Types of Heat Treatment of Forgings
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The heat treatment of forging is based on the type of forging and the raw materials used, and the process parameters of heat treatment are reasonably formulated. Mainly heating temperature, holding time and cooling speed, and through the full coordination of heating technology, that is, heating equipment, heating speed and heating atmosphere, and cooling technology, that is, cooling device, cooling medium, to obtain the metallography and mechanical properties required by the forging, to avoid heat treatment defects (deformation, cracks and abnormal organization, etc.). Therefore, heat treatment is a very important processing process of forging forming, and it is also an essential processing process in the forging forming process.
For automotive gear carburizing steel forgings, such as 20CrMnTiH material gear forgings, after forging forming air cooling to room temperature, usually need to be reheated, a normalization, so that the hardness is reduced, uniform organization, to improve cutting performance and reduce the deformation generated during carburizing process. However, because the cooling structure transformation during normalization is carried out in a continuous cooling process, that is, it is not carried out in a temperature range, so the obtained structure is not uniform. Especially when the forging section size and cooling conditions are different, the microstructure is not uniform. The uneven structure will inevitably bring uneven hardness and make the cutting performance worse, and also increase the deformation after carburizing.
In order to overcome these shortcomings of normalizing, isothermal normalizing is generally used instead of normalizing. The forgings that have been cooled to room temperature are reheated to a temperature slightly higher than the carburizing temperature (900℃ ~ 950℃ or above), and the cooling speed of 30℃/min ~ 42℃/min is cooled to 600℃ ~ 650℃ after a certain period of time, and the thermal insulation is for a certain period of time, until the austenite completes the pearlite transformation. After obtaining the ferrite and pearlite structures, remove from the furnace and air cool to room temperature. The uniform structure can be obtained by using isothermal normalizing, and the hardness fluctuation is small, and the hardness of the forging after isothermal normalizing can be adjusted by adjusting the heating temperature and isothermal temperature.
However, for tempered forgings with high metallographic structure and mechanical properties, before forging and final heat treatment (tempered), it is necessary to carry out an annealing or normalizing to uniform the organization, refine the grain, and improve the machinability, and then tempered heat treatment. For example, in order to ensure that the metallographic structure of its automobile engine connecting rod reaches level 1 to level 4, a forging enterprise increases the isothermal normalizing of waste heat before tempering heat treatment, so that the metallographic structure of the connecting rod reaches within level 3.
With the continuous progress of technology, the refinement and optimization of the heat treatment process will further promote the development of the forging manufacturing industry and improve the quality and competitiveness of products. Therefore, mastering and applying the heat treatment process can not only improve the overall performance of forgings, but also provide a solid guarantee for the continuous innovation and progress of the forging industry.
Classification of Heat Treatment of Forging
There are many kinds of heat treatment of steel forgings, and for small and medium forgings heat treatment, it can be divided into two categories according to the purpose of heat treatment.Preparatory Heat Treatment
Preparatory heat treatment, for subsequent forging metal cutting and subsequent heat treatment for carburized steel, bearing steel forgings, the main purpose of heat treatment is to provide the best cutting performance for forging metal cutting, reduce hardness, refine grains, improve unreasonable organization, but also for subsequent heat treatment to prepare the organization. In addition, some parts are also distinct from final heat treatment to obtain the required mechanical properties. Normalizing, isothermal normalizing and annealing are generally selected.For automotive gear carburizing steel forgings, such as 20CrMnTiH material gear forgings, after forging forming air cooling to room temperature, usually need to be reheated, a normalization, so that the hardness is reduced, uniform organization, to improve cutting performance and reduce the deformation generated during carburizing process. However, because the cooling structure transformation during normalization is carried out in a continuous cooling process, that is, it is not carried out in a temperature range, so the obtained structure is not uniform. Especially when the forging section size and cooling conditions are different, the microstructure is not uniform. The uneven structure will inevitably bring uneven hardness and make the cutting performance worse, and also increase the deformation after carburizing.
In order to overcome these shortcomings of normalizing, isothermal normalizing is generally used instead of normalizing. The forgings that have been cooled to room temperature are reheated to a temperature slightly higher than the carburizing temperature (900℃ ~ 950℃ or above), and the cooling speed of 30℃/min ~ 42℃/min is cooled to 600℃ ~ 650℃ after a certain period of time, and the thermal insulation is for a certain period of time, until the austenite completes the pearlite transformation. After obtaining the ferrite and pearlite structures, remove from the furnace and air cool to room temperature. The uniform structure can be obtained by using isothermal normalizing, and the hardness fluctuation is small, and the hardness of the forging after isothermal normalizing can be adjusted by adjusting the heating temperature and isothermal temperature.
Final Heat Treatment
the final heat treatment, so that the parts to obtain the required metallographic structure and mechanical properties, direct use for medium carbon steel and medium carbon alloy steel forgings, the purpose of heat treatment is to improve the mechanical properties of the product, after cutting processing, most no longer heat treatment, direct use. Heat treatment to ensure that the mechanical properties of the product requirements, that is, has a certain strength, hardness, but also has good plasticity and toughness, and can have moderate hardness, easy to cut processing. Such as engine crankshaft, connecting rod, etc., generally use tempering heat treatment (quenching and high temperature tempering).However, for tempered forgings with high metallographic structure and mechanical properties, before forging and final heat treatment (tempered), it is necessary to carry out an annealing or normalizing to uniform the organization, refine the grain, and improve the machinability, and then tempered heat treatment. For example, in order to ensure that the metallographic structure of its automobile engine connecting rod reaches level 1 to level 4, a forging enterprise increases the isothermal normalizing of waste heat before tempering heat treatment, so that the metallographic structure of the connecting rod reaches within level 3.
Sum up
Heat treatment of forgings plays a vital role in forging process. Through scientific and reasonable formulation of heat treatment process parameters, including heating temperature, holding time and cooling speed, combined with advanced heating and cooling technology, the metallographic structure and mechanical properties of forgings can be effectively improved. Preliminary heat treatment lays the foundation for subsequent processing and final heat treatment, which ensures high performance and reliability of forgings in practical applications. Whether it is normalizing and isothermal normalizing in preliminary heat treatment, or tempering processes in final heat treatment, these measures are designed to improve the overall performance of forgings, reduce defects, and meet the specific application needs.With the continuous progress of technology, the refinement and optimization of the heat treatment process will further promote the development of the forging manufacturing industry and improve the quality and competitiveness of products. Therefore, mastering and applying the heat treatment process can not only improve the overall performance of forgings, but also provide a solid guarantee for the continuous innovation and progress of the forging industry.
