Cellulosic electrodes

Cellulose is the principal substance in this type of electrode and comprising

typically ~ 40% of the flux constituents.

Cellulose is an organic material (naturally occurring) such as cotton and wood,

but it is wood pulp that is the principal source of cellulose used in the

manufacture of electrode coverings.

The main characteristics of cellulosic electrodes are:

. Cellulose breaks down during welding and produces carbon monoxide and

dioxide and hydrogen.

. Hydrogen provides part of the gas shielding function and gives a relatively

high arc voltage.

. The high arc voltage gives the electrode a hard and forceful arc with good

penetration/fusion ability.

. The volume of slag formed is relatively small.

. Cellulosic electrodes cannot be baked during manufacture or before welding

because this would destroy the cellulose; the manufacturing procedure is to

harden the coating by drying (typically at 70-100ºC).

. Because of the high hydrogen levels there is always some risk of H cracking

which requires control measures such as hot-pass welding to facilitate the

rapid escape of hydrogen.

. Because of the risk of H cracking there are limits on the strength/

composition and thickness of steels on which they can be used (electrode

are manufactured in classes E60xx, E70xx, E80xx and E90xx but both lower

strength grades tend to be the most commonly used).

. High toughness at low temperatures cannot be consistently achieved from

this type of electrode (typically only down to about -20ºC).

15.2.1 Applications of cellulosic electrodes

Cellulosic electrodes have characteristics that enable them to be used for

vertical-down welding at fast travel speed but with low risk of lack-of-fusion
because of their forceful arc.

The niche application for this type of electrode is girth seam welding of large

diameter steel pipes for overland pipelines (Transco (BGAS) P2, BS 4515 and

API 1104 applications). No other type of electrode has the ability to allow root

pass welding at high speed and still give good root penetration when the root

gap is less than ideal.

Because of their penetration ability these electrodes have also found application

on oil storage tanks – for vertical and circumferential seam welding of the

upper/thinner courses for which preparations with large root faces or square

edge preparations are used.

15.3 Rutile electrodes

Rutile is a mineral that consists of about 90% titanium dioxide (TiO2) and is

present in C and C-Mn steel rutile electrodes at typically ~50%.

Characteristics of rutile electrodes are:

. They have a very smooth and stable arc and produce a relatively thin slag

covering that is easy to remove.

. They give a smooth weld profile.

. They are regarded as the most user-friendly of the various electrode types.

. They have relatively high combined moisture content and because they

contain typically up to ~10% cellulose they cannot be baked and

consequently they do not give a low H weld deposit.

. Because of the risk of cracking they are not designed for welding of high

strength or thick section steel.

. (Although electrodes are manufactured in classes E60xx, E70xx, E80xx the

E60xx grade is by far the most commonly used).

. They do not give high toughness at low temperatures (typically only down

to about -20ºC).

The above listed characteristics mean that this type of electrode is used for

general-purpose fabrication of unalloyed, low strength steels in relatively thin

sections (typically ≤ ~13mm).

15.3.1 Rutile electrode variants

By adding iron powder to the covering a range of thick-coated electrodes have

been produced in order to enhance productivity.

Such electrodes give weld deposits that weigh between ~135 and 190% of their

core wire weight and so referred to as high recovery electrodes, or more

specifically for example a 170% recovery electrode.

The weld deposit from such electrodes can be relatively large and fluid and this

restricts welding to the flat position and for standing fillets for electrodes with

the highest recovery rates.

In all other respects these electrodes have the characteristics listed for standard

rutile electrodes.
15.4 Basic electrodes

Basic electrodes are so named because the covering is made with a high

proportion of basic minerals/compounds (alkaline compounds), such as calcium

carbonate (CaCO3), magnesium carbonate (MgCO3) and calcium fluoride

(CaF2).

A fully basic electrode covering will be made up with about 60% of these basic

minerals/compounds.

Characteristics of basic electrodes are:

. The basic slag that forms when the covering melts reacts with impurities,

such as sulphur and phosphorus, and also reduces the oxygen content of

the weld metal by de-oxidation.

. The relatively clean weld metal that is deposited gives a very significant

improvement in weld metal toughness (C-Mn electrodes with Ni additions

can give good toughness down to -90°C).

. They can be baked at relatively high temperatures without any of the

compounds present in the covering being destroyed, thereby giving low

moisture content in the covering and low hydrogen levels in weld metal.

. In order to maintain the electrodes in a low hydrogen condition they need to

be protected from moisture pick-up.

 By means of baking before use (typically at ~350°C), transferring to a

holding oven (typically at ~120°C) and issued in small quantities

and/or using heated quivers (‘portable ovens’) at the work station

(typically ~70°.

 By use of vacuum packed electrodes that do not need to be re-baked

before use.

. Basic slag is relatively viscous and thick which means that electrode

manipulation requires more skill and should be used with a short arc to

minimise the risk of porosity.

. The surface profile of weld deposits from basic electrodes tends to be

convex and slag removal requires more effort.

Metal powder electrodes contain an addition of metal powder to the flux

coating to increase the maximum permissible welding current level. Thus, for a

given electrode size, the metal deposition rate and efficiency (percentage of the

metal deposited) are increased compared with an electrode containing no iron

powder in the coating. The slag is normally easily removed. Iron powder

electrodes are mainly used in the flat and H/V positions to take advantage of

the higher deposition rates. Efficiencies as high as 130-140% can be achieved

for rutile and basic electrodes without marked deterioration of the arcing

characteristics but the arc tends to be less forceful which reduces bead

penetration.

15.4.1 Applications of basic electrodes

Basic electrodes have to be used for all applications that require good fracture

toughness at temperatures below ~ -20°C.

To avoid the risk of hydrogen cracking basic electrodes have to be used for

welding hardenable steels (most C-Mn and all low alloy steels) and for most

steels when the joint thickness is greater than about 15mm.

 Cellulosic
Characteristic Electrodes Rutile Electrodes Basic Electrodes
Principal Cellulose, ~40% of Rutile, ~50% of flux
Substance flux constituents constituents Basic minerals/compounds, ~60% of covering
Alkaline compounds such as calcium
Source of Rutile mineral, carbonate, magnesium carbonate, calcium
Substance Wood pulp primarily composed of TiO2 fluoride
Material Organic, from cotton Mineral, primarily
Composition and wood titanium dioxide (TiO2) Inorganic, alkaline compounds