DOC NO. 5Q&A-002-SA1 Revision: 0 Management · PDF fileQUALIFICATION & CERTIFICATION BOARD Date of Issue: 23 May 2011 Management System Manual IIW International Welding Specialist

DOC NO. 5Q&A-002-SA1 Revision: 0

QUALIFICATION & CERTIFICATION BOARD Date of Issue: 23 May 2011 Management

System Manual IIW International Welding Specialist (IWS) Sample Questions and Answers

SA1 – WELDING PROCESS AND EQUIPMENT

Page 1 of 12

ABN 69 003 696 526

QUESTION 1 1.1 Briefly describe the main arc initiation mechanisms for GTAW welding and give advantages and

disadvantages of each. 12 marks

Scratch Starting Method As with MMAW the electrode is “scratched” on the work surface to start the arc, and then moved away to the required arc length. • Lowest cost equipment. • Risk of tungsten contamination on work piece, and/or damage to electrode. • Welder must master the technique to avoid electrode sticking to the work. High Frequency (HF) Ignition A high voltage, high frequency current provides an ionized path for the welding current to follow when the electrode is brought within about 5mm of work piece. • Less skill required than with scratch starting, and less risk of contamination. • Enables stable AC welding with sinusoidal wave form. • Requires a HF unit (built in or added to power source). • HF may adversely affect other electrical and electronic equipment. Contact or Lift Arc Ignition The electrode is brought into contact with the work, and then lifted off and the arc ignites. This is done with electronic control that detects the short circuit situation and limits the current flow to very low level. As soon as the electrode is lifted the arc is started. • No HF even with AC (square wave form). • Low skill, low risk of contamination. • Highest cost, only available on some modern power sources.

1.2 With the aid of graphs, discuss the power source characteristics and applicable welding processes

for: a) Constant current drooping characteristic b) Constant voltage characteristic 8 marks Drooping: The output voltage decreases as the output current increases; Low (restricted) short-circuit current. Suitable for the following processes

MMAW TIG





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ABN 69 003 696 526

A Large change in arc length – (Voltage) causes a small change in amperage (melting rate).

Current

I1 I2

V2

V1

Vol

tage

A constant current machine will vary its output voltage to maintain a steady current while a constant voltage machine will fluctuate its output current to maintain a set voltage. Constant voltage: The output voltage remains substantially constant under output current alterations or a small change in arc volts results in a relatively large change in welding current. Suitable for constant feed electrode

• GMAW (MIG/MAG) • FCAW (Flux cored arc) • SAW (Sub-arc)

Small change in arc length – (Voltage) causes big change in amperage. Current

Vol

tage

V1 V2

I1 I2

V





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ABN 69 003 696 526

QUESTION 2 2.1 Briefly explain and describe the four main metal transfer mechanisms for GMAW welding with the

aid of sketches. 15 marks Dip (short arc, short circuiting) transfer:

Low voltage and low current (14-20V and ~60-200A on steel). Metal deposited during rapidly occurring short circuits (20-200/sec). Low heat input and narrow heat spread; weld metal 'freezes' rapidly. Spatter can occur if the short circuiting is violent. Uses: thin sections, positional welding, root pass to bridge a root gap. Globular transfer: Typically occurs at ~20-26V and >180A (on steel). Metal transfers in large globules at relatively low current density, sometimes during short circuits. The droplet size is characteristically greater diameter than the electrode. Uses: welding steel with CO2 shielding gas. Spray transfer:

High settings ~27V 180A (steel). Droplets < wire Ø. Axial transfer. High Ar content in shielding gas High heat input. High deposition rate. Deep penetration. Smooth weld surface. Minimal spatter. F and H positions





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ABN 69 003 696 526

Pulsed transfer Controlled spray transfer at the selected wire feed rate.

