In this article, we will see the 4 major types of welding electrode coatings. Before we see the types, let we first understand the basic of welding electrode coatings.
What is the welding electrode coating?
Electrode Coating is covered with a relatively high quality covering applied in a layer of 1 to 3mm thick. The weight of such a coating is from 15 to 30% of the electrode weight. The greatest of welding is done with coated electrodes. This restricts the process to a slow manual operation. If the flux coating is placed inside a long tube, the electrode can be in the form of a bare wire in the form of a coil.
Then the shielded arc process can be made continuous and automatic. The primary purpose of a light coating is to increase arc stability; the coating is also called as ionizing coatings. Since the electrode coating is brittle only straight stick electrodes can be used.
The function of Electrode coatings
- Improving the arc stability by providing certain chemicals which have this ability by ionizing the path of the arc
- Provide a protective gaseous atmosphere to prevent oxygen, hydrogen, and nitrogen picks up by the molten metal.
- Protective slag over hot metal
- Provide flux, which helps to remove oxides and other impurities from the molten metals
- Reduce spatter of weld metal – when coating burns off slower than the core.
- Acts as deoxidizer
- Slow down the cooling rate of the weld (due to the protective layer of slag) to prevent hardening.
- Coatings are normally insulators of electricity and so prevent the use of electrodes in narrow grooves etc.,
Classification of electrode coating
The classification standards of welding products distinguish several types of coated electrodes according to the kind of coating. An electrode coating is always composed of many constituents to provide various functions. They are
Mineral
Mineral products which act on fusion characteristics, contribute to the protection from the surrounding atmosphere of the drops. The weld pool by breaking up into a gaseous emission under the influence of the arc heat and constituting a slag. Physico-chemical characteristics have a major influence on the operational characteristics of the electrode.
Metal
Metal products which by being combined with metal resulting from the fusion of the electrode core. This makes it possible to adjust the analysis of the weld metal to obtain properties equivalent to those of the steel used in the welded joint.
Organic materials
Organic materials added in small quantity in basic coatings as an extrusion agent. These will be destroyed during high-temperature heating of these electrodes. They are present in much larger quantities in electrodes baked at low temperature (cellulose, rutiles, etc.). The decomposition of these products in the arc causes a release of hydrogen. The released hydrogen is beneficial for operational characteristics welding.
Binders
Binders which make it possible to obtain a solid coating which adheres to the metalcore. There are in general simple or complex silicates of sodium, potassium or lithium.
4 Major Types of Welding Electrode coatings
In industry, the following 4 major types of welding electrode coatings are the most commonly used. They are
- Rutile Coating
- Basic or low H2 Coating
- Cellulosic Coating
- Iron oxide Coating
Rutile electrode
The rutile type electrodes present the best properties in use: very good arc stability, transfer of metal in fine drops which generally results in a low level of spatters and lower fume emission than the basic electrodes, a very good bead wetting and a very easy restart from cold.
However, by its nature, this slag influences the content of residual elements in the deposited metal. The elements which are in general not desirable from the perspective of optimizing mechanical properties. They are
a. Oxygen
The oxygen content of the deposited metal can vary according to nature and the quantity of the deoxidizing elements present in the coating. The oxygen content cannot be lowered to the level which can be reached with a basic electrode. This results is a most significant inclusion content and consequently in lower ductile fracture energy during impact tests.
b. Titanium
The titanium content of the deposited metal cannot be adjusted, as we would wish, to optimize the mechanical properties. Indeed, the slag being mainly composed of rutile elements (titanium oxide TiO2), some titanium is inevitably transferred to the deposited metal in variable quantities. According to the oxidoreduction reactions and the metal-slag exchanges which occur in the arc and the interface with the weld pool. These reactions depend on all the chemical elements present, which must be balanced according to the various mechanical characteristics that the weld must meet (tensile strength, yield strength) and depend on the type of steel that we have to weld.
c. Niobium and Vanadium
The niobium and vanadium content of the deposited metal cannot be lowered beyond a certain point. Because these elements exist as impurities in the natural rutiles used in the manufacture of welding products. The use of synthetic rutiles, which are therefore very pure, is possible but not common. Because its cost is significantly higher than that of the natural rutiles.
d. Diffusible Hydrogen
The diffusible hydrogen content of welds created with rutile electrodes is always very high. Usually, this results from the presence of organic materials added to facilitate extrusion and to improve the arc’s characteristics. However, it is also a result of low baking temperatures which makes it possible to eliminate only a small proportion of the water incorporated with the silicate and it does not break up the extrusion agents.
Thus, rutile electrodes are valued for their user-friendliness and the creation of a weld bead, whereas basic electrodes are essential when the joints to be made must satisfy severe metallurgical quality standards.
Basic or low hydrogen Electrode Coating
The basic electrode coating is made up of calcium carbonates and fluorite. This coated electrodes must satisfy the required mechanical properties of the steels which it is intended to weld (tensile, impact strength, CTOD, creep, etc.). Many analytical combinations make it possible to obtain the tensile characteristics sought in the deposited metal, but the solutions that satisfy both the tensile and toughness characteristics are much more limited. This is increasingly true the higher the tensile properties.
Also, the chemical balance retained for an electrode must be the most robust possible, i.e. it must satisfy the various requirements despite the variations inherent in any industrial production, and that, in a broad field of welding conditions (thermal cycles). Lastly, a basic electrode must be designed so that the diffusible hydrogen content in the deposited metal is as low as possible to avoid any risk of cold cracking while minimizing or even precluding pre-heating and post-heating.
Cellulosic Electrode coating
The cellulosic electrode coating is made up of organic materials mostly cellulose. Cellulosic electrode coating is identical as rutile. But, the basic difference is the percentage of Titanium dioxide (Tio2) is less in the cellulosic coating. When cellulosic burns, it results is the evolution of a mixture of hydrogen and carbon monoxide. These gases provide a protective shield to the molten base metal.
When compared with the same thickness of the work a cellulosic coating results in deeper penetration as compared to rutile coating. Since the rate of evolution of hydrogen gas is high there is a danger of hydrogen embrittlement of the base metal.
Iron oxide electrode coating
The electrode coated with iron oxide improves arc behaviour, bead appearance; helps increase the metal deposition rate and arc travel speed. During welding evolution of hydrogen gas is less as compared with the cellulosic coated electrode. Also it as low penetration and good look weld penetration.
