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Increase of utilization factor of refractory alloy waste by electrometallurgy methods.

In technological process of manufacturing cast and deformed components of turbine engines (GTE) nickel-, cobalt- and iron-base cost intensive refractory alloys are used, which are not produced in Ukraine.

At the same time, national industry has big centers for manufacturing stationary (power engineering) and transportation (aviation, shipbuilding) gas turbines (SE <<Zorya--Mashproekt>>, Nikolaev, OJSC <<Motor--Sich>>, Company <<Progress>>, Zaporozhie, <<Turboatom>>, Kharkov) that explains actuality of solving technological tasks for efficient use of primary materials and recovery of the formed wastes. Choice of technology for conversion of cast and mechanical waste depends completely upon their clear preliminary ranking not only by grades of alloys and quantity thereof, but also by the degree of their contamination with foreign impurities (residues of forming and rod ceramic masses, harmful impurity elements, gases, etc.).

In this work authors carried out analysis of intraplant accountability documentation concerning formation and dynamics of accumulation of cast waste of various kinds of products (nozzle guide vanes and rotor blades, struts, fairings, etc.), using example of SE <<Zorya--Mashproekt>>, for the purpose of increasing efficiency of using cost intensive waste of nickeland iron-base refractory alloys and proposed improved system for classification and certification of the cast waste formed in production of components of power engineering and transportation GTE.

State of the issue, materials and technological processes. It should be noted that in recent years the main supplier of corrosion-resistant refractory alloys, used for manufacturing GTE blades and vanes with service life 5,000-10,000 h for power engineering and gas pumping power stations is Chelyabinsk Metallurgical Works; for ship and energy turbines with long service life (up to 100,000 h)--such known companies as INCO, First Rixson (both Great Britain), and Teledyne (USA). So, beginning from the year 2000, First Rixson has been supplying to the enterprise alloys CM88UVI (Russian analogue is alloy ChS88UVI), CM104VI (ChS104VI), and CM648VI (of VKh4L type) (Table 1).

An example of efficient implementation of low-waste production at foreign enterprises is a technological chain, established between the plant-manufacturer of alloys (First Rixson) and the enterprise-manufacturer of GTE (Howmet, Great Britain). So, for melting of a secondary charge billet First Rixson uses from 10 to 50 % of production wastes of the Howmet company (cast and mechanical ones), preliminary sorted by melts and cleaned of ceramics. Such sorting of the waste ensures for the secondary charge billet correspondence of chemical composition, including impurities, to the standard [1], established for refractory alloys of the type IN 738.

In the national practice it became possible to employ carefully controlled system of return of the cast production waste for manufacturing charge billets only for some especially expensive rhenium-containing alloys (of JS32 type) on <<Progress>> company (Zaporozhie).

For the rest Ukrainian enterprises of the branch it is economically inexpedient to return wastes to foreign metallurgical enterprises-manufacturers of alloys, while intraplant technology of using own cast production waste envisages till now addition to the primary billet of only 30-50 wt.% of conditioned waste in melting of items by the method of vacuuminduction remelting.

Such method of remelting can not ensure efficient refining of the melt from impurities, including refractory ceramics, alkaline metals, gases, etc. Development of both efficient refining technology for intraplant waste regeneration process, and rational system of waste classification and certification for each enterprise, according to the types of products, are necessary for increasing waste utilization factor in the mentioned system.

Analysis of terms of reference of foreign companies, established for refractory alloys [1, 2] concerning allowable quantity of impurity elements, including non-ferrous metals (lead, bismuth, tellurium, gallium, selenium, silicon, etc., totally up to 0.05 wt.%) proves the need of their strict control in the cast item. National standards, unfortunately, do not establish such strict requirements for primary billets of alloys.

According to existing standards, total amount of impurities, brought with initial charge materials, may constitute more than 1 % of the alloy mass. In addition, in the process of direct melting of items a significant source of contamination may be non-metallic inclusions, which get from the lining of a melting crucible, which represents molten magnesite (melting of the charge billet), and mullite-corundum crucibles (melting of items) [4]. In this connection of utmost importance is development and application in production cycle of new thermostable and heat-resistant ceramic materials [4].

