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Alloying titanium with oxygen from the gas phase in chamber electroslag remelting of titanium sponge.

In the group of promising structural materials, adopted in recent years by industry, a special position is occupied by titanium and its alloys (1), (2). The continuous expansion of the area of application of these materials in different branches of technology is explained by the favourable combination of the physical-chemical properties.

In addition to the increase of the volume of production of titanium, the quality of the material has also been improved. The decrease of the content of harmful impurities increases the plasticity and toughness of the material and reduces sensitivity to notches and other defects. On the other hand, the improvement of the purity of titanium reduces the level of the strength characteristics of the material and, consequently, complicates the production of commercial alloys.

The main impurities in commercial purity titanium, having a strong effect on the properties of titanium, included gases (oxygen, nitrogen and hydrogen). It is necessary to mention oxygen in which, in contrast to nitrogen and hydrogen, has not only a positive but also a negative effect on the properties of titanium (3-5). Having the small atomic radius, oxygen forms wide regions of interstitial solutions with [alpha]-and E[beta]-modifications of titanium.

The region of the -solutions includes the concentration from 0 to 34 at.% of oxygen without any phase transformations and disruption of the homogeneity of the structure of these solutions. The limiting solubility of oxygen in -titanium at the peritectic temperature of 1740[degrees]C is only 6 at.%. The different solubility of oxygen in the two titanium modifications is explained by the fact that the implantation of the oxygen atoms into the cavities of the BCC lattices of [beta]-titanium results in a considerably larger number of disruptions in comparison with the introduction of the same atoms into the octahedral cavities of the HCP lattice of [beta]-Ti.

Oxygen increases the temperature of the [alpha][right arrow] [beta] transformation and widens the temperature range of the [alpha]-phase, i.e., it is a [alpha]-stabiliser. Within the limits of the concentration range of the solid solutions, oxygen and titanium form compounds with the ordered structure, the so-called suboxides.

The formation of a solid solution of oxygen in Ti is associated with the extensive disruption of the crystal lattice reducing greatly the mechanical properties of the metal. The increase of the oxygen content increases the strength and hardness and reduces the plasticity of titanium. Controlling the oxygen content of the metal, it is possible to obtain the optimum ratio of the plasticity and strength characteristics of the titanium alloys.

Thus, oxygen may be regarded as a promising alloying element to produce new titanium alloys. This is especially important for medical components because in addition to the mechanical properties, corrosion resistance and biocompatibility are also important in this case. In contrast to other alloying components, for example, vanadium, oxygen is 'safer' (6).

The sources of oxygen as the alloying element may include titanium scrap (7-9), tailings from the cover of the systems for producing titanium sponge (10), (11), titanium oxides (12), (13), and also gaseous oxygen [14].

The application of gaseous oxygen for alloying titanium is economically most efficient. In the group of the metallurgical processes of melting titanium, the most suitable method for using this source of oxygen is chamber electroslag remelting (CESR) because alloying of titanium with oxygen from the gas phase during vacuum-arc and electron-beam remelting is very difficult as a result of the presence of vacuum in the melting space. CESR in contrast to canonic ESR can be used to produce almost any atmosphere in the melting space and efficiently refine and additionally alloy the metal (15-19).

In this article, special attention is given to investigating the possibilities of alloying titanium with oxygen directly from the gas phase during chamber electroslag remelting of titanium sponge with different initial oxygen content.

Consumable electrodes for CESR were produced by pressing titanium sponge produced at the ZTMK company. Both the standard sponge of grade TG100 with the oxygen con-tent of 0.0 35% (melts 3-6) and the sponge alloyed in advance with oxygen (14) to 0.11% (melts 1, 2, 7 and 8, Table 1) were used. The pressed electrodes with the diameter of 40 mm and 600 mm long were remelted in a solidification mould with the diameter of 60 mm (Fig. 1). Remelting was carried out in a chamber electroslag furnace, constructed on the basis of A-558 equipment (Fig. 2).

