Welding By Laxman Singh Sankhla, Jodhpur India.doc

TRAINING MANUAL MECHANICAL

ARC & GAS WELDING

CONTENTS PAGE

2.0 Introduction 33.0 Types of Welding 34.0 Electric Arc welding 34.1 Basic Principle of arc welding 4 4.2 Welding Station and Welding Equipment 74.3 Arc-Welding Hazards 94.4 Welding Operators Protection 104.5 Protective clothing for manual arc-welding 115.1 Safe Practices – Before Operation 125.2 Safe Practices – During Operation 125.3 Key Safety Points 126.1 Power supplies & General Arrangement of Arc Welding Equipment 14 6.2 Arc-Welding circuit 156.3 Polarity of the electrode: 157.1 Shielded Metal Arc Welding (SMAW) or Stick Welding Processes 157.2 Function of coating on electrode 198.1 Types of weld joint and Welds 208.2 Various Parts of welds 228.3 Butt weld and its terminology 238.4 Fillet Welds and its terminology 23 9.1 Weld Positions 2510.1 Weld joint preparation 26

Welding By Laxman Singh Sankhla, Jodhpur, India Mobile:+91-9928829491 Mail ID: [email protected] 1

http://en.wikipedia.org/wiki/Arc_welding


10.2 Factors affecting the method of edge preparation 2711.1 Weld joint fit up before welding 29 11.2 Striking the arc technique 3011.3 Tack welding 3212.1 Welding sequence 3312.2 Effect of the gap in weld penetration 3612.3 T- Joint 3713.1 Examples of Good & Bad Stick Welds 3813.2 Heat affected Zone 4014.1 Safety issues 4215.1 Welding Defects 4215.2 Porosity and its prevention 4515.3 Wormholes and its Prevention 4715.4 Incomplete Penetration or Fusion 4815.5 Trapped Slag 5015.6 Cracks and Distortion 5216.1 Geometrical imperfections 57

17.1 Introduction to Oxy-Acetylene Welding 61

17.2 Gas Welding Station 63

17.3 Protective Clothing for Gas Welding and Cutting 64

17.4 Safe Work Practices Electric & Gas Welding 65


http://en.wikipedia.org/wiki/Oxy-fuel_welding_and_cutting


17.5 Welding, Cutting and Brazing Accidents – Causes 66

18.1 Oxy-acetylene Welding Process 67

18.2 Basic Gas-Welding Equipment 69

18.3 Types of flame 73

19.1 Safe Work Practices 77

20.1 Fluxes 78

21.1 Welding Techniques 78

22.0 Comparison of oxy-acetylene and metallic arc-welding 81

23.1 Oxy-Acetylene Cutting 82



2.0 Introduction

Welding is used when a permanent joint which has high strength is required. It is often better to fabricate steel by welding than using a casting because the mechanical properties of steel are better than those of cast iron. In fusion welding the edges of the metal to be joined are melted. The molten metal runs together and so a joint is made whose strength is equal to that of metal being joined. Melting the workpieces and adding a filler material to form a pool of molten material (the weld pool) that cools to become a strong joint, with pressure sometimes used in conjunction with heat, or by itself, to produce the weld.

This is in contrast with soldering and brazing, which involve melting a lower-melting-point material between the workpieces to form a bond between them, without melting the workpieces.

Fabrication, when used as an industrial term, applies to the building of machines, structures and other equipment, by cutting, shaping and assembling components made from plate metal, formed and expanded metal ,tube stock,square stock ,sectional metals (I beams, W beams, C-channel...)

3.0 Types of Welding

1. Fusion Welding: Melting base metals(a) Arc Welding (AW): Heating with electric arc(b) Oxy-fuel Welding (OFW): Heating with a mixture of oxygen and acetylene (c) Resistance welding (RW): Heating the resistance by an electrical current(d) Other fusion welding: Electron beam welding and laser beam welding2. Solid State Welding: No melting, No fillers(a) Diffusion welding (DFW): Solid-state fusion at an elevated temperature(b) Friction welding (FRW): Heating by friction(c) Ultrasonic welding (USW): Moderate pressure with ultrasonic oscillating motion

4.0 Electric Arc welding

When electricity passes through an air gap from one conductor to another intense heat is produced. The temperature of an arc jumping


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http://en.wikipedia.org/wiki/Heat

http://en.wikipedia.org/wiki/Pressure

http://en.wikipedia.org/wiki/Melting


between two conductors is in the region of 3500 degree Celsius. In electric arc welding it is this arc which provides the source of heat.

During welding an electric current of high amperage and low voltage is fed into a metal electrode and the arc is drawn between the electrode and the metal to be joined. The high temperature of the arc melts the parent metal and the end of the electrode (or welding rod). Small drops of molten metal from the rod are deposited and this unites with the metal being welded.

Coated Electrodes: When a base wire is used as an electrode the arc is difficult to control and also the weld tends to be brittle and porous. When coating forms a slag over the weld which protects the weld from the atmosphere and reduces the rate at which the weld cools, making it less brittle.

4.1 Basic principles of Metallic-Arc Welding

The arc is produced by a low voltage, high-amperage electric current jumping an air gap between the electrode and the joint to be welded. The heat of the electric arc is concentrated on the edges of two pieces of metal to be joined. This causes the metal edges to melt. While these edges are still molten additional molten metal, transferred across the arc from a suitable electrode, is added. This molten mass of metal cools and solidifies into one solid piece.

As soon as the arc is struck, the tip of the electrode begins to melt, thus increasing the gap between electrode and work. Therefore, it is necessary to cultivate a continuous downward movement with the electrode holder in order to maintain a constant arc length of 3 mm during welding operation. The electrode is moved at a uniform rate along the joint to be welded, melting the metal as it moves.

The greatest bulk of electrodes used with manual arc welding are coated electrodes. A coated electrode consists mainly of a core wire having a concentric covering of flux and/or other material, which will melt uniformly with the core wire forming vaporized and partly molten screen around the arc stream. This shield protects the arc from contamination by atmospheric gases.

The liquid slag produced performs three important functions.

