DEVELOPMENT OF COATED ELECTRODES
- Can welding be done by bare wire?
- Why do we require coating at all?
- What are the different types of coating?
- What are the stages in the development of these coatings?
It was in 1801 when electric arc was first discovered by Sir Humphry Davy while conducting experiments in electricity. In the year 1889, Slavian off, made an attempt to weld with bare wires. But, welding with this gave rise to many difficulties like instability of arc, lack of shielding of molten metal which resulted in poor mechanical properties. In 1920, for the first time coated electrodes were developed. It was found that with a mixture of minerals covering the wire, virtually every aspect of the process could be controlled and improved. The first coated electrodes were called as iron oxide coated electrodes which contained iron oxide and natural silicates in the coating. The weld deposit with this electrode gave poor physical properties because it contained large quantities of gases like Hydrogen, Oxygen and Nitrogen. The coating was further improved upon by adding deoxidisers and denitriders in addition to iron oxide and silicates. These electrodes were known as acid coated electrodes.
But, weld deposit by these electrodes also had substantial quantity of gases and most of the useful elements like carbon and Mn were lost in transfer. The coatings employed in those early years of development were thin and their function was more of an arc stabilizer. They did very little by way of purifying or strengthening the weld. The weld was not better than the weld deposits obtained with bare wire electrodes.
Further, research in improving coatings gave rise to rutile coated electrodes which have better control in positional welding. In these electrodes the amount of oxygen and hydrogen which was entrapped impaired the physical properties of the weld metal. The rutile coated electrodes had limited use (upto 15 mm thickness of mild steel).The increased use of higher thickness and steels of high tensile properties limited the use of futile electrodes. The main source of hydrogen supply to the weld metal is the water forming compounds and the organic chemicals in the coating. Hence, efforts were made in developing low hydrogen electrodes.
The oxygen, nitrogen and hydrogen content of the weld metal and iron oxide content of the slag decreases as we move down the above list from iron oxide to basic coated types . Also the fillet weld profile clings from a concave profile in case of oxide coated electrodes to convex profile in case of basic coated electrodes.