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Electroslag casting with melting together.

Electroslag casting (ESC) is the method for producing special-purpose shaped billets, which are later used in the cast form. Cast electroslag billets are frequently used instead of the forged pieces, because in regard to their service properties they are not inferior to the latter and in certain respects even exceed them [1]. High quality of the electroslag metal is ensured due to refining of the molten metal in the process of remelting and its subsequent directed hardening in the water-cooled casting mould [2].

In ESC the moulds are used, internal configuration of which corresponds with necessary allowances to the shape of the external surface of the future shaped billet. They are made dismountable and have a more complex design than the moulds, used for melting of the conversion ingots by the method of classical electroslag remelting (ESR).

The ESC method differs from ESR not just by the shape of the ingots being melted, but also by a number of technological peculiarities. So, in the course of melting in ESC depth of the slag and the metal pools may significantly change, which exerts significant influence on the electroslag process mode and conditions of structure formation of the billet being melted [3]. In addition, process of the billet core formation differs from that in the places of formation of the branch pipes and other protruding parts [4]. In these places a new solidification front is formed, direction of movement of which does not coincide with the main one, provided additional cooling surfaces are available. In the zone, where these fronts meet, defects of shrinkage origin may form [5].

The ESC method with melting together, in which in the process of electroslag melting of the main billet its connection with the previously fabricated parts takes place, does not have this defect. In this case conditions of solidification of the metal close to the classical ESR are preserved, which stably ensures the highest quality of the cast metal [3].

ESC with melting together makes it possible to fabricate billets of a much more complex shape and bigger dimensions than in case of using conventional ESC. Application of this method allows increasing productivity of casting and reducing consumption of auxiliary materials and electric power. In this method the moulds of simple shape and smaller size are used, which is especially important under present economic conditions.

Indisputable merit of ESC with melting together is possibility of producing composite billets, the parts of which are made from steel of different grades or cast iron. In addition, the billets can be successfully adjusted for application under different conditions of operation of their separate parts, like, for example, composite rolls of rolling mills [6].

Depending upon mutual arrangement of the billet being melted and the parts to be melted to it, three different ESC methods with melting together may be singled out.

According to the first one, parts of the future billet, manufactured in advance, are installed into special holes, made in walls of the dismountable mould.

According to the second method these parts are installed inside the mould.

According to the third method, the prepared parts are placed under or above the mould, in which electroslag melting of the billet is performed.

In ESC with melting together almost always shunting of the working current, which passes through the metal pool, by the parts being melted together is performed. Depth of penetration depends upon the current, which passes through the part being melted together. In development of the ESC technology with melting together it is extraordinary important in many cases to have possibility to control value of this current in order to guarantee quality melting together of a part without its excessive penetration.

Classical example of the first ESC method with melting together is electroslag melting of crankshafts or their separate parts (cranks). Crankshaft is one of main parts of piston machines (internal combustion engines, pumps, compressors). Constructively they are made of main journals and crankpins and connecting them cheeks, located in different spatial positions.

[FIGURE 1 OMITTED]

At PA <<Bryansk Machine-Building Plant>> serial production of cranks for crankshafts of powerful ship diesels is mastered according to the technology of E.O. Paton Electric Welding Institute [7]. The cranks for them are fabricated by the ESC method with melting together. Each crank represents two cheeks, interconnected by the crankpin. Mass of electroslag casting billet of the crank is about 5 t, and diameter of the crankpin is about 0.5 m. After heat and mechanical treatments the cranks are assembled into crankshafts on main journals by means of shrink fit.

The cranks are manufactured in the mould, having shape of the cheek with a water-cooled niche in the place of a future crankpin. First billet of the cheek with a crankpin in the form of a boss is melted (Figure 1, a). Then the casting is taken from the mould and installed nearby outside the mould in such way that billet of the crankpin entered into the niche. Final operation consists in melting of the second cheek with simultaneous melting to it of the crankpin, manufactured together with the first cheek of the crank (Figure 1, b).

It is possible to produce by the ESC method with melting together, using consecutive melting of the cheeks with simultaneous melting to them of the manufactured beforehand crankpins, not just separate cranks but entirely the whole crankshaft, whereby main journals and crankpins are installed into respective holes in walls of the water-cooled mould designed for ESC of the cheeks (Figure 2, a). After melting of each second cheek the produced crank is turned around axis of the main journal at the required angle and melting of the next in turn cheek is performed (Figure 2, b). This operation is repeated till the whole crankshaft is fully manufactured. In this way crankshafts, designed for gas and engine compressors of several tons mass, are manufactured [8].

At the plant in city of Tientsin (People's Republic of China) similar technology is used for production of the diesel crankshafts of up to 3 m length [9].

