LECTURE-10 and 11

WELDING PROCESS, CLASSIFICATION, ADVANTAGES, APPLICATION

ARC WELDING, PRINCIPLES, EQUIPMENT, WELDING JOINTS, WELD POSITIONS,

POLARITY, ELECTRODES, TYPES.

WELDING PROCESS, CLASSIFICATION, ADVANTAGES, APPLICATION

ARC WELDING, PRINCIPLES, EQUIPMENT, WELDING JOINTS, WELD POSITIONS,

POLARITY, ELECTRODES, TYPES.

Welding is a process of joining two similar or dissimilar

metals by application of heat with or without application of

pressure and addition of filler material.

The heat may be obtained from the blacksmith’s fire,

electric arc, electric resistance or chemical reaction.

Modern methods of welding may be classified into two major types.

(i) Fusion process

(ii) Non fusion process

Welding processes

Advantages of welding:

1. Welding structures are normally lighter than riveted or

bolted structure.

2. Welded joints are strong, rigid and tight.

3. Alteration and additions can be easily made in the

existing structure.

4. Welding structure is smooth in appearance.

5. Welded joint has a great strength.

6. Cost effectiveness.

7. Simplicity of welded structures design.

8. Possiblity of welding at any part of a structure.

9. Consumes less time when compared to other type of

joints.

Disadvantages of welding:

– Internal stresses, distortions and change of

micro-structure in the weld region;

– Harmful effects: light, ultra violate radiation,

fumes, high temperature.

– Requires highly skilled labour

– Edge preparation is required before welding

Applications of welding

• Buildings and bridges structures

• Automotive, ship and aircraft

constructions

• Pipe lines

• Tanks and vessels

• Rail roads

• Machinery elements

Fusion welding or non-pressure weldingThe material at the joint is heated to a molten state and allowed to solidify. This includes gas welding, arc welding and thermit welding etc.

Non-fusion or pressure welding The pieces of metal to be joined are heated to a plastic state and then forced together by external pressure. This procedure is used in forge welding, resistance welding, Thermit welding and gas welding in which pressure is required.

Classification of welding processes

Classification of welding processes

I.Arc welding1. Carbon Arc Welding (CAW)2. Shielded Metal Arc Welding (SMAW)3. Submerged Arc Welding (SAW)4. Metal Inert Gas Welding (MIG, GMAW)5. Tungsten Inert Gas Arc Welding (TIG, GTAW)6. Electroslag Welding (ESW)7. Plasma Arc Welding (PAW)

II.Resistance Welding (RW)1. Spot Welding (RSW)2. Flash Welding (FW)3. Resistance Butt Welding (UW) 4. Seam Welding (RSEW)

Gas Welding (GW)1.Oxyacetylene Welding (OAW)2.Oxyhydrogen Welding (OHW)3.Pressure Gas Welding (PGW)

Solid State Welding (SSW)1.Forge Welding (FOW)2.Cold Welding (CW)3.Friction Welding (FRW)4.Explosive Welding (EXW)5.Diffusion Welding (DFW)6.Ultrasonic Welding (USW)

Thermit Welding (TW)Electron Beam Welding (EBW)Laser Welding (LW)

Weldability The property of a metal, which indicates the ease with which two similar or dissimilar metals are joined by fusion with or without the application of pressure.

Factors affecting the weldability of a metal:

1. Composition of the metal2. Melting point3. Thermal conductivity4. Thermal expansion5. Surface condition6. Change in micro structure

Arc welding is the welding process, in which heat is

generated by an electric arc struck between an electrode

and the work piece. Electric arc is luminous electrical

discharge between two electrodes through ionized gas.

Arc welding

The discharge of electric

Current across a gap of two

Electrodes.

Basic arc-welding circuit

Electric arc between the electrode and work piece closes the electric circuit.

The arc temperature may reach 10000°F (5500°C), which is sufficient for fusion of work piece edges and joining them.

