A cellulosic electrode covering consists of naturally
occurring organic compounds, minerals and metallic oxides where a cellulose (wood
powder and wood pulp) is the primary constituent, normally 30-40% of coating. In
their formation, the mixture of organic materials and metal oxides are bonded
on the core wire with the help of binders. These coatings on melting, gives
sound weld protection along with the formation of slag. Besides some limitation
on their usage, there are some factors that make them exceptional in intricate applications.
Described below are the key attributes of a cellulosic electrode.
1. Sound Gas Shielding
On melting, the organic material decomposes into gases; hydrogen,
carbon mono oxide, carbon dioxide that provide gaseous protection to the weldment
against atmospheric pollution.
2. High Arc Voltage
Apart from gas shield benefit, hydrogen also contributes to
make arc voltage up thus producing completely fused weld output with good
penetration.
3. Arc Characteristics
These electrodes produce highly stable and powerful arc which gives good fusion and
penetration results but on the other hand spattering may also occur. Well the
weld spatter is associated with coating thickness. Thinner the coating more
will be the spatter. Correct adjustment of current value for the particular
thickness is a bit complicated part in order to avoid severe spattering.
4. Electrode Polarity
This type produces quality results when apply with direct current reverse polarity (DCRP).
5 Ease of removing slag
Crumbly slag is produced in a very small quantity that can
be easily breakable. Presence of rutile in electrode coating assists to make
slag removable.
6. Weldable in Vertical Down Position
Ability of slag to turn into solid state quickly, make
cellulosic electrodes highly exceptional in vertical down welding. In this case
weld run is deposited with a bit faster travel and the question arises here
that it may create fusion problems. But as it stated earlier, powerful arc gives
good fusion results and hence making the cellulosic electrodes weldable also in vertical
down position. Moreover it is better to say that they can be applied in all weld
positions.
7. Best for root pass
These electrodes are best for root pass at faster rate and
quality weld outcome can be achieved as compared to other electrodes. These
electrodes attribute adequate penetration to the root section and that is the
primary reason why they have extensive use in many applications.
8. Formation of Hydrogen
The electrode coating has moisture content which on heating,
gives off hydrogen. As described above hydrogen assists in voltage enlargement
and maintenance of arc. That is why these type of electrodes are not baked as
moisture content can be lost. On the other hand hydrogen formation may cause hydrogen cracking due to which they cannot be applied on materials that are
susceptible to hydrogen induced cracking. Before applying cellulosic electrodes
following points must be considered in order to avoid cracking.
- Material type
- Material Composition
- Material thickness
- Material Resistance to hydrogen cracking
Last comments
Besides useful features, there are some limitations that
make cellulosic electrodes unfit for some applications. They don’t provide
adequate and uniform toughness in cryogenic applications. Due to cold cracking
they cannot be used in those applications where high deposition rate and
complete penetration is required. Also in that case opportunity for vertical
down welding using the cellulosic electrodes, cannot be availed.