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]
Table 1. Oxygen content in the experimental ingots of various melts

Melt Electrode Slag Furnace Mass
No. atmosphere fraction of
 oxygen %

1 Titanium [CaF.sub.2]+Ca Argon 0.110/0.083
 sponge, (2.5 %) ('still')
 alloyed
 with
 oxygen

2 As above [CaF.sub.2] As above 0.110/0.110

3 TG110 [CaF.sub.2] As above 0.035/0.053
 titanium
 sponge

4 As above [CaF.sub.2] Argon 0.035/0.075
 ('flowing')

5 As above [CaF.sub.2] Argon + 30% oxygen 0.035/0.110
 mixture,
 minimum
 consumption,
 'flowing'

6 As above [CaF.sub.2] Argon + 30% oxygen 0.035/0.230
 mixture,
 maximum
 consumption,
 'flowing'

7 Titanium [CaF.sub.2] Argon + 30% oxygen 0.110/0.220
 sponge, mixture,
 alloyed minimum
 with consumption,
 oxygen 'flowing'

8 As above [CaF.sub.2] Argon + 30% oxygen 0.110/0.270
 mixture,
 maximum
 consumption,
 'flowing'

Comment. The numerator gives the initial oxygen content, the
denominator the content after remelting.


Equipment was also fitted with cylinders with an argon-oxygen mixture and also devices for controlling the flowrate and pressure of gases (Fig. 3).

[FIGURE 3 OMITTED]

During melting, the excess pressure of gases (up to 25 kPa) was maintained in the system to compensate possible losses of gases. The source of gaseous oxygen was the first grade argon, containing 0.002% oxygen (GOST 10157-79), and especially prepared argon-oxygen mixture (30% oxygen).

Remelting was carried out under a flux produced from pure [CaF.sub.2], grade Ch (TU 6-09-5335-88) and under a [CaF.sub.2]+ Ca flux. The flux was melted directly in the solidification mould by the solid start method. The starting mixture was produced from titanium shavings and working flux.

The electrical parameters of remelting were maintained at: U = 40 V, I = 2.0-2.2 kA, ensuring high quality of the formation of the surface of the melted ingots. The argon-oxygen mixture was supplied through pipes in the sealing gasket of the upper flange of the water-cooled solidification mould (Fig. 1, 3).

Transverse templates were produced from the ingots (Fig. 4) and used for the preparation of specimens for the determination of the chemical composition and investigation of the structure of the metal in the cast conditioned. The structure was studied at a magnifications 50-500 on microscopes of Carl Zeiss, Axiovert 40 MAT, Neophot-21, a Neophot-2. The specimens were photographed with a digital camera and the digitised file was analysed using VideoTest Metall 1.0 computer program. Hardness was measured in a Rockwell device on the HRC scale and the results were transferred to the HB values, using tables. The gas content of the metal and the specimens was determined in LECO equipment.

[FIGURE 4 OMITTED]

The required amount of the argon and the argon-oxygen mixture blown into the working space for alloying titanium with oxygen was determined by the calculations. It was assumed that the mass in the melting rate is 6 g/s, and the pickup of oxygen is 100%. Consequently, to increase the oxygen content of the metal by 0.1% it is necessary to add 0.011 l/s of the argon-oxygen mixture (30% oxygen) and 15 l/s of commercial argon.

Taking into account the technical possibilities, remelting was carried out at the minimum (0.0 31 l/s, melts 5, 7) and maximum (0.32 l/s, melts 6 and 8) consumption of the argon-oxygen mixture. The comparison melts 2 and 3 were produced in the atmosphere of commercial purity argon and in the 'still' atmosphere (Table 1).

As indicated by the table, at all investigated variants of the CESR method, with the exception of the melt produced under [CaF.sub.2]+ Ca flux (melt 1), the results show a large increase of the oxygen content of titanium and also in the remelting the sponge in commercial purity argon with a low fraction of oxygen (melts 3, 4).

Evidently, the latter is associated with the capacity of the titanium sponge characterised by the developed (from 0.1 to 20 [m.sup.2]/g) specific surface with the magnesium chloride salt remaining on the surface after magnesium thermal reduction, to adsorb oxygen, nitrogen and atmospheric moisture even prior to remelting. For example, the surface of the sponge equal to 0.1[m.sup.2]/g contains no less than 0.005% oxygen, and 1 [m.sup.2] of the surface of titanium adsorbs up to 0.03 g of water vapour from the air (9). In addition, the moisture, oxygen and nitrogen are also brought in by commercial purity argon. All these increases the content of not only oxygen but also nitrogen in the metal after remelting, including after CESR under the [CaF.sub.2] flux.

