Normalizing Steel – Process, Temperature, Microstructure and Application

Normalizing steel is one of the basic processes heat treatment of steelIts main goal is to achieve a more uniform and fine-grained microstructure, reduce the effects of previous technological processes, and prepare the material for further thermal or mechanical treatment. Normalizing can affect the strength, hardness, impact strength, ductility, and machinability of steel, but the extent of these changes depends primarily on the carbon content, alloying elements, and the initial structure of the material.
The process involves heating the steel to a temperature that ensures the appropriate degree of austenitization, heating the material at that temperature, and then cooling it in still air. The cooling method is one of the fundamental differences between normalizing and complete annealing. Air cooling is faster than furnace cooling, which affects grain size and the resulting microstructure.
Normalizing is primarily used for unalloyed and low-alloy steels. It can be performed as the final heat treatment of a component or as an intermediate process preceding further operations such as hardening, heat treatment, or machining.

What is normalizing steel?

During normalizing, steel is heated to the austenitic range appropriate for its chemical composition. During the heating process, transformations occur that lead to the formation of austenite and a homogenization of the structure. The component is then removed from the furnace and cooled in air.
During cooling, austenite undergoes transformations leading – depending on the steel composition and cooling rate – primarily to the formation of ferritic-pearlitic or pearlitic structures. In alloy steels, the transformation process can be more complex, as alloying elements influence hardenability and the kinetics of phase transformations.
Normalizing is particularly useful after processes that may result in an inhomogeneous or coarse-grained structure, such as forging, hot rolling or casting.

The purpose of normalizing steel

Normalizing can serve several technological purposes. The precise purpose of the process depends on the steel grade, the material's condition, and its intended use.
The most important goals of standardization include:

• grain refinement – properly performed austenitization and subsequent cooling allow for obtaining a finer-grained structure,
• homogenization of the microstructure – especially after forging, rolling, casting and other high-temperature processes,
• improvement of mechanical properties – normalizing can increase strength and hardness compared to the fully annealed state, while maintaining useful ductility and impact strength,
• limiting the adverse effects of prior treatment – including structural inhomogeneities and excessive grain growth,
• preparation of steel for further heat treatment – e.g. hardening or thermal improvement,
• preparation of material for machining – in some steels, the appropriate structure after normalization can improve the conditions for further processing,
• obtaining more uniform properties in the cross-section of the element – especially important in the case of forgings, castings and semi-finished products after hot processing.

Factors influencing the effect of normalizing steel

The final properties of steel after normalizing depend on the entire heat treatment cycle. Even at the same temperature, two steel grades can develop different microstructures and properties upon air cooling.

Normalizing temperature of steel

The normalizing temperature is not a single constant for all steel grades. It is selected based on the material's chemical composition and critical temperatures associated with phase transformations.
For hypoeutectoid steels, i.e. steels containing less carbon than the eutectoid composition, normalizing is generally carried out above the critical temperature. Ac3In practice, the process temperature is usually sufficiently higher than Ac3 to ensure proper austenitization of the material.
In the case of hypereutectoid steels, the temperature selection rules require taking into account the line acm and the expected microstructure. Therefore, specifying a single range, e.g., 800–950°C, for all steels is only a gross simplification.
The actual normalizing temperature should be determined for a specific steel grade based on its material characteristics, the relevant standard, technological documentation or manufacturer's recommendations.

Normalizing steel – process flow

The normalization process can be divided into three basic stages: heating, annealing and cooling.

1. Heating

The component is heated to the normalizing temperature appropriate for the given steel grade. The heating rate should take into account the component size, geometry, cross-sectional thickness, and thermal conductivity of the material. For large or complex parts, heating too rapidly can cause significant temperature gradients and associated thermal stresses.

2. Heating

Once the required temperature is reached, the material is heated for a period of time sufficient to allow the temperature to equalize throughout the cross-section and to allow for the required structural transformations. The heating time depends on the component's dimensions, mass, steel grade, heating method, and furnace type.
The simple relationship between time and millimeters of thickness should not be considered a universal rule for all components. In industrial technology, parameters should be defined for specific materials and parts.

3. Air cooling

After annealing, the component is air-cooled. Cooling is faster than during typical complete annealing, in which the material cools slowly with the furnace. This difference in cooling rate influences the austenite transformation and the final microstructure of the material.
Cooling conditions also play a role. Component size, ambient temperature, air movement, and component orientation can all influence the actual heat dissipation rate.

