Annealing of steel: full, process and spheroidize annealing

What annealing does

Annealing is a heat treatment in which steel is heated, held long enough for the structure to change, and cooled slowly. The result is a softer, more ductile structure with lower internal stress.

The temperature is chosen against the steel's critical points. Ac1 is where austenite starts to form on heating. Ac3 is where the structure of a hypoeutectoid steel (below about 0.8 % carbon) becomes fully austenitic. Both points shift with carbon and alloy content, so the material standard or a published transformation chart for the grade is the reference for an actual cycle.

Full, process and spheroidize annealing

The three common types differ in how hot the steel goes and what structure they leave.

• Full annealing: heating about 30–50 °C above Ac3 for hypoeutectoid steels, holding, then cooling slowly in the furnace. It produces coarse ferrite and pearlite with low hardness. Heating far above Ac3 is avoided because austenite grains grow.
• Process or subcritical annealing: heating below Ac1, so no austenite forms. It restores ductility to cold-worked low-carbon steel between forming or drawing passes and relieves stress. Commonly quoted temperatures are about 540–650 °C.
• Spheroidize annealing: a long hold just below Ac1, cycling above and below Ac1, or heating just above Ac1 and cooling very slowly. Carbides become rounded particles in a ferrite matrix. This is the softest condition and is used before cold heading, cold extrusion and heavy machining of medium- and high-carbon steels.
• Stress-relief annealing: a lower subcritical treatment that reduces residual stress from welding, machining or straightening without a deliberate change in structure.

Cooling

Slow cooling is what separates annealing from normalizing and hardening. After a full anneal, parts are usually cooled in the furnace, at least through the transformation range, so the austenite transforms to soft, coarse pearlite.

After a subcritical anneal the cooling rate matters less to the structure, because no austenite has formed. It still matters for distortion and residual stress in large or complex parts.

Why a part is annealed

Annealing is usually an intermediate step. It prepares steel for the next operation rather than giving final properties.

• Machinability: lower hardness reduces tool wear and cutting forces in medium-carbon and alloy grades.
• Cold forming: spheroidized wire and bar can be cold headed, extruded or bent without cracking. Cold-heading wire standards such as EN 10263 list an annealed-to-spheroidized-carbides (+AC) condition.
• Restoring ductility: cold-worked steel is annealed between forming passes.
• Stress relief: reducing residual stress before final machining or after welding limits later distortion.
• Preparation for hardening: a uniform annealed structure responds more evenly to later quenching and tempering.