GB1559560A - Electroslag casting method - Google Patents

Description

(54) ELECTROSLAG CASTING METHOD (71) We, INSTITUT ELEKTROSVARKI IMENI E. O. PATONA AKADEMIT NAUK UKRAINSKOI SSR, of ulitsa Bozhenko 11, Kiev, Union of Soviet Socialist Republics, a Corporation organised and existing under the laws of the Union of Soviet Socialist Republics, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described, in and by the following statement :- This invention relates to electroslag casting and particularly to methods for the electroslag casting of heavy metal ingots used as billets for the manufacture of generator rotors and other large-sized components.

Recently, the demand for forgings made of 100-ton and larger ingots has greatly risen in various fields of industry, particu larly in power engineering. It is common knowledge that the quality of forgings depends in many respects on the ingot structure and determines the quality of the finished product. Practice has shown that conventional casting of heavy ingots by a single pouring of all the liquid metal into a mould inevitably leads to such defects as segregation, cavities, adverse distribution of non-metal impurities in the ingot, and surface oxidation. In the majority of cases the marked physical and chemical heterogeneity of ingots produced by the conventional method makes them unsuitable for the manufacture of large-sized components.

The metal structure in the centre of such an ingot is particularly poor. This is due to the fact that the liquid metal of the ingot solidifies mostly in the direction from the mould walls towards the centre. As the solidification front moves on, the conditions of heat removal worsen and the structure of the solidifying metal changes from a fine-textured dendritic chilled one adjacent the ingot surface to one composed of globular crystallites with extensive segregation in the centre.

What is desired is a method of portionwise electroslag casting which makes it possible to raise the efficiency by increas- ing the mass of portions of metal, to provide upward solidification of the ingot, to improve the quality of the ingot, to make the metal structure more homogeneous throughout the body of the ingot, to reduce defects related to segregation and shrinkage, to increase the chemical homogeneity of the metal structure in the head of the ingat and to eliminate the concentration there of sulphide and oxide impurities. and to reduce the power consumption in the process of portion-by-portion electroslag casting of heavy metal ingots.

The present invention provides a method for the electroslag casting of metal ingots in a cooled mould having non-consumable electrodes introduced therein, which method includes the steps of forming a slag pool in the cooled mould, heating said pool by the electrodes. pouring liquid metal by portions through a layer of the liquid slag upon the solidification of more than half of the metal of the preceding portion, and wherein the first portion of the liquid metal is poured until its mass attains 30 to 50 per cent of the total mass of the ingot being cast, while the pouring of the following portions is carried out until the mass of every portion attains 10 to 50 per cent of that of the first one.

After the first portion of metal (which may constitute up to 50 per cent of the total mass of the ingot) is poured, solidification is directed upwards from the bottom area of the mould on account of the greater heat removal in that region. A decrease in the mass of the second and following portions, in combination with heating of the slag pool, makes it possible to reduce the solidification rate in the direction of the mould walls and to provide for prevailing solidification from the bottom upwards. The above factors enhance the efficiency of the process practically without impairing the quality of ingots.

To improve the quality of the ingot, it is advisable that the mass of every, portion of metal subsequent to the second one be successively decreased as against that of the preceding portion. It is advisable to stop pouring every subsequent portion to the second one after the mass thereof attains 50 to 90 per cent of that of the preceding portion.

In a preferred procedure, the first portion of metal is poured until its mass attains 30 per cent of the total mass of the ingot, while every portion subsequent to the second one is poured until the mass thereof attains 90 per cent of that of the, preceding portion.

According to an alternative procedure, the first portion of liquid metal is poured until its mass attains 50 per cent of the total mass of the ingot, while the pouring of every following portion is terminated when the mass thereof amounts to substantially 50 per cent of that of the preceding one. Such process conditions provide for high homogeneity of the physical structure of the ingot.

According to the optimum alternative procedure, the first portion of liquid metal is poured until its mass attains 40 per cent of the total mass of the ingot, while the pouring of every portion subsequent to the second one is terminated when the mass thereof attains 77 per cent of that of the preceding portion.

The invention will ! be described further, by way of example only, with reference to the accompanying drawings, wherein : Figure 1 shows a cooled mould having electrodes introduced therein, in vertical section; Figure 2 is a plan view of the cooled mould with the electrodes connected to a power source ; Figure 3 is similar to Figure I and shows the cooled mould and the electrodes during the formation of a slag pool ; Figure 4 is similar to Figure 1 and shows the position of the electrodes and of a central tube during the pouring of a first portion of liquid metal into the cooled mould through a layer of molten slag; Figure 5 is similar to Figure 4 and shows the position of the electrodes on completion of the pouring of the first portion of liquid metal and a graphical illustration of the solidification directed mainly from the bottom area of the mould; Figure 6 is similar to Figure 4 and shows the pouring of a second portion of liquid metal into the mould through the layer of liquid slag; Figure 7 is similar to Figure 1 and shows the position of the electrodes after all the portions of. liquid metal have been poured into the mould, during the elimination of the contraction cavity; and Figure 8 is a vertical section through the mould and the ingot, illustrating the struc- ture of the ingot.

