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IIW IWI-B practice questions

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Question 1Welding processes and equipment

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

  1. To increase the deposition rate by ionizing the filler wire
  2. To cool the weld pool and reduce heat input
  3. To provide the current path between electrode and workpiece
  4. To exclude atmospheric contamination from the molten weld pool Correct answer.

Explanation

The shielding gas displaces air around the arc and weld pool, preventing absorption of oxygen and nitrogen that cause porosity and embrittlement. Gas composition also affects arc stability and transfer mode, but its primary duty is atmospheric exclusion.

Question 2Welding processes and equipment

Which power source characteristic is required for stable operation of the self-regulating GMAW process with a constant-speed wire feeder?

  1. Drooping characteristic with steep volt-ampere slope
  2. Constant current characteristic with rising slope
  3. Constant voltage (flat) characteristic Correct answer.
  4. Pulsed current with fixed frequency only

Explanation

A flat, constant-voltage characteristic lets arc length self-correct: if the nozzle-to-work distance shortens, current rises and burn-off increases until the original arc length is restored. This self-regulation is the basis of constant-speed GMAW.

Question 3Welding processes and equipment

In TIG welding of aluminium, why is alternating current commonly preferred over direct current electrode negative?

  1. It doubles the deposition rate compared with DCEN
  2. It eliminates the need for any shielding gas
  3. It prevents tungsten contamination entirely
  4. The electrode-positive half-cycle removes the refractory oxide layer Correct answer.

Explanation

Aluminium forms a tenacious oxide with a high melting point, and the electrode-positive half-cycle provides cathodic cleaning that breaks up this oxide. The electrode-negative half-cycle then limits tungsten heating and gives penetration, so AC balances cleaning and electrode life.

Question 4Welding processes and equipment

What is the main purpose of adding a small percentage of oxygen to an argon shielding gas for GMAW of austenitic stainless steel?

  1. To improve arc stability and weld pool wetting Correct answer.
  2. To raise the ferrite content above 15 percent
  3. To suppress chromium carbide precipitation
  4. To allow the use of CO2 as the only shielding gas

Explanation

Small oxygen or CO2 additions to argon improve arc stability, droplet transfer and bead wetting in stainless GMAW. The addition is kept low because excessive oxidizing potential degrades corrosion resistance and slag formation.

Question 5Welding processes and equipment

In SAW, why is the flux layer applied at a depth of roughly 25 to 40 mm ahead of the arc?

  1. To generate additional filler metal from the flux
  2. To prevent arc radiation and spatter escape and to ensure full arc burial Correct answer.
  3. To act as a thermal barrier that reduces penetration depth
  4. To preheat the plate surface by conduction

Explanation

Submerged arc welding relies on a deep flux blanket to bury the arc completely, which suppresses radiation, spatter and fume and stabilizes the arc cavity. Adequate depth also supports the molten flux wall that shields the pool.

Question 6Welding processes and equipment

Which phenomenon occurs in FCAW when the voltage is set too high for a given wire feed speed?

  1. Excessive penetration with a narrow, humped bead
  2. Increased spatter and stubbing of the wire into the pool
  3. Wire burn-back into the contact tip
  4. A wide, flat bead with increased risk of porosity and poor fusion Correct answer.

Explanation

Excessive voltage lengthens the arc, producing a wide, flat and often convex bead with poor penetration and greater exposure of the pool to atmosphere. Arc instability and porosity risk rise because the shielding envelope is stretched.

Question 7Welding processes and equipment

What distinguishes the spray transfer mode in GMAW?

  1. Transfer occurs only with CO2-rich shielding gas
  2. The arc is short and the wire repeatedly short-circuits into the pool
  3. Fine droplets are projected axially at high current density above a transition value Correct answer.
  4. Metal transfers in large globules at low current density

Explanation

Spray transfer occurs above a threshold current density at which a conical arc and fine axial droplets form, giving high deposition and deep penetration. It requires argon-rich shielding and is generally restricted to flat and horizontal positions.

Question 8Welding processes and equipment

In electroslag welding, how is the weld pool heated?

