Annealing Types Explained: Full, Spheroidize, Process and Stress Relief

One word, several treatments

Annealing is a family of treatments that heat steel and cool it slowly, or hold it at a moderate temperature, to make the structure softer, more uniform or less stressed. The right member of the family depends on what problem you are solving.

Full annealing

The steel is heated above its upper critical temperature, held for uniformity, then furnace-cooled very slowly. The result is a coarse ferrite–pearlite structure: low hardness, good ductility and a clean slate before further processing or hardening.

• Purpose: soften after forging or casting, refine an overheated structure, prepare for machining.
• Typical users: medium-carbon and alloy steels in the as-forged condition.

Spheroidize annealing

Held for many hours just below (or cycling around) the lower critical temperature, this treatment makes the carbides in high-carbon steel ball up into spheroids in a soft ferrite matrix instead of sharp lamellae.

• Purpose: the softest, most machinable condition for high-carbon and tool steels; also improves cold-forming response and prepares a uniform structure for later hardening.
• Typical users: bearing steel (52100 / 100Cr6), tool steels, high-carbon wire rod.

Process annealing

A subcritical treatment — heating below Ac1, commonly in the 550–700 °C region — applied between cold-working steps. It does not change the phase structure; it recrystallizes the deformed ferrite and restores ductility so cold drawing or rolling can continue.

• Purpose: restore formability of low-carbon steel during cold reduction.
• Typical users: wire, tube and strip producers.

Stress relief

The lowest-temperature member of the family: heat to roughly 550–650 °C for steels (below any structural change), hold, cool slowly. Locked-in stresses from welding, machining or cold work relax by local yielding and creep.

• Purpose: dimensional stability and reduced distortion risk, without softening the steel.
• Note: stress relief is not a substitute for post-weld heat treatment where a code requires one.

Choosing between them

Treatment	Temperature region	Main effect
Full anneal	Above Ac3, furnace cool	Soft, uniform ferrite–pearlite
Spheroidize	Just below/around Ac1, long hold	Softest structure for high-C steels
Process anneal	Below Ac1	Restores cold-work ductility
Stress relief	Below Ac1, lower range	Removes residual stress only

If a drawing just says "anneal", ask which one — the hardness, machinability and downstream heat treatment response differ substantially.

How the condition is specified and checked

Material standards name the delivery condition explicitly — annealed conditions appear as "+A" in European designations, with spheroidized variants called out separately for bearing and tool steels. Verification is by hardness: each standard pairs the annealed condition with a maximum Brinell hardness, because hardness is the practical guarantee that the structure really is the soft one intended. A batch that arrives above the stated maximum will machine poorly and harden unpredictably, whatever the certificate's condition label says. Conversely, over-annealing high-carbon steels wastes furnace time and can coarsen the structure that the final hardening cycle inherits — annealing is a controlled treatment with a target, not simply "as soft as possible".