• Spray transfer at current levels that would normally produce dip or globular transfer. • Positional welding in spray, improved weldability of thinner sections

Extends the current range in which spray transfer will occur. One form of improves the performance of dip (short arc) transfer. ”Synergic" power sources use electronic control to manage the complex parameters. Advantages include;

• Electronic control and feedback give a very stable arc with minimal spatter, and low risk of wire burn back or stubbing.

• Simplified selection of parameters with power source containing programs for specific materials, wire diameters and shielding gases.

• Larger diameter electrode wire diameters can be used (cheaper and easier to feed). 2.1 List an two advantages and two disadvantages of using argon based mixed shielding gas compared

to pure C02 shielding gas in MAG welding? 4 marks

Advantages: • Does not require a heating unit for the regulator • Less spatter • More stable arc Disadvantages: • More expensive • Shallower and narrower penetration profile, thus greater risk of lack of fusion • Less tolerant of dirty steel

2.2 Discuss the differences between the three flame settings used in oxy-acetylene welding and

brazing, explain the features that distinguish them from each other and list an application where it be used. Use sketches if necessary. 16 marks

Neutral flame

• 1:1 gas ratio approx. • High temperature (3200°C), concentrated zone at the end of the inner cone. • adequate heat content • minimum chemical reaction with parent and filler metals. • Use for fusion weld steels, cast iron, stainless steel, copper, aluminium and for brazing.





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ABN 69 003 696 526

2.3 List two reasons why GMAW wires may be copper coated? Give one drawback of the copper

coating. 3 marks

Reason • It improves the current pickup between contact tip and the electrode. • Helps prevent rusting of the wire when it is exposed to the atmosphere.

Drawback • Copper inclusions

2.4 List an advantage and a disadvantage of using pure C02 shielding compared to in argon based

shielding MAG welding? 2 marks

Advantages: • Cheaper • Deeper and wider penetration profile, thus lesser risk of lack of fusion Disadvantages: • Spatter • Heating unit required in the regulator • Unstable arc

QUESTION 3 3.1 Explain the effect of current and polarity of the GTAW process in terms of heat distribution,

penetration, electrode capacity, and cleaning action with the aid of a neat sketch. 10 marks Current Polarity

DCEN DCEP AC (Balanced)

Oxide Cleaning Action

No Yes Yes – once every ½ cycle

Heat balance 1/3 electrode 2/3 work

2/3 electrode 1/3 work

50/50

Penetration Deep, narrow Wide, shallow Medium Electrode capacity

Excellent Poor Medium





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ABN 69 003 696 526

3.2 Explain the principles of the SAW process with the aid of a neat sketch. 10 marks Process Principles

In the submerged arc welding (SAW) process the arc is established between a continuously fed wire electrode and the work. The arc is "submerged" under a layer of granular flux. Arc Characteristics

Once the arc is established the flux melts and forms a liquid bubble around the arc, periodically expanding, bursting and then reforming. Metal transfer is irregular, but confined to the flux bubble so no spatter is formed. A cavity at the front of the weldpool creates a strong rearward flow so that the weldpool is significantly elongated.

Arc

Weldpool

Weld

Solidified

Flux recycling system

Wire

Power source Granular flux

Work

Wire drive

Hopper

slag

Granular flux Solidified slag

Molten slag

Weldpool

Arc cavity

Electrode

Solidified weld metal





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ABN 69 003 696 526

QUESTION 4 4.1 Explain, with the aid of a sketch, the principles of the MMAW Process. 10 marks Process Principles

Electrode Core wire Flux coating

Arc

Slag coated metal droplet

Weld pool

Solidified slag Weld deposit

Gas shielding Power Source

The tip of the electrode is brought into contact with the work and then drawn a short distance away to form an arc. The heat of the arc (<6000°C) forms the molten weld pool on the work and the core wire of the electrode melts, transferring molten droplets across the arc to provide the filler metal. The arc and weld pool are shielded from the atmosphere by the decomposing flux coating on the electrode. The flux provides shielding by gases (such as CO, CO2,, H2) and molten slag. 4.2 State two functions of an electrode flux coating for MMAW. 2 marks

• To create a slag • To create a gas shield • To transfer elements into the weld pool – metallurgical • To transfer elements to the weld pool – volume increase • To protect the core wire.

