Electroslag remelting method and flux composition

An improved method is disclosed for electroslag remelting an electrode of an alloy containing at least one oxide forming ingredient, including melting such electrode in a novel flux composition containing about 1-35% boron oxide, by weight.

BRIEF SUMMARY OF THE INVENTION 
The present invention pertains to an improved electroslag remelting method, 
and more particularly to the use of from about 1% to 5% boron oxide in a 
flux composition through which an electrode is electroslag remelted. 
Ingots produced by the process of the present invention are characterized 
by significantly improved surface condition and internal quality. 
In the process of electroslag remelting, an electrode of the metal alloy to 
be remelted is partially immersed in a slag. The electrode of the metal 
alloy to be remelted may be obtained by various methods including, but not 
limited to, induction melting, vacuum induction melting, 
argon-oxygen-decarburization (AOD) melting and basic oxygen furnace (BOF) 
melting. Typically, the slag is heated to a temperature above the melting 
point of the metal by the slag's resistance to an electric current passed 
between a base plate of the mold and the metal electrode. Preferably, 
though not necessarily, the slag should become molten at a temperature 
below the melting point of the metal electrode. Therefore, the metal which 
is remelted passes through the molten slag bath and collects in a pool 
over the base plate of the mold. The collected metal is cooled and thereby 
solidified into an ingot within the mold. Electroslag remelting processes 
are described in U.S. Pat. Nos. 2,694,023; 3,067,473; 2,868,681; 3,551,137 
and in the Duckworth and Hoyle Text, "Electroslag Refining," Chapman and 
Hall, Ltd. (London, 1969). 
An objective of an electroslag remelting method is to produce an ingot 
having a smooth surface so that the ingot may be hot-worked in the 
"as-cast" form. An ingot having improved surface quality can be 
successfully hot-worked without the necessity of any surface preparation 
of the ingot after casting. Another objective of electroslag remelting 
methods is to produce an ingot having an internal structure which is free 
of entrapped slag, voids, macrosegregation and other defects which could 
make the ingot unsuitable for certain applications. 
It has been taught in the prior art that varying the composition of the 
electroslag remelting flux could affect the surface condition and internal 
quality of an ingot melted therethrough. For example U.S. Pat. No. 
3,857,702 teaches an electroslag remelting flux composition containing 
alumina, a fluoride and an alkaline earth metal oxide. Also, British Pat. 
No. 1,175,453 teaches the use of an electroslag remelting slag composition 
comprising a mixture of oxygen-free halides of alkali and alkaline earth 
metals for producing high quality non-ferrous ingots. Also, U.S. Pat. No. 
3,879,192 requires that an electroslag remelting flux composition contains 
at least 0.5% of at least one metallic element selected from the group 
consisting of metallic calcium, strontium, magnesium and barium, the 
remainder of the slag is composed of calcium fluoride, strontium fluoride, 
magnesium fluoride, barium fluoride or mixtures thereof. 
It is also taught in U.S. Pat. No. 4,161,398 that an ingot having improved 
surface condition and internal quality can be produced by electroslag 
remelting of a metal electrode produced under a slag consisting of the 
fluorides of barium and calcium only. It has been found that such slag may 
not have sufficient chemical capacity to dissolve certain oxides 
introduced into the system during certain electroslag melting processes. 
Such undissolved oxides may float on the top of the molten slag bath 
during remelting, and may cling to the mold wall producing an undesirable 
effect on both ingot surface and heat transfer from the ingot to the mold 
wall. 
Accordingly, an improved method for electroslag melting is desired which 
minimizes the effect of the formation of high melting point phase oxides 
which could adversely effect the electroslag remelting process and the 
ingots produced thereby. 
The present invention may be summarized as providing a new and improved 
method for electroslag remelting an electrode. This method includes 
melting an electrode of an alloy containing at least one oxide forming 
ingredient in a novel flux composition containing about 1 to 5% boron 
oxide, by weight. 
An objective of the present invention is to provide an improved method of 
electroslag remelting of electrodes to produce remelt ingots having 
superior surface condition and significantly improved internal quality. 
Another objective of the present invention is to provide a method for 
electroslag remelting wherein the slag composition is capable of fluxing 
undissolved oxides formed during the process which could otherwise have a 
detrimental effect upon ingot surface, internal ingot quality and heat 
transfer from the remelt ingot to the mold wall during the process. 
