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Production of increased strength titanium by its doping with oxygen in process of chamber electroslag remelting.

Titanium is one of the most distributed elements in the nature. Despite the fact that this metal was discovered as long as in 1791, it's application as of independent structural material and basis of alloys started just a little more than fifty years ago after successes achieved in the field of metallurgy of active metals and alloys.

Titanium alloys are used in the fields of technology, where high specific strength and resistance to impact loads are needed. At present in national and foreign machine building commercial titanium and (to a greater degree) its low-alloyed alloys are used because of higher strength characteristics.

In addition to improvement of the technology for production of the titanium sponge as an initial feedstock, quality of titanium and its alloys was improved. Due to sharp reduction of harmful impurities, in particular gases, plasticity and toughness of titanium and titanium-base alloys increases. However, high-purity titanium is not always widely used as a structural material, because at high plasticity it has low strength.

[FIGURE 1 OMITTED]

Significant strength increase is achieved by alloying of titanium using different elements, in particular, aluminium and vanadium. So, lately titanium-base alloys, having strength which exceeds 4-5 times that of the iodide titanium, are used.

At the same time, in case of application of the titanium alloys for items of the medical equipment the most important requirements are corrosion resistance and biocompatibility. At present for this purpose the VT6S-type titanium alloys are used. However, presence of vanadium (the alloying component) in the alloy can cause under certain conditions [1] formation of unsafe for a human organism chemical compounds. That's why development of the titanium alloys, alloyed by safe elements, is a rather actual task.

In this respect oxygen, as a titanium-strengthening element, is of interest. At high temperatures it easily dissolves both in [alpha]-Ti and [beta]-Ti [2], forming interstitial solutions. Maximal solubility of oxygen in titanium constitutes about 30 at.%. Oxygen effects most noticeably mechanical properties of titanium at its content in the metal up to 0.6 wt.% [3], whereby significant increase of strength characteristics at relatively insignificant worsening of plastic properties was registered.

So, by controlling content of oxygen in titanium one can effect its mechanical properties to a significant degree, whereby it is necessary to keep in mind that compounds of oxygen with titanium are harmless for a human organism and used in pharmaceutics and medicine [4].

Production of titanium, doped by oxygen, requires for development of a reliable metallurgical technology for introduction of oxygen into the metal and ensuring of its uniform distribution over height and section of the ingots and casts.

Such technology can be developed on the basis of the special electrometallurgy processes, in particular chamber electroslag remelting (CESR). The latter one in addition to the known flexibility in additional alloying allows ensuring high purity, as well as structural and chemical homogeneity of the material, due to uniform melting of the consumable electrode with the master alloy and simultaneous solidification of the ingot in the controllable atmosphere [5-8].

[FIGURE 2 OMITTED]

It is suggested to use so called titanium-oxygen master alloy as the oxygen-containing material. Sweepings of the reaction mass from the retort cover, subjected to a special seasoning in air for the purpose of their saturation with oxygen and nitrogen and then to vacuum separation for removal of magnesium and chlorine residues, are initial feedstock for its production [9]. From this material satellite-electrodes for CESR were made by the pressing method.

[FIGURE 3 OMITTED]

Remelting was performed in the chamber electroslag furnace, developed on basis of the A-550 unit (Figure 1). The unit allows degassing the working space down to the residual pressure 665-1330 Pa at low inflow of the atmospheric air and performing remelting in the inert gas at normal and excessive (up to the level (3-5) 104 Pa values of the pressure, which ensures exclusion of atmospheric air suction into the furnace melting space and preservation of the getter activity of condensate of the flux metal component.

The water-cooled chamber was installed directly on flange of the mould upper part, and before melting it was pumped off and then filled with argon. In the course of melting excessive pressure of argon (about 15 kPa) was maintained in the system for compensation of its possible losses.

The billets, pressed separately from the grade TG-110 titanium sponge and from sweepings of the reaction mass of 40 mm diameter and 300 mm length, were argon-arc welded into the consumable electrodes of 600 mm length (Figure 2).

