B. ARH et al.: ELECTROSLAG REMELTING: A PROCESS OVERVIEW
971–979
ELECTROSLAG REMELTING: A PROCESS OVERVIEW
ELEKTROPRETALJEVANJE POD @LINDRO – PREGLED PROCESA
Bo{tjan Arh, Bojan Podgornik, Jaka Burja
Institute of Metals and Technology, Lepi pot 11, Ljubljana, Slovenia
bostjan.arh@imt.si
Prejem rokopisa – received: 2016-06-15; sprejem za objavo – accepted for publication: 2016-09-02
doi:10.17222/mit.2016.108
The electroslag remelting process (ESR) is important because it provides better control of the solidification microstructure and
chemical homogeneity; it also enables greater cleanliness and better mechanical properties. The manufactured high-alloyed
steels and other alloys with a controlled chemical composition are used in aerospace, in thermal- and nuclear-power plants, in
chemical engineering, for military equipment, special tools, etc. An overview and the basics of the ESR process are presented in
this paper.
Keywords: electroslag remelting, solidified microstructure, chemical homogeneity, clean steel
Postopek elektropretaljevanja kvalitetnih jekel pod `lindro (EP@) ima velik pomen zaradi sposobnosti nadzora strjevalne
strukture in kemijske homogenosti saj postopek omogo~a doseganje ve~je ~istosti jekla in bolj{e mehanske lastnosti. Tako
izdelana visokolegirana jekla, in zlitine z garantirano kemijsko sestavo in lastnostmi, se namensko uporabljajo v letalstvu,
termoelektrarnah in jedrskih elektrarnah, v kemi~ni industriji, v medicini, za voja{ko opremo, za specialna orodja, itd. V pri-
spevku so opisane osnove tehnologije pretaljevanja zlitin pod `lindro.
Klju~ne besede: elektropretaljevanje pod `lindro, strjevalna struktura, kemijska homogenost, ~isto jeklo
1 INTRODUCTION
Nowadays, steelmaking technology enables the pro-
duction of high-purity steel melts. However, during ingot
casting the reoxidation of the melt occurs, thus increas-
ing the inclusion content. Segregations on the macro and
micro scales are also characteristic for ingot casting.
These cause anisotropy in the mechanical properties of
the steel. The ESR process almost completely removes
the macro-segregation phenomenon in heavy steel
ingots, thus ensuring a more homogeneous chemical
composition and a finer microstructure with fewer and
more evenly distributed non-metallic inclusions than in
cast ingots.1 The influence of ESR on remelted steel is
shown in Figure 1.2 This is why the ESR process is
essential for heavy steel ingots that are used for the
manufacturing of large generator and turbine shafts.3
The ESR process is suitable for high-quality ma-
terials such as:
• steel ball bearings, steel rollers, tool steels, wear-
resistant steels for low and high working temperatu-
res, high-speed steels for high performance,
• highly alloyed stainless steels, corrosion- and acid-
resistant steels and steels for high-temperature
applications,
• steels for aviation and aerospace technology, for
medical, pharmaceutical and chemical industries,
• Ni superalloys, Ti and Zr alloys for aerospace, medi-
cal and chemical industry, components,
• off-shore, power and aerospace engineering, reactor
comp.
ESR is a continuous process, where during the
remelting of the consumable electrode, refining and
solidification of the steel occur simultaneously. Cast,
rolled or forged ingots can be used as a consumable
electrode.
The ESR process is based on an electrical current
running via an electrode through the molten slag and
ingot. Due to the high electrical resistance of the slag,
the slag heats up and melts. The consumable electrode is
immersed in the liquid slag where the slag heat gradually
melts the tip of the electrode. Liquefied steel is dripping
Materiali in tehnologije / Materials and technology 50 (2016) 6, 971–979 971
UDK 669.187.56-046:621.745:66.02 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 50(6)971(2016)
Figure 1: Effect of ESR on the properties of remelted steels2
Slika 1: Vpliv ESR na lastnosti pretaljenih jekel2
from the electrode tip and is refined when passing
through the liquid slag, with oxides and sulphur being
bound in the slag. After passing through the slag, the
steel cools down and solidifies again into a remelted
ingot.3,4 The whole remelting process takes place in a
water-cooled copper mould, which allows the remelted
ingot to solidify quickly and very uniformly.
