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It happens frequently
that electrode users analyze the core wire of an electrode to
predict or consider the weld metal composition. This procedure
falls down completely. The analysis of the core wire of an
electrode is by no means the same as the deposit chemistry.
Many people presume
that if they are welding a specific base metal, they will have
an identical and satisfactory weld if they use an electrode
having a core wire of the same composition as the base metal. As
an example, they presume that by welding a base metal of Type SAE 4130 (chrome moly steel) with an electrode with a Type SAE
4130 steel core wire, the weld deposit will match exactly the
base metal and the weld will perform identically to the base
metal.
This is an understandable, but
completely illogical conclusion for many reasons. Following are
a few examples:
(1) The base metal is usually a
hot or cold worked material, having had grain refinement from
the working (such as rolling). The weld metal is a cast material
and thus cannot exactly resemble the base metal if the analysis
is the same, unless the electrode has additional properties to
compensate for this vast difference.
(2) Weld metals are prone to
pore-formation, which will also make a weld deposit differ from
a base metal even if the analysis of the core wire is identical.
(3) Some ingredients of the core
wire, such as chromium, are invariably lost in gaseous form into
the atmosphere during the arc transfer.
(4) Ordinary welds are prone to
contamination from many sources including:
(a) Carbon, phosphorous, and
sulphur content of the electrode or the base metal fused
into the weld deposit, which often cause interdentric
cracking in the weld deposit. These contaminants, and many
others, segregate following solidification of the weld metal
and follow the primary grain boundaries causing hot-cracks. Phosphorous also causes welds to be brittle at low
temperature.
(b) Ordinary weld deposits are
quite susceptible to oxygen contamination. Oxygen in solid
solution reduces the impact toughness and tensile strength of
steel. Welds made with other electrodes than Magna
generally contain more oxygen than do ordinary steel base
metals.
(c) Nitrogen
absorption of welds made with ordinary electrodes is a
matter of serious concern. Nitrogen in solid solution
absorbed from the atmosphere during welding lowers the
impact toughness of welds, lowers the elongation, and is
generally responsible for
"ageing", which is
a precipitate process in welds which causes impact toughness
and ductility to deteriorate to very low values. When one
considers that 78% of the air is nitrogen and that nitrogen
causes welds to be brittle, the need for prevention of
nitrogen contamination becomes obvious.
Magna has recognized that
a series of problems result from the old idea of presuming that
the same type core wire as base metal is adequate and will
supply good results for maintenance applications.
Magna research has proven
that in virtually any maintenance weldment, the electrodes must
have much higher alloy content and much higher physical
properties than the base metal.
Magna solutions
An electrode
consists of two parts: a core wire and a coating. Magna
uses high-purity core wire having generally a much higher
content of noble or semi-noble metals (such as nickel,
molybdenum, columbium, cobalt, silicon, manganese, vanadium,
chromium, and other
"super-metals") than
ordinary electrodes.
The highly researched super high
alloy Magna core wires with extra high alloy content,
stabilizing agents, highly deoxidized metals, and high purity
metals and other improvements completely change the character of
the arc. The core wires of Magna Maintenance Welding
Electrodes are carefully controlled so that metals or elements
that - in excess - can cause difficulty or possible weld
failure, such as carbon, sulphur, or phosphorous, are either
refined out or held in exceedingly low amounts. This enables
them to be stabilized by special additives which Magna
incorporates in the formulation of the electrode. Nothing has
been left to chance.
Magna
conducts continuous extensive research in electrode coating
chemistry and electrode coating technology. Magna employs
leading scientists and many highly qualified chemists and
technicians who perform studies in electrode coating technology.
Among the reasons for Magna Maintenance Welding
superiority is the advanced state of Magna'
s
Maintenance Electrode coating technology. It is believed that
the coatings of Magna electrodes are the most advanced in
the world with respect to maintenance applications. Magna
electrode coatings contribute to maintenance weld quality in
many special ways, including:
- Magna's
unique coatings deoxidize the weld metal. Oxygen contamination
is a major cause of weld failure. Magna electrodes
contain special deoxidizers which completely remove most
oxygen and reduce the balance to exceedingly finely dispersed
inclusions. The deoxidizer system is of a proprietary and
special nature not universally available.
- Magna coatings
actually produce a super shielding gas to protect the molten
weld metal. This gas envelope produced by the melting of the
coatings is especially designed to prevent the weld from being
contaminated by nitrogen, oxygen, hydrogen and other harmful
elements that often cause failure in ordinary electrode
deposits.
- Pore-resistant coatings.
