Heat treatment

Ahmed Samy

29 Jan, 2024

Imagine turning a soft, flexible piece of metal into a solid blade, ready to pierce and slice. This is not magic, but the art of heat treatment is a process that harnesses the power of fire to unlock the hidden potential of a material. Through carefully controlled cycles of heating and cooling, metals are transformed to the microscopic level, gaining strength, hardness, ductility, or even electrical conductivity.

What is heat treatment in engineering?

Heat treatment is a controlled process used to alter the physical and sometimes chemical properties of a material, typically a metal or alloy, by subjecting it to a specific temperature and time under controlled conditions. The primary goals of heat treatment are to improve or change the material's mechanical properties, such as hardness, strength, toughness, and ductility.

This process consists of heating a metal or alloy to a specific predetermined temperature, holding at this temperature for required time, and finally cooling from this temperature. All these operations are carried out in solid state. Sometimes, it becomes necessary to repeat these operations to impart some characteristics.

Heat treatment of metals is an important operation in the final fabrication process of many engineering components. The object of this process is to make the metal better suited, structurally and physically, for some specific applications. For example, an annealing treatment may be necessary between deep drawing operations particularly when excessive cold working has been carried out.

Note that:

Heat treatment may be defined as heating and cooling operation(s) applied to metals and alloys in solid state so as to obtain the desired properties.

All metals can be subjected to thermal cycling. But the effect of thermal cycling may differ from one metal to another. For example, heat treatment has significant impact on steels, and their properties may be changed considerably by definite heating and cooling cycles. In contrast, there is hardly any effect of thermal cycling on properties of hot rolled copper.

Purpose of heat treatment

Heat treatment may be undertaken for the following purposes:

Improvement in ductility and toughness.
Relieving internal stresses.
Refinement of grain size.
Enhance wear resistance.
Improve corrosion resistance.
Increasing hardness or tensile strength and achieving changes in chemical composition of metal surface as in the case of case-hardening.

Other beneficial effects of heat treatment include improvement in machinability, alteration in magnetic properties, modification of electrical conductivity, improvement in toughness and development of recrystallized structure in cold-worked metal.

What are the factors affecting the heat treatment process?

Several factors can influence the outcomes of heat treatment processes. These factors need to be carefully controlled to achieve the desired material properties. Some of them are the temperature up to which the metal/alloy is heated, the time that the metal/alloy is held at the elevated temperature, the rate of cooling, and the atmosphere surrounding the metal/alloy when it is heated.

Heat treatment diagram

Any heat treatment process can be represented graphically with temperature and time as coordinates. Figure 1 describes a simple heat treatment cycle, whereas Figures 2 and 3 represent some complex heat treatment cycles.

Figure 1 is the simplest possible heat treatment cycle in which the metal/alloy is heated, held at the elevated temperature for some time, and then cooled to room temperature.

Figure (1)

Figure 2 shows a typical heat treatment cycle suitable for a precipitation hardenable alloy. In this case, the alloy is heated and held at predetermined high temperature. This step is termed as solutionizing. The alloy is then cooled rapidly to room temperature by quenching. The quenched alloy is heated and held at a moderately high temperature above the room temperature, followed by slow cooling. The last step, i.e. heating to and holding at a moderately high temperature is termed as ageing or more specifically as artificial ageing. It is because some precipitation hardenable alloys get hardened even at room temperature. Such alloys are known as natural age hardenable alloys. Duralumins are natural age hardenable alloys.

Figure (2)

Figure 3 represents a typical heat treatment cycle for carburizing process. The low carbon steel is heated in the temperature range of austenitic region, in contact with some carbonaceous material. It is held at this temperature for some time and then quenched. The quenched steel is reheated to a temperature slightly lower than the one employed in the first step. After holding for some time, it is rapidly cooled to room temperature. In the last step, the steel is again heated to about 750 Celsius (which is just above the lower critical temperature), held at this temperature and then quenched. By these three steps, a hard case and tough core are obtained in the carburized steel.

75 0^{\circ} C(which is just above the lower critical temperature), held at this temperature and then quenched. By these three steps, a hard case and tough core are obtained in the carburized steel.

Figure (3)

Heat treatment process variables

Heat treatment temperature, holding time, and rate of heating and cooling are some of the parameters which affect the heat treatment processes, and are commonly referred to as heat treatment process variables. The required magnitude of these variables depend on the chemical composition, size and shape of the object and the final properties desired in the metal/alloy.

In general, the object to be heat treated is put into a heat treatment furnace at room temperature. This furnace is then heated up to a pre-decided temperature. The average rate of heating is the total increment in temperature divided by the total time taken. The rate of heating can also be calculated for various stages by considering appropriate ranges of temperature.

