Types, methods and process of steel annealing

The purpose of full annealing is to remove the previous microstructure at room temperature and soften the previously hardened material, generally to facilitate later deformation or processing. The process itself involves heating the steel and aims to ensure the reliability of welded steel connections. Normalizing annealing, stress relieving annealing, or full annealing - these are just a few elements that make up steel annealing. How to ensure a uniform fine-grained structure and what does the whole process involve? We write about this below!

Working with metals, and annealing steel

Full annealing is a process of transforming the distorted, cold-worked lattice structure back into a stress-free structure through heating. It is a solid-state process, usually followed by slow cooling in a furnace.

Stages of annealing 

Recovery is the first step in annealing. It is a low-temperature process that does not cause significant changes in the microstructure. Its main function is to alleviate internal stress. Recovery is a time and temperature-dependent process. Changes in mechanical properties are minimal, and the primary application of recovery is stress relief to prevent stress corrosion cracking or minimize distortions caused by residual stress. Recrystallization occurs at higher temperatures as new fine crystals appear in the microstructure. They typically appear in the most deformed areas, such as grain boundaries or slip planes. Recrystallization occurs during the nucleation of unstressed grains and the growth of these nuclei to absorb the cold-worked material. The recrystallization temperature refers to the approximate temperature at which a heavily cold-worked material will fully recrystallize within 1 hour. It can be noted that the greater the deformation, the lower the recrystallization temperature. The recrystallization temperature of zinc, lead, and tin is lower than room temperature, so cold working is not possible.

Heat treatment of steel

The entire annealing process involves heating to the appropriate temperature, followed by slow cooling in the furnace throughout the entire range of transformation. The purpose of annealing is to obtain refined grains, soften them, improve electrical and magnetic properties, and sometimes improve machinability. Annealing is a slow process approaching equilibrium and approaching phase modes. Annealing includes several thermal cycles, classified according to the highest temperature reached:

• subcritical annealing: heating below the critical temperature A1;

• critical annealing: heating above A1, but below the upper critical temperature A3 is used for hypereutectoid steel, and Acm for hypereutectoid steel;

• full annealing: heating above the critical upper limit of temperature A3.

Thermal engineering of steel alloy systems

The oldest concept of hardness assessment based on phase transformation diagrams is the critical cooling rate. The critical cooling rate of steel can be defined as the rate of continuous cooling necessary to prevent undesirable transformations. In the case of steel, it is the minimum rate at which austenite must be continuously cooled to stop changes above the Ms temperature or the slowest cooling rate that gives 100% martensite. Since the cooling curve from which the critical cooling rate is derived is not linear, there are various ways to determine it. Use the TTT curve for the tested alloy, examples include austenite cooling time between the austenitizing temperature and the quenching bath temperature; average cooling rate calculated from (austenitizing temperature – quenching bath temperature) / time elapsed for cooling; or at a given temperature (nose method). The so-called nose method provides an estimate of the actual critical cooling value approximately 1.5 times faster. In addition, the critical cooling rate was obtained from significantly different TTT or CCT maps. Therefore, the method of calculating the critical cooling rate should be specified.

Annealing of gray cast iron

The entire annealing process consists of two stages. 

  • The first one is performed above the critical temperature range. It decomposes carbides and homogenizes the matrix. This causes the dispersion of segregating elements, which can lead to local stabilization of carbides and pearlite. 
  • The second stage is carried out at a temperature below the critical temperature range. It transforms the matrix into ferrite, precipitating all C in the solution onto the existing graphite. 

 

Subcritical annealing of spheroidal cast iron is not recommended as it may lead to a deterioration of mechanical properties due to frame formation. Moreover, the rate of ferrite decreases sharply below a certain temperature of 650°C, and annealing at medium temperature may take less time and provide better performance. High Si content promotes carbide decomposition. The influence of minor elements on the formation of carbides and perlites has been described earlier.

