Five free practice questions on materials, welding and weld inspection: unit F of paper A (4A) of the 4th Class Power Engineer certificate exam, written to the SOPEEC syllabus. They are in the exam's format: four options, one correct answer. Answer each one before opening the explanation. No account, no email.
01The questions
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Stainless steel resists corrosion in a plant mainly because of its
- chromium content
- nickel content
- molybdenum content
- low carbon content
Show the answer
A. Correct. Chromium in sufficient amount forms a thin, tough, self-repairing chromium-oxide film on the surface, which keeps oxygen and water off the metal beneath. Without enough chromium a steel is not stainless, whatever else it contains.
Why not the others
B. Nickel is added to many stainless steels to give a tough, ductile, non-magnetic (austenitic) structure, but nickel alone does not make steel stainless. The protective surface film is chromium oxide.
C. Molybdenum improves resistance to pitting in chloride water and adds strength at high temperature, but many stainless steels contain none. The corrosion resistance that makes steel stainless comes from chromium.
D. Low-carbon ('L') grades limit chromium carbides forming at welds, which protects the chromium's effect, but plain low-carbon steel still rusts. The protective film comes from the chromium content.
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A shop is welding the long longitudinal and circumferential seams of a 75 mm thick steam-drum shell. The shell can be turned on rollers so every seam is welded flat, and high deposition with few stops is wanted. Which process suits these seams?
- oxy-acetylene welding
- gas tungsten arc welding (GTAW)
- shielded metal arc welding (SMAW)
- submerged arc welding (SAW)
Show the answer
D. Correct. Submerged arc welding feeds a continuous wire under a blanket of granular flux, so it can run high currents and deposit a lot of metal per pass with few stops. Its loose flux limits it to flat or nearly flat work, which turning rolls provide on drum seams.
Why not the others
A. Oxy-acetylene welding puts heat in slowly over a wide area and is used for thin sections and small repairs. It cannot practically fuse 75 mm plate; heavy drum seams are welded by SAW.
B. GTAW gives very clean, controlled welds and is often used for root passes and thin tubes, but its deposition rate is low. Filling a 75 mm seam with it would be very slow; SAW suits long, flat, heavy seams.
C. SMAW can weld thick plate in any position, but it is manual, deposits slowly and stops at every electrode change, adding start-stop defects. Long flat seams on a turning shell suit machine SAW.
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Which sequence is correct for a liquid penetrant test of a weld?
- clean, apply penetrant, allow dwell time, remove excess, apply developer, inspect
- clean, apply developer, apply penetrant, allow dwell time, remove excess, inspect
- clean, apply penetrant, remove excess at once, apply developer, inspect
- apply penetrant, allow dwell time, remove excess, apply developer, clean, inspect
Show the answer
A. Correct. The surface must be clean and dry so penetrant can enter any opening. Dwell time lets it soak into fine flaws; the excess is then removed from the surface only, and the developer draws penetrant back out of flaws so they show as indications to inspect.
Why not the others
B. Developer applied first would sit over the openings and keep penetrant out, and it would be wiped off with the excess. Developer goes on after the excess penetrant is removed, to draw penetrant back out of any flaw.
C. Removing the excess at once gives the penetrant no time to soak into fine flaws, so tight cracks can be missed. A dwell time is allowed after applying penetrant and before removing the excess.
D. Skipping the first cleaning leaves oil, scale and dirt that keep penetrant out of flaws, and cleaning after developing would wipe away the indications before they are inspected. Cleaning comes first.
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A yellow brass (copper-zinc) fitting from a hot-water line is examined after years of service. Its shape is unchanged, but it has reddish, copper-coloured patches where the metal is porous and can be crumbled with a knife. This is
- graphitization
- dezincification
- erosion-corrosion
- high-temperature scaling
Show the answer
B. Correct. In dezincification the zinc is leached out of the brass, leaving a weak, porous layer or plug of copper in the original shape. Brasses with less zinc, or with an inhibitor added, resist it, which is why fittings for such water are chosen with care.
Why not the others
A. Graphitization is the matching attack on grey cast iron: the iron is leached away and a soft graphite skeleton is left in the original shape. It happens to cast iron, not to brass. Losing zinc from brass is dezincification.