1. Protects the solidifying weld metal from any further contamination from the atmosphere.

2. Prevents rapid cooling of the weld metal.



3. Controls the contour of the completed weld.

The core wire melts in the arc and tiny globules of molten metal shoot across the arc into the molten pool (arc crater in parent metal) during welding. These tiny globules are explosively forced through the arc stream. They are not transferred across the arc by the force of gravity; otherwise it would not be possible to use the manual arc-welding process for overhead welding.

The chemical coating surrounding the core wire melts or burns in the arc. It melts at a slightly higher temperature than the metal core and, therefore, extends a little beyond the core and directs the arc. This extension also prevents sideways arcing when welding in deep grooves.

Shielded metal-arc welding with the transformer welding machine depends upon this fundamental fact: that when one side of the welding circuit is attached to a piece of steel, a welding electrode connected to the other side and two brought into contact, an arc will be established. If the arc is properly controlled, the metal from the electrode will pass through the arc and be deposited on the steel. When the electrode is moved along the steel at the correct speed, metal will deposit in a uniform layer called a bead. Electrodes consist of a core of steel wire, usually called mild since it contains a low percentage of carbon (0.10-0.14). Around this core is applied a special coating which assists in creating the arc and at the same time protects the molten metal as it transfers across the arc.



The arc stream and basic features of manual gas shield metal-arc welding



4.2 Welding Station and Welding Equipment



Mains operated arc-welding equipment



This equipment is designed to change the high-voltage Alternating Current mains supply into a safe, low-voltage, heavy-current supply suitable for arc welding. Output can have an AC or DC. For safety the output voltage is limited between 50 and 100 V; however, the output current may be as high as 500 A. A welding set is basically a transformer to breakdown the high mains voltage and a tapped choke to control the current flow to suit the gauge of electrode used

4.3 Arc-Welding Hazards



Hazards that may arise from mains operated welding equipment are set out in table:

To eliminate these hazards as far as possible, the following precautions to be taken.

1. Make sure that the equipment is fed from a switch-fuse so that it can be isolated from the main supply.

2. Make sure that the trailing primary cable is armoured against mechanical damage.

3. Make sure that all cable insulation is undamaged and all terminals are secured and undamaged

4. Make sure that all equipment is adequately earthed.

5. Make sure that current regulator has an ‘off’ position so that in the event of an accident the welding current can be stopped without having to follow the primary cable back to the isolating switch.

6. Make sure that the ‘external welding circuit’ is adequate for the heavy currents it has to carry.



4.4 Welding Operators Protection

Welding, without the proper precautions, can be a dangerous and unhealthy practice. Because many common welding procedures involve an open electric arc or flame, the risk of burns is significant. PPE are designed to protect the wearer's body or clothing from injury by blunt impacts, electrical hazards, heat, chemicals, and infection, for job-related occupational safety and health purposes

Following is a list of basic PPE:

Fire-resistant gloves, Aprons, Safety shoes, Helmet

Ultraviolet radiation filter plate (arc welding), Goggles with filter lenses

Protective long sleeve jackets to avoid exposure to extreme heat and flames.


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http://en.wikipedia.org/wiki/Chemicals

http://en.wikipedia.org/wiki/Injury

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4.5 Protective clothing for manual arc-welding



Arc welding with a welding helmet, heavy leather gloves, and other protective clothing.



5.1 Safe Practices – Before Operation

Check all connections

Work lead firmly attached to work

Contact surfaces of work clamps free of metal splatter particles

Coiled welding cable spread out

Inspect work and electrode lead cables

5.2 Safe Practices – During Operation

Proper working surface

Clean material

Good housekeeping practices

Use metal tongs and pliers to handle hot metal

Hot metal marked “HOT” with soapstone

5.3 Key Safety Points

Welder is properly installed and grounded

Never weld without adequate ventilation

Take proper precautions to prevent fires


http://en.wikipedia.org/wiki/File:AlfredPalmerwelder1.jpg


Protect entire body with fire retardant clothing, shoes and gloves

Wear eye protection at all times

Weld only in a fire safe area

Never do any welding, cutting or hot work on used drums, barrels, tanks or similar containers

Mark metal “HOT” with soapstone

Keep a well stocked first aid kit handy



6.1 Power supplies & General Arrangement of Arc Welding Equipment



6.2 Arc-Welding circuit

Circuit Diagram of an Alternating Current Arc-Welding Set

6.3 Polarity of the electrode: Electrode can be charged either positively or negatively. The positively charged anode will have a greater heat concentration, and as a result, changing the polarity of the electrode has an impact on weld properties. If the electrode is positively charged, the base metal will be hotter, increasing weld penetration and welding speed. Alternatively, a negatively charged electrode results in more shallow welds

7.1 Shielded Metal Arc Welding (SMAW) or Stick Welding Processes

Shielded metal arc welding [SMAW] or stick welding process is an electrical machine [which may be DC or AC] supplies current to an electrode holder which carries an electrode. An earth cable connects the work piece to the welding machine to provide a return path for the current. The weld is initiated by tapping [striking] the tip of the electrode against the work piece which initiates an electric arc. The high temperature generated [about 6000 Deg.C] almost instantly produces a molten pool and the end of the electrode. The electrode continuously melts into this pool and fills the groove. The operator


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needs to control the gap between the electrode tip and the work piece while moving the electrode along the joint.

The most common welding power supplies are constant current power supplies and constant voltage power supplies. In arc welding, the length of the arc is directly related to the voltage, and the amount of heat input is related to the current. Constant current power supplies are most often used for manual welding processes such as gas tungsten arc welding and shielded metal arc welding.


http://en.wikipedia.org/wiki/Voltage

http://en.wikipedia.org/wiki/Electrical_current


In the shielded metal arc welding process [ SMAW ] the 'stick' electrode is covered with an extruded coating of flux. The heat of the arc melts the flux which generates a gaseous shield to keep air away from the molten pool and also flux ingredients react with unwanted impurities such as surface oxides, creating a slag which floats to the surface of the weld pool. This forms a crust which protects the weld while it is cooling. When the weld is cold the slag is chipped off.