By the ESC method with melting together of the pre-manufactured parts, installed into the mould hole, billets of the compressor unit connecting rods from steel 40 [10], cutter holders of big machine tools [11], forks of bulldozer blades for heavy industrial tractors [12], and a number of other parts of complex shape are produced.

[FIGURE 2 OMITTED]

An example of another application of the ESC method with melting together is electroslag cladding (ESCl) by a thick layer of external surface of the rolls or other cylindrical components, which allows not just repairing worn rolls, but manufacturing new bimetal ones. Originally such cladding was performed using an annular mobile mould and consumable electrodes in the form of a paling of bars, installed in the middle of the gap between the mould and the billet being surfaced [13].

In such ESCl method non-uniform penetration was registered in cross-section of the billet being surfaced. Opposite the consumable electrodes penetration was much higher than in intervals between them. This phenomenon can be avoided if to use instead of the paling of bars a continuous tubular consumable electrode [14], manufactured by the method of spun casting, and rolled pipes.

However, when for ESCl any types of consumable electrodes are used, non-uniform penetration over height of the billet being surfaced occurs. As electroslag process progresses, depth of penetration increases, whereby one does not manage to prevent this phenomenon by programmed change of power, because in the classical process with application of the consumable electrodes a direct connection exists between the power, released in the slag pool, and amount of the melted in it metal [14].

It became possible to significantly change energy situation in the slag pool due to application in the course of ESCl of the current-leading mould (CLM), which allows introducing into the slag pool energy irrespective of the amount of the metal being melted in it, and to refuse in this way from application of the consumable electrodes. In ESCl of cylindrical billets the CLM makes it possible to maintain more simply unchangeable depth of penetration over length of the billet being melted [15].

Using CLM ESCl is performed by application of a lumpy additive material in the form of shot, chips, chaff or small chunks (Figure 3, a) [16] and by feeding into the slag pool separate portions of molten metal, melted beforehand in another unit, for example, in the induction furnace (Figure 3, b) [9].

Using ESC with melting together (the third method) bosses are, as a rule, produced on the semifinished big item in the form of different branch pipes. For this purpose the mould, designed for formation of the cast branch pipe, is installed directly on the body of the future item (Figure 4). Preliminary in the body in the places, where future branch pipes will be located, technological holes are drilled, and above them the mould is coaxially installed. A branch pipe is formed by the method of ESR of a consumable electrode, manufactured from the same steel as the item body. In the process of the branch pipe melting its melting together with the body occurs. In the course of further melting of the produced semifinished item the holes of necessary diameter are drilled over axis of the branch pipes.

[FIGURE 3 OMITTED]

This method of ESR with melting together, developed at the E.O. Paton Electric Welding Institute, is used in serial production of steam separators and pressurizers for nuclear power plants. On one body of a steam separator there are 432 branch pipes of 105 mm diameter from 22K steel, and on the pressurizer body--108 branch pipes of 150 mm diameter from the same steel. For their melting a special rig is used, which ensures precise positioning of the mould relative axis of the future branch pipe and performance of ESCl in it [17, 18].

In this way branch pipes with flanges are melted on body of the Dn 800 valve from steel 20 for second circuit of the NPP power unit. Preliminary the very body is melted by the ESC method [10]. In similar way cylindrical branch pipes are melted on covers of Dn 400 valves from steel 20 for pipelines of RBMK 1000 units of NPP and on the cover of the steam chest from 15Kh1M1F steel [19]. In similar way parts of the hub type on a hollow shaft are manufactured from 38KhS steel, for which purpose a manufactured beforehand hollow shaft is installed into the mould and a flange is melted in it with simultaneous melting to the shaft [20].

[FIGURE 4 OMITTED]

Wide introduction of special-purpose parts for ship building, chemical, power engineering and other branches of industry, produced by the ESC method with melting together, was enabled by high quality of these items and economic efficiency of their production. Available in the literature data, obtained on the basis of different investigations, and (which is especially important) positive results of operation convincingly confirmed this.

The ESC method with melting together combines in one technological process ESC and ESW. The billets, produced by this method, actually consist of two parts with different structures of the metal (pre-manufactured parts to be melted together and the basic casting) and zone of their connection. Mechanical properties of this billet first of all depend on the joining zone metal quality.

The parts to be melted together have, as a rule, simple shape and are manufactured from forged pieces or rolled metal, which ensures their high quality, and in some cases--by the ESC method and even ESC method with melting together.

Quality of cast electroslag metal of the billet core is doubtlessly high, because it was repeatedly confirmed by the most careful and diverse investigations within many decades [1].