$� �$ Heating of an object is carried out in a single furnace, there are many instances when more than one furnace is used. For example, if the chemical composition of the alloy to be heat treated is such that either the base metal or alloying elements are prone to oxidation or the alloy exhibits tendency towards grain growth at high temperature, the heat treatment will result in poor yield and unsatisfactory mechanical properties.

What are the stages of the heat treatment process?

Here are the 3 stages of heat treatment process:

Heating.
Holding or soaking.
Cooling.

1. Heating

The material is heated to a specific temperature, which is different for each material and depends on the desired outcome. For example, steel might be heated to 815°C to harden it.

2. Holding or Soaking

The material is held at that temperature for a certain amount of time to allow the desired changes to occur. This time can range from a few seconds to several hours.

3. Cooling

The material is then cooled at a controlled rate. The cooling rate can have a significant impact on the final properties of the material. For example, rapid cooling can make a metal harder, while slow cooling can make it softer.

What are the types of heat treatment?

Stress Relieving.
Annealing.
Normalizing.
Hardening.
Tempering.

Stress Relieving heat treatment

Stress-relieving heat treatment is a process used to reduce internal stresses in a material, typically metal, that result from various manufacturing processes such as welding, machining, cold working, or casting. The primary objective is to improve the material's dimensional stability, reduce the risk of distortion, and enhance its mechanical properties.

Internal stresses under certain conditions can have adverse effects. For example, steels with residual stresses under corrosive environment fail by stress-corrosion cracking, whereas, in general, failure by stress- corrosion cracking occurs under the combined action of corrosion and externally applied stresses. These stresses also enhance the tendency of steels towards warpage and dimensional instability. Fatigue strength is reduced considerably when residual tensile stresses are present in the steel.

The process of stress relieving consists of heating metal or alloy uniformly to a temperature below the lower critical temperature (recrystallization temperature), holding at this temperature for sufficient time, followed by uniform cooling. Uniform cooling is of utmost importance as non-uniform cooling will itself result in the development of internal stresses. Thus the very purpose of stress relieving will be lost.

The specific temperature and time required for stress relieving will vary depending on the material, its thickness, and the amount of internal stress present. However, as a general rule, stress relieving temperatures are typically around 500°F to 1200°F (260°C to 650°C) for steel and 300°F to 700°F (150°C to 370°C) for aluminum.

Note that:

Internal stresses are those stresses which can exist within a body in the absence of external forces, these are also known as residual stresses or locked-in stresses.

The problems associated with internal stresses are more difficult in brittle materials than in ductile materials.

No microstructural changes occur during the process.

This process differs from other subcritical treatments in which structural improvement takes place.

Stress Relieving heat treatment applications:

This process find applications in welded structures, machined parts, castings, high-pressure components

Welded structures:
Stress relieving is often used to prevent cracking.

Machined parts:
Stress relieving can improve the dimensional stability of machined parts.

Castings:
Stress relieving can prevent distortion and cracking.

High-pressure components:

Stress relieving is often used for components that will be subjected to high pressure, such as pressure vessels.

Annealing heat treatment

Annealing heat treatment process involves heating to a predetermined temperature, holding at this temperature, and finally cooling at a very slow rate used to soften metals and increase their ductility. This makes them more malleable and easier to shape without breaking, crucial for various metalworking applications.

The temperature, to which metal is heated, and the holding time are determined by various factors such as the chemical composition, size and shape of metal component and final properties desired.

The various purposes of annealing heat treatment process are to:

Relieve internal stresses developed during solidification, machining, forging, rolling or welding.
Improve or restore ductility and toughness.
Enhance machinability.
Eliminate chemical non-uniformity.
Refine grain size.
Reduce the gaseous contents in alloy.

Annealing heat treatment applications:

Wire drawing: Annealing softens the wire before it is drawn through dies to decrease the risk of breakage.

Sheet metal forming: Annealed sheets are more pliable and can be easily bent, stamped, or drawn into desired shapes.

Welding: Annealing heat treatment after welding helps relieve stresses and prevent cracking around the weld zone.

Machining: Annealed metals are easier to machine, resulting in smoother finishes and longer tool life.

Types of annealing heat treatment

Annealing heat treatment can be classified into groups, based on temperature of treatment, phase transformation that takes place during treatment, and the purpose of the treatment. All the groups based on these three criteria are interrelated.

Depending on heat treatment temperature, annealing treatment can be subdivided into three classes: full annealing, partial annealing and subcritical annealing.

1. Full annealing

The alloy is heated above its upper critical temperature (the A3 temperature in the case of steel) and then slowly cooled in a controlled manner. To achieve complete recrystallization, eliminate internal stresses, and refine grain structure, resulting in improved mechanical properties.