This heat melts both the base metal and the electrode, producing a pool of molten metal sometimes called a "crater."

The crater solidifies behind the electrode as it is moved along the joint. The result is a fusion bond.

Arc welding equipment

Arc welding equipment

i. AC or DC machine ii. Electrode

iii. Electrode holder iv. Cables, cable connections

v. Cable lug vi. Chipping hammer

viii. Earthing clamps ix. Wire brush

x. Helmet xi. Safety goggles

xii. Hand gloves xiii. Apron, sleeves etc.

AC or DC Machine

• Generators driven by electric motors or IC engines obtain DC welding supply.

• For AC welding supply, transformers are mostly used to step down the voltage (200-400 volts) to welding voltage of 50-90 volts.

• 100A - 200A - Light manual welding

• 300 - 400A - Average manual welding

• 800 - 3000A - Automatic welding

A.C. arc welding D.C.Welding

The A.C. Welding transformer has no moving parts and is simpler.

The D.C welding generator has rotating parts and is more

complicated. The transformer costs less and

its maintenance cost is low. The generator costs more and its maintenance cost is high.

Since the distribution of heat is equal, therefore there is no need for changing the polarity. Hence only ferrous metals are usually welded by A.C.

Heat distribution is different in two poles, i.e., 2/3 in positive and 1/3 in negative. By changing the polarity, all types of metals can be welded by D.C.

All types of electrodes can not be used in A.C. arc welding because the current constantly reverses with every cycle.

All types of electrodes, bare or coated can be used in D.C. arc welding because the polarity can

be changed to suit the electrode.

The problem of 'arc blow' does not arise, as it is very easy to control.

In D.C. the 'arc blow' is serve and cannot be controlled easily.

The arc is never stable. The arc is more stable.

In can be used only when A.C. supply from the mains is available.

In the absence of A.C. main supply, an engine driven D.C. generator set can be used.

A.C. is more dangerous D.C. is comparatively less dangerous

The transformer consists of a rectangular steel tank.

An oil cooled double wound step down transformer

reduces the supply main voltage to a welding voltage

of 80. All windings are totally enclosed in the steel

tank.

Rotating the hand wheel can vary the output of the

transformers and the weld current can be directly

read at the window on the tank cover. The set is

connected 400-440 volts 3 phase, 50 cycles ac

supply.

AC or DC machine

It is filler metal in the form of a wire or rod used as a terminal in an electric current for the purpose of producing an electric arc.

The electrodes are classified as consumable electrode and non-consumable electrode.

A consumable electrode is similar in composition to the metal being welded and it melts to become a part of weld.

A non-consumable electrode may be made of carbon, graphite or tungsten, which do not consume during the welding operation.

Electrode

Specific electrodes are available for welding:1.Mild steel2.Cast iron3.Stainless steel4.Copper, bronze, brass, etc5.High tensile steels

The coding consists of a prefix letter, a code number of digits in numerals and in some cases one or more suffix letters, each indicating a specific property or characteristic or both of electrode.

e-g. E 616 – 515E - The prefix letter indicate the method of manufacturing6 -The first digit indicate the type of covering1 -The second digit indicates the welding position6 -The third digit indicates the welding current5 & 1-The fourth and fifth digit indicates the tensile strength of the weld metal5- The sixth digit indicates the percentage elongation of deposited metal.

Specification of electrode

Angularity of electrode• The quality of weld is determined to a

marked degree by the angular deposition

of the electrode to the work. In flat welding,

electrode should not be held at not more

than 20º from the vertical and 3 mm away

from the work.

Polarity

In DC welding machine, the heat generated by the flow

of current is split into two parts, in the ratio of 66% at

the positive pole and 33 percent at the negative pole.

In straight polarity the electrode is negative and the

work is positive.

In the reverse polarity the electrode is positive and the

work is negative.

Wire Brush

Wire brush is a cleaning tool.

It is used for removing loose slag, spatter and oxides.