The mould with the slag pool is moving upwards as
the new ingot is formed. The design of the mould can be
in the form of fixed long moulds or collar-type moulds.
The use of collar-type moulds with movable moulds or a
movable base plate, gives the possibility of producing
ingots of any required length (Figure 2).5,6 Furthermore,
the ESR enables the production of ingots with the
desired shape, i.e., round, square, rectangular.
In Slovenia, among other steelmaking processes,
ESR technology is used in the Metal Ravne steelworks.
2 ATMOSPHERE CONTROL
Due to the ever-increasing demands for material
properties, different variations of the ESR process were
developed to ensure these demands.6–8
IESR (electroslag remelting under a protective
atmosphere of inert gas at atmospheric pressure) is a
variation of the ESR where an inert gas (argon) protects
the slag and metal from oxidation and the absorption of
nitrogen and hydrogen from the air. The oxidation of the
electrode is almost entirely avoided, thus providing
better cleanliness of the ingot. However, due to the ab-
sence of oxygen in the furnace atmosphere, desulphuri-
zation is not optimal.
PESR (electroslag remelting under increased
pressure) is a variation of the ESR with an increased
pressure of nitrogen and the melt solidifies under
pressure. In this way a large amount of nitrogen can be
introduced into the steel melt. The pressure depends on
the alloy composition and the desired nitrogen content in
the remelted ingot.
VAC-ESR (electroslag remelting under vacuum) is a
variation of the ESR that provides vacuum degassing of
the melt. Dissolved gases such as hydrogen and nitrogen
are removed, and the remelted metal is protected from
oxidation. The process is suitable for the remelting of
superalloys and titanium alloys.
3 PROCESS PARAMETERS
The heat required to run the ESR process is generated
by the Joule effect in the slag bath. The electrical
characteristics, heat balance and electrode/ingot
diameter, influence the quality of the remelted ingot.7,9
An energy input of between 1000 kwh/t and 1500 kwh/t
of steel is usually required for ESR. The slag bath is
considered to be a variable resistor, whose resistance is
determined by the electrode distance, the effective slag
resistivity and by the electrical current path. The usual
slag depth is of the order of 100 mm.
The shape of the liquid pool is influenced by the heat
input in the process.10,11 The greater the distance between
the consumable electrode and the remelted ingot, the
smoother the heat distribution in the slag. When
determining the electrode distance, it is important to take
into consideration that a shorter current path means a
higher current with concentrated heat generation under
the electrode tip and an undesirable deepening of the
metal pool. On the other hand, a longer current path
demands a high voltage, which causes more even heat
generation and a flatter, more favourable pool profile.
ESR operating voltages are usually in the region of 40 V
or less.
A choice of AC versus DC circuit ESR9 is possible.
For ingots of 20 cm in diameter or more, single phase
AC gives optimum refinement and melt rate. The
DC-ESR requires a lower melt rate for metal refinement.
However, when the metal refinement is not the main
criterion, the DC-ESR provides the highest melting rates
per unit of power consumption. The present practice is to
utilize a single-phase AC power supply and low
electrode/ingot diameter ratio (0.4 to 0.7).11 Typically, a
frequency of 50 Hz or 60 Hz is used in AC operation.
However, for the largest ingots, where reactivity be-
comes more important, it is better to use low-frequency
power (5-10 Hz) for improved efficiency.
