MEM05026C Apply welding principles SAMPLE · (WTIA). For detailed information on recognition procedures, students will need to ... Unit parthway. MEM05026C Apply welding principles

MEM05 Metal and EngineeringTraining Package

Learner guideVersion 2

Training and Education SupportIndustry Skills Unit

Meadowbank

Product Code: 5807

MEM05026CApply welding principles

SAMPLE

MEM05026C Apply welding principles

© TAFE NSW (Training & Education Support Industry Skills Unit, Meadowbank) 2014

AcknowledgmentsThe TAFE NSW Training and Education Support Industry Skills Unit, Meadowbank would like to acknowledge the support and assistance of the following organisations and people in the production of this learner guide.

BOC Gases AustraliaCIGWELD/Thermadyne AustraliaFronius AustraliaLincoln Electric Company AustraliaSilverwater Welding SuppliesStandards AustraliaWelding Industries of Australia

Writers:Ed Harkness (From existing TAFE publications)Stephen Davies (TES Industry Skills Unit)

Reviewers:John Anderson (Hunter Institute)Vince Aranda (Illawarra Institute)Peter Stauffer (Hunter Institute)

Project Manager:Stephen DaviesEducation Programs ManagerTAFE NSW

EnquiriesEnquiries about this and other publications can be made to:

Training and Education Support Industry Skills Unit, Meadowbank Meadowbank TAFE Level 3, Building J, See Street, MEADOWBANK NSW 2114

Tel: 02-9942 3200 Fax: 02-9942 3257

© TAFE NSW (Training and Education Support, Industry Skills Unit Meadowbank) 2014

Copyright of this material is reserved to TAFE NSW Training and Education Support, Industry Skills Unit Meadowbank. Reproduction or transmittal in whole or in part, other than for the purposes of private study or research, and subject to the provisions of the Copyright Act, is prohibited without the written authority of TAFE NSW Training and Education Support, Industry Skills Unit Meadowbank.

ISBN: 978-1-74236-515-2

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Table of Contents

Introduction ................................................................................. 11

1. General introduction ........................................................................... 11

2. Using this learner guide ....................................................................... 11

3. Prior knowledge and experience ........................................................... 13

4. Unit of competency overview ............................................................... 13

Topic 1: Welding regulations ........................................................ 21

Introduction .......................................................................................... 21

Workplace health and safety (WHS) ......................................................... 23

Review questions ................................................................................... 24

Topic 2: Welding safety ................................................................ 27

Introduction .......................................................................................... 27

Hazards ................................................................................................ 27

Hazardous locations ............................................................................... 28

Industrial gas cylinders ........................................................................... 34

Scaffolding ........................................................................................... 37

Skin and eye damage prevention ............................................................. 38

Protective clothing and equipment (PPE) .................................................. 39

X-ray and gamma-ray radiation ................................................................ 40

Industrial injuries ................................................................................... 40


Topic 3: Electrical terms ............................................................... 45

Introduction .......................................................................................... 45

Types of welding current ......................................................................... 46

Alternating current ................................................................................ 47

High frequency machines (HF) ................................................................. 49

Constant current (drooping voltage) welding machine ................................. 50

Constant potential welding machine .......................................................... 51

Motor generators ................................................................................... 51

Other equipment .................................................................................... 54

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Topic 4: Arc welding conditions .................................................... 63

Introduction .......................................................................................... 63

Current/amperage .................................................................................. 63

Polarity ................................................................................................. 65


Topic 5: Manual metal arc welding electrodes .............................. 71

Introduction .......................................................................................... 71

Covered electrodes ................................................................................. 71

Electrode classification ............................................................................ 72

Type of flux coatings ............................................................................... 75

Storage, handling and conditioning electrodes ............................................ 78


Topic 6: Elements in carbon steel ................................................. 83

Introduction .......................................................................................... 83

Weldability of carbon steels ..................................................................... 83

Influence of elements on weldability of carbon steels .................................. 86


Topic 7: Heat treatment ............................................................... 93

Introduction .......................................................................................... 93

Effects of welding heat on carbon steels ................................................... 94

Effects of heat on mechanical properties .................................................... 95

Heat treatments method ......................................................................... 95

