Metal Classification, Ferrous Properties and Heat Treatment

From natural ore to an engineering material

Metals are natural resources obtained by extracting and processing ores. An ore is a rock or mineral containing a metal compound from which a metal can be extracted. The ore is not usually a ready-to-use piece of metal: extraction separates the metal from its compound, and further processing produces material suitable for manufacture.

A designer then chooses a metal for the job it must do. A car body panel needs to be shaped and withstand knocks; a drill bit needs to retain a hard cutting edge. Calling both materials ‘metal’ does not explain why they are suitable.

Ferrous, non-ferrous and alloy describe different things

Ferrous metals contain iron, while non-ferrous metals do not contain iron. Mild steel and cast iron are ferrous; copper and aluminium are non-ferrous. Ferrous metals are usually magnetic, but ‘usually’ matters: iron content, rather than a magnet test alone, defines the category.

An alloy is a mixture of metals, or a metal mixed with another element, made to change its properties. Alloying overlaps with the ferrous/non-ferrous classification. Steel is a ferrous alloy of iron and carbon; brass is a non-ferrous alloy of copper and zinc.

Pure iron is generally too soft for many engineered products. Adding carbon produces steels with more useful properties. As carbon content increases, steel generally becomes harder and stronger, but less tough and harder to form. This creates a trade-off: a hard cutting edge is useful on a tool, whereas easily formed, tough sheet is useful for a car body.

What physical properties tell a designer

Physical properties describe behaviour such as responses to heat and electricity.

Melting point is the temperature at which a material changes from solid to liquid. Casting requires the metal to melt, so melting point affects the temperature and energy needed for manufacture. For comparison, aluminium melts at approximately 660°C, whereas titanium melts at about 1668°C: processing titanium requires much higher temperatures.

Thermal conductivity describes how readily heat passes through a material. High conductivity is useful where heat must be transferred efficiently, such as cookware and radiators. Copper is an excellent thermal conductor.

Electrical conductivity describes how readily electric current flows through a material. Copper and aluminium conduct electricity very well. Copper’s conductivity, combined with its ability to be drawn into wire, explains its use in electrical wiring. A metal being strong does not automatically make it a good electrical conductor.

How metals respond to forces

Mechanical properties describe a material’s response to forces and loads. Several can matter at once, but they are not interchangeable.

Tensile strength is resistance to being pulled apart. Imagine pulling opposite ends of a metal rod: the relevant strength is its resistance to failure under that tension. By contrast, a material under compression is being squashed. Cast iron’s good compressive strength is particularly useful in machine bases.

Toughness is the ability to absorb energy and withstand impact without fracturing. A tough metal may dent or bend rather than crack when struck. Mild steel is tough; cast iron is comparatively brittle, so it is less suitable where sudden impacts are a major concern.

Plasticity means that deformation can be permanent without the material breaking. If a metal strip stays bent after the force is removed, it has undergone plastic deformation. This is useful when manufacturing a permanent shape.

Malleability is the ability to be hammered, rolled or pressed into shape without breaking. It is especially important for sheet forming. Ductility is a related property describing the ability to be stretched or drawn into shapes such as wire.

Hardness is resistance to scratching, wear and indentation. A cutting tool needs hardness so that its edge does not quickly wear away. However, hardness is not toughness: a hard tool can still chip if it is too brittle.

Comparing ferrous materials

The ferrous family includes cast iron; mild, medium-carbon and high-carbon steels; stainless steel; high-speed steel; and high-tensile steel. They should not be treated as one interchangeable material.

HSS earns its extra cost where cutting generates substantial heat; for low-heat hand tools such as files and chisels, cheaper high-carbon steel is usually sufficient.

Medium-carbon steel sits between mild and high-carbon steel in carbon content and working properties. It can be hardened and then tempered, making it useful when a component needs more strength and wear resistance than mild steel without sacrificing as much toughness as a very hard cutting tool.

High-tensile steel describes a performance requirement rather than a single carbon-content class. Alloying and heat treatment can produce high tensile strength alongside other useful properties. One example is 4140 chromium-molybdenum steel, used for shafts and gears in its hardened-and-tempered condition. Its strength and toughness are valuable, but welding requires specialist control and its processing adds cost. High-tensile steel is not automatically stainless steel: tensile strength and corrosion resistance are different properties.

Mild steel may have sufficient strength for a product while being easier to form and cheaper than a harder steel. Conversely, selecting mild steel for a cutting edge would sacrifice wear resistance. Material selection therefore means balancing strengths and weaknesses against the actual use, not simply choosing the ‘strongest’ metal.

Heat treatment changes performance

Heat treatment changes a metal’s properties through controlled heating and cooling. Unlike alloying, it does not normally involve changing the overall composition. The cooling method is crucial: slow cooling and rapid cooling can produce very different results.

Annealing involves heating the metal and then cooling it slowly. It softens the metal, relieves internal stresses and makes it easier to work. This is useful when further bending or shaping is needed. It can also restore workability after deformation has made a metal harder.

Hardening involves heating suitable steel and then cooling it rapidly by quenching in a suitable liquid, such as water or oil. Hardness increases, helping a cutting edge resist wear, but the steel also becomes more brittle. Medium- and high-carbon steels can be hardened this way; mild steel does not respond in the same way because its carbon content is too low.

Tempering follows hardening. The hardened steel is reheated to a temperature below that used for hardening and then cooled, commonly in air. This reduces brittleness and increases toughness, at the cost of some hardness. A tool needs this balance: an edge that is extremely hard but chips easily is not a successful cutting edge.

Case hardening gives low-carbon steel a hard outer layer while retaining a tough core. Carbon is added to the surface, followed by heating and quenching to harden that carbon-rich layer. A gear, for example, benefits from a wear-resistant surface without the whole component becoming brittle.

The important distinction is the intended result: annealing supports further shaping; hardening improves wear resistance; tempering makes hardened steel less brittle; case hardening combines a hard surface with a tough interior.

Classification

• Metals are extracted from natural ores.
• Ferrous: contains iron. Non-ferrous: does not contain iron.
• Alloy: a mixture used to change properties; can be ferrous or non-ferrous.
• Increasing steel’s carbon content generally increases hardness and strength but reduces toughness and ease of forming.