Optimum melting rates and energy inputs depend on
the ingot diameter. A. S. Ballantyne and A. Mitchell12
considered the optimum conditions for the maximum
permissible melt rate at the lowest possible power with
the Equation (1):
Melt rate = constant × power × fill ratio (area) ×
× mould area / electrode distance (1)
Many operators considered melt rate as proportional
to the ingot diameter, which is obtained at a melt rate of
the order of 0.004 kg/min/mm.7 The relationships
between the melt rate, current and voltage for a 240-mm
diameter ingot are shown in Figure 3. For a given
B. ARH et al.: ELECTROSLAG REMELTING: A PROCESS OVERVIEW
972 Materiali in tehnologije / Materials and technology 50 (2016) 6, 971–979
Figure 2: Schematic representation of the ESR unit: a) retracting
ingot, b) rising mould5,6
Slika 2: Shematski prikaz ESR naprave: a) navzdol vle~en ingot,
b) dvigajo~a kokila5,6
current and ingot size there is an optional voltage that
corresponds to a maximum melt rate.7
The ESR process can be controlled by a computer:
from melt initiation, through power build-up, steady melt
rate period, reduced melt rate period to maintain pool
profile, hot-tapping sequences and melting termination.13
4 SLAG
The slag plays an important role in the ESR process;
it generates Joule heat for the melting of the electrode,
refines the liquid metal through the absorption of
non-metallic inclusions, desulphurization, protects the
metal from contamination, provides lubrication for the
copper mould/solidifying steel shell interface, and
controls the horizontal heat transfer between solidifying
metal and mould.
Slags for ESR are usually based on calcium fluoride
(CaF2), lime (CaO) and alumina (Al2O3). Silica (SiO2),
magnesia (MgO) and titania (TiO2) may be present,
depending on the alloy to be remelted and refined. The
CaF2 content increases the solubility of basic components
in the slag (CaO and MgO) and thus increases the
effective sulphide capacity of the slag.7 In order to per-
form its intended functions, the slag must have some
well-defined properties, which are:
• its melting point must be lower than that of the metal
to be remelted,
• it must be electrically efficient,
• its composition must ensure the desired chemical
reactions,
• it must have suitable viscosity at the remelting
temperature.
As presented in Table 1 the concentrations of cal-
cium fluoride may vary from 0 % to 100 % of mass frac-
tions.7 The remaining slag constituents are mostly used
for decreasing the basicity.
The slag chemical composition is changed during the
ESR process, due to the formation of volatile fluoride,
the precipitation of high-melting-point phases and the
reaction in the ESR process.14 The changes in composi-
tion affect the slag’s metallurgical properties and even-
tually affect the quality of the final product. The quantity
of consumed slag steel depends on the remelted ingot
diameter.6
Table 1: Composition of some ESR slags, in mass fractions (w/%)7
Tabela 1: Sestava nekaterih ESR `linder, v masnih odstotkih (w/%)7
CaF2 CaO MgO Al2O3 SiO2 Comments
100
Electrically inefficient,
use where oxides are not
permissible
70 30
Difficult starting, high
conductivity, use where
Al not allowed, risk of
H2 pick-up
70 20 10 Good all-round slags,
medium resistivity70 15 0 15
50 20 30 Good all-round slag,higher resistivity
70 30
Some risk of Al pick-up,
good for avoidance of H2
pick-up, higher resisti-
vity
40 30 30 Good general-purpose
slags60 20 20
80 10 10 Moderate resistivity,relatively inert
60 10 10 10 10 Low melting point,"long" slag
50 50 Difficult starting,efficient electrically
Many of the slags used in ESR can be described with
the ternary fluorspar-lime-alumina system.7 The phase
diagram shown in Figure 4 has been extensively investi-
gated and defined by K. Mills.15 The main feature is an
eutectic corresponding to compositions with roughly
B. ARH et al.: ELECTROSLAG REMELTING: A PROCESS OVERVIEW
Materiali in tehnologije / Materials and technology 50 (2016) 6, 971–979 973
Figure 3: Effect of current and voltage on melt rates7
Slika 3: Vpliv toka in napetosti na hitrost taljenja7
Figure 4: Phase diagram of the CaF2-Al2O3-CaO system according to
K. Mills15
Slika 4: Fazni diagram sistema CaF2-Al2O3-CaO po K. Millsu15
equal proportions of lime and alumina. This identifies
the slags with liquidus temperatures in the range
1350–1500 °C, which make them suitable for melting of
a wide range of alloys, including steels and super alloys.
In the case of slag with 70 % fluoride and 30 % alumina
the lime is excluded as much as possible in order to pre-
vent hydrogen pick-up, while there are no problems with
the presence of the two liquids. The binary lime-alumina
system on the other hand, has only a limited range of
slags with suitable melting characteristics, while the
binary calcium fluoride-lime system is used in cases
where a high degree of desulphurization is required.
However, its disadvantage is having a low resistivity.
High lime contents also increase the risk of moisture
retention or hydrogen pick-up.
