The function of wire brush is to remove the slag particles after chipping by chipping hammer. Sometimes, if available a power wire wheel is used in place manual wire brush.

On the other hand, when the work is made negative and electrode as positive then polarity is known as reverse or negative polarity, as shown in Fig. 7.16 (b).

These are available in the form of continuous wire or rods. They must be used only with straight polarity in D.C. welding. Bare electrodes do not provide any shielding to the molten metal pool from atmospheric oxygen and nitrogen.

Carbon and graphite electrodes can be used only for D.C. welding, while tungston electrodes can be used for both D.C. and A.C. welding. The filler material is added separately when these types of electrodes are used. Since, the electrodes do not consumed, the arc obtained is stable.

It is used when the thickness of the plates is from 8 to 16 mm. Both the edges are bevelled to form an angle of about 70° to 90°, as shown if Fig. 7.17 (b).

After reading this article you will learn about:- 1. Meaning of Electric Arc Welding 2. Procedure of Electric Arc Welding 3. Electric Current for Welding 4. Significance of Polarity 5. Equipments 6. Edge Preparation of a Joint 7. Electrodes.

An oxidizing agent is a substance which supplies oxygen or any other electronegative element. In other words, it is a substance which removes hydrogen or any other electropositive element. In a chemical reaction, an oxidising agent, after carrying out oxidation, is itself reduced. Oxidising agents are also called oxidants or can be referred to as oxidizers. The oxidation state of oxidising agents decreases as they are reduced in an oxidation-reduction reaction. Oxidising agents are electron acceptors.

The function of cables or leads is to carry the current from machine to the work. These are flexible and made of copper or aluminum. The cables are made of 900 to 2000 very fine wires twisted together so as to provide flexibility and greater strength.

Hydrogen peroxide

The electric arc is produced when two conductors are touches together and then separated by a small gap of 2 to 4 mm, such that the current continues to flow, through the air. The temperature produced by the electric arc is about 4000°C to 6000°C.

The deoxidising ingredients are graphite, powdered aluminum, wood flour, calcium carbonate, starch, cellulose, dolomite, etc.

(vii) The electrode has a minimum percentage elongation in tensile test of deposited metal is 20 percent on a gauge length of 5.65 √So and average impact test value of deposited metal is 47J at 27°C. [411].

The shape of edges may be plain, V-shaped, U-shaped, re­shaped, etc. The choice of various edge shapes depends upon the kind, thickness of metal to be welded. Some different types of grooves for edges of the work are shown in Fig. 7.17. BaDD

The D.C. welding machine obtains their power from an A.C. motor or diesel/petrol generator or from a solid state rectifier.

In reverse polarity, about 67% of heat is liberated at the electrode (positive terminal) and 33% on the work (negative terminal).

The function of electrode holder is to hold the electrode at desired angle. These are available in different sizes, according to the ampere rating from 50 to 500 amperes.

Reducing agent example

2. It produces low melting temperature slag, which dissolves the impurities present in the metal like oxides and nitrides, and floats on the surface of the weld pool.

If the number of electrons lost is equal to the number of electrons gained, then the two half-reactions are simply added to get the overall equation for the redox reaction. But, if the number of electrons lost during the oxidation half-reaction is different from the number of electrons gained during the reduction half-reaction, the two half-reactions are multiplied by a suitable integer.

The arc welding is a fusion welding process in which the heat required to fuse the metal is obtained from an electric arc between the base metal and an electrode.

The fourth, fifth and sixth digits of the code number represents tensile strength, maximum yield stress and percentage elongation with impact value.

It is used when the thickness of the plates is more than 16mm and where welding can be performed on both sides of the plate. Both the edges are bevelled to form a double-V, as shown in Fig. 7.17 (c).

These are sometimes also called as conventional electrodes. A coating (thin layer) of flux material is applied all-round the welding rod, and hence termed as coated electrode. The flux, during welding, provides a shielding to the molten metal zone from the atmospheric oxygen and nitrogen. This flux also prevents formation of oxides and nitrides. Flux chemically react with the oxides present in the metal and forms a low melting temperature fusible slag.

