SU1562356A1 - Method of thermal heat treatment of metals - Google Patents

Abstract

The invention relates to metallurgy, more specifically to the heat treatment of metals and alloys. The purpose of the invention is to improve the quality of the metal by increasing the intensity of heat exchange in the quenching medium. The method includes heating above A ₀ , holding and cooling in a quenching medium placed in an electric field with a strength of 1 kV / cm to 0.8 punch strength of the quenching medium. The method allows to obtain a higher hardenability and a smaller structure of the hardened part. 1 ill., 1 tab.

Description

The invention relates to metallurgy, in particular to the heat treatment of metals.

The purpose of the invention is to improve the quality of the metal by increasing the intensity of heat exchange in the hardening medium.

An example. Samples of steel 45 with a diameter of 40 mm and a height of 60 mm are heated in an electric muffle furnace to a temperature of 820 ° C for 40 minutes and quenched in oil.

The quenching bath is a cylindrical glass vessel with an electrically conductive ring (tire) over the entire surface of the walls. The quenching bath and the sample are in an electric field with a strength of 1 kV / cm to 0.8 breakdown intensity of the cooling medium (Epp 220 kV / cm).

The drawing shows the application of an electric field to the quenching bath and the metal to be treated, including the quenching bath 1 for oil, the metal to be treated 2, the electrode 3, the quenching medium 4 and the voltage source 5.

An electrically insulated busbar is placed into the quenching bath, to which a busbar is connected to one pole of the source of electrical voltage, the other is connected to the metal to be treated.

The quenching properties of the medium are determined by the cooling rate of the metal being treated. When a metal is immersed in a quenching bath, boiling of the liquid takes place on the surface of the heated metal and natural convection of the liquid near the heated metal.

The intensity of cooling of the metal surface is determined by the heat of the liquid – vapor phase transition for a given liquid, the rate of supply

cn

OE 1C

03

ate

O5

Waves of liquid to the surface of heat and imen, as well as the rate of vapor vapor absorption processes, the type of ipen (film, bubble).

The electric field allows the boiling heat flux on the conductive wall 1.4 times to be intensified, which is achieved due to the smaller critical radius of the embryo of the vapor bubble, as well as an increase in the bubble growth rate. 8 electric field only bubble boiling occurs. These effects are explained by a decrease in the surface tension of a liquid in an electric field.

The intensification of the processes of desorption, the supply of fresh liquid to the cooled surface, as well as convective heat transfer is explained by electroconvection, i.e. the movement of a fluid in an electric field. Intensification of heat exchange by convection reaches (} -8) o / 0, where d0 is the heat transfer coefficient in the absence of an electric field.

The intensification and deceleration of heat exchange by the electric field is determined by the direction of the gradients of temperature and electric intensity. If vTJ4vE2 (vectors are unidirectional), then convective heat transfer is intensified, if convection is suppressed to the same extent, as intensified. This makes it possible to worsen the heat transfer conditions if a positive potential is applied to the tire and the metal to be treated is grounded. Such a task may arise if it is necessary to replace the quenching medium (oil) with another medium (water). If an electric potential is applied to the metal being processed and the bus (bath) is grounded, then heat exchange will be intensified. This task is faced with the need to increase the hardening depth and cooling rate. The magnitude of the change in heat transfer (cooling rate) is determined by the magnitude of the electric field created, it depends on the geometric dimensions of the metal being processed, the size and shape of the quenching bath and the electrical properties of the quenching medium. To obtain a given effect, the magnitude of the electric field is selected when processing the end

five

0

five

0

five

0

five

0

five

Old technological process. However, it cannot be higher than the breakdown for a given metal E, therefore it is chosen from the conditions of labor protection E, 8 Епр.

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Quenching media in practice are not clean. The lower limit of the applied electric field is determined by the magnitude of its effective action. The intensity of the effect of an electric field on heat transfer is determined by the square of the intensity of the electric field. At low voltages of the electric field (less than 1 kV / cm), the effect of the floor on heat transfer in the fluid is so small that no changes in the structure and properties of the metal were found in the heat treatment experiments (see experiment, mode 2). Thus, the range of exposure to an electric field to heat treatment is 1 kV / cm 0.8 E,.

The experiments were carried out under the following conditions.

Mode 1 is conducted without exposure to an electric field (E - 0).

Mode 2 is conducted with an electric field (negative potential applied to the metal) E 0.5 kV / cm; mode 3 - the same, E 1, 1 kV / cm; mode 4 is the same, E 10 kV / cm, mode 5 is the same, E 177 kV / cm (this is the ultimate technical field strength for affecting the heat transfer in pure transformer oil). When an attempt is made to increase the voltage, the leakage current quickly increases, and then with a further increase in the voltage, the dielectric breakdown is reached.

Mode 6 is conducted with an electric field (a positive potential is applied to the metal), E 10 kV / cm.

The measurement is carried out on three samples in each case from the edge to the center. The hardness measurement points are separated from each other at a distance of 3 mm. Only 18 images. Measurement results are averaged.

;

The table shows the results of experiments, from which it can be seen that the maximum depth of the hardened layer of the samples processed according to mode 5. The microstructure studies showed that the treatment according to modes 2 and 6; beyond the boundary conditions, does not improve the structure; it does not practically differ from the samples treated in mode 1. Mode 3 contributes to a finer structure.

The best results were obtained when processing according to mode 5 (fine martensitic needles), with normal.

orientation of the blocks, and the structure is homogeneous.

The table also shows the properties of the samples after processing by the well-known method (mode 1, nagoi to 820 ° C, holding for 4 min and cooling in oil without applying an electric field).

The proposed method (modes) improves the quality of the metal in structure and hardness, has a higher homogeneity of the final product (metal), and the costs (cost of the source of electrical voltage and electrical energy for leakage currents) are small.

Claims (1)

The invention The method of heat treatment of metals, including heating to a temperature The temperature is higher than Ac, exposure and subsequent cooling characterized in that, in order to improve the quality of the metal by increasing the intensity of heat exchange in the quenching medium, cooling is carried out in a medium in an electric field with a strength of 1 kV / cm to 0, 8 of the breakdown intensity of the cooling medium, the negative potential being reported to the metal being treated.