As shows analysis, carried out at enterprises of the branch, amount of conditioned wastes, formed in melting of items of the considered assortment, constitutes on average 23 % of total mass of the consumed charge (Table 2). Off-grade wastes, to which relate pouring basins with residues of ceramic nets for filtration of the metal, and blades and vanes, rejected in LUMMA-control because of defects of internal cavities, constitute on average 28 % of the consumed charge mass, irrespective of the alloy grade. So, results of statistical analysis showed that total amount of wastes, formed in melting of GTE blades and vanes, constituted on average 60-70 % of the loaded charge mass, of which it was allowed till now to use repeatedly not more than 30-40 % because of absence of systemic record-keeping of the wastes and efficient regeneration process.

For economically substantiated solution of the problem of as full as possible recovery of conditioned and off-grade wastes of refractory alloys, rejected, as one of the reasons, because of their chemical composition, the authors propose technology of two-stage remelting of refractory alloys with inclusion into the technological chain of developed at SE <<Zorya-Mashproekt>> jointly with PTIMA of so called method of directed zone remelting [5].

Horizontal vacuum resistance furnace PMP-4M was used as a casting unit. Standard ceramic filter K657-2783ChI, installed into upper part of a specially designed multilevel corundum mould, which allowed performing filtration directly in it, was used for ensuring primary mechanical filtration of the melt from course non-metallic inclusions.

Refining of the melt from non-metallic inclusions takes place due to presence in the horizontal furnace of three thermal zones (heating, melting, and solidification). Rate of solidification under stationary conditions is regulated by speed of movement of the moulds with molten metal along the furnace from one zone into the other. According to thermodynamic characteristics of the alloys temperature of the melt is maintained at the level, not exceeding 1460 [degrees]C. Thermal gradient at the grain growth front constitutes 15-20 [degrees]C/cm that enables casting-off of impurity elements to the boundary of solidification front and their subsequent removal by machining.

In the process of the melt solidification within assigned thermal and time parameters precipitation in intergrain space of lump-like carbide phases, which are concentrators of stresses and stimulate origination of cracks, was not registered.

An important factor is correct choice of refractory materials and technology of producing crucibles, filters and casting moulds, provided effect of the melt refining from gases and non-metallic inclusions in the process of solidification and after hardening of the billet is preserved. Criterion of serviceability of refractory materials is, first of all, degree of contact interaction between the refractory and the melt.

A series of experiments was carried out for investigation of interphase interaction of such materials as quartz, distensilimanit, corundum, zircon and alumomagnesian spinel with melts of refractory alloys of the grades ChS70, ChS88U and ChS104. Experimental meltings were carried out in the commercial vacuum-induction furnace UPPF-2 according to the technological conditions, established at the enterprises-manufacturers of GTE blades and vanes for each grade of the alloys (pressure in the furnace was 1.2-2.5 Pa, temperature of pouring into the moulds-1560-1580 [degrees]C, and temperature of the mould-800 [degrees]C). Depth of change of the cast layer (so called contact zone), measured by means of the metallographic microscope, was selected as the main parameter, which characterized degree of the interaction. Structural-chemical peculiarities of the contact zone were investigated by methods of X-ray microspectral analysis, auger-spectroscopy, and optical metallography; microhardness of the cast along cross section of the specimens was also studied. The investigations showed that depth of the zone constituted from 0.5-5.0 in interaction with alumomagmnesian spinel to 150-200 mm for quartz, and diminished along the row quartz--distensilimanit--zircon--corundumalumomagmnesian spinel.

Analysis of structural changes of the contact zone showed absence in it of carbide precipitations, which proved carbon impoverishment of the interaction zone. In the course of X-ray microspectral analysis reduction of content of such elements as aluminium, titanium and chromium in the near-surface zone was registered. Probably mechanism of interaction of the alloy with the mould is stipulated by oxidation of the alloy components (carbon, aluminium and titanium by oxygen) released in dissociation of SiO2 from the mould and redistribution of these elements in the near surface zone of the casts.

