Quenching and tempering of steel and the effect of quenching cooling rate on deformation

Quenching and Tempering of Steel

Hardening; for springs, not only should they be fully quenched, but the amount of martensite in the center should not be less than 90%; for structural parts. It should also be fully quenched.

1. Steel tempering process The tempering of steel can be divided into three categories: low temperature tempering, medium temperature tempering and high temperature tempering. Tempering at different temperatures can result in different tempering structures and properties. 3j5 handsome‘5D3 Zhong Yingg． ． goI clock machinery technology tea machinery miscellaneous. ~) Issue 1, 1997 ③ High-temperature tempering: The tempering temperature is 500℃~680℃, the tempered structure is sorbite, and the tempering hardness is HRC23~35. After tempering, it can obtain comprehensive mechanical properties with good strength and toughness and plasticity. Quenching and high-temperature tempering are usually called "quenching and tempering". It is widely used in various important structural parts, especially suitable for connecting rods, bolts and shaft parts that work under alternating loads. In addition, it can also be used as a pre-heat treatment for precision parts (such as screw rods, measuring tools, molds, etc.) to obtain a fine and uniform tempered sorbite structure, which provides a structural basis for controlling the final heat treatment deformation and obtaining better final properties. For steel types with a tendency to temper brittleness, tempering within the brittleness temperature range should be avoided as much as possible. 2. Determination of tempering time In the tempering process, the tempering time has three functions: first, to ensure that the structural transformation can be fully carried out; second, to reduce or eliminate the internal stress of the workpiece; third, to enable the workpiece to obtain the required tempering performance. At a certain tempering temperature, after the workpiece is completely burned, it takes about 0.5 hours to complete the structural transformation. That is to say, from the perspective of tissue transformation alone, it only needs to be kept warm for about 0.5 hours. However, from the perspective of residual stress relaxation, it generally takes 2 to 3 hours. That is to say, from the perspective of completing the structural transformation and minimizing the residual stress, tempering for generally small and medium-sized workpieces for 2 to 3 hours is enough. However, tempering has an impact on the properties of the tempered steel after all, so the tempering time must be considered in conjunction with the tempering temperature. It can generally be thought that tempering time and tempering temperature are two parameters that compensate each other, that is, increasing the tempering temperature can shorten the holding time; extending the tempering time can appropriately reduce the tempering temperature. The above are just basic guidelines for determining tempering time. At present, the tempering time in production is generally determined based on the thickness of the effective section of the workpiece. There are many reference data for calculating tempering time, which should be selected according to the specific situation. Reasonable corrections should be made in practical applications. 3． Cooling after tempering After tempering, it is usually air-cooled out of the furnace. For steel containing elements such as cf, Ni, Mn, etc., in order to prevent the occurrence of temper brittleness after high-temperature tempering, oil cooling should be used.

2. Quenching and tempering process, the purpose is to improve the hardness and wear resistance and make it have a certain toughness. The quenching and tempering process is shown in Figure 7. Figure 7 Quenching and tempering process curve of crankshaft guide rail According to the above content, the quenching heating temperature should be Ac1+30℃~50℃, but the actual temperature selection should also consider the heating method, workpiece size and shape, workpiece performance requirements, cooling conditions, factors such as original organization. For salt bath heating methods, workpieces with complex shapes and large changes in cross-sectional dimensions, the heating temperature should be slightly lower; for heating methods mainly based on radiation heating, workpieces with simple shapes and high hardness requirements, and occasions with gentle cooling, you can choose Higher temperature heating. Because the shape of the crankshaft guide rail is relatively simple, the spheroidized pearlite structure is obtained after spheroidizing annealing, and high hardness and wear resistance are required after quenching, so the quenching heating temperature is selected as Ac+50℃~70℃, that is, 780℃~ 8o0℃(Ac1=730℃). The holding time of quenching heating must also be determined according to the heating conditions, so it should generally be tested experimentally first. The basic principle for determining the heating and holding time is not to be too long. This prevents oxidation and decarburization of the workpiece surface and also reduces energy consumption. The workpiece is not allowed to decarburize, so it is advisable to use a salt bath for heating. The salt bath should be deoxidized, and the thermal insulation time should be based on heat penetration. Because the workpiece will not be processed after quenching and tempering, the deformation control is strict, and a certain degree of toughness is expected, so nitrate salt isothermal cooling is used. After being isothermally heated in nitrate salt at 180°C to 200°C for a period of time, the quenched parts can retain a larger amount of retained austenite, thereby reducing deformation and giving them a certain degree of toughness. Practice has proved that this process is feasible. For the determination principles of tempering temperature, tempering time and cooling method after tempering, please refer to relevant information and the aforementioned content. After annealing, quenching and tempering, all technical indicators of the workpiece meet the requirements, and the service life and effect are satisfactory.