  1. By exothermic reaction of the flux with the base metal
  2. By an electric arc maintained between wire and plate
  3. By electrical resistance of a molten conductive slag bath Correct answer.
  4. By induction from an external coil around the joint

Explanation

Electroslag welding passes current through a molten slag bath whose electrical resistance generates the heat that melts wire, plate faces and flux. There is no arc once the process is established, which is why it is classified separately from arc welding.

Question 9Welding processes and equipment

Which parameter change most directly increases penetration in GTAW with a given electrode and gas?

  1. Increasing shielding gas flow rate
  2. Increasing travel speed
  3. Increasing arc length by raising voltage
  4. Increasing welding current Correct answer.

Explanation

In GTAW, current controls heat input and therefore penetration depth more directly than any other single variable. Higher current widens and deepens the pool, whereas gas flow, arc length and travel speed affect other aspects of bead shape.

Question 10Welding processes and equipment

Why is direct current electrode positive unsuitable for GTAW of most metals with a thoriated tungsten electrode?

  1. It produces excessive cathodic cleaning of the base metal
  2. It prevents the establishment of the arc entirely
  3. It causes the electrode to overheat and melt because the arc energy is concentrated at the tungsten Correct answer.
  4. It reverses the shielding gas flow direction

Explanation

With electrode positive, electrons bombard the tungsten and deposit most of the arc energy there, so the electrode overheats and erodes. DCEP is therefore limited to very low currents or special applications, and AC or DCEN is used instead.

Question 11Welding processes and equipment

What is the role of the liner in a GMAW torch assembly?

  1. To cool the contact tip during high-duty operation
  2. To guide the wire from the feeder to the contact tip with minimal friction Correct answer.
  3. To direct shielding gas onto the weld pool
  4. To conduct welding current to the electrode wire

Explanation

The liner is a conduit, usually of steel or plastic, that guides the wire smoothly from the wire drive to the contact tip. Poor liner condition causes erratic feed, so it is a routine maintenance item.

Question 12Welding processes and equipment

In resistance spot welding, what does increasing the electrode force generally do to the contact resistance at the sheet interface?

  1. It converts the process from resistance to arc heating
  2. It has no measurable effect on contact resistance
  3. It decreases contact resistance by improving surface conformity Correct answer.
  4. It increases contact resistance by roughening the surfaces

Explanation

Higher electrode force flattens asperities and increases real contact area at the faying interface, lowering contact resistance. This concentrates heating in the weld nugget rather than at the electrode-sheet interface, improving nugget quality.

Question 13Welding processes and equipment

Which current waveform feature is characteristic of a modern inverter-based welding power source compared with a conventional transformer-rectifier?

  1. Fixed 50 or 60 Hz output frequency determined by the mains
  2. Inability to control pulse parameters electronically
  3. Higher operating frequency allowing faster dynamic response and reduced weight Correct answer.
  4. Requirement for a large iron-cored transformer at mains frequency

Explanation

Inverter sources rectify the mains and switch it at high frequency before transforming, which shrinks the transformer and permits very fast current control. This enables precise pulsing, waveform shaping and improved arc stability.

Question 14Welding processes and equipment

Why is a trailing shield sometimes used in TIG welding of titanium?

  1. To reduce the arc voltage for better control
  2. To apply flux ahead of the arc
  3. To extend inert gas protection over the cooling weld and heat-affected zone Correct answer.
  4. To increase the welding current capacity of the torch

Explanation

Titanium absorbs oxygen and nitrogen at temperatures well above the melting point and remains reactive while cooling, so protection must continue behind the torch. A trailing shield extends the inert atmosphere over the cooling bead and HAZ.

Question 15Welding processes and equipment

What is the function of the gas lens in a GTAW torch?

  1. To produce smooth laminar gas flow over a wider protected area Correct answer.
  2. To filter moisture from the shielding gas
  3. To mix two shielding gases in a fixed ratio
  4. To increase gas turbulence for better cooling

Explanation

A gas lens uses fine mesh screens to straighten the gas flow, producing laminar coverage that extends further from the nozzle. This improves protection and allows the electrode to protrude further for better access.

Question 16Welding processes and equipment

In a synergic pulsed GMAW power source, what does the synergic control do?

