RU2149196C1 - Method of vacuum electric-arc remelting of ingots - Google Patents

Abstract

FIELD: special electrometallurgy, particularly, vacuum electric-arc remelting of highly reactive metals and alloys. SUBSTANCE: method is used in smelting of ingots of the second remelting from titanium alloys. Prior to ending of smelting process, electrode is lowered down to value of arc gap of 15-25 mm and smelting is carried out at arc current intensity of 10-14 kA up to melting of marks. Marks are made on side surface of upper part of electrode during its preparation for smelting. Marks are made in the form of longitudinal grooves. EFFECT: provision of fixed smelting of electrode upper part before elimination of contraction cavity to obtain flat end of electrode and to reduce casting defects and to enhance ingot-to-product yield. 2 cl, 2 dwg, 1 ex

Description

 The invention relates to the field of special electrometallurgy, in particular to a vacuum arc remelting of highly reactive metals and alloys, and can be used to obtain ingots of the second remelting from titanium alloys.

 The choice of a technological scheme for producing ingots depends on their purpose and quality requirements for semi-finished products. For the bulk of the ingots, the main requirement is the purity of the metal from internal defects, as well as the uniformity of the chemical composition and the efficiency of the process.

 A known method of the second vacuum arc remelting of titanium alloy ingots with a diameter of 650 ... 850 mm, including preparing consumable electrodes (first remelting ingots) for melting, the initial melting period, the main melting period and the melting end period (Titanium alloys. Melting and casting of titanium alloys. Responsible editor V.V.Dobatkin, Moscow, Metallurgy, 1978, pp. 295 ... 306 [1]) - prototype.

 The preparation of the first remelting ingots for melting involves trimming the crown, removing bevels from the clover, grooving or trimming of the oxidized sections, and removal of chloride from the side surface of the ingot.

 The initial period consists in the excitation of the electric arc between the electrode and the tray and a smooth increase in the arc current from 5 kA to 25 ... 37 kA for 25 ... 30 minutes (dilution of the liquid bath on the pan).

 The main melting period consists in melting the electrode at an arc current of 25 ... 37 kA and an arc voltage of 46 ... 50 V with the solenoid on with the optimum level of alternating magnetic field strength of 40 ... 80 Oe and automatic process control (ARP).

The final melting period - the removal of the shrink shell - lead to a decrease in the arc current for a long time from 1010 ^{± 2.0} to 3 $\genfrac{}{}{0ex}{}{\text{± 2.0}}{\text{-0.5}}$ kA (and voltage, respectively, from 30 to 22 I) with the disconnection of the ARDP.

 The disadvantages of this method is that, secondly, the arc gap between the end of the electrode and the molten metal bath is practically not controlled, and secondly, melting at high currents of the arc leads to the advance melting of the middle part of the consumable electrode (see [1] , Fig. 117, 118, p. 298). Concavity (pit) in the middle of the electrode section reaches a depth of 80 ... 120 mm and more. The absence of a flat end does not allow us to precisely establish the arc gap before the modes of shrink shell removal, which leads to uneven fusion of the electrode end, premature ingot withdrawal from the mold wall, liquid metal flowing into the formed gap and creating a so-called “bridge” in the upper part of the ingot (liquid entry baths under the end of the electrode), causing the formation of defects of foundry origin: shrink shell, shrink pores, etc., which significantly reduces the yield.

 The problem to which this invention is directed is to increase the yield of lost wax by reducing casting defects by providing a fixed fusion of the upper part of the electrode before removing the shrink shell and obtaining a flat end of the electrode by reducing the arc gap and the arc current to optimal values.

The problem is solved in that in the method of vacuum arc remelting of ingots, mainly titanium alloys, before the end of the melting process - removing the shrink shell - the consumable electrode is lowered down to an arc gap of 15 ... 25 mm and melted with an arc current of 10 .. .14 kA before the fusion of the marks, and the marks are made on the side surface of the upper part of the electrode in preparation for melting. Labels are in the form of longitudinal grooves
The main difference of the proposed method compared to the prototype is that before removing the shrink shell (VUR) on the fused end of the electrode form a flat end. It is formed by changing two parameters: by reducing the arc current strength to 10 ... 14 kA and by reducing the arc gap (arc length) to 15 ... 25 mm.

