heat treatment types

The Different Types of Heat Treatments

Heat treatment types are crucial processes in metallurgy that modify the properties of metals and alloys. These treatments involve controlled heating and cooling to achieve desired characteristics like strength, hardness, or ductility. Common methods include annealing, normalizing, quenching, and tempering. Read on to discover how these processes can transform materials for various applications.

Different Ways to Use Heat Treatment

It’s important to understand the different types of heat treatment when you’re using a heat treat oven or industrial kiln. Here are the nine heat treatment types you should know about: 

Hardening

Hardening heat treatments increase metal strength through heating and cooling in a prescribed manner depending on the type of metal. The process begins by heating the metal in a furnace, oven, or kiln to a temperature above its critical point (typically 750°C-1300°C, or 1382°F-2400°F), altering its internal structure while remaining solid. The metal is held at this temperature, often for about an hour per inch of thickness, to ensure uniform transformation. Rapid cooling, usually by quenching in water, oil, or brine, follows. This swift cooling "freezes" the high-temperature structure, creating a harder, more wear-resistant material. In steels, this results in the formation of martensite, a metastable phase.

The hardening process's effectiveness depends on factors like carbon content (for steels), alloying elements, and specific heating and cooling parameters. While hardness and wear resistance increase, the metal often becomes more brittle. 

To address this, hardening is frequently followed by tempering, a lower-temperature treatment that balances hardness with flexibility. Different metals may require varied techniques; for instance, some aluminum alloys use precipitation hardening. Advanced methods like induction or laser hardening offer more localized treatment. Precise control of the process is crucial to achieve desired properties without damaging the metal.

Related: What’s the Energy Cost to Run a Heat Treating Kiln?

Quenching

Quenching is a heat treatment process where pre-heated metals are rapidly cooled to attain specific physical or mechanical characteristics. The heated metals are typically cooled in oil, but air, water, and brine can also be used, depending on the material and the desired outcomes. The choice of quenching medium affects the cooling rate, which is crucial for achieving the desired properties. For instance, water provides a faster cooling rate than oil, while air cooling is the slowest.

During the quenching process, the metal is heated to a specific temperature above its critical point, which varies depending on the alloy composition. For steels, this is typically between 815°C and 870°C (1500°F to 1600°F), well above the austenization temperature but below the melting point. The metal is maintained at this temperature for a predetermined duration to ensure uniform heat distribution and complete phase transformation. It is then rapidly cooled in the chosen medium, which "freezes" the high-temperature microstructure, often resulting in the formation of martensite in steels. 

Annealing

Annealing is a heat treatment technique designed to alter the microstructure of metals, making them more ductile and less prone to internal stresses and hardness. This modification facilitates easier shaping and reduces the risk of cracking. While particularly advantageous for heat treating steels, annealing is also great for various metals including copper, aluminum, and brass. The process begins by heating the metal to a specific temperature where its crystalline structure becomes malleable yet remains solid. For steels, this temperature is typically between 750°C and 900°C (1382°F to 1652°F), depending on the specific alloy. Maintaining the metal at this temperature allows for the reduction of internal stresses, homogenization of the structure, and in some cases, recrystallization of grains.

The metal is then cooled slowly and in a controlled manner to room temperature, ensuring the formation of a more ductile crystalline structure. This cooling rate is typically slower than in other heat treatments like normalizing or quenching, often taking several hours or even days for large workpieces. For steels, the slow cooling allows the formation of larger, more stable grains and the precipitation of carbides, resulting in a softer, more workable material. The cooling rate and final temperature can be adjusted to achieve specific properties. For instance, process annealing (used for low-carbon steels) might cool to around 600°C (1112°F) before air cooling, while full annealing cools slowly to room temperature. This controlled cooling is crucial to achieving the desired improvements in the metal's properties, including increased ductility, reduced hardness, and improved machinability.

Tempering

Hardening can indeed cause metal to become brittle, particularly in thinner sections. For many metals, including tool steels and other alloys used in blades, the next step after hardening is usually tempering. Tempering is crucial as it reduces brittleness by relieving the internal stresses that develop during the rapid cooling of the hardening process. This stress relief occurs through the controlled decomposition of martensite (the hard, brittle structure formed during quenching) into a more stable structure.

To temper metal, the hardened piece is reheated to a temperature between room temperature and its critical point, typically ranging from 150°C to 700°C (302°F to 1292°F). The exact temperature depends on the desired balance between hardness and toughness. The metal is kept at this temperature for a specific time, which can vary from a few minutes to several hours, depending on the size of the workpiece and the desired properties. 

While oil baths are common for tempering, molten salt baths or controlled atmosphere furnaces and ovens are more typically used than molten salt or sand due to extreme safety and environmental concerns. The tempering process not only reduces brittleness but also enhances the metal's toughness, dimensional stability, and fatigue resistance. It's important to note that while tempering generally decreases hardness, it significantly improves the overall balance of mechanical properties, making the metal more suitable for its intended application.