According to the tendency to reduce CaF2 in slags,
the investigations of the slag S 2015 (Wacker Cheime)
with 30 % CaF2 were performed. The results also
showed that tested slags with 4.7 % CaF2 can be satis-
factorily applied in the ESR-process of UTOP Mo6
steel.16
A certain amount of SiO2 addition into the ESR slag
in the case of the drawing-ingot-type ESR process is
important for improving the lubrication performance,
controlling silicon and aluminium content in the liquid
steel and modifying oxide-type inclusions.17 Further-
more, the addition of SiO2 suppresses the crystallization
temperature of CaF2-Al2O3-CaO slags. Furthermore, the
MgO and SiO2 in fluoride-containing slags affect the
slag’s surface tension.18
Although CaF2 is a crucial component in any ESR
slag and it greatly decreases the melting temperature of
the slag systems, it is insoluble in oxide phases. An
example of an ESR slag microstructure is shown in
Figure 5, where the lighter dendrite-like phase is a stable
CaO-Al2O3 phase, the darker phase is fluorspar, and the
small white dots the undissolved magnesia particles. The
fluorspar phase contains only calcium and fluoride and is
not dissolved in other microstructural constituents.
Slag properties, such as electrical conductivity, ther-
mal conductivity, density, viscosity and surface tension
play an important role in effective melting and metal
refining. K. Mills19 has produced a table of the physical
properties of slags for practical purposes (Table 2). Slag
resistivity affects the operating characteristics and eco-
nomics of ESR. Alumina increases the resistivity of the
slag and promotes good heat generation, thus enabling a
reduction of the slag bulk content, which also reduces
the heat loss due to the reduced area of contact between
the slag and the mould wall.
L. A. Kamenski et al.20 refer to "long" and "short"
slags when discussing slag viscosity. Long slags remain
fluid over a wide range of temperatures and are likely to
give thin slag skins and therefore good ingot surfaces.
Short slags rapidly become viscous on cooling and are
likely to give thick slag skins and poor ingot surfaces.
High calcium fluoride contents promote short slags,
whereas silica and magnesia favour long slags.
The slag plays an important role in ESR, from the
control-of-inclusions point of view.21 The chemical and
physical properties of slag also have a great effect on the
removal of inclusions.
5 THERMODYNAMICS
In the case of ESR of steel in an air atmosphere,
chemical reactions take place and change the chemical
composition of the as-cast ingot.22 The levels of some
B. ARH et al.: ELECTROSLAG REMELTING: A PROCESS OVERVIEW
974 Materiali in tehnologije / Materials and technology 50 (2016) 6, 971–979
Figure 5: Microstructure of ESR slag
Slika 5: Mikrostruktura ESR `lindre
Table 2: Phase diagram of the CaF2-Al2O3-CaO system and the
physical properties of slag at 1600 °C, according to K. Mills19
Tabela 2: Fazni diagram sistema CaF2-Al2O3-CaO in fizikalne
lastnosti `lindre pri 1600 °C, po K. Millsu19
Electrical
conduc-
tivity
Viscosity Density Surfacetension
Total
normal
emissivity
Contour  (
–1
cm–1)
 (10–1
Ns m–1)

(g cm–3)
s
(mN m–1) TN
A 6 0.15 2.47 285
0.96B 5 0.2 2.48 300
C 4 0.25 2.49 310
D 3.5 0.3 2.5 320
0.9
E 3 0.4 2.55 335
F 2.5 0.6 2.6 350
0.85
G 2 0.8 2.7 400
H 1 1.0 2.8 450 0.81
elements, such as Co, Ni, Cr, Mo, W, C remain un-
changed after remelting. However, the content of Si, O,
and S can be changed from 10 % to 80 %, while the
content of Al and Ti can vary depending on the melting
conditions (decrease or increase). Therefore, some
measures need to be taken to prevent the losses of
elements. This can be achieved by using special ESR
variations as discussed before. Another way is control of
the slag composition and regular additions to the slag,
which is desirable due to steady melting conditions. The
oxidation of the elements can be prevented by deoxida-
tion of the slag during the melting process by additions
of aluminium.
