Metallic Materials: Structure, Properties and Heat Treatment
Metallic Materials covers the iron-carbon system, the heat treatments that manipulate it, and the non-ferrous alloys that dominate electrical, marine and aerospace work. The exam expects you to read a specification, name the microstructure it produces, and predict what happens when the process drifts.
The Fe-Fe3C framework
Ferrite is BCC with 0.022% C solubility at 727 C; austenite is FCC with 2.14% at 1147 C; cementite carries 6.67% C. At 0.76% C and 727 C austenite decomposes eutectoidally to pearlite. Hypoeutectoid steels show proeutectoid ferrite at grain boundaries; hypereutectoid grades show a cementite network, which is a continuous crack path and the reason tool steels are spheroidise annealed.
Transformation and hardenability
TTT diagrams are isothermal; CCT diagrams apply to continuous cooling, and the critical cooling rate is read from the CCT nose. Martensite is displacive and diffusionless, so carbon content, not alloying, sets as-quenched hardness. Manganese, chromium, molybdenum and nickel push the CCT curve right and improve hardenability, measured by Jominy end-quench testing to ISO 642 and reported as hardness against distance from the quenched end.
Non-ferrous essentials
6061 aluminium is solution treated near 530 C, quenched and artificially aged to the T6 temper; 5083 is non-heat-treatable, strengthened only by work hardening, so welding softens it. Austenitic stainless steels (304, and 316 with molybdenum for pitting resistance) are not hardenable by quenching; exposure to roughly 450-850 C during welding precipitates chromium carbides and sensitises the grain boundaries, avoided by using 304L/316L or the stabilised grades 321 and 347.
Worked example
A 40 mm shaft in 0.40% C steel must reach about 300 HB in the core. Normalising produces ferrite plus pearlite at roughly 180 HB, so a quench-and-temper route is required: austenitise at 850 C, oil quench (the section is comfortably within the oil-cooled hardenability band for this grade), then temper at about 400 C for one hour. The result is tempered martensite in the region of 380-420 HV near the surface, with the core lower but still above target. Tempering above 550 C would fall below the hardness requirement while improving toughness.
Traps
Confusing hardness with hardenability; believing alloying raises as-quenched hardness, when only carbon does at full martensite; using isothermal TTT data to design a continuous quench; forgetting that tempering trades strength for toughness and relieves residual stress; treating stainless steel as corrosion-proof; overlooking heat-affected-zone softening in work-hardened alloys.