A consumable electrode – a filler metal rod coated with chemicals for flux and shielding (230-460mm long and 2.5-9.4mm in diameter)



The filler metal must be comparable with base metalCurrent: 30-300A and Voltage: 15-45VCheaper and portable than oxyfuel weldingLess efficient and variation in current due to the change in length of consumableelectrodes during the processThe SMAW process can not be used on steel thinner than about 3mm and being a discontinuous process it is only suitable for manual operation. It is very widely used in fabrication shops and for on site steel construction work.

Electric current is used to strike an arc between the base material and consumable electrode rod, which is made of steel and is covered with a flux that protects the weld area from oxidation and contamination by producing CO2 gas during the welding process. The electrode core itself acts as filler material.

The process is versatile and can be performed with relatively inexpensive equipment, making it well suited to shop jobs and field work


http://en.wikipedia.org/wiki/Carbon_dioxide

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http://en.wikipedia.org/wiki/Flux_(metallurgy)


Shown in the picture is the electrode and it’s holder. The cover on the electrode is flux. Electrical power source can also be seen in the behind.

7.2 Function of coating on electrode 1. To facilitate striking the arc and to enable it to burn stably. 2. Serve as an insulator for the core wire. 3. It provides a flux for the molten pool, which picks up impurities and forms a protective slag which is easily removed. 4. It stabilises and directs the arc and the globules of molten core metal 5. It provides a protective non-oxidising or reducing gas shield (smoke like gas) around the arc to keep oxygen and nitrogen in the air away from the molten metal

6. It increases the rate of melting (i.e. metal deposition) and so speeds up

the welding operation7. It enable the use of alternating current.8. It gives good penetration.9. It increases or decreases the fluidity of the slag for special

purposes. It can, for example, reduce the fluidity of electrodes used for overhead welding.

The flux covering the electrode melts during welding. This forms the gas and slag to shield the arc and molten weld pool. The slag must be chipped off the weld bead after welding. The flux also provides a method of adding scavengers, deoxidizers, and alloying elements to the weld metal.



Finished weld The slag removed (weld bead exposed)

Major defects in this process are: Undercutting, Incomplete penetration, incomplete fusion, Porosity, Slag Inclusions, Cracks and burn through.

8.1 Types of weld joint and welds • Types of Joints– Butt joint– Corner joint– Lap joint– Tee joint– Edge joint• Types of Welds– Fillet weld– Groove weld– Plug and slot welds--Spot and Seam welds--Flange and Surfacing welds



Basic Types of Weld



8.2 VARIOUS PARTS OF WELDS



8.3 Butt weld and its terminology : A butt weld is made between two pieces of metal usually in the same plane, the weld metal maintaining continuity between the sections.



Butt weld terminology

8.4 Fillet Welds and its terminology: These welds are roughly triangular in cross section and between two surfaces not in the same plane and the weld metal is substantially placed alongside the components being joined. Deposited weld metal is external to the profile of the welded elements

Fillet weld terminology



9.1 Weld PositionsCommon welding joint types – (1) Square butt joint, (2) Single-V preparation joint, (3) Lap joint, (4) T-joint.



10.1 Weld joint preparation



Plate edge preparation

10.2 Factors affecting the method of edge preparation:

1. Metal thickness2. Heat source (gas or electric arc)3. Cost (a V-edge can be prepared more cheaply than U-edge, but

the V-edge uses more filler material)4. Gap (a small gap uses less filler material but may not allow

adequate penetration)

Approximate guide to plate edge preparation



11.1 Weld joint fit up before welding



Weld joint fit up before welding. The weld groove which is to be filled by welding can be seen.

11.2 Striking the arc technique



• Scratch Start Technique•

• Drag electrode across workpiece like striking a match; lift electrode slightly after touching work. If arc goes out, electrode was lifted too high. If electrode sticks to workpiece, use a quick twist to free it.1. Electrode2. Workpiece3. Arc

Scratch Start Technique

1. Electrode2. Workpiece3. Arc



Tapping Technique

Bring electrode straight down to workpiece; then lift slightly to start arc.If arc goes out, electrode was lifted too high.If electrode sticks to workpiece, use a quick twist to free the

electrode


http://en.wikipedia.org/wiki/File:SMAW.welding.navy.ncs.jpg


11.3 Tack welding: When the plates are set up for welding they must be in perfect alignment. They are tack welded to hold them in this position. These tack welds are very short runs at intervals along the joint and should be made with a high current so that proper penetration is obtainedThe maximum pitch and length of the tack welds are obtained from the formulae: Pitch = 100mm+16 x plate thickness in mm Length of each track = 3 x plate thicknessHence, for a plate 12 mm thick: Pitch = 100+16 x 12 = 292 mm (say 300 mm) Length of each tack = 3 x 12 = 36 mm

Weld joint fit up for a pipe to reducer joint, the root gap is clearly visible.

Double Vee Weld groove fit up for plates bend in to pipes



Spatter : The metal drops and particles expelled during arc or gas welding. They do not form part of the weld. Spatter loss is the metal lost due to spatter. Spatter stick to surrounding surfaces should be minimized by correcting the welding conditions and should be eliminated by grinding when present.

Arc strikes appear as localized remelted metal from inadvertent or careless arc manipulation. They must be avoided and any traces removed because small cracks and their localized heat affected zone may become the origin of dangerous fatigue failures.

12.1 Welding sequence


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Welded layers



Butt welds

12.2 Effect of the gap in weld penetration



12.3 T- Joint

13.1 Examples of Good & Bad Stick Welds



Example of Good and Bad Stick Welds

• Good Weld

Example of Bad Stick Welds

• Amperage Too High

Amperage too low



Travel too fast

Travel too slow



Arc too short

Arc too long

13.2 HEAT AFFECTED ZONE

When the weld pool is cooling and solidifying, the majority of the heat flows through the parent metal alongside the joint. The steel is thus subjected to heating and cooling cycles similar to those experienced in heat treatment practice.

As shown in Figure, the structure of the steel will be changed in this region (called the heat affected zone, HAZ)



Heat-affected zone (HAZ)The blue area results from oxidation at a corresponding temperature of 600 °F (316 °C). This is an accurate way to identify temperature, but does not represent the HAZ width. The HAZ is the narrow area that immediately surrounds the welded base metal.

