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Intensification of molten metal flows in the liquid pool in induction heating.

In surfacing of parts of worn components, and also surfacing the surface of the blanks for projecting elements, local remelting of the surface layer of metallic components, the optimum method of the removal of defects or local alloying of the surface layer is, in addition to the electrometallurgical surfacing technologies, the technology based on induction heating [1-8] offering new possibilities in the production and repair of components.

The extensive application of induction heating for these applications has been delayed by the high consumption of electric energy. Therefore, it was important to develop a method with the reduced electric energy consumption. The studies in this direction are also important because of the fact that induction heating is the ecologically efficient method.

One of the methods of reducing the energy consumption in induction heating is the intensification of molten metal flows in the liquid pool because this is accompanied by the increase of the intensity of heat exchange between the already molten metal and the metal to be melted. This shortens the melting time and the applied power. The development of the method of intensification of molten metal flows in the liquid pool is also the subject of the present work.

The E.O. Paton Electric Welding Institute, Kiev has developed a method which creates suitable conditions for intensification of the macroflows of molten metal in the central part of the liquid pool directed to the process the surface of the metallic component in induction heating. This is achieved by optimising the ratio of the dimensions of the induction coil and the body of the heater, lining the induction heater with a refractory material, selection of the shape of the crucible, covering the surface of the metal with the refractory cover and, most importantly, special position of the induction coil in relation to the heated component.

To fulfil this task, investigations were carried out to develop a special method of calculating the induction coil--heated component system which would make it possible to simulate electromagnetic processes taking place in the system. The method is based on the stationary movement of the liquid along the closed trajectories situated inside the geometrically single-connected region, under the effect of the vortex field of the volume forces (9).

The density of electromagnetic forces, acting on the electrically conducting liquid, contains the vortex and potential components. The ratio between these components in the individual points of the volume of the liquid differs. Only the vortex component of the electromagnetic forces excites the movement of the liquid.

By analogy with certain types of induction melting furnaces ('cold crucibles'), the proposed method uses the principle of transfer of the energy of the alternating electromagnetic field through the metallic wall consisting of the individual parts (sections) electrically insulated from each other (6).

In conventional induction furnaces, the induction coil is placed in the deposition in which it completely or partially affects the pool of molten metal, and the end part of the coil do not project outside the limits of the pool (Fig. 1).

[FIGURE 1 OMITTED]

Figure 1 shows part of the molten metal pool and part of the induction coil, distributed in relation to the vertical axis of symmetry. Two zones of the vortex electromagnetic forces form in the investigated part of the pool. The zones formed closed macro flows of molten metal (upper and lower), moving in the direction of these forces and having the opposite directions [5--7].

The intensity of movement of the macro-flows of molten metal depends on the hydraulic resistance and the shape of the crucible. In the absence of mechanical obstacles, the region of the metal, affected by movement, may extend far outside the limits of the zone of vortex forces, causing movement of the flows. For this position of the induction coil on the vertical axis of symmetry the macroflows in the lower part of the pool are directed downwards, and those in the upper part of the pool are directed upwards with the formation of a convex section on the surface of molten metal.

In the case of different intensity of the two vortex zones of the force field, the boundary of the vortex speed is displaced into the side of the zone with the less intensive circulation of molten metal. In the presence of the asymmetric hydraulic boundary conditions, the speed field is also deformed, underlying separating the vortices is displaced in the direction of the vortex with more constricted movement conditions. The volume of the macroflow of molten metal, circulating in the zone with the lower intensity of circulation of the melt or situated in the constricted conditions, decreases, and the volume of the macroflow in the second zone increases (in comparison with the symmetric case).

The asymmetry of the force field may be caused also by the nonuniformity of the gap between the induction coil and the liquid metal pool, the nature of distribution of the density of the macroflow in the induction coil, the displacement of the crucible in relation to the induction coil, and the shape of the crucible. For example, when using a conical crucible (in a cylindrical induction coil), the maximum magnetic induction is displaced into the region with a smaller gap (a wide part of the crucible). This increases the relative value of the intensity of circulation of the melt and the length of the vortex zone of the force field, acting in the narrow part of the system. On the other hand, the hydraulic conditions are more favourable in the wide part of the crucible. This is confirmed by the experimental data, presented in (3), which indicates that the resultant effect of the conicity of the crucible on the movement of the melt is not strong.

The molten metal flows may be used as the liquid metal working tool for the treatment of metallic components. In this case, the crucible with the orifice in the lower part is placed on the treated component. The macroflows of molten metal in the crucible treat the component, depositing it with metal, melting the metal or welding to it the metal located in the crucible.