Microstructure of steel after normalizing

One of the most important effects of normalizing is the modification of the steel's microstructure. In typical hypoeutectoid steels, a properly conducted process produces a structure containing ferrite and pearlite. As the carbon content increases, the pearlite content increases.
Compared to the slow cooling during complete annealing, the structure obtained during normalizing is typically finer. A finer pearlitic structure and appropriate grain size can lead to increased strength and hardness of the material.
However, the final microstructure depends not only on the normalizing temperature, but also on:

• carbon content,
• type and content of alloying elements,
• austenite grain size,
• heating time,
• dimensions of the element,
• cooling rates,
• microstructure of the material before normalizing.

The effect of normalization on the mechanical properties of steel

Normalizing can significantly affect the mechanical properties of steel, but the magnitude of these changes depends on its composition and initial condition. The process should not be considered a universal method that produces identical results in all grades.

Tensile strength

The finer microstructure achieved after normalizing can lead to an increase in tensile strength compared to fully annealed steel. The effect depends, among other factors, on the carbon content and alloying elements.

Yield point

Microstructure changes and grain refinement can also affect the yield strength. This property is particularly important for structural steels intended for load-bearing components.

Hardness

Normalized steel is typically harder than fully annealed steel because faster cooling promotes the development of a finer structure. However, normalizing does not achieve the hardness characteristic of properly hardened martensitic steel.

Impact strength and ductility

Grain refinement and structural homogenization can positively impact impact strength and material behavior under load. However, the specific effect cannot be determined without considering the steel grade and process parameters.

Machinability

The effect of normalizing on machinability is not the same for all steels. In low-carbon steels, some increase in hardness can improve chip formation and reduce excessive material ductility. In other cases, soft annealing or spheroidizing annealing prior to machining may be more beneficial.

Normalizing vs. Annealing

Normalizing and annealing are processes that involve heating and controlled cooling of steel, but they differ primarily in the cooling conditions and the resulting microstructure.
After normalizing, steel is generally cooled in air, whereas during complete annealing, cooling is much slower and usually occurs in conjunction with the furnace. As a result, normalized steel typically has a finer structure and higher hardness and strength than the corresponding fully annealed material.
If the main goal is to reduce residual stresses without fundamentally changing the material structure, stress relief annealing.

Normalizing and hardening

Steel hardening It differs from normalizing primarily in the cooling rate and the desired structural effect. During hardening, the aim is to cool the austenite to obtain a high-hardness structure, most often with a significant martensite content.
Normalizing uses slower air cooling and is not used to create a martensitic structure in typical carbon steels. However, it can be a step in preparing the material for subsequent hardening or heat treatment.

Normalizing vs. Letting Go

Letting go is a process generally performed after hardening. Its purpose is to modify the structure of the hardened steel and achieve the required combination of hardness, strength, and ductility.
Normalizing, on the other hand, can be a standalone heat treatment or a step preparing the material for subsequent processes. Therefore, both processes have different technological goals.

Normalizing and stress relief annealing

Normalizing leads to the austenitization of the steel and the subsequent formation of a new microstructure upon cooling. Stress relief annealing, on the other hand, is performed at a lower temperature and is primarily intended to reduce residual stresses while minimizing the impact on the basic microstructure and properties of the material.

Normalizing and other methods of heat treatment of steel

Normalizing, annealing, hardening, and tempering differ in process temperature, cooling method, and desired effect. The table below summarizes the key differences between these processes.

Normalization of various steel grades

Unalloyed structural steels

Normalizing is widely used for unalloyed structural steels. The process can be used after rolling, forging, or casting to refine and homogenize the structure and achieve the required mechanical properties.

Carbon steels

In carbon steels, the normalizing effect is strongly related to the carbon content. As the carbon content increases, the pearlite content increases, and the hardness, strength, and ductility of the material change. The group of steels with increased carbon content includes C55S, C67S, C75S, C85S, and C100S spring steels.

Low-alloy steels

In the case of low-alloy steels, the influence of chromium, molybdenum, nickel, manganese, and other elements on the transformations occurring during cooling must be taken into account. The increased hardenability of some grades means that the steel's behavior during air cooling can differ significantly from that of ordinary carbon steel.

High-alloy steels

The concept of normalization should not be automatically applied to all high-alloy steels. For stainless steels, heat-resistant steels, tool steels, and other special steels, heat treatments appropriate to the specific group and grade are applied. The parameters should always be based on the required microstructure and material properties.

Normalizing forgings

Normalizing is particularly important in the production of forged parts. During forging, steel is subjected to high temperatures and significant plastic deformation, and the resulting structure may be non-uniform.
Properly selected normalizing refines the grain and unifies the microstructure of the forging. Depending on the material type, it can be the final stage of heat treatment or a preparation for hardening and tempering.