14.1 Safety issues



1. Heat and sparks

Welding procedures involve an open electric arc or flame, the risk of burns is significant. To prevent them, welders wear protective clothing in the form of heavy leather gloves and protective long sleeve jackets to avoid exposure to extreme heat, flames, and sparks.

2. Eye damage

The brightness of the weld area leads to a condition called arc eye in which ultraviolet light causes inflammation of the cornea and can burn the retinas of the eyes. Goggles and helmets with dark face plates are worn to prevent this exposure of UV light. To protect bystanders, transparent welding curtains often surround the welding area. These curtains, made of a polyvinyl chloride plastic film, shield nearby workers from exposure to the UV light from the electric arc, but should not be used to replace the filter glass used in helmets.

Sunglasses and blowtorching goggles are not adequate for arc welding protection.

3. Inhaled matter

Flux-cored arc welding and shielded metal arc welding produces smoke containing particles of various types of oxides. The size of the particles in question tends to influence the toxicity of the fumes, with smaller particles presenting a greater danger. Additionally, many processes produce carbon dioxide and ozone. that can prove dangerous if ventilation is inadequate. Furthermore, the use of compressed gases and flames in many welding processes pose an explosion and fire risk; some common precautions include limiting the amount of oxygen in the air and keeping combustible materials away from the workplace.

15.1 Welding Defects /Imperfections

One or more of the following faults may be visible when an imperfect weld is examined:

(a) Undercutting, is caused by the burning away of the side walls of the joint recess or a reduction of the parent metal thickness at the fusion face.

(b)Lack of penetration and excessive penetration, both of which lead to a weak joint. It can be caused by insufficient heat or too large a filler rod.


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(c) Lack of fusion is caused when the filler rod is too small in diameter and melts before the parent metal melts, so that the two do not fuse together

(d) Inclusion and porosity: Inclusions can result from a number of causes including incorrect welding current, incorrect flux coating, slag traps in multi-run welds, and poor operating technique. Porosity is caused by moisture and inadequate cleaning of the materials being joined.

Typical welding defects

Other welding defects include the following:

Hydrogen induced HAZ cracking, reheat cracking, hot cracking /solidification cracking

Any of these defects are potentially disastrous as they can all give rise to high stress intensities which may result in sudden unexpected failure below the design load or in the case of cyclic loading, failure after fewer load cycles than predicted.



Essentially a discontinuity or flaw is called a defect if it exceeds the acceptance limits established by engineering based on Fitness for Service criteria.



15.2 Porosity and its prevention

Porosity is a collective name describing cavities or pores caused by the absorption of nitrogen, oxygen and hydrogen entrapment of gas in molten metal pool which is then released on solidification to become trapped in the weld metal causing porosity



Nitrogen and oxygen absorption in the weld pool usually originates from poor gas shielding. Leaks in the gas line, too high a gas flow rate, draughts and excessive turbulence in the weld pool are frequent causes of porosity.

Uniformly distributed porosity Surface breaking pores

Grease and oil on the surface of the workpiece or filler wire are also common sources of hydrogen.

Surface coatings like primer paints and surface treatments such as zinc coatings, may generate copious amounts of fume during welding. The risk of trapping the evolved gas will be greater in T joints than butt joints especially when fillet welding on both sides

Prevention

The gas source should be identified and removed as follows:

1. Air entrainment

Avoid weld pool turbulence

Use filler with adequate level of deoxidants

Reduce excessively high gas flow

Avoid draughts

2. Hydrogen



Clean and degrease the workpiece surface

3. Surface coatings

Clean the joint edges immediately before welding

15.3 Wormholes and its Prevention

Characteristically, wormholes are elongated pores which produce a herring bone appearance on the radiograph.

Cause Wormholes are indicative of a large amount of gas being formed which is then trapped in the solidifying weld metal. Excessive gas will be formed from gross surface contamination or very thick paint

Prevention

Eliminating the gas and cavities prevents wormholes.

Gas generation

Clean the workpiece surfaces

Remove any coatings from the joint area

When clustered at the center of a weld, they are not considered dangerous fatigue promoters, or highly detrimental to fatigue resistance, although they may reduce the static stress carrying capacity of the welded member.



Porosity susceptibility of materials

Principal gases causing porosity and recommended cleaning methods

Detection of Porosity and remedial action

If the imperfections are surface breaking, they can be detected using a penetrant or magnetic particle inspection technique. For sub surface imperfections, detection is by radiography or ultrasonic inspection. Radiography is normally more effective in detecting and characterising porosity imperfections. However, detection of small pores is difficult especially in thick sections.

Remedial action normally needs removal by localized gouging or grinding but if the porosity is widespread, the entire weld should be removed. The joint should be re-prepared and re-welded as specified in the agreed procedure.

15.4 INCOMPLETE ROOT FUSION OR PENETRATION

Identification


Material Gas Cleaning

C Mn steel Hydrogen, Nitrogen and Oxygen

Grind to remove scale coatings

Stainless steel Hydrogen Degrease + wire brush + degrease

Aluminium and alloys

Hydrogen Chemical clean + wire brush + degrease + scrape

Copper and alloys

Hydrogen, Nitrogen Degrease + wire brush + degrease

Nickel and alloys Nitrogen Degrease + wire brush + degrease


Incomplete root fusion is when the weld fails to fuse one side of the joint in the root. Incomplete root penetration occurs when both sides of the joint are unfused. Typical imperfections can arise in the following situations:

An excessively thick root face in a butt weld

Too small a root gap

Misplaced welds

Failure to remove sufficient metal in cutting back to sound metal in a double sided weld

Incomplete root fusion when using too low heat input.

• too small a bevel angle, • too large an electrode in MMA welding



Effect of electrode size on root fusion

Large diameter electrode

Small diameter electrode

The risk of incomplete root fusion can be reduced by using the correct welding parameters and electrode size to give adequate arc energy input and deep penetration. Electrode size is also important in that it should be small enough to give adequate access to the root, especially when using a small bevel angle. It is common practice to use a 4mm diameter electrode for the root so the welder can manipulate the electrode for penetration and control of the weld pool. However, for the fill passes where penetration requirements are less critical, a 5mm diameter electrode is used to achieve higher deposition rates.

