New Technology of Electroslag Casting of Solid Horn-Shaped Cores

New Technology of Electroslag Casting of Solid Horn-Shaped Cores

E.N. Eremin

Omsk State Technical University, Omsk, Russia

ABSTRACT

New technology is proposed for production of solid horn-shaped cores, including electroslag melting of consumable electrode in melting crucible, modifying of molten metal by refractory particles and its subsequent pouring together with slag into chill mould. Equipment for realizing this process is described. Influence of nanoparticle of titanium carbonitride on structure and properties of KhN67VMTY alloy is shown. Zones of transcrystallization in casting are eliminated, sizes of dendrites are abruptly descreased, acquiring a favourable shape in the entire volume of the metal solidified. It is specially noted that morphology and topography of phases is bettered by modifying what stipulates increased high-temperature strength of metal and increases structural stability and continuous toughness of alloy in 2,6-3,4 times.

INTRODUCTION

In chemical, oil, gas and other branches of industry sharply bent pipe taps of various diameters are used. They are manufactured by means of a hydraulic press, which pushes heated cut-of-length round billets over a special tool – a broach. The main element of the broach is the horn-shape core, manufactured from high-temperature material, because it operates at high-temperatures and experiences high mechanical loads, received during movement of a tap over the core [1]. The broach itself consists of three parts: the core, a spacer, and a shank joined by means of welding. In commercial production of the cores Cr-Ni alloys are used of the type 25-50, characterized by good weldability. However, mechanical and high-temperature strength properties of these materials don't ensure high level of strength and wear resistance of the cores. Because of this reason term of their operation makes is only 200-250 h, thus making it possible to manufacture only 16000-18000 taps [2].

RESULTS AND DISCUSSION

Results of the carried out studies confirm these data and show that even provided technology of the core manufacturing by means of melting in the induction furnace is observed and its chemical composition corresponds to technical conditions established for the used steel, properties of the cast metal don't ensure necessary durability of horn-shaped cores in the composition of a broach.

Studies of the Dn 76 broach, cast according to the mass production technology and destroyed in the process of operation in transverse direction, showed that destruction was coursed by brittle, coarse grain fracture (Figure 1).

In the macrostructure studied in arbitrary section of the element two zones were detected: over periphery big columnar dendrites and in the center relatively equiaxial grains.

In the microstructure characteristic precipitates were registered against the background of coarse grains of γ-solid solution, indicative of high-temperature oxidation of the metal in the process of operation in the atmosphere of fuel combustion products with increased sulfur content. So, it advisable to use high-temperature nickel alloys for manufacturing such cores. At the same time using for this purpose ofcomplexly alloyed nickel alloys is limited by their very poor weldability.

a

bc

Figure 1. Broken commercially produced core: a – character of fracture;
b – macrostructure; c – microstructure (200)

So, the most acceptable material is alloy KhN67VMTYu recommended for manufacturing welded items [3].

The main method for producing castings from high-temperature alloys is casting by the lost-wax process in hot ceramic moulds in vacuum induction furnaces, which stipulates very high labor input and extremely low profitability of the production.

At the same time poor casting-technological properties of these alloys and high requirements to the castings quality inevitably result in low output of efficient metal and significant percentage of waste when using traditional methods of casting.

One can see from mentioned above that problem of new technologies development for producing components from high-temperature alloys requires for immediate solution.

Lately, one of the leading positions in production of items from alloy steels is occupied by the processes based on electroslag metal melting, in particular, elec-troslag chill mould casting (ECMC) [4]. It is explained by economic advantages stipulated by cheaper equipment, low operation expenses and, the main point, higher quality of the produced metal.

Traditional electroslag melting of high-temperature alloys, which is performed using the method of the electrode remelting in crucible with subsequent slag-metal melt over-pouring into the mould, has a number of difficulties [5, 6] stipulated, first of all, by reoxidation of liquid metal, because of which required chemical composition is not ensured in the cast. Besides, cast electroslag metal has columnar course-grain structure with high length of primarydendrite arms. All these factors don't enable obtaining required service characteristics of the cast metal. That's why improvement of the electroslag processes technology is urgent task.

