How to ensure the heat treatment quality of forgings?

In order to ensure the heat treatment quality of forgings, it is very important to select appropriate process parameters when formulating the process. At present, the formulation of forging heat treatment process is basically based on the actual production experience of the factory. With the development of science and technology, it is possible to determine the process parameters preliminarily through calculation and then improve them through production practice under the current technical conditions. To determine the process parameters by means of actual measurements is time-consuming and costly for forgings, and sometimes it is impossible. Therefore, it is a significant work to develop the calculation technology of forging heat treatment process parameters. All countries are competing to carry out this work, and some achievements have been made. ,

In the calculation work, the first thing to determine the calculation model in line with the reality, the calculation conditions can only consider the main factors affecting the process parameters, ignore some minor factors, on the other hand, in the actual production of the influence factors are changeable, so the calculation method can only be approximate. Even so, the calculation results still have important significance to guide the actual production. The following is a general introduction to the relevant calculations. Heating and cooling calculation at constant temperature of surrounding medium. Heating calculation; Cooling calculation; Calculation of final cooling time of forgings.

Calculation of the distribution of forgings along the cross section. The cooling curves of different parts of forging are superimposed on the continuous cooling transition curve to understand the cooling structure of each part.

Based on the cooling curves of different parts of forgings with a diameter in a medium, the microstructure distribution and the depth of quenched layer of forgings with any diameter in the same medium after quenching are obtained.

It is very important to control the cooling rate of forging tempering. The main factor to be considered is the residual stress after forging tempering. The cooling rate after tempering directly affects the residual stress value. It is found that there is an elastoplastic transition temperature between tempering temperature and room temperature of forgings. This temperature varies with different types of steel, which is generally believed to be about 400-450℃. The residual stress is mainly generated in the cooling process above 400-450℃, steel is in a plastic state above 400℃, too fast cooling rate will produce great thermal stress, plastic deformation, so that the residual stress value increases.

When the steel is in an elastic state below 400℃, the cooling rate has no significant effect on the residual stress. So above 400℃ to slow cooling, 400℃ below can be cold faster, if necessary, can be isothermal between 400-450℃ for a period of time, will reduce the internal and external temperature difference of the forging elastoplastic state, is conducive to reducing the residual stress. For some important forgings, the residual stress should be less than 10% of the yield point.

Slow cooling above 400 ° C produces a second type of tempering brittleness for some steels. In the general heat treatment of small and medium pieces, in order to prevent tempering brittleness, after tempering forging should be quickly cooled in oil or water. However, this method is not suitable for large items. For large pieces, it mainly relies on alloying, reducing the content of harmful elements such as phosphorus in steel and vacuum carbon deoxygenation to reduce or even eliminate the tempering brittleness, and rarely adopts the rapid cooling method, so as to avoid the stress is too large and cause the workpiece cracking.