On the basis of the results obtained, taking into account application as binding agents in manufacturing of GTE blades and vanes of mainly hydrolyzed ethylsilicate or silica gels (sources of faintly structured amorphous [SiO.sub.2]), we developed method for binding [SiO.sub.2] into more thermostable compounds [6].

The authors have experimentally shown that finely dispersed powder of metal aluminium may serve as efficient modifier for binding [SiO.sub.2]. This allows transforming [SiO.sub.2] in heat treatment of the moulds into alumosilicate-mullite. So, depth of changed layer of the cast, poured into corundum mould with binding ethylsilicate, containing 14-16% [SiO.sub.2], constituted 30-40 mm in contrast to the cast, poured into the modified mould (20-25 [micro]m).

Long- and short-term strength tests of the specimens showed that temperature below 800 [degrees]C and depth of the contact zone within 40 mm do not exert significant influence on values of mechanical characteristics, but fatigue strength of the specimens, poured into moulds from different materials, clearly depends upon depth of the co7ntact zone. So, utmost endurance on the basis of [2-10.sup.7] cycles at temperature 800 [degrees]C of alloy ChS70 constituted for the specimens, poured into moulds without a modifier, 400-420 MPa, while with application of a modifier--430-440 MPa. Similar trend was registered in tests of specimens from alloy ChS88U [7].

On the basis of the results of investigations, carried out for the purpose of choosing refractory materials for manufacturing filters and crucibles, it was recommended to use corundum and alumomagnesian spinel. Technological process for manufacturing shell forms from modified ceramics was developed under manufacturing conditions.

For increasing degree of cleaning of a secondary charge billet in manufacturing of the GTE special purpose parts (blades of the first and second stages) it was recommended to carry out after zone cleaning second stage of refining remelting of produced billet by developed in PTIMA, method of combined melting [8], which envisages remelting of wastes in vacuuminduction installation, erected on the basis of a serial commercial furnace UPPF-3M.

Melting of the charge and overheating of the melt is performed in a ceramic crucible by means of induction heating, and refining and thermal and time treatment--due to additional electron beam heating of the melt by the electron beam gun [9]. Technological remelting modes vary depending upon the ratio of conditioned and off-grade wastes, used in manufacturing of a secondary billet. Power of the vacuum-induction inductor was increased in melting (Figure) up to the maximum (50 kW) for the purpose of the melting time reduction. After induction of the liquidmetal pool electron beam gun was switched on and the pool was heated by the beam in addition to the vacuum-induction heating for removal of slag from the surface and intensification of the melt refining process for fuller evaporation of slag from the surface. Integral temperature of the pool after these manipulations constituted 1600-1700 [degrees]C.

Efficiency of the new method was tested in refining of conditioned wastes of refractory corrosion-resistant alloys, used for manufacturing blades of aviation, ship and stationary engines [10, 11]. It was established that combined melting ensured efficient reduction in the alloys of gases, impurity elements, and non-metallic inclusions in all investigated types of alloys and enabled refining of the structure and cleaning of the grain boundaries in comparison with the castings, produced according to the standard technology.

So, sulfur and phosphorus relate to the most harmful impurities, which form low-melting eutectics from sulfides and phosphates of certain metals, that's why their content in the alloys is strictly limited by valid standards by the concentration 0.010-0.001 %. It is important to stress that after two-stage remelting according to the presented conditions in the alloy ChS88U the trend was registered to reduction of phosphorus content (from 0.0050 in the primary charge billet to 0.0037 % in the secondary one) and content of sulfur did not exceed assigned by the standard level when ratio of conditioned and off-grade wastes in the charge was 1:1.

