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IIW IWT practice questions

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Question 1Welding processes and equipment

In gas metal arc welding, what is the primary function of the shielding gas?

  1. It excludes atmospheric oxygen and nitrogen from the molten weld pool Correct answer.
  2. It cools the contact tip and prevents it from overheating
  3. It conducts the welding current between the contact tip and the workpiece
  4. It provides the filler metal that fills the joint gap

Explanation

The shielding gas flows around the arc and weld pool to displace air, preventing oxidation, nitriding and porosity in the solidified weld. It is not the current path and does not supply filler metal.

Question 2Welding processes and equipment

Which power source characteristic is required for the manual metal arc (SMAW) process to maintain a stable arc when the operator varies arc length?

  1. A rising (constant-current) characteristic with steep droop Correct answer.
  2. A flat constant-voltage characteristic
  3. A high-frequency pulsed output only
  4. A purely sinusoidal alternating output

Explanation

Manual metal arc welding uses a droop characteristic so that large changes in arc length produce only small current changes, giving the welder a forgiving, stable arc. A flat characteristic would cause current to swing widely with arc length.

Question 3Welding processes and equipment

What does the term 'duty cycle' mean for an arc welding power source?

  1. The percentage of a ten-minute period during which the machine can operate at a stated current without overheating Correct answer.
  2. The number of times per second the output polarity reverses
  3. The ratio of arc voltage to open-circuit voltage
  4. The proportion of total weld length deposited in the flat position

Explanation

Duty cycle is defined over a ten-minute base period, so a 60 percent rating means six minutes of welding at the rated current followed by four minutes of cooling. Exceeding the rated current reduces the permissible duty cycle.

Question 4Welding processes and equipment

In the GTAW process, the electrode is normally made of tungsten. Why is a separate filler wire usually added rather than using a consumable electrode?

  1. To increase the deposition rate above that of GMAW
  2. To permit welding without any shielding gas
  3. To keep the tungsten electrode non-consumable and the arc independent of filler addition Correct answer.
  4. To allow the polarity to be reversed without damaging the torch

Explanation

GTAW uses a non-consumable tungsten electrode so the arc is established independently of the filler, allowing precise control of heat input and separate filler addition. The tungsten is not intended to melt into the weld.

Question 5Welding processes and equipment

Which of the following best describes the function of the contact tip in a GMAW torch?

  1. It provides the arc starting high-frequency pulse
  2. It meters the shielding gas flow to the nozzle
  3. It guides the wire into the joint without electrical contact
  4. It transfers welding current to the wire electrode close to the arc Correct answer.

Explanation

The contact tip is a replaceable copper component that conducts current into the wire electrode at a fixed distance from the arc. Wear of the bore leads to poor current transfer and arc instability.

Question 6Welding processes and equipment

What is the main purpose of using a backing gas when welding the root of a stainless steel pipe?

  1. To preheat the pipe root before welding
  2. To remove the need for a root gap
  3. To prevent oxidation of the root bead by excluding oxygen from the underside Correct answer.
  4. To increase the welding current capacity of the torch

Explanation

A backing or purge gas protects the hot root surface from oxygen, preventing oxidation and loss of corrosion resistance on the inside of the pipe. Without it, the root may show heavy oxide discolouration and reduced properties.

Question 7Welding processes and equipment

Which welding process uses a continuously fed consumable wire electrode with shielding provided by a flux core rather than an external gas?

  1. Oxy-fuel welding
  2. Submerged arc welding
  3. Flux-cored arc welding Correct answer.
  4. Gas tungsten arc welding

Explanation

Flux-cored arc welding uses a tubular wire containing flux that generates shielding gases and slag, so it can operate without a separate shielding gas supply. This makes it useful in windy field conditions.

Question 8Welding processes and equipment

What is the purpose of the slag formed during shielded metal arc welding?

  1. It conducts the welding current across the arc gap
  2. It protects the solidifying weld from atmospheric contamination and slows cooling Correct answer.
  3. It supplies additional alloying elements to the wire before melting
  4. It acts as a permanent part of the finished weld joint

Explanation

Slag covers the cooling bead, limiting oxygen and nitrogen pickup and reducing the cooling rate. It must be removed between passes and after welding.

Question 9Welding processes and equipment

A welder is using a power source rated at 300 A with a 60 percent duty cycle. They intend to weld continuously at 350 A. What is the correct engineering assessment?