The 'flowing' atmosphere of technical purity argon, in comparison with the 'still' atmosphere, increases the oxygen content of titanium produced by CESR by a factor of 1.5 (melts 3 and 4). The application for alloying of the argon-oxygen mixture with 30% of oxygen increases the oxygen content 2-7 times (melts 5-8).

Attention should be given to the fact that the degree of pickup of oxygen does not increase with the increase of the gas flowrate. In fact, the degree of pickup decreases. For example, at the minimum flowrate (0.031 l/s), the degree of pickup is 27 (melt 5) and 39 (melt 7) and at the maximum flowrate (0.3 l/s) it is 6 (melt 8) and 7% (melt 6). Evidently, this is associated with the fact that the supply of oxygen to the oxidised metal is not the limiting member in the process of oxidation of the titanium electrode.

It should be mentioned that in alloying titanium with gaseous oxygen the nitrogen content of the metal was higher (up to 0.020-0.030%). However, the content was within the range of the requirements of GOST 19807-91 for titanium grades VT1-00 and VT1-0 (up to 0.04%of nitrogen) and ASTM B-337 for commercial purity titanium Grade 1-Grade 3 (0.03-0.05%).

The indirect indicator of the content and distribution of the impurity in titanium is the hardness of the metal. Figure 5 shows the hardness in the horizontal cross-section of the experimental ingots. As indicated by the graph, the hardness of the metal of the investigated specimens increases with increasing oxygen content. For example, the highest hardness is characteristic of the specimens with the oxygen content of 0.27 % (melt 8), and the lowest hardness for the specimens with the mass fraction of oxygen of 0.0 53% (melt 3). The hardness in the radial direction remains approximately constant, indicating the uniform distribution of the impurities in the horizontal cross-section of the ingots.

The microstructure of the specimens taken from the centre of the ingots is shown in Fig. 6. It may be seen that the oxygen content of titanium has a strong effect on the formation of the structure of the metal. For example, for titanium with the oxygen content in the range 0.0 53-0.110% (melts 1-5) the characteristic feature is the formation of the coarse dendritic structure in which the differences between the individual areas are visible and a small magnification (Fig. 6a). The dendritic areas are etched uniformly without a distinctive substructure (Fig. 6b). In some cases, the areas inside dendrites showed a plate-shaped substructure with low intensity (Fig. 6c), typical of commercial purity titanium in the cast condition.

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]

The increase of the oxygen content up to 0.2% and higher (Fig. 6d, e) results in the formation of the structure of shear trans-formation, supporting the increase of the strength characteristics. The formation of these structures in cast titanium can be explained by the larger mass fraction of oxygen which affects the kinetics of the phase polymorphous transformation in the metal during cooling of the ingot.

Conclusions

1. The chamber electroslag remelting methods, used for metallurgical processing, guarantees the supply of oxygen from the gas phase into titanium during remelting of the sponge.

2. The experiments have shown that it is possible to increase the oxygen content in the metal by a factor of 2-7 in comparison with the initial content.

3. The results of investigations of the structure and hardness measurements show that the process ensures high chemical and structural homogeneity of the titanium ingots.

References

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(14.) OvchinnikovA.V., et al., Ti2--7 v SNG, Yalta, 15-18.4.2007, Kiev, 2007, 170-1673.

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S.N. Ratiev, O.A. Ryabtseva, A.A. Troyanskii, A.D. Ryabtsev, S.I. Davydov and L.Ya. Shvartsman

Donetsk National Technical University, Donetsk ZTMK Company, Zaporozh'e
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Author:Ratiev, S.N.; Ryabtseva, O.A.; Troyanskii, A.A.; Ryabtsev, A.D.; Davydov, S.I.; Shvartsman, L.Ya.
Publication:Advances in Electrometallurgy
Geographic Code:4EXUR
Date:Apr 1, 2010
Words:2436
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