Last reviewed 16 Sept 2026 · 11 min read
Ferrous metals
Ferrous metals contain iron as the main constituent; their properties depend largely on carbon content.
| Metal | Carbon content (approx.) | Properties | Uses |
|---|---|---|---|
| Pig iron | About 3–4% (with Si, Mn, S, P impurities) | Crude, brittle; product of the blast furnace | Raw material for cast iron and steel |
| Cast iron | About 2–4% | Strong in compression, weak in tension, brittle, not malleable or weldable, good castability, corrosion resistant | Pipes, manhole covers, columns (historic), machine bases, sanitary fittings, railings |
| Wrought iron | Very low (below about 0.15%, typically much lower) with slag fibres | Nearly pure, tough, ductile, malleable, weldable (forge), corrosion resistant; equal strength in tension and compression but low | Chains, rivets, bolts, ornamental work (largely replaced by mild steel) |
| Steel | Up to about 2% (structural steels much lower) | Strong in tension and compression, ductile, weldable (low carbon) | Reinforcement, structural sections, sheets, pipes, wires |
Types of cast iron
- Grey cast iron — graphite flakes; machinable, brittle; most common castings.
- White cast iron — carbon as cementite; very hard and brittle.
- Malleable cast iron — white iron heat-treated — some ductility.
- Ductile (spheroidal graphite, SG) iron — graphite nodules (by magnesium addition) — much tougher and ductile — ductile iron pipes.
Classification of steels by carbon
| Steel | Carbon (approx.) | Uses |
|---|---|---|
| Mild (low carbon) steel | About 0.15–0.25% | Reinforcement bars, structural sections, sheets |
| Medium carbon steel | About 0.3–0.6% | Rails, axles, machine parts |
| High carbon steel | About 0.6–1.5% | Tools, springs, cutting instruments, high-tensile wires |
Effect of increasing carbon: strength and hardness increase; ductility, toughness and weldability decrease.
Manufacture of steel
Iron ore is reduced in a blast furnace with coke and limestone to produce pig iron (hot metal). Steel is then made by removing excess carbon and impurities:
- Bessemer process (historic) — air blown through molten iron.
- Open hearth process (historic).
- Basic oxygen furnace (LD process) — pure oxygen blown onto molten iron — major modern route.
- Electric arc furnace (EAF) — melting scrap (and sponge/direct reduced iron) with electric arcs — flexible, used widely in India.
- Induction furnace — small-scale melting of scrap and sponge iron.
Molten steel is refined (secondary metallurgy), continuously cast into billets, blooms and slabs, and hot rolled into bars, sections, plates and coils.
Properties of mild steel
| Property | Typical value |
|---|---|
| Modulus of elasticity (E) | 2 × 10⁵ N/mm² |
| Yield strength (structural E250 / reinforcement Fe 250) | 250 N/mm² |
| Density | 7850 kg/m³ |
| Coefficient of thermal expansion | About 12 × 10⁻⁶ per °C (close to that of concrete — basis of RCC compatibility) |
| Poisson's ratio | About 0.3 |
| Modulus of rigidity (G) | About 0.77 × 10⁵ N/mm² |
Stress–strain curve (tension)
For mild steel:
- Proportional limit — stress proportional to strain (Hooke's law).
- Elastic limit — full recovery on unloading.
- Upper and lower yield points — sudden extension at nearly constant stress (yield plateau).
- Strain hardening — stress rises again with further strain.
- Ultimate tensile strength — maximum stress.
- Necking — local reduction of area.
- Fracture — breaking point (ductile cup-and-cone fracture).
High-strength deformed (HSD/TMT) bars and cold-worked steel usually have no definite yield point; the 0.2% proof stress (stress at which permanent strain of 0.2% remains) is taken as the yield strength.
Reinforcing steel
Types
| Type | Features |
|---|---|
| Mild steel plain bars (Fe 250) | Plain round bars; lower strength; good ductility; used for stirrups/ties in some cases and where bending is severe |
| Cold twisted deformed (CTD) bars | Older high-yield bars made by cold twisting — reduced ductility; largely replaced |
| Thermo-mechanically treated (TMT) bars | Hot-rolled bars quenched by water sprays (forming a hard tempered martensite outer rim) and self-tempered, with a softer ferrite–pearlite core — high strength with good ductility and weldability — most common today |
| Epoxy-coated and galvanised bars, stainless steel bars | Corrosion protection in aggressive environments |
Grades (IS 1786)
High-strength deformed bars are produced in grades Fe 415, Fe 415D, Fe 415S, Fe 500, Fe 500D, Fe 500S, Fe 550, Fe 550D, Fe 600 (the number is the minimum 0.2% proof stress/yield stress in N/mm²).
- "D" grades have higher ductility requirements (greater elongation and higher ratio of tensile strength to yield strength) — preferred for earthquake-resistant structures; "S" grades are for special seismic applications.
- Indicative minimum elongation: about 14.5% for Fe 415, 12% for Fe 500, and higher for D grades (e.g. about 16% for Fe 500D); minimum ratio of tensile strength to yield strength about 1.10 for Fe 415 and 1.08 for Fe 500, with higher ratios for D grades.