Heat treatment of steel and annealing

Popular types of heat treatment include:

  • Annealing (full annealing): annealing is one of the most common methods of heat treatment of steel. It is used to soften steel and increase ductility. During this process, the steel is heated and slowly cooled to room temperature in the lower range of the austenite phase field. The resulting microstructure consists of coarse ferrite or coarse ferrite and pearlite, depending on the carbon and alloy content in the steel. This process relieves stresses in the metal, resulting in a large grain structure and soft edges, allowing the metal to dent or bend instead of cracking under impact or stress, and also facilitates grinding or cutting of the annealed metal.
  • Normalization: steel is normalized by heating to the austenitic phase field at a temperature slightly higher than that used for air cooling after annealing. Many steels are normalized to establish uniform ferritic and pearlitic microstructures and uniform grain size.
  • Process annealing (recrystallization annealing): process annealing occurs at a temperature slightly lower than the eutectoid temperature of 1341°F (727°C). This treatment is suitable for cold-rolled low-carbon steel sheets to restore ductility. In aluminum-hardened steel, recrystallized ferrite will have an ideal crystal structure for deep drawing into complex shapes, such as oil cans and compressor housings. The crystal texture is achieved by developing a favorable orientation of ferrite grains, i.e., the crystallographic axes of ferrite grains are oriented in a preferred, not random, orientation.
  • Spheroidization: to produce steel in the finest possible state, spheroidization is usually carried out by heating just above or just below the eutectoid temperature of 1341°F (727°C) and maintaining it at this temperature for an extended period. This process decomposes the lamellar pearlite into small spheres of cementite in a continuous ferrite matrix. To achieve a very uniform dispersion of cementite spheres, the initial microstructure is usually martensite. This is because carbon is more evenly distributed in martensite than in lamellar pearlite. Cementite plates must first dissolve and then distribute carbon in the form of spheres, while cementite spheres can come directly from martensite.
  • Stress Relieving: Steel products with residual stress can be heated to a eutectoid temperature close to 1341°F (727°C). Heating at this temperature will achieve stress relief.
  • Hardening: this is a process of very rapid cooling of high-carbon steel after heating, thereby "freezing" steel particles into a very hard martensitic form, making the metal even harder. In every steel, there is a balance between hardness and ductility, where the harder it becomes, the less durable or impact resistant it will be. To produce higher strength bainitic and martensitic compositions, steel must be heated to the austenitic phase field and rapidly hardened by oil or water quenching. In this process, high-strength low-alloy steel (HSLA) is produced, which is then hardened. It should be noted that microalloy additions such as Nb, V and Ti can also produce HSLA steels. These microalloyed steels gain strength through thermomechanical treatment, not heat treatment.
  • Tempering: when hardened steel (martensitic steel) is tempered by heat treatment to a temperature close to the eutectoid temperature of 1,341°F (727°C), the dissolved carbon in the martensite forms cementite particles, and the steel becomes more ductile. Tempering reduces the stresses in the metal caused by the hardening process, reducing the hardness of the metal, while better withstanding impacts without cracking. Hardening and tempering are used in various steels to achieve the desired combination of strength and toughness. Often, mechanical and thermal treatment are combined in so-called thermomechanical treatments to achieve better properties and more efficient material processing. These processes are common for high-alloy special steels, superalloys, and titanium alloys.

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    Aluminum alloys, heat treatment and annealing

    Depending on the alloy system and previous treatments, there are different types of annealing treatments for various purposes. 

    • Full Annealing (tempering O): provides the softest, yet most ductile and machinable conditions for formable alloys and those not susceptible to heat treatment. Cold work hardening is reduced or eliminated by heat treatment at temperatures from about 250°C to 450°C for a period from seconds to an hour. The exact time and temperature depend on the amount of previous cold work and the concentration of dissolved substance. To eliminate the effects of precipitation hardening, annealing treatment dissolves small hardened deposits. The annealing temperature should not exceed 415°C to avoid oxidation and grain growth. The rate of heating and cooling must be controlled to avoid precipitation hardening/softening in alloys subjected to heat treatment. For all alloys, relatively slow cooling is recommended to minimize distortions. However, for heat treatable alloys, slow cooling may lead to the formation of coarse-grained deposits. 
  • Partial annealing: Intermediate mechanical properties: annealing (tempering H2) of cold-worked alloys unsuitable for heat treatment is referred to as partial annealing or reverse annealing. During this process, the material undergoes regeneration, partial recrystallization, or complete recrystallization. Partial annealing provides better bendability and deformability than alloys with a similar level of H1 tempering strength. Strict temperature control is important for achieving uniform and consistent mechanical properties.

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    Annealing of castings is not common. However, this process can be used to ensure maximum dimensional stability during high-temperature work and high strength. When used, heat treatment at 315-345°C for 2-4 hours is required to ensure optimal stress relief and precipitation of phases formed by the excess dissolved substance remaining in the solid solution during casting. As seen above, there are many types of annealing of iron alloys and other metals. Stabilizing annealing, recrystallizing annealing, and other techniques will improve the mechanical properties of steel.

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