  1. The duty cycle at 350 A will be lower than 60 percent, so continuous operation risks overheating Correct answer.
  2. The machine will automatically limit the current to 300 A
  3. The duty cycle remains 60 percent regardless of current
  4. The duty cycle will rise because a higher current produces a more efficient arc

Explanation

Duty cycle ratings apply at a stated current; increasing current raises resistive heating, so the permissible duty cycle falls. Continuous welding at above the rated current can cause thermal protection trips or damage.

Question 10Welding processes and equipment

Which shielding gas mixture is commonly chosen for GMAW of carbon steel to promote a stable spray transfer and good penetration?

  1. Argon with 15 to 25 percent carbon dioxide Correct answer.
  2. Argon with 2 percent oxygen only
  3. Pure helium
  4. Pure carbon dioxide

Explanation

Argon-rich mixtures with carbon dioxide provide a stable spray transfer, good wetting and penetration in carbon steel. This is a common general-purpose mixture for the process.

Question 11Welding processes and equipment

During GTAW of aluminium with alternating current, what is the purpose of the electrode-positive half cycle?

  1. It stabilises the arc by cooling the electrode
  2. It provides cathodic cleaning that breaks up the surface oxide film Correct answer.
  3. It prevents tungsten contamination of the weld
  4. It maximises penetration by concentrating heat in the workpiece

Explanation

On the electrode-positive half cycle, electrons leave the workpiece surface and help break up the tenacious aluminium oxide, giving the characteristic cleaning action. The electrode-negative half cycle provides most of the penetration.

Question 12Welding processes and equipment

A welder increases wire feed speed on a constant-voltage GMAW set without changing the voltage setting. What is the expected effect on the arc?

  1. Both current and voltage remain unchanged
  2. Welding current increases and the arc length tends to shorten Correct answer.
  3. Voltage rises and the arc becomes longer
  4. Welding current decreases and the arc lengthens

Explanation

In a constant-voltage system the wire burn-off rate self-regulates: pushing more wire into the arc draws more current until burn-off matches feed rate, which shortens the arc. The voltage setting largely fixes arc length.

Question 13Welding processes and equipment

Which factor most directly governs the deposition rate in submerged arc welding?

  1. Welding current Correct answer.
  2. Flux particle size
  3. Travel speed alone
  4. Arc voltage

Explanation

Deposition rate in SAW rises approximately with the square of welding current, so current is the dominant variable. Voltage mainly affects bead shape and flux consumption, and travel speed affects bead size.

Question 14Welding processes and equipment

Why is hydrogen usually avoided as a shielding gas component for welding high-strength steels?

  1. It reacts with the tungsten electrode to form a brittle compound
  2. It can dissolve into the weld metal and cause hydrogen-assisted cold cracking Correct answer.
  3. It reduces the arc temperature below the melting point of steel
  4. It prevents the formation of a slag layer

Explanation

Hydrogen has high solubility in molten steel and low solubility in ferrite, so it can remain trapped and promote cold cracking, particularly in high-strength or thick sections. Hydrogen additions are therefore restricted to austenitic stainless and similar applications.

Question 15Welding processes and equipment

In the SAW process, what happens if the flux layer is too shallow ahead of the arc?

  1. The slag becomes easier to remove
  2. The arc may flash through and cause porosity or arc instability Correct answer.
  3. The welding current automatically reduces
  4. The deposition rate increases because the arc is exposed

Explanation

Adequate flux depth is needed to contain the arc and generate shielding gas and slag. If the layer is too shallow, the arc can break through, causing porosity, spatter and an unstable arc.

Question 16Welding processes and equipment

A welder is welding thin sheet in the vertical-up position and wants to avoid burn-through. Which GMAW transfer mode is most suitable?

  1. Spray transfer at high current
  2. Pulsed spray at maximum wire feed
  3. Globular transfer at high voltage
  4. Short-circuiting transfer with low heat input Correct answer.

Explanation

Short-circuiting transfer operates at low current and voltage, depositing metal in a controlled manner with low heat input, which suits thin sheet and out-of-position work. Spray modes require higher currents that risk burn-through.

Question 17Welding processes and equipment

What is the main reason for using direct current electrode negative (DCEN) in GTAW of steel?