2. Partial annealing

Also known as incomplete annealing or intercritical annealing. The alloy is heated to a temperature between its lower and upper critical temperatures (A1 and A3 or Acm in the case of steel) and subsequently cooled. To Balance the mechanical properties by allowing partial recrystallization and phase transformations. This process is often used to achieve a specific combination of hardness and ductility.

3. Subcritical annealing

Is a process in which the maximum temperature to which alloy is heated is always less than the lower critical temperature (A1 in the case of steel). To relieve stresses and enhance certain mechanical properties without inducing phase transformations. It is particularly useful for avoiding grain growth.

Various types of annealing processes classified on the basis of annealing temperature (in case of steel)

Note that:

Annealing can form either the final treatment or a preparatory step for further treatment.

In subcritical annealing, no phase transformation takes place. only thermally activated phenomena, such as recovery, recrystallization, grain growth, agglomeration of carbide and softening occur in this process.

Depending on the specific purpose, annealing is classified into various types: Full annealing, isothermal annealing, diffusion annealing, partial annealing, recrystallization annealing, process annealing, and spheroidizing annealing. The prefix with the word annealing describes the basic purpose of the type of annealing.

Full Annealing

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The soaking time is crucial to ensure that the entire cross-section of the material reaches the same temperature and allows for the homogenization of the microstructure.

Full annealing can involve either air cooling or furnace cooling, depending on the material and its requirements.

Air cooling is a slower process and may be necessary for materials that require a more gradual transformation.

Furnace cooling involves cooling the material in the furnace to maintain a controlled environment and prevent rapid cooling.

Isothermal Annealing

Isothermal annealing is a heat treatment process involves heating the material to a specific temperature, known as the annealing temperature and holding it at that temperature for a specified period, followed by rapidly cooling to temperature less than lower critical temperature. The cooling rate can influence the final properties of the material.

Isothermal annealing is particularly useful for achieving specific microstructural changes, such as the formation of certain phases, spheroidization of cementite in steels, or refinement of grain structure.

The main effects of isothermal annealing include:

Recrystallization and Grain Refinement: Isothermal annealing is often used to induce recrystallization, which is the process of forming new, strain-free grains within a material. This helps in refining the grain structure and improving mechanical properties.

Phases Transformation: Some materials undergo phase transformations at elevated temperatures. Isothermal annealing allows these transformations to occur, leading to changes in the material's microstructure.

Stress Relief: The process can help relieve internal stresses that may have developed during previous manufacturing processes, like casting, forging, or machining.

Note that:

The duration of soaking is a critical factor in achieving the desired changes.

This holding or dwell time allows for the transformation of the microstructure to occur.

Diffusion Annealing

Diffusion annealing, also known as homogenizing annealing is a heat treatment process that involves heating a material to a specific temperature above the upper critical temperature and holding it at that temperature for a certain period to allow the diffusion of atoms or molecules within the material, followed by slow cooling.

Diffusion annealing employed to remove any structural non-uniformity and achieve specific microstructural changes.

Diffusion annealing is employed for various purposes, including:

Homogenization: Achieving a more uniform distribution of elements in the material.

Grain Growth: Controlling the size and distribution of grains in the material.

Recovery and Recrystallization: Removing defects and recrystallizing the material for improved mechanical properties.

Stress Relief: Reducing internal stresses in the material.

Partial Annealing

Partial annealing, is also referred to as intercritical annealing or incomplete annealing is a heat treatment process involves heating the material to a specific temperature between lower critical and upper critical temperature and holding it at that temperature for a certain period, followed by cooling to a specific intermediate temperature before being quenched or air-cooled to room temperature.

Partial annealing primarily used to transform coarse grains to fine grains.

Applications of partial annealing include:

Stress Relief: Partial annealing can be used to relieve internal stresses in a material, making it less prone to distortion or cracking during subsequent manufacturing processes.

Modification of Mechanical Properties: By controlling the annealing temperature and cooling rate, specific mechanical properties such as hardness, ductility, and toughness can be tailored to meet the requirements of a particular application.

Microstructure Control: Partial annealing allows for the adjustment of the material's microstructure, which can affect its overall performance.

Note that:

The term "partial" indicates that the material is not fully annealed but undergoes a specific heat treatment cycle to achieve desired properties.

Partial annealing differs from full annealing in that the material is not cooled to room temperature. Instead, it is often cooled to a specific intermediate temperature before being quenched or air-cooled to room temperature.

Recrystallization Annealing

Recrystallization annealing is a heat treatment process involves heating the material to a specific temperature above the recrystallization temperature and holding it at that temperature for a certain duration, followed by slow cooling to room temperature.