  1. It locks the shielding gas flow to the arc voltage
  2. It automatically adjusts pulse parameters to the selected wire feed speed Correct answer.
  3. It synchronizes the wire feeder to the mains frequency
  4. It synchronizes two welding torches on one fixture

Explanation

A synergic control links pulse frequency, peak current, background current and duration to the wire feed speed chosen by the operator. One knob then sets the whole pulse regime, which keeps one droplet per pulse over a range of currents.

Question 17Welding processes and equipment

Which defect is most likely if the contact tip in GMAW is worn oversize?

  1. Reduced penetration from lower current density
  2. Excessive shielding gas consumption
  3. Erratic arc and poor directional control from poor current transfer Correct answer.
  4. Increased wire feed speed beyond the set value

Explanation

An oversize or worn contact tip allows the wire to wander and makes electrical contact inconsistent, giving arc instability and loss of directional control. The tip should be replaced when the bore wears beyond the wire diameter tolerance.

Question 18Welding processes and equipment

What is the principal reason for using a backing bar or backing strip in root-pass welding?

  1. To serve as permanent filler metal for the root
  2. To preheat the joint to interpass temperature
  3. To support the molten root and prevent burn-through or excessive penetration Correct answer.
  4. To increase the arc voltage automatically

Explanation

A backing bar or strip supports the molten root pool, helping to control penetration and prevent burn-through, and can also improve root bead shape. Some backing strips are consumable and become part of the joint.

Question 19Welding processes and equipment

Which of the following is the main advantage of narrow-gap welding with a mechanized GMAW or SAW system?

  1. It eliminates the need for shielding gas
  2. It removes the need for interpass temperature control
  3. It allows welding without any joint preparation
  4. It reduces the weld metal volume and distortion in thick sections Correct answer.

Explanation

Narrow-gap welding uses a deep, narrow preparation with weaving or tandem torches to fill thick sections using much less weld metal. The result is lower heat input, less distortion and shorter welding time than a conventional V-groove.

Question 20Welding processes and equipment

In a mechanized SAW system, what is the purpose of the arc voltage control (AVC) function?

  1. To regulate the wire feed speed to match the travel speed
  2. To vary the welding current independently of the wire feed speed
  3. To maintain a constant nozzle-to-work distance by adjusting the welding head height Correct answer.
  4. To control the flux recovery unit

Explanation

AVC senses arc voltage and raises or lowers the welding head to keep arc length constant, which is critical when the workpiece surface is uneven. Stable arc length in turn keeps current and bead shape consistent.

Question 21Welding processes and equipment

What causes a phenomenon known as arc blow in DC arc welding?

  1. Too high a wire feed speed in GMAW
  2. Magnetic forces from residual magnetism or asymmetric current paths deflecting the arc Correct answer.
  3. Excessive shielding gas flow disturbing the arc
  4. Moisture in the electrode coating

Explanation

Arc blow arises from magnetic fields generated by the welding current interacting with residual magnetism or with the geometry of the current return path. The arc deflects away from the intended line, causing uneven fusion and spatter.

Question 22Welding processes and equipment

In FCAW, what is the difference between self-shielded and gas-shielded flux-cored wires?

  1. Gas-shielded wires always contain no flux
  2. Self-shielded wires require an external argon blend; gas-shielded wires do not
  3. Self-shielded wires can only be used with AC current
  4. Self-shielded wires rely on the flux to generate shielding; gas-shielded wires require an external gas supply Correct answer.

Explanation

Self-shielded flux-cored wires contain ingredients that decompose to provide a protective gas envelope, so no external cylinder is needed and they tolerate wind. Gas-shielded wires need an external CO2 or mixed gas supply and are less tolerant of draughts.

Question 23Welding processes and equipment

Which of the following best describes the purpose of preheating in welding a thick low-alloy steel section?

  1. To slow the cooling rate and reduce the risk of hydrogen-assisted cold cracking Correct answer.
  2. To remove the need for post-weld heat treatment in all cases
  3. To increase the cooling rate to refine the grain structure
  4. To lower the welding current required for the same penetration

Explanation

Preheating reduces the temperature gradient between weld and parent metal, slowing cooling and allowing hydrogen to diffuse out. This lowers the hardness of the heat-affected zone and reduces susceptibility to cold cracking.