Heating methods ................................................................................... 98

Temperature measuring devices ..............................................................101

Review questions ..................................................................................104

Topic 8: Welding terms and symbols .......................................... 107

Introduction .........................................................................................107

Types of welds .....................................................................................107

Parts of a weld and general terms ...........................................................108

Parts of a preparation ............................................................................111

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Weld positions ......................................................................................113

Welding symbols ...................................................................................115


Topic 9: Preparing weld joints .................................................... 125

Introduction .........................................................................................125

Types of edge preparations .....................................................................126

Fusible inserts ......................................................................................129

Methods of preparing joints ....................................................................130

Jigs and fixtures ....................................................................................131


Topic 10: Metal cutting and gouging ........................................... 135

Introduction .........................................................................................135

Preparing metal parts for welding ............................................................135

Guided cutting ......................................................................................140

Oxygen-fuel gas gouging ........................................................................142

Applications ..........................................................................................142

Automated thermal cutting ....................................................................145

Electrode holder ....................................................................................153

Operating techniques .............................................................................154

Plasma arc cutting .................................................................................155

Hazards ...............................................................................................161

Maintenance of equipment ......................................................................162

Mechanical cutting .................................................................................162


Topic 11: Welding procedures .................................................... 167

Introduction .........................................................................................167

Proving a welding procedure ...................................................................167

Types of procedures...............................................................................168


Topic 12: Weld testing ................................................................ 177

Introduction .........................................................................................177

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Types of weld testing .............................................................................177

Destructive testing ................................................................................177

Non-destructive testing methods .............................................................184

Weld faults ...........................................................................................188

Topic 13: Welding alloy steels .................................................... 199

Introduction .........................................................................................199

Physical and Mechanical properties ..........................................................199

Alloy steels ...........................................................................................199

Effects of alloying elements on steel ........................................................203

Welding processes for joining low alloy steels ............................................208

Welding processes for HSLS ....................................................................209


Topic 14: Non-ferrous metals ..................................................... 219

Introduction .........................................................................................219

Aluminium and aluminium alloys .............................................................219

Copper and copper alloys .......................................................................225

Nickel and nickel alloys ..........................................................................228

Cutting non-ferrous metals .....................................................................230

Safety precautions ................................................................................231


Topic 15: Gas tungsten arc welding (GTAW) .............................. 237

The process ..........................................................................................237

Equipment ...........................................................................................239

Polarity/current .....................................................................................249

Weld defects .........................................................................................253

GTAW safety .........................................................................................255


Topic 16: Gas metal arc welding (GMAW) and Flux cored arc welding (FCAW) ................................... 261

Introduction .........................................................................................261

The GMAW process ................................................................................261

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Welding guns ........................................................................................264

Welding variables ..................................................................................267

Correcting weld and machine faults .........................................................278

Safety .................................................................................................280

Flux cored arc welding (FCAW) ................................................................281

Flux cored wires ....................................................................................282

Shielding gases .....................................................................................286


How the process works ..........................................................................291

Equipment ..........................................................................................291

Consumables ........................................................................................296

SAW variables .......................................................................................297

Edge preparations .................................................................................300

Typical SAW faults .................................................................................303

Safety ................................................................................................304

Electro-slag welding (consumable guide) ..................................................306

Topic 18: Oxyacetylene welding (OAW)...................................... 311

The OAW process ..................................................................................311

Equipment for OAW ...............................................................................312

Welding torches ....................................................................................318

OAW weld faults ....................................................................................324

Low temperature welding processes .........................................................324

Consumables ........................................................................................326

Uses ....................................................................................................327

Resource Evaluation Form .......................................................... 333

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Page 15 of 336© TAFE NSW (TES, Industry Skills Unit Meadowbank) 2014


Recognition of prior learning (RPL)

RPL is not applicable for this competency unit. MEM05026C Apply welding principles is a welder certification unit and therefore will require candidates to formally undertake an externally set examination as previously indicated.

