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IIW IWI-C practice questions

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Question 1Welding processes and equipment

In GMAW, what is the primary function of the shielding gas?

  1. It provides the filler metal that builds up the joint
  2. It excludes atmospheric oxygen and nitrogen from the molten weld pool and arc zone Correct answer.
  3. It conducts the welding current from the contact tip to the workpiece
  4. It cools the contact tip and nozzle during high-duty operation

Explanation

The shielding gas displaces air around the arc and molten pool, preventing oxygen and nitrogen pickup that would cause porosity and embrittlement. Current conduction is the job of the wire and arc plasma, and filler comes from the electrode wire, not the gas.

Question 2Welding processes and equipment

Which power source characteristic is required for stable manual metal arc (SMAW) operation with a rutile electrode?

  1. A constant-voltage characteristic with a steeply drooping arc
  2. A drooping (constant-current) characteristic giving a nearly vertical volt-ampere curve Correct answer.
  3. A flat characteristic with no open-circuit voltage limit
  4. A rising characteristic in which voltage increases with current

Explanation

Manual arc welding relies on a drooping characteristic so that arc length variations produce only small current changes, which keeps the electrode from overheating and stabilizes the arc. A constant-voltage source is used for continuous wire processes where self-regulation of arc length is wanted.

Question 3Welding processes and equipment

In SAW, why is the flux layer applied ahead of the arc rather than behind it?

  1. To bury the arc and molten pool, suppressing spatter and fume while the arc melts the flux into a protective slag Correct answer.
  2. To prevent the wire from touching the workpiece
  3. To add alloying elements only after solidification has finished
  4. To preheat the plate before the arc arrives

Explanation

Submerged arc welding pours granular flux ahead of the arc so the arc burns entirely beneath the flux blanket. This shields the pool from air, confines spatter and radiation, and generates the slag that protects the bead during cooling.

Question 4Welding processes and equipment

What is the main reason for using a backing bar or ceramic backing tape in one-sided root-pass welding?

  1. To reduce the hydrogen content of the shielding gas
  2. To increase the welding current capacity of the machine
  3. To magnetize the joint and improve arc stiffness
  4. To support the molten root and prevent burn-through while shaping the root bead Correct answer.

Explanation

Backing retains the molten root pool so full penetration can be achieved without burn-through, and it helps form a smooth root profile. It does not change the power source output or the gas chemistry.

Question 5Welding processes and equipment

In GTAW, which electrode and polarity combination is normally used for welding aluminium?

  1. DCEN with a thoriated tungsten electrode
  2. DCEP with a pure tungsten electrode and a helium shield
  3. AC with a tungsten electrode, using the half-cycle that provides cathodic cleaning Correct answer.
  4. DCEN with a zirconiated electrode and no shielding gas

Explanation

Alternating current is standard for aluminium because the electrode-positive half-cycle breaks up the tenacious oxide film (cathodic cleaning) while the electrode-negative half-cycle limits heat at the tungsten. Pure DCEN does not clean the oxide and DCEP overheats the electrode.

Question 6Welding processes and equipment

A welder reports that the GMAW arc is unstable, stubbing into the pool with frequent burn-back. Which parameter should be checked first?

  1. Shielding gas cylinder colour code
  2. Contact-tip-to-work distance being too long
  3. Wire-feed speed relative to the set voltage Correct answer.
  4. Workpiece thickness relative to the joint type

Explanation

Stubbing and burn-back indicate that the wire-feed speed is too high for the voltage setting, so the wire reaches the pool before it melts. Adjusting the WFS-to-voltage balance restores stable spray or short-circuit transfer.

Question 7Welding processes and equipment

Why is a water-cooled torch often specified for high-current GTAW?

  1. To remove heat from the torch body and cable so the torch can carry higher current without overheating Correct answer.
  2. To allow the use of AC instead of DC
  3. To increase the open-circuit voltage of the power source
  4. To ionize the shielding gas more efficiently

Explanation

High welding current generates large resistive and arc heat in the torch head and cable. Water cooling carries that heat away, allowing higher duty cycles and currents without damaging the torch or burning the operator's hand.

Question 8Welding processes and equipment

In FCAW, what distinguishes a self-shielded (gasless) wire from a gas-shielded flux-cored wire?

