Engineering Materials and Testing

Manufacturing begins with choosing a material and knowing how it behaves. GATE asks about crystal structures, the iron–carbon diagram, heat treatment and material tests.

Crystal structures

FCC metals have many slip systems and are very ductile. Imperfections (vacancies, dislocations, grain boundaries) control strength: dislocation motion is how metals deform plastically.

Iron–carbon diagram

Key points (approximate):

• Eutectoid at 0.8% C and 727 °C: austenite → pearlite (ferrite + cementite).
• Eutectic at 4.3% C and 1147 °C: liquid → ledeburite.
• Steels have up to about 2.1% C; cast irons have more.
• Hypo-eutectoid steels (< 0.8% C) are ferrite + pearlite; hyper-eutectoid steels are pearlite + cementite.

Lever rule: the fraction of a phase in a two-phase region is the length of the opposite tie-line segment divided by the whole tie line.

Heat treatment

• Annealing: heat and cool slowly in the furnace → soft, ductile.
• Normalising: cool in still air → finer grains, a little stronger than annealed.
• Hardening (quenching): cool fast from austenite → martensite, very hard and brittle.
• Tempering: reheat hardened steel to a moderate temperature → tougher, slightly softer.
• Case hardening (carburising, nitriding, induction hardening): hard surface, tough core.

Mechanical properties and tests

• Tensile test: E, yield strength, UTS, % elongation, % reduction in area.
• Hardness: Brinell (ball, BHN = load/curved area), Vickers (diamond pyramid), Rockwell (depth of indentation; scales B and C).
• Impact test (Charpy, Izod): toughness, the energy to fracture a notched bar. BCC steels show a ductile-to-brittle transition at low temperature.
• Fatigue and creep tests: life under cyclic load, and slow deformation under constant load at high temperature.

Practice