What Are The 4 Types Of Heat Treatment?
Heat treatment is a group of industrial processes used to alter the physical, and sometimes chemical, properties of a material. The most common application is metallurgical. Heat treatments are also used in the manufacture of many other materials, such as glass. Heat treatment involves the use of heating or chilling, normally to extreme temperatures, to achieve the desired result such as hardening or softening of a material.
The four main heat treatment processes used in metallurgy are annealing, normalizing, hardening, and tempering. These processes provide benefits such as increasing strength and hardness, improving machinability and ductility, and relieving internal stresses. Proper heat treatment is crucial for optimizing the mechanical properties of metals and alloys, and enables materials to withstand their intended engineering applications.
Heat treatment has many important applications across industries like construction, transportation, power generation, and manufacturing. It enables the reliable and efficient production of everything from consumer goods to industrial equipment. With a growing focus on lightweight, high-performance materials, the use of optimized heat treating processes is essential.
Annealing
Annealing is a heat treatment process that alters the physical and sometimes chemical properties of a material to increase ductility and relieve internal stresses (https://www.twi-global.com/technical-knowledge/faqs/what-is-annealing). The process involves heating a material above its recrystallization temperature, holding it at that temperature for a specified time, then slowly cooling.
The main purposes of annealing are to soften the material, improve machinability, and make the material more ductile. It also relieves internal stresses that can build up in a material from processes like welding, cold working, or casting (https://en.wikipedia.org/wiki/Annealing_(materials_science)).
The annealing process consists of three main steps: heating the material to a target temperature, holding it at that temperature for a specified time, then cooling it back to room temperature. The rate of heating and cooling can be tailored to produce desired properties.
Annealing is commonly used on ferrous alloys like steels, non-ferrous alloys like copper and brass, and materials like glass and plastics. It can be performed on cast or wrought products, welded fabrications, cold worked metals, and more.
Normalizing
Normalizing is a heat treatment process that aims to improve the mechanical properties and microstructure of steel after it has been subjected to thermal or mechanical processing such as casting, rolling, forging, and welding (https://www.metalsupermarkets.com/what-is-normalizing/).
The purpose of normalizing is to soften the steel and make it more uniform in structure. It refines the grain size, increases ductility and toughness, and eliminates internal stresses. The process modifies steel’s microstructure by reforming displaced grains and defects caused by previous machining (https://www.thermexmetal.com/what-is-normalizing).
The normalizing process involves heating the steel above its upper critical temperature, soaking it to ensure uniform temperature, then allowing it to cool in open air. The cooling must be slow enough to prevent the formation of martensite. Typical soaking temperatures range from 865–925°C for hypoeutectoid steels and 760–810°C for hypereutectoid steels.
Normalizing is commonly applied to low carbon machine steel parts that require improved machinability and uniformity. It is also used prior to hardening to refine the grain size and improve hardenability.
Hardening
Hardening, also known as quench hardening, is a heat treatment process that maximizes the hardness and strength of ferrous metals like steel. The purpose is to make the metal harder and more resistant to plastic deformation.
The hardening process consists of heating the metal above its critical temperature, soaking it at that temperature for a sufficient time to allow the phase transformation to occur uniformly throughout the part, and then rapidly cooling or quenching the hot metal in water, oil, or air. This rapid cooling does not allow time for the iron atoms to diffuse out of the crystal structure of the austenite phase, causing them to be “frozen” in place as martensite, an extremely hard metastable structure [1].
Hardening is commonly applied to steels, alloy steels, and tool steels to maximize their hardness and wear resistance. Examples include hardening parts like gears, shafts, bearings, cutting tools, and dies. Hardened steel can achieve a hardness of over 50 HRC on the Rockwell scale.
Tempering
Tempering is a type of heat treatment used to increase the toughness and reduce the brittleness caused by the hardening process (Ref: https://www.thyssenkrupp-materials.co.uk/technical-knowledge-hub/tempering). It involves reheating quenched metal or alloy to a temperature below the lower critical temperature to impart some toughness (Ref: https://fractory.com/tempering-explained/).
The purpose of tempering is to reduce some of the brittleness imparted by quenching and produce a more ductile and tough metal (Ref: https://en.wikipedia.org/wiki/Tempering_(metallurgy)). It helps relieve internal stresses induced by the hardening quench. The exact temperature and time of tempering depends on the desired mechanical properties.
Tempering is commonly applied to quenched steels and other iron-based alloys after hardening to increase their toughness and durability. It allows tuning the hardness versus toughness properties as needed. Tempered materials see widespread use in applications ranging from tools and dies to springs and bearings.