  1. Self-shielded wires operate only in the short-circuiting transfer mode
  2. Self-shielded wires rely on flux ingredients that generate shielding gas and slag, so no external gas is supplied Correct answer.
  3. Self-shielded wires require pure argon shielding
  4. Self-shielded wires cannot be used outdoors

Explanation

Self-shielded flux-cored wires contain ingredients that decompose in the arc to form protective gas and slag, so they can be used without a gas cylinder. This makes them popular for field work where wind would disperse external shielding.

Question 9Welding processes and equipment

What is the principal reason for using helium instead of argon in GTAW or GMAW on thick aluminium?

  1. Helium eliminates the need for oxide cleaning
  2. Helium has higher thermal conductivity and ionization potential, giving a hotter arc and deeper penetration Correct answer.
  3. Helium produces a more oxidizing arc
  4. Helium is denser and blankets the pool better

Explanation

Helium transfers heat more effectively to the workpiece and produces a higher-energy arc, which improves penetration on thick or highly conductive metals such as aluminium. Argon is denser and quieter but gives a cooler arc.

Question 10Welding processes and equipment

For which application is the electroslag welding (ESW) process most appropriate?

  1. Welding thin sheet in the flat position at high speed
  2. Making vertical single-pass welds in thick plate using a molten slag bath for resistance heating Correct answer.
  3. Joining dissimilar thin-walled tube with pulsed current
  4. Producing small precise fillet welds in the overhead position

Explanation

ESW is a vertical, high-deposition process for thick sections in which current passes through a conductive slag bath that melts the wire and joint faces. It is not suited to thin sheet, positional work, or precision small welds.

Question 11Welding processes and equipment

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

  1. The number of starts the machine can make per hour
  2. The ratio of arc voltage to open-circuit voltage
  3. The maximum current the machine can deliver for one second
  4. The percentage of a 10-minute period during which the machine can deliver its rated current without overheating Correct answer.

Explanation

Duty cycle is expressed as a percentage of a standard 10-minute period at a stated ambient temperature, typically 40 degrees Celsius. A 60 percent rating at 300 A means 6 minutes of welding at 300 A followed by 4 minutes of cooling.

Question 12Welding processes and equipment

A welder is making a root pass on pipe with GTAW and observes a dark, sooty deposit on the aluminium bead. What is the most likely cause?

  1. A tungsten electrode that is too short
  2. Insufficient cleaning action, often from too little electrode-positive current or contaminated gas Correct answer.
  3. Excessive travel speed with correct cleaning action
  4. Too much post-flow shielding gas

Explanation

The black soot is smut left when the aluminium oxide is not fully broken down by cathodic cleaning. Increasing the cleaning half-cycle, checking gas purity and flow, and cleaning the joint before welding resolve it.

Question 13Welding processes and equipment

Which feature of an inverter-based welding power source most directly improves control of the arc?

  1. A fixed-output rectifier with no feedback loop
  2. A mechanical tap changer on the transformer
  3. A large iron-core transformer with a low switching frequency
  4. High-frequency switching of the primary current, allowing fast regulation of output waveform and current Correct answer.

Explanation

Inverter machines rectify mains and switch it at high frequency through a small transformer, so the output can be regulated thousands of times per second. This gives fast arc response and allows waveform shaping, unlike traditional transformer or tapped designs.

Question 14Welding processes and equipment

In the SAW process, which combination usually gives the highest deposition rate?

  1. Low current with a small-diameter wire and slow travel
  2. High current with a large-diameter wire and multiple wires or hot-wire addition Correct answer.
  3. Low voltage with a small wire and high travel speed
  4. Pulsed current with a single thin wire

Explanation

Submerged arc deposition rises strongly with current and wire cross-section, and tandem or multi-wire arrangements plus hot-wire addition multiply the melt-off rate. Low-current or small-wire procedures deposit much less metal per unit time.

Question 15Welding processes and equipment

What is the purpose of the 'hot start' function on an SMAW power source?

  1. To switch automatically from AC to DC after the arc starts
  2. To increase the open-circuit voltage permanently during welding
  3. To preheat the electrode flux coating before welding
  4. To deliver a brief current boost at the start of the arc to prevent electrode sticking and ensure fusion at the strike point Correct answer.