Magna electrode coatings contain scavengers, cleansers,
degreasers, and have an ability to absorb foreign matter,
dirt, contamination, and impurities, float them away, and hold
them in the slag for easy removal. This special feature
enables Magna maintenance welds to be made without the
porosity that is common with ordinary electrodes.
- Magna
Maintenance Electrodes provide a slag layer around the molten
metal globules during transfer, and then form a protective
chemical slag blanket over the complete weld deposit. With
most electrodes, the slag is usually little more than a
residue of the electric welding process. Magna
Maintenance Electrodes have a completely different type
coating which forms a protective blanket that not only
provides a resistance to oxidation and other contamination but
emphatically retards the cooling rate. A
"Widmanstatten"
structure occurs when ordinary electrodes are used which allow
the weld to cool too rapidly. The Widmanstatten structure
caused by rapid cooling with ordinary production electrodes is
harmful. Rapid cooling causes the ferrite to form needle-like
plates which are transverse to the pearlite.
The Magna slag blanket
holds the heat and retards the cooling to permit the complete
precipitation of the ferrite in the grain boundaries in such a
way that the ferrite surrounds the pearlite grains. The Magna
protective slag blanket effectively retards the cooling rate and
promotes a more refined and more desirable grain structure.
- Hydrogen gas
inclusion (commonly referred to as
"fish-eyes") is a
major problem in maintenance welding. Hydrogen's main threat
to welding comes from the chemically combined water which is
present in the coatings of many production welding electrodes. This water decomposes into hydrogen and oxygen in the arc
transfer process. Iron has a high solubility for hydrogen even
at moderate temperatures, so considerable amounts of hydrogen
enter weld deposits. The hydrogen which enters the weld when
production welding electrodes are used can be completely
removed by heating the weld to 482 oF (250 oC)
and holding the part at this heat for 15 hours.
This procedure can be carried
out in production factories as another step in manufacture.
However, it is totally impractical in maintenance welding. This
is why the Magna Research Department has given
consideration to the problem of hydrogen inclusion in
maintenance welds.
It has repeatedly been
demonstrated that hydrogen contamination of welds cause cracking
and underbead cracking (this is a type of cracking in the heat
affected zone adjacent to or under the weld, caused by the
hydrogen contamination during welding). Hydrogenous welds cause
a pronounced reduction in ductility and elongation and are crack
sensitive.
Magna has built into the
special coatings a resistance to hydrogen transfer across the
arc. Electrodes such as
Magna 305,
Magna 303 and many others are based on all
mineral coatings with special additives that tend to repel
hydrogen. These coatings, in manufacture, are baked at high
temperatures to remove even the last traces of hydrogen. These
special coatings are another reason Magna electrodes
result in more reliable maintenance welds.
Magna coatings are not
mere simple cellulose or rutile formulations. They contain many
supplements and special features. Some of these are:
(1) Higher purity, higher
quality binders.
(2) Higher purity, higher
quality chemicals. There are many grades of chemicals available
to electrode manufacturers including the lower quality technical
grades, U.S. pure, Pharmaceutical grades, etc. Magna
quality requires unusually high grades of chemicals.
(3) Magna coatings are
produced with special mixing equipment, using a variety of
mixers to attain different results with different chemicals. The
particle size of chemicals is carefully studied. The mixing of
the coatings is carefully monitored so that every batch is
identical.
(4) Magna introduces many
additional metals such as strontium, sodium, aluminum,
graphite, as well as stabilizing compounds and various other
additives such as fluorides, carbonates and calcium, through the
unique coatings to improve both maintenance weld quality and
weld ability.
(5) Magna upgrades the
quality of the deposit by adding finely ground metal to the
coating. Such metals as molybdenum, chromium, cobalt, nickel and
many others enrich the weld deposit.
(6) The
concentricity of all Magna Maintenance Welding Electrodes
is controlled with such surgical preciseness that the maximum
core-plus-one-covering dimension by more than 5 per cent of the
minimum core-plus-one-covering dimension. This precise
concentricity control prevents
"finger-nailing",
uneven burn-off, erratic performance and spatter which occurs
with so many welding electrodes because of poor concentricity.
(7) Magna employs
carefully controlled amounts of ferrite formers in the coatings
in order to enable the Magna deposits to resist
hot-cracking. Magna electrode coatings are highly
sophisticated coatings, many containing more than 20
ingredients. They are the result of specific research to design
coatings especially engineered for the special problems of
maintenance welding. It is believed that they represent the
highest state of the art today for the purpose for which they
have been designed. They supply weld deposit additions that
provide increased physical properties and increased resistance
to cracking or costly weld failures. The coatings are so rich in
extra metals and supplements that the final alloying process is
actually only completely finished at the tip of the electrode. |