Normalizing steel castings

In steel castings, normalizing can be used to modify the structure formed during solidification and cooling. Castings, especially those with large cross-sections, can exhibit significant microstructural and grain size heterogeneity.
Normalizing allows for a more uniform structure in the appropriate grades and prepares the casting for further technological processes.

Normalizing after forging and rolling

The conditions during the final forging or rolling phase have a significant impact on the microstructure of the semi-finished product. If the temperature was high or the material cooled uncontrolled, excessive grain growth or structural inhomogeneity may occur.
Normalizing can in such cases serve as a preparatory treatment, leading to a more predictable structure before further thermal or mechanical treatment.

Application of steel normalization

Normalizing is used primarily where it is necessary to homogenize the microstructure, refine the grain or prepare the material for subsequent production stages.
Typical applications include:

• forgings and forged elements,
• steel castings,
• shafts and axles,
• elements of machines and devices,
• structural parts,
• semi-finished products after hot rolling,
• elements prepared for hardening and tempering,
• parts subjected to further machining.

Advantages of normalizing steel

• possibility of grain crushing,
• homogenization of the microstructure,
• improvement of selected mechanical properties,
• the ability to prepare the material for subsequent heat treatment processes,
• a relatively simple method of air cooling,
• can be used after forging, rolling and casting,
• obtaining higher hardness and strength than after typical complete annealing in many carbon steels.

Disadvantages and limitations of normalization

• the process is not suitable for every type of steel,
• air cooling may cause differences in cooling rates in elements with large or variable cross-sections,
• deformation of elements with complex geometry is possible,
• normalizing does not provide the hardness characteristic of hardening,
• is not a process intended primarily for the maximum removal of residual stresses,
• in steels with increased hardenability, cooling in air may lead to different structures than in typical carbon steels,
• Incorrect temperature can lead to excessive grain growth and deterioration of material properties.

What does the correct normalization effect depend on?

The effectiveness of normalizing depends on the correct selection of the entire technological cycle, not just the maximum temperature. Consideration must be given to the steel grade, its chemical composition, initial structure, component dimensions and geometry, heating rate, austenitizing temperature and time, and actual cooling conditions.
It is particularly important to avoid excessively high temperatures or excessively long soaking times, as these can lead to austenite grain growth. Conversely, insufficient temperatures or too short soaking times can prevent the required structural uniformity from being achieved.
Normalizing steel is a heat treatment process involving heating the material to the appropriate austenitizing temperature range, annealing, and then air-cooling. The process is primarily used to refine the grain, homogenize the microstructure, and obtain properties suitable for further processing or use.
Compared to full annealing, normalizing utilizes faster cooling, which is why it leads to a finer structure and greater hardness and strength in many carbon steels. However, it is not a substitute for hardening, tempering, or stress relief annealing—each of these processes pursues a different technological goal.

Methods of heat and thermochemical treatment of steel

Softening is just one group of processes used to modify the microstructure and properties of steel. Depending on the desired effect, normalizing, stress relieving, hardening, tempering, and thermochemical processes such as carburizing and nitriding are also used.

Steel heat treatment methods

• Normalizing steel – grain refinement and microstructure unification.
• Stress relief annealing – reduction of residual stresses.
• Softening of steel – reducing hardness and improving susceptibility to further processing.
• Full annealing – austenitization and slow cooling.
• Isothermal annealing – transformation carried out while holding at a properly selected temperature.
• Ball annealing – spheroidizing – change in the morphology of cementite or other carbides.
• Recrystallization annealing – removal of the effects of crushing and restoration of plasticity.
• Homogenizing annealing – reducing segregation and homogenizing the material composition.
• Hardening and heat treatment – shaping the microstructure and properties by hardening and tempering.
• Tempering steel – controlled modification of steel properties after hardening.
• Induction hardening – surface induction heating and rapid cooling.
• Flame hardening – surface hardening after local heating with a flame.
• Oversaturation – obtaining a supersaturated solid solution by an appropriate thermal cycle.
• Aging – controlled phase separation leading to material strengthening.

Methods of thermochemical treatment of steel

• Nitriding of steel – diffusional enrichment of the surface layer with nitrogen.
• Carburizing steel – diffusional enrichment of the surface with carbon.
• Carbonitriding – simultaneous enrichment of the surface layer with carbon and nitrogen.
• Nitrocarburization / nitrocarburization – enrichment of the layer primarily with nitrogen and carbon.
• Drilling – diffusion saturation of the surface with boron.
• Diffusion chrome plating – diffusion enrichment of the layer with chromium.
• Diffusion aluminizing – diffusion enrichment of the layer in aluminum.