To ensure more efficient application of the macroflows of liquid metal formed in the crucible for the treatment of metallic components, it is necessary to intensify the macroflows of molten metal of the liquid pool in the direction of the vertical axis of symmetry of the crucible towards the treated component.

The most intensified movement of the molten metal macroflows is observed in the case in which only one vortex zone of the force field exists in the volume of the melt of one part of the crucible in relation to the vertical axis of symmetry. Consequently, only one macroflow forms. The entire energy of the magnetic field of the induction coil forms a single powerful macroflow, instead of two macroflows, formed in the conventional furnaces.

This movement of the macroflows can be generated in melting on a bottom skull or in the liquid pool during the process of solidification of the ingot in induction furnaces with the cold crucible or in the electromagnetic solidification mould.

The most efficient technological parameters and efficiency update with the induction coil in the position in which the asymmetry of the induction coil--crucible electromagnetic system in the hydrodynamic boundary conditions ensures the suppression of one vortex circuit by another.

This method was developed at the E.O. Paton Electric Welding Institute, Kiev. It is based on the mutual displacements of the induction coil in which both the lower and upper circuits can be suppressed. To intensify the suppression of the upper circuit, the bottom of the crucible can be rounded.

To suppress the upper macroflows of molten metal, the induction coil must be displaced upwards to the distance in which the horizontal axis of symmetry of the induction coil coincides with the upper edge of the molten pool metal (Fig. 2). In this case, the molten metal macroflows in the central part of the pool in the crucible are directed downwards, to the bottom of the crucible. A concave region (not a meniscus-shaped convex region) forms on the surface of molten metal. The convex sections are situated in the vicinity of the crucible walls.

[FIGURE 2 OMITTED]

This effect was detected for the first time in the experiments with the displacement of the induction coil in relation to the crucible in the direction of height. To investigate this effect, theory was proposed at a calculation method developed which makes it possible to simulate the distribution of the electro-magnetic field of the induction coil on the surface of the melt using the impedance boundary conditions (9).

The experimental results show that the process of the component using the liquid metal flows, directed downwards along the vertical axis of symmetry of the crucible, the most efficient crucible has the form of a cone or a paraboloid of rotation, with the open narrow lower part resting of the component to be treated (Fig. 2).

The investigations of the model of the vortex movement of the liquid metal carried out in [6] yielded important results showing that the tangential electromagnetic forces are of secondary importance for the circulation of the metal in the conventional classic furnaces (less than 3% contribution to the circulation of the melt). The main contribution is pro-vided by the normal forces, determined by the electromagnetic pressure of the field of the induction coil on the metal.

The strength of the radial component of the magnetic field of the induction coil in the classic deposition is equal to 0 in the plane of symmetry of the induction coil along its axis and remains the same on the axis of the induction coil, irrespective of the distance between the observation point and the centre of the induction coil (7).

In any other plane, parallel to the plane of symmetry of the induction coil, the strength of the radial component of the magnetic field increases from 0 to the maximum value in the vicinity of the turns of the induction coil and subsequently decreases with increasing radius of the coil. Therefore, the electromagnetic pressure, determined by the strength of the radial component of the magnetic field, increases from zero at the axis of the crucible with the melt to the maximum at the edge of the crucible, exciting the moment of the melt along the crucible wall downwards, and along the axis of the crucible upwards.

This direction of movement of the melt, used as the working tool for the treatment of the components situated in the lower part of the crucible, is detrimental. To suppress this movement to the maximum extent, it is necessary to combine the surface of the melt with the plane of symmetry of the induction coil along its height.

In the vicinity of this plane, the electromagnetic pressure on the melt is exerted also by the forces are directed from the side surface to the axes of the crucible and associated with the axial component of the strength of the magnetic field. Since the axial component of the strength of the magnetic field and the strength of the electrical field have the highest values, the electromagnetic forces will be of the same magnitude. In particular, these forces caused the formation of the axial flows of the metal in the crucible directed in the upper part of the crucible upwards with the formation of a meniscus, and in the lower part of downwards with the formation of the metal jet essential for the treatment of the component.

It is evident that to suppress the upper vortex in the crucible it is necessary (in addition to the proposed method) to place the crucible in the induction coil in deposition in which the upper surface of the melt is on the level of the plane of symmetry of the induction coil in the direction of its height. It is convenient to restrict mechanically the possibility of movement of the melt in the upper circuit, for example, by placing a restricting wall in the plane of symmetry of the induction coil on the surface of the melt. The wall has the form of a cover preventing lifting of the metal which forms and avoid-ably as a result of compression of the melt by the forces determined by the effect of the axial component of the strength of the magnetic field.