An advantage of the present invention is that a remelt ingot can be 
produced by electroslag remelting which ingot is characterized by 
excellent surface quality in the "as-cast" condition which precludes the 
necessity for surface conditioning, such as grinding, grit blasting, and 
the like prior to further hot-working of the remelt ingot. 
Another advantage of the present invention is the production of an ingot by 
electroslag remelting which ingot exhibits an internal structure free of 
entrapped slag, voids, macrosegregation and other defects which could 
otherwise result in rejection of the material for certain applications. 
A further advantage of the present invention is to provide a novel flux 
composition which is suitable for electroslag remelting. 
These and other objectives and advantages of this invention will be more 
fully understood and appreciated with reference to the following 
description.

DETAILED DESCRIPTION 
The method of the present invention pertains to the electroslag remelting 
of electrodes containing oxide forming ingredients. Such oxide formers may 
include manganese, titanium, vanadium, tungsten, scandium, yttrium and 
rubidium. The oxides of such strong oxide forming ingredients are 
substantially fluxed by the flux composition of the present invention, and 
more particularly such fluxing is attributed to the boron oxide. For 
example, the present invention is applicable to the electroslag remelting 
of an electrode consisting essentially of about 10 to 80% copper, 5 to 40% 
nickel and up to about 80% manganese, by weight. More specifically, the 
present invention may be directed to the electroslag remelting of 
manganese base alloy electrodes, such as 72% manganese, 18% copper and 10% 
nickel. It should be understood, however, that the flux compostion of the 
present invention is useful in the electroslag remelting of all electrodes 
containing oxide forming ingredients which are fluxed by boron oxide. 
An initial step in the process of electroslag remelting of an electrode is 
to provide a flux composition. The flux composition of the present 
invention contains about 1 to 5% boron oxide, by weight. The remainder of 
the flux composition may consist of barium fluoride, calcium fluoride, 
magnesium fluoride, magnesium oxide, calcium oxide, aluminum oxide, and 
mixtures thereof. The specific amount of boron oxide added to the flux 
composition depends upon the amount of metal oxides to be dissolved into 
the slag during remelting, with consideration for the boron residual 
tolerable in the remelted ingot. For example, over about 275 ppm of boron 
in an ingot having about 70-75% manganese, 9-11% nickel, and about 17-19% 
copper may not be tolerable, because excess boron results in increased 
thermal expansion, and decreased hot workability of the ingot. 
Understandably, the amount of boron oxide in the flux composition 
increases as the amount of metal oxides to be dissolved increases. 
An exemplary flux composition of the prior art, consisting essentially of 
barium fluoride and calcium fluoride for the melting of copper-nickel 
alloy electrodes and manganese-copper-nickel alloy electrodes is disclosed 
in U.S. Pat. Nos. 4,161,398 and 4,161,399. A flux composition used in an 
electroslag remelting method of the present invention likewise may include 
barium fluoride and calcium fluoride. However, the flux composition of the 
present invention further includes about 1 to 5% boron oxide. 
In a preferred embodiment of the present invention a flux composition is 
provided by first preparing a mixture consisting essentially of about 20% 
to 80% barium fluoride, about 20% to 80% calcium fluoride, and about 1% to 
5% boron oxide, by weight. Such mixture is then fused. Certain flux 
composition may be fused at a temperature within the range of from about 
450.degree. F. below the melting point of the electrode of the alloy to be 
remelted to about 200.degree. F. above the melting point of the electrode 
to be melted in order to form such flux composition. In more preferred 
applications, the mixture is fused at a temperature within the range of 
from about 350.degree. F. below the melting point of the electrode to 
about 100.degree. F. above the melting point of the electrode. 