- Bend and rebend tests check ductility of bars.
- Deformations (ribs/lugs) improve bond with concrete.
Structural steel
- IS 2062 — hot-rolled medium and high tensile structural steel — grades such as E250 (yield 250 MPa), E350, E410, E450 (with quality sub-grades A, BR, B0, C related to impact properties and weldability).
- Rolled sections: ISMB (medium weight beams), ISMC (channels), ISA (angles), ISJB, ISLB, ISHB, T-sections, plates, flats, tubes (hollow sections).
- Prestressing steel — high-tensile plain wires and stranded wires (e.g. 7-wire strands) with tensile strengths commonly in the range of about 1500–1900 MPa, low relaxation.
- Weathering steel (e.g. Corten type) — forms a stable protective rust patina — bridges, facades.
- Stainless steel — contains at least about 10.5% chromium (often with nickel) — forms a passive chromium oxide film — corrosion-resistant reinforcement, fittings, cladding.
- Cold-formed light gauge sections — purlins, light frames.
Heat treatment of steel
| Process | Procedure | Effect |
|---|---|---|
| Annealing | Heating above critical temperature, slow cooling in furnace | Softens, relieves internal stresses, improves ductility and machinability |
| Normalising | Heating above critical temperature, cooling in still air | Refines grain structure, improves strength and toughness uniformly |
| Hardening (quenching) | Heating and rapid cooling in water or oil | Increases hardness and strength; makes steel brittle |
| Tempering | Reheating hardened steel to a moderate temperature and cooling | Reduces brittleness, relieves quenching stresses, improves toughness |
| Case hardening (carburising, nitriding) | Hardening the surface layer only | Hard wear-resistant surface with tough core |
Corrosion of steel
Corrosion is the electrochemical deterioration of steel in the presence of moisture and oxygen, forming rust (hydrated iron oxides), which occupies several times the volume of steel — causing cracking and spalling of concrete cover.
Accelerated by: chlorides (sea water, deicing salts), carbonation of concrete (loss of alkalinity), acids, industrial pollution, stray currents, contact with dissimilar metals (galvanic corrosion), cracks and inadequate cover.
Prevention
- Protective coatings — paints (primers such as zinc chromate, red oxide), epoxy coatings (fusion-bonded epoxy-coated bars), bituminous coatings.
- Galvanising — zinc coating (sacrificial protection).
- Metallic coatings — zinc/aluminium metallising, electroplating (chromium, nickel).
- Cathodic protection — sacrificial anodes (zinc, magnesium, aluminium) or impressed current systems — pipelines, marine structures, bridge decks.
- Corrosion-resistant steels — stainless steel, weathering steel.
- In concrete: adequate cover, dense low-permeability concrete (low w/c), limiting chlorides, corrosion inhibitors, crack control, blended cements.
- Design details — avoiding water traps, ensuring drainage and ventilation, avoiding dissimilar metal contact.
Non-ferrous metals
| Metal | Properties | Uses |
|---|---|---|
| Aluminium | Light (density ~2700 kg/m³), corrosion resistant (protective oxide film), good conductor, ductile, E about 70 GPa (one-third of steel), high thermal expansion, easily extruded | Window and door frames, curtain wall systems, roofing and cladding sheets, composite panels, electrical conductors, foils (insulation), structural members in special cases |
| Copper | Excellent electrical and thermal conductor, ductile, corrosion resistant | Electrical wiring, water and gas pipes, roofing, damp-proof courses, lightning conductors |
| Zinc | Corrosion resistant, low melting point | Galvanising steel, roofing sheets, alloys (brass) |
| Lead | Very heavy, soft, malleable, corrosion resistant, toxic | Damp-proof courses and flashings (traditional), radiation shielding, sheathing of cables; no longer used for drinking water pipes due to toxicity |
| Tin | Soft, corrosion resistant | Tinning of steel, solder, alloys |
| Nickel and chromium | Corrosion resistant, hard | Plating, stainless steel alloying |
Alloys
| Alloy | Composition (main) | Uses |
|---|---|---|
| Brass | Copper + zinc | Fittings, taps, door handles, hinges, valves |
| Bronze | Copper + tin | Bearings, statues, hardware, marine fittings |
| Gunmetal | Copper + tin + zinc (sometimes lead) | Valves, pumps, bushes, fittings |
| Duralumin | Aluminium + copper + magnesium + manganese | High strength-to-weight — aircraft, lightweight structures |
| Solder | Lead + tin (lead-free solders use tin with silver/copper) | Joining pipes and electrical connections |
| German silver (nickel silver) | Copper + nickel + zinc | Decorative fittings, cutlery |
| Stainless steel | Iron + chromium (≥ ~10.5%) + nickel | Sanitary fittings, cladding, handrails, corrosion-resistant reinforcement |
| Invar | Iron + about 36% nickel | Very low thermal expansion — surveying tapes, precision instruments |