employed: For workpieces with significant differences in cross-sectional dimensions, the areas with a faster cooling rate should be wrapped and protected using materials such as asbestos rope or iron sheeting, in order to reduce the cooling rate of those areas and thus ensure uniform cooling across all parts of the workpiece. ——Cooling time of the workpiece in water: For workpieces deformed primarily due to structural stress, 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.