In the heat treatment process of
forging, heating and cooling are two key links, which have an important impact on the final mechanical properties and service life. This paper will discuss the calculation methods required for uniform heating and cooling of
forgings, focusing on how to accurately predict the cooling time and speed based on the diameter of the forgings, the quenching medium and their corresponding temperature changes.
In the heat treatment process of forgings, heating is a crucial step, especially when uniformly heating infinitely long cylindrical forgings. If all parts of the forging have the same temperature and no temperature difference before heating, the temperature t at each point on the forging section will be a function of the radius r during the heating process. This means that in the initial stage of heating, the temperature change between the forging center and the outside is gradual, and the heat is transferred from the outside to the inside, resulting in a change in the temperature distribution.
Normally, in order to simplify the calculation and analysis, when the length of the forging is at least three times its diameter, it can be considered as an infinitely long cylinder. This assumption allows us to ignore the end effect and focus on the temperature change on the forging section.
Cooling calculation When forging cooling, an important issue is to be able to know in advance the cooling of different diameters of forgings in different quenching media, including:
(1) The cooling time required when the forging center is cooled to a certain temperature
(2) Cooling curves for forging centers of different diameters at different quenching temperatures
(3) the cooling rate of the forging center
(4) The relationship between the cooling time of any part of the forging and the diameter of the forging.
(1) The cooling time required when the forging center cools to a certain temperature. Since the forging determines the final cooling time according to the temperature of the core, a more reliable curve of the relationship between the cooling time and the diameter of the forging is required. And because the cooling time of the same diameter forging cold to a certain temperature is also related to the quenching temperature, a series of cooling curves at different quenching temperatures are required.
(2) suitable for different quenching temperature of different diameter forging center cooling curve. According to the existing measured data and heat transfer theory derived for different quenching temperature of not more than 2m, diameter of the forging in water, oil and air cooling center cooling curve.
(3) forging center cooling speed. Usually the cooling rate at 700 ° C is used to determine the hardenability of the steel, which requires a curve of the relationship between the cooling rate and the diameter.
In the calculation of forging final cooling time, the cooling time is often calculated by the cooling time per millimeter, especially for the quenching medium with strong cooling capacity, the temperature of the heart of the small part is too low, and the final cold of the large part is insufficient. Forgings are usually 1/3R from the surface of the sampling for mechanical properties testing, some particularly important parts also need to take the core and surface samples, so it is necessary to understand the relationship between the cooling of different parts of the forging and the forging diameter. The normalizing treatment of forging processing is commonly used in the following 6 kinds.
(1) Complete annealing - eliminate the small non-uniform structure and Wei's structure formed by the forging process, refine the grain, and eliminate the residual stress of forging processing and reduce hardness.
(2) Spheroidizing annealing -- to obtain spherical cementite and ferrite structure, it is not only low hardness, but also easy to get bright processing surface during cutting, and it is not easy to produce deformation cracks during subsequent quenching.
(3) Isothermal annealing -- not only can extend the annealing time, but also can obtain a uniform structure and reduce hardness. In important large forging processing, it can also be used to evacuate hydrogen to prevent white spots. The post-forging heat treatment of aluminum alloy and copper alloy is also common by annealing process. The purpose is to eliminate work hardening, stress and improve plasticity.
(4) Normalizing - can get finer pearlite, can improve the forging processing machine performance suitable for machine processing. (Low carbon steel (including stainless steel, heat-resistant steel), medium carbon steel and low carbon alloy steel.
(5) Normalizing and high temperature tempering - eliminate the stress generated during normalizing cooling, improve plasticity and toughness.
(6) Tempering - forging processing has good comprehensive machine performance.
In the heat treatment process, if the instantaneous stress generated reaches the yield point of the material at this temperature, the forging will produce plastic deformation and relax the stress. If the instantaneous stress is greater than the breaking strength of the material, it is possible to crack the forging until it breaks. Even if the instantaneous stress is less than the strength limit of the material, because there are always some metallurgical defects inside the large forgings, there is a great concentration of stress at these defects, to further expand the original defects, and even cause the fracture of the forgings. Therefore, controlling the instantaneous stress is an important issue in the formulation of heat treatment process for large forgings.
Heat treatment residual stress has a dual effect on the workpiece, there are good effects, there are bad effects, if the residual stress and the working stress symbol of the part is the same, can reduce the strength of the part, if the opposite, it will increase the strength of the part. For some important large forgings, the allowable participating stress should be less than 10% of the yield point of the material.
In summary, the heat treatment process of forgings is not only related to the microstructure and mechanical properties of materials, but also directly affects the service life and safety of forgings. Through the in-depth analysis of the relationship between cooling time, speed and forging diameter, we can develop a more scientific and reasonable heat treatment program, and effectively reduce the residual stress and crack risk that may occur in the heat treatment process. In the face of evolving industrial needs, mastering these heat treatment principles and calculation methods will provide a solid foundation for improving the overall quality and reliability of forgings.