Question 24Welding processes and equipment

In plasma arc welding, what distinguishes the transferred mode from the non-transferred mode?

  1. Non-transferred mode requires no power supply
  2. In transferred mode the arc terminates on the nozzle
  3. Transferred mode uses only alternating current
  4. The arc is established between the electrode and the workpiece in transferred mode Correct answer.

Explanation

In transferred plasma, the pilot arc is transferred to the workpiece and the current flows between electrode and work, giving high energy density for welding. Non-transferred arcs terminate on the nozzle and are used mainly for cutting or surfacing.

Question 25Welding processes and equipment

What is the primary effect of increasing the electrode extension (stick-out) in GMAW at constant voltage?

  1. The transfer mode changes from spray to globular automatically
  2. The shielding gas flow becomes laminar
  3. The arc voltage rises and the current falls
  4. The resistance heating of the wire increases, so the current rises and deposition increases Correct answer.

Explanation

A longer electrode extension increases the resistive heating of the wire between contact tip and arc, which raises the burn-off rate and the current drawn in a constant-voltage system. This increases deposition but reduces penetration because less energy reaches the plate.

Question 26Welding processes and equipment

In SAW, what is the primary function of the granular flux layer covering the arc and weld pool?

  1. It cools the wire electrode before it enters the arc
  2. It conducts the welding current from the contact tube to the workpiece
  3. It acts as a mechanical support that prevents the root from sagging
  4. It shields the arc from the atmosphere and contributes alloying elements to the weld metal Correct answer.

Explanation

Granular flux in SAW melts to form a slag that excludes air from the arc and pool, and the molten flux also transfers alloying elements and stabilizes the arc. This dual shielding and metallurgical role is why flux composition is selected together with the wire.

Question 27Welding processes and equipment

Which shielding gas or mixture is most commonly specified for GMAW of austenitic stainless steel to promote a stable spray transfer with good penetration?

  1. Argon with 2-3% oxygen or a small addition of CO2 Correct answer.
  2. Pure helium
  3. Pure carbon dioxide
  4. Argon with 20-25% CO2

Explanation

Argon-based mixtures with small oxidizing additions such as 2% oxygen or 2-3% CO2 give a stable spray arc and good wetting in stainless steel while limiting carbon pickup and oxidation. Higher CO2 contents push the process toward short-circuiting or globular transfer and increase carbon pickup.

Question 28Welding processes and equipment

A fabricator wants to weld 6 mm carbon steel plate in the flat position at high deposition rate and is comparing single-wire SAW with a tandem (two-wire) SAW system. Which statement best describes the practical consequence of the tandem arrangement?

  1. Each wire has its own power source and the arcs are spaced so deposition rate rises substantially without a proportional heat input increase Correct answer.
  2. The two arcs share one power source, so deposition rate rises only marginally
  3. The trailing arc must run in DC electrode positive to avoid arc interference
  4. Tandem operation requires the flux to be electrically conductive to couple the two arcs

Explanation

In tandem SAW each wire is fed and powered independently, and the arcs are separated along the joint so the second arc can use the residual heat of the first. This raises deposition rate considerably, and the heat input per unit of deposited metal can be kept moderate rather than rising in proportion.

Question 29Welding processes and equipment

A welder using the short-circuiting GMAW transfer mode notices the arc extinguishing and reigniting irregularly with heavy spatter. Which parameter adjustment is the standard remedy?

  1. Increase the welding voltage while keeping the wire feed speed constant
  2. Reduce the voltage toward the minimum that still gives a stable short circuit Correct answer.
  3. Increase the wire feed speed without changing the voltage
  4. Decrease the inductance so the current rises faster during each short circuit

Explanation

Short-circuiting transfer needs a voltage low enough that the arc extinguishes and the wire dips into the pool at a controlled frequency. Excess voltage lengthens the arc, producing irregular re-ignition and spatter, so lowering the voltage toward a stable setting is the usual correction.

Question 30Welding processes and equipment

In a GTAW installation, the high-frequency unit is used in two distinct ways. Which description correctly distinguishes the continuous high-frequency mode from the start-only mode?

