Heat treatment basics: hardening, tempering and annealing

What heat treatment is

Heat treatment means heating and cooling metal in a controlled way to change its properties, such as hardness, toughness or how easy it is to machine. It is common for tools, dies, shafts and gears.

The common processes

• Annealing: heating and slowly cooling to soften the metal and relieve stress, so it is easier to machine or form.
• Hardening: heating to a high temperature and cooling quickly, to make steel hard and wear resistant. Hardened steel is also brittle.
• Tempering: reheating hardened steel to a lower temperature, to trade a little hardness for the toughness it needs in use.
• Case hardening: making only the outer layer hard while the core stays tough, useful for gears and shafts.
• Stress relieving: a gentler heating to reduce stress after welding or heavy machining and limit distortion later.

Points to plan for

• Say the hardness you need, for example in HRC, and the steel grade. The right process depends on both.
• Parts can distort in heat treatment. For precise parts, machine roughly first, heat treat, and finish machine or grind after.
• Some workshops do heat treatment in-house, and many send it out. Ask who does it and how long it will take.

Ask for a record

For important parts, ask for a hardness test result. It is a simple check that shows the treatment did what you asked.

Kinds of work

• Die making
• Mold making
• CNC machining
• Casting
• Fabrication