This unit, along with all other AS 1796 Certifications require a formal examination / assessment process as outlined in section 2 of the Standard. Students who have successfully achieved a TAFE NSW AS 1796 Certification are eligible to receive full recognition for certificates 1-9 from external industry associations such as the Australian Welding Institute (AWI) and Welding Technology Institute of Australia (WTIA). For detailed information on recognition procedures, students will need to contact either or both of the two industry associations on their respective websites: http://www.austwelding.com.au/ and

http://www.wtia.com.au/

Unit parthway

MEM05026C Apply welding principles is a unit in the MEM05, Metal Engineering Industry Training Package. It is a unit of study in Certificate III and IV in Engineering and Diploma in Engineering-Advanced Trade. It may also be selected as a unit of study in other Engineering courses, packaged to include a group of fabrication and specialist units.

In conjunction with further units of study, welder certification provides students with the knowledge and skills to carry out first class welding operations in an engineering workshop or site. A pathway of training in the use of applying welding principles could include the following competency units:

MEM05043B Perform

welds to code standards

using GMAW

MEM05026C Apply Welding

Principles

MEM05042BPerform

weld to code stnadards

using FCAW

MEM05044BPerform welding to code

standards using GTAW

MEM05045BPerform pipe welds to code

standards using MMAW

MEM05046BPerform

weld to code standards

using MMAWSAMPLE

Page 16 of 336 © TAFE NSW (TES, Industry Skills Unit Meadowbank) 2014


Elements and performance criteria:

Element Performance criteria1. Apply all statutory and

regulatory requirements to welding procedures.

1.1. Statutory and safety requirements are applied to welding

2. Interpret all welding terms, codes and symbols.

2.1. Welding terms and symbols are correctly interpreted.

3. Determine the effects of heat treatment on metal in relation to welding.

3.1. Reasons for performing heat treatment are identified.

3.2. Processes such as pre-heat/post-heat treatment, stress relieving, normalising and annealing are appropriately applied.

Required knowledge and skillsThe following describes the knowledge and skills required for this unit. These include:

Required knowledge:

• Any applicable industry standards, national/Australian standards, NOHSC guide-lines, state/territory regulatory codes of practice/standards for the applicable welding processes

• Safe work practices and procedures

• Hazards related to welding

• Safety equipment and procedures related to welding activities

• Welding terminology

• Welding codes and symbols

• Logical sequence of welding processes

• Tools, equipment, techniques used in welding

• Effect of heat treatment on metal.

Required skills

• Interpreting welding specifications including terms, codes and symbols

• Planning the sequence of welding operations

Access and equity considerations

• All workers in Metal and Engineering industries should be aware of access, equity and human rights issues in relation to their own area of work

• All workers should develop their ability to work in a culturally diverse environment

• In recognition of particular issues facing Aboriginal and Torres Strait Islander communities, workers should be aware of cultural, historical and current issues impacting on Aboriginal and Torres Strait Islander people

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• Assessors and trainers must take into account relevant access and equity issues, in particular relating to factors impacting on Aboriginal and/or Torres Strait Islander clients and communities.

Work health and safety (WHS)

Your teacher will encourage you to assist in identifying/eliminating hazards and to devise control measures for potential risks to yourself and others that may arise during practical sessions. The Work, Health and Safety Act 2011 (WHS Act) and WHS Regulations are enforced throughout all Australian States and Territories.

This legislation is aimed at providing consistency, certainty and clarity across Australia making it easy to understand workplace health and safety responsibilities. The WHS Act and its regulations will require both teachers and learners to take reasonable steps to control and monitor potential risks in the classroom, practical workshops and in the workplace. Detailed information relating to the WHS Act and regulations can be found on the following websites:

• WorkCover Authority of NSW www.workcover.nsw.gov.au

• Safe Work Australia www.safeworkaustralia.gov.au

What you will needStudents will require the following:

• Pens, pencils and calculator

• Learner resource MEM05026C Apply welding principles

Wallet cardUpon successful completion of this knowledge based unit and at least one (1) of the following practical competency units (see list below) you will be eligible to receive a TAFE NSW issued wallet card which recognises your certification to AS 1796.

• MEM05042B Perform welds to code standards using flux core arc welding process

• MEM05043B Perform welds to code standards using gas metal arc welding process

• MEM05044B Perform welds to code standards using gas tungsten arc welding process.