As far as yield of efficient metal is concerned, which was estimated by the ratio of the metal mass in the billets to the mass of the charged wastes, in refining of conditioned wastes by the VIR + EBR method it constituted 98 %, while in refining of conditioned wastes in the mixture with the off-grade ones--89.5 %. Although remelting of off-grade wastes is accompanied by significantly higher irretrievable losses of metal in comparison with the conditioned ones, application of the developed technology is efficient, because it allows to return into the production about 90 % of the wastes.

CONCLUSIONS

1. Analysis of intraplant accountability documentation concerning formation and dynamics of accumulation of cast waste in production of cast GTE parts (rotor blades and nozzle guide vanes, struts, fairings, etc.) on the basis of SE <<Zorya--Mashproekt>> was carried out. According to the obtained data on formation of conditioned and off-grade wastes (quantity, degree of contamination) norms of their ratio in remelting into secondary charge billet were drawn and recommendations of allowable content of the types of wastes according to each grade of alloys, taking into account responsibility of each part, were given.

2. It was shown that significant increase (up to 90 % of the charge mass) of the off-grade waste utilization factor was achieved by development and testing of the technology of zone remelting of the refractory alloy waste in horizontal melting unit PMP-4 with application of original design of a multilevel ceramic mould, which allowed performing primary mechanical refining of the melt due to the inserted ceramic filter. Thermal gradient at the grain growth front, which equals 15-20 [degrees]C/cm, enables casting-off of impurity elements to the boundary of solidification front and their further removal by machining of the billet.

3. The investigations showed that main reason of the melt interaction with material of the mould was [SiO.sub.2] of the binder--hydrolyzed ethylsilicate. For elimination of this phenomenon technology for binding [SiO.sub.2] (mullite) was developed, which is a thermo-chemically stable element and does not interact with the melt.

[1.] Standard AMS 2280A: Trace elements control. Nickel alloys castings. Issued 01.07.1992.

[2.] (2001) Instruction I ZhAKI. 105.015-89: Quality system. Castings of vacuum pouring heat-resistant alloys. Rules of acceptance and methods of control. Introd. at NPP Mashproekt in 1989. Nikolaev.

[3.] (2001) Instruction M ZhAKI. 105, 509-2001: Heat-resistant cast alloys for gas turbine blades. Certificate of alloy ChS88UVI. Introd. at NPP Mashproekt in 2001. Nikolaev.

[4.] Stepanov, V.M., Shaev, O.V., Trefilov, A.F. (1981) Study of possibility of application of mullite-carbocorundum crucibles for casting of gas turbine engine blades in UPPF furnaces. In: Aircraft materials. Advanced processes of casting of cooled blades, Issue 6, 16-19. Moscow: ONTI VIAM.

[5.] Dobkina, Yu.G. (2001) Special structure of multilevel shape with filtration of melt for recovery of superalloys. Protsessy Litia, 1, 68-74.

[6.] Simanovsky, V.M. (2001) Theoretical principles of producing of mould and rods on the base of modified ceramics. Ibid., 2, 41-47.

[7.] Simanovsky, V.M. (2000) Study of interaction between metal-mould contact zone for heat-resistant alloys. Ibid., 3, 83-85.

[8.] Myalnitsa, H., Dobkina, Yu. (2002) Thermal stability of superalloys structure after cast waste recovery. In: Proc. of 6th World Congr. on Recovery, Recycling, Reintegration (Geneva, Switzerland, 2002), 5.

[9.] Anikin, Yu.F., Zhezhera, A.D., Ladokhin, S.V. et al. (1998) Unit for joint induction and electron beam melting of metals and alloys. Metall i Litie Ukrainy, 5/6, 8-10.

[10.] (1995) Producing of high-temperature cast blades of aircraft gas-turbine engines. Ed. by S.I. Yatsyk. Moscow: Mashinostroenie.

[11.] (1997) Current technologies in producing of gas turbine engines. Ed. by A.G. Bratukhin et al. Moscow: Mashinostroenie.