  1. It prevents arc blow by reversing the magnetic field
  2. It allows a larger diameter tungsten electrode to be used at low current
  3. It concentrates heat in the workpiece and gives deeper penetration Correct answer.
  4. It gives the strongest oxide cleaning action

Explanation

With the electrode negative, electrons bombard the workpiece, depositing most of the arc energy there and producing a deep, narrow weld. The trade-off is that oxide cleaning is minimal.

Question 18Welding processes and equipment

A welding power source has a rated output of 250 A at 100 percent duty cycle. Approximately what current could be used at a 60 percent duty cycle, assuming the same thermal loading?

  1. About 190 A
  2. About 150 A
  3. About 320 A Correct answer.
  4. About 250 A

Explanation

Using the square-root relationship between current and duty cycle, I2 = 250 times the square root of (100/60), which gives roughly 320 A. This reflects that resistive heating scales with the square of current.

Question 19Welding processes and equipment

In pulsed GMAW, what is the role of the background current?

  1. It maintains the arc between pulses and controls overall heat input Correct answer.
  2. It provides the main droplet detachment and penetration
  3. It determines the polarity of the output
  4. It controls the shielding gas flow rate

Explanation

In pulsed transfer the peak current detaches droplets, while the background current keeps the arc alive and reduces average heat input. Adjusting the background level and duration tailors heat input and cooling.

Question 20Welding processes and equipment

Why is pure argon often preferred over argon-carbon dioxide mixtures for GMAW of aluminium?

  1. Carbon dioxide is an oxidising gas that would contaminate the aluminium weld Correct answer.
  2. Argon is the only gas that can be ionised
  3. Carbon dioxide would prevent the arc from starting
  4. Argon gives a much higher arc voltage than any mixture

Explanation

Aluminium is highly reactive and carbon dioxide would introduce oxygen, causing oxidation and porosity. Argon or argon-helium mixtures are therefore used to protect the weld.

Question 21Welding processes and equipment

Which parameter combination best reduces the risk of lack of fusion in a multi-pass SAW joint on thick plate?

  1. Minimum voltage with maximum flux depth
  2. Adequate current density with controlled travel speed and correct bead overlap Correct answer.
  3. Maximum travel speed with minimum current
  4. Minimum current with maximum number of passes

Explanation

Lack of fusion is prevented by sufficient heat input to melt the sidewall and previous bead, achieved through adequate current and controlled travel speed with proper bead placement. Excessively fast travel or low current leaves cold laps.

Question 22Welding processes and equipment

A GTAW arc wanders and the weld shows inconsistent penetration on a long steel plate. Which cause should be investigated first?

  1. Contaminated tungsten electrode tip only
  2. Excessive backing gas pressure
  3. Insufficient shielding gas flow at the regulator
  4. Arc blow from residual magnetism or an unbalanced ground connection Correct answer.

Explanation

On long or previously magnetised steel components, residual magnetism can deflect the arc, producing wander and inconsistent penetration. Ground placement and demagnetisation should be checked before other causes.

Question 23Welding processes and equipment

In a synergic pulsed GMAW power source, what does the 'synergic' control do when the operator changes wire feed speed?

  1. It sets the travel speed of the carriage
  2. It changes the shielding gas composition automatically
  3. It switches polarity to suit the position
  4. It automatically adjusts pulse parameters to maintain one droplet per pulse Correct answer.

Explanation

A synergic program links wire feed speed to pulse peak, background and duration so that droplet detachment remains regular, typically one droplet per pulse. This simplifies setting the machine for a given wire and gas.

Question 24Welding processes and equipment

A thick aluminium component is being welded by GTAW and the welder switches from pure argon to an argon-helium mixture. What is the expected effect?

  1. Higher heat input and deeper penetration for the same current Correct answer.
  2. Lower arc voltage and a cooler weld
  3. Stronger oxide cleaning on DCEN
  4. Reduced need for a filler wire

Explanation

Helium has a higher ionisation potential and thermal conductivity than argon, raising arc voltage and heat input for a given current, which improves penetration in thick aluminium. The trade-off is a less stable arc and higher gas cost.

Question 25Welding processes and equipment

During GMAW, the contact tip bore becomes oval after prolonged use. Which effect on the process should be expected?

