Covered Electrodes

In addition to establishing the arc and delivering filler metal to the weld, the electrode intro-duces other materials into and around the arc and weld pool through its covering. The main purpose of using a flux covering is to protect the molten metal from atmospheric contamina-tion, the flux performs the following functions leading to the formation of a successful weld.

– weld-metal protection

– arc-stabilisation

– provides scavengers, de-oxidants, and fluxing agents to cleanse the weld and prevent excessive grain growth in the weld deposit.

– provides a slag blanket to protect hot metal from air, enhance mechanical strength, bead profile and surface cleanliness of weld bead.

– coating melts slower than the core wire, forming a cup the electrode end which addi-tionally protects droplets of molten metal and makes touch welding possible and spatter loss is reduced.

– provides a means of adding alloying elements to enhance weld metal properties or adding iron powder to increase deposition efficiency.

In the following paragraph these factors will be briefly discussed.

4.3.1 Weld-Metal Protection

• Flux melts with the core wire and covers the surface of the molten metal drops and the weld-pool (see Fig. 4.3), excluding oxygen and nitrogen to come in their contact.

As the pool progressively solidifies, the flux forms a slag blanket over the weld-bead and continues to protect it from oxidation till it cools to room temperature.

Molten-metal drop Slag-blanket

Weld-bead

Molten flux layer covers the molten drop of metal

Base plate

Fig. 4.3 Molten flux covers molten metal droplet and forms a slag blanket over the weld bead excluding oxygen and nitrogen to come in their contact

• The flux must also be completely detachable. This is very important especially when multiple layers are to be deposited. Ideally we require a slag which automatically detaches itself off the weld deposit. This requirement is difficult to reconcile with the need to adhere to the weld-metal during the cooling period. Slag detachability is also influenced by compounds added to the flux to achieve other objectives. A compromise

between the antagonistic effects of the compounds added to achieve different objectives is the only solution.

• Additional protection from atmospheric oxygen and nitrogen contamination is pro-vided by adding compounds in the coating which decompose by the heat of the arc and form an additional gaseous shield around the arc and weld-pool. They may be carbonates (giving carbon dioxide) or cellulose (giving hydrogen and carbon monox-ide).

4.3.2 Arc Stability

• There are two major aspects of arc stability. It is the ease of initiating and maintain-ing an electric arc durmaintain-ing weldmaintain-ing, and reignitmaintain-ing the arc durmaintain-ing each half cycle in a.c.

welding. For this to occur the gases in the arc gap must ionise rapidly and at lowest possible potential. Additions of titanium oxide, potassium silicate, calcium carbonate facilitate arc stabilisation. This is in addition to their normal purpose of acting as a flux.

Thus arc stability depends upon:

– O.C.V. of power source

– Transient voltage recovery characteristics of the power source

– Size of molten drops of filler metal and slag in the arc

– Arc path ionisation

– Electrode manipulation

A stable arc is also the one which is maintained straight along the electrode, axis and does not waver to find the shortest path especially on the sides of a vee edge preparation during welding in a groove, i.e. it must stay firmly fixed in the direction dictated by the welder.

4.3.3 Control of Weld-Metal Composition

This is one of the advantages of SMAW that it permits the control of weld metal composition by adding alloying elements to the flux covering. From a given combination of flux and weld-metal compositions, the alloying elements are distributed between the two in more-or-less the same proportion. If the flux or slag is low in, say, manganese, this metal transfers from the weld to the slag until the correct proportion is reached. Thus elements can be added to or taken from the weld deposit simply by altering the flux composition. The amounts of alloying elements to be added to produce a particular weld-metal composition can be calculated by the electrode manufacturer. In general, there are three major factors that control weld-metal com-position. These are: alloying, deoxidation, and contamination control.

Alloying. When the core wire used has the same composition as desired in the weld, we need not add any alloying elements, except to ensure that the elements are not lost during welding. The electrodes used with low carbon, carbon-manganese, and low alloy steels, alloyed core wires turn out to be expensive. Alloying is to be done in the weld pool. Thus low carbon steel core wires could be used and manganese, chromium, molybdenum, etc. could be added through the flux. This helps in producing a large variety of electrodes with the same core wire, especially when small quantities of specific composition are needed.

Deoxidation. During the welding of steel, if the molten weld-metal pool contains ex-cessive oxygen, it gives rise to the formation of carbon monoxide bubbles which get trapped in the solidifying weld metal to form porosity:

FeO + C = Fe + CO

This also causes loss of carbon which reduces the strength of the weld. This reaction can be supressed by adding deoxidants in the coating. A commonly used deoxidant for steel is silicon (added to the coating as ferro-silicon). Oxygen reacts with silicon in preference to steel as follows:

2FeO + Si = 2Fe + SiO₂

Silicon oxide formed floats to the weld-pool surface and forms slag. For welding copper the deoxidant used could be phosphorus or zinc to remove the oxygen and could be added to the filler metal and not to flux.

