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IIW IWS practice questions

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Question 1Welding processes and equipment

In shielded metal arc welding (SMAW), what is the primary function of the electrode covering (flux coating)?

  1. Shielding the molten metal from atmospheric contamination and stabilizing the arc Correct answer.
  2. Providing mechanical support to keep the electrode rigid during welding
  3. Conducting the welding current from the electrode holder to the core wire
  4. Increasing the melting point of the core wire so it feeds more slowly

Explanation

The covering decomposes and burns to generate shielding gas and slag that protect the molten weld pool from oxygen and nitrogen, while also stabilizing the arc and adding alloying elements. The core wire itself conducts the current, so the covering is not a conductor.

Question 2Welding processes and equipment

Which gas is most commonly used as the primary shielding gas for GMAW of carbon steel in short-circuiting transfer?

  1. Pure helium
  2. Pure nitrogen
  3. Pure argon
  4. Carbon dioxide or an argon-CO2 mixture Correct answer.

Explanation

Carbon dioxide and argon-CO2 blends are the standard shielding gases for GMAW of carbon steel because they provide good arc stability and penetration at low cost. Pure argon tends to give poor short-circuit transfer characteristics in steel.

Question 3Welding processes and equipment

In TIG (GTAW) welding, what is the role of the tungsten electrode?

  1. It generates the shielding gas by decomposition
  2. It carries the current and sustains the arc without being consumed Correct answer.
  3. It supplies the filler metal to the weld pool
  4. It melts and becomes part of the weld deposit

Explanation

The non-consumable tungsten electrode carries the welding current and maintains the arc; filler metal, when used, is added separately. Tungsten's very high melting point allows it to survive the arc without significant consumption.

Question 4Welding processes and equipment

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

  1. The proportion of a 10-minute period that the machine can operate at its rated output without overheating Correct answer.
  2. The percentage of total weld length that must be inspected
  3. The ratio of open-circuit voltage to arc voltage
  4. The number of times per minute the arc can be restarted

Explanation

Duty cycle is defined over a standard 10-minute period and expresses the fraction of that time the power source can deliver its rated current continuously without exceeding its thermal limit. It is a thermal rating, not an inspection or restart parameter.

Question 5Welding processes and equipment

Which welding process uses a continuously fed consumable electrode and a granular flux layer that is not manually removed during welding?

  1. Gas tungsten arc welding
  2. Submerged arc welding Correct answer.
  3. Gas metal arc welding
  4. Shielded metal arc welding

Explanation

Submerged arc welding (SAW) feeds a bare consumable wire into a deep blanket of granular flux that shields the arc completely, so the arc is not visible. The other listed processes use gas shielding or a coated electrode rather than a granular flux blanket.

Question 6Welding processes and equipment

In resistance spot welding, the heat required to form the weld nugget is generated primarily by which mechanism?

  1. Joule heating from the resistance at the faying interface and contact points Correct answer.
  2. Friction between the electrode tips and the workpiece surface
  3. Chemical reaction of a flux coating between the sheets
  4. An electric arc struck between the electrodes

Explanation

Resistance spot welding passes a high current through the stacked sheets, and the highest resistance occurs at the faying interface, so Joule heating (I squared R) concentrates there and forms the nugget. No arc is struck and no flux is used.

Question 7Welding processes and equipment

For which polarity is a TIG torch normally connected when welding aluminium with alternating current?

  1. Direct current electrode positive only
  2. Direct current electrode negative only
  3. The torch is connected to neither terminal and the arc is initiated by a capacitor
  4. The torch is connected to the AC terminal so polarity alternates each half cycle Correct answer.

Explanation

Aluminium is normally welded with AC in GTAW: the electrode-negative half cycle gives good penetration and electrode life, while the electrode-positive half cycle provides cathodic cleaning of the oxide film. The polarity therefore alternates every half cycle.

Question 8Welding processes and equipment

Which of the following is a characteristic feature of the FCAW process compared with GMAW?

  1. It requires a non-consumable tungsten electrode
  2. It operates only in the vertical-down position
  3. It uses a tubular wire containing flux, which can provide self-shielding without an external gas supply Correct answer.
  4. It uses a granular flux blanket that hides the arc

Explanation

Flux-cored arc welding uses a tubular wire filled with flux, and self-shielded variants need no external shielding gas, making the process suitable for site work. GMAW uses a solid wire and always requires a shielding gas.

