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 hightemperature 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 of com-
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plexly alloyed nickel alloys is limited by their very poor weldability.
a
b
c
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, electroslag 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 primary dendrite 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 start-
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ed 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 to be
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
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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 metal mould 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 multicomponent 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 carbon content by several thousandth of one percent share, thus exceeding technical conditions requirements.
Table 1. Chemical composition of KhN67VMTYu alloy
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 0.07 18.92 4.46 4.43 2.42 1.24 0.46 0.003 0.007 0.42
modifying
Metal modified 0.07 18.97 4.46 4.44 2.65 1.25 0.46 0.003 0.007 0.42
by 0.5 % (TiCN
+ Ti + Y)
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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 temperMechanical properties
Highature, °C
temperature
σt
σy
δ
ψ
850
MPa
%
strength  , h
200
Initial alloy
20
850
20
850
20
850
Metal without modifyng
Metal modified by 0.5
% (TiCN + Ti + Y)
858
526
784
591
914
628
560
–
478
–
616
–
14.8
12.7
8.6
10.2
20.2
14.8
16.6
12.1
21.6
81
28
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  200 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.
850
a
b
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
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(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).
a
b
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 cross1-152
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 hornshaped cores according to GOST 17375-83 was organized. Industrial tests showed
essential increase of the core service life in the broach assembly, which makes it possible to produce more than 30,000 taps.
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