The Basics of Normalizing Annealing

Normalizing annealing is primarily used to obtain a refined and uniform pearlitic-ferritic structure in steel, as well as predictable mechanical properties. The concept of normalizing implies both stress relief and structural transformation. Unlike stress relieving, normalizing introduces structural changes; and unlike softening or spheroidizing, it does not result in the softest possible structure for a given steel. Often, normalized steel undergoes further heat treatment or machining. Steel products that are normalized or shaped (e.g., rolled) under normalizing conditions are marked with the additional symbol +N (or +TN), while those that are normalized and tempered are marked +NT (or +TNT).

Purpose of Steel Normalizing

Deformation or improper temperature distribution during thermal processes may lead to a non-uniform steel structure with large grains and carbides, which can cause distortions in further processing. Normalizing ensures complete recrystallization, relieves internal stresses, and eliminates unfavorable structural changes, thereby ensuring the required properties are achieved.

Normalizing is used for toughening steel, nitriding steel, and boiler steel as a preparatory step before hardening to reduce stresses and quenching distortion. Carburizing steels are normalized before carburizing to reduce deformation. It is also one of the most commonly applied treatments for carbon and structural steels, sometimes used as a final treatment—certain structural steels may be delivered in the +N or +NT condition. Bars, forgings, and larger components are typically normalized.

Normalizing after hot rolling is applied to some sheets, such as electrical steel sheets.

In bearing steel, normalizing corrects microstructural defects like carbide networks or banding, coarse grains, and excessive variation in carbide size. These flaws arise due to improper final plastic working temperature, too slow cooling, or spheroidizing errors, resulting in an overly coarse, non-uniform spheroidized structure. Properly worked bearing steel products are not normalized. Normalizing is conducted at 900–920°C, preferably in a protective atmosphere. Some defects cannot be corrected by normalizing and require homogenization.

Stainless, acid-resistant, and free-cutting steels are rarely normalized.

Normalizing and a Typical Steel Processing Route

1. Steel is in a softened condition, with a spheroidized structure—carbides dispersed in a ferrite matrix.
2. After plastic working (especially hot working), the steel may exhibit a non-uniform structure with large grains and dispersed carbides. Internal stresses may be present, and the structure requires homogenization.
3. Steel is subjected to normalizing. The resulting structure is fine-grained and uniform pearlite or ferrite.
4. The normalized product may then be hardened, thermally improved, carburized, or put into service.

Normalizing Annealing – Temperature

Carbon Steel – Normalizing of carbon steel involves heating the steel to a temperature 30–50°C above A_C3 (for hypoeutectoid steel and cast steel) or 50°C above A_CM (for hypereutectoid steel), soaking until uniform austenite is achieved, and then cooling in still air. Some hypereutectoid steels prone to grain growth are normalized at A_CM or slightly below.

Alloy Steels – These are normalized according to manufacturer specifications and relevant standards, based on their chemical composition. Examples include:

• Case hardening steel 1.5918 / 17CrNi6-6: normalized at 850–890°C before carburizing to reduce deformation.
• Heat treatable steel 35HGS / 35HGSA: normalized at 860–880°C before hardening to reduce quenching stresses and distortion.
• Heat treatable steel 50CrMo4 / 1.7228: normalized at 840–880°C before hardening.
• Nitriding steel 41CrAlMo7 / 1.8509: normalized at 900–940°C before hardening.
• Structural steel 34CrMo4 / 1.7220: normalized at 850–880°C before hardening.
• Boiler steel 21CrMoV5-7 / 1.7709: normalized at 950–980°C before hardening.
• Boiler steel 13CrMo4-5 / 1.7335: normalized at 910–940°C before hardening.
• Heat-resistant steel X11CrMo5 / 12CrMo19-5: normalized at 930–980°C before hardening.

Normalizing Rolling

Normalizing rolling is hot rolling where the final rolling phase occurs within the austenitizing temperature range, followed by air cooling. This process both shapes the product and removes internal stresses in one cycle.

Combined Normalizing and Spheroidizing

In some facilities, normalizing is conducted in a single cycle along with spheroidizing annealing in continuous furnaces for large cross-section forgings. The refinement of pearlite achieved through normalizing accelerates the spheroidizing process.

Structural Transformations

During normalizing, steel transforms into austenite with initially fine grains, which upon cooling turns into a pearlitic-ferritic structure. The initial structure before normalizing is generally irrelevant.

Regeneration of Steel

Normalizing annealing can correct certain defects typically resulting from hardening.

Overheated steel (heated to too high a temperature, causing excessive grain growth) can be regenerated by normalizing. However, burnt steel (with oxidation at grain boundaries) cannot be repaired this way.

Distorted or warped steel due to improper hardening can be repaired as follows: normalize the steel, cool it in air, straighten it, and then harden it again.

Full Annealing

Full annealing may be considered a variant of normalizing. It differs only in the cooling phase, which is significantly slower and occurs with the furnace or in ash. Full annealing produces a structure closest to equilibrium—ferrite with pearlite, pure pearlite, or pearlite with cementite. It is mainly used for alloy steels.