Question 9Welding processes and equipment

A power source is rated at 300 A with a 60% duty cycle. Approximately what current can it deliver continuously for an 8-hour shift without exceeding its thermal rating?

  1. About 232 A Correct answer.
  2. About 300 A
  3. About 150 A
  4. About 180 A

Explanation

Using the standard relation I2 = I1 times the square root of (duty cycle 2 divided by duty cycle 1), a 100% duty cycle current is 300 times the square root of 0.6, which is about 232 A. The machine can therefore run continuously at roughly this reduced current.

Question 10Welding processes and equipment

In GMAW, which metal transfer mode is characterized by low heat input, low spatter, and is typically used for thin sheet in all positions?

  1. Short-circuiting transfer Correct answer.
  2. Spray transfer
  3. Globular transfer
  4. Pulsed transfer at high background current

Explanation

Short-circuiting transfer occurs at low current and voltage, and the wire repeatedly touches the pool to transfer metal, giving low heat input suitable for thin sheet and positional work. Spray transfer requires high current and gives high heat input, limiting its use to thicker sections.

Question 11Welding processes and equipment

Why is direct current electrode negative (DCEN) generally preferred for TIG welding of stainless steel?

  1. It provides stronger cathodic cleaning of the oxide layer
  2. It prevents distortion by lowering the heat input to the plate
  3. It makes the arc wander so that the pool spreads evenly
  4. It concentrates more heat in the workpiece and reduces the risk of tungsten contamination Correct answer.

Explanation

With DCEN the workpiece receives most of the arc heat, giving deeper penetration, while the tungsten stays relatively cool and resists melting or transferring into the pool. Stainless steel does not need the strong oxide cleaning that aluminium requires.

Question 12Welding processes and equipment

In SAW, why must the welding parameters be selected so that the arc is fully covered by the flux?

  1. To allow the operator to observe the arc for better control
  2. To eliminate the need for any current return path
  3. To increase the open-circuit voltage of the power source
  4. To prevent exposure of the arc and pool to the atmosphere and to avoid flash and spatter escaping Correct answer.

Explanation

The flux blanket excludes air from the arc and pool, and it also suppresses radiation and spatter; if the arc breaks through the flux, shielding is lost and defects or operator hazards result. The arc is deliberately not visible in SAW.

Question 13Welding processes and equipment

During resistance spot welding of two 1 mm steel sheets, the weld nugget forms primarily at which location?

  1. At the interface between the electrode tip and the outer sheet surface
  2. In the water cooling channel of the electrode
  3. At the faying interface between the two sheets Correct answer.
  4. Uniformly through the full thickness of both sheets

Explanation

Because the faying interface has the highest contact resistance and the current density is greatest there, Joule heating peaks at that plane and the nugget grows outward from it. The electrode contact surfaces are water-cooled and have lower resistance.

Question 14Welding processes and equipment

A welder must join thin aluminium sheet with GTAW and wants good oxide cleaning without overheating the tungsten. Which setup is most appropriate?

  1. DCEN with a very large electrode diameter relative to current
  2. AC with a zirconiated or thoriated tungsten electrode Correct answer.
  3. DCEP with a pure tungsten electrode
  4. DCEN with a pure tungsten electrode

Explanation

AC provides alternating cleaning and penetration half cycles, and zirconiated or thoriated tungsten tolerates the electrode-positive half cycle better than pure tungsten. This balances oxide removal against electrode life.

Question 15Welding processes and equipment

In FCAW, what is the main consequence of using a self-shielded wire in a drafty outdoor location?

  1. The weld quality improves because the draft cools the pool
  2. The polarity must always be changed to DCEP for the draft to be overcome
  3. The weld is unaffected because no external gas can be blown away Correct answer.
  4. The flux must be replaced by a granular blanket

Explanation

Self-shielded flux-cored wires generate their own shielding from the decomposing flux, so wind does not strip a gas envelope and the process is well suited to outdoor site work. Gas-shielded processes would need windbreaks under the same conditions.

Question 16Welding processes and equipment

Which combination best describes the function of the electrode coating in SMAW when welding in the vertical-up position?

  1. It provides a very fluid slag that runs freely down the joint
  2. It increases the deposition rate by raising the arc voltage automatically
  3. It eliminates the need for any weaving technique
  4. It provides a fast-freezing slag that supports the pool against gravity Correct answer.

