Steel softening – methods, temperature, microstructure and application

Softening of steel It encompasses heat treatment processes that reduce a material's hardness, increase its plasticity, or achieve a microstructure that is more favorable for further processing. Softening is primarily used prior to machining, plastic forming, and subsequent heat treatment processes, such as hardening.
This process is particularly important for high-carbon, alloy, and tool steels, whose high hardness or unfavorable carbide content can make turning, milling, drilling, or other machining operations difficult. Softening can also be used in materials hardened by cold working, where it is necessary to eliminate the effects of cold work and restore the material's ability to undergo further deformation.
There is no single universal method for softening steel. Depending on the material type, carbon content, alloying elements, initial structure, and desired effect, various methods are used, including full annealing, isothermal annealing, spheroidizing annealing, and recrystallizing annealing.

What is steel softening?

Softening exploits the relationship between the mechanical properties of steel and its microstructure. Properly selected heating, annealing, and cooling processes can alter the size and shape of structural components, the distribution of carbides, the material's hardening state, and the level of residual stresses.
Depending on the process used, the following may occur: transformation of austenite into ferrite and pearlite, spheroidization of cementite or other carbides, recrystallization of the strain-hardened material, and a reduction in the dislocation density.
The effect is typically a reduction in the material's hardness and resistance to further processing, and an increase in ductility. However, the exact mechanism depends on the method used, so the term "softening" should not be identified with a single thermal cycle.

Why does steel soften?

Material delivered after rolling, forging, drawing, hardening, or other technological processes does not always have the properties necessary for the next production operation. Excessive hardness increases tool wear and cutting forces, while excessive strain hardening can limit the ability to further stamp, bend, or draw.
The primary objectives of softening steel are:

• reducing the hardness of the material,
• improved machinability in appropriate steel grades,
• increasing plasticity and ability to be further shaped,
• removing or limiting the effects of deformation hardening,
• obtaining a favorable form of cementite or carbides,
• preparation of steel for hardening and further heat treatment,
• homogenization of the microstructure before subsequent production stages.

Methods of softening steel

The choice of softening method depends primarily on the type of steel and the desired technological effect. High-carbon steel intended for hardening requires a different process than low-carbon sheet metal highly hardened during cold rolling.

1. Full annealing

Full annealing involves heating the steel to a range that ensures appropriate austenitization, heating the material and then cooling it very slowly, usually together with the furnace.
For hypoeutectoid steels, the process temperature is selected above the critical temperature Ac3. The exact temperature, soaking time, and cooling rate depend on the steel grade and the dimensions of the component.
Slow cooling allows for a relatively soft ferrite-pearlite structure to be obtained. Compared to normalization of steel cooling is much slower, so the resulting structure and mechanical properties are different.
Typical Application: structural steels and other grades requiring reduction of hardness, improvement of plasticity or preparation for further processing.

2. Isothermal annealing

Isothermal annealing utilizes a controlled transformation of austenite at a constant temperature. After austenitization, the steel is cooled to the appropriate transformation temperature and then held at that temperature until the required microstructure is achieved.
Unlike classical annealing with very slow, continuous cooling, the transformation takes place under controlled isothermal conditions. This allows for a more uniform structure and, in certain steels, a shorter process time.
The method is used, among others, in some alloy and tool steels where slow cooling during traditional annealing would be time-consuming.
Effect: obtaining a controlled microstructure and properties enabling further mechanical or thermal processing.

3. Ball annealing – spheroidizing

Ball annealing, also called spheroidizing, is particularly important for high-carbon, bearing, and tool steels. Its purpose is to change the morphology of cementite and other carbides to produce more rounded, spheroidal particles arranged in a ferritic matrix instead of a lamellar or mesh-like arrangement.
Spheroidization can be performed using various thermal cycles, including prolonged heating near the Ac1 temperature or cycling through a specific temperature range. The parameters depend on the chemical composition and initial structure of the specific species.
Changing the shape and arrangement of carbides can significantly reduce the hardness of high-carbon steel and improve its machinability. A properly prepared structure also provides a favorable initial condition prior to subsequent hardening.
Typical Application: tool steels, bearing steels and other high carbon steels intended for further mechanical processing and hardening.

