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
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.