  1. Continuous high-frequency maintains arc stability in AC welding, while start-only high-frequency assists ignition and then switches off Correct answer.
  2. Continuous high-frequency superimposes a pulsed current to control bead shape, while start-only mode controls gas preflow
  3. Continuous high-frequency is used only for DC electrode negative welding, while start-only mode is used for AC
  4. Continuous high-frequency is required for aluminum, while start-only mode is required for stainless steel

Explanation

In AC GTAW the arc extinguishes and reignites each half-cycle, and continuous high-frequency helps re-establish it, improving stability. In the start-only setting the high frequency operates briefly to initiate the arc and is then disconnected, which avoids electrical interference during welding.

Question 31Materials and their behaviour during welding

In the IIW carbon equivalent formula CE = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15, which element carries the largest individual weighting?

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

Explanation

In this formula carbon enters with a coefficient of 1, while manganese is divided by 6, the Cr+Mo+V group by 5 and the Ni+Cu group by 15. Carbon therefore has by far the strongest influence on the calculated equivalent, which reflects its dominant effect on hardenability of the heat-affected zone.

Question 32Materials and their behaviour during welding

A structural steel has a carbon equivalent (CE, IIW) of 0.48. Which precaution is most appropriate before welding a thick butt joint?

  1. Use a hydrogen-free process only at ambient temperature
  2. Weld without any preheat because CE is below 0.5
  3. Apply a suitable preheat and control heat input Correct answer.
  4. Increase cooling rate by quenching the joint after each pass

Explanation

A CE of 0.48 is above the commonly used threshold of about 0.40-0.45, above which hardenability and cold-cracking risk rise noticeably, especially with increasing thickness. Preheat combined with controlled heat input lowers the cooling rate and gives hydrogen time to escape, which is the standard industrial response.

Question 33Materials and their behaviour during welding

Which transformation product in the coarse-grained heat-affected zone is most strongly associated with hydrogen-assisted cold cracking?

  1. Spheroidised pearlite
  2. Untempered martensite Correct answer.
  3. Coarse grain boundary ferrite
  4. Polygonal ferrite

Explanation

Rapid cooling of a hardenable coarse-grained HAZ produces untempered martensite, which is hard, brittle and sensitive to hydrogen. Combined with residual stress and diffusible hydrogen, this microstructure is the classic condition for cold cracking.

Question 34Materials and their behaviour during welding

Which three factors are classically combined in the explanation of hydrogen-induced cold cracking?

  1. Sulphur, phosphorus and slag
  2. Current, voltage and travel speed
  3. Porosity, undercut and spatter
  4. Diffusible hydrogen, a susceptible microstructure and tensile stress Correct answer.

Explanation

Cold cracking requires hydrogen dissolved in the weld metal or HAZ, a hard susceptible microstructure and a tensile stress acting on the joint. Removing any one of the three prevents the cracking, which is why preheat, low-hydrogen consumables and stress control are used together.

Question 35Materials and their behaviour during welding

A steel contains 0.22% C, 1.3% Mn, 0.5% Cr and no other alloying additions. Using CE = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15, what is the approximate carbon equivalent?

  1. 0.36
  2. 0.22
  3. 0.72
  4. 0.54 Correct answer.

Explanation

The calculation is 0.22 + 1.3/6 + 0.5/5 = 0.22 + 0.217 + 0.10, giving approximately 0.54. This value indicates a steel that requires preheat and heat-input control for thicker sections.

Question 36Materials and their behaviour during welding

Why does increasing plate thickness raise the required preheat temperature for a given steel?

  1. Thicker plate has a lower melting point
  2. Thicker plate reduces the hydrogen content of the consumable
  3. Thicker plate lowers the yield strength of the HAZ
  4. Thicker plate increases heat extraction from the joint, raising the cooling rate Correct answer.

Explanation

A thick section acts as a large heat sink, so heat is conducted away from the weld more rapidly and the HAZ cools faster. The resulting harder microstructure and shorter time for hydrogen escape make preheat more necessary as thickness increases.

Question 37Materials and their behaviour during welding

Which carbon steel is generally regarded as having the best weldability with conventional arc processes?