  1. Automatic compensation by the wire feed motor
  2. Higher shielding gas flow to the weld
  3. A shorter, more stable arc
  4. Poor current transfer and erratic arc with increased spatter Correct answer.

Explanation

An oval or worn contact tip allows the wire to move and lose reliable electrical contact, causing current fluctuation, arc instability and spatter. Replacing the tip restores consistent current transfer.

Question 26Welding processes and equipment

In gas tungsten arc welding (GTAW) of thin stainless steel sheet, which electrode tip geometry is normally prepared to stabilise the arc and control bead width?

  1. A sharply tapered point with a small included angle
  2. A flat, truncated tip with a defined included angle Correct answer.
  3. A blunt tip with no prepared taper at all
  4. A balled tip formed by melting the electrode

Explanation

For DCEN GTAW on steels, the tungsten electrode is ground to a truncated cone: a defined included angle with a small flat land at the tip. The flat land stabilises the arc root and limits arc wander, while the taper controls the current-carrying capacity and the bead profile. A completely pointed electrode concentrates the arc and can melt back or wander, and a balled tip belongs to AC operation on aluminium.

Question 27Welding processes and equipment

A welder is setting up gas metal arc welding with a solid wire on a constant-voltage power source. Which combination of variables primarily determines the welding current that flows?

  1. The contact tip-to-work distance only
  2. The open-circuit voltage selected on the machine
  3. The shielding gas flow rate in litres per minute
  4. The wire feed speed set by the operator Correct answer.

Explanation

On a constant-voltage characteristic, the arc self-regulates: the wire feed speed sets the burn-off rate and therefore the current drawn, while the voltage setting mainly fixes arc length. Increasing wire feed speed pushes more wire into the arc, lowering the arc length and raising current until burn-off balances the feed. Voltage and gas flow influence arc stability and bead shape but do not set the current level in the same direct way.

Question 28Welding processes and equipment

In submerged arc welding of a thick carbon steel butt joint, the operator notices the arc is unstable with heavy spatter and erratic penetration, and the flux appears partly unmelted around the arc. Which cause should be investigated first?

  1. The flux is contaminated with moisture and the hopper feed is restricted Correct answer.
  2. The welding current has been set too low for the wire diameter
  3. The travel speed has been increased beyond the normal range
  4. The wire extension has been shortened below the recommended value

Explanation

Submerged arc welding depends on a continuous, dry, free-flowing flux blanket that fully covers the arc; damp or restricted flux produces an exposed arc, spatter and erratic penetration. Moisture also introduces hydrogen, so the flux must be stored and baked according to the manufacturer's instructions and fed smoothly. Current, travel speed and wire extension changes give different, more predictable symptoms than an exposed, spattering arc.

Question 29Welding processes and equipment

A welder must join thin aluminium sheet in the flat position with GTAW and wants to minimise distortion and burn-through. Which measure is most appropriate?

  1. Preheat the entire sheet to about 150 degrees Celsius before welding
  2. Switch to a larger electrode and increase the shielding gas flow rate
  3. Use a copper backing bar and reduce current with a faster travel speed Correct answer.
  4. Increase the welding current and travel faster to limit heat input

Explanation

Aluminium conducts heat away rapidly and has no colour change on heating, so thin sheet is prone to burn-through when too much heat is applied. A copper backing bar acts as a heat sink and supports the root, while reduced current and faster travel keep heat input low. Preheating is used for thick aluminium sections, not for thin sheet where it would worsen distortion and burn-through.

Question 30Welding processes and equipment

A welding engineer compares two power sources for a semi-automatic GMAW cell: one with a flat, constant-voltage characteristic and one with a drooping characteristic. Which statement correctly describes the consequence for arc self-regulation?

  1. The flat characteristic gives poor self-regulation and requires manual voltage correction
  2. Both characteristics give identical self-regulation because the wire feed is motor driven
  3. The flat characteristic gives strong self-regulation because a small arc-length change produces a large current change Correct answer.
  4. The drooping source gives better self-regulation because current falls as voltage rises

Explanation

Self-regulation in GMAW relies on a steeply sloping or flat voltage characteristic: if the arc lengthens, the current falls sharply and the wire burns off more slowly, restoring the arc length. A drooping characteristic produces only a small current change for a given voltage change, so the process cannot self-correct and the arc becomes unstable with a constant-speed wire feed. This is why constant-voltage or slightly sloping sources are standard for GMAW.