Stress Relieving

If you’re still learning about the heat treatment process, stress relieving is an important type of heat treatment to understand. 

Stress relieving is a heat treatment process where metal is heated to a moderate temperature, typically 450°C to 650°C (842°F to 1202°F) for steels, below its critical point. The metal is held at this temperature for a specific time based on its thickness and composition, then cooled slowly in still air. This process primarily reduces internal stresses from manufacturing without significantly altering the metal's strength or hardness. It's particularly useful for parts that have undergone stress-inducing processes like machining, welding, or cold-working.

The primary benefits of stress relieving include enhanced dimensional stability, reduced risk of stress corrosion cracking, and slight improvement in flexibility. It doesn't typically increase strength or hardness but prevents distortion and cracking during subsequent machining or use. This process is crucial in industries requiring tight tolerances and prevention of unexpected deformation, such as aerospace, automotive, and precision manufacturing. By alleviating internal stresses, stress relieving ensures parts remain stable and less prone to failure during their service life.

Nitriding

Nitriding is a surface-hardening technique that forms a durable outer layer on metal using nitrogen. Unlike methods that harden the entire piece, nitriding selectively strengthens the surface, making it ideal for tools like wrenches that endure frequent use and abrasion. The process can be performed using gas (typically ammonia), liquid (salt baths), or plasma methods.

In gas nitriding, the metal is placed in a sealed chamber heated to 495°C-565°C (925°F-1050°F). 

Ammonia gas circulates and decomposes, releasing atomic nitrogen that reacts with the metal surface, forming nitrides with iron and alloying elements. The case depth increases with time at temperature. Post-process cooling or quenching isn't required, as the nitride layer permanently bonds with the metal surface. This process significantly enhances surface hardness, wear resistance, and fatigue strength, while also improving corrosion resistance. Nitriding is particularly effective on alloy steels containing nitride-forming elements like chromium, molybdenum, and aluminum.

Cyaniding

Cyaniding is a heat treatment process that improves fatigue strength, wear resistance, and surface hardness of small to medium-sized ferrous metal components. The name "cyaniding" comes from the use of sodium cyanide in the treatment.

The process begins by immersing the metal in a molten salt bath heated to 1550°F-1750°F (843°C-954°C). This bath typically contains sodium cyanide, along with other salts like soda ash, carbonate, and chloride. The metal remains in the bath for a period (usually 15-30 minutes, depending on desired case depth) during which the cyanide ions decompose at high temperatures. This decomposition releases both nitrogen and carbon, which diffuse into the metal surface. As these elements saturate the surface, they form a hard case layer. After treatment, the metal is quenched in oil, water, or brine to rapidly cool and complete the hardening process. The result is a component with a hard, wear-resistant surface layer and a tough core, beneficial for parts subject to high stress and wear.

Case Hardening

Case hardening is an excellent choice for creating components with a hard, wear-resistant exterior and a tough, ductile interior. This process is frequently applied to low-carbon steels, which initially lack the carbon content necessary for high surface hardness. Case hardening involves adding carbon, nitrogen, or both to the surface layer of the metal through heat treatment and exposure to carbon or nitrogen-rich environments.

Typically, this procedure is performed after initial machining to enhance the durability of the finished parts. The most common method, CARBURIZING  involves heating the steel to 850°C-950°C (1562°F-1742°F) in a carbon-rich atmosphere. Other methods include nitriding and carbonitriding. The process parameters, including temperature, time, and cooling rate, are carefully controlled to achieve the desired case depth and to maintain a balance between surface hardness and core toughness. This prevents the part from becoming overly brittle. After case hardening, some final machining or grinding may be necessary to achieve precise final dimensions, as the process can cause slight changes in size.

Aging / Precipitation Hardening

Aging, also known as precipitation hardening, is a heat treatment process that increases the strength and hardness of certain metal alloys. It's commonly used for aluminum, magnesium, nickel, and some steel. The process works by forming extremely small, uniformly dispersed particles (precipitates) within the alloy's crystal structure.

The precipitation hardening process typically involves three steps. First, solution treatment, where the alloy is heated to a specific high temperature to dissolve alloying elements into a solid solution, then rapidly cooled (quenched) to keep these elements in a supersaturated solid solution. Second is aging, where the alloy is held at an intermediate temperature, allowing controlled precipitation of fine particles. 


Aging can be natural (at room temperature) or artificial (at elevated temperatures). The choice between natural and artificial aging depends on the alloy composition and desired properties. Some alloys, like certain aluminum series, are designed to age naturally over days or weeks, while others require artificial aging at specific temperatures for optimal properties. The precipitates formed during aging impede dislocation movement in the crystal structure, thereby increasing the alloy's strength and hardness.

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