Annealing Processes in Steel: Full, Process, Spheroidise, and Normalising

Introduction

Annealing is a broad term describing any heat treatment that reduces hardness, relieves residual stresses, or improves ductility and machinability. In ferrous metallurgy, the term encompasses several distinct processes — each targeting a specific outcome — that differ in austenitising temperature, cooling rate, and resulting microstructure. Understanding which annealing process to apply in which situation is fundamental to efficient steel manufacturing.

This article covers full annealing, process (subcritical) annealing, spheroidising annealing, normalising, and stress relieving, with technical detail on temperatures, microstructures, and industrial applications.

The Iron-Carbon Phase Diagram as the Basis for Annealing

All annealing processes are designed in relation to the critical temperatures of the Fe-C phase diagram:

• Ac1 (lower critical temperature): ~727°C. Below this, steel is in the α-ferrite + cementite field. Above it, austenite begins to form.
• Ac3 (upper critical temperature): ~912°C for pure iron; decreases with increasing carbon to 727°C at 0.77% C. Above Ac3, hypoeutectoid steels are fully austenitic.
• Acm: The upper boundary for hypereutectoid steels — above which only austenite exists (cementite dissolves). Increases with carbon above 0.77% C.

The temperatures are labelled “Ac” (chauffage = heating). On cooling, the equivalent temperatures are denoted “Ar” (refroidissement = cooling) and are slightly lower due to hysteresis (typically 10–30°C below Ac values at moderate cooling rates).

Full Annealing

Process Description

Full annealing involves heating the steel to 30–50°C above Ac3 (for hypoeutectoid steels) or above Ac1 (for hypereutectoid steels), soaking at temperature for sufficient time to achieve complete, homogeneous austenitisation, then cooling very slowly — typically in the furnace at 10–30°C per hour. This produces the softest possible equilibrium microstructure for a given carbon content.

Resulting Microstructure

Slow cooling through the eutectoid temperature allows maximum time for carbon diffusion, producing coarse pearlite: wide ferrite and cementite lamellae with interlamellar spacings of 200–500 nm. For hypoeutectoid steels, proeutectoid ferrite forms first at austenite grain boundaries before pearlite nucleates. The coarse lamellar structure is the softest microstructure for any given carbon content — hardness 140–220 HBN for 0.1–0.8% C steels.

Applications

• Maximising machinability of medium-carbon steels before precision machining
• Eliminating banding (compositional segregation aligned with rolling direction) in plate and bar
• Restoring ductility after cold working to near full values
• Preparing steel for subsequent cold forming operations

Process (Subcritical) Annealing

Process Description

Process annealing is performed below Ac1 — typically at 550–700°C — on cold-worked low-carbon steels (typically <0.25% C). No phase transformation occurs. Instead, three metallurgical recovery mechanisms operate:

1. Recovery: Thermal energy allows dislocations to rearrange into lower-energy configurations (sub-grain walls). Dislocation density decreases modestly; strength decreases slightly; ductility improves slightly.
2. Recrystallisation: New, strain-free grains nucleate and grow, consuming the deformed work-hardened grains. Strength drops dramatically; ductility is restored. Recrystallisation temperature depends on prior cold work level — greater deformation lowers the recrystallisation temperature.
3. Grain growth: After recrystallisation is complete, continued heating coarsens the new grains. Excessively coarse grains degrade surface finish in subsequent forming and promote “orange-peel” surface texture.

In wire drawing, cold rolling of strip, and tube drawing, process annealing between passes restores ductility for the next deformation stage. The process is also called “inter-strand annealing,” “box annealing” (in batch bell furnaces), or “strand annealing” (continuous in-line annealing of wire).

Spheroidising Annealing

Why Spheroidise?

In medium and high-carbon steels (0.35–2.14% C), the lamellar cementite in pearlite and the grain-boundary cementite network in hypereutectoid steels are detrimental to machinability and cold formability. Spheroidising converts lamellar and grain-boundary cementite into discrete spheroidal carbide particles dispersed in a ferrite matrix. This is the softest achievable microstructure for high-carbon steels and is specified before:

• Cold heading of medium and high-carbon fasteners
• Cold drawing of high-carbon wire
• Precision machining of bearing and tool steels to close tolerances

Process Methods

Several spheroidising routes exist:

1. Sub-critical spheroidising: Hold just below Ac1 (700–720°C) for 8–40 hours. Cementite lamellae fragment and spheroidise by surface energy minimisation. Simple but very slow.
2. Cycling spheroidising: Repeatedly cycle between just above Ac1 and just below, typically 10–20 cycles. Repeated austenite-ferrite transformations fragment lamellae rapidly. Faster than sub-critical; widely used for bearing steels (52100) and cold heading wire.
3. Isothermal spheroidising after austenitising: Austenitise at low temperature (780–790°C for high-carbon steel, to avoid full carbide dissolution), then isothermally hold at 680–720°C. Rapid but requires precise temperature control.

