Materials and Their Properties

This is a vocabulary topic with a purpose: the words describe how a material fails, and a question about which material suits a job is asking which failure matters.

The properties

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The three most confused are strength, hardness and toughness.

Strength is how much force it takes to break it.

Hardness is resistance to surface damage. A hard material is usually brittle, which is why a glass-hard file will snap rather than bend.

Toughness is the ability to absorb energy and deform without fracturing. A tough material is usually not very hard.

So hardness and toughness pull against each other, and choosing a material is usually choosing where on that scale to sit. A hammer face is hardened so it does not deform; a hammer handle is tough so it does not snap. Same tool, two opposite requirements.

Ductile and malleable

Ductile: drawn into a wire. Think of a wire being pulled longer and thinner.

Malleable: hammered or rolled into sheet. Think of a hammer flattening it.

Both describe deforming without breaking, and the opposite of both is brittle.

The mnemonic that works: ductile for drawing into wire.

Copper is both, which is why it is made into wire and into sheet.

Alloys

An alloy is a metal mixed with one or more other elements to get properties that neither has alone.

Steel is the one to know: iron plus a small amount of carbon. More carbon means harder and more brittle; less means softer and tougher, which is the same trade-off again.

Cast iron has more carbon than steel, which makes it hard, good in compression, and brittle - it cracks rather than bends, which is why a cast iron part that has been dropped is scrap.

Heat treatment

Heating and cooling a metal changes its properties without changing its composition.

Quench fast to harden, cool slowly to soften. That is the core of it.

Tempering after hardening is the normal sequence, because a fully hardened part is too brittle to use. A chisel is hardened so it will cut and then tempered so it will not shatter.

Case hardening gives both at once - a hard wearing surface over a tough interior - which is what a gear tooth and a camshaft lobe need.

Corrosion

Corrosion is chemical attack on a metal, and rust is the specific case for iron and steel.

Rust needs both oxygen and moisture. Remove either and it stops, which is why oiling a surface protects it and why a part sealed in dry air does not rust.

Protection methods:

• Paint or coating, which excludes air and water.
• Galvanizing, which is a zinc coating that corrodes in preference to the steel.
• Alloying, as in stainless steel's chromium.
• Oil or grease on working surfaces.

Aluminum does not rust - rust is specific to iron - but it does oxidize, and its oxide layer is tight and protective rather than flaky. That is why aluminum survives outdoors and steel does not.

Dissimilar metals in contact corrode faster in the presence of moisture, which is why fasteners are chosen to match what they fasten.

What you can skip

Across the 37 questions on this topic in our bank:

• Tensile strength figures never appear here; tensile stress is calculated in the structures lesson.
• Composites. Composite materials come up twice and carbon fiber once. The metal properties in this lesson carry the topic.

Where people lose points

Treating hard and strong as the same thing.

Thinking a hard material is tough. It usually is not.

Swapping ductile and malleable.

Saying aluminum rusts. It oxidizes; rust is iron.

Thinking annealing hardens. It softens.

Forgetting that tempering follows hardening and reduces brittleness deliberately.

Work one in under a minute

A cold chisel must hold a sharp edge and survive being struck repeatedly. What heat treatment does it need, and why not just harden it?

Harden it first, so the edge is hard enough to cut and hold its shape.

Then temper it, because a fully hardened chisel is brittle and would shatter under a hammer blow.

Tempering trades a little hardness for toughness, which is exactly the trade the tool needs: hard enough to cut, tough enough to be hit.

That is the hardness-against-toughness scale being deliberately positioned, which is what heat treatment is for.

Where this leads

Material choice explains where steel goes in a concrete beam, why a hammer has two different hardnesses, and much of the tool content in Auto and Shop.