Explanation

Hot start temporarily raises current for a fraction of a second when the arc is struck, which helps establish the arc and avoid sticking on cold plate. It is a transient function, not a permanent change to the welding output.

Question 16Welding processes and equipment

A GMAW procedure on carbon steel produces a bead with poor penetration and a convex profile. Which change is most likely to improve penetration?

  1. Increase travel speed without changing other parameters
  2. Increase wire-feed speed and current while keeping voltage matched Correct answer.
  3. Increase contact-tip-to-work distance
  4. Increase voltage while keeping wire-feed speed constant

Explanation

Penetration in GMAW is driven mainly by welding current, which rises with wire-feed speed. Raising current while keeping the voltage matched to the wire feed increases heat input at the root and flattens the bead.

Question 17Welding processes and equipment

Which safety hazard is specific to the SAW process compared with open-arc processes?

  1. Intense ultraviolet radiation from a visible open arc
  2. High-frequency starting voltage
  3. Spatter striking the welder's face shield
  4. Exposure to flux dust and fumes, and the risk of molten flux entering footwear Correct answer.

Explanation

Because the arc is buried, UV and spatter exposure are lower than in open-arc welding, but handling granular flux creates dust and the flux can be hot enough to burn feet. Proper ventilation and protective footwear are therefore important.

Question 18Welding processes and equipment

What is the main function of the contact tip in GMAW?

  1. To cool the molten weld pool
  2. To measure the welding current for the control circuit
  3. To mix the shielding gas with the wire
  4. To guide the wire and transfer welding current to it at a fixed distance from the arc Correct answer.

Explanation

The contact tip is a replaceable copper component that directs the wire toward the joint and conducts current into it. Its wear and its distance from the work strongly affect arc stability and current pickup.

Question 19Welding processes and equipment

In pulsed GMAW, what does the background current do during the low-current phase of the cycle?

  1. It cleans the oxide from aluminium
  2. It maintains the arc and preheats the wire without transferring a droplet Correct answer.
  3. It switches the process to short-circuiting transfer
  4. It creates the spray transfer of each droplet

Explanation

Pulsed GMAW alternates a high peak current that detaches one droplet per pulse with a low background current that keeps the arc alive and the wire warm. This gives spray-like transfer at average currents well below the normal spray threshold.

Question 20Welding processes and equipment

Which condition most strongly promotes arc blow in DC welding?

  1. Welding near the end of a long joint with the work connection far away, creating asymmetric magnetic fields Correct answer.
  2. Using a short arc length and a small electrode
  3. Welding with AC current at high frequency
  4. Using a ceramic backing bar

Explanation

Arc blow arises from unbalanced magnetic fields around the arc, which are worst near the ends of a joint or when the work-return connection is remote and current paths are asymmetric. Repositioning the earth clamp and using backstep techniques reduce it.

Question 21Welding processes and equipment

What is the purpose of adding oxygen or carbon dioxide to an argon shielding gas for GMAW of carbon steel?

  1. To prevent the formation of slag
  2. To increase the hydrogen content of the weld
  3. To lower the cost of the gas only
  4. To stabilize the arc, improve metal transfer and wetting, and reduce spatter Correct answer.

Explanation

Small additions of oxidizing gases improve arc stability and droplet transfer on steel and promote good bead wetting. Pure argon on steel gives an unstable, spattery arc, so oxidizing additions are standard.

Question 22Welding processes and equipment

In resistance spot welding, what does the electrode force primarily control?

  1. The thickness of the zinc coating on the sheet
  2. The cooling-water flow rate through the electrodes
  3. The electrical resistance at the sheet interface and the containment of the molten nugget Correct answer.
  4. The welding current waveform frequency

Explanation

Electrode force presses the sheets together, establishing contact resistance at the faying surface and confining the molten nugget during current flow. Too little force causes expulsion, while too much can reduce contact resistance and nugget size.

Question 23Welding processes and equipment

Which consumable is used as the electrode in the plasma arc welding process?

  1. A solid steel wire fed through a contact tip
  2. A consumable flux-cored wire
  3. A non-consumable tungsten electrode with a constricting nozzle Correct answer.
  4. A coated stick electrode

Explanation

Plasma welding uses a tungsten electrode inside a torch with a small orifice that constricts the arc into a high-energy plasma jet. Filler, if used, is added separately; the electrode itself is not consumed.