Microstructure and Hardness

Fully spheroidised microstructure consists of globular carbides 0.5–2 µm diameter distributed uniformly in ferrite matrix. Hardness: 170–230 HBN for 1.0% C steel — comparable to full annealing of lower carbon grades. Cold formability is excellent; machinability of high-carbon steels is transformed.

Normalising

Process Description

Normalising heats steel to 40–60°C above Ac3, soaks briefly, then cools in still air (or forced air for heavy sections). The faster air cooling relative to furnace annealing produces a finer pearlite structure — shorter interlamellar spacing (100–200 nm) — and a more uniform grain size. Normalised steel is harder and stronger than full-annealed steel of the same composition, but has better and more uniform mechanical properties than as-cast or as-forged steel.

Why Normalise Instead of Anneal?

• Lower cost: No furnace cool required — cycle time is 2–4× faster than full annealing.
• Better machinability in some medium-carbon steels: finer pearlite chips more cleanly than coarse pearlite.
• Conditioning for heat treatment: Normalising produces a homogeneous, fine-grained baseline microstructure before quench-and-temper heat treatment.
• Structural applications: Normalised steel has a specified, reproducible microstructure accepted by many structural and pressure vessel codes (e.g. ASTM A36, EN 10025 S355N) as a final condition.

Stress Relieving

Stress relieving is performed at 450–650°C — well below Ac1 — for 1–4 hours, followed by slow furnace cooling. No phase transformation or recrystallisation occurs; instead, thermally activated dislocation recovery and creep reduce residual stresses by 50–80% without significantly changing hardness or microstructure. It is essential after:

• Welding: reduces HAZ residual stresses to below yield strength, improving fatigue life and preventing SCC
• Machining of precision components: removes machining stresses before final grinding
• Straightening and forming operations
• Casting (for grey and ductile iron castings before finish machining)

For pressure vessels, ASME VIII Div. 1 mandates post-weld heat treatment (essentially stress relief) for carbon steel above specific thicknesses and design temperatures.

Process Selection Guide

Frequently Asked Questions

Q: Why does normalising sometimes produce a harder microstructure than expected?
A: In alloy steels with high hardenability (high CE), air cooling may be fast enough to form bainite or even martensite at the surface of large sections, significantly increasing hardness above the pearlitic structure expected from normalising. For these steels, isothermal annealing below the pearlite nose is required to guarantee a soft, fully pearlitic microstructure.

Q: Is it possible to stress relieve and over-temper simultaneously?
A: Yes — if a quenched and tempered steel is stress relieved at a temperature above the original tempering temperature, it will be over-tempered, losing hardness and strength. Stress relief temperature must always be at least 25°C below the original tempering temperature.

Q: What is “bright annealing”?
A: Annealing in a controlled protective atmosphere (H₂, N₂, or vacuum) that prevents oxidation and decarburisation, producing a bright, oxide-free surface. Used for stainless steels, high-carbon steels, and precision components where surface condition is critical.

Conclusion

The family of annealing processes — full, process, spheroidising, normalising, and stress relieving — provides the heat treater with a complete toolkit for optimising softness, ductility, grain structure, and residual stress in steel. Selecting the right process requires understanding the starting condition, target microstructure, and subsequent processing or service requirements. See also: Complete Guide to Quenching and Iron-Carbon Phase Diagram Guide.

References

• ASM Handbook Vol. 4A: Steel Heat Treating Fundamentals and Processes. ASM International, 2013.
• Bhadeshia, H.K.D.H. and Honeycombe, R., Steels: Microstructure and Properties. 4th ed. Butterworth-Heinemann, 2017.
• Verhoeven, J.D., Steel Metallurgy for the Non-Metallurgist. ASM International, 2007.

metallurgyzone