Possibility of improving ECMC was studied. Remelting of the electrode, which was represented by rolled alloy KhN67VMTYu of 60 mm diameter, was started from liquid start and performed under high-fluoride flux ANF-21 on the A-550U unit with the TShS-3000-1 power source. This unit has small overall size and is convenient in operation. Due to these properties it is widely used in research works.

Copper water-cooled melting unit, which guaranteed «sterile» conditions for melting high-temperature alloys, was used as crucible. For increasing its efficiency on internal surface of the shell slotted recesses were made, which formed together with slag skull gas cavities stipulating high thermal resistance of the wall, thus allowing necessary amount of liquid metal to be accumulated in it.

Because of significant «secondary» oxidation of the alloying elements during high-temperature alloys pouring in the presence of air, the scheme of the melt bottom pouring without intermediate crucibles into the mould installed along the axis of the melting unit under penetrated inoculator-plug was used. For this purpose pouring device was used consisting of the copper water-cooled bottom plate with through conical hole, in which current-carrying inoculator-plug, made from the same material as the consumable electrode, was located. In central part of the plug blind hole was made for installation of the cooling element.

After the required mass of the melt was accumulated, the element was removed from the plug, cross connection between the hole and the melt stopped tobe cooled and was penetrated, which resulted in drain of the melt into the mould with significant head portion for receiving liquid flux, which, representing a hot top, equalized temperature of the cast being crystallized and prevented formation of shrinkage cavity in it.

Scheme of the complex for ECMC is given in Figure 2.

Figure 2. Complex for ECMC: 1– unit A-550U; 2 –electrode; 3 – melting unit;
4 – bottom plate; 5 – metal mould; 6 – cooling element; 7 – transformer TShS-3000

Technology of the core manufacturing includes melting of the electrode in the melting unit under flux, modifying of the accumulated metal, and its subsequent pouring together with the slag into the metal mould. Conditions of the remelting are as follows: voltage 42 V, current 2850-3000 A.

Lately, modifying with high-temperature particles is used for improving quality of the cast metal items made from refractory alloys [7-10]. In this work carbon nitride titanium (TiCN), titanium, and yttrium ultra-dispersion powders were used for modifying. Modifying agent was chosen according to the methodology described in [11]. Modifying agent was produced by mixing powder components with subsequent cold pressing into pellets of 25-30 mm diameter and thickness of 10-20 mm. Modifying agent was introduced at the temperature 1650 °C for 2 min before the drain, thus ensuring uniform distribution of disperse inoculator-particles all over the volume of liquid metal in the melting unit. Metal pouring into the metal mould was performed at the temperature 1600 °C.

After the melt was poured, a mould was disconnected from the melting unit and transferred by a jib to the site for final crystallization and cooling of the cast. Then the mould was disassembled and cast billet removed. External surface of the cast was even, slag skull made up about 1 mm, thus allowing small allowance for further machining. Disassembled metalmould with the cast in the slag skull and appearance of the manufacture cores are given in Figure 3.

a b

Figure 3. Disassembled metal mould with a casting (a) and appearance of the cast cores (b)

From produced in this way casts samples for experiments and metallographic analyses were made. Properties of non-modified and modified to various degree metals were compared.

The KhN67VMTYu alloy relates to the group of cast complex alloyed multi-component high-temperature alloys, in which together with significant size of the macrograin and high grain diversity of crystals various phases are formed during crystallization, namely solid solutions, eutectic systems, carbides, and intermetallic compounds. The main strengthening phase is γ’-phase, which represents intermetallic Ni, (Al, Ti). That's why properties of the cast alloy are determined, first of all, by its chemical and phase compositions.

Chemical analysis data prove (Table 1) that majority of the main alloying elements of the cast metal produced by the bottom pouring method, change insignificantly and correspond to the metal brand. Expected increase of the carbon mass fraction after introduction of the modifier practically was not registered. This may be explained by the fact that free and bound carbon content in TiCN does not exceed 1.2 and 12.0 % of the powder mass, respectively. That's why introduction of 0.06 % TiCN may increase carboncontent by several thousandth of one percent share, thus exceeding technical conditions requirements.