Explanation

For vertical-up welding, fast-freezing slag systems hold the molten pool in place so that it solidifies before it can run down the joint. Highly fluid slags are used for flat-position, high-deposition work instead.

Question 17Welding processes and equipment

Which of the following best explains why spray transfer in GMAW is normally restricted to the flat position?

  1. The high current produces a large, fluid weld pool that gravity would pull out of position Correct answer.
  2. The shielding gas becomes toxic when the torch is tilted
  3. The arc becomes unstable because of the earth's magnetic field
  4. The wire feed motor cannot operate at angles other than horizontal

Explanation

Spray transfer requires current above a transition value, which produces a large and fluid pool; in the flat position gravity holds it in place, but in vertical or overhead positions it would sag or drop. This is why positional work uses short-circuiting or pulsed modes.

Question 18Welding processes and equipment

A workshop needs to weld thick carbon steel plate with high deposition rate and no visible arc, and the joint can be rotated to the flat position. Which process is the most suitable choice?

  1. Submerged arc welding Correct answer.
  2. GTAW with cold wire feed
  3. Resistance seam welding
  4. SMAW with a rutile electrode

Explanation

SAW offers very high deposition rates on thick plate and is used in the flat position with the arc buried under granular flux, so no arc is visible. It is well suited to mechanized welding of rotated or flat work.

Question 19Welding processes and equipment

In resistance spot welding, increasing the electrode force beyond the optimum most likely causes which effect?

  1. No change in nugget size because force does not affect resistance
  2. An arc to form between the electrodes and the sheet
  3. A larger nugget because contact resistance increases
  4. A smaller nugget because contact resistance and current density at the interface decrease Correct answer.

Explanation

Higher electrode force flattens the contact asperities, lowering contact resistance and spreading the current over a larger area, which reduces current density and shrinks the nugget. Excessive force can therefore cause undersized welds.

Question 20Welding processes and equipment

In GTAW, what is the main reason for using a high-frequency arc-starting unit rather than scratch starting?

  1. It automatically sets the welding current for the operator
  2. It increases the deposition rate of the filler wire
  3. It removes the need for a shielding gas
  4. It initiates the arc without touching the workpiece, avoiding tungsten contamination and electrode damage Correct answer.

Explanation

High-frequency or lift-start circuits establish the arc across a gap, so the tungsten never touches the workpiece and cannot pick up contamination that would cause arc wander. Deposition rate and gas supply are unaffected.

Question 21Welding processes and equipment

A SAW procedure on 20 mm plate shows incomplete fusion at the sidewall. Which change is most likely to correct the problem?

  1. Reducing the welding current and increasing the travel speed
  2. Decreasing the flux layer thickness to zero
  3. Changing the current return to a single point far from the joint
  4. Increasing the welding current and adjusting the wire position toward the sidewall Correct answer.

Explanation

Incomplete sidewall fusion in SAW usually results from insufficient heat input or the arc being directed away from the joint face. Raising current increases penetration, and repositioning the wire toward the sidewall directs heat where fusion is lacking.

Question 22Welding processes and equipment

A GMAW short-circuiting weld on 1.5 mm sheet shows excessive spatter and frequent stubbing of the wire into the pool. Which parameter adjustment is most appropriate?

  1. Reduce the shielding gas flow to zero
  2. Increase the voltage slightly and ensure the inductance is set for smooth short-circuiting Correct answer.
  3. Switch to spray transfer by raising the current well above the transition level
  4. Increase the wire feed speed while keeping voltage constant

Explanation

Stubbing and spatter in short-circuiting transfer typically indicate that voltage is too low relative to wire feed speed, so the wire drives into the pool before it can detach. Raising voltage slightly and tuning inductance smooths the short-circuit cycle.

Question 23Welding processes and equipment

In AC GTAW of aluminium, the electrode-positive half cycle provides oxide cleaning but also heats the tungsten. What is the practical consequence for electrode selection?