4.3 List two limitations of MMAW. 2 marks

Limitations of MMAW

• Low deposition rates compared to other manual processes. • A high skill level required high quality welds, especially for work such as positional welding on

structural steel work or for pressure pipe welding. Many welders require several years experience to become truly competent in this type of work.

• Consumables must be stored and handled correctly for low hydrogen welds. Joint set-up and cleanliness is critical for top quality welds.





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4.4 Identify a surface characteristic imparted by surfacing or spraying. 1 mark

• Wear resistance • Heat resistance • Oxidation resistance • Corrosion resistance • Electrical resistance • Electrical conductance • Restoration of size

4.5 Explain, with the aid of a sketch, the principles of the FCAW Process. 10 marks Flux-cored arc welding (FCAW) is a semi-automatic or automatic arc welding process. FCAW requires a continuously-fed consumable tubular electrode containing a flux and a constant voltage or, less commonly, a constant electric current welding power supply. An externally supplied shielding gas is sometimes used, but often the flux itself is relied upon to generate the necessary protection from the atmosphere. The process is widely used in construction because of its high welding speed and portability.

FCAW Equipment

The FCAW Process

Welding gun

Power source

Wire feed unit

Shielding gas

l

Weld

Shielding gas nozzle Electrod

Contact tip

Arc

Gas shielding

QUESTION 5 5.1 Define the following terms:

i) Duty Cycle (Power Source). 2 marks

The percentage of time, for an arbitrary test period, during which power supply can be operated at its rated output without overloading.

ii) Deposition Efficiency of a Welding Process 2 marks

Deposition efficiency in arc welding is the ratio of the weight of the weld metal deposited to the nett weight of the filler material used.

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

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

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

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

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

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

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





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ABN 69 003 696 526

5.2 State the effects of the following on weld quality i) Arc length that is too long using GMAW 2 marks

• Increase spatter levels • Increase the chance of undercut. • Produce an erratic arc. • Increase the chance of contamination from the surrounding air. • Increase the voltage.

5.3 Explain the process principles of Friction Welding with the aid of a sketch. 4 marks

Friction welding is a solid-state joining process that produces a weld under compressive force between the contacting surfaces of work pieces moving or rotating relative to one another. Heat is produced and material is plastically displaced from the faying surfaces.

5.4 What is the fundamental difference between brazing and soldering. 2 marks

Brazing - the filler metal melting temperature is higher than 450°C Soldering - the filler metal melting temperature is lower than 450°C

5.5 Explain the principles of Explosive Welding. 3 marks

Explosive welding is a solid state welding process (that is, no fusion or liquid state) that uses a progressive explosive detonation to force two metals to impact together at high speed and pressure to form a metallurgical bond. The explosive force brings the two metals progressively together at the collision front while expelling a jet of metal that forms just ahead. The jet cleans the surfaces from oxides and contaminants.

5.6 Explain with the aid of a sketch, the hot plate welding process used for weldable plastics. 5 marks The technique involves heating the ends of the parts to be joined against an electrically heated platen until they are sufficiently molten. The heater plate is then removed and the parts pressed together. A cooling cycle follows, allowing the weld to develop strength. The platens are normally stainless steel or aluminium coated with PTFE to prevent the molten polymer sticking to them. Holding fixtures are required to hold the parts in alignment for welding. To make the weld, the joining surfaces are brought into contact with the heated platen and held with sufficient pressure to allow the surfaces to melt. Once the surfaces are melted, the holding pressure is released, the platen removed and the two surfaces pushed together to form the joint. Pressure is maintained until the molten material has solidified.





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ABN 69 003 696 526

5.7 Explain with the aid of a sketch, the principle of carbon arc air gouging. 5 marks

Travel. Electrode at 35°-70° to work surface.