The produced built-up electrodes were remelted into copper water-cooled mould of 115 mm diameter and 500 mm length. The following versions were considered: melt 1--electrode from 100 % TG-110 titanium sponge; melt 2--electrode from 100 % reaction mass (RM); melt 3--built-up electrode from 50 % TG-110 titanium sponge and 50 % RM; melt 4--electrode from 100 % TG-110 titanium sponge.

[FIGURE 4 OMITTED]

Powder of calcium fluoride Ca[F.sub.2] of grade Ch (TU 6-09-5335-88), calcinated at the temperature 973 K for 3 h, and metal calcium were used as the flux-forming materials. The flux was melted directly in the mould using <<solid>> start. Remelting was performed using flux from pure Ca[F.sub.2] (melts 1-3) and flux of the Ca[F.sub.2] + Ca system (melt 4). Electrical parameters were maintained at the constant level (U = 47 V, I = = 3 kA), ensuring good quality of surface formation of the ingots being melted.

The ingots were cooled in the mould for about 30 min and then they were <<undressed>>. They had smooth side surface (Figure 3). Slag cap and skull separated easily. The ingots were subjected to ultrasonic control (USC) at the frequency 5 MHz using the Krautkramer Branson instrument USN 52, then they were cut along longitudinal axis, and samples were taken for investigation of chemical composition and structure of the metal in the cast state. The structure was investigated at magnifications 50-500 on the microscopes <<Neophot-21>> and <<Neophot-2>>. The samples were photographed by digital camera, and the digitized file was analyzed using computer program Image Tool for obtaining quantitative characteristics of the structure. Hardness was measured on the Rockwell instrument according to HRC scale and then using the tables converted into HB. Gas content in metal of the samples was determined on the LECO instrument.

The results of chemical analysis and mechanical tests of the metal of experimental ingots are presented in the Table. Hardness values are presented in Figure 4.

It is established that hardness of the metal of Experimental ingots increases as content of oxygen in them gets higher. So, the highest hardness is characteristic of the specimens with 0.4 wt.% O (melt 2), and the lowest--with 0.07 wt.% O (melt 4), whereby in the radial direction (from the surface to the center) hardness in all ingots at all levels remains approximately constant, which proves uniform distribution of the impurities over horizontal section of the ingots. At the same time, the trend was registered of hardness increase from head of the ingot to its bottom. This is connected with transition of the impurities from the flux into the metal at initial stages of the ingot formation.

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]

Microstructure of metal of the sample, cut out from the middle of the ingots, is presented in Figures 5 and 6.

As one can see from the Figures, the metal of all melts has single-phase [alpha]- or [alpha]'-structure, dispersity of which depends upon weight share of oxygen. Microstructure of the metal, when content of oxygen in it constitutes 0.1 % (melt 1), is typical for the commercial titanium. Increase of weight share of oxygen up to 0.3 % (melt 3) does not cause significant change of the [alpha]-phase morphology. Increase of dispersity of the [alpha]-phase packages was registered.

At further increase of oxygen content in titanium up to 0.4 % (melt 2) the microstructure acquires a typical acicular character, which allows its classifying as [alpha]'-phase. Formation of this structure is accompanied by sharp increase of hardness. At low weight share of oxygen (0.07 %) in titanium, which can be ensured by CESR using the flux containing metal calcium (melt 4), the microstructure close to the typical one for commercial titanium is formed similar to the metal structure of melts 1 and 3.

Results of the tests (see the Table) show that changes of mechanical characteristics correlate with the structural ones. In the metal with increased content of oxygen (0.3-0.4 %) significant increase of the strength characteristic values was discovered. At the same time, in melt 3 level of ductility, close to that of the commercial titanium, is preserved, while formation of [alpha]'-phase in the metal of melt 2 causes complete loss of ductility.

So, analysis of presented microstructures of the experimental metal proves influence of oxygen content in titanium within considered range on change of their morphology and formation of phases.

CONCLUSIONS

1. It is established that oxygen may be used as economically doping element that allows significant increasing strength level of titanium due to reduction of the natural reserve of its plastic characteristics.

