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Selecting fluxes for arc slag remelting in production of titanium billets.

The method of arc slag remelting (ASR), developed at the E.O. Paton Electric Welding Institute, Kiev in the 1970s [1], combines the treatment of molten metal with the electric arc, burning in a controlled gas atmosphere and in a liquid synthetic slag through each of the current passes in the process of remelting of the consumable electrode. The layer of the synthetic slags, covering the metallic pool, results in the de-concentration of heat of the cross-section of the pool and reduces the depth of the metal pool making it flatter in comparison with vacuum-arc (VAR) and electron beam remelting (EBR). In addition, as a result of the formation of the slag skull, the billets produced by ASR in contrast to VAR are characterised by a smooth side surface and no machining is required prior to subsequent processing.

[FIGURE 1 OMITTED]

To apply the method of ASR to the production of titanium billets, it was necessary to ensure isolation of the arc from contact with air and produce a controlled gas atmosphere in the arcing zone. It was also necessary to reduce the cost of titanium and its alloys and made them fully capable of competition with stainless nickel-containing steels and alloys [2, 3].

Two methods were tested (Fig. 1): ASR using a flux gate--the simplest device placed directly on the upper end of the solidification mould; ASR in a chamber furnace.

In the latter case, experiments were carried out with the existing furnaces for vacuum-arc remelting with the vacuum system switched off and also in specially designed chamber furnaces for ASR in which the process can be realised in a controlled gas atmosphere.

In ASR, the electric arc burns in a shielding gas or slag vapours. The presence of the slag vapours, containing chemical elements with low ionisation potentials, results in the stabilisation and stable burning of the arc. In contrast to VAR, in ASR the metallic corona does not form on the billet. This is explained by the fact that in ASR the metal splashes (small droplets) fall either into the slag pool or the edge of the slag, distributed around the perimeter of the slag pool.

The depth of the metal pool in melting the billets by ASR is smaller in comparison with EBR and the pool is flatter. This is explained by the fact that the surface of the pool is heated more uniformly. The dissipation of heat through the pool is also supported by the transfer of droplets into the pool by several flows throughout the entire cross-section of the electrode.

In ASR the surface of the liquid slag pool is one of the electrodes. The slag pool is an electrolyte melt with a large number of complex and simple ions. The temperature of the slag pool may reach 2000[degrees]C. Some components of the slag evaporate at these temperatures, others breakup (dissociate) into the component ions and in this form may take part in the current transfer process. However, since the temperature of the arc discharge is considerably higher than the temperature of the slag pool, the intensity of the evaporation, dissociation and ionisation processes is greatly higher in this case.

It should be mentioned that the components included in the composition of ASR slags exert different effects on the arcing conditions. The positive effect is exerted by alkali and alkali-earth metals with a low ionisation potential [4-7]. They are easily ionised, form cations and free electrons which take part subsequently in charge transfer.

The total ionisation potential of a mixture of different gases and vapours, present in ASR, is determined by the component with the lowest ionisation potential and depends on the concentration of this component.

For example, the ionisation potentials of potassium, sodium, calcium and argon are equal to 4.33, 5.11, 6.10 and 15.7 V. Thus, the chemical composition of the fluxes has a strong effect on the electrical characteristics of the process (stability of arcing, the height of ignition peaks, the presence of breaks in the current curve, open circuit voltage of the power source, etc).

The presence of easily ionised elements in the slag results in a large decrease of the voltage gradient in the arc column, increase of the strength of the effect of elements with low ionisation potential on the effective ionisation potential of the gas mixture. The latter tends to the potential of the most easily ionised element in the arc atmosphere.

In most cases, ASR of titanium is carried out using ESR (electroslag remelting) fluxes, containing Ca[F.sub.2] and a number of halides of alkali and rare-earth materials (Table 1). The role of the fluxes in ASR slightly differs from the conventional role of the refining component in ESR and ASR of steels and iron-based alloys. The role of the slag as the heat carrier is almost completely balanced.

The main zone of energy generation is the arc. The investigations, including those carried out in the plant conditions [4], show that in comparison with ESR, ASR used in melting of billets of steels and alloys reduces the consumption of energy by a factor of 1.5 and also halves the requirement on the synthetic flux per 1 t of metal.

The main functions of the slag in ASR of titanium, in addition to stabilising the arc, include the formation of the surface, formation of a skull layer, protection of the metal against the surrounding atmosphere, formation of a protective condensate coating on the consumable electrode. If the flux composition is correctly selected, slag regeneration may take place.