• MEM05045B Perform pipe welds to code standards using manual metal arc welding process

• MEM05046B Perform welds to code standards using manual metal arc welding process.

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X-ray and gamma-ray radiationSome non-destructive weld testing methods involves radiography. Radiographic testing is done by trained personnel who are responsible for ensuring the area is safety-signposted. All other persons must remain outside the designated area during testing. Exposure to radiographic rays can cause long term damage and must be avoided at all times.

Canaryyellow

Black

Radiation warning symbol

Industrial injuriesWhile good housekeeping and safe work practices prevent most industrial accidents, injuries sometimes happen. Typical industrial injuries include:

• burns (minor and major)

• cuts

• abrasions

• bone fractures

• neck and spinal damage.

Many cuts burns and abrasions are minor and are treated with little loss of work time. However all injuries, no matter how small, should be reported to the first aid officer to help plan future safety procedures.

More serious injuries should be treated by qualified personnel. On-the-spot first aid should not be a substitute for qualified assistance. The following steps, in the event of an accident are a guide only:

• act quickly

• switch off any electrical circuits involved

• do not touch burns or remove clothing from burnt areas

• send for medical assistance

• stay with the victim at all times

• never move the victim if a back or neck injury is suspected.

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Review questionsThese questions have been included to help you revise what you have learnt in Topic 2: Welding safety.

Short-answer questions

1. List four (4) sources of fumes generated from thermal cutting or welding operations.

______________________________________________________________

______________________________________________________________

______________________________________________________________

2. List three (3) precautions that must be taken when welding in a confined space.

______________________________________________________________

______________________________________________________________

______________________________________________________________

3. Explain how gas cylinders should be supported when they are lifted by a crane.

______________________________________________________________

______________________________________________________________

______________________________________________________________

4. Briefly explain why MMAW electrodes must be removed from holders when hung on scaffold staging where there are wire ropes.

______________________________________________________________

______________________________________________________________

5. Name three (3) harmful rays emitted from an electric arc.

______________________________________________________________

______________________________________________________________

______________________________________________________________

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Multiple choice questions (tick the correct answer)

6. The first precaution against fire or explosion in a hazardous location is: □ no welding or cutting is permitted □ high frequency equipment must not be used □ there must be an inspection of the area □ the tradesperson must hold a current pressure welding certificate

7. The background colour of the identifying shoulder label on an oxygen cylinder is: □ red □ white □ yellow □ green

8. Confined spaces require: □ degreasing before welding □ adequate ventilation □ workers to take salt tablets regularly to avoid dehydration □ small diameter electrodes to be used

9. Welding filter lens must be: □ shade 13 minimum □ approved and identified by Australian Standards □ made in Australia □ green

10. The primary circuit, high frequency units and welding leads: □ are serviced by welding tradespersons □ are safe to use in wet conditions □ can be assumed free of all hazards □ can all be hazardous

True or False questions (circle the correct response)

11. A container that has held flammable or toxic materials must be examined by a responsible person before welding. True False

12. All lead lighting in confined spaces must be 32 volts maximum. True False

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13. Acetylene cylinders are colour coded peacock blue. True False

14. Radiographic weld tests present no danger to the operator. True False

15. LPG is a fuel gas. True False

16. Argon cylinders are fitted with right hand threaded connections. True False

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Topic 3: Electrical terms

IntroductionWelding operators must have a basic knowledge of electricity and understand the electrical requirements of various arc welding processes. This topic is intended to assist you understand the basic terminology and application of electricity associated with arc welding processes. The electrical terms in common use are briefly explained below:

Current (Symbol I or A)

Also referred to as amperage, current is the measurement of electron flow through a given circuit. The term current measures the amount of electricity that flows through a circuit in a second. The level of current supplies the heat energy required for welding. An amp is a unit of electric current.

Voltage (Symbol V or E)

Voltage or electric potential is the driving force (force or pressure) that enables a current to flow within an electrical circuit. A volt is a unit of electric pressure.

Resistance (Symbol O or Ω)

Resistance is the restriction to current flow in an electrical circuit. Every component in the circuit, including the conductor, has some degree of resistance to current flow. The unit of resistance is the ohm. The welding arc has its heat energy released and resulting high temperature from the resistance in the arc column.