Question 31Materials and their behaviour during welding

In the iron-carbon equilibrium diagram, what is the maximum carbon solubility in austenite (gamma iron)?

  1. 0.8 % at 723 °C
  2. 6.67 % at room temperature
  3. 2.11 % at 1147 °C Correct answer.
  4. 0.022 % at 723 °C

Explanation

Austenite can dissolve far more carbon than ferrite because its FCC lattice provides larger interstitial sites. The maximum solubility reaches about 2.11 % at 1147 °C, the eutectic temperature. Above this carbon level, cementite or ledeburite forms instead of a single austenite phase.

Question 32Materials and their behaviour during welding

A plain carbon steel with 0.45 % C is welded and cooled slowly. Which microstructural constituent dominates the resulting weld metal?

  1. Martensite
  2. Ferrite with a small amount of pearlite
  3. Pearlite with pro-eutectoid ferrite Correct answer.
  4. Pearlite with pro-eutectoid cementite

Explanation

Hypo-eutectoid steels (below 0.8 % C) cooled slowly form pro-eutectoid ferrite first along prior austenite grain boundaries, followed by pearlite. At 0.45 % C the pearlite fraction clearly dominates while ferrite remains present. Martensite requires fast cooling, and pro-eutectoid cementite only appears in hyper-eutectoid steels.

Question 33Materials and their behaviour during welding

Why does the hardenability of steel increase with rising carbon content up to about 0.8 %?

  1. Carbon raises the Ms temperature so martensite forms sooner
  2. Carbon increases the thermal conductivity of austenite
  3. Carbon promotes graphite precipitation that pins dislocations
  4. Carbon lowers the critical cooling rate by shifting the CCT curve to longer times Correct answer.

Explanation

Dissolved carbon delays the diffusion-controlled transformation of austenite, pushing the CCT diagram to longer times and lowering the critical cooling rate. As a result, thicker sections can be hardened. Above the eutectoid composition, hardenability gains level off because undissolved carbides reduce the carbon in solution.

Question 34Materials and their behaviour during welding

Which transformation product forms when austenite is cooled just below the Ms temperature but above the Mf temperature?

  1. Coarse pearlite
  2. Partially transformed martensite with retained austenite Correct answer.
  3. Spheroidite
  4. Bainite

Explanation

Cooling to a temperature between Ms and Mf transforms only the fraction of austenite permitted by the thermodynamic driving force at that temperature. The remainder stays as retained austenite, which may transform later or persist at room temperature. Full martensite requires cooling below Mf.

Question 35Materials and their behaviour during welding

What is the main metallurgical reason for preheating a thick-section carbon-manganese steel before welding?

  1. To refine the prior austenite grain size in the weld metal
  2. To reduce the arc voltage required for stable transfer
  3. To increase the deposition rate of the electrode
  4. To reduce the cooling rate and lower the risk of hydrogen-assisted cold cracking Correct answer.

Explanation

Preheating slows the cooling rate, which gives hydrogen more time to diffuse out and produces a softer, more crack-resistant microstructure in the heat-affected zone. It also reduces the thermal gradient and hence residual stress. The combined effect lowers the susceptibility to cold cracking in hardenable steels.

Question 36Materials and their behaviour during welding

Which carbon equivalent formula is commonly used for assessing weldability of carbon-manganese steels and includes the term Cr+Mo+V?

  1. CE = C + Mn/6 + (Cr+Mo+V)/5 + (Cu+Ni)/15 Correct answer.
  2. CE = C + Mn/20 + Ni/10
  3. CE = C + Si/4 + P/2
  4. CE = C + 2Mn + 4Si

Explanation

The IIW carbon equivalent formula weights each alloying element by its effect on hardenability relative to carbon. Manganese is divided by 6, Cr+Mo+V by 5, and Cu+Ni by 15. A CE above roughly 0.4 % signals a need for preheat and controlled heat input.

Question 37Materials and their behaviour during welding

A steel has a carbon equivalent of 0.55 %. What is the most appropriate practical response for arc welding?