Question 24Welding processes and equipment

A fabricator must weld 25 mm thick carbon steel plate in the flat position with high deposition and full penetration from both sides. Which process selection is most appropriate?

  1. Manual metal arc with 2.5 mm rutile electrodes
  2. Submerged arc welding with a multi-wire or tandem setup Correct answer.
  3. Short-circuiting GMAW with 0.8 mm wire
  4. GTAW with a single 2.4 mm tungsten and no filler

Explanation

SAW with multiple wires gives the high deposition rate and deep penetration needed for thick plate in the flat position. The other options are low-deposition or unsuitable for 25 mm sections.

Question 25Materials and their behaviour during welding

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

  1. About 0.008 % Correct answer.
  2. About 4.3 %
  3. About 2.1 %
  4. About 0.8 %

Explanation

Ferrite is a body-centred cubic phase and can hold only a very small amount of carbon in solid solution, roughly 0.008 % at room temperature and about 0.02 % at 723 °C. The much larger figures quoted for austenite (about 2.1 %) and for the eutectic (4.3 %) apply to other phases or to the liquid.

Question 26Materials and their behaviour during welding

Which transformation occurs when austenite of eutectoid composition (about 0.8 % C) is cooled slowly through the A1 temperature?

  1. Austenite transforms into delta ferrite and liquid.
  2. Austenite transforms into coarse primary cementite only.
  3. Austenite transforms into martensite by a diffusionless shear.
  4. Austenite transforms into pearlite, a lamellar mixture of ferrite and cementite. Correct answer.

Explanation

At the eutectoid point, austenite decomposes on slow cooling into the two equilibrium phases ferrite and cementite, which grow cooperatively as alternating lamellae known as pearlite. Martensite needs rapid quenching, and the delta ferrite plus liquid reaction lies far above the A1 on the iron-rich side.

Question 27Materials and their behaviour during welding

A carbon-manganese steel weld metal contains 0.10 % C, 1.2 % Mn and 0.5 % Si. Which element has the strongest effect on hardenability per unit addition in such weld metal?

  1. Carbon Correct answer.
  2. Manganese
  3. Sulphur
  4. Silicon

Explanation

Carbon has by far the largest multiplying effect on hardenability, so even small increases in carbon strongly raise hardness and cracking risk in the heat-affected zone and weld metal. Manganese also increases hardenability but requires much higher additions to achieve a comparable shift in the continuous cooling diagram.

Question 28Materials and their behaviour during welding

Which of the following is the principal reason for preheating a thick, high-carbon steel joint before welding?

  1. To refine the grain size of the weld metal
  2. To slow the cooling rate and reduce the risk of hydrogen-assisted cold cracking Correct answer.
  3. To remove nitrogen from the weld pool
  4. To increase the deposition rate of the electrode

Explanation

Preheat reduces the temperature difference between the weld and the surrounding plate, lowering the cooling rate and giving hydrogen time to escape while keeping the heat-affected zone softer and more ductile. It is a cracking-prevention measure rather than a means of raising productivity or changing the deposit chemistry.

Question 29Materials and their behaviour during welding

What is the essential difference between the crystal structure of austenite and that of ferrite?

  1. Austenite is face-centred cubic; ferrite is body-centred cubic. Correct answer.
  2. Austenite is body-centred tetragonal; ferrite is hexagonal close-packed.
  3. Austenite is hexagonal close-packed; ferrite is face-centred cubic.
  4. Austenite is body-centred cubic; ferrite is face-centred cubic.

Explanation

Austenite (γ-Fe) has a face-centred cubic lattice, which gives it high ductility and a large interstitial solubility for carbon. Ferrite (α-Fe) is body-centred cubic, with lower packing density and a very limited carbon solubility.

Question 30Materials and their behaviour during welding

Which of the following statements correctly describes the formation of martensite?

  1. It forms by long-range diffusion of carbon out of austenite during slow cooling.
  2. It forms only when the steel is held at 700 °C for several hours.
  3. It is an equilibrium phase that appears on the iron–carbon diagram at 723 °C.
  4. It forms by a diffusionless, displacive transformation when the cooling rate exceeds the critical value. Correct answer.

Explanation

Martensite results when austenite is quenched fast enough that carbon atoms cannot diffuse, so the lattice transforms by a cooperative shear into a supersaturated body-centred tetragonal structure. Because it is not an equilibrium phase, it does not appear on the equilibrium diagram and cannot be produced by prolonged holding.