LI. MAKSYUTA (1), Yu.G. KVASNITSKAYA (1), V.M. SIMANOVSKY (1) and G.F. MYALNITSA (2)

(1) Physical-Technological Institute of Metals and Alloys, NASU, Kiev, Ukraine

(2) Zorya--Mashproekt>> Company, Nikolaev, Ukraine
Table 1. Full chemical composition of refractory alloys ChS88U and IN
738 LC of different producers according to valid standards [1-3]

Element NI 88UVI ChS88UVI IN 738 LC
 <<First Rixson>> Russia, Stupino

 Weight share of element, %

Ni Base 59.88 Base
C 0.09 0.084 0.10
Cr 15.43 15.56 16.00
Co 10.97 8.59 8.30
Mo 2.13 0.76 1.70
Fe 0.08 0.49 0.13
Al 3.16 3.90 3.50
Ti 4.76 3.99 3.40
B 0.093 0.011 0.10
W 5.35 6.38 2.70
Nb 0.26 0.29 0.90
Zr 0.04 0 0.40
Hf 0.50 0.029
Y 0.03 --
Ce 0.015 --
Cu 0.01 -- < 0.20
Si 0.03 0.07 < 0.10
Mn 0.01 0.06 < 0.20

Element NI 88UVI ChS88UVI IN 738 LC
 <<First Rixson>> Russia, Stupino

 Weight share of element, %

Ta 0 0.030 1.600
P 0.006 0.005 0.005
S 0.001 0.002 0.001
Ga < 0.002 0.001 --
In 0 1 x [10.sup.-5] --
Mg < 0.005 0.002 < 0.005
Ag < 1 x [10.sup.-4] 1 x [10.sup.-5] < 1 x [10.sup.-5]
N 8 x [10.sup.-4] 0.006 0.002
O 0.0015 0.0014 6.[10.sup.-4]
As < 0.0015 3.8 x [10.sup.-4] --
Bi < 5 x [10.sup.-4] 6.27 x [10.sup.-4] < 1 x [10.sup.-5]
Pb < 5 x [10.sup.-5] 7.5 x [10.sup.-4] < 3 x [10.sup.-5]
Sb < 2 x [10.sup.-4] 1.4 x [10.sup.-4] --
Se < 1 x [10.sup.-4] 3.7 x [10.sup.-5] --
Sn < 0.002 0.0024 --
Te < 5 x [10.sup.-5] 0.0016 < 5 x [10.sup.-5]
Tl < 2 x [10.sup.-5] 2 x [10.sup.-6] N/D
Zn < 4 x [10.sup.-4] 6 x [10.sup.-5] Same

Note. Weight share of Hg, Ge, Au, K, Na, U, Th constitutes
[less than or equal to] 50 ppm.

Table 2. Structure of consumption of alloys in manufacturing of GTE
blades and vanes, %

Grade of Yield of Off-grade wastes Conditioned
alloy efficient wastes
 alloy, %

 Blades and Pouring Blades and
 vanes basins vanes
ChS70 34 8.2 20.3 9.3
ChS88U 32 9.6 19.4 12.6
ChS91 43 6.6 20.8 7.3
ChS104 32 8.1 20.8 9.2
EK9 37 7.2 20.0 8.5
EP648 42 5.3 20.5 6.1
Mean value 37 7.4 20.3 8.7

Grade of Conditioned Machining Melting and Total
alloy wastes waste irretrievable amount of
 losses waste

 Runners,
 feeders

ChS70 17.1 8 3.1 66
ChS88U 13.2 10 3.2 68
ChS91 10.0 9 3.3 57
ChS104 16.7 10 3.2 68
EK9 16.3 8 3.0 63
EP648 12.9 10 3.2 58
Mean value 14.3 9.1 3.2 63

Amount of loaded charge is assumed as 100 %.
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Title Annotation:ENERGY AND RESOURCE SAVING
Author:Maksyuta, Li.; Kvasnitskaya, Yu.G.; Myalnitsa, V.M.; Simanovsky G.F.
Publication:Advances in Electrometallurgy
Date:Jan 1, 2007
Words:3266
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