2. It is shown that the CESR as a metallurgical process makes it possible to introduce by means of the additional alloying necessary amount of oxygen into titanium and ensure in this way chemical homogeneity of the ingot metal.

[1.] (1978) Concise chemical reference book. Ed. by V.A. Rabinovich. Leningrad: Khimiya.

[2.] (1999) Constitution diagrams of binary metal systems: Reference Book. Ed. by N.P. Lyakishev. Vol. 3. Book 1. Moscow: Mashinostroenie.

[3.] (1979) Metallurgy and technology of welding of titanium and its alloys. Ed. by S.M. Gurevich. Kiev: Naukova Dumka.

[4.] Nikolaev, G.I. (1987) Metal of century. Moscow: Metallurguiya.

[5.] Ryabtsev, A.D., Troyansky, A.A. (2001) Production of ingots of titanium, chrome and alloys on their base in chamber furnaces using the "active" metal-containing fluxes. Problemy Spets. Elektrometallurgii, 4, 6-10.

[6.] Ryabtsev, A.D., Troyansky, A.A. (2005) Electroslag remelting of metals and alloys using the fluxes containing active additives in furnaces of chamber type. Elektrometallurguiya, 4, 25-30.

[7.] Ryabtsev, A.D., Troyansky, A.A., Korzun, E.L. et al. (2002) Metal alloying with nitrogen from gas phase in ESR process. Advances in Electrometallurgy, 4, 2 -6.

[8.] Ryabtsev, A.D., Troyansky, A.A., Pashinsky, V.V. et al. (2002) Application of electroslag technology for refining titanium and titanium alloys from nitrogen-rich inclusions. Ibid., 3, 8-11.

[9.] Davydov, S.I., Shvartsman, L.Ya., Ovchinnikov, A.V. et al. (2006) Some features of titanium alloying with oxygen. In: Proc. of Int. Sci.-Techn. Conf. on Ti-2006 in CIS Countries (Suzdal, Russia, May 21-24, 2006). Kiev: Naukova Dumka, 253-257.

A.D. RYABTSEV (1), S.I. DAVYDOV (2), A.A. TROYANSKY (1), L.Ya. SHVARTSMAN (2), O.A. RYABTSEVA (1), V.V. PASHINSKY (1) and K.L. FEOFANOV (2)

(1) National Technical University, Donetsk, Ukraine

(2) Company <<ZTMK>>>, Zaporozhie, Ukraine
Results of experimental metal investigation

Type of electrode Slag Weight share of impurities, %
 composition

 Fe Si Ni N

TG-110 titanium -- 0.056 0.003 0.032 0.004
sponge

Melt 1 (TG-110) Ca[F.sub.2] 0.090 0.002 0.032 0.015

Melt 2 (RM) Ca[F.sub.2] 0.090 0.003 0.018 0.016

Melt 3 (50 % Ca[F.sub.2] 0.080 0.004 0.019 0.011

TG-110 + 50 % RM) Ca[F.sub.2] 0.044 0.003 0.034 0.015
Melt 4 (TG-110) + Ca

Type of electrode Weight share HA [[sigma] [delta],
 of .sub.t], %
 impurities, % MPa

 N I

TG-110 titanium 0.011 0.044 105 -- --
sponge

Melt 1 (TG-110) 0.055 0.100 150-200 438 12.0

Melt 2 (RM) 0.098 0.400 300-360 708 --

Melt 3 (50 % 0.110 0.300 200-260 637 10.8

TG-110 + 50 % RM) 0.033 0.070 135-180 480 16.0
Melt 4 (TG-110)
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Title Annotation:ELECTROSLAG TECHNOLOGY
Author:Ryabtsev, A.D.; Davydov, S.I.; Troyansky, A.A.; Shvartsman, L.Ya.; Ryabtseva, O.A.; Pashinsky, V.V.;
Publication:Advances in Electrometallurgy
Article Type:Report
Date:Jul 1, 2007
Words:1980
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