Filler Metals for Welding: Part Two

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:

1. Covered electrodes
2. Solid (bare) electrode wire or rod
3. Fabricated (tubular or cored) electrode wire
4. A category of miscellaneous or others.

Covered electrodes

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:

1. Specific composition of the deposited weld metal
2. Specific mechanical properties of the deposited weld metal
3. Elimination of weld metal porosity
4. Elimination of weld metal cracking
5. Desirable weld deposit contour
6. Desirable weld metal surface finish, i.e., smooth, with even edges
7. Elimination of undercut adjacent to the weld
8. Minimum spatter adjacent to the weld
9. Ease of manipulation to control slag in all positions
10. Providing a stable welding arc
11. Providing penetration control, i.e. deep or shallow
12. Providing for initial immediate arc striking and re-striking capabilities
13. Providing a high rate of metal deposition
14. Eliminate noxious odors and fumes
15. Reducing the tendency of the coating to pick up moisture when in storage
16. Reducing electrode overheating during use
17. Providing a strong tough durable coating
18. Providing easy slag removal
19. 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 cellulose
• 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 metal oxides
• Mineral silicates, to provide slag and give strength to the electrode covering
• 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 electrode wires.

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., are involved.

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 other forms.

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.

1. 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.
2. Tubular wire production method provides versatility of composition and is not limited to the analysis of available steel billets.
3. 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.