Special attention in development of the ESC technological process with melting together is paid to achievement of the required metal properties in the area of connection, including heat-affected zone of metal of the part being melted together. In ESC with melting together mass of the cast part of the billet significantly exceeds mass of the weld in ESW. That's why despite the fact that in casting with melting together significantly higher amount of heat is released than in ESW, rate of melting together is low and, according to our estimates, equals 0.2-0.4 m/h, while in ESW of the metal of 100 mm thickness it is not less than 0.8 m/h [21].

Low rate of melting together enables more stretched in time thermal cycle than in usual ESC and longer stay of the metal of the part being melted together under high temperature conditions. Such thermal cycle causes unfavorable change of metal structure of the transitional layer and requires for additional investigation.

Available data prove that under ESW conditions of low-alloyed and alloyed structural steels, as well as in ESC with melting together of billets from these steels, in the heat-affected zone significant growth of the grain and formation of Widmanstaetten structure and ferrite banks over the grain boundaries were registered. This causes significant reduction of ductility and toughness of the metal in the overheating zone. For restoration of the metal properties in this zone only high-temperature tempering after welding of low-alloyed steels with low content of carbon is sometimes used. However, in majority of cases for achievement of the required properties it is necessary to heat an item up to the temperature of full or partial austenite transformation.

In ESC with melting together the task of ensuring high property parameters in the overheating zone is successfully solved. So, in cranks of crankshafts from 20G steel of powerful ship diesels quality of the ESC metal of the cheeks and crankpins in the melting together zone after annealing and subsequent normalization with tempering significantly exceeds that of the metal of cranks, manufactured by conventional casting, not just in regard to strength, ductility, and toughness [7], but also in regard to fatigue strength [22], whereby properties of the metal in the melting together zone don't differ at all from those of the electroslag cast metal of the cheeks, which was not subjected to overheating. Detailed investigation of quality of the ESC crankshafts after their long operation on ships did not show any defects [23].

In the course of numerous investigations of quality of the transitional zone metal and branch pipes, melted on bodies of NPP equipment from 22K steel, it was registered that on this steel as well full restoration of the metal properties in the overheating zone is achieved after normalization with tempering. Investigations of the transitional zone metal, its mechanical properties, susceptibility to embrittlement and resistance to the low-cycle fatigue showed full identity of properties of the metal of this zone, the base metal of the item body and the cast electroslag metal of the branch pipe [17, 18].

Similar results in regard to removal of consequences of overheating were also achieved in manufacturing by the ESC method with melting together of the hub-type billets from alloyed steel with increased content of carbon of 38KhS grade [20]. After annealing and oil hardening with subsequent tempering mechanical properties of the metal in the fusion zone did not differ from those of the metal in other parts of the billet and exceeded properties, required by the standard for this grade of steel. Moreover, some specimens from the fusion zone had noticeably higher value of impact strength than specimens from areas of the billet, which were not subjected to overheating during manufacturing.

In production of the composite rolls (steel/cast iron) by cladding in CLM of chromic cast iron by the additive in the form of shot [24] values of rates of cladding and melting together in ESC are close (about 0.4 m/h). As far as melting point of the used for surfacing chromic cast iron is lower than that of steel, heat-affected zone of deposit on a steel billet has a lower maximum temperature, and metal with Widmanstaetten structure was not discovered in it. Mechanical properties of the heat-affected zone metal are not given; just certain increase of its hardness is mentioned.

So, consequences of unfavorable action of the extended thermal cycle of ESC on steel parts being melted together, as shows experience, are removed by heat treatment with heating of the cast above temperature of austenite transformation and subsequent tempering. In many cases it coincides with conditions of heat treatment of the used steel, required for acquisition by it of the necessary properties.

In development of the ESC technology with melting together one has to take into account different sensitivity of steels to action of thermal cycles on them and carefully develop conditions of their heat treatment for achievement of the required level of mechanical properties.

CONCLUSIONS

1. It is established that the most universal method for producing high-quality massive steel billets of complex shape is ESC with melting together, application of which at present may be very efficient for development of many branches of mechanical engineering.

2. It is shown that the ESC method with melting together guarantees obtaining of mechanical properties of the metal of billets at the level of those made from cast electroslag metal.

3. It is determined that ESC with melting together is the most economical in comparison with other methods of ESC as to the cost of the rigging and auxiliary materials and consumption of electric power and metal.

4. It is shown that in development of technological process for ESC with melting together one has to find possibility to affect level of the working current portion, which is shunted by the parts being melted together.

5. It is determined that for each specific grade of steel, used in manufacturing of billets by the ESC with melting together method, it is necessary to develop conditions of heat treatment, which would ensure obtaining of necessary mechanical properties.

[1.] (1981) Electroslag metal. Ed. by B.E. Paton, B.I. Medovar. Kiev: Naukova Dumka.

[2.] Medovar, B.I., Latash, Yu.V., Maksimovich, B.I. et al. (1963) Electroslag remelting. Moscow: Metallurgizdat.

