Beside national organizations, the International Standards Organization (ISO)
also issues filler metal specifications. Many of the less industrialized
nations utilize specifications of the industrialized countries or
ISO standards. The ISO standards are available from the welding or
standardization association of each country.
Filler metals can be classified into four basic categories. These are:
- Covered electrodes
- Solid (bare) electrode wire or rod
- Fabricated (tubular or cored) electrode wire
- A category of miscellaneous or others.
The covered electrodes are the most popular type of filler metal used in
arc welding. The composition of the covering on the electrode determines
the usability of the electrode, the composition of the deposited weld metal,
and the specification of the electrode. The composition of coatings on
covered arc welding electrodes has been surrounded in mystery and little
information has been published. The formulation of electrode coatings
is very complex and while it is not an exact science it is based on
well-established principles of metallurgy, chemistry, and physics,
tempered with experience.
The original purpose of the coating was to shield the arc from the oxygen
and nitrogen in the atmosphere. It was subsequently found that ionizing
agents could be added to the coating which helped to stabilize the arc
and made electrodes suitable for alternating current welding.
It was found that silicates and metal oxides helped to form slag,
which would improve the weld bead shape because of the reaction at the
surface of the weld metal. The deposited weld metal was further refined
and its quality improved by the addition of deoxidizers in the coating.
In addition, alloying elements were added to improve the strength and
provide specific weld metal deposit composition. Finally, iron powder
has been added to the coating to improve the deposition rate.
An electrode coating is designed to provide as many as possible
of the following desirable characteristics:
- Specific composition of the deposited weld metal
- Specific mechanical properties of the deposited weld metal
- Elimination of weld metal porosity
- Elimination of weld metal cracking
- Desirable weld deposit contour
- Desirable weld metal surface finish, i.e., smooth, with even edges
- Elimination of undercut adjacent to the weld
- Minimum spatter adjacent to the weld
- Ease of manipulation to control slag in all positions
- Providing a stable welding arc
- Providing penetration control, i.e. deep or shallow
- Providing for initial immediate arc striking and re-striking capabilities
- Providing a high rate of metal deposition
- Eliminate noxious odors and fumes
- Reducing the tendency of the coating to pick up moisture when in storage
- Reducing electrode overheating during use
- Providing a strong tough durable coating
- Providing easy slag removal
- Providing a coating that will ship well and store indefinitely
Some of these characteristics may be incompatible and therefore compromises
and balances must be provided and designed into the coating. Of course,
the requirements must be achieved at the minimum possible cost.
In addition, the formulation must be manufacturable with conventional
extrusion equipment at high production rates. No single electrode type
will meet all of the foregoing requirements, and there is no one single
"universal electrode". Instead there is a variety of electrode types each
having certain desirable characteristics.
The coatings of electrodes for welding mild and low alloy steels may
have from 6 to 12 ingredients such as:
- Cellulose, to provide a gaseous shield with a reducing agent. The
gas shield surrounding the arc is produced by the disintegration of
- Metal carbonates, to adjust the pH of the slag and to provide
a reducing atmosphere
- Titanium dioxide, to help form a highly fluid but quick-freezing
slag. It will also provide ionization for the arc
- Ferromanganese and ferrosilicon, to help deoxidize the molten weld
metal and to supplement the manganese content and silicon content of the
deposited weld metal
- Clays and gums, to provide elasticity for extruding the plastic
coating material and to help provide strength to the coating
- Calcium fluoride, to provide shielding gas to protect the arc,
adjust the pH of the slag, and provide fluidity and solubility of the
- Mineral silicates, to provide slag and give strength to the electrode
- Alloying metals, including nickel, molybdenum, chromium, etc., to
provide alloy content to the deposited weld metal
- Iron or manganese oxide, to adjust the fluidity and properties
of the slag. In small amounts iron oxide helps stabilize the arc
- Iron powder, to increase the productivity by providing additional
metal to be deposited in the weld
By using combinations and different amounts of these constituents it
is possible to provide an infinite variety of electrode coatings. The
binder used for most electrode coatings is sodium silicate, which is
chemically combined and harden to provide a tough, strong coating.
The design of the coating provides the proper balance to give the electrode
specific usability characteristics and to provide specific weld deposit
chemistry and properties. In general, the different electrodes that meet
a particular classification have somewhat similar compositions.