Electroslag metal with a homogeneous chemical composition is produced using fluxes not containing the oxides of easily reduced elements of chromium, manganese, silicon, etc. Special attention has been paid to fluorides used as components of the flux. However, some of these compounds (fluorides of lithium, potassium, sodium) are characterised by high conductivity and susceptibility to the formation of complex compounds with titanium, chromium, zirconium, etc.

As a result of the low melting and boiling point of the chlorides of alkali-earth metals which is lower than that of the identical fluorides, the majority of them cannot be used as the base of the flux in ASR. In addition, some of these compounds (for example, potassium chloride) are susceptible to hydration [8]. The properties of the compounds, included in the fluxes, are shown in Table 1.

The highest chemical activity in relation to the molten metal and the simplest structure are typical of salt slags. They do not contain any oxides contaminating the metal with nonmetallic inclusions and oxidising the metal.

In the middle of the 1950s, B.I. Medovar and S.M. Gurevich proposed for the first time in the world and used halide-type fluxes for welding of high-alloyed steels, with the fluxes containing no oxidation compounds [3]. The boiling point of the flux should be sufficiently high (not lower than 2000[degrees]C) to ensure that the slags can be superheated above the melting point of titanium. The basic component of these fluxes is Ca[F.sub.2].

One of the first welding fluxes based on Ca[F.SUB.2] was ANF-1, i.e., fluorspar refined to the required grain size and baked at a high temperature. Similar fluxes appeared abroad and were used only after a delay of a number of years. Almost immediately the fluoride fluxes were also used in ESR of steels alloyed with easily oxidised elements. The experimental investigations, carried out at the E.O. Paton Electric Welding Institute, show that the flux for ESR of titanium can consist of special purity refractory fluorides of alkali-earth metals: calcium fluoride, strontium fluoride and barium fluoride [5], and also the fluorides of rare-earth metals (FREM) based on La[F.sub.4]. The fluorides of alkali-earth metals are characterised by high melting [micro]more than 1200[degrees]C) and boiling (higher than 2200[degrees]C) points. The fluxes based on Ca[F.SUB.2] were tested experimental conditions for ASR of titanium: Ca[F.sub.2]-FREM; Ca[F.sub.2]-Ba[F.sub.2]Ca[Cl.sub.2]; Ca[F.sub.2]-Ba[F.sub.2]Ca[Cl.sub.2]-Sr[F.sub.2]. They are also used efficiently for the production of titanium billets by ASR.

The new possibilities of producing defect-free titanium billets by ASR are offered by a new method developed on the basis of the application of a current-conducting solidification mould combined with active slags, containing metallic calcium. Consequently, hard high-nitrogen inclusions, if they are present in the consumable electrodes, can be dissolved to a greater extent during remelting.

However, according to [5], this method can be applied only in a chamber furnace with a controlled atmosphere and using new flux compositions because the method is associated with the problems with withdrawal of the billets and, in addition to this, there are new requirements on the fluxes used. New data on the application of chamber furnaces in the production of titanium billets were published in [6, 7].

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

When using FREM as flux components, it is necessary to calculate accurately the density of the produced flux compositions, otherwise the density of the slag is higher than the density of the metal. The nomogram for the application of FREM and calcium fluoride is shown in Fig. 2. The nomogram was spotted using the experimental results obtained and the E.O. Paton Electric Welding Institute, Kiev with the application of FREM in the production of ASR titanium billets.

ASI was used for producing a pilot-plant batch of billets of VT1-0 titanium using new flux compositions. ASR was carried out in a R-951 furnace using stationary solidification moulds with the square cross-section of 200 x 200 mm with a flux gate. The flux was poured in through a siphon. The melting zone was shielded with a flux gate and by argon blowing (Fig. 3). After removing the slag skull, the billets were characterised by a satisfactory surface. The external appearance of the produced ASI billets is shown in Fig. 4. The results of chemical analysis of the produced metal are presented in Table 2.

[FIGURE 5 OMITTED]

The hardness HB of the ASR metal, melted using the flux with the FREM addition, is higher than that produced using pure Ca[F.sub.2], evidently as a result of the alloying of titanium with rare-earth metals leading to the production of these elements from the slag during ASR.