The oxygen potential of the slag determines the
chemistry of the ESR process.7 It affects the non-metallic
inclusions and sulphur removal. Oxygen reacts with
some elements in the metal and suppresses hydrogen
pick up. In the slag oxygen is mostly bound as FeO,
MnO and SiO2. To estimate the oxygen content in the
steel it is necessary to find the relationship between FeO
in the slag and oxygen in the remelted ingot.22,23 How-
ever, due to the very low solubility of FeO in CaF2 slags,
its activity is extremely high. The oxygen content can be
estimated by thermodynamic analyses of the reactions
between active components and oxygen. Silicon and
manganese are elements that can react with the oxygen
present in the steel and from the slag.23,24 When silicon is
the strongest deoxidizer, the oxygen content of the steel
is determined by the Si content.23 At constant tempe-
rature and Si content in the steel, the oxygen content of
the metal is higher at higher activity of the SiO2 in the
slag, or by lowering the basicity of the slag.
Aluminium losses in the remelted ingot are small,
especially at high alumina content in the slag. On the
other hand, the presence of Al2O3 in the slag reduces the
oxidation of silicon. The reaction between the silicon in
the electrode and Al2O3 in the slag also controls the
oxidation of aluminium in the remelted ingot.25 Thus, Al
content in the remelted ingot depends on the content of
Al2O3 in the slag and the content of silicon in the
electrode, temperature and chemical composition of the
steel.7,25 The content of Al in the remelted ingot de-
creases when CaF2-Al2O3-CaO slags with increased SiO2
content are used. When aluminium is used for
deoxidation, up to 15 % of added Al is transferred to the
molten steel. The content of titanium in the remelted
steel will depend on the content of Al and Ti in the
consumable electrode, the content of Al2O3 and TiO2 in
the slag and the oxygen potential in the gas phase above
the slag (Figure 6).26 The equilibrium between the Al
and Ti content in the electrode at different TiO2 contents
is presented in Figure 6.
For the content of Al in electrode, the titanium loss
can be minimized by the addition of TiO2 to the slag. At
high contents of Al, aluminium reduces TiO2 in the slag
and also regulates the ratio of Ti:TiO2.
In the early stages of ESR development, the removal
of sulphur was considered as one of the major objectives.
The rate of desulphurization increases with the basicity
of the slag. Sulphur transfer takes place mainly at two
interfaces, according to the following two reactions:23
Slag/metal reaction: [S] + (O2–) = (S2-) + [O] (2)
Gas/slag reaction: (S2–) + 3/2 {O2} = {SO2} + (O
2) (3)
A thermodynamic analysis of the reactions shows
that the desulphurisation is related to the concentration
of O2– ions in the slag, the partial pressure of oxygen in
the gas phase and the chemical composition of the
steel.27 The transfer of sulphur from the metal to the slag
is promoted by the high slag basicity and low concen-
tration of oxygen in the metal. On the other hand, the
sulphur transfer from slag to gas is promoted by a high
partial pressure of oxygen in the atmosphere and the low
basicity of the slag. The ability of the slag to take sul-
phur is defined in terms of its sulphur capacity. The sul-
phur capacity for the fluorspar-lime-alumina system
increases as the CaF2 content is increased and by in-
creasing the amount of lime to the saturation point.28
In the case of ESR under inert gas, the sulphur
remains in the slag and builds up there as the process
continues. In such cases the sulphur capacity is the ruling
factor, and the slag bulk must be adjusted in order to
continue its desulphurising action to the end of the pro-
cess, i.e., the slag/metal ratio assumes greater import-
ance.7
6 SOLIDIFICATION AND STRUCTURE
The solidification structure of an ESR ingot is a func-
tion of the local solidification time and the temperature
gradient at the liquid/solid interface.4,7 To achieve a
directed dendrite primary structure, a relatively high
temperature gradient at the solidification front must be
maintained during the entire remelting period.
