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Special features of equipment for the continuous production of centrifugal electroslag castings.

The method of centrifugal electroslag casting (CESC) with the vertical axis of rotation, developed at the E.O. Paton Electric Welding Institute, can be used to produce cast billets of high-quality with the properties not inferior to those of forgings [1, 2]. The advantages of the method in comparison with the deformation production methods include the possibility of producing large and shaped billets from steels and alloys, including from alloys with low deformability.

As a result of the relatively simple procedure, the method is used on an increasing scale in engineering production. At the same time, the productivity of this method is considerably lower than that of the deformation methods as a result of both the specific features and existing practice with the production of a single casting from a single melt which delays the application of the method in series production.

To increase the productivity of the process and, consequently, reduce the cost of production of the billets, it is necessary to change the nature of production in transition to semi-continuous or continuous casting. This production can be realised using the CESC complex fitted with an electroslag furnace with non-consumable and consumable electrodes [3]. In this system, the consumable electrodes can be replaced during melting.

The concept of the construction of electroslag furnaces with the replacement of consumable electrodes during melting with preheating of the lower ends of the changeable electrodes, immersed in the slag pool, has been included in a number of foreign and domestic electroslag remelting furnaces [4]. However, these furnaces are unjustifiably cumbersome and expensive. At the same time, the problem of producing defect-free billets in the ESR furnaces with the change of the consumable electrodes has not been completely solved.

Many investigators oppose even short-term breaks in supply of energy into the slag pool which are unavoidable in operation with the replacement of the electrodes during melting, assuming that even short-term disruptions of the heat balance in ESR which leads to the formation of micro--and macrosegregation bands in the billets have a detrimental effect on the fatigue strength of the metal containing these defects [4].

Since the CESC processes use crucible melting, the liquid metal temperature and the condition of the bottom surface of the crucible can be easily regulated by the ESR parameters prior to and after replacing the electrodes. Therefore, the task of the application of the furnace with the replacement of the electrode for CESC is greatly simplified.

[FIGURE 1 OMITTED]

The proposed equipment with the change of consumable electrodes during electrode melting (Fig. 1) consists of a stationary column with the base, an upper carriage with a mechanism for clamping consumable electrodes and a lower carriage with a rotating mechanism carrying non-consumable electrodes, and also current-carrying busbars and connectors of the flexible water-cooled cables with couplings. The current-carrying busbars and the cables of the upper and lower carriages are connected to the same current source. The use of the lower carriage with the rotating mechanism which carries the non-consumable electrodes an rotates of them into the non-working position and back during melting, widens the technological parameters of equipment.

Another special feature of this equipment is the possibility of regulation in the lower carriage of the distance between the non-consumable electrodes in two coordinates in the horizontal plane and also selection of the number and dimensions of these electrodes. The variation of the disposition of the non-consumable electrode in the slag pool makes it possible to use changeable ceramic crucibles of different dimensions.

[FIGURE 2 OMITTED]

Figure 2 shows the design of a centrifugal machine with the vertical axis of rotation which can be used in the centrifugal electroslag casting system. The machine has the following characteristics:
External diameter of the mould, mm          1800
Height of the mould, mm                     1200
Total load on the faceplate, kg     up to 12 000
Nominal frequency of rotation, 1/min        1500
Frequency of rotation of the mould,
   1/min                                  80-800
Power of the electric drive, kW              110
Weight of the casting, kg up to             3000
Weight of the centrifugal
   machine, kg                              4950


The centrifugal equipment has the form of a frame welded from channels and secured to the base using anchoring bolts. The drive consists of an asynchronous motor and a microprocessor frequency converter L300P (Hitachi). The driver transfers the torque to the drive roll connected with the clutch. Three bearing sections are secured in the frame under the angle of 120[degrees]. The bearing sections contain one drive and two driven rolls without the possibility of radial displacement. The faceplate with the diameter of 2000 mm is rotated using the drive roll by a friction pair. All the rolls are covered with protective shields. The central bearing section fixes the faceplate in the axial position.