C. Erosion-corrosion wears metal away where fast or turbulent flow strips the protective film, leaving grooves and thinned areas, so the shape changes. Here the shape is unchanged and the metal has turned to porous copper: dezincification.
D. High-temperature scaling forms an oxide layer on metal heated far above hot-water temperatures, as on furnace parts. Porous, copper-coloured brass at hot-water temperature is dezincification, the loss of zinc from the alloy.
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After its seams are welded, a thick carbon-steel drum is heated evenly in a furnace to a set temperature below the steel's lower critical (transformation) temperature, held there for a time, then cooled slowly. The purpose of this treatment is to
- relieve the residual stresses left as the welds shrank
- harden the weld metal so that it resists erosion in service
- close up any gas porosity left in the welds
- refine the grain structure, as normalizing does
Show the answer
A. Correct. Welds shrink as they cool and lock high residual stresses into a heavy drum. Holding it evenly below the transformation temperature lets the steel relax those stresses, and slow cooling stops new ones forming. This is post-weld heat treatment (stress relieving).
Why not the others
B. Stress relieving softens rather than hardens: holding below the transformation temperature and cooling slowly lowers hardness in the weld and heat-affected zone. Hardening would need rapid quenching, which welded pressure parts avoid.
C. Porosity is gas trapped as the weld metal froze; heat treatment below the melting range cannot close it. Rejectable porosity is ground out and rewelded. The furnace cycle described relieves residual stress.
D. Normalizing heats steel above its upper critical temperature, where the structure transforms, and cools it in still air to refine the grain. The treatment described stays below the lower critical temperature, so the structure is not transformed; its purpose is to relieve residual stress.
02Answer key
| Question | Answer | Learning outcome |
|---|---|---|
| Q1 | A | Describe the various types of ferrous materials |
| Q2 | D | Describe metal arc welding processes |
| Q3 | A | Describe the process of Penetrant Testing for detecting weld or material defects |
| Q4 | B | Describe the various types of non-ferrous materials |
| Q5 | A | Describe heat treatment of welds |
03What unit F covers
Metals and how they are joined and checked: toughness, fatigue and creep, how carbon and alloying elements change steel, corrosion such as dezincification, brazing and soldering, oxy-fuel cutting and welding, SMAW and submerged arc welding, stress relieving, hoop stress and reinforced openings, weld defects such as undercut and cracks, and visual, penetrant, radiographic and ultrasonic testing.
The SOPEEC 4th Class syllabus names this unit “Material and Welding” (ABSA numbers it A6) and lists three objectives under it. SOPEEC does not publish how many exam questions come from each unit, and neither do the ABSA, Technical Safety BC, TSASK or TSSA pages we checked. The right-hand column is how many questions our bank holds on each objective, Standard and Advanced together, not a count from the exam.
| Syllabus objective | In our bank |
|---|---|
| F-24 Describe the mechanical properties of engineering materials and the ability of alloying elements to change the mechanical properties of materials, and identify nonferrous materials as used in engineering. | 10 |
| F-25 Describe the welding processes that are relevant to the plant and Power Engineer. | 12 |
| F-26 Describe the inspection and testing methods of welds and materials used in plants. | 12 |
04What the questions turn on
Name the failure from its conditions
Toughness is the ability to absorb an impact without cracking. Fatigue cracks grow under many repeated stresses well below the tensile strength, often from a weld toe. Creep is slow stretching under steady stress at high temperature, as in superheater tubes.
Longitudinal seams carry hoop stress
In a cylinder under internal pressure the hoop stress, pD/2t, is twice the lengthwise stress, pD/4t. Hoop stress acts across the longitudinal seam, so that seam gets the closest attention in design and inspection.
Surface checks miss buried flaws
Visual inspection sees only the surface, so slag buried between passes needs a volumetric test such as radiography or ultrasonics. Angle-beam ultrasonics suits flat flaws like lack of sidewall fusion and needs no radiation exclusion zone.
Penetrant steps in order
Clean the surface, apply penetrant, allow the dwell time, remove only the surface excess, then apply developer. The developer draws penetrant back out of surface-breaking flaws so they show against a contrasting background.
05The other units
Paper 4A has twelve units, A to L. Each has its own page of free questions:
The 4th Class Power Engineering practice exam (4A) page describes the full question bank. How these questions are written and checked: how our questions are made. Found a mistake? Tell us and we will fix it.