Question 31Materials and their behaviour during welding

Which alloying element is deliberately added to stainless steels to provide passivity and corrosion resistance?

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

Explanation

Chromium forms a thin, adherent chromium oxide film on the steel surface that gives stainless steels their passive, corrosion-resistant behaviour; roughly 12 % Cr is the usual threshold. Nickel stabilises austenite and molybdenum improves pitting resistance, but neither creates the passive film on its own.

Question 32Materials and their behaviour during welding

In the context of weldability, what does the term 'carbon equivalent' (CE) estimate?

  1. The total alloy content of the welding consumable
  2. The carbon content lost through oxidation during welding
  3. The carbon pick-up from the shielding gas
  4. The tendency of a steel to form hard martensite and cold cracks in the heat-affected zone Correct answer.

Explanation

Carbon equivalent formulas combine carbon with other alloying elements, weighted according to their hardenability contribution, into a single number that indicates how readily the steel may form hard transformation products and crack on cooling. It is used to select preheat and heat input, not to describe consumables or gas reactions.

Question 33Materials and their behaviour during welding

A steel plate shows a carbon equivalent of 0.45 %. Which combination of measures is most appropriate to reduce cold cracking risk?

  1. Reduce the number of passes and use higher carbon filler
  2. Increase restraint and use a hydrogen-containing process
  3. Apply preheat, control heat input and use low-hydrogen consumables Correct answer.
  4. Quench the joint immediately after welding

Explanation

A CE of 0.45 % indicates moderate to high hardenability, so the standard precautions are preheat, controlled heat input and low-hydrogen consumables to limit the combined effect of hydrogen, hard microstructure and restraint. Additional restraint or quenching would increase rather than reduce cracking risk.

Question 34Materials and their behaviour during welding

During welding of a fully austenitic stainless steel, what is the main metallurgical concern associated with the heat-affected zone?

  1. Sensitisation by chromium carbide precipitation at grain boundaries Correct answer.
  2. Formation of coarse pearlite
  3. Formation of acicular ferrite
  4. Spheroidisation of cementite

Explanation

In the 450–850 °C range, chromium carbides can precipitate at grain boundaries in austenitic stainless steel, depleting the adjacent zone of chromium and making it susceptible to intergranular corrosion; this is sensitisation. Pearlite, acicular ferrite and cementite spheroidisation are features of carbon and low-alloy steels, not fully austenitic stainless grades.

Question 35Materials and their behaviour during welding

A high-strength low-alloy steel is welded with excessive heat input. Which consequence is most likely in the coarse-grained heat-affected zone?

  1. Grain refinement and improved toughness
  2. Formation of a fully austenitic structure at room temperature
  3. Complete elimination of the heat-affected zone
  4. Coarsening of prior austenite grains with reduced toughness Correct answer.

Explanation

High heat input raises the peak temperature and holds the coarse-grained zone above the austenitising temperature for longer, allowing prior austenite grains to grow. The resulting coarse microstructure, often with brittle constituents, lowers toughness and increases the risk of cracking.

Question 36Materials and their behaviour during welding

What is the effect of sulphur in steel weld metal?

  1. It strongly increases hardenability
  2. It stabilises the austenitic structure
  3. It promotes hot cracking by forming low-melting films at grain boundaries Correct answer.
  4. It acts as a strong deoxidiser

Explanation

Sulphur has very low solubility in solid steel and forms low-melting sulphide films at grain boundaries, which can open during solidification shrinkage and cause solidification (hot) cracking. Manganese is added to combine with sulphur as higher-melting manganese sulphide and reduce this risk.

Question 37Materials and their behaviour during welding

Which mechanism explains why hydrogen can cause delayed cracking in a hardened heat-affected zone?

  1. Hydrogen increases the carbon content of the weld metal
  2. Hydrogen oxidises the grain boundaries at high temperature
  3. Hydrogen diffuses into the lattice and accumulates at stress concentrations, reducing cohesive strength Correct answer.
  4. Hydrogen reacts with iron to form a low-melting eutectic

Explanation

Hydrogen dissolved in the weld metal diffuses toward regions of high triaxial stress, such as the hardened heat-affected zone, where it embrittles the lattice and can initiate cracks that grow over hours or days. This is why low-hydrogen consumables, preheat and post-weld heat treatment are used.

