Ultra high temperature measurement in heat treatment (furnaces)

Table of contents

During fabrication, metals and metal alloys go through various processes such as casting, welding and rolling that can damage the structure of the metal and affect their mechanical properties. This problem can be resolved by various sorts of heat treatments (also called thermal processing) in which the metals are heated and cooled in different types of furnaces according to exact time and temperature parameters, depending on the type of metal and the required end-product. By definition, a furnace is an oven that can reach temperatures above 1000°C. A good quality, ultra high temperature thermocouple will provide accurate temperature data to ensure the precise control required. Kamet’s high temperature thermocouples (Types B,S,R, C or D) are a good match.

In this article we will look at how our high temperature thermocouples are well suited to various heat treatment processes with the focus on annealing furnaces, vacuum furnaces and atmosphere furnaces.

Types of heat treatment and furnaces

Each (combination of) heat treatment type and furnace is suited to a specific type of metal or chosen to achieve a particular property in the final product. The goal may, for example, be to improve the metal’s hardness, ductility and/or corrosion resistance. The choice of furnace and heat treatment process depends on factors such as material type, desired surface finish, and the need to prevent oxidation (among others). Each furnace and treatment process has its advantages and is suited for specific applications.

Types of Heat treatment

  • Hardening: increase hardness and strength
  • Tempering: reduce brittleness
  • Annealing: softening, enhancing ductility, relieve internal stress
  • Normalizing: uniform grain structure
  • Quenching: rapid cooling to reduce crystal grain size (increases hardness)
  • (low-pressure) Carburizing: enhance the surface properties of iron or steel

Types of furnace

 Atmosphere furnaceVacuum furnace *Annealing furnace
Types of heat treatmentAnnealing, hardening, low-pressure carburizingAnnealing, hardening, quenchingAnnealing, hardening, tempering, normalizing
FunctionOperate with specific gas compositions eg nitrogen and hydrogen are blended for annealingThe vacuum eliminates all gasses which minimizes oxidation. Metal parts that need to withstand high heat and stress require heat treatment in a vacuum.Soften metals and increase ductility by modifying microstructure. First a temperature is maintained that is hotter than recrystallization but lower than melting point. Then a long, controlled cooling process occurs for the formation of more ductile grains.  
AdvantageReduces the risk of oxidization or decarburizationAbsence of air makes it suited to sensitive materialsRelieves internal stress making metals more machinable
Maximum temperature1700°CUp to 3000°C1300 ℃
Type of thermocoupleTypes R or SDepending on the temperature of the heat process, a choice can be made between types N, S, R, B and C. Above 1300°C only type C is suited for use in a vacuum furnace.Type K can be used up to 1000℃. Type R and S are more expensive but offer more accuracy in the extreme conditions of furnace environments.
Specific properties of thermocouple assemblyResistance to high heat in oxidizing. reducing or carbonizing atmospheresWithstand very high temperatures and vacuum.Very accurate temperature readings within a large temperature range. Oxidation resistant.

*Extra information on the use of Kamet’s thermocouples in vacuum furnaces can be found on this page.

Using high temperature thermocouples in furnaces

In the table above you see three of the most common furnaces and the thermocouples most suited to the heat treatment processes that take place in them.

 When selecting a thermocouple for use in a furnace, it is important to consider the:

  • most appropriate sheath and housing
  • temperature range of the furnace
  • duration of the heat treatment process
  • atmospheric effects
  • positioning of the thermocouple
  • desired metallurgical results
  • required tolerances and/or degree of accuracy
  • required lifespan (sometimes operators choose for single-use thermocouples in furnace applications)
  • function of the thermocouple, for example, is it for monitoring and/or for process control?

Ultra high temperature thermocouples (type C/D or B,R or S) have many unique features that make them an excellent choice for use in the extreme heat of furnaces. For more details on their properties and function, you can read here.

Temperature control

High temperature thermocouples can be used to feed data to a temperature controller which maintains the desired heat by adjusting the power input to the furnace. Accurate temperature control requires data from thermocouples positioned in various points of the furnace. Temperature control can affect atmosphere control and thus the metallurgy of the end product. For example in carburizing, if the product is not at a uniform temperature and the center is cooler, then there will be less diffusion and a potentially softer part. Both the positioning of the thermocouple and the calculation of the required setpoints and tolerances is essential for optimal results.

Position of thermocouples for temperature control

Temperature uniformity throughout the furnace ensures superior product quality. Thermocouple positioning is therefore an important factor to consider with various possibilities. A position near the heating elements/source provides accurate readings on the temperature throughout the process. A second option is placement near to the material being processed and in the case of continuous furnaces this would be near the entry/exit points of the materials. In this way the temperature profile of the products can be monitored during heating and cooling. In addition, thermocouples are often placed at certain critical points in the furnace chamber, such as where specific chemical reactions or structural transformations occur. Thermocouples in all three locations allow for accurate monitoring of temperature variations within the furnace and contribute to the important goal of temperature uniformity.

Defining and meeting tolerances for temperature control

A temperature setpoint is calculated based on the controller, thermocouple and required metallurgical results. Certain heat treatment processes may have very tight tolerances and targets with regards to maximum temperature and the rate of heating/cooling. In these cases the accuracy of the thermocouple readings is critical and will determine both the choice of thermocouple and sheath type. Furthermore the placement of thermocouples in different positions is strongly advised when tolerances are tight (as explained in the paragraph above).

Thermocouples from Kamet

In conclusion, heat treatment is an exact science involving extremely high temperatures (generally above 1000°C) and, as such, ultra high temperature thermocouples play an essential role in ensuring the precise heat parameters are maintained that will result in the required quality of metal product.

Kamet’s offers all the parts necessary for a complete furnace-proof thermocouple assembly including a variety of sheath types and a range of diameters for thermocouple (extension) cable and lead wires. We can also provide both hot-end junctions and cold-end terminations.

Our high-temperature thermocouples meet the highest quality standards, meeting the demanding requirements of heat treatment furnaces. Kamet’s thermocouple types R / S / B / D/ / C conform not only to these standards but to precise calibration requirements, especially in terms of tolerance. Our thermocouples are an excellent investment for extremely high-temperature heat processing applications. You can read more detailed information about Kamet’s ultra high temperature thermocouples here. Our brochure on high temperature thermocouples may also be of interest.

Download the high temperature thermocouple brochure

In this brochure, you will find technical drawings and product customization options, including all the steps to configure your needed thermocouple.

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About Kamet

Kamet can provide you with more than 35 years of market and industrial knowledge and experience in the high temperature thermocouple world. We look forward to helping you with the ideal thermocouple assemblies for your heat treatment processes. Our website offers an extensive knowledge base of FAQ’s along with pages of product information. Should you nonetheless have specific questions, please reach out to us and our in-house expert will be quick to respond.

Download the high temperature thermocouple brochure

Download the high temperature thermocouple brochure

In this brochure, you will find technical drawings and product customization options, including all the steps to configure your needed thermocouple.

"*" indicates required fields

By clicking the "Send" button, you are agreeing to our terms and conditions and the privacy policy.