Open circuit voltage (OCV)

Open circuit voltage is the potential available when the welding machine is turned on but before the arc is struck. The OCV can be measured with a voltmeter at the output terminals of the welding machine before welding takes place. The level of OCV is carefully controlled by manufacturers of welding machines to avoid electric shock to the operator. Modern machines are fitted with a voltage reduction device (VRD) to provide added safety to the operator.

Examples of open circuit voltages:

• a.c. manual arc welding machines 50-80 OCV (80 volts maximum)

• d.c. manual arc welding machines 60-115 OCV (115 volts maximum).

Some types of a.c. manual metal arc welding machines have dual OCV ranges. These machines allow the operator to deal with a wider range of situations, such as:

• the use of small diameter electrodes which require a high OCV and low current

• special electrode types such as hydrogen controlled, have basic fluxes. They can be difficult to strike and require a high OCV between 70-80 volts to establish the arc

• large electrodes with larger cross sectional area require less OCV.

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Arc voltage

Arc voltage is the measure of voltage across a welding machines terminals while the machine is working and welding is taking place. The arc voltage for most machines is in the range of 20-35 volts.

Voltage reduction device(VRD)

VRD stands for voltage reduction device and is safety device fitted to a welding machine. The purpose of a VRD is to reduce the maximum unloaded open circuit voltage (OCV) across the machines terminals to a safe voltage. When a welding circuit resistance is greater than 200 ohms (for example when touching the human body) the machine's VRD will sense this and limit the OCV to around 12 volts.

When a load less than 200 ohms resistance is applied to the machines output, the VRD will sense this and allow the machines maximum OCV to flow to enable welding to take place. Australian Standards 1674.2 2003 states the voltage across the machines terminals must be less than 35 volts for d.c. and 25 volts for a.c.

Types of welding current

Direct current When the electrons move in the same direction at all times, this is called direct current (d.c.).

In a direct current machine, the electrons flow constantly from the terminal marked negative (-) to the terminal marked positive (+).

The way these terminals are connected to the welding circuit determines the polarity of the connection.

+

Time

Direct current

The diagram above shows the current and voltage delivered over a period of time. The level stays constant because the current flows in only one direction.

Australian Standard 1101.3-2005 defines this as either electrode positive or electrode negative. The word pole simply means the end of the arc – polarity means which terminal is connected to the electrode end of the arc.

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The normal connection is electrode negative (-) because of the direction of electron flow, the job is at the positive (+) receiving end of the electron flow in the arc and more heat is released at that point.

Heat distribution in a direct current arc is approximately 2/3rds at the positive pole and 1/3rd at the negative pole. For this reason, consumable wire electrodes such as those used with the GMAW process are connected to the positive (+) terminal to allow the wire to rapidly melt-off to form the weld.

When using the GTAW process the opposite is in place, the non-consumable tungsten electrode is connected to the negative (-) terminal. This arrangement allows 2/3rds of the heat energy to flow from the electrode tip into the workpiece where it is required.

Alternating current When the electron flow reverses its direction at regular intervals, it is called an alternating current (a.c.). Australian standard cycle time for alternating current is 50 cycles per second, or 50Hz (hertz).

1 cycle = 1/50 of a second or 20 ms

Time

Typical a.c. sine wave

The current acts for the same length of time for both the positive (+) and negative (-) parts of the cycle (10ms) ie. the time distribution between positive and negative is 50/50. This gives a 50/50 heat distribution as well.

a.c. phases The single wave graph shown below illustrates a single alternating current phase. This is the type of current available from a 240v power point. Power generation plants that supply mains power, deliver three phase current to make better use of the distribution system.

Single phase ACSingle phase a.c.

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The main industrial supply is 415v 3-phase. Some of the transformer type welding machines will only use 415v single phase input. This is clearly marked on power source compliance plates, which also gives the rating and duty cycle of the machine.

Three phase ACThree phase a.c.

Rectified a.c.a.c. can be changed to d.c. by passing the current through a heavy-duty semi-conducting diode. This will allow the current to pass in one direction but not in the reverse. The result is a single-directional current flow.

