Flux Covering Ingredients and their Functions

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

Function

Ingredients

1.

Fluxing agents

Silica, CaO, Flourspar.

2.

Slag formers

Rutile, Titania, Potassium titanate, limenite, Asbestos, Alumina, Silica flour, Iron oxide, Calcium fluoride (Flourspar) Feldspar, Manganese dioxide, Wollastonite.

3.

Arc stabilisers

Potassium oxalate, Potassium silicate, Zirconium car­bonate, Potash, Feldspar, Lithium carbonate, Titania.

4.

Gas forming materials

Cellulose, Limestone, Woodflour, Calcium carbonates, other carbonates.

5.

Alloying

Ferro-manganese, Ferro-chrome, Ferromolybdenum, Electronickel, Ferro-titanium, Metal powders.

6.

Deoxidisers

Ferrosilicon, Ferromanganese.

7.

Binders

Sodium silicate, Dextrin, Potassium silicate, Gum arabic, Sugar, Asbestos.

8.

Slipping agents (for easy extrusion)

Glycerine, China clay, Kaolin clay, Talc, Bentonite clay, Mica.

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:

— Cellulosic,

— Rutile,

— Oxidising Iron-oxide and

— Basic

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

Classification

Coating Ingredients

Gas shield

Gas content of weld deposite ml/00 g

Applications

S. No.

Type

AWS/ASTM

Diffusible*

hydrogen

Residual

hydrogen

1.

Cellulosic

E—6010

Typically 40% cellulose 25% TiO2 ; 20% MgSiO3 ; 15% Fe-Mn bonded with sodium or potassium silicate.

Approximately 40% H2 : 40% CO + CO2 and 20% H2O

15—30

1—5

General purpose elec­trode for carbon steel. Most commonly used type in U. S.A. Pipe welds. More heavily coated rods are used for deep penetration. Most heavily coated arc cutting electrodes.

2.

Rutile

E—6012 and E—6013

Typically 4% cellulose 50% TiO2 ; 10% CaCO3 ; 6%

SiO2 ; 20% Mica ; 10% Fe-Mn bonded with sodium or potassium silicate.

10—30

0.5—4.0

General purpose weld­ing of carbon steel ; most generally used type in U. K. and other countries.

3.

Iron oxide (Deoxidized)

E—6020

Oxides and carbonate of iron and manganese with mineral silicates and ferro­manganese.

10—20

0.5—4.0

Give sound deposit with satis factory mechanical properties. Easy slag removal and good appea­rance of weld bead. Declining use.

4.

Basic low hydrogen

E—7015 and E—7016

Typically 60% CaCO3 ; 30% CaF2 ; 2.5% Fe-Mn ; 4% Fe-Si ; 2.5% Fe-Ti bonded with sodium or potassium silicate.

Approximately 80% CO and 20% CO2

0.5—7.5 (dried immediately before use at 150°C)

0.0—2.0

Lowest hydrogen content. Good notch-ductility. Used for carbon steel where notch-ductility must be optimum: critical ship structures and sub-zero temperature applications. Low alloy steel electrodes: stain­less steel 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

Core-wire

diameter

mm

Lengths of electrode

Welding Current (Amperes)

Light work

Normal work

Heavy work

2.5

250/300/350

55

70

85

3.2

350/450

90

110

130

4.0

350/450

140

165

180

5.0

350/450

180

210

240

6.0

350/450

200

255

315

6.3

350/450

220

260

320

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