4. Recrystallization annealing

Recrystallization annealing It is used primarily for materials hardened during cold forming, e.g. rolling, drawing or stamping.
During cold deformation, the dislocation density increases, the material hardens, and its hardness and strength increase, while ductility decreases. Once a high degree of cold work is reached, further shaping may become difficult.
Recrystallization annealing creates new grains, free from the effects of previous strain hardening. This leads to a reduction in hardness and strength due to strain hardening, and the restoration of ductility.
Typical Application: sheets, strips, wires and other semi-finished products subjected to intensive cold working.

Comparison of steel softening methods

Method Basic mechanism Main effect Typical application Full annealing Austenitization and slow transformation during cooling Hardness reduction and softer structure Structural steels and semi-finished products before further processing Isothermal annealing Austenite transformation at a controlled, constant temperature Uniform microstructure and controlled properties Selected alloy and tool steels Ball/spheroidizing annealing Change in cementite and carbide morphology Hardness reduction and improved machinability High-carbon, bearing, and tool steels Recrystallization annealing Formation of new grains after prior work hardening Removal of hardening effects and restoration of ductility Cold-worked sheets, strips, and wires

Comparison of steel softening methods

How does the microstructure change during softening?

Microstructural changes depend on the type of annealing. Therefore, it is impossible to identify a single structure characteristic for all softening processes.
Depending on the technology used, the following may occur:

• formation of a ferritic-pearlitic structure during slow cooling after austenitization,
• controlled austenite transformation during isothermal annealing,
• spheroidization of cementite and other carbides in high-carbon and alloy steels,
• recovery and recrystallization of the structure after prior cold plastic working,
• reducing the dislocation density and the effects of strain hardening.

Spheroidizing annealing – why is it important for tool steels?

In tool steels and other high-carbon steels, hardness alone isn't the only parameter that determines machining suitability. The form, size, and distribution of carbides are also crucial.
A structure with appropriately distributed, nearly spherical carbides can facilitate machining and provide a suitable starting condition for subsequent austenitization and hardening. Therefore, tool steels are often supplied in the soft-annealed condition.
Actual hardness after annealing should be determined for the specific steel grade. A single HB or HV range should not be adopted for all tool steels.

Steel softening and machinability

One of the most important reasons for applying softening is to prepare the material for turning, milling, drilling, broaching, or other machining operations. Reducing excessive hardness can reduce cutting edge stress and tool wear.
However, the relationship between hardness and machinability is not linear. The lowest possible hardness material is not always the easiest to machine. Very soft and ductile steels can experience problems with chip formation, built-up edge, and surface finish.
Therefore, the appropriate process is selected not only to achieve the lowest possible hardness, but above all to obtain a microstructure appropriate for the specific material and subsequent production technology.

Softening and carbon content in steel

Carbon content has a significant impact on the choice of softening method. In low-carbon steels, removing the effects of cold-working hardening can be particularly important. In medium-carbon steels, the choice of process depends, among other things, on subsequent mechanical and thermal treatment.
In high-carbon steels, the morphology of cementite and other carbides is becoming increasingly important. Therefore, processes leading to carbide spheroidization are often used in tool, bearing, and spring steels.

Softening of carbon steels

For carbon steels, the choice of treatment depends on the carbon content and the desired final condition. For example, C45 steel may require different treatment than high-carbon spring or tool steels.
In steels with higher carbon content, spheroidizing annealing can be used prior to machining and subsequent hardening. Obtaining the proper cementite form facilitates further processing and prepares the structure for the next thermal cycle.

Softening of alloy steels

Alloying elements can significantly influence transformation temperatures, hardenability, the type of carbides formed, and the rate of diffusion processes. Therefore, parameters appropriate for plain carbon steel should not automatically be applied to chromium, chromium-molybdenum, nickel, or high-alloy steels.
In alloy steels, controlling the temperature, time, and cooling rate is particularly important. For tool steels, soft annealing parameters are usually determined individually for each grade.

Softening of stainless steels

When it comes to stainless steels, the term "softening" requires more caution. Austenitic, ferritic, martensitic, and duplex steels have different microstructures and require different heat treatments.
For example, austenitic stainless steels can significantly harden during cold working. Restoring their ductility requires carefully selected heat treatment appropriate for the specific grade. Therefore, a single, universal temperature range for "softening stainless steel" should not be used.