Single diode rectifier

The graph shown above is for a single diode rectifier. It only uses one half-cycle and the full power of the input is not used. This is called half-wave rectification. If diodes are used in pairs the whole of the ac input is put to use. This called full wave rectification.

a.c. full-wave single phase (all available current is used in the negative cycle)

More advanced welding power sources used for MMAW, GMAW and GTAW will use the full three phases and full wave rectification. Output level is much more steady and similar to the direct current generator. These machines use a bridge rectifier which is made up of three diodes to cope with the three phases.

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0

Typical three phase current

High frequency machines (HF)Alternating current (ac) welding machines work well when using flux coated electrodes requiring a current of about 80 amps or more, but when current values are lowered for smaller electrodes, striking the arc becomes difficult.

A high frequency (HF) unit generates a high voltage (3000v) but minimal current at 2 million Hz (2 MHz). This secondary superimposed voltage allows the welding arc to ignite (strike) without the electrode physically touching the surface of the job. It will also maintain an arc when the voltage and current values are low. High frequency current does not add to the heat input of the main welding arc.

Welding leads are covered with a special type of insulation to avoid the high frequency arc through and to prevent being degraded by the ozone generated in the corona discharge surrounding them while the current is flowing.

High frequency is used for:

• welding with small diameter electrodes

• gas tungsten arc welding with a.c.

• initiating an arc when using gas tungsten arc welding with d.c.

High frequency safetyWhen using high frequency with welding equipment, the operator must take care to avoid accidental, unintentional ‘arcing’ causing eye injury. As indicated above, the arc can start without the electrode physically striking the work surface.

Care should be taken to protect the high frequency unit from wet weather or moisture conditions as this greatly increases the risk of fatal electric shock. While high frequency current does not have high amperage it will assist in directing the main welding current through a persons body.

Electrode leads should be kept clear of metal surfaces as the high frequency pilot arc will discharge through the insulation causing rubber-covered leads to break down. Securely fasten the work clamp to the work and ensure that both the work and work table are isolated from building frames or support structures.

Failure to do this may result in the welding current damaging both the machine and the primary wiring potentially creating an electrical hazard for workshop personnel.

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Constant current (drooping voltage) welding machineAll manual metal arc welding machines which maintain constant current have a drooping volt/amp characteristic curve. The open circuit voltage (OCV) is higher than the voltage needed for welding, but is needed at that value to enable easy striking of the electrode.

Once the arc is established, the voltage will drop to a pre-set level for welding. This is called the arc voltage. This characteristic holds the current constant even during small variations in arc length, which naturally occur because the electrode is manually fed into the arc.

When the electrode makes contact with the job it results in a ‘short-circuit’ which allows maximum current to flow. As the contact is broken the arc establishes and the pre-set current and voltage come into action for welding.

The graph below shows the volt-amp conditions during welding. It can be seen from the graph that a comparatively large change in voltage will only have a small effect on the current setting. This effectively maintains a near-constant current to the arc.

Current (amps)

Voltage

Volt-amp curve in a drooping voltage machine

Some modern electronically controlled power sources deliver true constant current whereby a change in arc length results in no change in welding current. A typical volt-amp characteristic of a true constant current power source is shown in the diagram below.

U (V)

I (A)no change in welding current with change in arc length

Constant current power source

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It can be seen that there is no change in welding current despite a change in arc length.

Constant potential welding machineGas metal arc welding (GMAW) machines have a relatively flat volt-amp output curve. These machines have a lower open circuit voltage than those used for manual metal arc welding and are not suitable for this process.

To produce consistently uniform welds, GMAW power sources must be able to produce and maintain a constant voltage and arc length throughout the welding operation. This consistency is achieved by using a constant voltage (CV), also called constant potential power source in conjunction with a wire feed mechanism.

Constant voltage machines are designed to produce a relatively flat voltage/current characteristic. For example, as the wire “stick out” length increases with this type of machine the welding current will automatically decrease. Conversely, as wire “stick out” length decreases, welding current will increase but voltage remains constant. An example of the flat voltage/current type relationship is illustrated below.

OCV

Volta

ge

Current

Operating point

CV flat volt /current characteristic

Note: Modern inverter power sources can produce drooping and constant potential power supply depending on the welding process selected.

