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How Does Heat Treatment Change a Metal's Properties?

  • Intermediate
  • Article
  • Ages 16+
  • 5 min read
A closed heat-treatment furnace with an amber window and an automated gripper holding steel above a quench tank.

Heat treatment changes a metal's properties by changing its internal structure. For steel, heating to the austenite state and cooling at different rates gives different structures: fast cooling forms much martensite and high hardness, while slower cooling forms ferrite and pearlite with lower hardness.

Contents
Same starting point, different cooling rates, different structures.
  • Fast cooling
  • Slow cooling

What you will learn

  • Describe why cooling rate changes the structure and hardness of steel.
  • Explain what the Jominy end quench test measures.
  • Name other factors and the price of making steel harder.

Before you start

Know what hardness and brittleness mean, see the article on strength, hardness and ductility. This article uses steel as its example.

What does heat treatment change inside a metal?

It changes the internal structure through controlled heating and cooling. The University of Cambridge Jominy end quench package describes a steel sample that is normalised to remove structure differences from earlier forging, then austenitised. From that state, how it cools decides which structure the steel transforms into.

The package uses the idea of hardenability, the ability of a steel to transform partially or completely from austenite into some fraction of martensite at a given depth below the surface when cooled under a given condition.

How does cooling rate decide structure and hardness?

The faster the cooling, the more martensite and the harder the steel. The Jominy end quench test is the standard method for measuring the hardenability of steels. The cooling rate varies along the sample, from very rapid at the quenched end to rates equivalent to air cooling at the other end.

The proportions of the phases at any position depend on the cooling rate, with more martensite where cooling is fastest. Ferrite and pearlite form where cooling is slower. Hardness is measured at intervals from the quenched end: high hardness occurs where there is much martensite, and lower hardness indicates transformation to bainite or ferrite and pearlite.

The quenched end is hardest, and hardness falls with distance from it.
  • Quenched end, cooling fastest
  • Far end, cooling slowly
  • Hardness falls gradually

What else matters, and what is the price?

Steel composition matters too, and making steel harder has a price. The package says carbon controls the hardness of the martensite. Hardenability depends on the chemical composition and can also be affected by prior processing conditions, such as the austenitising temperature.

Hardness cannot be the only target. The Jominy end quench package warns that high-carbon steels are prone to distortion and cracking during heat treatment. Composition, component size and processing conditions must therefore be considered together, rather than simply choosing the fastest cooling rate.

Knowing hardenability helps choose the combination of alloy and heat treatment for components of different size, to minimise thermal stresses and distortion. Steels with low hardenability may be used for small components such as chisels and shears, or for surface-hardened parts such as gears.

In the Jominy example, regions with more martensite have higher hardness.
  • Less martensite, lower hardness
  • More martensite, higher hardness

Real-world example

Think of a chisel and a gear. The package names both as parts that can use steels of low hardenability, because the part is small or only the surface needs hardening. With the same steel, a different cooling rate changes the structure and the hardness.

Common misunderstandings

Heat treatment only changes the colour of the metal. It changes the internal structure, and so the hardness.

Harder is always better. Hardness alone does not describe the risk of distortion and cracking during heat treatment.

Heating alone is enough. According to the package, the cooling rate decides the proportions of martensite, ferrite and pearlite.

Frequently asked questions

What does the Jominy test measure?

It measures the hardenability of steel: its ability to harden in depth when quenched.

Why is the quenched end harder?

Cooling is fastest there, so the most martensite forms.

What does carbon do?

The package says carbon controls the hardness of the martensite.

Summary

  • Heat treatment changes steel's properties by changing its internal structure through heating and cooling.
  • Fast cooling forms much martensite and high hardness; slower cooling forms ferrite and pearlite.
  • Composition, especially carbon, also matters; distortion and cracking must be considered during heat treatment.

Terms in this article

English and Vietnamese terms, with the meanings used here. Hover, tap or Tab to a term with a dotted underline for an explanation as you read.

heat treatmentxử lý nhiệt
Controlled heating and cooling to change a metal's structure and properties.
austenite
The steel state after heating, from which it transforms on cooling.
martensite
The structure formed when steel cools quickly, giving high hardness.
hardenabilityđộ thấm tôi
The capacity of steel to harden in depth when quenched.
quenchtôi
Rapid cooling of heated steel.

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Sources

These sources are written for older readers. If they feel hard, read them with an adult.