Question 38Materials and their behaviour during welding

Which of the following best describes the solidification structure of a typical ferritic steel weld metal?

  1. Columnar grains growing epitaxially from the fusion boundary toward the weld centre Correct answer.
  2. Equiaxed grains growing from the centre outward
  3. A single crystal with no grain boundaries
  4. A completely amorphous structure

Explanation

Weld metal solidifies by epitaxial growth from the partially melted base metal grains at the fusion boundary, producing columnar grains that grow in the direction of maximum heat extraction. Subsequent solid-state transformation may refine or alter this structure, but the primary solidification pattern is columnar.

Question 39Materials and their behaviour during welding

Which element is added to steel to improve its resistance to pitting and crevice corrosion in chloride environments?

  1. Aluminium
  2. Molybdenum Correct answer.
  3. Carbon
  4. Silicon

Explanation

Molybdenum, typically around 2–3 % in duplex and austenitic grades, significantly improves resistance to pitting and crevice corrosion in chloride-containing environments. Silicon, aluminium and carbon do not provide this specific benefit and may even be detrimental at high levels.

Question 40Materials and their behaviour during welding

A low-alloy steel contains 0.15 % C, 1.4 % Mn, 0.5 % Cr and 0.3 % Mo. Using typical hardenability considerations, which statement is most accurate?

  1. The steel's hardenability depends mainly on its silicon content
  2. The steel will not transform to martensite under any cooling rate
  3. The steel has moderate to high hardenability and may require preheat and controlled heat input Correct answer.
  4. The steel has low hardenability and requires no preheat

Explanation

The combination of manganese, chromium and molybdenum raises the hardenability of this low-carbon steel well above that of a plain carbon steel of the same carbon content, so preheat and controlled heat input are likely to be needed for thicker sections. Silicon has only a minor influence on hardenability.

Question 41Materials and their behaviour during welding

What is the main metallurgical consequence of nitrogen pick-up in the weld metal of a carbon-manganese steel when shielding is poor?

  1. Improved impact toughness
  2. Increased porosity and reduced toughness Correct answer.
  3. Lower yield strength
  4. Complete suppression of ferrite formation

Explanation

Nitrogen absorbed from the atmosphere can form gas porosity and, in solid solution, causes strain ageing and a loss of toughness in ferritic weld metal. Adequate shielding and correct gas flow are therefore essential to prevent nitrogen pick-up.

Question 42Materials and their behaviour during welding

Which of the following is the most likely cause of hot cracking in a fully austenitic stainless steel weld?

  1. Excessive preheat temperature
  2. A fully austenitic solidification mode with low ferrite and high impurity content Correct answer.
  3. Low heat input
  4. High ferrite content in the weld metal

Explanation

Fully austenitic solidification leaves a continuous network of grain-boundary liquid enriched in sulphur and phosphorus, which is prone to hot cracking because ferrite is not present to dissolve those impurities. A small controlled ferrite content in the deposit helps to reduce this susceptibility.

Question 43Materials and their behaviour during welding

Which statement correctly describes the effect of nickel in austenitic stainless steel?

  1. It promotes ferrite formation
  2. It increases susceptibility to hot cracking by raising sulphur content
  3. It stabilises the austenitic structure and improves toughness and ductility Correct answer.
  4. It forms the passive oxide film

Explanation

Nickel is an austenite stabiliser that allows the face-centred cubic structure to remain stable at room temperature, giving the steel good ductility and toughness. Chromium is responsible for the passive film, and nickel does not raise sulphur content.

Question 44Materials and their behaviour during welding

A steel weld is made with a consumable containing about 0.08 % C and 1.0 % Mn, and the base metal is a 0.25 % C steel. Which phenomenon should be considered when assessing the weld metal composition?

  1. Oxidation of manganese to manganese dioxide in the weld pool
  2. Formation of graphite nodules in the weld metal
  3. Complete elimination of carbon by the arc
  4. Dilution of the weld metal by base metal melting Correct answer.

Explanation

Part of the base metal melts into the weld pool, so the final weld metal composition is a mixture of filler and base metal, a process called dilution. With a higher-carbon base metal, dilution raises the weld metal carbon content above the consumable value and can affect hardenability and cracking risk.