  1. Pure tungsten is preferred because it has the highest melting point of all tungsten types
  2. The electrode diameter should be reduced to concentrate heat at the tip
  3. Zirconiated or thoriated tungsten is preferred because it maintains a stable arc and resists melting under the positive half cycle Correct answer.
  4. Cerium-doped tungsten must be avoided because it cannot carry alternating current

Explanation

The positive half cycle delivers heat to the tungsten, so electrodes with higher electron emissivity and better high-temperature stability, such as zirconiated or thoriated types, are used to maintain a stable arc and resist tip melting. Pure tungsten is used mainly for AC on aluminium with lower current demands and is less tolerant.

Question 24Welding processes and equipment

A fabricator must produce a long, leak-tight seam on thin coated steel sheet at high speed. Which process is most appropriate?

  1. GTAW with manual filler addition
  2. SMAW with a basic electrode
  3. Resistance seam welding with overlapping spot welds Correct answer.
  4. SAW with a granular flux blanket

Explanation

Resistance seam welding uses rotating electrode wheels to produce a continuous series of overlapping spot welds, giving a leak-tight seam at high production speed on thin sheet. The arc processes listed are slower and less suited to continuous seam production.

Question 25Welding processes and equipment

A welding engineer observes that a power source rated at 250 A, 60% duty cycle is being run continuously at 250 A for long production runs and is tripping on thermal overload. What is the most appropriate corrective action?

  1. Reduce the welding current to approximately 194 A for continuous operation or shorten the welding periods Correct answer.
  2. Disconnect the cooling fan to reduce dust ingress
  3. Change the electrode polarity to reduce the duty cycle requirement
  4. Increase the current further so that the overload trips faster and resets

Explanation

At 100% duty the permissible current is 250 times the square root of 0.6, about 194 A; running continuously at 250 A exceeds the thermal rating. Reducing current or limiting welding periods to the rated duty cycle restores operation within the rating.

Question 26Welding processes and equipment

In the SAW process, what is the primary role of the granular flux layer that is laid ahead of the arc and recovered after welding?

  1. It acts as a mechanical backing that supports the root bead against gravity
  2. It shields the arc and molten pool from the atmosphere and contributes alloying elements through slag-metal reactions Correct answer.
  3. It cools the wire electrode before it enters the arc, preventing overheating
  4. It conducts the welding current from the contact nozzle into the workpiece

Explanation

In submerged arc welding the arc burns beneath a blanket of granular flux, which melts into slag and gas that exclude air from the arc zone. The molten slag also takes part in metallurgical reactions, so elements such as manganese and silicon can be transferred to or removed from the weld metal. Unmelted flux is collected and reused.

Question 27Welding processes and equipment

A welder using the GTAW process on DCEN notices the tungsten electrode becoming contaminated with a small ball of molten metal at the tip after touching the pool. What is the most appropriate immediate action?

  1. Reduce the welding current by half and continue welding without touching the electrode
  2. Switch to DCEP so that the electrode-positive polarity cleans the tungsten automatically
  3. Stop welding, remove the electrode, break off the contaminated tip by grinding it to a taper, and reinstall it Correct answer.
  4. Increase the shielding gas flow rate and continue welding with the same electrode

Explanation

Once the tungsten has dipped into the molten pool, the contaminated tip cannot be restored by process adjustments; it must be removed mechanically. The electrode is taken out and ground longitudinally to a clean taper, then reinstalled and the arc re-established. Continuing with a contaminated tip produces arc wander and tungsten inclusions in the weld.

Question 28Welding processes and equipment

In the GMAW process, which parameter change most directly shifts the transfer from globular to spray mode when welding 1.2 mm solid wire in argon-rich shielding gas?

  1. Reducing the wire feed speed so that the arc length becomes longer
  2. Replacing the argon-rich gas with pure carbon dioxide at the same current
  3. Increasing the wire feed speed and hence the welding current above the transition value Correct answer.
  4. Increasing the contact-tip-to-work distance while keeping all other settings constant

Explanation

Spray transfer requires a current above a threshold value at which the electromagnetic pinch force detaches fine droplets from the tip before they grow large, so raising the wire feed speed raises the current into that range. This threshold depends strongly on the shielding gas composition, being much lower with argon-rich mixtures than with CO2. Below the threshold the transfer is globular.

Question 29Welding processes and equipment

A fabricator is welding a 6 mm fillet on structural steel with FCAW and observes a rough, rope-like bead with deep ripples and poor sidewall wetting. Which corrective measure addresses the most likely cause?

