What is a heat treatment furnace? An easy-to-understand explanation of types, construction and examples

A heat treatment furnace is a piece of equipment that has heating and cooling functions to perform heat treatment.
Heat treatment can improve the structure of metallic materials and impart various properties to them. Through heat treatment processes in a furnace, processes such as hardening, tempering, annealing and normalising can be carried out.
This article provides a detailed explanation of heat treatment furnaces, including the types and design of heat treatment furnaces.

What is a heat treatment furnace?

A heat treatment furnace is a specialised piece of equipment used to carry out heat treatment, with the ability to control the heating and cooling of workpieces.

Heat treatment is a type of process that improves quality by heating and cooling workpieces and can change the internal properties of the material without changing its shape.
In heat treatment, the processes of heating, soaking and cooling the workpiece after heating are critical, and it is necessary to manage the heating time, soaking time and cooling time.
Therefore, a heat treatment furnace is designed with a structure that allows a combination of heating, soaking and cooling processes that are appropriate to the heat treatment process. There are systems where the heat treatment runs automatically, managing the time for each process.
Heat treatment of metals is an essential technology for industries dealing with metallic materials, with the most active heat treatment being applied to steel materials.
Steel materials can be given a variety of properties through heat treatment, such as increased strength, toughness and resilience.
In addition, heat treatment processes also exist for non-ferrous metals such as aluminium and titanium, and specialised heat treatment furnaces are used for each type of material.

Differences between a heat treatment furnace and an industrial furnace

A heat treatment furnace is one type of furnace among many, but here we will explain the differences between it and other types of furnace.

The term “furnace” generally refers to any device used to heat materials, including cooking ovens and incinerators, which are also types of furnace.
Among these, furnaces used for industrial purposes are called industrial furnaces, which are used to process various materials such as metal, ceramics and glass.
Within industrial furnaces, there are many types based on the materials and processing methods involved. Specialised industrial furnaces are designed for specific materials or types of processing.

Types of industrial furnaces	Main functions	Related processing methods
Heat treatment furnace	Changing the properties and improving the quality of metal materials	Heat Treatment
Heating furnace	Heating of metal materials	Forging, pressing
Sintering furnace, Firing furnace	Forming metals and ceramics by sintering or firing	Powder metallurgy (sintering, firing)
Melting furnace	Melting of metal materials	Casting
Drying furnace	Drying and removing moisture or solvents from material surfaces	Drying

Of the industrial furnaces listed above, melting furnaces, heating furnaces, sintering furnaces and firing furnaces are primarily used to process metal raw materials into specific product forms.
In particular, casting, forging and stamping are processes used to form the raw shapes of many metal products. In these processes, industrial furnaces are used to change the metallic state of the material to one that is easier to process, and then the material is formed into specific shapes.

In contrast, drying and heat treatment furnaces are used after the product shape has been roughly formed. For example, a heat treatment furnace is used to finish castings, forgings, stampings and more.
For certain products, heat treatment may also be applied after processes such as cutting or bending to change the shape. Once the major processing steps have been completed, the product is placed in the heat treatment furnace to impart its final properties.
A heat treatment furnace is not used for direct shaping or processing, but is designed to cause internal changes in the material.

The basic function of heat treatment

There are various methods of heat treating and furnaces are available to accommodate these methods.

Basic function of heat treatment

There are various methods of heat treating and furnaces are available to accommodate these methods.

The fundamental principle of heat treatment is to heat and cool metal materials to bring out specific properties, a process that has been developed through much trial, error and experience.
For example, when forging a Japanese sword, the blade is heated until it turns red, then forged and finally cooled by immersion in water. This process is the heat treatment step that gives the Japanese sword its strength.
In modern times, heat treatment processes are systematised as a technical framework, and processes such as heating and cooling are established to achieve the desired metal structure.

Steel materials are the most effective materials for heat treatment. Steel can acquire different properties depending on the state of its microstructure and the influence of elements such as carbon.
Heat treatment is a technique that controls changes in microstructure and the content of elements such as carbon. As a result, the metal structure undergoes changes in fine microstructures such as ferrite, pearlite, cementite and austenite, resulting in improved properties such as elasticity, toughness and hardness.

Types of heat treatment

These are the main types of heat treatment for steel materials. In some cases, several heat treatments are combined depending on the properties required.