 The presence of marks on the side surface of the upper part of the electrode allows you to accurately determine the moment of transition from the maximum arc current of the main melting period to the moment of fixing the flat end of the electrode and the VUR mode. When the electrode melts during the main melting period at an arc current of 25 ... 37 kA, a mark appears on the screen in the form of a luminous spot on the dark projection of the electrode end (circle). The current is gradually reduced and at the same time the electrode is lowered until the liquid metal bath darkens. Then, with further fusion of the electrode, a second mark appears (a bright spot on a dark circle). Knowing the length of the longitudinal grooves (marks), you can quite accurately establish the arc gap to the end of the removal of the shrink shell.

 The method of setting the arc gap of 15 ... 25 mm before removing the shrink shell in the process of melting the consumable electrode is the know-how of production.

 The essence of the proposed method of vacuum arc remelting of ingots is illustrated by graphic materials.

In FIG. 1 shows the upper part of the sacrificial electrode with diametrically opposite to each other marks l ₁ and l ₂ , front view. The electrode is welded to the electrode holder.

 In FIG. 2 shows the melting parameters of the electrode, where: 1 - diagram of the recording voltage on the arc; 2 is a diagram for recording the current strength of an arc; 3 is a diagram of a stock movement record. Intervals of melting modes are also shown here: a - beginning of melting; b - the main mode of melting; in - WUR mode; g - the formation of a flat end of the electrode in front of the VUR.

 Example.

 The consumable electrode is melted on a DTV-8.7-G10 vacuum electric arc furnace.

On a consumable electrode (ingot of the first remelting) of a W6 alloy with a diameter of 670 mm and a length of 2100 mm, two marks l ₁ and l ₂ (longitudinal grooves) located diametrically opposite to each other were made with a mill. One groove l ₁ = 120 mm for fixing the beginning of the ASD; another groove l ₂ = 240 mm - for fixing the moment of transition from the arc current of 32 kA (working arc current in the main melting period) to 10 ... 14 kA (fusion of the concave electrode end and the formation of a flat end). The electrode was loaded into a mold with a diameter of 770 mm. After loading and alignment of the electrode, it was welded to the electrode holder (cinder). The furnace was evacuated, the power source was turned on, the arc current was set to 5 kA and the arc gap was 40 mm (arc length). After 15 minutes, the arc current increased to 10 kA and after the molten metal bath was placed on the pallet (15 min), the solenoid was turned on and the arc current was gradually increased to 5 kA for 5 minutes (operating mode). After reaching the working mode of melting the electrode, the ARDP controller was turned on and melted until a luminous point from the groove l ₂ appeared on the screen. The length of the arc at this moment was 120 mm. The ARDP controller has been disabled. The arc current was gradually reduced to 10 kA, and the rod was lowered by 60 mm (see Fig. 2, item 3). When another luminous point l _{1 appeared, they} switched to the mode of removing the shrink shell, the arc gap during this period varied within 15.. . 20 mm. After the end of the VUR, after 1 hour, the arc gap was measured by lowering the rod with the remainder of the electrode to a short circuit (8 mm). After cooling the ingot (3 hours), the furnace was opened, the remaining electrode (55 mm) and nastilya (6 ... 8 mm) were measured. Thus, the arc gap at the end of the VUR is (8 + 6 ... 8) = 14 ... 16 mm, which corresponds to the optimal arc gap, which ensures a high-quality and safe end of the melting.

 The obtained ingot was of good quality, the yield increased by 0.5% due to the reduction of casting defects.

 The proposed method of melting ingots in comparison with the known ones allows for fixed fusion of the upper part of the electrode before removing the shrink shell, which makes it possible to obtain a flat end of the electrode, reduce the formation of casting defects and, in general, increase the yield of smelted ingots. In addition, the flat end of the electrode allows you to maintain a quasi-stationary mode of combustion of the vacuum arc by reducing the temperature gradient from the center to the periphery.

Claims (2)

 1. The method of vacuum arc remelting of ingots, mainly titanium alloys, including preparing the consumable electrode for melting, the initial melting period, the main melting period and the end of the melting process - removing the shrink shell, characterized in that before the end of the melting process the electrode is lowered down to the arc gap 15 - 25 mm and melt it at an arc current of 10 - 14 kA until the marks are fused, and the marks are made on the side surface of the upper part of the electrode in preparation for melting.  2. The method according to claim 1, characterized in that the marks are made in the form of longitudinal grooves.

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