In addition, to reduce the relative value of the radial component of the magnetic field in the zone of the crucible, it is convenient to use the induction coil with the axial length not smaller than the diameter of the crucible.

To test the specimens of induction heaters, experiments were carried out with a device which included two frequency converters PVV with a power of up to 100 kW, frequency 8 kHz, a TVD 3000 transformer, used for quenching components in the condensers ESVP 800/1000.

The condensers were connected parallel to the primary winding of the transformer (Fig. 3), and the total capacitance of the condensers was 26.6-29.2 [micro]F. The transformation coefficient was equal to 5, the maximum voltage in the induction coil was 160 V. Equipment A550 was used for moving the heater with the molten metal in relation to the induction coil.

[FIGURE 3 OMITTED]

The proposed method of calculating the induction coil was used in designing and producing a two-winding induction coil with the height of (80[+ or -]5) mm, diameter 190 mm. The conductors were produced from the copper pipes with the cross-section of 10x15 mm (Fig. 4). The windings of the induction coil were connected in parallel and were matched. This design of the induction coil has made it possible to change in a wide range of the weight of the component for melting and the position of the induction coil in relation to the molten metal. The induction coil was used in all tested heaters.

[FIGURE 4 OMITTED]

In the improved lined induction heater, the treatment of the horizontal surfaces of the metallic components was carried out by the formation of the vertical macroflows of liquid metal, directed downwards to the treated component along the vertical axis of symmetry of the induction coil. The macroflows formed after melting of the blank.

The possibilities of melting the steel component with the liquid metal macroflows and the melting rate can be estimated on the basis of the penetration data for the sheets of St3sp steel, wt.%: 0.14--0.22 C; 0.15--0.30 Si; 0.40--0.65 Mn;

The melting time of the steel sheet in relation to the thickness was as follows:
sheet thickness, mm 20, 40, 60, 80
melting time of the sheet, min 0.2-0.3; 1.5-2.0; 4.0-5.0.


The dynamic pressure of the molten metal jet, in relation to the diameter of the penetrated depression or orifice, was 100--1500 N/[m.sup.2].

To isolate the molten metal from the cooled walls of the heater and defined the dimensions of the zone of the effect of the liquid metal on the surface of the treated component, the lining was produced using a mixture of magnesite with kaolin and sand (5-10% each), mixed using water glass with water. The mixture was deposited on the melted component and dried using the induction heater. The dimensions of the lining were smaller than the dimensions of the heater and, consequently, after introducing the blank with the lining into the heater, it was possible to place dry sand in the gap between the wall of the heater and the coating ensuring the reliable thin wall lining. This lining did not fracture during long-term holding of the molten steel in it. The cracks, formed in the lining, have no effect on the technological process, and the dry sand prevented the risk of escape of molten metal, ensuring easy removal of the heater from the lining after completing heating.

Holes in the form of a funnel (Fig. 5) with the dimensions presented in Table 1 formed in the sheets below the heater. These dimensions can be recommended for melting steel components to a depth of 20, 40, 60 and 80 mm using the macroflow, formed at the frequency of the induction current of 8 kHz. However, it should be mentioned that these data can be regarded as optimum because even they resulted in arrests of the process associated with the switching of the containers, and in the experiments with a sheet 80 mm thick it was not possible to reach the maximum power.

[FIGURE 5 OMITTED]
Table 1. Main parameters of the process of melting of steel sheets

Sheet Hole Parameters of Melting
thickness, diameter, molten metal productivity,
mm mm kg / min

 Upper Lower Volume, Mass, kg
 [cm.sup.3]

20 65-56 38-28 45-50 0.35-0.39 1.00-1.95

40 73-65 25-20 80-100 0.625-0.780 0.39-0.42

60 110-93 30-22 240 1.88 0.48-0.38

80 130-125 15 600 4.68 0.335


The experimental results show that the depth of penetration of the component depends only shape and dimensions of the blank, i.e., on the shape and dimensions of the cavity in the lining.

Figure 5 shows the solidified molten metal pool, which may be used as an example of melting the given elements on the sheet.

If the size of the orifice in the lining (refractory material) is not sufficiently large, the macroflow of molten metal maybe restricted under the heater (Fig. 6a), and if the size of the orifice is as required, but the rising macroflows of liquid metal freely leave the funnel in the treated component (Fig. 6b). This increases the depth of the funnel with other conditions being equal.