Dibenzoyl peroxide

We can say redox reactions can be considered as the sum of two half-reactions; one is oxidation half-reaction, and the other is reduction half-reaction. To explain these half-reactions, let us take an example of the oxidation of sodium by chlorine to form sodium chloride.

A metal electrode is used which supplies the filler metal. The electrode may be flux coated or bare. In case of bare electrode, extra flux material is supplied. Both direct current (D.C.) and alternating current (A.C.) are used for arc welding.

The cleaning of the surface should be carried out mechanically by wire brush or power wire wheel, and then chemically by carbon tetrachloride. Proper shape to the edges of the plate should be given to produce a proper joint.

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A.C. machine usually works with 50 hertz or 60 hertz power supply. The efficiency of A.C. welding transformer varies from 80% to 85%. The energy consumed per Kg. of deposited metal is 3 to 4 kWh for A.C. welding while 6 to 10 kWh for D.C. welding. A.C. welding machine usually work with low power factor of 0.3 to 0.4, while motor in D.C. welding has a power factor of 0.6 to 0.7. The following table 7.9 shows the voltage and current used for welding machine.

Oxidizing agent example

The welding machine used can be A.C. or D.C. welding machine. The A.C. welding machine has a step-down transformer to reduce the input voltage of 220- 440V to 80-100V. The D.C. welding machine consists of an A.C. motor-generator set or diesel/petrol engine-generator set or a transformer-rectifier welding set.

The functions of cable connectors are to make a connection between machine switches and welding electrode holder. Mechanical type connectors are used; as they can he assembled and removed very easily. Connectors are designed according to the current capacity of the cables used.

In redox reactions, some substances receive or accept electrons, and some give or donate. The species that donates the electrons is known as a reducing agent, and the species that accepts them is known as an oxidizing agent or oxidizer. In simple words, an oxidizer is a chemical species that oxidizes another chemical species by receiving electrons from it. Some common oxidizers are oxygen, ozone, fluorine, chlorine, hydrogen peroxide, etc.

What is oxidation and reduction

(ii) The electrode covering has an appreciable amount of titanium with basic materials and will produce fluid slag. [3].

The six digit code number indicated the performance characteristics and mechanical properties of the weld metal deposit.

For example, during the oxidation of sodium by chlorine, the number of electrons lost by each sodium atom is one, while those gained by each chlorine molecule are two. Therefore the oxidation half-reaction is multiplied by 2 and then added to the reduction half equation to get the equation for the overall redox reaction.

These electrodes do not consumed during the welding operation, hence they named, non-consumable electrodes. They are generally made of carbon, graphite or tungsten. Carbon electrodes are softer while tungsten and graphite electrodes are hard and brittle.

Both D.C. (direct current) and A.C. (alternating current) are used to produce an arc in electric arc welding. Both have their own advantages and applications.

It is used when the thickness of the plate is from 3 to 5 mm. Both the edges to be weld should be spaced about 2 to 3mm apart as shown in Fig. 7.17 (a).

When the arc is obtained, intense heat so produced, melts the work below the arc, and forming a molten metal pool. A small depression is formed in the work and the molten metal is deposited around the edge of this depression. It is called arc crator. The slag is brushed off easily after the joint has cooled. After welding is over, the electrode holder should be taken out quickly to break the arc and the supply of current is switched off.

The alternating current for arc is obtained from a step down transformer. The transformer receives current from the main supply at 220 to 440 volts and step down to required voltage i.e., 80 to 100 volts. The direct current for arc is usually obtained from a generator driven by either an electric motor, or patrol or diesel engine.

Select the proper current and polarity. The spot are marked by the arc at the places where welding is to be done. The welding is done by making contact of the electrode with the work and then separating the electrode to a proper distance to produce an arc.

An open circuit voltage (for striking of arc) in case of D.C. welding is 60 to 80 volts while a closed circuit voltage (for maintaining the arc) is 15 to 25 volts.

First of all, metal pieces to be weld are thoroughly cleaned to remove the dust, dirt, grease, oil, etc. Then the work piece should be firmly held in suitable fixtures. Insert a suitable electrode in the electrode holder at an angle of 60 to 80° with the work piece.