  1. A 0.80% C high-carbon tool steel
  2. A 1.2% C cast iron
  3. A 0.08% C low-carbon steel Correct answer.
  4. A 0.45% C medium-carbon steel

Explanation

Low-carbon steels with roughly 0.08% C have low hardenability, so the HAZ does not readily form martensite and cold-cracking risk is low. Medium- and high-carbon steels need preheat and often post-weld heat treatment.

Question 38Materials and their behaviour during welding

What is the main reason for specifying a post-weld heat treatment on a thick-section Cr-Mo steel weldment?

  1. To temper hard HAZ microstructures and relieve residual stress Correct answer.
  2. To increase the cooling rate of the HAZ
  3. To increase the diffusible hydrogen content
  4. To promote grain coarsening in the weld metal

Explanation

Post-weld heat treatment tempers martensite and other hard transformation products while relaxing residual stresses, which reduces susceptibility to cold cracking and improves toughness. It also supports hydrogen effusion if applied promptly.

Question 39Materials and their behaviour during welding

Which phenomenon is characteristic of hot cracking (solidification cracking) in the weld metal?

  1. Cracks that initiate from the root pass under restraint only at room temperature
  2. Cracks filled with oxide scale from the plate surface
  3. Cracks that form along solidifying grain boundaries at high temperature Correct answer.
  4. Cracks that appear hours after welding at low temperature

Explanation

Hot cracking occurs during solidification when low-melting films, often enriched in sulphur and phosphorus, remain along grain boundaries and cannot accommodate the shrinkage strain. The crack surfaces are typically oxidised because they form at high temperature.

Question 40Materials and their behaviour during welding

Which change to a weld metal composition best reduces its susceptibility to solidification cracking?

  1. Reducing the sulphur and phosphorus content Correct answer.
  2. Increasing the carbon content to 0.6%
  3. Increasing the hydrogen content of the shielding gas
  4. Increasing sulphur and phosphorus levels

Explanation

Sulphur and phosphorus form low-melting eutectics that widen the solidification temperature range and weaken grain boundaries, so reducing them improves crack resistance. Clean steelmaking and consumable selection are the usual controls.

Question 41Materials and their behaviour during welding

A weld metal contains 0.10% C, 0.9% Mn, 0.03% S and 0.03% P. Which statement best describes its likely solidification cracking behaviour?

  1. High risk because carbon is above 0.05%
  2. Very high risk because manganese is present
  3. Low risk because sulphur and phosphorus are low Correct answer.
  4. Risk cannot be related to composition at all

Explanation

With sulphur and phosphorus both around 0.03%, the amount of low-melting eutectic at grain boundaries is limited, so solidification cracking risk is comparatively low. Manganese also helps by combining with sulphur to form higher-melting MnS instead of low-melting FeS.

Question 42Materials and their behaviour during welding

Which alloying element is added to austenitic stainless steel weld metal primarily to reduce solidification cracking by promoting a small amount of ferrite?

  1. Ferrite is promoted by chromium and controlled by nickel equivalent balance Correct answer.
  2. Titanium
  3. Sulphur
  4. Carbon

Explanation

In austenitic stainless weld metal a small fraction of delta ferrite, obtained by balancing the chromium and nickel equivalents, disrupts continuous austenite grain boundaries and improves resistance to solidification cracking. This balance is commonly assessed with a constitution diagram.

Question 43Materials and their behaviour during welding

What does the Schaeffler diagram predict for stainless steel weld metal?

  1. The expected ferrite content from chromium and nickel equivalents Correct answer.
  2. The impact toughness at -40 degrees C
  3. The thermal conductivity of the base plate
  4. The hydrogen content of the shielding gas

Explanation

The Schaeffler diagram plots chromium equivalent against nickel equivalent and indicates the resulting phase balance, including the expected ferrite level, for stainless steel weld metal. This helps predict both cracking resistance and properties such as toughness and corrosion behaviour.

Question 44Materials and their behaviour during welding

Which condition is required for sensitisation of an austenitic stainless steel?