Heat treatment methods	Heat treatment process	Properties obtained
Hardening	Heat and soak the material until it transforms into austenite, then rapidly cool it to transform it into martensite.	・Hardening ・Increased strength ・Decreased toughness
Tempering	Reheat at a temperature lower than the hardening temperature and soak to form martensite.	・Toughness adjustment ・Hardness adjustment ・Microstructure stabilisation ・Removal of residual stress
Annealing	Heat, soak, then slowly cool with furnace cooling or air cooling.	・Increased ductility ・Softening of microstructure ・Improved machinability ・Microstructure stabilisation ・Removal of residual stresses
Normalising	Heat above the austenite temperature, soak at a constant temperature, then air cool.	・Removal of internal stresses ・Formation of fine microstructure ・Increased toughness

Hardening and tempering are heat treatments that are often used together, primarily to increase the strength and hardening of steel materials.
Hardening is a heat treatment that hardens steel materials. While this makes the material harder, it also reduces its toughness, making it more brittle.
To overcome this, tempering is used to change the microstructure, slightly adjusting the hardness and increasing and stabilising the toughness, making the material more practical to use.

Annealing, on the other hand, is a heat treatment that softens steel materials. After heating, slow cooling or furnace cooling is used to convert the material to a softer microstructure.
While the hardness decreases as the material softens, its machinability and ductility improve, making it easier to cut or draw.
Normalising also involves heating and slow cooling of the material, but does not soften the material as much as annealing. Instead, it primarily improves mechanical properties such as removing internal stresses and increasing toughness, which helps to improve impact resistance.

These heat treatments are mainly processes for steel materials, but there are also heat treatments for other non-ferrous materials such as aluminium, copper alloys and titanium. The timing of heating and cooling and the resulting properties are different for each material.
There are also specialised heat treatment furnaces for non-ferrous metals, and heat treatments must be tailored to each material.

Types of heat treatment furnaces

Heat treatment furnaces can be classified based on differences in heating methods, cooling methods and processes. Below is an overview of the general classification.

Classification of heat treatment furnaces by heat source

Heat treatment furnaces are broadly divided into two types based on the heating method: [combustion furnaces] and [electric furnaces].

・Combustion furnace：
Direct heating by combustion of heavy oil or gas (burner type, combustion gas type) or indirect heating.

・Electric furnace：
Heating by electric resistance, high-frequency induction heating, arc heating, electron beam or plasma heating.

For combustion furnaces, in addition to the direct heating method using burners, there is also a method where high temperature combustion gases are injected into the furnace from an external source.
The type of heat source also changes depending on the atmosphere gases inside the furnace during heat treatment.

In electric furnaces, in addition to heating by passing current through electrical resistance, heat sources are also used for specific processes such as high frequency hardening or laser hardening.

Classification of heat treatment furnaces by cooling method

Proper cooling after heating is crucial in heat treatment, and there are also classifications based on cooling method.

Cooling methods include water cooling and oil cooling, which are used for rapid cooling, as well as air cooling, which cools in the open air, and slow cooling, which is gradual cooling.
Slow cooling in heat treatment furnaces is sometimes referred to as furnace cooling, where the temperature in the furnace is controlled during cooling.
The cooling method required varies depending on the heat treatment, so some heat treatment furnaces are capable of using more than one cooling method.

Classification of heat treatment furnaces by process

Heat treatment furnaces can be divided into batch and continuous furnaces based on the process they perform.

Batch furnaces perform heating, soaking and cooling independently, with a single process taking place at a time.
Heating takes place inside the furnace and soaking can also take place if required. The parts are then moved out of the oven for cooling.
This method is suitable for small batch production and requires the use of trolleys, rollers or conveyors for loading into the furnace and unloading during cooling.

Continuous furnaces pass workpieces through continuous heating, soaking and cooling processes within the same furnace, making them ideal for mass production of large numbers of workpieces.
The continuous furnace has separate processing chambers for each step, and workpieces that have completed one step (e.g. heating) are transferred to the next step (e.g. soaking) and finally to cooling.
Continuous furnaces are also suitable for treating long products such as wire and tube, and are highly efficient for processing these materials.

Classification of heat treatment furnaces by type of heat treatment

Heat treatment furnaces are constructed according to the type of heat treatment they are designed to perform, with some furnaces capable of performing multiple heat treatments.

Heat treatments include hardening, tempering, annealing and normalising, with specialised furnaces typically used for each process. For example, hardening is carried out in a hardening furnace, while annealing is carried out in an annealing furnace.
In addition, continuous furnaces can be configured to perform both hardening and tempering in a continuous process without interruption.

Structure of heat treatment furnaces

The structure and overall size of heat treatment furnaces varies between batch and continuous furnaces, but the basic components of a heat treatment furnace remain the same. The following is an introduction to the structural components of a heat treatment furnace.

Heating chamber

The heating chamber is the space in which the parts are heated. The heat generated by the heat source fills the chamber to raise the temperature to the required level.
Continuous heat treatment furnaces have multiple heating chambers to accommodate processes such as hardening and tempering.
In addition, fans are installed inside the heating chamber to prevent temperature unevenness by circulating the heat.

