Ferrous metals and their properties: HSC Industrial Technology Metal and Engineering
“Describe the composition, properties and applications of ferrous metals, including the effect of carbon content on steels, and select appropriate ferrous metals for engineering and fabrication tasks”
Ferrous metals are based on iron and are usually magnetic and prone to rust. Carbon content is the key variable: low-carbon (mild) steel is soft, ductile and easily welded; medium and high-carbon steels are harder, stronger and can be heat treated; cast iron is high in carbon, hard and brittle. Alloy and stainless steels add elements for strength or corrosion resistance. You choose a ferrous metal by matching its strength, hardness, workability and corrosion resistance to the job.
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What this dot point is asking
In the Metal and Engineering focus area, NESA expects you to understand ferrous metals: those based on iron. You need to describe their composition, explain how carbon content changes the properties of steel, name the common ferrous metals and their uses, and select the right one for a fabrication or engineering task. Because ferrous metals dominate engineering, this knowledge underpins both the written examination and the material choices in your Major Project folio.
What makes a metal ferrous
A ferrous metal is one in which iron is the main element. Iron on its own is soft, so it is alloyed with carbon and other elements to make the family of steels and cast irons that engineering relies on. Two properties shared by most ferrous metals matter in practice: they are magnetic, and unless protected they rust, because iron reacts readily with oxygen and moisture. Recognising these traits is one way to identify a metal as ferrous.
The role of carbon
The amount of carbon dissolved in iron is the single biggest control on a steel's properties. As carbon content rises, the metal becomes harder and stronger but less ductile and harder to weld:
- Low-carbon (mild) steel: roughly up to a quarter of a per cent carbon. Soft, ductile, tough and easy to cut, form and weld, so it is the everyday metal for fabrication, frames and sheet.
- Medium-carbon steel: stronger and harder, used for shafts, gears and components that carry load.
- High-carbon (tool) steel: hard and able to hold a cutting edge once heat treated, used for tools, blades and springs, but brittle and hard to weld.
This carbon scale is the most examined idea in the focus area, because it explains why you cannot simply swap one steel for another.
Cast iron
Cast iron contains a high proportion of carbon. It is hard, wear-resistant and excellent at absorbing vibration, and it casts well into complex shapes, which is why it is used for machine bases and bodies. The trade-off is that it is brittle and shatters under impact rather than bending, so it is poor where shock or tension loads occur.
Mechanical properties
When you select a ferrous metal you are really selecting a balance of properties:
- Strength: resistance to load before failure.
- Hardness: resistance to wear, scratching and indentation.
- Toughness: ability to absorb impact without fracturing.
- Ductility and malleability: ability to be drawn into wire or formed without cracking.
- Brittleness: the opposite of toughness, seen in cast iron and high-carbon steel.
Carbon content and heat treatment shift these properties, so the same base metal can be soft and tough or hard and brittle depending on how it is processed.
Alloy and stainless steels
Adding further elements creates alloy steels with tailored properties. Chromium and nickel produce stainless steel, which resists corrosion and is used for food, marine and decorative work. Other alloying elements raise strength, hardness or heat resistance. These steels cost more but solve problems that plain carbon steel cannot.
Selecting a ferrous metal
Choose by asking what the part must do. For a welded frame, mild steel is cheap, strong enough and easy to join. For a cutting tool, high-carbon or tool steel holds an edge. For a corrosion-prone or hygienic application, stainless steel is worth the cost. For a vibration-damping machine base, cast iron suits. Always state the property that drives the choice and the alternatives you rejected, because that justification is what the marking guidelines reward.
Carbon content is the master control on steel. As carbon rises, hardness and strength increase but ductility and weldability fall. This is why low-carbon mild steel is the easy-to-weld workhorse of fabrication, while high-carbon tool steel holds a cutting edge but is brittle and difficult to weld.
- Treating all steel as the same
- Mild, medium and high-carbon steels behave very differently; always specify the type.
- Forgetting rust
- Ferrous metals corrode unless protected; account for finishing or use stainless where moisture is present.
- Welding high-carbon steel like mild steel
- High-carbon steels crack if welded without care; note their poor weldability.
- Using cast iron under impact
- Cast iron is brittle and shatters under shock; reserve it for compression and vibration damping.
Exam-style questions
Questions in the style of NESA exam questions on this dot point, each with a worked answer. They are written by ExamExplained unless tagged "Past paper"; the year shows the paper a question is modelled on.
2019 HSC-style1 markWhich of the following lists the materials in order of increasing carbon content? A. Tool steel, mild steel, cast iron B. Cast iron, mild steel, tool steel C. Mild steel, cast iron, tool steel D. Mild steel, tool steel, cast ironShow worked answer →
The correct answer is D: mild steel, tool steel, cast iron.
Carbon content rises in this order: mild (low carbon) steel has roughly 0.1 to 0.3 per cent carbon, tool (high carbon) steel has roughly 0.6 to 1.4 per cent, and cast iron has more than about 2 per cent carbon. So from least to most carbon the order is mild steel, then tool steel, then cast iron.
More carbon increases hardness and brittleness but reduces ductility, which is why cast iron is hard and brittle while mild steel is soft and ductile. Only option D lists this increasing sequence correctly, so D is correct.
2019 HSC-style1 markHow is cast iron different from bright steel? A. It contains less carbon B. It is more difficult to file C. It shatters more easily on impact D. It requires a lubricant when machiningShow worked answer →
The correct answer is C: it shatters more easily on impact.
Cast iron has a high carbon content (over about 2 per cent), much of it present as graphite flakes. This makes it hard and rigid but brittle, so it has little ability to absorb a sudden shock and will crack or shatter on impact rather than bend. Bright (mild) steel, with far less carbon, is tough and ductile and bends instead.
Cast iron contains more carbon, not less (A); it is generally not harder to file than hardened steel; and it is usually machined dry because the graphite acts as a lubricant, so D is wrong. That leaves C as correct.
2019 HSC-style1 markWhich of the following occurs in the Bessemer steel-making process? A. Air is forced through molten pig iron. B. Pig iron ingots are cast in sand moulds. C. Steel is produced in continuous lengths. D. Silicon and manganese are added to the molten mix.Show worked answer →
The correct answer is A: air is forced through molten pig iron.
In the Bessemer process, a blast of air is blown through the molten pig iron in a converter. The oxygen in the air burns off excess carbon and impurities such as silicon and manganese, lowering the carbon content of the iron and converting it into steel.
Casting ingots in sand (B) and producing continuous lengths (C) describe other processes such as casting and continuous casting, and adding silicon and manganese (D) is the reverse of what the air blast removes, so A is correct.