  1. Exposure in the range of roughly 450-850 degrees C, allowing chromium carbide precipitation at grain boundaries Correct answer.
  2. Rapid quenching from 1050 degrees C
  3. Cold working followed by immediate pickling
  4. Exposure to a strongly reducing atmosphere at room temperature

Explanation

Sensitisation occurs when austenitic stainless steel is held in the intercritical range, typically about 450-850 degrees C, where chromium carbides precipitate along grain boundaries. The adjacent zones become chromium-depleted and lose corrosion resistance, so intergranular attack can occur.

Question 45Materials and their behaviour during welding

How does low-temperature transformation (LTT) of austenite affect residual stress in a restrained weld?

  1. It has no influence on residual stress
  2. It increases residual tensile stress in the weld metal
  3. It generates compressive residual stress in the weld metal because transformation occurs at low temperature when the surrounding material is already rigid Correct answer.
  4. It eliminates all residual stress in the base metal

Explanation

When martensitic transformation occurs at a low temperature, the surrounding cooler material is already stiff, so the volume expansion of the transforming weld metal is constrained and produces compressive stress in the weld. This principle underlies low-transformation-temperature consumables used to resist cold cracking and distortion.

Question 46Materials and their behaviour during welding

Which statement correctly describes the effect of hydrogen in steel at elevated temperature compared with low temperature?

  1. Hydrogen causes cold cracking at high temperature and blistering only at low temperature
  2. Hydrogen improves ductility at all temperatures
  3. Hydrogen causes cold cracking mainly at low temperature and can cause high-temperature hydrogen attack at elevated temperature Correct answer.
  4. Hydrogen has no effect on steel at any temperature

Explanation

Diffusible hydrogen trapped in a hard microstructure at low temperature promotes cold cracking, while at elevated temperature hydrogen can react with carbides to form methane, causing internal decarburisation and fissuring known as high-temperature hydrogen attack. The two damage mechanisms occur in different temperature regimes.

Question 47Materials and their behaviour during welding

Which welding consumable type is preferred for welding a hardenable steel where hydrogen-assisted cold cracking is a concern?

  1. A bare wire with no shielding
  2. A rutile-coated electrode stored in a humid workshop
  3. A basic low-hydrogen electrode kept in a holding oven Correct answer.
  4. A cellulosic electrode with a high moisture tolerance

Explanation

Basic low-hydrogen electrodes have a coating that minimises hydrogen pickup, and baking plus storage in a holding oven keeps moisture out of the coating. This reduces diffusible hydrogen in the weld and lowers the risk of cold cracking in hardenable steels.

Question 48Materials and their behaviour during welding

Which of the following best describes the effect of increasing heat input on the HAZ of a hardenable steel?

  1. It reduces the cooling rate and generally lowers HAZ hardness, but may coarsen the grain structure Correct answer.
  2. It has no effect on the HAZ
  3. It raises the cooling rate and increases HAZ hardness
  4. It eliminates the need for any preheat

Explanation

Higher heat input slows cooling, which usually reduces the hardness of the HAZ and lowers cold-cracking risk. However, the longer time at high temperature can coarsen the austenite grains, which may reduce toughness, so heat input must be balanced rather than simply maximised.

Question 49Materials and their behaviour during welding

What is lamellar tearing primarily associated with?

  1. Very fine grain size in the HAZ
  2. Excessive preheat of the base plate
  3. Low hydrogen content in the consumable
  4. High sulphur content and through-thickness strain in plate containing inclusion stringers Correct answer.

Explanation

Lamellar tearing occurs when through-thickness contraction strain acts on plate containing elongated non-metallic inclusions, particularly sulphide stringers, which provide weak planes parallel to the surface. It is controlled by reducing sulphur, improving inclusion shape control and designing joints to limit through-thickness restraint.

Question 50Materials and their behaviour during welding

Which steelmaking practice most directly improves resistance to lamellar tearing in thick plate?

  1. Adding hydrogen during casting
  2. Increasing the sulphur content to improve machinability
  3. Calcium treatment and inclusion shape control with low sulphur Correct answer.
  4. Increasing the carbon content to 0.5%

Explanation

Calcium treatment modifies elongated sulphide inclusions into compact globular forms, and low sulphur reduces their volume fraction, so through-thickness ductility improves. This is the standard metallurgical route to lamellar-tearing-resistant plate.