Recrystallization annealing is used in metallurgy to eliminate or reduce the effects of cold working (such as deformation or strain hardening) in metals and alloys and induce the formation of new, strain-free grains (crystals) in the material.

Recrystallization annealing is primarily decrease hardness or strength and increase ductility.

The benefits of recrystallization annealing include:

Softening: It reduces hardness and increases ductility by eliminating the effects of cold working.

Improved Grain Structure: Recrystallization promotes the growth of new, smaller grains, resulting in a more uniform and refined grain structure.

Dimensional Stability: The process helps in relieving internal stresses, contributing to improved dimensional stability of the material.

Note that:

Recrystallization temperature, and also recrystallization annealing temperature, depends on chemical composition, amount of prior deformation, holding time and initial grain size.

Process Annealing

Process annealing is a heat treatment process that involves heating a metal to a specific temperature below the lower critical temperature and holding it at that temperature for a certain amount of time, followed by slow cooling.

Process annealing is primarily used to reduce hardness and increase ductility.

Process annealing is commonly used for the following purposes:

Relieving internal stresses: It helps reduce residual stresses that may have developed during previous manufacturing processes.

Improving machinability: The heat treatment can enhance the material's machinability by softening it and reducing hardness.

Recovery of ductility: Process annealing can increase the material's ductility and formability.

Refining grain structure: The slow cooling during process annealing can result in a refined grain structure, improving the material's mechanical properties.

Normalizing heat treatment

Normalizing is a heat treatment process involves heating the material, usually steel, to about 40-50°C above its critical transformation temperature and holding it at this temperature for a specific period to ensure uniform temperature throughout the entire cross-section of the material, followed by air-cooled in still air or slightly agitated air to room temperature.

Normalizing heat treatment is primarily used to reduce internal stresses, refine the grain size, and achieve a more uniform and desirable microstructure of materials, particularly steel.

Note that:

Air cooling helps in achieving a more uniform structure and reducing the likelihood of internal stresses.

Cooling rate can be controlled either by changing air temperature or air volume.

In case of steel, after normalizing, the resultant microstructure should be pearlitic.

For such steels, cooling in air does not lead to normalized.

Normalized steels are generally stronger and harder than fully annealed steels.

Normalizing Vs Annealing

Normalized steels are harder than annealed ones. Relatively rapid cooling in the case of normalizing results in higher degree of supercooling. Therefore, austenite decomposes at relatively lower temperatures, resulting in better dispersion of ferrite-carbide aggregate. Also, the amount, of pearlite is more. Both of these factors result in higher strength and hardness. So, where these properties are desired, annealing treatment cannot be employed, and normalizing should be done.

Prolonged heat treatment time and higher energy consumption make the annealing treatment more expensive than normalizing. Thus, normalizing is the preferred treatment in industries. Cooling rates are not critical for normalizing as in the case of annealing. They can be increased considerably in order to cut short the total time for treatment.

The only point to keep in mind is that quenching should only produce balanced fine components. After reaching a certain temperature, which is well below the minimum critical temperature, the steel may be quenched. Natural steel has a lower impact transition temperature than annealed steel. This is mainly due to the fine grain size of natural steel. Annealing improves the machinability of medium-carbon steel, while normalizing improves the machinability of low-carbon steel.

Hardening heat treatment

Hardening is a heat treatment process used to increase the hardness, strength, and improve wear resistance of a material. The process involves heating the material to a predetermined temperature above its critical temperature, usually known as hardening temperature, and holding it at that temperature for a certain duration, followed by rapid cooling such as quenching in water, oil or salt baths. This rapid cooling is known as quenching and is a crucial step in achieving the desired hardness.

Note that:

The term rapid cooling simply does not mean that cooling rate is higher than that adopted in annealing and normalizing. What it really means is that cooling rate is equal to or more than the upper critical cooling rate.

It's worth noting that normalizing is different from annealing and hardening. Annealing involves slow cooling to achieve a soft and ductile microstructure, while hardening involves rapid cooling to increase hardness. Normalizing falls between these two processes in terms of cooling rate and resulting microstructure.

Tempering heat treatment

Tempering is a heat treatment process used to improve the toughness, ductility, and strength of hardened steel. This process is typically performed after the steel has undergone a quenching process, which involves rapid cooling from a high temperature. The quenching process results in a hard but brittle material, and tempering is employed to reduce the brittleness while maintaining hardness to achieve the desired mechanical properties.

After quenching steel, the hardened steel is then reheated to a temperature below its critical point (the temperature at which the steel became fully hardened during quenching) and then the steel is held at the tempering temperature for a specific duration followed by slow cooling in still air or by using a controlled cooling process. The slow cooling helps to relieve internal stresses and prevents the material from becoming too brittle.