Air jet under electrode

Electrode clamp with air holes. DCEP or AC

Copper coated electrode. Stickout ~150mm max.

Molten material

Torch

An arc is struck between a consumable carbon-graphite electrode and this produces a molten pool on the workpiece. The molten metal is then blown away by jets of compressed air, the forward movement of the electrode producing a cut or groove in the metal.





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ABN 69 003 696 526

QUESTION 6 The following questions have multiple-choice answers and are worth 2 marks each. Only one answer is correct. Tick only one. If more than one box is ticked, no marks will be awarded. 6.1 The development of the GMAW/MAG welding processes has been mainly due to?

a. Improvement of productivity and ease of mechanisation. b. Applicability to welding of stainless steels. c. Use of inert gases.

d. Improvement of mechanical strength of weld. 6.2 Which welding process you recommend for Cr-Mo steel pipes, 20 mm diameter and 2 mm wall

thickness:

a. Manual Metal Arc Welding (MMAW) b. Gas Tungsten Arc Welding (GTAW) c. Plasma Keyhole Welding d. Submerged Arc Welding (SAW)

6.3 Which of the following welding processes is only suitable for automation?

a. Manual Metal Arc Welding (MMAW) b. Submerged Arc Welding (SAW) c. Electron beam welding (EBW) d. Gas Tungsten Arc Welding (GTAW)

6.4 In arc welding processes, penetration is mostly affected by?

a. Preheat b. Voltage c. Arc length d. Current 6.5 The welding power source must: a. Decrease the voltage of the power supply and increase the current. b. Maintain the parameters of the power supply. c. Increase the efficiency of the primary consumption. d. Decrease the power supply current. 6.6 Which of the following power source types produces direct current? a. A rectifier b. A diesel generator c. An inverter d. All of the above 6.7 What is the difference between MIG and MAG Welding? a. The base material b. The type of shielding gas used c. The wire electrode d. The power source





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ABN 69 003 696 526

6.8 When welding austenitic stainless steels, backing gas is often used to prevent oxidation of the surface of the material thereby reducing the corrosion resistance. Which of the following types is not used as backing gas for stainless steel?

a. Argon b. Argon and CO2 mixture c. Helium

d. Nitrogen and hydrogen mixture, eg. 90/10 6.9 What determines the selection of a wire feed roller for GMAW/MAG? a. The type (chemical composition) of the wire b. Wire feed rate c. The pre-bending of the wire d. Diameter of the roller 6.10 Which MMAW electrodes produce the deepest penetration in the base metal?

a. Acid coated electrodes b. Rutile coated electrodes c. Basic coated electrodes d. Cellulose coated electrodes 6.11 In MMAW with covered electrodes when the arc length increases,

a. Current increases. b. Penetration increases. c. Arc voltage increases. d. Spattering decreases.

6.12 In submerged arc welding: a. Only alternating current is used b. Only direct current is used. c. The polarity has no importance. d. Alternating and/or direct current can be used. 6.13 Which of the following statements are not correct for the requirements of Laser Beam Welding,

LBW? a. Very accurate fit-up is necessary b. Filler metal is always used

c. Mirrors or optic fibres are used to focus the laser beam d. The process calls requires automation 6.14 Which joining methods can be used for surfacing: a. TIG-Hotwire b. Acetylene/Oxygen process c. Thermal spraying d. Answers a), b) and c) are correct. 6.15 What are the four main process parameters of the resistance spot welding process? a. Material, thickness, current type and pressure b. Current, pressure, time, electrode type and size c. Time, pressure, material type, electrode size and type d. Polarity, pressure, material type, coating type

Documents

DOC NO. 5Q&A-002-SA1 Revision: 0 Management · PDF fileQUALIFICATION & CERTIFICATION BOARD Date of Issue: 23 May 2011 Management System Manual IIW International Welding Specialist