Engineering Materials and Heat Treatment
What this domain tests
Engineering Materials and Heat Treatment asks you to classify ferrous and non-ferrous metals, relate composition to properties, read standard designations, and choose a heat-treatment cycle for a service requirement. Expect questions on carbon steels, cast irons, alloy steels, hardenability, annealing, normalising, hardening, tempering, and surface-hardening methods.
Core rules
- Plain-carbon steel properties rise with carbon up to about 0.8% C (eutectoid); beyond that, a cementite network makes it harder and more brittle. Low-carbon steel (0.1–0.3% C) is ductile and weldable; medium-carbon (0.3–0.6%) responds to hardening and tempering; high-carbon (0.6–1.2%) suits springs, dies and cutting tools.
- Annealing: heat above the critical temperature, hold, then cool very slowly in the furnace - softens, relieves stress, refines grain. Normalising: heat above critical, cool in still air - finer pearlite, better machinability and strength than annealed.
- Hardening: austenitise, then quench fast enough to form martensite. Tempering must follow: reheat below the lower critical temperature to trade some hardness for toughness.
- Case hardening (carburising, nitriding, induction) hardens the skin while keeping a tough core.
Worked example
A 0.45% C steel shaft is required to be tough with a hard wearing surface. It is austenitised at about 830–850 °C, quenched in oil to avoid distortion cracks, then tempered at 550 °C and air-cooled. The quench produces martensite; tempering relieves quenching stress and produces tempered martensite. If the drawing calls for a 0.8 mm case, carburise at 900–930 °C, cool, re-harden and temper.
Traps
Saying hardening alone is the final treatment; confusing annealing (furnace cooling) with normalising (air cooling); assuming higher carbon always means better toughness; forgetting that nitriding needs alloy steels and does not require a quench.