B. ARH et al.: ELECTROSLAG REMELTING: A PROCESS OVERVIEW
Materiali in tehnologije / Materials and technology 50 (2016) 6, 971–979 975
Figure 6: Effect of Al:Ti ratio in the electrode on TiO2 content in the
slag26
Slika 6: Vpliv odvisnosti Al:Ti v elektrodi na vsebnost TiO2 v
`lindri26
Macrostructure of the ESR ingots is different from
the macrostructure of conventionally cast ingots due to
the different heat transfer and heat removal. The growth
direction of the dendrites is a function of the metal pool
during solidification. Thus, the gradient of dendrites with
respect to the ingot axis increases with melting rate. In
extreme cases the growth of directed dendrites can come
to a stop. The ingot core then solidifies non-directionally
in equiaxed grains, which leads to segregation and micro
shrinkage. Even in the case of directional dendritic soli-
dification, the micro segregation increases with the den-
drite arm spacing. A solidification structure with
dendrites parallel to the ingot axis yields optimal results.
However, this is not always possible. A good ingot sur-
face requires a minimum energy input and accordingly a
minimum melting rate.
Increasing the melting rate increases the difference
between the gradient of the solidus and liquidus iso-
therms and leads to increased pool depth.29 Hence, grains
grow in radial direction instead of vertical direction.
Figure 7 shows the direction of the grain growth depen-
dent on the melting rate, which affects the pool depth.
Figure 8 presents the predicted grain structures for
different melting rates up to 600 kg/h.30 Increasing the
melting rate causes a finer grain structure and changes
the growth direction of the columnar structure from the
axial to radial growth and deeper liquid pool at very high
melting rates. Increasing the molten slag temperature
also results in a coarser columnar grain structure and a
reduced thickness of the refined equiaxed grain layer,
both at the surface and the bottom of the ESR ingot.
In spite of directional dendritic solidification, defects
such as tree-ring patterns, freckles and white spots can
occur in a remolten ingot.4,7,31 Macro-segregation and
porosity structures in the middle of the ingot are very un-
common for ESR ingots.
A major attribute of ESR is its ability to produce ma-
terial with reduced micro-segregation. This is linked with
the local solidification time and dendrite-arm spacing.7
ESR material normally freezes in a columnar manner,
which gives less micro-segregation than equiaxed struc-
tures. The greater the temperature gradient, the smaller is
the distance between the dendritic arm spacing and the
lower is the chemical heterogeneity in the micro areas. In
ESR, temperature gradients are greater than for conven-
tional casting. Therefore, the secondary dendrite-arm
spacing will be smaller in ESR than in conventional
B. ARH et al.: ELECTROSLAG REMELTING: A PROCESS OVERVIEW
976 Materiali in tehnologije / Materials and technology 50 (2016) 6, 971–979
Figure 7: The ingot grain growth at a melting rate of: a) 0.9 kg/min
and b) 1.2 kg/min32
Slika 7: Rast zrna v ingotu pri hitrostih taljenja: a) 0,9 kg/min in
b) 1,2 kg/min34
Figure 9: Segregations in the hot-work tool steel: a) consumable elec-
trode, b) remelted ingot
Slika 9: Izceje v orodnem jeklu za delo v vro~em: a) elektroda pred
pretaljevanjem, b) pretaljen ingot
Figure 8: The predicted grain structure of ESR ingot30 for different
melting rates; a) 136 kg/h, b) 271 kg/h, c) 407 kg/h and d) 542 kg/h
Slika 8: Napovedana zrnatost ESR-ingota30 pri razli~nih hitrostih
taljenja: a) 136 kg/h, b) 271 kg/h, c) 407 kg/h in d) 542 kg/h
ingots. The effect of decreasing the segregation effect is
shown in Figure 9, where a comparison of microstruc-
tures before (Figure 9a) and after (Figure 9b) ESR pro-
cessing was made for a hot-work tool steel. The micro-
structure in both cases is tempered martensite. The
difference in segregation bands is obvious. While the
segregations are evident in the consumable electrode
(Figure 9a) they are almost completely eliminated in the
remelted ingot (Figure 9b).
Figure 10 shows the effect of local solidification
time on the dendrite spacing.32 The dendrite-arm spacing
is decreased as the cooling rate is increased.
