Ferrous Metals: Mild Steel, Stainless Steel, Cast Iron & HSS

This lesson covers ferrous metals — metals that contain iron as their primary element. Understanding ferrous metals, their properties and applications is essential for AQA GCSE Design and Technology (8552), Section 3.1.6.

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What Are Ferrous Metals?

Ferrous metals contain iron (Fe) as the main element. The word "ferrous" comes from the Latin "ferrum", meaning iron.

Key Characteristics of Ferrous Metals

Characteristic	Detail
Iron content	All ferrous metals contain iron
Magnetic	Most ferrous metals are magnetic (attracted to a magnet)
Corrosion	Most ferrous metals rust when exposed to moisture and oxygen (except stainless steel)
Strength	Generally very strong, especially steel
Cost	Relatively cheap and widely available

AQA Exam Tip: The two defining features of ferrous metals are: (1) they contain iron and (2) they are magnetic. If an exam question asks how to identify a ferrous metal, the answer is to test it with a magnet.

graph TD
  FE["Ferrous Metals<br/>iron + carbon, magnetic, rust"]
  FE --> MS["Mild Steel<br/>0.1–0.3% carbon"]
  FE --> SS["Stainless Steel<br/>iron + chromium 10.5%+ + nickel"]
  FE --> CI["Cast Iron<br/>2–4% carbon"]
  FE --> HSS["High Speed Steel<br/>tungsten + chromium alloy"]
  MS --> MSU["Car body panels, RSJ beams,<br/>nuts/bolts, wire, fences"]
  SS --> SSU["Cutlery, sinks, surgical<br/>tools, food processing"]
  CI --> CIU["Engine blocks, manhole covers,<br/>vices, machine bases"]
  HSS --> HSSU["Drill bits, lathe tools,<br/>milling cutters, taps & dies"]

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Mild Steel (Low Carbon Steel)

Mild steel contains approximately 0.1–0.3% carbon with the remainder being iron. It is the most commonly used metal in the world.

Properties

Property	Detail
Carbon content	0.1–0.3%
Strength	Good tensile strength
Hardness	Relatively soft compared to other steels
Ductility	Excellent — can be drawn into wire
Malleability	Excellent — can be hammered and pressed into shape
Weldability	Easy to weld using MIG, TIG or arc welding
Machinability	Easy to cut, drill, file and machine
Corrosion	Rusts easily — requires protective coating (paint, galvanising, plating)
Magnetic	Yes
Cost	Cheap — the most affordable structural metal

Applications of Mild Steel

Application	Why Mild Steel Is Used
Car body panels	Malleable — easily pressed into shape; cheap for mass production
Nuts, bolts, screws	Strong, cheap, easy to thread
Structural steelwork	Beams, columns, girders — high strength at low cost
Wire	Ductile — drawn into wire for fencing, paper clips, staples
Sheet metal products	Cabinets, enclosures, shelving
Garden tools	Spades, rakes, wheelbarrow frames

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Stainless Steel

Stainless steel is an alloy of iron with at least 10.5% chromium (plus varying amounts of nickel and other elements). The chromium forms an invisible, self-healing oxide layer on the surface that prevents rust.

Properties

Property	Detail
Chromium content	≥10.5% (typically 18% in common grades)
Corrosion resistance	Excellent — does not rust under normal conditions
Strength	High tensile strength
Hardness	Harder than mild steel; more difficult to machine
Hygiene	Non-porous surface is easy to clean and sterilise
Appearance	Attractive silver finish; can be polished to a mirror shine
Magnetic	Some grades are magnetic; others (austenitic, e.g. 304/316) are not
Cost	Significantly more expensive than mild steel

Applications of Stainless Steel

Application	Why Stainless Steel Is Used
Kitchen sinks	Corrosion-resistant, hygienic, easy to clean
Cutlery	Food-safe, attractive, durable
Surgical instruments	Sterilisable, corrosion-resistant, hard
Chemical processing equipment	Resists chemical attack
Watch cases	Attractive, durable, hypoallergenic
Outdoor furniture	Will not rust in rain
Food processing machinery	Meets hygiene standards

AQA Exam Tip: Stainless steel is the exception among ferrous metals — it does not rust. Always explain that this is due to the chromium oxide layer that forms on the surface and protects the iron beneath.

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Cast Iron

Cast iron contains 2–4% carbon — much more than steel. It is made by melting iron in a blast furnace and pouring (casting) it into moulds.

Properties

Property	Detail
Carbon content	2–4%
Compressive strength	Excellent — very strong when squashed
Tensile strength	Poor — brittle; cracks rather than bending
Hardness	Very hard; difficult to machine
Castability	Excellent — flows well when molten, filling complex moulds
Vibration damping	Absorbs vibration well
Corrosion	Rusts, but forms a protective rust layer over time
Magnetic	Yes
Cost	Cheap in large quantities (sand casting is economical)

Applications of Cast Iron

Application	Why Cast Iron Is Used
Engine blocks	High compressive strength; absorbs vibration; complex shapes cast easily
Manhole covers	Very strong in compression (vehicles drive over them); cheap to cast
Vices and anvils	Extremely hard and rigid
Frying pans / cookware	Excellent heat retention and distribution
Decorative railings	Complex shapes easily cast; traditional aesthetic
Machine tool beds	Vibration damping improves machining accuracy

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High Speed Steel (HSS)

High Speed Steel is a tool steel alloy containing tungsten, chromium, vanadium and molybdenum. It retains its hardness at high temperatures.

Properties

Property	Detail
Hardness	Very hard — retains hardness up to ~600 °C
Heat resistance	Does not soften at the high temperatures generated during cutting
Wear resistance	Excellent — maintains a sharp cutting edge for longer
Brittleness	More brittle than mild steel — can chip if misused
Cost	Expensive compared to carbon steel tools

Applications of HSS

Application	Why HSS Is Used
Drill bits	Stays sharp at high drilling speeds and temperatures
Lathe tools	Cuts metals without losing hardness from friction heat
Milling cutters	Withstands high-speed rotation and cutting forces
Saw blades	Cuts harder metals at higher speeds than carbon steel
Taps and dies	Cuts threads in metal; needs to maintain a sharp edge

AQA Exam Tip: HSS is always the answer when a question asks about cutting tools that need to maintain hardness at high temperatures. The key phrase is "retains hardness at elevated temperatures."

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Comparing Ferrous Metals

Metal	Carbon %	Key Property	Key Weakness	Primary Use
Mild steel	0.1–0.3	Ductile, malleable, cheap	Rusts	Structural, car bodies
Stainless steel	Low C + ≥10.5% Cr	Corrosion-resistant	Expensive, hard to machine	Cutlery, sinks, surgical
Cast iron	2–4	Hard, strong in compression	Brittle	Engine blocks, manhole covers
HSS	Tool steel + W, Cr, V, Mo	Retains hardness when hot	Expensive, brittle	Drill bits, lathe tools

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Summary