A preferred flux composition of the present invention may also be prepared 
by mixing the barium fluoride, calcium fluoride and boron oxide, in the 
ranges mentioned above, fusing the mixture and then cooling the fused 
material and crushing the resulting solid. Preferably, however, the fused 
material may be poured into an electroslag remelting mold as a molten 
liquid in order that the electrode of the alloy to be melted may be 
partially immersed directly into the molten flux and in order that 
resistance heating of the molten flux to the melting temperature of the 
electrode may be initiated. It should be understood, however, that the 
flux composition may be mixed directly in the electroslag remelting mold, 
perhaps around an electrode already disposed in the mold, and may be fused 
therein by passing an electric current between the mold and the electrode 
to be melted. It should also be understood that complete or partial 
additions of flux ingredients, particularly additions of boron oxide, may 
be made intermittently or continuously to the basic slag during remelting 
using feed systems and devices well known to those skilled in the art of 
electroslag remelting. It should be further understood that the boron 
oxide in the flux composition may be formed. For example, by adding boron, 
ferroboron, nickelboron, or the like, or by adding borides of compatible 
metals to the slag, boron oxide may be formed and thus added to the flux 
composition. Alternatively, purposeful additions of boron may be provided 
in the electrode being remelted, which would continuously serve as a 
source for the boron oxide which would be continuously formed as the 
electrode, containing boron, is remelted. Therefore, the provision of 
about 1% to 5% boron oxide in the flux composition of the present 
invention should not be limited by the method in which the boron oxide is 
added to, or formed in, the flux composition. 
To electroslag remelt an alloy, typically in the form of an electrode, the 
electrode is inserted into the flux composition. It should be noted that 
the electrode may be of practically any shape or form. With the electrode 
in the flux composition, sufficient heat is generated in the flux 
composition to melt the electrode. Such heat is typically generated by 
passing electric current between the mold and the electrode. The rate of 
remelting of the electrode is determined by the amount of Joule heat 
generated in the flux composition, and this, in turn, is determined by the 
amperage of the electric current passed between the mold and the 
electrode. For the production of small ingots, i.e., about four inches in 
cross-section, a typical remelting rate is from about 100 to about 250 or 
more pounds per hour. Large ingots, i.e., at least about twenty-four 
inches in cross-section, are typically prepared at a melting rate of from 
about 500 to about 4,000 pounds per hour. It should be understood, 
however, that the melting rates may vary. 
The electric current passed between the mold and the electrode may be 
either alternating current (AC) or direct current (DC). When direct 
current is employed the electrode of the alloy to be remelted may serve as 
either the cathode or the anode, and the base plate of the mold may serve 
as the other electrode. Therefore, either straight or reverse polarity may 
be established during electroslag remelting with direct current. It should 
also be understood that the remelting of an electrode may be performed 
with the flux being in contact with air, or under a protective atmosphere. 
Such protective atmosphere may include, but is not limited to inert gases 
such as argon, helium, nitrogen or mixtures thereof. 
During electroslag remelting by the process of the present invention molten 
droplets of the alloy of the electrode being remelted pass from the 
electrode through the flux composition. As the process continues, it is 
understandable that the slag advances upward in the mold on the surface of 
the remelted molten metal. The electrode being remelted may be maintained 
in the molten flux composition throughout the process. The remelted metal 
may be collected in the mold and cooled to a temperature below its melting 
point, preferably by progressive solidification, thereby forming an 
electroslag remelted ingot. The ingot formed by electroslag remelting, in 
accordance with the present invention, may be of a variety of shapes and 
forms. 
During electroslag remelting, oxides may be present in or on the electrode 
to be remelted. Additionally, oxides may be formed on the electrode 
surface due to the exposure of the heated portions of the electrode to air 
during remelting. The presence of from about 1% to about 5% boron oxide in 
the flux composition of the present invention results in the fluxing of 
such oxides during the electroslag remelting process. For example, in the 
electroslag remelting of a manganese base alloy electrode, such as 72% 
manganese, 18% copper and 10% nickel, manganese oxides may be introduced 
into the system during electroslag remelting. More particularly, high 
melting point phase of manganese oxides may be formed which do not 
dissolve in the base slag, and may float on the top of the molten slag in 
the electroslag remelting mold. Such high melting point phase oxides may 
also cling to the mold wall and produce an undesirable effect on the 
surface of the remelted ingot being produced. Furthermore, the presence of 
such oxides on the mold wall, typically in thick flux layers, may 
adversely affect heat transfer from the ingot to the crucible wall and 
could, therefore, have a detrimental effect on the internal quality of the 
ingot being produced. For the electroslag remelting of manganese base 
alloy electrodes in a flux composition of the present invention the flux 
product, MnO-B.sub.2 O.sub.3, is liquid at the electroslag remelting 
temperatures and is easily dissolved into a molten flux composition 
consisting of boron oxide, barium fluoride and calcium fluoride. 
Therefore, the use of boron oxide in the flux composition of the present 
invention dissolves such oxides preventing their adherence to mold walls 
and therefore results in a significant improvement in the surface quality 
and internal structure of the remelted ingot. 
