RU2151200C1 - Process of electric resistance heating of blanks - Google Patents

Abstract

FIELD: heat treatment of metals and alloys. SUBSTANCE: invention can find use in mechanical engineering to raise plastic properties of steel blanks passed through plastic metal working in particular. Blanks of predominantly medium carbon steel are heated in two stages by electric current of certain density with pause in supply of current during heating. EFFECT: provision for control over physical and mechanical characteristics, intensification of process of thermal treatment, decreased power consumption. 1 c, 4 dwg, 1 tbl

Description

 The invention relates to the heat treatment of metals and alloys and can be used in mechanical engineering to increase the ductility of steel billets, in particular wire from medium carbon steel after cold drawing.

 A known method of wire drawing, including cold drawing and electrical heating with a given current density in two stages (see RF patent N 2043800).

 The reasons that impede the achievement of the required technical result when using the known method include the fact that the known method can be used in relatively limited cases, since it is necessary to choose the heating zone of the wire by current, taking into account the time of its movement.

 The closest method of the same purpose to the claimed object according to the totality of features is the method of electrical contact heating of workpieces, mainly from medium-carbon steels, including the zone heating of the workpiece by passing current of a given density for a given time with the heating zone moving along the length of the workpiece (see RF patent N 2044781 , C 21 D 1/40, 1995, prototype).

In the known method, heating is performed by a current with a density of 49-103 A / mm ² for 0.5-1.6 s.

 For reasons that impede the achievement of the required technical result when using the known method adopted as a prototype, the known method does not allow to obtain high plastic properties of the workpiece (wire) and more fully intensify the heat treatment process.

 As for the ranges of current density and the time of its action, known from the prototype, the heat treatment of the wire in the indicated time ranges does not achieve the required technical result, described below, and the task is not performed, which is confirmed by the information given in the present description of the application, in particular, the results conducted research (see chart and table).

 The invention consists in the following.

 The technical result that can be obtained by carrying out the invention is to significantly increase the ductility of the wire by more than 4 times with a decrease in strength of not more than 40%, intensification of the processing process, and reduction of energy consumption.

 The specified technical result in the implementation of the invention is achieved by the fact that in the known method of electrothermal treatment of a metal billet mainly from medium-carbon steel, including heating the billet with an electric current of a given density of a certain duration, the heating is performed in two stages with equal durations in the first and second stages, comprising 1-3 s at pauses between steps lasting 0.24-4 s.

When passing a current density of 6.5 A / mm ² with heating durations of the first and second steps of 1 s, the pause duration is 0.25-2 s.

 Information confirming the possibility of carrying out the invention to obtain the above technical result is given below.

Through wire samples (length of the order of 250 mm) from medium carbon steel grade st. 70 with a diameter of 3.2-3.5 mm, charged, for example, after cold drawing from a wire rod with a diameter of 6.5 mm, passed a high-density electric current (TVP) of 35-65 A / mm2 ^of industrial frequency with a pause of current supply in the interval 0.25-4 s after the first stage of heating with a current of 1-3 s.

 The table, as well as the diagram of FIGS. 1 and 2, summarizes the results of the applicant's repeated studies of the plastic and strength properties of the wire after its processing by the proposed method.

The table for criteria 1-5 shows the relative ductility: δ _t / δ _b.t. - the ratio of the relative deformation of the workpiece treated with current δ _t to the relative deformation without current treatment δ _b.t. and relative strength σ _t / σ _b.t - the ratio of the tensile strength of the workpiece treated with current σ _t to the tensile strength of the workpiece without current treatment σ _bt Results according to criteria 1-3 were obtained at a current density of 65 A / mm ² , and according to criteria 4.5 at a current density of 35 A / mm ² . For all criteria, the duration of current heating in the first stage t _n1 and the second t _n1 were chosen equal. The pause duration t _p varied from 0-4.5 s.

 The results of the study of plastic properties by the criterion "relative ductility" are presented in FIG. 1. Curves 1-5 correspond to the criteria 1-5 of the table. Figure 2 presents the results of studies of strength properties by the criterion of relative strength. Here, curves 1-5 correspond to criteria 1-5 of the table. A pause current heating diagram is shown in FIG. 3, without a pause - in figure 4. The workpiece was cooled in air.

Example 1. A wire sample of steel 70 with a diameter of 3.2 mm, obtained after cold drawing from a wire rod with a diameter of 6.5 mm, was heated by alternating current of industrial frequency with a density of 65 A / mm ² stepwise, at the first heating stage for 1 s and at the second heating steps for 1 s, with a heating pause between steps of 0.5 s. Heating and cooling were carried out in air. As a result of processing, the relative ductility increased by 2.3 times, and the relative strength was 0.9.

 Example 2. It differs from example 1 in that the current heating at each stage was 2 s, with a pause of 0.5 s. Relative ductility was 2.58, and relative strength 0.89.

 Example 3. It differs from example 1 in that the current heating at each stage was 3 s, with a pause of 0.5 s. Relative ductility was 2.9, and relative strength 0.82.

Example 4. It differs from example 1 in that the heating was carried out by a current with a density of 35 A / mm ² at each stage for t _n1 = t _n2 = 1 s, with a pause of 0.5 s. The relative ductility was 2.38, and the relative strength was 0.9.

Example 5. It differs from example 1 in that the heating was carried out by a current with a density of 35 A / mm ² at each stage for t _n1 = t _n2 = 2 s, with a pause of 0.5 s. The relative ductility was 2.18, and the relative strength was 0.92.

It can be seen from the test results that, starting from a pause time of 0.25 s and up to 4 s when passing a current with a density of 65 A / mm ² and 35 A / mm ^{2, the} ductility of the wire increases by more than 4 times, with a relative decrease in the tensile strength not more than than 40%.

Due to the short periods of heat treatment and high heating rates of the workpiece with a current of about 400-550 ^o C / s, the heat treatment of the TVP can be performed in both static and dynamic modes of motion of the workpiece during short-term heating to temperatures of 500-600 ^o C. For long workpieces, heat treatment TVP is not limited by the speed of movement of the workpiece.

 Conducted multiple tests have established that the specified time interval is optimal. The pause time favorably affects a significant increase in the ductility of steel.

 Thus, the above research data indicate the possibility of carrying out the invention to obtain the above result.

 Therefore, the claimed invention meets the requirement of "industrial applicability" under applicable law.

Claims (2)

1. The method of electric contact heating of workpieces, including heating the workpiece by passing an electric current of industrial frequency of a given density and a certain duration, characterized in that a current of a density of 35 - 65 A / mm ^{2 is} passed, heating is carried out in two stages with a duration of 1 - 3 s at each stage and with a pause between steps with a duration of 0.25 - 4.0 s. 2. The method according to claim 1, characterized in that when passing a current with a density of 65 A / mm ² with a heating time of 1 step at each stage, the pause duration is 0.25 - 2.0 s.

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