  1. Increase the welding current or reduce the travel speed to improve wetting and bead shape Correct answer.
  2. Decrease the welding current and increase the travel speed to reduce the weld pool size
  3. Switch the shielding gas from CO2 to argon with a higher oxygen content
  4. Increase the electrode extension to several times the recommended value

Explanation

A rough, rope-like bead with poor sidewall wetting in FCAW usually indicates insufficient heat input or excessive travel speed, so the pool freezes before it can wet the joint faces. Raising the current or slowing the travel speed increases heat input and produces a flatter, better-wetting bead. Electrode extension and gas composition must also stay within the procedure limits.

Question 30Welding processes and equipment

In resistance spot welding of two 1.0 mm low-carbon steel sheets, the weld nugget grows outward from the faying interface. Which combination of changes will most reliably increase the nugget diameter without causing expulsion?

  1. Reduce the electrode force and increase the squeeze time before the current is applied
  2. Increase the welding current slightly and extend the weld time while maintaining adequate electrode force Correct answer.
  3. Increase the electrode force and shorten the weld time simultaneously
  4. Decrease the welding current and increase the hold time after the current ceases

Explanation

Nugget growth depends on heat generated at the faying interface, which rises with current squared and with weld time, but the electrode force must be sufficient to contain the molten metal and prevent expulsion. A modest increase in current together with a longer weld time therefore enlarges the nugget while adequate force keeps it sound. Force, current and time must be balanced as a set.

Question 31Materials and their behaviour during welding

In the iron-carbon equilibrium diagram, what is the maximum carbon solubility in ferrite (α-iron) at room temperature approximately?

  1. 0.02 %
  2. 0.8 %
  3. 0.008 % Correct answer.
  4. 2.1 %

Explanation

Ferrite is a body-centred cubic solid solution of carbon in α-iron and can dissolve only a very small amount of carbon, roughly 0.008 % at room temperature and about 0.02 % at 723 °C. The 0.8 % figure is the eutectoid composition and 2.1 % is the maximum solubility in austenite.

Question 32Materials and their behaviour during welding

Which crystal structure does austenite (γ-iron) possess?

  1. Body-centred tetragonal
  2. Body-centred cubic
  3. Hexagonal close-packed
  4. Face-centred cubic Correct answer.

Explanation

Austenite is the face-centred cubic high-temperature phase of iron, which has a higher packing density and therefore dissolves far more carbon than ferrite. Body-centred cubic describes ferrite, and body-centred tetragonal describes martensite.

Question 33Materials and their behaviour during welding

A carbon-manganese steel is welded and the weld metal cools rapidly. What effect does increased carbon content have on the hardenability of the steel?

  1. It decreases hardenability and suppresses martensite formation
  2. It has no measurable effect on hardenability
  3. It increases toughness of the heat-affected zone
  4. It increases hardenability and promotes martensite formation Correct answer.

Explanation

Carbon is the strongest single alloying element for increasing hardenability, so raising carbon shifts the continuous cooling transformation curve to longer times and makes martensite easier to form on rapid cooling. This raises the risk of hard, crack-sensitive microstructures in the heat-affected zone.

Question 34Materials and their behaviour during welding

What is the primary mechanism by which hydrogen causes cracking in welded high-strength steels?

  1. Hydrogen reacts with iron to form a liquid grain-boundary phase
  2. Hydrogen increases the carbon content of the weld metal
  3. Hydrogen raises the ductile-to-brittle transition temperature of ferrite only
  4. Diffusing hydrogen accumulates at stress concentrations and reduces cohesive strength Correct answer.

Explanation

Hydrogen dissolved in the weld metal diffuses to regions of high triaxial stress such as grain boundaries and inclusions, where it lowers the local cohesive strength and initiates delayed cracking. This is why preheating, low-hydrogen consumables and post-weld heat treatment are used to control hydrogen-assisted cold cracking.

Question 35Materials and their behaviour during welding

Which of the following best describes the solidification mode that produces equiaxed grains in the centre of a weld pool?

  1. Directional heat flow into the base metal
  2. Rapid cooling of a fully austenitic weld pool
  3. Constitutional undercooling ahead of the advancing solid-liquid interface Correct answer.
  4. Epitaxial growth from the fusion boundary

Explanation

Equiaxed grains nucleate in the weld pool when constitutional undercooling develops ahead of the columnar front, allowing new grains to form in the liquid. Epitaxial and directional growth instead produce columnar grains aligned with the heat-flow direction.