For casting an ingot, use is made of a cooled mould 2 mounted on a bottom plate 1, the mould 2 being provided with a water jacket 3, as shown in Figure 1. Placed on the bottom plate 1 is a metal starting bar 4 the chemical composition of which approximates that of the metal of the ingot to be cast ; Non-consumable graphite electrodes 5 are introduced within the mould 2, their number being divisible by three. In the embodiment illustrated, there are three electrodes 5. The electrodes 5 are connected to a three-phase commercial frequency power source G and are uniformly spaced about the circumference of the mould cross-section, as shown in Figure 2. The electroslag casting of an ingot is then carried out as follows.

The electrodes 5 are lowered till their ends touch the starter bar 4. A slag having a high refining ability, or a mixture of stock components thereof, is poured into the mould 2. The threephase power source G supplies the electrodes 5 with current of 10000 to 20000 amperes at 50 to 90 volts.

A layer of liquid slag appears in the mould and a slag pool 7 is formed, as shown in Figure 3. Hence-forward the layer of liquid slag is continuously heated by the electrodes 5. The slag pool 7 may be formed in any alternative manner, e. g. by pouring a premelted liquid slag into the mould 2. A solidified slag lining 8 is formed on the wall of the mould 2 and in the gap between the starting bar 4 and the mould wall. The height of the slag lining 8 is equal to that of the slag pool 7 in the mould 2.

Once the slag pool 7 is formed, liquid metal is poured by portions. The first portion 9 (Figure 4) is poured via a central tube into the mould 2 through the layer of liquid slag 7. As the mould 2 is filled, the electrodes 5 are gradually raised, their ends being kept in the slag pool, as shown in Figure 4. The pouring of the first portion of liquid metal is terminated when the mass attains 30 to 50 per cent of the total mass of the ingot being cast. After completion of the pouring of the first portion 9, the electrodes 5 continue heating the slag pool 7. The slag layer 7, which is continuously heated, transfers heat to the upper part of the liquid metal of the first portion 9.

Simultaneously, as a result of very active heat removal through the tbottom plate I liquid metal solidifies on the surface of the starting bar 4 and adjacent the cooled wall of the mould 2, the metal solidifying mainly from the bottom region of the mould 2, as shown in Figure 5.

When more than half of the metal of the first portion 9 has solidified, the second portion of liquid metal is poured through the liquid slag layer, as shown in Figure 6.

The second portion of the liquid metal mixes with the still unsolidified metal of the first portion 9 and so homogenizes the chemical composition of the ingot being cast. The second portion is poured until its mass attains 10 to 50 per cent of that of the first portion 9. As the height of the ingot increases, heat removal from the metal through the bottom plate 1 worsens.

Under these conditions the reduction in the mass of the second portion and the continuous heating of the slag pool considerably decrease the solidification rate in the direction of the mould walls and allow the metal to continue to solidify mainly from the bottom area upwards.

After more than half of the second portion metal has solidified, the third portion of the liquid metal is poured through the liauid slag layer, the mass of the third portion being equal to or less than that of the second portion. Subsequently, one or more further portions of liquid metal are poured in a similar manner until the casting of the ingot is completed. After pouring the last portion of liquid metal into the mould 2, the contraction cavity is eliminated by continued heating of the slag pool above the ingot 10 by the electrodes 5, as shown in Figure 7.

The structure of an ingot cast by pouring four portions of liquid metal is shown in Figure 8, which roughly indicates the rela- tionship between the masses Mi to M, of the portions and the ingot mass.

Hereinafter, examples of the utilization of the above method are described.

E, rarwple I When casting 200-ton forging ingots use is made of a water-cooled mould 2500 mm in diameter and 6000 mm high. The mould is mounted on a bottom plate having a starting bar placed thereon. Three or six graphite electrodes are introduced into the mould so that their ends touch the starting bar. Each electrode is 250 to 500 mm in diameter. Slag or a mixture of stock components thereof is charged into the mould.

The electrodes are supplied with electric power, the current being 10000 to 20000 amperes and the voltage 50 to 90 volts.

This causes the slag to melt, i. e. a slag pool is formed and continuously heated by the electrodes. Upon forming the slag pool, the first portion of liquid metal is poured into the mould through the liquid slag.

The mass of the first portion is 60 tons, i. e.

30 per cent of the total mass of the ingot being cast. After the pouring of the first portion, the electrodes continue heating the slag pool, and eight to fifteen hours later, when more than half of the first portion has solidified, the second, portion of liquid metal is poured. The mass of the second portion amounts to 20 tons, i. e. 33 per cent of the first portion. After the pouring of the second portion, the electrodes continue heating the slag pool, and three to ten hours later, the third portion of liquid metal is poured into the mould. The mass of the second, third, and every following portion is 20 tons. After the pouring of the last portion, the contraction cavity is eliminated. With such a technique, no segregation is developed, even in steels which solidify over a wide temperature range.

