Materials and Workshop Practice
What this domain tests
Materials behaviour and the hand skills of the workshop: steels and their properties, corrosion, welding and its defects, fits and tolerances, bearings, seals, pipework and blanking. The examiner wants to know why a component failed and how you would make or repair a part safely on board.
Core rules and concepts
- Steels. Mild steel, higher tensile steel, alloy and stainless grades. Know carbon content effect on weldability, why higher tensile steel needs care in repair (crack sensitivity, approved welding procedures), and the purpose of grain refinement.
- Non-ferrous materials. Copper-nickel for sea water piping, aluminium bronze for pumps and propellers, bearing white metal, brasses prone to dezincification.
- Corrosion. General, galvanic (needs dissimilar metals plus electrolyte - the classic sea water pipe with a steel bracket), crevice, pitting, stress corrosion cracking, and erosion-corrosion. Protection by coating, cathodic protection (sacrificial anodes and impressed current), inhibitors and material selection.
- Welding. Common processes (SMAW, MIG/MAG, TIG), joint preparation, preheat where required, and defects: porosity, slag inclusion, lack of fusion, undercut, cracks. Non-destructive examination by visual, dye penetrant, magnetic particle, ultrasonic and radiography.
- Fits and tolerances. Clearance, interference and transition fits; measurement with micrometer, vernier and dial gauge; the effect of temperature on measurement; alignment by feeler, laser or clock gauge.
- Bearings and seals. White metal bearing clearance checks, bedding and scraping, oil seals, O-rings, gland packing, mechanical seals.
- Pipework and blanking. Spool pieces, spectacle flanges, blind flanges, hydrostatic testing, and the rule that any temporary repair is recorded and time-limited.
Worked example
A sea water pipe shows a pinhole at a welded bracket and the surrounding metal is thinned. Reasoning: the bracket created a crevice and possibly a galvanic couple, with erosion-corrosion accelerating local thinning. Action: isolate and drain, blank or renew the section, use the correct material for the spool and an approved welding procedure, check thickness of adjacent pipework, and record. The scoring answer names the mechanism, not just "corrosion".
Traps that cost marks
- Naming "corrosion" without saying which type and why.
- Welding higher tensile steel without a procedure or preheat consideration.
- Confusing magnetic particle with dye penetrant (one is for ferromagnetic surfaces).
- Measuring without accounting for temperature or instrument zero.
- Leaving a temporary repair undocumented.