Question 45Materials and their behaviour during welding

Which of the following statements about the effect of phosphorus in steel is correct?

  1. It acts as an austenite stabiliser equivalent to nickel
  2. It improves weld metal toughness
  3. It is deliberately added to increase hardenability
  4. It increases the risk of solidification cracking and temper embrittlement Correct answer.

Explanation

Phosphorus segregates strongly to grain boundaries during solidification and can form low-melting films that promote solidification cracking; it is also associated with temper embrittlement in some steels. Specifications therefore limit phosphorus to low levels, and it is not deliberately added as a hardenability element.

Question 46Materials and their behaviour during welding

A 12 % Cr martensitic stainless steel is to be welded. Which combination of precautions is most appropriate?

  1. Preheat around 200–300 °C, control of hydrogen, and post-weld heat treatment after cooling below the martensite finish temperature Correct answer.
  2. Use of austenitic filler only, with no consideration of heat treatment
  3. No preheat and immediate water quenching
  4. High preheat above 400 °C and slow cooling without any post-weld treatment

Explanation

Martensitic stainless steels are hardenable and hydrogen-sensitive, so moderate preheat, low-hydrogen practice and a post-weld heat treatment (tempering) carried out after the weld has cooled sufficiently to complete the martensitic transformation are standard. Excessive preheat can prevent martensite formation and leave retained austenite.

Question 47Materials and their behaviour during welding

In the context of the iron–carbon diagram, what is the significance of the A3 line for a hypoeutectoid steel?

  1. It marks the temperature at which ferrite begins to form from austenite on cooling Correct answer.
  2. It is the temperature at which austenite transforms completely to martensite
  3. It is the temperature at which graphite precipitates
  4. It marks the melting point of cementite

Explanation

The A3 line separates the austenite region from the two-phase austenite-plus-ferrite region, so on cooling ferrite starts to form at this boundary for hypoeutectoid compositions. It is distinct from the A1 eutectoid temperature and from the martensite start temperature, which is not shown on the equilibrium diagram.

Question 48Materials and their behaviour during welding

Which of the following best explains why the heat-affected zone in a steel weld can be harder than the weld metal itself?

  1. The heat-affected zone always has a higher carbon content than the weld metal
  2. The heat-affected zone may transform to martensite on rapid cooling while the weld metal has a different composition and cooling path Correct answer.
  3. The weld metal always contains more ferrite than the heat-affected zone
  4. The heat-affected zone is not affected by welding heat at all

Explanation

The heat-affected zone is base metal that has been austenitised and then cooled at a rate determined by the surrounding plate, which can produce hard martensite, especially in higher-carbon or higher-carbon-equivalent steels. The weld metal composition differs because it is a mixture of filler and diluted base metal, so its hardenability and transformation behaviour can be different.

Question 49Materials and their behaviour during welding

A duplex stainless steel weld is made with excessive heat input and slow cooling. Which microstructural imbalance is most likely, and why is it a concern?

  1. Formation of martensite throughout the weld metal
  2. Excessive ferrite with no austenite, causing loss of toughness
  3. Excessive austenite and formation of intermetallic phases such as sigma, reducing corrosion resistance and toughness Correct answer.
  4. Complete transformation to pearlite, reducing strength

Explanation

Slow cooling through the 600–1000 °C range promotes the formation of austenite and, with prolonged exposure, intermetallic phases such as sigma phase, which embrittle the steel and impair corrosion resistance. Duplex welds therefore require controlled heat input and cooling to maintain a balanced ferrite–austenite microstructure.

Question 50Materials and their behaviour during welding

In a carbon-manganese steel weld, which microstructural constituent forms at the lowest transformation temperature and is generally the hardest and least tough?

  1. Martensite Correct answer.
  2. Acicular ferrite
  3. Polygonal ferrite
  4. Coarse grain boundary ferrite

Explanation

Martensite forms when austenite is cooled so rapidly that carbon cannot diffuse, producing a supersaturated, body-centred tetragonal structure by a diffusionless shear transformation. Because it is heavily distorted and carries high internal strain, it is the hardest and least tough of the constituents listed. Ferrite and acicular ferrite form by diffusional or displacive mechanisms at higher temperatures and are considerably softer.