The slag is float on the top of the weld and can easily be brushed off after solidification of weld. The quality of weld produced by coated electrode is much better as compared to that of bare electrodes.

The first digits of the code number essentially explain the type of covering used on the electrode and this covering signifies the performance characteristics.

Oxidising agents or oxidizers may contribute to combustion. Therefore the symbol for an oxidant or oxidising agent is a circle with flames on top of it. Oxidizers are considered dangerous, which is why they have a hazard symbol. Given below is the symbol of an oxidizer:

Reducing agent

The functions of protective clothings used are to protect the hands and clothes of the welder from the heat, spark, ultraviolet and infrared rays. Protective clothing used are leather apron, cap, leather hand gloves, leather sleeves, etc. The high ankle leather shoes must be wear by the welder.

The electrode coating may consists two or more ingredients. Different type of coatings used for different type of metals to be welded.

The slag forming ingredients are silicon oxide (Sio2), Manganese oxide (Mno2), iron oxide (FeO), asbestos, mica, etc. In some cases, aluminum oxide (Al2o3) is also used but it makes the arc less stable.

In addition to the coding given above, all electrodes must conform to the test requirements of IS: 814 (part I and II) – 1974. Each packet of electrodes must have a marking indicating coding and specification.

The alloying elements used for improvement of weld strength are vanadium cobalt, molybdenum, aluminum, chromium, nickel, zirconium, tungsten, etc.

This concludes our article about oxidants. If you want to study similar Chemistry topics, download the Testbook App. It is available free on the Google play store and provides all the necessary study materials, which will help you with exam preparation. So, visit the store now to download the app and enjoy the benefits.

The function of screen and face shield is to protect the eyes and face of the welder from the harmful ultraviolet and infrared radiations produced during welding. The shielding may be achieved from head helmet or hand helmet.

In these reactions, electrons are transferred from one substance. Thus, redox reactions may also be referred to as electron transfer reactions. The substance which loses electrons is called a reducing agent, and the other which accepts the electrons is called an oxidising agent.

Learn more about an oxidizing agent as we go through its definition. Also, learn how we can identify it. Its properties, applications, and some examples are also discussed here.

Oxidizing agent and reducing agent

These electrodes get melted during welding operation, and supply the filler material. They are generally made with similar composition as the metal to be welded.

The constituents of typical electrode coatings and their functions are given in table 7 11. Some of them are discussed here:

The function of chipping hammer is to remove the slag after the weld metal has solidified. It has chisel shape and is pointed at one end.

Mildoxidisingagent

(iv) The electrode is suitable for welding with direct current, with electrode being +ve or -ve. It is also suitable of welding with an alternating current with open circuit voltage less than 90 volts. [5].

In straight polarity, about 67% of heat is distributed at the work (positive terminal) and 33% on the electrode (negative terminal). The straight polarity is used where more heat is required at the work. The ferrous metal such as mild steel, with faster speed and sound weld, uses this polarity.

The efficiency and quality of welded joint also depends upon the correct preparation of the edges of the plates to be welded. It is necessary to remove all scales, rust, grease, paint, etc. from the surface before welding.

Hence, the welds obtained by these electrodes are of lower strength, lower ductility and lower resistance to corrosion. They find limited use in minor repair and poor quality work. They used to weld wrought iron and mild steel. In modern practice they are not used or rarely used. They are also known as plain electrodes.

It is used when the thickness of the plate is more than 20mm. The edge preparation is difficult but the joints are more satisfactory. It requires less filler metal, as shown in Fig. 7.17 (d) and (e).

The A.C. welding machine has a step down transformer which receives current from main A.C. supply. This transformer step down the voltage from 220 V-440V to normal open circuit voltage of 80 to 100 volts. The current range available up to 400 amperes in the steps of 50 ampere.

The reverse polarity is used where less heat is required at the work as in case of thin sheet metal weld. The non-ferrous metals such as aluminum, brass, and bronze nickel are welded with reverse polarity.

The ingredients to improve arc characteristics are sodium oxides (Na2O), Calcium oxides (CaO), magnesium oxides (MgO), and titanium oxide (TIO2).

When the work is made positive and electrode as negative then polarity is called straight or positive polarity, as shown in Fig. 7.16 (a).