Chipping Hammer

A chipping hammer is used for removing slag and spatter.

It is advisable to wear chipping goggles while using

chipping hammer so that particles of slag may not strike

the eyes.

Helmet

A helmet is used by the welder for

protection of his eyes, face and the

throat from harmful rays of electric arc

and from the hot flying particles or

spatter.

Hand Gloves

For protection of wrist and hands,

gloves and worn by the welders.

They are made of leather or asbestos.

They should be as flexible as possible.

Welding joints

i. Lap joint

ii. Butt joint

iii. Corner joint

iv. Edge joint

v. T-joint

Lap joint

This used to joint two overlapping plates so that the edge of each plate is welded in the other.

These joints are employed on plates

having thickness less than 3 mm.

The lap joints may be single transverse, double transverse and parallel lap joints.

Butt joint

It is used to joints two ends or two edges or surfaces located

in the same plane with each other.

In butt welds, the plate edges do not require beveling if the

thickness of plate is less than 5 mm.

On the other hand, if the plate thickness is 5 mm to 12.5

mm, the edges should be beveled to V or U-groove and

plates having thickness above 12.5 mm should have a V

or U-groove on both sides.

Types of butt joints

1.Square butt joint

2.Single V-butt joint

3.Double V-butt joint

4.Single U-butt joint

5.Double U-butt joint

Corner joint

It is used to joint two edges of two plates whose

surfaces are at right angles to each other.

It is used for both light and heavy gauge sheet metal.

In some cases corner joint can be welded, without any

filler metal, by melting off the edges of the parent metal.

Edge joint

It consists of joining the edge of parallel plate by means of weld.

The edge joint is obtained by joining two parallel plates.

It is economical for plates having thickness less than 6 mm.

This joint is unsuitable for members subjected to direct tension or bending.

T-joint

It is used to weld two plates or sections, which

are right angles to each other.

It is widely used to weld stiffeners in aircraft and

other thin walled structures.

These joints are suitable upto 3 mm thickness.

Welding Positions

1.Flat position

2.Horizontal position

3.Vertical position

4.Overhead position

Flat position

In this position, the filler metal is deposited

from the upper side of the joint with the face of

the weld horizontal.

Horizontal position

In this position, the weld is deposited upon the

side of a horizontal and against a vertical

surface.

Vertical position

In this position, the line of welding is in a

vertical plane and the weld is deposited

upon a vertical surface.

Overhead position

In this position, the weld is deposited from the

under side of the joint and the face of the weld

is horizontal.

It is the reverse of flat welding.

Bead - It describes the neatness of ripples formed by the metal while it was in semi liquid state. It denotes the appearance of the finished weld.

Face of the weld - The exposed surface of the weld

Flux - A fusible material used to prevent formation of oxides, nitrites and promote fusion of metal.

Welding Terminologies

Base metal - The metal to be welded. It is also known as parent metal.

Fillet - Weld metal in the internal corner point formed by two pieces of metal, giving the joint additional strength to with stand unusual stresses.

Tack - A relatively small weld to hold parts together to assist assembly or maintain alignment of exist during welding.

• Spatter - Globules of weld metal or filler expelled during welding and adhering to the surface of the parent metal or solidified weld metal. Use of shorter arc, reduced current, and correct electrode minimize the spatter.

• • Slag - Molten flux absorbs oxide

from the surface of the parent metal and floats on the molten metal.

Arc Crater The blast of the arc forces the molten metal out of the pool, thus forming a small depression in the parent metal around which molten metal is piled up. This is known as arc crater Arc LengthThe distance through the centre of the arc from the tip of the electrode to the bottom of the arc crater is the arc length. Arc length is vital variable and should be 3 to 4 mm Arc Blow A large current produces arc blow. The space around the arc and on the adjacent metal is always influenced by magnetic field, which tends to deflect the arc. This is known as arc blow. It can be minimized by changing the angle of electrode and direction of welding.