The effect of time and temperature on the softening of steel

Temperature and time are closely related. Too low a temperature may not lead to the required microstructural changes, while too high a temperature or excessively long heating may cause undesirable grain growth or other structural changes.
Process parameters should include:

• grade and chemical composition of steel,
• critical material temperatures,
• output structure,
• degree of prior strengthening,
• dimensions and geometry of the element,
• type of furnace and heating method,
• expected final microstructure,
• type of further mechanical or heat treatment.

Effect of cooling rate

Cooling rate is one of the parameters that determines the final structure of steel. Classic full annealing uses slow cooling to allow for the transformations that lead to a softer structure.
In isothermal annealing, the austenitized material is brought to a specific temperature at which a controlled transformation occurs. In recrystallization annealing, on the other hand, the cooling method has a different significance, as the primary goal of the process is to remove the effects of previous work hardening, not to transform the austenite.
For this reason, one cooling rate value should not be used as appropriate for all softening processes.

Atmosphere during soft annealing

During annealing, not only temperature and time are important, but also the furnace atmosphere. High temperatures can lead to surface oxidation, scale formation, and, under certain conditions, changes in the chemical composition of the surface layer.
Depending on the technological requirements, furnaces operating in an air atmosphere, protective atmospheres, or vacuum are used. Protective atmospheres or vacuum can be particularly important for precision components and alloy steels, where oxidation reduction and high surface quality are required.

Application of steel softening in industry

Tool production

Tool steels are often mechanically treated before final hardening. Soft annealing, particularly spheroidizing, produces a structure that facilitates milling, turning, drilling, and other operations performed before final heat treatment.

Bearing production

In high-carbon bearing steels, it is important to achieve the appropriate carbide shape and distribution before hardening. Spheroidizing annealing is therefore a crucial step in material preparation.

Production of sheets, strips and wires

Materials subjected to cold rolling or drawing gradually harden. Recrystallization annealing restores the appropriate ductility and allows the shaping process to continue.

Machine industry

Softening can be used when preparing semi-finished products for shafts, axles, gears, transmission components, molds and other parts requiring intensive machining before final heat treatment.

Softening and stress relief annealing

Softening should not be equated with stress relief annealingThe primary purpose of stress relief is to reduce residual stresses while minimizing the change in the basic microstructure of the material.
During softening, the aim is primarily to change the hardness, plasticity or microstructure in a way that facilitates subsequent technological operations.

Softening and normalizing

Normalizing steel involves austenitization and subsequent air cooling. In many carbon steels, this leads to a finer structure and greater hardness and strength than slow annealing.
If the primary goal is to make high-carbon tool steel as easy to machine as possible, spheroidizing annealing may be a more appropriate process. However, if grain refinement and structural homogeneity after forging or rolling are required, normalizing may be the appropriate process.

Softening and hardening

Softening and steel hardening They pursue fundamentally different goals. Hardening produces a high-hardness structure, while softening is used to reduce hardness or obtain a structure favorable for further processing.
However, these processes often occur in a single technological sequence. Tool steel may first be soft annealed, then mechanically processed, and only then hardened and forgiven.

The most important benefits of softening steel

• reducing excessive hardness,
• improving machinability in appropriate grades,
• increasing the plasticity of the material,
• restoration of plasticity after cold working,
• obtaining the appropriate carbide morphology,
• preparation of tool and high-carbon steels for hardening,
• the possibility of carrying out subsequent stages of shaping and mechanical processing.

Process limitations

• parameters must be selected for a specific steel grade,
• too high a temperature can lead to unfavorable grain growth,
• incorrect cooling may result in an incorrect microstructure,
• spheroidizing processes may require long heating times,
• not every softening method is suitable for every type of steel,
• in the case of high-alloy steels, the type and stability of the carbides present and the transformations specific to a given grade should be taken into account.

Steel softening is not a single process with universal parameters, but rather a group of heat treatments selected based on the chemical composition, initial microstructure, and intended use of the material. Full annealing, isothermal annealing, spheroidizing annealing, and recrystallizing annealing utilize different mechanisms, but their common goal is to achieve a material condition more favorable for subsequent technological operations.

For high-carbon and tool steels, spheroidizing annealing is particularly important, as it allows for a change in the carbide morphology and prepares the material for machining and subsequent hardening. In materials strengthened by cold working, recrystallization is crucial, eliminating the effects of cold work and restoring ductility.
The temperature, soaking time, and cooling method should always be specified for a specific steel grade. This allows for achieving the required microstructure without uncontrolled grain growth, undesirable transformations, or deterioration of material properties.

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.