Heat treatment

Controlled thermal cycles that give metals and alloys the mechanical properties required.

Heat treatment is a thermal cycle carried out under set conditions and temperatures, followed by faster or slower cooling, so that a metal or alloy (usually steel) takes on the crystalline structures that give it specific mechanical and/or technological properties.

Understanding how heat treatment affects an alloy requires its phase diagram, but that alone is not enough: phase diagrams describe equilibrium structures at a given temperature, and their curves are obtained with very slow heating and cooling.

The rate of cooling or heating therefore plays a major role. It affects not only the transition temperatures but the very nature of the structure obtained, which may include metastable constituents (such as martensite in steels) that do not appear in the phase diagram.

The main heat treatments a metal or alloy can undergo:

• Full annealing
• Process annealing
• Spheroidise annealing
• Recrystallisation annealing
• Isothermal annealing
• Normalising
• Direct quenching
• Interrupted quenching
• Martempering
• Austempering (bainitic)
• Tempering
• Stress relieving
• Isothermal quench and temper
• Solution treatment

Heat treatments for steel fall into two broad groups, depending on whether a phase transition occurs:

• With phase transition: carried out above the eutectoid temperature: annealing (simple or isothermal), normalising, quenching, martempering, bainitic hardening and austempering
• Without phase transition: carried out below the eutectoid temperature: soft annealing, tempering and stress relieving