Besides a more homogeneous composition and com-
pact solidification structure, the removal of non-metallic
inclusions is an important characteristic of ESR.33
Normally, inclusions easily initiate micro-voids and
cracks at the inclusion/steel interface, which can be the
origin of fatigue fracture or other defects. Also, ESR
steel is not an exception.34 Many factors influence the
formation of non-metallic inclusions in ESR steel,
including furnace atmosphere, content of inclusions in
the consumable electrode, slag amount and its compo-
sition, power input, melting rate, filling ratio, etc. Most
non-metallic inclusions occur due to the reactions bet-
ween oxygen and elements such as manganese, silicon
and aluminium.35 Deoxidization of the slag during
electroslag remelting36 has an important influence on the
non-metallic inclusions formation in the ESR ingot. The
results of the experimental work show that the lowest
number of inclusions is attained in ESR with the lowest
viscosity and the highest interfacial tension.37 However,
the absence of large inclusions is typical for ESR, as
shown in Figure 11.38
The removal of non-metallic inclusions during ESR
takes place at the tip of the electrode, where mainly
absorption and dissolution of non-metallic inclusions in
the slag take place.6,39 As the electrode tip is heated
towards its melting point, the inclusions in the electrode
are re-dissolved before the metal melts. Any other in-
clusions, such as larger exogenous inclusions in the
electrode, are not dissolved in the solid metal and will be
exposed to the slag when the electrode tip becomes
molten. If the slag composition is suitable, the tempe-
rature is high enough and the dwell time is long enough
the non-metallic inclusions will dissolve in the slag.40
However, at this point there may be further reactions due
to the difference in equilibrium constants, as well as the
possibility of the flotation of large inclusions. The metal
at this point is free from non-metallic inclusions, but
may have in solution elements that produce inclusions by
reaction during the freezing time (sulphur removal reac-
tion).41,42 The removal efficiency of inclusions increases
with the reduced melting speed.43
The research work of Y.-W. Dong et al.44 was focused
on the impact of fluoride-containing slag and interac-
tions at the slag-metal interface on the non-metallic
inclusions in steel. Results indicate that a multi-compo-
nent slag (CaF2, CaO, Al2O3, SiO2, MgO) has a better
capacity for controlling the amount of inclusions. Most
non-metallic inclusions for multi-component slag are
MgO-Al2O3 inclusions, while mainly Al2O3 inclusions
exist when using conventional 70 % CaF2 – 30 % Al2O3
slag. Furthermore, the maximum inclusion size for
multi-component slags was found to be smaller than for
conventional binary slag.
7 CONCLUSION
The main purpose of the remelting process is to
control the non-metallic inclusions in the steel, remove
segregations and shrinkage, and produce more homoge-
nous ingots. The low speed of remelting, combined with
the water-cooled mould, ensures a particularly homoge-
B. ARH et al.: ELECTROSLAG REMELTING: A PROCESS OVERVIEW
Materiali in tehnologije / Materials and technology 50 (2016) 6, 971–979 977
Figure 11: Inclusion size and frequency for inclusions >2.5 μm in
air-melted and ESR with 3 % Cr-Mo-V steels38
Slika 11: Velikost in {tevilo vklju~kov > 2,5 μm po ESR-pretaljevanju
in pretaljevanju na zraku jekla 3 % Cr-Mo-V38
Figure 10: Microstructures of ingots of ESR nickel alloy 718, with
dendrite-arm spacing at different cooling rates: a) slower cooling,
b) faster cooling and c) fastest cooling32
Slika 10: Mikrostruktura EP@ ingota nikljeve zlitine 718 in razdalj
med dendritnimi vejami: a) po~asnej{e ohlajanje, b) hitrej{e ohlajanje
in c) najhitrej{e ohlajanje 32
neous and balanced, stable solidification. The segrega-
tions within a remolten ingot are thus much lower (or
even eliminated) compared to open cast continuous cast
billets or conventional cast ingots. For this reason most
segregation-sensitive steels are ESR processed for
homogenisation.
The slag plays an important role in the ESR process,
as it absorbs non-metallic inclusions, removes sulphur,
influences the ingot surface and the melting rate as well
as the overall economics of the process. That is why the
chemical composition and physical properties of the
chosen ESR slag is of paramount importance for a
high-quality ESR ingot.
In order to further improve non-metallic inclusion
reduction, specialised versions of the ESR process like
IESR, PESR and VAC-ESR have been developed.
The application of the ESR process is appropriate for
tool and high-speed steels as well as special stainless
steels and special alloys intended for the most demand-
ing applications.
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