[FIGURE 6 OMITTED]

The large increase of the intensity of the macroflows of liquid metal as a result of the application of the lining was utilised in penetrating for the first time the steel sheets with a thickness of 60 and 80 mm. In melting the sheet with a thickness of 60 mm, the specific consumption of electric energy was 4 kW h per kilogram of molten steel. However, it is not possible to compare this value with a specific consumption of energy when using the heaters without the lining since the large number of experiments carried out to melt the sheet with a thickness of 60 mm using the heater without the lining have not been successful.

The experimental results show that the intensity of the steel jet depends on the position of the induction coil in relation to the surface of the treated component and the blank. For example, in the case of the 80 mm thick sheet, ridges were detected around the funnel under the heater indicating the lifting in relation to the cold steel in the direction of the perimeter of the funnel.

The experiments were carried out at the same current frequency in the induction coil (8 kHz). Therefore, the effect of frequency on the formation of the molten metal macroflows was not evaluated in the experiments. It may be claimed that the decrease of the current frequency increases the pressure of the macroflows and the efficiency of the effect of the current frequency on the treated metal. This is associated with the increase of the strength of the magnetic field of the induction coil and, correspondingly, the pressure on the liquid metal at the same power of the induction coil.

The experiments with the investigation of the melting of the steel sheets show the extent of intensification of the macroflows of liquid metal in the given method of induction heating when they are capable of freely melting relatively thick steel sheets. However, this method is no design for producing holes in sheets. The application of the proposed method of positioning the induction coil in relation to the molten metal pool makes it possible to save electric energy in induction heating of components (by a factor of 1.2-1.4 in comparison with the currently available methods).

It is promising to use the proposed method of induction heating for developing technologies of depositing parts of worn components, projecting elements, local remelting of the surface layer of metallic components in order to eliminate defects or carry out local alloying of the surface layer.

Conclusions

1. Analysis of the electrodynamic processes in the liquid metal, sustained on the horizontal surface of the metallic component in the field of the induction coil, shows the following conditions of the formation in the liquid metal of intensified macroflows of molten metal, directed to the component: the upper surface of the melt should be situated on the level of the plane of symmetry of the induction coil; it is recommended to use the induction coil with the axial length close to the diameter of the coil; it is recommended to prevent the formation of the upper convex meniscus of the metal as a result of the application of the restricting cover.

2. Experiments have confirmed the possibility of formation of molten metal macroflows directed from top to bottom on the vertical axis of symmetry of the induction coil on the treated component in induction heating.

3. The improved lined heater enables the macroflows of molten metal to penetrate freely of the horizontal section of the treated component to a depth of up to 80 mm.

4. The proposed ecologically efficient method of induction heating may be used for developing the technologies of depositing parts of worn components, surfacing the projecting elements, local remelting of the surface layer of metallic components in order to remove defects or carry out local alloying of the surface layer, because the proposed method is characterised by a considerably lower energy consumption in comparison with the currently available induction heating methods.

References

(1.) Farbman S.A. and Kolobnev I.F., Induction furnaces for melting of metals and alloys, Metallurgiya, Moscow, 1968.

(2.) Tir L.L., Magnitnaya Gidrodinamika, 1973, No. 3, 144-146.

(3.) Svilo A.V. and Tir L.L., ibid, 1973, No. 3, 144-146.

(4.) Fomin N.I., Determination of the parameters of the induction coil-crucible-charge system in the induction furnaces with a cold crucible, i:Investigations inthe area of industrial heating, Tr. VNIIETO, No. 7, Energiya, Moscow, 1975, 65-71.

(5.) Tir L.L., ibid, No. 7, 1975, 72-77.

(6.) Tir L.L. and Gubchenko A.P., Induction melting furnaces for the processes with improved accuracy and cleanness, Energoatomizdat, Moscow, 1988.

(7.) Furui M., et al., ISIJ Intern., 1993, No. 3, 400-404.

(8.) Gorislavets Yu.M., Induction equipment for the electromagnetic treatment of metals and alloys, Dissertation, Kiev, 1988.

(9.) Pis'mennyi A.S., Induction heating for welding and related technologies, E.O. Paton Electric Welding Institute, Kiev, Kiev, 2005.

A.S. Pis'mennyi, V.M. Baglai, A.A. Pis'mennyi and S.V. Rymar

E.O. Paton Electric Welding Institute, Kiev
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Title Annotation:VACUUM INDUCTION MELTING
Author:Pis'mennyi, A.S.; Baglai, V.M.; Pis'mennyi, A.A.; Rymar, S.V.
Publication:Advances in Electrometallurgy
Date:Apr 1, 2010
Words:3801
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