Effect of process parameters on heat treatment deformation

Whether it is conventional heat treatment or special heat treatment, heat treatment deformation may occur. When analyzing the impact of heat treatment process parameters on heat treatment deformation, it is more important to analyze the impact of the heating process and the cooling process. The main parameters of the heating process are uniformity of heating, heating temperature and heating speed. The main parameters of the cooling process are uniformity of cooling and cooling rate. The influence of uneven cooling on quenching deformation is the same as the uneven cooling caused by asymmetric cross-sectional shape of the workpiece. This section mainly discusses the influence of other process parameters.

Deformation caused by uneven heating---Excessive heating speed, uneven temperature of the heating environment and improper heating operation can cause uneven heating of the workpiece. Uneven heating has a significant impact on the deformation of slender workpieces or thin parts. The uneven heating mentioned here does not refer to the inevitable temperature difference between the surface and the core of the workpiece during the heating process, but specifically refers to the temperature gradient that exists in each part of the workpiece due to various reasons. In order to reduce deformation caused by uneven heating, high-alloy steel workpieces with complex shapes or poor thermal conductivity should be heated slowly or preheated. However, it should be pointed out that although rapid heating can lead to an increase in the deformation of long-axis workpieces and flaky plates; however, for workpieces where volume deformation is dominant, rapid heating can often reduce deformation. This is because when only the working part of the workpiece needs to be quenched and strengthened, rapid heating can keep the core of the workpiece at a lower temperature and higher strength, and the working part can reach the quenching temperature. In this way, the stronger core can prevent large deformation of the workpiece after quenching and cooling. In addition, rapid heating can use higher heating temperature and shorter heating and holding time, which can reduce the deformation caused by the weight of the workpiece due to long stay in the high temperature stage. Rapid heating only causes the surface layer and local areas of the workpiece to reach the phase transition temperature, which accordingly reduces the volume change effect after quenching, which is also beneficial to reducing quenching deformation.

Effect of heating temperature on deformation---Quenching heating temperature affects quenching deformation by changing the temperature difference during quenching and cooling, changing the hardenability, Ms point and the amount of retained austenite. Increasing the quenching heating temperature increases the amount of retained austenite, lowers the Ms point, reduces the deformation caused by structural stress, and makes the cavity of the sleeve workpiece tend to shrink; but on the other hand, increasing the quenching heating temperature increases the quenching penetration properties, increase the temperature difference during quenching and cooling, increase thermal stress, and tend to expand the inner hole. Practice has proved that for low-carbon steel workpieces, if the inner hole shrinks after quenching at normal heating temperature, the shrinkage will be greater if the quenching heating temperature is increased. In order to reduce shrinkage, the quenching heating temperature must be lowered; for workpieces made of medium-carbon alloy steel, if If the inner hole expands after quenching at normal heating temperature, the quenching heating temperature will increase even more. In order to reduce the expansion of the hole cavity, the quenching heating temperature also needs to be lowered. For Cr12 high alloy mold steel, increasing the quenching heating temperature will increase the amount of retained austenite and the cavity will tend to shrink.

Effect of quenching cooling rate on deformation---Generally speaking, the more intense the quenching cooling, the greater the temperature difference between the inside and outside of the workpiece and in different parts (parts with different cross-sectional sizes), resulting in greater internal stress, resulting in increased heat treatment deformation. The deformation of hot die steel specimens (150 long * 100 wide * 50 high) after quenching and tempering at different cooling rates. Among the three media, oil cooling is the fastest, followed by hot bath cooling, and air cooling is the slowest. After the workpiece is quenched at three different cooling rates, the deformation in length and width tends to shrink, with little difference in deformation; however, the deformation caused by air cooling quenching and hot bath quenching with slow cooling rates in the thickness direction is much smaller. Its deformation and expansion is less than 0.05%, while shrinkage deformation occurs during oil quenching, and its maximum deformation reaches about 0.28%. However, when the change in cooling rate causes the phase transformation of the workpiece to change, the increase in cooling rate does not necessarily cause an increase in deformation, but sometimes reduces the deformation. For example, when low carbon alloy steel shrinks after quenching because the core contains a large amount of ferrite, increasing the quenching cooling rate to obtain more bainite in the core can effectively reduce shrinkage deformation. On the contrary, if the workpiece expands due to the acquisition of martensite in the center after quenching, reducing the cooling rate thereby reducing the relative amount of martensite in the center can reduce the expansion. The influence of quenching cooling rate on quenching deformation is a complex issue, but the principle is that the quenching cooling rate should be minimized while ensuring the required structure and performance.