Soaking chamber

The soaking chamber is where the temperature of the heated parts is controlled and slow cooling takes place.
In small batch furnaces, the heating chamber may serve as both the heating and soaking chamber.

Cooling chamber

This is the chamber where the parts are cooled after heating. The cooling medium used can be water, oil, air or other substances.
Water and oil are used for rapid cooling, with systems such as shower nozzles or immersion tanks used to rapidly cool the parts.
In the case of air cooling, an open space where the parts are moved from the heating chamber can also be used as a cooling chamber.

Heat source device

The heat source device is responsible for raising the temperature in the heating chamber. Burners are used for combustion-based furnaces, while heaters are used for electric furnaces.
The required power of the heat source device is calculated based on the amount of heat required for the workpieces, the heat loss from the surface of the equipment, the heat loss when the doors are opened, and the required heating speed.

Atmosphere gas

Atmosphere gas is the gas used to replace the air in the furnace, depending on the treatment.

Atmosphere gases include inert gases such as nitrogen and argon, oxidising gases such as oxygen and steam, reducing gases such as hydrogen and ammonia, and carburising gases used for surface carburising of steel.

There are atmosphere gas generation devices that supply pre-fabricated gas, and devices that generate and supply mixed gases within the heat treatment furnace.

Temperature control system

Heat treatment requires precise temperature management at each stage – heating, soaking and cooling – so a highly accurate temperature control system is essential.
The system includes equipment to measure the furnace temperature in the heating, soaking and cooling chambers, and to control the heat source and atmosphere gases in response to temperature changes, all managed by computer control.
For temperature control, simple on/off control is the simplest and most cost-effective option, but more recently, precise temperature settings are often achieved using PID (Proportional-Integral-Derivative) control systems.

Conveyor system

This system is used to move workpieces within the heat treatment furnace. Batch-type systems may require manual movement of the parts.
In continuous-type systems, conveyors, rollers, pushers and elevators for vertical movement are used to continuously move workpieces into the furnace or to transfer them to the cooling tanks.

Case studies

Pot type Aluminum heat treatment furnace (T5、 T6)

Workpieces are placed in a basket for solution annealing and ageing. The separation of the solution annealing and ageing furnaces allows for smooth heat treatment without loss of time.

Temperature	170～600℃
Effective length	W1000×L1000×H1000
Atmosphere Gas	Air
Purpose of heat treatment	Solution treatment / Aging
Products to be treated	Aluminum
Capacity	600kg/ch
Electricity	120kW

Continuous strand furnace

This is a bright annealing furnace designed for non-ferrous metals (titanium alloys). It allows the heating tubes, water cooling tubes and cooling tubes to be replaced individually.

Temperature	700～1000℃
Effective length	10A×L2800×6pcs
Atmosphere Gas	Ar
Purpose of heat treatment	Bright annealing
Products to be treated	Titanium alloy
Capacity	150kg/ch
Electricity	46kW

Request for materials with specifications and examples of strand furnaces.

Frequently Asked Questions

Q.What are the key considerations when quenching machine parts?

Consider the size (diameter, thickness) of the parts to be heat treated. The quenching process varies depending on the size and shape of the part, which also affects the depth of heat penetration. It is generally accepted that steel exhibits toughness when quenched and tempered. When designing equipment, it is advisable to consider factors such as hardenability, quenching hardness and depth of quenching.

Q.What safety measures are required when operating a heat treatment furnace?

A heat treatment furnace is equipped with an internal heat source and it is essential to manage the furnace temperature, humidity levels and fire protection systems.
Some gases in the atmosphere can be toxic, so gas leak detection systems and worker safety measures are critical.
Safety features include automatic pressure relief valves that release pressure when it exceeds a certain level, interlocks that prevent the machine from operating unless certain conditions are met, and emergency stop buttons in the event of an emergency.
For worker safety, fixed and movable guards cover hazardous parts of the equipment, and photoelectric sensors automatically stop the machine if someone enters a hazardous area.
In addition, automatic shutdown features are essential to ensure safety during events like earthquakes or power outages, effectively preventing secondary damage, such as fires, in the event of a disaster.

Summary

Heat treatment furnaces can manage and control complex processes through a variety of equipment.
Heat treatment includes processes such as quenching, tempering, annealing and normalising, and there are furnaces designed for each of these processes. It is important to select the appropriate furnace based on the heat treatment process required.

As a manufacturer of industrial furnaces, Sun Furnace can provide you with a customized atmosphere gas generation system for your application based on our experience in manufacturing more than 1,500 industrial furnaces. We also offer free technical consultations, so please feel free to contact us.

Learn more about Sun Furnace annealing furnaces
Read more about Sun Furnace’s stranding furnace
Read more about Sun Furnace stainless steel heat treatment furnace
Learn more about Sun Furnace’s non-ferrous heat treatment furnace
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