Comparison of Processes
Heat treatment processes differ in the heating temperature used, the medium in which they are heated, the rate of heating and cooling, and more. Here are some key differences:
Annealing involves heating to a specified temperature, holding for a period of time, then slowly cooling. The purpose is to relieve residual stresses and soften the material. Normalizing is similar to annealing but involves air cooling instead of furnace cooling. The goal is to refine the grain structure.
Hardening requires rapidly heating to a temperature above critical, holding briefly, then quenching rapidly in water or oil. This produces a hard but brittle martensitic structure. Tempering then reheats the metal to a lower temperature which reduces brittleness and restores some ductility.
Compared to annealing and normalizing, hardening and tempering use much faster heating rates, higher temperatures, and rapid quenching. While annealing and normalizing aim to relieve stresses, hardening and tempering create internal stresses and harden the metal through martensitic transformation. Understanding these key differences allows selection of the optimal process for the desired material properties. (https://www.welderdestiny.com/comparing-heat-treatments.html)
Effects on Properties
The different heat treatment processes affect the properties of steel in various ways. Annealing makes steel softer and more ductile by reducing hardness and strength.[1] It also improves machinability and reduces internal stresses. Normalizing provides a more uniform and refined grain structure, enhancing ductility while decreasing hardness and brittleness.[2]
Hardening makes steel harder and stronger by producing martensite, but reduces ductility and toughness.[3] It also increases wear resistance. Tempering reduces the brittleness of hardened steel while maintaining strength and hardness. It does this by decreasing the amount of martensite and introducing fine carbides.[1] Tempering improves toughness and ductility.
In summary, annealing and normalizing soften the steel for improved machinability and ductility. Hardening and tempering increase hardness and strength for applications needing wear resistance.
[1] https://www.sciencedirect.com/science/article/pii/S2214785320364385
[2] https://www.scirp.org/pdf/jmmce20100800003_84384625.pdf
[3] https://thermo-fusion.com/key-benefits-of-heat-treating-metals/
Applications
Heat treatment is utilized across many industries to alter the properties of metals and alloys for different applications and requirements (https://matmatch.com/learn/process/heat-treatment). The most common applications involve using different heat treatment processes like annealing, hardening, and tempering to improve the hardness, toughness, strength, machinability, or ductility of metals.
For example, annealing is commonly used to soften metals and improve machinability for manufacturing. It helps relieve internal stresses and make metals easier to machine, bend, or form (https://www.sst.net/applications-for-heat-treating/). Hardening and tempering help strengthen and toughen steels for tools, gears, engine components, and other high-stress applications. Normalizing helps refine the grain structure for more uniform properties. The appropriate heat treatment process depends on the material and its intended use.
Heat treatment allows tailoring properties like hardness, ductility, tensile strength, fatigue life, and corrosion resistance for the specific needs of the application (https://www.iqsdirectory.com/articles/heat-treating.html). Steels can be optimized for strength, wear resistance, toughness, or other requirements by choosing the right thermal processing technique.
Other Types
While annealing, normalizing, hardening, and tempering are the most common heat treatment processes, there are a few other less common types of heat treatment worth mentioning:
https://www.sst.net/different-types-of-heat-treatment/
Austempering is a heat treatment process that is similar to quenching and tempering, but involves holding the metal at an intermediate temperature rather than cooling completely to room temperature. This produces bainite microstructures in the metal that have desirable properties. It requires careful control of the time and temperature.
Martempering is another process similar to conventional tempering, but again involves holding the metal at an intermediate temperature after quenching rather than cooling completely to room temperature. This helps reduce distortions and cracks from the quenching process.
While used less frequently than the major heat treatment types, austempering and martempering can produce unique microstructures and properties in metals that have specialized applications.
Conclusion
Heat treatment is an essential process in metallurgy and materials engineering. The four main types – annealing, normalizing, hardening, and tempering – allow engineers to tailor the properties and microstructure of metals and alloys for specific applications and performance requirements.
Proper heat treatment plays a crucial role in optimizing strength, toughness, hardness, ductility, and other material characteristics. The effects of heat treatment are wide-reaching, enabling us to design high-performance alloys for critical applications in aerospace, automotive, energy, and countless other industries.
While the four processes discussed provide the foundation, continued research and development lead to innovative variations leveraging these core concepts. Overall, heat treatment remains indispensable for developing modern materials that build our infrastructure and technology.