  1. Apply preheat and control interpass temperature with a low-hydrogen process Correct answer.
  2. Increase the cooling rate to promote a tough fine-grained structure
  3. Use a cellulose-coated electrode to increase arc energy
  4. Weld without preheat using a high heat input

Explanation

A CE of 0.55 % indicates high hardenability and a strong susceptibility to cold cracking. Preheat combined with controlled interpass temperature and a low-hydrogen consumable reduces the hydrogen level and softening the HAZ. Cooling must not be accelerated, since fast cooling would promote hard martensite.

Question 38Materials and their behaviour during welding

Which of the following is a typical characteristic of the heat-affected zone in a single-pass weld on a quenched and tempered steel?

  1. Uniform coarse-grained ferrite throughout the zone
  2. Complete elimination of residual stresses
  3. A hardness peak in the coarse-grained region adjacent to the fusion line Correct answer.
  4. A carbon content higher than the base metal

Explanation

The region heated just below the solidus reaches high austenitising temperatures, coarsening the grains and, on cooling, forming hard martensite or bainite. This produces a hardness peak near the fusion line. Further from the fusion line, peak temperatures fall and hardness decreases.

Question 39Materials and their behaviour during welding

In austenitic stainless steel welds, what is the main consequence of sensitisation in the temperature range 450–850 °C?

  1. Chromium carbides precipitate at grain boundaries and locally deplete chromium Correct answer.
  2. Sigma phase dissolves and increases toughness
  3. Carbon diffuses into the ferrite and raises hardness
  4. Nickel segregates to the weld centreline and causes hot cracking

Explanation

Between about 450 and 850 °C, chromium carbides precipitate along grain boundaries, consuming chromium from the adjacent matrix. The chromium-depleted zones lose passivity and become susceptible to intergranular corrosion. Low-carbon grades or stabilised grades are used to avoid this.

Question 40Materials and their behaviour during welding

Why is a small amount of delta ferrite often desirable in the weld metal of austenitic stainless steels?

  1. It prevents sigma phase formation entirely
  2. It raises the Ms temperature of the weld metal
  3. It increases the coefficient of thermal expansion
  4. It helps resist solidification cracking and improves resistance to stress-corrosion cracking Correct answer.

Explanation

A ferrite content of roughly 3–10 FN in austenitic weld metal reduces the risk of solidification cracking by altering the solidification mode and accommodating residual elements such as sulphur and phosphorus. It also improves resistance to chloride stress-corrosion cracking. Excess ferrite, however, can transform to sigma phase during high-temperature service.

Question 41Materials and their behaviour during welding

Which alloying element is added to austenitic stainless steels to stabilise the austenite and improve resistance to reducing acids?

  1. Nickel Correct answer.
  2. Molybdenum
  3. Chromium
  4. Titanium

Explanation

Nickel is a strong austenite stabiliser and is the main element that makes the 300-series stainless steels fully austenitic. Chromium provides passivity, molybdenum improves pitting resistance, and titanium is used for stabilisation against sensitisation.

Question 42Materials and their behaviour during welding

Which aluminium alloy series is generally considered non-heat-treatable and relies on solid-solution strengthening and work hardening?

  1. 7xxx
  2. 5xxx Correct answer.
  3. 2xxx
  4. 6xxx

Explanation

The 5xxx series (Al-Mg) cannot be strengthened by precipitation hardening and gains strength from magnesium in solid solution plus strain hardening. The 2xxx, 6xxx and 7xxx series are heat-treatable because they contain copper, magnesium-silicon or zinc in sufficient amounts to form strengthening precipitates.

Question 43Materials and their behaviour during welding

What is the principal problem caused by the high thermal conductivity of aluminium during fusion welding?

  1. The arc becomes unstable at low currents
  2. Hydrogen solubility increases sharply in the solid state
  3. Rapid heat extraction requires high heat input and preheat to ensure fusion Correct answer.
  4. The oxide film becomes more stable at high temperature

Explanation

Aluminium conducts heat away from the weld pool about five times faster than steel, so heat is rapidly dissipated into the surrounding metal. Higher heat input or preheating is needed to achieve adequate penetration and fusion. The stable oxide film and hydrogen porosity are separate issues.

Question 44Materials and their behaviour during welding

Which type of porosity is most commonly associated with hydrogen in aluminium weld metal?