This design greatly reduces the load on the shaft and the bearings of the central bearing section from disbalance in rotation. The centre of gravity of the mould and the casting, i.e., the point of application of the perturbing centrifugal forces, is situated between the drive and driven rolls. Another special feature is the fact that the central shaft can be deflected during operation from the vertical axis through some angle which compensates the inaccuracy of positioning the faceplate and the horizontal plane.

To ensure safety, the centrifugal machine is positioned at some distance from the crucible furnace in a container and its working space is restricted by a roll-away protective jacket. The machine is fitted with systems for water-air cooling and lubricating of the bearings with a liquid lubricant.

The CESC operates as follows. A slag pool is produced in a ceramic crucible by the dry start or by pouring a portion of slag prepared in a separate flux-melting furnace. This is followed by the ESR of consumable electrodes. During this period, the non-consumable electrodes occupy the non-working deposition (Fig. 1a). After building up the required portion of liquid metal in the crucible the consumable electrodes are removed from the melting zone. The liquid metal together with the slag are transported using a rolling carriage to the centrifugal machine positioned 5 m from the furnace, and they are poured into the rotating mould of the centrifugal machine. A small portion of liquid metal (10-15%) is left in the crucible. After pouring in a new portion of slag, the rotating mechanism, fitted with the electromechanical drive, is used for rotating the block of non-consumable electrodes into the working position (Fig. 1b). The slag pool is preheated with non-consumable electrodes.

Preheating of the slag is accompanied by replacing the consumable electrodes (Fig. 1c). The non-consumable electrodes are subsequently removed from the melting zone and consumable electrodes are introduced. Removal from the melting zone is ensured using the rotating mechanism (Fig. 3).

A special feature of the rotating mechanism is the optimum design of the current-carrying busbars in the form of hinges. Consequently, the minimum break in the process of replacing the electrodes can be efficiently ensured. A break in the electroslag process taking into account the transport time of the crucible to the centrifugal machine and back is 2-3 min.

In addition to remelting the new consumable electrodes, the first casting is solidified and removed from the mould. After building up the required portion of liquid metal, the operations are repeated.

Relatively small blanks for components are produced efficiently by CESC whose productivity is lower than that of the deformation methods. Therefore, the centrifugal electroslag casting systems should be fitted with a centrifugal machine with a relatively high power.

[FIGURE 3 OMITTED]

This method can be used to produce large seamless blanks, for example in ring and wheel rolling production which have present are produced by deformation-welding methods. In alternating loading, the welded joint is a weak link of the entire structure.

When using the centrifugal electroslag casting system fitted with a crucible furnace and a centrifugal machine with a relatively high power, productivity can be increased by increasing the weight of the casting and by using multi-position moulds.

With transition to continuous production of castings, the proposed centrifugal electroslag casting system increases the productivity of the process and also the economic parameters and efficiency as a result of improving the heat balance of the melting process in a constantly heated crucible, increasing the service life of the lining working without thermal cycles of heating and cooling, reducing the duration of preparatory operations carried out previously prior to every melt (dressing of the bottom surface of the crucible to remove the remnants of the solidified slag, repair of a bottom electrode in the mono-filar scheme, verification of the elements of the water cooling systems, etc).

References

[1.] Goryachek A.V., et al., Metallurgyia mashinostroeniya, 2008, No. 1, 26-28.

[2.] Skripnik S.V., et al., Sovremen. Elektrometall., 2008, No. 3, 15-17.

[3.] Goryachek O.V., Equipment for electroslag remelting of metals and alloys, Patent No. 74472, Ukraine, MPC S 22 V 9/18.9/187, 15.12.2005.

[4.] Paton B.E. and Medovar B.I., Electroslag furnaces, Naukova Dumka, Kiev, 1976.

S.V. Skripnik

Titan NPF Company, Kiev
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Title Annotation:ELECTROSLAG TECHNOLOGY
Author:Skripnik, S.V.
Publication:Advances in Electrometallurgy
Article Type:Report
Geographic Code:4EXUR
Date:Jan 1, 2010
Words:1508
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