Question

Briefly describe the tests made on a piece of metal to determine its suitability for use in engineering. Explain clearly what is meant by any four of the following metallurgical terms:

(a) Work hardening

(b) Case hardening

(c) Annealing

(d) Normalising

(e) Nitriding

Answer 1

Tests on a metal to determine its suitability for engineering use

Tensile test: a specimen is pulled in a testing machine and a load/extension curve obtained; from it are derived ultimate tensile strength, yield stress, proof stress, elongation (%) and reduction of area, which indicate strength and ductility.
Compression test: measures behaviour under crushing/compressive load, important for cast iron and structural columns.
Hardness test (Brinell, Rockwell, Vickers/Pyramid, Shore): a hardened indenter is pressed into the surface; the depth/area or impression indicates resistance to surface indentation/wear.
Impact/toughness test (Izod, Charpy): a pendulum strikes a notched specimen and the energy absorbed in fracture measures toughness and resistance to shock.
Fatigue test: a specimen is repeatedly loaded in a rotary-bending or push-pull machine to determine the endurance limit and behaviour under cyclic stress, vital for shafting and crankshafts.
Bend/transverse test: a bar is loaded across a span to check flexibility and soundness of the material.
Chemical analysis/spectrographic test: determines composition (carbon, alloying elements, impurities, sulphur/phosphorus) which controls properties.
Microscopic/metallographic examination: a polished and etched specimen is examined to reveal grain size, structure, inclusions, cracks and segregation.
Non-destructive tests: ultrasonic, magnetic particle, dye penetrant and radiographic examination detect internal flaws without damaging the part.
Approximate tests: spark test (distinguishes steel by spark colour), file test, impact test on a block.

Explanation of four metallurgical terms

Part (a)

Work hardening (strain hardening): cold working, i.e. plastic deformation at low temperature, which permanently increases the dislocation density of the metal lattice. This makes the metal harder and stronger but less ductile (more brittle). Excessive cold working can cause cracking. Example: cold-drawn wire, rolling, riveting, swaging, hammering a shaft. It may be reduced by annealing.

Part (b)

Case hardening: a surface-hardening heat treatment for low-carbon steels. The component is heated in a carbon-rich medium (packing in charcoal, gas carburising, or heating in a sodium cyanide bath) so that carbon diffuses into the surface layers, then quenched. The result is a hard, wear-resistant surface case over a tough, ductile (low-carbon) core. Used for gear teeth, camshafts, pins and pins/rollers that need a hard-wearing surface.

Part (c)

Annealing: heat treatment in which the metal is heated to a suitable temperature and held, then cooled slowly. It removes internal stresses, softens the metal, refines or coarsens grain, and restores ductility and machinability after cold working or hardening. Example: annealing cold-worked boiler tubes, softening hardened steel, stress relief of castings by slow cooling.

Part (d)

Normalising: the metal (steel) is heated to a temperature above its upper critical point (about 30-50 C above the transformation temperature), held to allow full transformation, then cooled in still air. This produces a fine, uniform, equiaxed pearlite/ferrite structure, refines grain size, and removes internal stresses and the effects of prior working, giving a good combination of strength and ductility without the brittleness or softness extremes. Used after forging/rolling for structural steel.

Part (e)

Nitriding: a surface-hardening process in which nitrogen is introduced into the surface of special alloy steels (nitriding steels with aluminium/chromium/molybdenum) by heating in an ammonia or nitrogenous atmosphere (usually 490-525 C) for many hours. Nitrogen diffuses in and forms hard nitrides, producing an extremely hard, wear-resistant, corrosion-resistant case with a tough core, with very little distortion. Used for crankshafts, cylinder liners, gear components and valves.

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