Question 36Materials and their behaviour during welding

In the context of the Schaeffler diagram, which element is used together with chromium to estimate the ferrite content of stainless steel weld metal?

  1. Molybdenum equivalent
  2. Nickel equivalent Correct answer.
  3. Manganese equivalent
  4. Carbon equivalent

Explanation

The Schaeffler diagram plots chromium equivalent against nickel equivalent to predict the microstructure of stainless steel weld metal. Chromium and ferrite-forming elements raise the chromium equivalent, while nickel and austenite-forming elements raise the nickel equivalent.

Question 37Materials and their behaviour during welding

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

  1. It prevents sensitisation of the heat-affected zone
  2. It reduces the risk of solidification cracking and helps resist hot cracking Correct answer.
  3. It eliminates the need for shielding gas
  4. It increases the chromium content of the weld

Explanation

A controlled ferrite content of roughly 5 to 10 FN in austenitic weld metal helps to accommodate residual sulphur and phosphorus and reduces the risk of solidification cracking. Ferrite also improves resistance to stress-corrosion cracking in some environments.

Question 38Materials and their behaviour during welding

What is the main cause of hot cracking (solidification cracking) in the weld metal of carbon and low-alloy steels?

  1. Low sulphur and phosphorus contents
  2. High residual hydrogen content
  3. Excessive preheating of the base metal
  4. Segregation of low-melting-point impurities such as sulphur and phosphorus at grain boundaries Correct answer.

Explanation

During solidification, solute elements such as sulphur and phosphorus are rejected into the remaining liquid and form low-melting films along grain boundaries. When thermal contraction strains act on these weak boundaries, hot cracks form.

Question 39Materials and their behaviour during welding

Which phase transformation occurs when austenite is cooled rapidly enough to avoid diffusion during welding of a carbon steel?

  1. Ferrite transforms to austenite
  2. Austenite transforms to martensite Correct answer.
  3. Austenite transforms to ferrite and pearlite
  4. Cementite transforms to graphite

Explanation

When the cooling rate is fast enough that carbon cannot diffuse, austenite transforms by a diffusionless shear mechanism into martensite. Martensite is hard and brittle and increases the risk of cold cracking in the heat-affected zone.

Question 40Materials and their behaviour during welding

A weld is made on a quenched and tempered structural steel with a high carbon equivalent. Which measure most directly reduces the risk of cold cracking in the heat-affected zone?

  1. Increasing the restraint of the joint
  2. Increasing the heat input and cooling rate simultaneously
  3. Using a higher hydrogen consumable
  4. Applying preheat and controlling interpass temperature Correct answer.

Explanation

Preheat and interpass temperature control slow the cooling rate, allow hydrogen to escape and reduce the formation of hard martensite in the heat-affected zone. Together with low-hydrogen consumables this is the standard method of avoiding cold cracking.

Question 41Materials and their behaviour during welding

Which of the following describes the term 'carbon equivalent' as used in welding metallurgy?

  1. The ratio of carbon to iron in the weld pool
  2. A value combining carbon and other alloying elements to indicate hardenability and cracking susceptibility Correct answer.
  3. The carbon content of the electrode coating
  4. The percentage of carbon that evaporates during welding

Explanation

Carbon equivalent formulas combine carbon with other alloying elements such as manganese, chromium and molybdenum to express the steel's hardenability as an equivalent carbon content. It is used to decide preheat requirements and assess cold-cracking risk.

Question 42Materials and their behaviour during welding

What is the principal effect of nitrogen pick-up in the weld metal of a steel when shielding is poor?

  1. It decreases hardness and improves ductility
  2. It acts as a powerful deoxidiser
  3. It removes carbon from the weld metal
  4. It forms nitrides, increases strength and hardness, and can cause porosity Correct answer.

Explanation

Nitrogen from the atmosphere dissolves in the weld pool and forms hard nitrides, raising strength and hardness while lowering toughness; excessive nitrogen also causes porosity. Good shielding and correct gas flow prevent nitrogen pick-up.

Question 43Materials and their behaviour during welding

Which of the following is a typical consequence of excessive heat input when welding a quenched and tempered steel?