Figure 5 shows fragments of the longitudinal macrosections of the upper part of the ASR billets, melted in accordance with the technological conditions of the melts No. 1 and 2, and also the external appearance of the end of a consumable electrode after completing melting. Melting of the electrode is characterised by the distinctive uniform droplet formation of the metal throughout the entire cross-section of the electrode (Fig. 6).

[FIGURE 6 OMITTED]

The experimental results show that the production of the titanium billets by the ASI method may be realised in the existing ESR furnaces using conventional ESR solidification moulds, fitted with a flux gate. The application of fluxes with the additions of FREM (up to 30%) in ASR titanium results in the high quality of the billets surface and a high density macrostructure with no defects.

References

[1.] Paton B.E., et al., A method of remelting consumable electrode, Author's Cert. No. 520784, SSSR, MPK S 21 c 5/56, 07.07.82.

[2.] Paton B.E., et al., Probl. Spets. Elektrometall., 1995, No. 4, 3-6.

[3.] Medovar B.I. and Gurevich S.M., Avt Svarka, 1955, 31-41.

[4.] Paton B.E., et al., Probl. Spets. Elektrometall., 2000, No. 4, 18-20.

[5.] Medovar L.B., et al., Probl. Spets. Elektrometall., 2000, No. 4, 80-29.

[6.] Troyanskii A.A. and Ryabtsev A.D., Elektrometallurgiya, 2005, No. 1, 11-17.

[7.] Troyanskii A.A. and Ryabtsev A.D., Lit. Proiz., 2007, No. 1, 11-17.

[8.] Gurevich S.M., et al., Production of ingots and cast billets from titanium alloys by electroslag remelting, in: Electroslag remelting, Proceedings of the Second national conference on electroslag remelting, Metallurgiya, Moscow, 1964, 184-188.

L.B. Medovar, V.Ya. Saenko and V.A. Ryabinin

E.O. Paton Electric Welding Institute, Kiev
Table 1. Properties of the materials used for flux
compositions in ASR titanium [8]

              Temperature,           Density, g/[cm.sup.3], at
               [degrees]C                T, [degrees]C

Component    melting    boiling      20     1000/[T.sub.m]

LiCl            606        1382      --          1.50
LiF             842        1676      1.80        1.73
MgF2            1263       2227      3.14        --
CaF2            1418       2500      3.20        2.60
SrCl            868        1950      3.20        2.70
SrF2            1486       2477      4.24        --
BaCl2           958        1560      3.14        3.11
BF2             1320       2200      4.91        4.22
NaF             995        1700      1.963       1.96
NaCl            800        1440      --          1.42 1.55
YF3             1152       2227      5.07        --
LaF3            1493       2327      5.94        --
AlF3            1040       1260      --          --
[Na.sub.3]
  Al[F.sub.6]   1000       --                    3.036 3.036
Ti              1671       3260      4.54        --

                Density, g/[cm.sup.3], at
                  T, [degrees]C

Component        1000/[T.sub.boil]

LiCl                 1.33
LiF                  --
MgF2                 2.19
CaF2                 2.40
SrCl                 2.65
SrF2                 3.28
BaCl2                --
BF2                  3.78
NaF                  --
NaCl                 --
YF3                  --
LaF3                 4.43
AlF3                 --
[Na.sub.3]
  Al[F.sub.6]        --
Ti                   4.08

Table 2. Results of ASR of VT1-0 titanium in a square
solidification mould 200x200 mm with a flux gate

Melt    Electrical
No.     regime of melt       Flux                 [O], %

        I, kA    U, V                    Bottom   Middle   Top

  1      3.0      98     Ca[F.sub.2]     0.097     0.08    0.12
  2      2.5     100     Ca[F.sub.2] +   0.099     0.10    0.11
                          FREM (30%)

Melt              [N], %                 Hardness, HB
No.
        Bottom   Middle    Top      Bottom   Middle   Top

  1     0.021    0.024     0.025      133     135     139
  2     0.030    0.030     0.035      151     152     157

Comment. According to the requirements of GOST 19807-77, the
oxygen content of VT1-0 is [less than or equal to] 0.12%
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Article Details
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Title Annotation:ELECTROSLAG TECHNOLOGY
Author:Medovar, L.B.; Saenko, V.Ya.; Ryabinin, V.A.
Publication:Advances in Electrometallurgy
Article Type:Report
Geographic Code:4EXUR
Date:Jan 1, 2010
Words:2331
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