Best practice in prevention

Use the correct current level and not too large an electrode size for the root

When using a joint configuration with a joint gap, make sure it is of adequate size and does not close up during welding

Do not use too high a current level causing the weld pool to bridge the gap without fully penetrating the root.

15.5 TRAPPED SLAG

Slag is normally seen as elongated lines either continuous or discontinuous along the length of the weld. As slag is the residue of the flux coating, it is principally a deoxidation product from the reaction between the flux, air and surface oxide. The slag becomes trapped in the weld when two adjacent weld beads are deposited with inadequate overlap and a void is formed. When the next layer is deposited, the entrapped slag is not melted out. Slag may also become entrapped in cavities in multi-pass welds through excessive undercut in the weld toe or the uneven surface profile of the preceding weld runs. As they both have an effect on the ease of slag removal, the risk of slag imperfections is influenced by the type of flux and welder technique



The type and configuration of the joint, welding position and access restrictions all have an influence on the risk of slag imperfections.

• they normally appear as straight lines along the centreline of the weld bead, as shown in Fig., but may occasionally appear as transverse cracking depending on the solidification structure

Lack of cleanliness traps slags in the weld

Large slag inclusion creates fusion problem

It can be found by x-ray and ultrasonic inspections.

• solidification cracks in the final crater may have a branching appearance

• as the cracks are 'open', they are easily visible with the naked eye On breaking open the weld, the crack surface in steel and nickel alloys may have a blue oxidized appearance, showing that they were formed while the weld metal was still hot

• Segregation of impurities to the centre of the weld also encourages cracking. Concentration of impurities ahead of the solidifying front weld forms a liquid film of low freezing point which, on solidification, produces a weak zone. As solidification



proceeds, the zone is likely to crack as the stresses through normal thermal contraction build up. An elliptically shaped weld pool is preferable to a tear drop shape. Welding with contaminants such as cutting oils on the surface of the parent metal will also increase the build up of impurities in the weld pool and the risk of cracking.

Factors which increase the risk include:

insufficient weld bead size or shape

welding under high restraint

material properties such as a high impurity content or a relatively large amount of shrinkage on solidification.

Slag inclusions

Identification

Radiograph of a butt weld showing two slag lines in the weld root

Slag is normally seen as elongated lines either continuous or discontinuous along the length of the weld. This is readily identified in a radiograph, as shown in Figure Slag inclusions are usually associated with the flux processes

Best practice to prevent slag inclusions:

The following techniques can be used:

• Use welding techniques to produce smooth weld beads and adequate inter-run fusion to avoid forming pockets to trap the slag



• Use the correct current and travel speed to avoid undercutting the sidewall which will make the slag difficult to remove

• Remove slag between runs paying particular attention to removing any slag trapped in crevices

• Use grinding when welding difficult butt joints otherwise wire brushing or light chipping may be sufficient to remove the slag.

15.6 Cracks and Distortion

A crack is produced by a fracture which can arise from the stresses generated on cooling or acting on the structure. It is the most serious type of imperfection found in a weld and must be removed. Cracks not only reduce the strength of the weld through the reduction in the cross section thickness but also can readily propagate through stress concentration at the tip, especially under impact loading or during service at low temperature



Craters are visually inspectable depressions indicating improper weld terminations, usually with the presence of radial cracks. They should be avoided or eliminated through improved welding skill or repaired if present. Cracks can appear of two different kinds. Hot cracks form when the material solidifies, generally because of the presence of low melting constituents. Cold cracks are generated later, when the material is cold and under stress.

Usually none are tolerated (at the prescribed detection level), so that they must be removed by careful grinding (if superficial) or repaired by welding. The most insidious ones are those not open to the surface that may require specialized techniques to be detected and evaluated.

Globular volumetric three dimensional discontinuities, porosity or inclusions, are usually found deep inside the weld

Defects - solidification cracking

Identification

Visual appearance

Solidification cracks are normally readily distinguished from other types of cracks due to the following characteristic factors:

• they occur only in the weld metal • they normally appear as straight lines along the centreline

of the weld bead, as shown in Figure, but may occasionally appear as transverse cracking depending on the solidification structure



• solidification cracks in the final crater may have a branching appearance

• as the cracks are 'open', they are easily visible with the naked eye

Solidification crack along the centre line of the weld

On breaking open the weld, the crack surface in steel and nickel alloys may have a blue oxidised appearance, showing that they were formed while the weld metal was still hot.

Causes

The overriding cause of solidification cracking is that the weld bead in the final stage of solidification has insufficient strength to withstand the contraction stresses generated as the weld pool solidifies. Factors which increase the risk include:

• insufficient weld bead size or shape • welding under high restraint

• material properties such as a high impurity content or a relatively large amount of shrinkage on solidification.

Joint design can have a significant influence on the level of residual stresses. Large gaps between component parts will increase the strain on the solidifying weld metal, especially if the depth of penetration is small. Therefore, weld beads with a small depth-to-width ratio, such as formed in bridging a large gap with a wide, thin bead, will be more susceptible to solidification cracking, as shown in Fig. 2. In this case, the centre of the weld which is the last part to solidify, is a narrow zone with negligible cracking resistance

Weld bead penetration too small



Segregation of impurities to the centre of the weld also encourages cracking. Concentration of impurities ahead of the solidifying front weld forms a liquid film of low freezing point which, on solidification, produces a weak zone. As solidification proceeds, the zone is likely to crack as the stresses through normal thermal contraction build up. An elliptically shaped weld pool is preferable to a tear drop shape. Welding with contaminants such as cutting oils on the surface of the parent metal will also increase the build up of impurities in the weld pool and the risk of cracking.

Steels

Cracking is associated with impurities, particularly sulphur and phosphorus, and is promoted by carbon whereas manganese and silicon can help to reduce the risk. To minimise the risk of cracking, fillers with low carbon and impurity levels and a relatively high manganese content are preferred. As a general rule, for carbon-manganese steels, the total sulphur and phosphorus content should be no greater than 0.06%.

Best practice in avoiding solidification cracking

Apart from the choice of material and filler, the principal techniques for minimising the risk of welding solidification cracking are:

• Control joint fit-up to reduce gaps. • Before welding, clean off all contaminants from the

material

• Ensure that the welding sequence will not lead to a build-up of thermally induced stresses.