  1. Elongated intergranular cracks
  2. Solidification cracking at the crater
  3. Spherical gas pores distributed throughout the weld Correct answer.
  4. Lack of sidewall fusion

Explanation

Hydrogen has a much higher solubility in liquid aluminium than in the solid, so it is rejected during solidification and forms spherical pores. Moisture, hydrocarbons and hydrated oxide on the surface are the usual sources. Thorough cleaning and dry shielding gas are the main preventive measures.

Question 45Materials and their behaviour during welding

Which characteristic distinguishes the welding behaviour of austenitic stainless steel from that of carbon steel regarding distortion?

  1. Austenitic stainless steel has a lower coefficient of thermal expansion and less distortion
  2. Austenitic stainless steel has higher thermal conductivity so heat is concentrated
  3. Austenitic stainless steel does not undergo solidification shrinkage
  4. Austenitic stainless steel has a higher coefficient of thermal expansion and tends to distort more Correct answer.

Explanation

Austenitic stainless steels have a coefficient of thermal expansion roughly 50 % higher than carbon steel, combined with lower thermal conductivity. This produces steeper thermal gradients and greater expansion and contraction, giving more distortion and higher residual stress. Distortion control measures such as balanced sequences and fixtures are therefore important.

Question 46Materials and their behaviour during welding

Which of the following is the main reason for the high susceptibility of copper and copper alloys to heat-affected-zone cracking during welding?

  1. High thermal conductivity conducts heat away and promotes brittle phases in the HAZ Correct answer.
  2. Copper alloys contain no alloying elements
  3. Copper has a very high hydrogen solubility in the solid state
  4. Low thermal conductivity causes overheating

Explanation

Copper conducts heat very effectively, so the weld pool is narrow and the surrounding metal stays relatively cold, producing steep gradients. Certain copper alloys, such as those containing bismuth or lead, also form low-melting grain-boundary films that crack under contraction stress. Preheating and careful filler selection are used to reduce the risk.

Question 47Materials and their behaviour during welding

What is the effect of sulphur and phosphorus on the solidification cracking susceptibility of steel weld metal?

  1. They refine the dendritic structure and reduce cracking
  2. They form low-melting-point films at grain boundaries and increase cracking Correct answer.
  3. They raise the solidus temperature and widen the freezing range
  4. They act as strong deoxidisers and remove oxygen

Explanation

Sulphur and phosphorus segregate strongly to the interdendritic liquid during solidification and form low-melting eutectics such as iron sulphide. These films remain liquid while the surrounding solid is under contraction stress, so they act as crack paths. Manganese is added to combine with sulphur as higher-melting MnS and reduce this risk.

Question 48Materials and their behaviour during welding

Which mechanism best explains reheat cracking in the coarse-grained heat-affected zone of low-alloy Cr-Mo steels during post-weld heat treatment?

  1. Absorption of nitrogen from the atmosphere
  2. Rapid cooling below the Ms temperature
  3. Relaxation of residual stress by grain-boundary sliding with precipitation-hardened grain interiors Correct answer.
  4. Hydrogen diffusion to the weld metal

Explanation

During post-weld heat treatment, residual stresses relax by creep, but the coarse-grained matrix is strengthened by fine precipitates while the grain boundaries are relatively weak. Strain concentrates at the boundaries and causes intergranular cracking. Avoiding excessive restraint and controlling the PWHT cycle reduce the risk.

Question 49Materials and their behaviour during welding

What is the typical effect of increasing heat input on the toughness of the coarse-grained heat-affected zone in a high-strength low-alloy steel?

  1. Toughness increases because the grains are refined
  2. Toughness increases because martensite is eliminated
  3. Toughness decreases because prior austenite grains coarsen Correct answer.
  4. Toughness is unaffected by heat input

Explanation

Higher heat input raises the peak temperature and lengthens the time spent at high temperature, allowing prior austenite grains to grow. The coarse grains produce coarse transformation products with poor toughness. For this reason, high-strength steels are often welded with limited heat input to maintain HAZ toughness.

Question 50Materials and their behaviour during welding

Which element is primarily responsible for hot shortness in steel and is therefore kept low in weldable structural steels?

  1. Manganese
  2. Nickel
  3. Sulphur Correct answer.
  4. Chromium

Explanation

Sulphur forms low-melting iron sulphide films at grain boundaries, causing hot shortness during hot working and promoting solidification cracking in welds. Manganese is added to form MnS, which has a higher melting point and is less harmful. Structural steel specifications therefore limit sulphur content.