  1. Softening of the heat-affected zone and loss of strength Correct answer.
  2. Formation of a narrow, hard heat-affected zone
  3. Increased susceptibility to hydrogen cracking
  4. Reduced grain size in the heat-affected zone

Explanation

Excessive heat input tempers the heat-affected zone beyond the original tempering condition, lowering its hardness and strength. It also widens the heat-affected zone and can reduce toughness, so heat input must be controlled within a qualified range.

Question 44Materials and their behaviour during welding

In austenitic stainless steel, what does sensitisation of the heat-affected zone involve?

  1. Dissolution of delta ferrite into the matrix
  2. Formation of martensite from austenite
  3. Absorption of hydrogen into the austenite lattice
  4. Precipitation of chromium carbides at grain boundaries with local chromium depletion Correct answer.

Explanation

In the temperature range of roughly 450 to 850 °C, carbon and chromium combine to form chromium carbides at grain boundaries, depleting the adjacent matrix of chromium. This lowers corrosion resistance and can lead to intergranular corrosion in service.

Question 45Materials and their behaviour during welding

Which alloying element is primarily responsible for the corrosion resistance of stainless steel?

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

Explanation

Chromium forms a thin, adherent passive oxide film on the steel surface that protects it from corrosion. A chromium content of at least about 10.5 to 12 % is normally required for stainless behaviour.

Question 46Materials and their behaviour during welding

A dissimilar joint is made between carbon steel and austenitic stainless steel. What happens to carbon at the fusion boundary during welding?

  1. Carbon diffuses from the carbon steel into the stainless steel, forming a decarburised zone Correct answer.
  2. Carbon diffuses from the stainless steel into the carbon steel
  3. Carbon forms a liquid eutectic that causes hot cracking
  4. Carbon is completely vaporised at the interface

Explanation

At elevated temperature, carbon migrates from the higher-carbon, lower-chromium carbon steel into the stainless steel, which has a strong carbon affinity due to chromium. This creates a decarburised, soft zone on the carbon steel side and a carburised, hard zone on the stainless side.

Question 47Materials and their behaviour during welding

Which of the following weld defects is most characteristic of aluminium and its alloys if the oxide film is not removed before welding?

  1. Hydrogen-induced cold cracking
  2. Sensitisation of the heat-affected zone
  3. Solidification cracking in the ferrite
  4. Lack of fusion and oxide inclusions Correct answer.

Explanation

Aluminium forms a tenacious, high-melting aluminium oxide film that is not broken down at normal arc temperatures, so it can prevent fusion and become trapped as inclusions. Proper cleaning and, in AC TIG, cathodic cleaning action are used to remove it.

Question 48Materials and their behaviour during welding

What is the effect of a high nickel content in austenitic stainless steel weld metal on its solidification behaviour?

  1. It promotes an austenitic solidification mode and increases the risk of solidification cracking Correct answer.
  2. It promotes ferrite formation and reduces cracking risk
  3. It raises the melting point to that of pure iron
  4. It prevents carbide precipitation entirely

Explanation

High nickel relative to chromium favours primary austenitic solidification, in which low-melting impurities segregate along grain boundaries and make the weld metal more susceptible to solidification cracking. A balanced composition with some ferrite is generally preferred.

Question 49Materials and their behaviour during welding

Which of the following best describes the mechanism of lamellar tearing in welded joints?

  1. Hot cracking of the weld metal during solidification
  2. Grain growth in the heat-affected zone
  3. Hydrogen embrittlement of the weld metal
  4. Cracking along inclusions parallel to the plate surface under through-thickness restraint Correct answer.

Explanation

Lamellar tearing occurs in rolled plate when through-thickness shrinkage strains open up weak interfaces between non-metallic inclusions aligned parallel to the rolling plane. It is controlled by joint design to reduce through-thickness restraint and by using low-sulphur, inclusion-controlled steel.

Question 50Materials and their behaviour during welding

What is the purpose of adding titanium or niobium to stainless steel welding consumables?

  1. To increase the ferrite content of the weld metal
  2. To increase the hydrogen content of the weld
  3. To improve the electrical conductivity of the electrode
  4. To stabilise carbon as carbides and reduce sensitisation Correct answer.

Explanation

Titanium and niobium have a stronger affinity for carbon than chromium does, so they form stable carbides and keep chromium in solution. This prevents chromium depletion at grain boundaries and improves resistance to intergranular corrosion.