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

The word “anneal” in a heat treatment drawing is dangerously vague. There are five distinct annealing treatments — full annealing, process (subcritical) annealing, spheroidize annealing, stress-relief annealing, and normalizing — and they operate at different temperatures, produce different microstructures, and serve completely different functions. Specifying the wrong one costs machining hours, dimensional stability, and in tool steels, can make the material essentially unmachineable. This guide explains the mechanism, temperature, outcome, and correct application for each treatment, with a decision tree for the most common industrial cases.

1. The Iron-Carbon Diagram Context

All annealing treatments are defined relative to the critical temperatures of the iron-carbon system:

Treatments above Ac3 (or Ac1 for hypereutectoid steels) involve full austenitization — the microstructure transforms completely to austenite, and what forms on cooling depends on the cooling rate. Treatments below Ac1 work within the two-phase or single-phase ferrite/carbide region without any austenite formation — microstructure change is limited to carbide morphology changes and recovery/recrystallization.

2. Full Annealing

Full annealing involves heating above Ac3 (for hypoeutectoid steels, up to ~0.77% C) or above Ac1 (for hypereutectoid and tool steels), holding at temperature until the austenite is fully homogenized, and then cooling extremely slowly — typically in the furnace at 10–30°C/hour — to below approximately 600°C before the part can be removed to air.

When to use full annealing: After rough forging of medium-carbon alloy steels (SCM440, SNCM439) to restore machinability before rough turning. After normalizing produces a hardness too high for machining. As a intermediate softening step in multi-step forming operations.

3. Process (Subcritical) Annealing

Process annealing (sometimes called subcritical annealing or in-process annealing) is performed entirely below Ac1 — no austenite forms. The temperature range is typically 600–700°C. The microstructure change is recrystallization of cold-worked ferrite: dislocations introduced by cold working rearrange and annihilate, restoring the ductility and reducing the hardness lost to work hardening, without any phase transformation.

Because no austenite forms, there is no hardening risk on cooling — parts can be air cooled from 650°C without concern. This makes process annealing faster and cheaper than full annealing: the furnace cool phase (the rate-limiting step in full annealing) is eliminated.

Application: Low-carbon sheet steel and wire (SPHC, SPCC equivalent) during multi-pass cold drawing or cold rolling operations. Between drawing passes for stainless steel wire. NOT useful for medium or high-carbon steels where the goal is carbide morphology change rather than recrystallization — for those, spheroidize annealing is required.

4. Spheroidize Annealing

Spheroidize annealing converts lamellar pearlite (alternating layers of ferrite and cementite) into spheroidal (globular) carbides dispersed in a ferritic matrix. Spheroidal carbides present less stress concentration than lamellar cementite and are easier to cut around — the result is the softest, most machinable, most cold-formable condition achievable for medium and high-carbon steels.

The mechanism: heating to just above or at Ac1 for hypereutectoid steels (~750–800°C) causes partial dissolution of carbides. At this temperature, remaining undissolved carbides act as nuclei. Extremely slow cooling (or cycling above and below Ac1) allows the dissolved carbon to diffuse to those nuclei and grow them into spheroids rather than reform into lamellae. The process takes 4–16 hours or more for large sections.

Spheroidize annealing is the standard delivery condition for bearing steel (SUJ2), carbon tool steel (SK85), and die steel (SKD11) bar stock. When a drawing specifies “spheroidize annealed bar,” it means the steel was heat treated at the mill to this condition before shipping, and the hardness values above are the acceptance criteria.

5. Stress-Relief Annealing

Stress-relief annealing removes residual stress without changing the microstructure. The temperature is below Ac1 — typically 550–650°C for steels — and the mechanism is time-temperature-dependent dislocation movement: dislocations rearrange by thermally activated processes (recovery) without sufficient energy to cause recrystallization or carbide morphology change. The result is a reduction in residual stress of 70–90% with no change in hardness, grain size, or phase composition.

Typical applications:

• After welding: Weld residual stresses can exceed 300–400 MPa in medium-carbon steels. Stress relief at 600–650°C for 1–2 hours reduces residual stress to below 50 MPa, preventing delayed cracking and reducing distortion on subsequent machining.
• After casting: Cast steel (SC450, SC480) contains thermal residual stresses from non-uniform solidification. Normalizing or stress relief at 550–600°C before rough machining is standard practice.
• Between rough and finish machining: Rough machining of precision tool steel components (EDM dies, precision jigs) introduces residual stresses that cause dimensional shift on finish machining. A stress-relief cycle at 550–600°C between rough and finish operations stabilizes the part.
• After straightening of bars and shafts: Cold straightening introduces residual stresses. Stress relief at 550–600°C before grinding prevents distortion during grinding.

6. Normalizing — Not Annealing, but Often Confused

Normalizing is technically not an annealing treatment — it is a grain refinement and homogenization treatment. The steel is heated above Ac3 (hypoeutectoid) or above Acm (hypereutectoid) to fully austenitize, then cooled in still air. Air cooling is faster than furnace cooling, producing fine pearlite rather than coarse lamellar pearlite. The result is a more uniform, finer-grained structure — but the hardness may be equal to or higher than full annealed condition, particularly for medium and high-carbon steels.

Normalizing is the correct treatment when:

• As-cast grain structure needs refining before machining (SC cast steel — mandatory per JIS G5111)
• Banded or segregated forging microstructure needs homogenization before final heat treatment
• A “reference” or baseline condition is needed before hardening (normalizing establishes a uniform starting microstructure that responds predictably to quench hardening)

Normalizing is not a substitute for annealing when maximum softness is required. For medium and high-carbon steels, normalizing will produce a hardness in the 180–250HBW range depending on carbon content — adequate for light machining but not for the precision cavity milling and EDM wire-cut operations used in tool and die manufacturing.

7. Common Mistakes

8. Selection Guide by Situation

9. FAQ

Q: Is “bright annealing” a different type?

Bright annealing refers to the furnace atmosphere, not the type of annealing. A bright anneal is performed in a controlled reducing or inert atmosphere (hydrogen, nitrogen, or vacuum) to prevent surface oxidation — the steel exits the furnace with a bright, scale-free surface. The temperature and cooling cycle can correspond to full annealing, process annealing, or spheroidize annealing depending on the steel and objective. Bright annealing is standard for stainless steel strip and tube to avoid the scale that forms in air-atmosphere annealing.

Q: Does spheroidize annealing weaken the steel permanently?

No. Spheroidize annealed steel is in its softest condition, but this condition is fully reversed by subsequent hardening. For tool steels (SKD11, SK85) and bearing steels (SUJ2), the spheroidized condition is the standard pre-heat-treatment state — the steel is machined in spheroidized condition, then hardened by austenitizing and quenching to achieve the specified service hardness (58–65HRC). The spheroidize-annealed hardness (180–255HBW) and the hardened hardness (58–65HRC) are completely different states of the same material.

Q: How long does each treatment take?

Process annealing: 1–3 hours total (fast cool). Stress-relief annealing: 2–6 hours depending on section. Full annealing: 12–48 hours (dominated by furnace cooling). Spheroidize annealing: 8–24 hours (long hold near Ac1 needed for full spheroidization). Normalizing: 3–6 hours (fast air cool). The furnace cooling phase in full and spheroidize annealing is the rate-limiting step — for large tool steel blocks (300+ mm), the furnace cool from 800°C to 600°C at 20°C/hour alone requires 10 hours.