• Select welding parameters and technique to produce a weld bead with an adequate depth to width ratio, or with sufficient throat thickness (fillet weld), to ensure the weld bead has sufficient resistance to the solidification stresses (recommend a depth to width ratio of at least 0.5:1).



• Avoid producing too large a depth to width ratio which will encourage segregation and excessive transverse strains in restrained joints. As a general rule, weld beads whose depth to weld ratio exceeds 2:1 will be prone to solidification cracking.

• Avoid high welding speeds (at high current levels) which increase the amount of segregation and the stress level across the weld bead.

• At the run stop, ensure adequate filling of the crater to avoid an unfavourable concave shape

Detection and remedial action

Surface breaking solidification cracks can be readily detected using visual examination, liquid penetrant or magnetic particle testing techniques. Internal cracks require ultrasonic or radiographic examination techniques.

Most codes will specify that all cracks should be removed. A cracked component should be repaired by removing the cracks with a safety margin of approximately 5mm beyond the visible ends of the crack. The excavation is then re-welded using a filler which will not produce a crac Distortion and cracking

To reduce the amount of distortion and residual stresses, the amount of heat input should be limited, and the welding sequence used should not be from one end directly to the other, but rather in segments.

The other type of cracking, hot cracking or solidification cracking, can occur with all metals, and happens in the fusion zone of a weld. To diminish the probability of this type of cracking, excess material restraint should be avoided, and a proper filler material should be utilized.

Cold cracking is a defect that occurs in welding and requires all the following preconditions:

susceptible microstructure (e.g. martensite) hydrogen present in the microstructure (hydrogen

embrittlement) service temperature environment (normal atmospheric

pressure): -100 to +100 °F high restraint

Eliminating any one of these will eliminate this condition.


http://en.wikipedia.org/wiki/Hydrogen_embrittlement

http://en.wikipedia.org/wiki/Hydrogen_embrittlement

http://en.wikipedia.org/wiki/Hydrogen

http://en.wikipedia.org/wiki/Martensite

http://en.wikipedia.org/wiki/Welding

http://en.wikipedia.org/w/index.php?title=Hot_cracking&action=edit&redlink=1


16.1 Geometrical imperfections:

It refers to those characteristic of the weld, like incorrect fit up, misalignment, and poor bead shape (undercut, underfill, overlap, melt through and distortion) as determined by visual inspection. They are an indication of inadequate workmanship and may be cause for concern if exceeding requirement limits.

Under fill - A condition in which the weld face or root surface extends below the adjacent surface of the base metal.

Root concavity –

Overlap - The protrusion of weld metal beyond the weld toe or weld root. There may be fusion problem.

Undercut - A groove melted into the base metal adjacent to the weld toe or weld root and left unfilled by weld metal. Clean weld but irregular melting of edges because of high welding current produce undercuts.



Misalignment -

Porosity

Undercut

Slag



Crack

Excess penetration

Incomplete Fill



•

Gas Welding and Cutting

17.1 Oxy-Acetylene Welding Process


http://en.wikipedia.org/wiki/Oxy-fuel_welding_and_cutting


It is widely used for welding pipes and tubes, as well as repair work. It is also frequently well-suited, and favored, for fabricating some types of metal-based artwork. Used for iron or steel welding but also to brazing, braze-welding, metal heating (for bending and forming), and also oxyfuel cutting.

Combustion of acetylene in oxygen produces a welding flame (temperature of about 3100 °C). The flame, since it is less concentrated than an electric arc, causes slower weld cooling, which can lead to greater residual stresses and weld distortion, though it eases the welding of high alloy steels. A similar process, generally called oxyfuel cutting, is used to cut metals.

Oxy-fuel welding/cutting generally requires two tanks, fuel and oxygen


http://en.wikipedia.org/wiki/Oxygen

http://en.wikipedia.org/wiki/Acetylene

http://en.wikipedia.org/wiki/File:Welding.jpg


FuelsAcetylene (HC2H). is the primary fuel for oxy-fuel welding and is the fuel of choice for repair work and general cutting and welding.There is about 1700 kPa (250 lbf/in²) pressure in the tank when full. Acetylene when combined with oxygen burns at a temperature of 3200 °C to 3500 °C (5800 °F to 6300 °F), highest among commonly used gaseous fuelsAcetylene has a two stage reaction. The primary chemical reaction involves the acetylene disassociating in the presence of oxygen to produce heat, carbon monoxide, and hydrogen gas: C2H2 + O2 → 2CO + H2. A secondary reaction follows where the carbon monoxide and hydrogen combine with more oxygen to produce carbon dioxide and water vapor. When the secondary reaction does not burn all of the reactants from the primary reaction, the welding processes produces large amounts of carbon monoxide, and it often does. Carbon monoxide is also the byproduct of many other incomplete fuel reactions.

Other gases that may be used for gas welding are propylene, liquified petroleum gas (LPG), propane, natural gas, hydrogen, and MAPP gas (liquefied petroleum gas mixed with methylacetylene-propadiene)


http://en.wikipedia.org/wiki/Allene

http://en.wikipedia.org/wiki/Methylacetylene

http://en.wikipedia.org/wiki/MAPP_gas


http://en.wikipedia.org/wiki/Natural_gas

http://en.wikipedia.org/wiki/Propane

http://en.wikipedia.org/wiki/Liquified_petroleum_gas

http://en.wikipedia.org/wiki/Liquified_petroleum_gas

http://en.wikipedia.org/wiki/Propylene

http://en.wikipedia.org/wiki/Carbon_dioxide



http://en.wikipedia.org/wiki/Fahrenheit

http://en.wikipedia.org/wiki/Celsius

http://en.wikipedia.org/wiki/Oxygen

http://en.wikipedia.org/wiki/Pressure

http://en.wikipedia.org/wiki/KPa



17.2 Gas Welding Station



17.3 Protective Clothing for Gas Welding and Cutting

The essential features of good quality welding goggles

17.4 Safe Work Practices Electric & Gas Welding



Safety Check:

Ensure electrical cord, electrode holder and cables are free from defects

No cable splices within 10 feet of electrode holder.