Table 1. Chemical composition of KhN67VMTYualloy

Object of study / Fraction of elements, wt.%
C / Cr / W / Mo / Ti / Al / Si / S / P / Mn
Initial alloy / 0.07 / 19.38 / 4.62 / 4.56 / 2.74 / 1.36 / 0.48 / 0.006 / 0.009 / 0.43
Metal without modifying / 0.07 / 18.92 / 4.46 / 4.43 / 2.42 / 1.24 / 0.46 / 0.003 / 0.007 / 0.42
Metal modified by 0.5 % (TiCN + Ti + Y) / 0.07 / 18.97 / 4.46 / 4.44 / 2.65 / 1.25 / 0.46 / 0.003 / 0.007 / 0.42
Standard alloy / ≤0.08 / 17-20 / 4-5 / 4-5 / 2.2-2.8 / 1.0-1.5 / ≤0.6 / ≤0.01 / ≤0.015 / ≤0.5

Mechanical properties of the metal at room temperature and long-term strength at 850 °C were determined on the samples cut out in longitudinal direction and subjected to thermal treatment: hardening at 1170 °C for 3 h, air cooling; ageing at 850 °C, holding for 10 h, air cooling. Obtained results, represented in Table 2, prove that modifying significantly increases both mechanical properties of the alloy and its long-term strength.

Table 2. Results of the KhN67VMTYu alloy tests

Object of study / Test temperature, °C / Mechanical properties / High-temperature strength , h
σt / σy / δ / ψ
MPa / %
Initial alloy / 20 / 858 / 560 / 14.8 / 16.6 / -
850 / 526 / – / 12.7 / - / 81
Metal without modifyng / 20 / 784 / 478 / 8.6 / 12.1 / -
850 / 591 / – / 10.2 / - / 28
Metal modified by 0.5 % (TiCN + Ti + Y) / 20 / 914 / 616 / 20.2 / 21.6 / -
850 / 628 / – / 14.8 / 96

The best results were registered when general addition of the modifier made up about 0.5 % of the liquid metal mass. In this case metal strength increase by 16 %, caused by modifying, is accompanied by 2.6-3.4 times increase of high-temperature strength thus exceeding properties of the initial alloy. This may be explained by the fact that quality and service properties of high-temperature alloys are determined, besides the chemical composition, by condition of the grain boundaries, their size, homogeneity degree, morphology, and topography of inclusions [3, 12, 13].

Study of the cast non-modified electroslag metal (Figure 4) showed that it has transcrystalline macrostructure, consisting of long narrow columnar crystals over the periphery and equiaxial crystals in the center. It is explained by intensive heat withdrawal by metal mould.

ab

c d

Figure 4. Structure of the core made of non-modified alloy KhN67VMTYu:
a – macrostructure; b – microstructure (100); c – carbides (500); d – γ’-phase (8000)

Length of the columnar grains achieves 30 mm, grain diameter in the center is 10 mm. The microstructure represents austenite with precipitation of carbides, eutectics, and γ’-phase. Segregation connected with formation of the carbon-nitride sites (Ti, Mo, W)(CN) and binary carbides of (Ni, Cr)3(Mo, W)3C type was detected. Needle-like line-lattice inclusions of long length carbides, located mainly over grain boundaries, were registered in the structure.

Study of structural transformations at 850 °C showed that strengthening γ’-phase Ni3(Ti, Al) has chaotically arranged orbicular forms, which frequently have irregular configuration. Size of the particles is unstable – from 0.3 to 1.2 μm. It is the evidence of its coagulation and dissolution in γ-solid solution. Besides, precipitates of the η-phase lamellas of Ni3Ti type were registered. It is the evidence of the beginning of γ’-phase regeneration into η-phase. Process of regeneration goes according to the general scheme: dissolution of the γ’-phase particles and diffusion of titanium atoms to the growing η-phase lamella. Needle-like lamellar shape of the phases enables brittle fracture of the alloy [3, 13]. Besides, these phases weaken solid solution by extracting refractory elements from it.