Contamination. The most harmful contaminant entering the molten weld-pool through the flux is hydrogen which leads to the formation of hydrogen cracks. Hydrogen is present in the electrode flux covering both as combined and absorbed moisture. Absorbed moisture can be removed by drying the electrodes before welding. The extent of chemically combined mois-ture depend upon the compounds used in the coating. Hydrogen has very high solubility in iron at elevated temperature. As the metal solidifies the solubility goes down and hydrogen bubbles are formed and are entrapped. As the metal cools and contracts, the pressure in the bubble exceeds the metal strength at that temperature forming cracks. Oxidising iron-oxide electrodes have been found to give beneficial results in solving the problem of hydrogen crack-ing.

• Other contaminants could be due to careless handling of the electrodes. Grease, oil, damped sulphurous fumes absorbed from the surroundings etc. may be transferred to the weld pool and cause harm. Careful handling of electrodes is, therefore, neces-sary.

4.3.4 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 car-bonate, Potash, Feldspar, Lithium carcar-bonate, 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 Glycerine, China clay, Kaolin clay, Talc, Bentonite clay, (for easy extrusion) 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

 Metal Arc (SMA) Welding

75

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

Coating Ingredients Gas shield

Gas content of weld deposite ml/00 g

Applications

S.No. Type AWS/ASTM Diffusible* Residual

hydrogen hydrogen

1. Cellulosic E–6010 Typically 40% cellulose 25% 15–30 1–5 General purpose

elec-TiO₂ ; 20% MgSiO₃ ; 15% trode for carbon steel.

Fe-Mn bonded with sodium Most commonly used

or potassium silicate. 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 Typically 4% cellulose 50% 10–30 0.5–4.0 General purpose

weld-E–6013 TiO₂ ; 10% CaCO₃ ; 6% ing of carbon steel ;

SiO₂ ; 20% Mica ; 10% most generally used

Fe-Mn bonded with sodium type in U.K. and other

or potassium silicate. countries.

3. Iron oxide E–6020 Oxides and carbonate of 10–20 0.5–4.0 Give sound deposit with

(Deoxidized) iron and manganese with satis factory mechanical

mineral silicates and ferro- properties. Easy slag

manganese. removal and good

appea-rance of weld bead.

Declining use.

4. Basic low E–7015 and Typically 60% CaCO₃ ; 30% Approximately 0.5–7.5 (dried 0.0–2.0 Lowest hydrogen content.

hydrogen E–7016 CaF₂ ; 2.5% Fe-Mn ; 4% 80% CO and immediately Good notch-ductility.

Fe-Si ; 2.5% Fe-Ti bonded 20% CO₂ before use at Used for carbon steel

with sodium or potassium 150°C) where notch-ductility

silicate. must be optimum:

critical ship structures and sub-zero temperature applications. Low alloy steel electrodes: stain-less steel electrodes.

*Electrodes giving upto 10 ml diffusible hydrogen per 100 gm deposited metal are called hydrogen controlled eletrodes.

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 proper-ties 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 suit-able 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)

C.F. = D d

d D

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 coat-ings 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 in-crease weld deposition rates.

4.3.5 Current Ranges for SMAW Electrodes These ranges are given in Table 4.3.

Table 4.3. Current ranges for SMAW electrodes Core-wire Lengths of Welding Current (Amperes)

diameter electrode Light work Normal work Heavy work

mm

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

4.3.6 Electrode Core-wire Composition

According to AWS A5.1–81, the core wire for the electrodes in this specification is usually a rimmed or capped steel having a typical composition of 0.1% C, 0.45% Mn, 0.03% S, 0.02% P, and 0.01% Si. IS : 2879-1975 recommends rimming quality steel with the following composi-tion (maximum percent) 0.1% C, 0.38–0.62% Mn, 0.03% S, 0.03% P, 0.03% Si, 0.15% Cu.

4.3.7 Factors Affecting Electrode Selection

Each situation needs a number of factors to be considered before specifying a particular elec-trode. These factors are:

(a) composition of metal to be welded

(b) mechanical properties desired in the joint

(c) weldability problems – any risk of weld metal cracking (d) heat input limitations

(e) welding power source available a.c. or d.c.

(f) welding position (g) type of joint

(h) parent metal thickness

In document Welding Sciences and Technology - Ibrahim Khan (Page 82-89)