[3.] Paton, B.E., Medovar, B.I., Bojko, G.A. (1974) Electroslag casting. Review. Moscow: NIIMASh.

[4.] Shevtsov, V.L., Kumysh, Marinsky, G.S. et al. (1975) Slag and metal filling of cooled moulds in electroslag casting of complex shape products. Problemy Spets. Elektrometallurgii, 2, 26-31.

[5.] Shevtsov, V.L., Majdannik, V.Ya., Zhadkevich, M.L. et al. (1998) Electroslag casting of billets of casings of high pressure Christmas tree. Ibid., 4, 3-12.

[6.] Ksyondzyk, G.V. (1968) Means and prospects of electroslag cladding. In: New mechanized cladding methods. Kiev: PWI.

[7.] Medovar, B.I., Bojko, G.A., Popov, L.V. et al. (1979) Electroslag casting in production of crankshafts of large ship diesels. Problemy Spets. Elektrometallurgii, 10, 37-41.

[8.] Bezhin, V.V., Dubinsky, R.S., Medovar, B.I. et al. (1983) Production of billets of gas-motor-compressor crankshafts by electroslag casting method. In: Electroslag technology. Kiev: Naukova Dumka.

[9.] Medovar, B.I., Medovar, L.B., Saenko, V.Ya. (1999) Development of electroslag process in special electrometallurgy. Avtomatich. Svarka, 9, 7-12.

[10.] Alikin, A.P., Bojko, G.A. (1983) Electroslag casting in chemical machine-building. In: Electroslag technology. Kiev: Naukova Dumka.

[11.] Yuzhanin, Zh.I. (1976) Combined (ESR + rolling) tool holder billets of heavy turning mills. Problemy Spets. Elektrometallurgii, 5, 45-46.

[12.] Mironov, Yu.M. (2005) Electroslag furnaces for melting and casting: Manual. Cheboksary: ChGU.

[13.] Ksyondzyk, G.V. (1966) Circular electroslag cladding of cylindrical parts in vertical position. Avtomatich. Svarka, 5, 63-67.

[14.] Ksyondzyk, G.V. (1973) Some principles of parent metal penetration in circular electroslag cladding. In: High-performance processes of cladding and consumables. Kommunarsk.

[15.] Kuskov, Yu.M. (1999) Cladding in current-carrying mould: the prospective direction of electroslag technology development. Avtomatich. Svarka, 9, 76-80.

[16.] Kuskov, Yu.M. (2005) Electroslag process and technology of cladding by discrete materials in current-carrying mould: Syn. of Thesis for Dr. of Techn. Sci. Degree. Kiev: PWI.

[17.] Paton, B.E., Tupitsyn, L.V., Sobolev, Yu.V. et al. (1977) New advanced technological process for production of branch pipes on equipment bodies. Energomashinostroenie, 1, 27-29.

[18.] Fojta, A., Rozkoshny, K. (1988) Experience of electroslag melting of pressurizer branch pipes. Problemy Spets. Elektrometallurgii, 1, 37-43.

[19.] Kriger, Yu.N., Nechaev, E.A., Karpov, O.S. (1985) Elect roslag melting in power engineering. Ibid., 3, 24-28.

[20.] Tsygurov, L.G., Kovalev, V.G., Bojko, G.A. (1991) Technological diagrams of ESC of part billets of hub type. Ibid., 1, 11-15.

[21.] (1980) Electroslag welding and cladding. Ed, by B.E. Paton. Moscow: Mashinostroenie.

[22.] Goncharov, I.T., Egorov, S.P. (1984) Study of fatigue strength of cranks of ship diesel crankshafts made by electroslag casting method. In: Special electrometallurgy, Issue 21, 39-41.

[23.] Popov, L.V., Antsiferov, S.S., Bojko, G.A. et al. (1983) Experience of application of electroslag casting technology at the Production Association <<Bryansk Machine-Building Plant>>. In: Coll. on Electroslag Technology. Kiev: Naukova Dumka.

[24.] Ksyondzyk, G.V. (1984) Transition zone in electroslag shot cladding of chromium cast iron on steel. In: Coll. on New Surfacing Processes, Properties of Deposited Metal and Transition Zone. Kiev: PWI.

M.L. ZHADKEVICH, V.L. SHEVTSOV and L.G. PUZRIN

E.O. Paton Electric Welding Institute, NASU, Kiev, Ukraine
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Title Annotation:ELECTROSLAG TECHNOLOGY
Author:Zhadkevich, M.L.; Shevtsov, V.L.; Puzrin, L.G.
Publication:Advances in Electrometallurgy
Article Type:Report
Date:Jul 1, 2007
Words:3543
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