Ensure welding unit is properly grounded. This helps to avoid over heating.

All defective equipment shall be repaired or replaced before using.

Remove all jewelry – rings, watches, bracelets, etc…

Ensure PPE e.g.. welding hood, gloves, rubber boots or safety shoes, apron are available and in good condition.

Ensure fire extinguisher is charged and available.

Ensure adequate ventilation and lighting is in place.

Set Voltage Regulator to Manufacture ’ s specifications.

Avoid electrical shock DON’T wrap cables around any body part.

Ensure fittings are tight.

Inspect hoses for cuts and frayed areas.

Set gauges to desired PSI.

Ensure that sufficient PPE is made available.

Locate welding screens to protect employee’s – DON’T block your exit.

Ensure that adequate ventilation and lighting are in place.

By increasing ventilation around the welding environment, the welders will have much less exposure to harmful chemicals from any source.



17.5 Welding, Cutting and Brazing Accidents – Causes

Inadequately trained personnel

Poor housekeeping practices

Poor shop layout

Inadequate lighting and ventilation

Improper storage and movement of compressed gas cylinders

Exposure of oxygen cylinders and fittings to oil/grease

Pointing welding or cutting torches at a concrete surface

Electric shock due to improper grounding

Inhalation of toxic fumes or vapors

Proximity of combustible solids, liquids, or dusts

Presence of explosive mixtures of flammable gases and air

Presence of an oxygen-enriched atmosphere



18.1 Oxy-acetylene Welding Process

The flame is applied to the base metal and held until a small puddle of molten metal is formed. The puddle is moved along the path where the weld bead is desired. Usually, more metal is added to the puddle as it is moved along by means of dipping metal from a welding rod or filler rod into the molten metal puddle. The metal puddle will travel towards where the metal is the hottest. This is accomplished through torch manipulation by the welder.

The amount of heat applied to the metal is a function of the welding tip size, the speed of travel, and the welding position. The flame size is determined by the welding tip size. The proper tip size is determined by the metal thickness and the joint design

The welder will modify the speed of welding travel to maintain a uniform bead width. Uniformity is a quality attributes indicating good workmanship. Trained welders are taught to keep the bead the same size at the beginning of the weld as at the end. If the bead gets too wide, the welder increases the speed of welding travel. If the bead gets



too narrow or if the weld puddle is lost, the welder slows down the speed of travel. Welding in the vertical or overhead positions is typically slower than welding in the flat or horizontal positions.

.The welder must add the filler rod to the molten puddle. The welder must also keep the filler metal in the hot outer flame zone when not adding it to the puddle to protect filler metal from oxidation. Do not let the welding flame burn off the filler metal. The metal will not wet into the base metal and will look like a series of cold dots on the base metal. There is very little strength in a cold weld. When the filler metal is properly added to the molten puddle, the resulting weld will be stronger than the original base metal.



18.2 Basic Gas-Welding Equipment

Acetylene and Oxygen Cylinders



Safety with cylinders

When using fuel and oxygen tanks they should be fastened securely upright to a wall or a post or a portable cart. An oxygen tank is especially dangerous for the reason that the oxygen is at a pressure of 21 MPa (3000 lbf/in² = 200 atmospheres) when full, and if the tank falls over and its valve strikes something and is knocked off, the tank will effectively become an extremely deadly flying missile propelled by the compressed oxygen, capable of even breaking through a brick wall.

For this reason, never move an oxygen tank around without its valve cap screwed in place.


http://en.wikipedia.org/wiki/Atmosphere


On oxyacetylene torch system there will be three types of valves, the tank valve, the regulator valve, and the torch valve. There will be a set of these three valves for each gas. The gas in the tanks or cylinders is at high pressure. Oxygen cylinders are generally filled to approximately 2200 psi. The regulator converts the high pressure gas to a low pressure stream suitable for welding.

Never attempt to directly use high-pressure gas.

Pressure Regulators


http://en.wikipedia.org/wiki/Gas_cylinder

http://en.wikipedia.org/wiki/Tank

http://en.wikipedia.org/wiki/Regulator

http://en.wikipedia.org/wiki/Valve


High-Pressure Welding Torch


The pressure adjusting screw:– Turning clockwise allows the

gas allows to flow.– Turning counter clockwise

reduces or stop the gas flow.


Oxyacetylene welding/cutting is not difficult, but there are a good number of subtle safety points that should be learned such as:

More than 1/7 the capacity of the cylinder should not be used per hour. This causes the acetone inside the acetylene cylinder to come out of the cylinder and contaminate the hose and possibly the torch.

Acetylene is dangerous above 15 psi pressure. It is unstable and explosively decomposes.

Proper ventilation when welding will help to avoid large chemical exposure.

18.3 Types of flame

The welder can adjust the oxy-acetylene flame to be

1. Carbonizing (as known as reducing) flame2. Neutral flame

3. Oxidizing flame

Adjustment is made by adding more or less oxygen to the acetylene flame.



Neutral Flame:

The normal welding flame produced when equal volumes of oxygen and acetylene are burnt together. The inner cone is a blunt ‘U’

The neutral flame is the flame most generally used when welding or cutting. The welder uses the neutral flame as the starting. This flame is attained when welders, as they slowly open the oxygen valve on the torch body, first see only two flame zones. At that point, the acetylene is being completely burned in the welding oxygen and surrounding air. The flame is chemically neutral. The two parts of this flame are the light blue inner cone and the darker blue to colorless outer cone. The inner cone is where the acetylene and the oxygen combine. The tip of this inner cone is the hottest part of the flame. It is approximately 6,000 °F (3,300 °C) and provides enough heat to easily melt steel. In the inner cone the acetylene breaks down and partly burns to hydrogen and carbon monoxide, which in the outer cone combine with more oxygen from the surrounding air and burn.


http://en.wikipedia.org/wiki/Carbon_monoxide



Carburising (Reducing) flame:

An excess of acetylene creates a carbonizing flame and characterised by a ragged ‘feather’ of unburnt acetylene gas around the inner cone. Since this flame diffuses carbon into the surface of low carbon steels it can be used for ‘flame-hardening’ processes, the flame being moved slowly over the metal surface followed by a quenching spray.