Motor generatorsThere are two types of motor generators, these include:

1. 415v mains supply connected to an electric motor which drives a generator producing a suitable range of welding currents. This type machine features a variable voltage for finer control over arc conditions.

2. Petrol or diesel engine driven generators produce welding current. Most of the currently produced by this type of machine is based on heavy-duty alternators. Current output can be rectified to d.c. as needed.

Both a.c. and d.c. welding supplies are available as welding output current. Many of them provide auxiliary outlets for power tools and wire feeder units.

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Review questions

These questions have been included to help you revise what you have learnt in Topic 3: elementary electricity.

Short answer questions

1. Define the following electrical terms: Voltage.

________________________________________________________________

________________________________________________________________ Amperage.

________________________________________________________________

________________________________________________________________

Open circuit voltage (OCV).

________________________________________________________________

________________________________________________________________

2. Explain why all welding machine terminals must have tight, metal to metal connections. ________________________________________________________________ ________________________________________________________________

3. Name three (3) types of power sources capable of delivering direct current (d.c.) for welding.

________________________________________________________________

________________________________________________________________

________________________________________________________________

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Multiple choice questions (tick the correct answer).

4. Which of the following power sources does not have a choice of polarity? □ rectifier □ inverter □ transformer □ generator

5. Which of the following power sources can be used on locations without on-site power? □ rectifier □ inverter □ transformer □ generator

6. Welding work clamps are rated by: □ size of clamp opening □ voltage range □ size of welding cable □ current carrying capacity

7. A high frequency unit is attached to a welding machine to □ increase weld metal deposition □ enable the arc to be struck without touching the work □ prevent undercut □ change a.c. to d.c.

8. An alternating current (a.c.) sine wave has a: □ positive half cycle and a negative half cycle □ positive cycle only □ negative cycle only □ flat characteristic

True/false questions (circle the correct response).

9. All welding power sources have the same open circuit voltage.

True False

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10. The maximum open circuit voltage for a.c. welding machines is 80v.

True False

11. Voltage is the unit of pressure in an electrical circuit.

True False

12. d.c. inverters can be used for MMAW, GTAW and GMAW.

True False

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Topic 4: Arc welding conditions

IntroductionWith electric arc welding the variables depend on the process selected. In this topic we compare the effects of changes to weld conditions on weld quality and stability of the arc using the manual metal arc welding process.

Current/amperageThe current/amperage setting controls the heat energy available to the welding arc. Electrode manufacturers will always label the electrode packet with the type of electrode and current range settings recommended for that type and size.

Using recommended current settings will give the deposited metal its full strength and other important properties. Most general purpose (GP) electrodes are used at currents near the top of their recommended range. Lower settings make electrodes difficult to run consistently. Using too low a current/amperage setting for a given type and size electrode can:

• reduce arc stability making it difficult to strike and maintain the arc• reduce penetration• produce poor weld appearance• cause problems removing slag from the toes of the weld• reduce welding speeds• increase the risk of slag inclusions• produce excessively convex bead shapes.

When operating within the recommended current range, penetration and bead shape improve as current settings increase. However, using too high a current/amperage setting for a given type and size electrode can cause:

• porosity due to overheating of the electrode and deterioration of the flux coating

• increased spatter levels• poor weld appearances• undercut• a reduction in alloying elements from the deposited metal• increased consumption rate of the electrode.

Attempts to gain increased penetration by increasing current settings beyond the recommended maximum can be counter-productive. If deep penetration is required then a deep-penetration electrode should be used.

VoltageA minimum arc voltage is needed to maintain the welding arc. MMAW electrodes will operate successfully within a fairly narrow voltage range. Within this range there is a noticeable effect upon weld penetration, bead width and blending at the toes of the weld run.

The general effect of increasing arc voltage is to spread and flatten the weld bead, which can be said for all arc processes. Basic types of a.c. transformer and a.c./d.c. transformer-rectifier machines have no means of varying the arc voltage. Some transformer machines have dual-voltage terminals however this only gives a choice

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Documents

MEM05026C Apply welding principles SAMPLE · (WTIA). For detailed information on recognition procedures, students will need to ... Unit parthway. MEM05026C Apply welding principles