Solid electrode wires
Solid metal wires were first used for oxy fuel gas welding to add filler
metal to the joint. These wires or rods were provided in straightened
lengths approximately 1 m long. The earliest electrodes for arc welding
were also solid and bare, usually in the length of 12 to 14 in.
long (300-350 mm). Later on, solid wire was provided in coils for
"bare wire" automatic arc welding and later for submerged arc and
electro-slag welding. The latest process to use solid bare wire is
gas metal arc welding, which uses relatively small-diameter
The manufacture of wire or rod for welding electrodes is essentially
the same except that straightening and cut operation is added for a
welding rod. The drawing of steel wires and nonferrous wires is
essentially the same; however, different amounts of reduction per
drawing die, different drawing lubricants, different heat treatments, etc.,
The solid steel electrode wires may not be "bare". Many suppliers provide
a very thin copper coating on the wire. The copper coating is for several
purposes. It improves the current pickup between contact tip and the
electrode. It aids drawing and helps prevent rusting of the wire when
it is exposed to the atmosphere.
Solid electrode wires are also made of various stainless steel
analyses, aluminum alloys, nickel alloys, magnesium alloys,
titanium alloys, copper alloys, and other metals.
When the wire is cut and straightened it is called a welding rod,
which is a form of filler metal used for welding or brazing which does
not conduct the electrical current. If the wire is used in the electrical
circuit it is called a welding electrode and is defined as a component
of the welding circuit through which current is conducted. A bare
electrode is normally thought of as being a wire; however, it can take
Several different systems are used to identify the classification of a
particular electrode or welding rod. In all cases a prefix letter is used:
- Prefix R: Indicates a welding rod
- Prefix E: Indicates a welding electrode
- Prefix RB: Indicates use as either a welding rod or for brazing filler metal
- Prefix ER: Indicates either an electrode or welding rod
The system for identifying bare carbon steel electrodes and rods for
gas shielded arc welding is as follows:
- ER: Prefix indicates an electrode or welding rod
- 70: Indicates the required minimum as-welded tensile
strength in thousands of pounds per square inch (psi)
- S: Indicates solid electrode or rod
- C: Indicates composite metal cored or stranded electrode or rod
- 1: Suffix number indicates a particular analysis and usability factor
The system for identifying solid bare carbon steel for
submerged arc is as follows:
- The prefix letter E is used to indicate an electrode
- This is followed by a letter, which indicates the level of
manganese, i.e., L for low, M for medium, and H for high manganese.
- This is followed by a number, which is the average amount of
carbon in points or hundredths of a percent.
The composition of some of these wires is almost identical with some
of the wires in the gas metal arc welding specification.
Flux-cored or tubular electrodes
The outstanding performance of the flux-cored arc welding process is made
possible by the design of the cored electrode. This inside-outside electrode
consists of a metal sheath surrounding a core of fluxing and alloying
compounds. The compounds contained in the electrode perform essentially
the same functions as the coating on a covered electrode, i.e.,
deoxidizers, slag formers, arc stabilizers, alloying elements, and
may provide shielding gas.
There are three reasons why cored wires are developed to
supplement solid electrode wires of the same or similar analysis.
- There is an economic advantage. Solid wires are drawn from steel
billets of the specified analyses. These billets are not readily available
and are expensive. Also, a single billet might provide more solid electrode
wire than needed.
- Tubular wire production method provides versatility of composition
and is not limited to the analysis of available steel billets.
- Tubular electrode wires are easier for the welder to use than
solid wires of the same deposit analysis, especially for welding pipe
in the fixed position.
Mild Steel electrodes
Carbon steel electrodes are classified by the American Welding Society
specification, "Carbon steel electrodes for flux-cored-arc welding". This
specification includes electrodes having no appreciable alloy content for
welding mild and low alloy steels.
The system for identifying flux-cored electrodes follows the same
pattern as electrodes for gas metal arc welding, but is specific for
tubular electrodes. As an example, for E70T-1:
- E: Prefix indicates an electrode.
- 70: Indicates the required minimum as-welded tensile strength in thousands of
pounds per square inch (psi).
- T: Indicates tubular, fabricated, or flux-cored electrode.
- 1: Suffix number indicates the chemistry of the deposited weld
metal, gas type, and usability factor.