This flame is characterized by three flame zones; the hot inner cone, a white-hot "acetylene feather", and the blue-colored outer cone. This is the type of flame observed when oxygen is first added to the burning acetylene. The feather is adjusted and made ever smaller by adding increasing amounts of oxygen to the flame. A welding feather is measured as 2X or 3X, with X being the length of the inner flame cone. The unburned carbon insulates the flame and drops the temperature to approximately 5,000 °F (2,800 °C).



Reducing flame: The reducing flame is typically used for hardfacing operations or backhand pipe welding techniques. The feather is caused by incomplete combustion of the acetylene to cause an excess of carbon in the flame. Some of this carbon is dissolved by the molten metal to carbonize it. The carbonizing flame will tend to remove the oxygen from iron oxides which may be present, a fact which has caused the flame to be know as a "reducing flame"

Oxidising Flame: Occurs when excess oxygen is present. It is roaring, noisy flame with a sharp, conical, brilliant white inner cone. It is used only for welding some copper and aluminium alloys.


http://en.wikipedia.org/wiki/Hardfacing


19.1 Safe Work Practices

• Blow out cylinder valve

• Turn on cylinder valve first and then adjust the regulator pressure screw.

• Never stand in front or behind a regulator when opening the cylinder valve

• Open cylinder valve slowly• Purge oxygen and acetylene passages • Light the acetylene • Never use oil or grease• Do not use oxygen as a substitute for air • Keep your work area clean

Fire Prevention

Establish fire barriers

Work on fire-resistant floor

Work in environment free of flammable liquids and vapors

Check for necessity of Hot Work Permit

Establish Fire Watcher- qualified person proficient in operation of available fire

extinguishers- duty to detect and prevent spread of fire

Fill any cracks in floor

Remove or protect combustibles

Have fire extinguishers available and ready



20.1 Fluxes: No flux is required when welding the steel as sufficient protection against atmospheric oxidation is provided by the product of combustion of the burnt welding gases. However, when using cast iron, brass, bronze and aluminium filler rods, a flux must be used. Flux cleans the faces of the parent metals being joined, also prevents their re-oxidation by atmospheric oxygen. The Flux is usually supplied as powder or as a paste and must be matched to the filler rod are used and parent metal. Some times pre-coated filler rods are used, similar in appearance to thin welding electrodes.

21.1 Welding Techniques

The two principle gas welding techniques are

(a) Leftward Welding; (b) Rightward welding

Leftward welding is used for sheet metal and thin plates up to 5 mm thick. Above 5 mm thickness Rightward welding; has the following advantages:

1. Since the flame is directed towards the previously deposited metal, this metal is continuously annealed giving improved mechanical properties to the joint.

2. Less agitation of the weld pool results in less chance of oxidation.

3. More economical as smaller vee angles can be used.

4. Better vision and control for the operator.



Leftward Welding



Rightward Welding



22.0 Comparison of oxy-acetylene and metallic arc-welding



23.1 Oxy-Acetylene Cutting:

Although this is not a welding process, it uses an oxy-acetylene flame. The rate of oxidation of plain carbon steel increases with temperature, and when mild steel reaches 890 Degree Celsius it will burn to magnetic iron oxide in an atmospheric of pure oxygen. The nozzle of the cutting torch is arranged to pre-heat the metal being cut to ignition temperature. At this temperature, a jet of pure oxygen is directed from the centre of the nozzle on to the hot metal. This burns though the metal and blast particles of iron oxide from the cut. Gas cutting is widely used in fabrication industry for cutting thick plate and for edge preparation prior to welding. The cutting torch may be guided by hand or by machine.

A cutting torch head is used to cut metal. Only iron and steel can be cut using this method. It is similar to a welding torch, but can be identified by having three pipes that go to a 90 degree nozzle and by the oxygen-blast trigger.



Cutting a rail just before renewing the rails and the ballast

The outer jets are for preheat flames of oxygen and acetylene. The central jet carries only oxygen for cutting.

The flame is not intended to melt the metal, but to bring it to its ignition temperature.

Cutting is initiated by heating the edge or leading face (as in cutting shapes such as round rod) of the steel to the ignition temperature (bright cherry red) using the pre-heat jets only. Once this temperature is attained, high pressure oxygen is supplied to the heated parts by pressing the "oxygen-blast trigger". This oxygen reacts with the metal, forming iron oxide and producing heat. It is this heat which continues the cutting process. The cutting torch only heats the metal to start the process; further heat is provided by the burning metal.

The melting point of the iron oxide is around half of that of the metal; as the metal burns, it immediately turns to liquid iron oxide and flows away from the cutting zone. The oxygen chemically combines with the iron in the ferrous material to instantly oxidize the iron into molten iron oxide, producing the cut. Initiating a cut in the middle of a workpiece is known as piercing.


http://en.wikipedia.org/wiki/Iron_oxide

http://en.wikipedia.org/wiki/Ignition_temperature

http://en.wikipedia.org/wiki/File:Railway-cutting-2-a.jpg


Oxygen Rich Butane Torch Flame

Fuel Rich Butane Torch Flame

It is worth noting several things at this point:

1. The oxygen flow rate is critical — too little will make a slow ragged cut; too much will waste oxygen and produce a wide concave cut. The oxygen cutting pressure should match the cutting tip oxygen orifice.

The oxidation of iron by this method is highly exothermic. The metal is first heated by the flame until it is cherry red.

2. Since the melted metal flows out of the workpiece, there must be room on the opposite side of the workpiece for the spray to exit. When possible, pieces of metal are cut on a grate that lets the melted metal fall freely to the ground.


http://en.wikipedia.org/wiki/File:OxygenRichBlowTorchFlame.jpg

http://en.wikipedia.org/wiki/File:FuelRichBlowTorchFlame.jpg


A cutting torch is used to cut a steel pipe

Side of metal, cut by oxygen - propane cutting torch


http://upload.wikimedia.org/wikipedia/commons/0/00/Cutted_metal.jpg


‘The End’


Welding By Laxman Singh Sankhla, Jodhpur India.doc

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