Example 2 200-ton forging ingots of steels which solidify over a narrow temperature range are cast similarly, by maintaining the following parameters of the process. The first portion of liquid metal is poured until the mass thereof attains 80 tons i. e. 40 per cent of the total mass of the ingot. The second portion of liquid metal is poured after 6 to 15 hours, after more than half of the first portion has solidified. The second and all the following portions of liquid metal are poured until the mass of each of them attains 20 tons, the time between pourings being 3 to 10 hours. Such a technique is highly efficient for casting heavy ingots of a steel which solidifies over a narrow tem, perature range. Casting of a 200-ton ingot is thus carried out in seven pourings.

Exemple 3 A 200-ton forging ingot is cast by pouring the first portion until its mass attains 50 percent of the total mass of the ingot being cast, i. e. 100 tons. The pouring of the second portion is commenced when over half of the first portion has solidified and is terminated when the mass thereof attains 50 per cent of that of the first portion, i. e. 50 tons. The mass of every following portion is successively reduced as against that of the preceding one. In particular, the third, fourth, and further portions are poured with the time between pourings being successively diminished from 10 to 0.5 hours, while the mass of every portion is reduced to approximately 50 per cent of that of the preceding one.

Thistechnique, while ensuring rather high efficiency (only eight pourings), provides better conditions for the solidification of the ingot because a change in the mass of every following portion conforms to the changes in the heat removal as the height of the ingot increases, and favours directional solidification.

Exsngle 4 A 200-ton forging ingot is cast in the foregoing manner with the exception of the metering of portions. The first portion of liquid metal is poured into the mould until its mass attains 30 per cent of the total mass of the ingot, i. e. 60 tons. The second portion of liquid metal is poured until its mass attains 30 tons, i. e. 50 per cent of the mass of the first portion. The masses of the portions of liquid metal following the second one are successively reduced by 10 per cent as against the mass of a preceding portion.

E, ra ple S A 200-ton ingot is cast in the foregoing manner, maintaining the same parameters, with the exception of the metering of portions. In particular, the first portion of liquid metal is poured into the mould until its mass attains 40 per cent of the total mass of the ingot, i. e. 80 tons. The second portion of liquid metal is poured until its mass attains 50 per cent of that of the first portion, i. e. 40 tons. The mass of every portion of liquid metal following the second one is successively reduced by 23 per cent as against the mass of the preceding portion, while accordingly diminishing the intervals of time between the pourings from 20 to 2 hours. The embodiment is preferably when high casting efficiency and high quality of heavy ingots are simul- taneously required.

The foregoing method for the electroslag casting is most promising for producing high-quality heavy forging ingots from 40 to 350 tons used, in particular, for manufacturing rotors of turbines having unit ratings exceeding one thousand megawatts.

The principal advantage of this method over the prior art practice is the provision of a higher efficiency of the portion-byportion casting of ingots by increasing the mass of portions combine with the homogeneity of chemical and physical structure practically throughout the body of the ingot due to an upwardly directed solidification.

Claims (9)

WHAT WE CLAIM IS :-

1. A method of electroslag casting of an ingot in a cooled mould having nonconsumable electrodes introduced therein, including forming a liquid slag pool in the cooled mould, heating the pool by means of the electrodes, and pouring successive portions of liquid metal into the mould through the liquid slag, the mass of the first portion being 30 to 50 per cent of the total mass of the ingot being cast, each following portion being poured only after the solidification of more than half of the metal of the preceding portion and having a mass no greater than that of the preceding portion, the mass of the second portion being 10 to 50 per cent of that of the first portion.

2. A method as claimed in claim 1, wherein the masses of the portions are successively reduced.

3. A method as claimed in claim 2, wherein the mass of every portion subse- quent to the second one is 50 to 90 per cent of that of the preceding portion.

4. A method as claimed in claim 3, wherein the mass of the first portion is 30 per cent of the total mass of the ingot being cast, and the mass of every portion subsequent to the second one is 90 per cent of that of the preceding portion.

5. A method as claimed in claim 3, wherein the mass of the first portion is 50 per cent of the total mass of the ingot being cast, and the mass of every following portion is substantially 50 per cent of that of the preceding portion.

6. A method as claimed in claim 3, wherein the mass of the first portion is 40 per cent of the total mass of the ingot being cast, and the mass of every portion subsequent to the second one is 77 per cent of that of the preceding portion.

7. A method of electroslag casting substantially as described herein with reference to the accompanying drawings.

8. A method of electroslag casting substantially as described in any of Examples 1 to 5.

9. An ingot cast by a method according to any preceding claim.