In the absence of boron oxide in the flux composition of the present 
invention, the formed oxides, such as manganese oxides, remain solid and 
do not completely dissolve into barium fluoride/calcium fluoride slag. It 
is understandable that any oxides present or formed on the surface of the 
electrode of the alloy to be remelted would mechanically drop into the 
slag during electroslag remelting and float thereon. Such floating oxide 
bodies could further deteriorate the surface condition of the remelted 
ingot by adhering to or being included in the top end of the electroslag 
remelted ingot at the end of the electroslag remelting process. Such 
inclusions at the top end of the electroslag remelted ingot could 
interfere with heat transfer during hot-topping of the ingot and could 
lead to a large pipe cavity and hence poor yields. 
EXAMPLE I 
A manganese-copper-nickel alloy electrode was prepared in a vacuum 
induction furnace by methods known to those skilled in the art. Such 
electrode had the following compositional specification: 
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ELEMENT SPEC. 
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carbon .10 max. 
manganese 71.0-73.0 
phosphorus -- 
sulphur .025 max. 
silicon .10 max. 
nickel 9.0-11.0 
copper 17.0-19.0 
iron .70 max. 
chromium .10 max. 
cobalt .10 max. 
aluminum -- 
nitrogen -- 
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A slag composition was prepared by fusion of 70 weight % barium fluoride 
plus 30 weight % calcium fluoride slag and reagent anhydrous boron oxide 
in a graphite crucible to produce a slag having a nominal 66.7% barium 
fluoride, 28.6% calcium fluoride, and 4.7% boron oxide composition. 
Precautions were taken to keep the boron oxide dry and free of moisture 
before adding the slag. This slag was solidified, crushed and then used 
for electroslag melting of the above identified manganese-copper-nickel 
alloy electrodes. During melting of the slag the temperature thereof is 
preferably controlled to less than about 2200.degree. F. to prevent 
vaporization and/or decomposition of the slag components, and possible 
reactions with crucible materials, typically graphite. The electrode was 
remelted in the flux composition using direct current reverse polarity 
melting with about 2,000 amperes. A reproduction of a photograph showing 
the internal structure and external surface condition of an ingot of this 
alloy electroslag remelted in the flux composition described above is 
shown in FIG. 1. It can be readily observed from FIG. 1 that the use of a 
barium fluoride, calcium fluoride, boron oxide slag in accordance with the 
electroslag remelting process of the present invention leads to improved 
surface condition, as well as improved internal and end quality for 
electroslag remelted ingots. 
EXAMPLE II 
A manganese-copper-nickel alloy electrode having the same compositional 
specification as the electrode set forth for Example I was remelted in a 
flux composition. The flux composition of Example II consisted of 70 
weight % barium fluoride and 30 weight % calcium fluoride. No boron oxide 
was used in the flux composition of Example II in order to provide an 
ingot which could be readily compared to that produced by the process set 
forth in Example I. A reproduction of a photograph showing the internal 
structure an external surface condition of the manganese-copper-nickel 
alloy ingot electroslag remelted using this flux composition, without 
boron oxide, is shown in FIG. 2. It should be noted that the internal 
structure and external surface condition of the electroslag remelted ingot 
shown in FIG. 2 is considered an improvement over certain ingots produced 
with various other flux compositions. However, it can be readily 
appreciated that the remelt ingot produced in accordance with the process 
of the present invention results in significant improvements in external 
surface conditions and internal structure. 
Although the electrodes shown in the drawing are generally cylindrical, the 
present invention is applicable to the melting and forming of electrodes 
and ingots that are square, rectangular and hollow, or of various other 
configurations such as slabs, plates and blooms. 
Additional manganese-copper-nickel alloy electrodes were remelted under the 
conditions set forth for the above examples, except that the flux 
compositions were as follows: 
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EXAMPLE III EXAMPLE IV 
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barium fluoride 
69.3% 68.3% 
calcium fluoride 
29.7% 29.3% 
boron oxide 1.0% 2.4% 
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The surface condition and internal quality of the "as-cast" electroslag 
remelted ingots of Examples III and IV remelted in the slag compositions 
set forth above, showed marked improvement over ingots remelted in the 
slag composition of Example II which did not contain boron oxide. 
Whereas the particular embodiments of this invention have been described 
for the purposes of illustration, it will be apparent to those skilled in 
the art that numerous variations of the details may be made without 
departing from the invention.