Introduction of 0.5 % modifier into the melt causes essential change of both obtained structure and morphology and topography of phases being precipitated. Drastic refining of the macrograin takes place, while columnar shape of the grains and their heterogeneity practically disappear (Figure 5).

ab

c d

Figure 5. Structure of the core made of modified alloy KhN67VMTYu:
a – macrostructure; b – microstructure (1000); c – carbides (500);d – γ’-phase (l0000)

CONCLUSIONS

The modifier affects positively morphology of carbides applying discrete character to the precipitates. The carbides acquire compact equiaxial shape. Grain boundary has a form of a wavy line, which envelops particles located in the alloy matrix. Dendrite structure of the cast metal is fine and homogeneous over the ingot cross-section. Essential changes in the γ’-phase structure take place. It acquires rectilinear cubic-like outline and ordered arrangement. Such condition of the structure proves that grain boundaries are «fixed» by the particles, and because of this reason their migration during heating is insignificant and, as a result, level of properties at high temperatures should be higher. Really, the tests showed that mechanical properties and long-term strength of the modified metal are high and homogeneous over the core length, and exceed properties of the commercial alloy produced by the metallurgical integrated works.

According to the developed ECMC technology production of solid horn-shaped cores according to GOST 17375-83 was organized. Industrial tests showed essential increaseof the core service life in the broach assembly, which makes it possible to produce more than 30,000 taps.

REFERENCES

1. Galperin, A.I. Machines and equipment for producing of curvilinear pipeline sections. Moscow: Nedra,1983. – 203 p.
2. Shuvalov, Yu.B. Hot drawing of steeply curvedpipe-branches on hollow horn-shaped cores. Khimich. i Neft. Mashinostroenie. –1991. – № 2. –Р. 36-37.
3. Khimushin, F.F. High-temperature steels and alloys. Moscow: Metallurgiya, 1969. – 752 p.
4. Paton, B.E., Medovar, B.I. Electroslag cruciblemelting and pouring of metal. Kiev: Naukova Dumka, 1988. – 214 p.
5. Bashnin, Yu.A., Isakina, V.N., Maslenkova, E.A. Effect of remelting processes on structure and properties ofsteel. Moscow: Metallurgiya, 1991. – 240 p.
6. Klichevets, M.I., Shelgaeva, A.V., Vedernikov, G.G. et al.Electroslag melting of high-temperature nickel alloyKhN73MBTYu. In: Special electrometallurgy. – 1980. – Issue 41. –P. 60-68.
7. Eremin E.N.Application of modifying for improvement of properties of nitrogen-containing stainless steel ring-shaped billets, produced by the method of centrifugal electroslag casting. – Sovremennaya electrometallurgy, 2006. - №3. – P.3-7.
8. Eremin E.N.Modification of weld metal in electroslag welding of KhN77TYuR refractory alloy. – The Paton welding journal. - № 9. - 2007. - P. 38-41.
9. Eremin E.N.Application of electroslag remelting for regeneration of waste of heat-resistant alloys. – Sovremennaya electrometallurgy, 2005. - №9. – P.38-41.
10. Fatkulin, O.Kh., Ofitserov, A.A. Modification ofhigh-strength nickel alloys by disperse particles of refractory compounds. Litejnoe Proizvodstvo. – 1993. –№ 4. – P. 13-14.
11. Eremin, E.N. Principles of complex modification ofcast electroslag metal. In: Analysis and synthesis of mechanical systems. Omsk: OmGTU. – 1999. – P. 76-78.
12. Latash, Yu.V., Klyuev, M.M., Negoda, G.P. et al.Structure,phasecompositionandpropertiesofalloyEI437B of electroslag pouring. In: Special electrometallurgy. –1984. – Issue 55. – P.7-11.
13. Logunov, A.V., Petrushin, P.V., Kukshova, E.A. et al.Prediction of effect of structure factors on mechanical properties of high-strength alloys.Metallovedenie iTerm. Obrab. Metallov. – 1987. – №6. – P. 16-20.

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