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

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.