Depending upon the welding situational requirements a number of chemical compounds are used in formulating a flux. In Table 4.1 these compounds are listed with their major functions and secondary benefits for the welding of steels. The electrode flux covering performs the following functions:
1. Provide a protecting atmosphere
2. Forms slag of suitable characteristics to protect molten metal from oxidation
3. Facilitate over head and position welding
4. Stabilise the arc
5. Add alloying elements to the weld metal
6. Refine the metallurgical structure
7. Reduce weld spatter
8. Increase deposition efficiency
9. Remove oxides and impurities
10. Determine the depth of arc penetration
11. Affect weld-bead shape
12. Slow down the weld cooling rate
13. Contributes weld metal from powdered metal in the coating.
Table 4.1 Electrode Covering Ingredients with Functions
Modern coated electrodes were first developed by Oscar Kjellberg of Sweden in 1907. Since that time considerable research has been done on electrode coating to obtain:
— good tensile and impact properties matching the base metal.
— most satisfactory electrode running characteristics.
— low cost formulation.
All this research has led to the development of a few standard covering types which have been coded and classified in the international specifications for electrodes as follows:
— Oxidising Iron-oxide and
Table 4.2 compares the characteristics of these electrodes.
Cellulosic coverings. These coatings contain large quantities of organic materials. Cellulose exceeds 30% by weight. Other organic materials like wood flour, charcoal, cotton, starches and gums are also used to partially replace cellulose. It produces gaseous atmosphere of approximately the following composition,
55% CO, + 42% H2 + 1.5% H2O + 1.0% CO2
The presence of hydrogen increases the voltage across the arc column making it more penetrating. For a given current cellulosic electrodes give 70% more deeper penetration than other electrodes. As most of the covering decomposes, the slag layer formed is thin and is easily removed. Hydrogen content of the weld is high. It is not recommended for welding high
Table 4.2. Characteristics of different types of electrodes
*Electrodes giving upto 10 ml diffusible hydrogen per 100 gm deposited metal are called hydrogen controlled eletrodes.
Shielded Metal Arc (SMA) Welding 75
strength steels. Because the coating does not contain much of ionisation compounds, they work well on d. c. To make them suitable for working on a. c. potassium, silicate is added to the coating.
Rutile coverings. Here the main ingredient is titanium-oxide. This compound is a good slag former and arc stabiliser. These electrodes are general purpose. By varying the amount of fluxing agents, viscosity and surface tension can be adjusted to give electrodes either for flat position only or for all position welding. Mechanical properties are adequate. Flux requires combined moisture to retain binding strength. The moisture, if excessively driven off, binding of the flux will suffer. It is retained and, therefore, hydrogen content of the weld deposit is high (25—30 ml/100 g.). This is higher than the quantity allowable (10 ml/100 g) for high strength steel welds.
Oxidising type covering. This covering contains mainly iron-oxide and silicates with or without manganese oxides. During welding it forms heavy solid slag with oxidising properties giving rise to welds which are low in carbon and manganese. The resultant deposit is soft and low in strength. Its use is limited to sheet metal fabrication.
Basic coverings. These coverings contain calcium carbonate and calcium fluoride (fluorspar) as bonding agents, and deoxidants. This results in a basic slag which is fairly fluid. The solidified slag is heavy, friable glassy brown. They are mainly used for welding high strength steels. Use of compounds containing combined moisture is avoided. They are baked at 400- 450°C temperature which is high enough to drive-off nearly all the combined moisture. With the arc heat calcium carbonate forms carbon-dioxide and carbon monoxide gases. The gas evolution rate is substantially lower. It is, therefore, necessary to maintain a short arc to avoid oxygen and nitrogen contamination.
The arc characteristics can be modified by using easily ionisable metals in the coating. The use of potassium silicate as a binder instead of sodium silicate makes the electrode suitable for a. c. welding also. But for high quality welding d. c. is preferred.
Flux covering thickness. This varies with each class and brand of electrode, and is usually expressed as coating factor, which is the ratio of coating diameter to the core wire diameter (see Fig 4.4)
Fig. 4.4 SMAW electrode
These electrodes are often classified as light coated, medium coated and heavy coated depending on their coating factor as given below Light coated 1.2 — 1.35
Medium coated 1.4 — 1.70
Heavy coated 1.8 — 2.20
As the coating thickness increases the weldpool becomes deeper and narrower, and the electrode is said to have deep penetration characteristics. Electrodes with very thick coatings are used for cutting metals.
Alloying elements and iron powder. Subtantial amounts of alloying elements are sometimes added to the coating so as to obtain a desired composition of the weld deposit. Iron powders can be added to the coatings in amounts from 10—50% of the coating weight to increase weld deposition rates.
4.3.3 Current Ranges for SMAW Electrodes
These ranges are given in Table 4.3.
Table 4.3. Current ranges for SMAW electrodes