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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 machining. The process involves heating the steel and is intended to bet on the reliability of welded steel joints. Normalizing, stress-relief, or complete annealing are some elements that makeup steel annealing. How to take care of obtaining a homogeneous fine-grained structure, and what does the whole process consist of? How to carry out the cooling process? What are annealing processes and internal stresses? We write about it below!

Working with metals

Full annealing transforms a distorted, cold-formed 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 microstructure. The primary function is to relieve internal stress. Recovery is a time- and temperature-dependent method. Changes in mechanical properties are minimal, and the primary use of recrystallization is stress relieving to prevent corrosion cracking or minimize strain due to residual stress. Recrystallization occurs at higher temperatures as new fine crystals appear in the microstructure. They usually 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-treated material. Recrystallization temperature refers to the approximate temperature at which a heavily cold-treated material fully recrystallizes within 1 hour. It can be noted that the greater the strain, the lower the recrystallization temperature. The recrystallization temperature of zinc, lead, and tin is more melancholy than room temperature, so cold working is impossible.

Heat treatment of steel

The entire annealing process involves heating to a suitable temperature, followed by slow cooling in a furnace throughout the transformation range. 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 involves 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 supereutectoid steels, and Acm for supereutectoid steels;

– total annealing: heating above the critical upper-temperature limit of A3.

Thermal engineering of steel alloy systems

The critical cooling rate is the oldest concept for evaluating hardenability based on phase transformation diagrams. The critical cooling rate of a 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 produces 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 alloy under test; examples include austenite cooling time between austenitizing temperature and quenching bath temperature; average cooling rate calculated from (austenitizing temperature – quenching bath temperature) / elapsed cooling time; or at a given temperature (nose method). The so-called nose method provides an approximate estimate of the actual critical cooling value 1.5 times faster. In addition, the required cooling rate was obtained from significantly different TTT or CCT maps. Therefore, it is necessary to provide a method for calculating the critical cooling rate.

Annealing of gray cast irons

The entire annealing process consists of two stages. 

– The first is performed above the critical temperature range. It decomposes the carbides and homogenizes the matrix. This causes the dispersion of segregation elements, leading to carbides and pearlite local stabilization. 

– The second stage is carried out below the critical temperature range. It transforms the matrix into ferrite, precipitating all the C in the solution onto the existing graphite. 

Subcritical annealing of ductile iron is not recommended because it can deteriorate mechanical properties due to frame formation. In addition, the rate of ferrite decreases rapidly below a specified temperature of 650°C, and mid-temperature annealing can take less time and provide better performance. High Si content promotes the decomposition of carbides. The effect of fine elements on carbide and pearlite formation was described earlier.

Heat treatment of steel

Popular types of heat treating include:

– Annealing (full annealing): annealing is one of the most common heat treatment methods for steel. It is used to soften steel and increase flexibility. During this process, the steel is heated and slowly cooled to room temperature in the lower region of the austenite phase field. The resulting microstructure consists of coarse ferrite or coarse ferrite and pearlite, depending on the carbon and alloy content of the steel. This process relieves stresses in the metal, resulting in a large grain structure and soft edges, allowing the metal to indent or bend instead of cracking under impact or pressure and making it easier to grind or cut 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 perlitic microstructures and uniform grain sizes.

– Process annealing (recrystallization annealing): Process annealing occurs at a temperature slightly below the eutectoid temperature of 1341°F (727°C). This treatment is suitable for cold-rolled steel sheets with low carbon content to restore flexibility. In aluminum-hardened steel, the recrystallized ferrite will have an ideal crystalline structure for deep drawing into complex shapes such as oil canisters and compressor housings. The crystalline texture is achieved by developing a favorable orientation of the ferrite grains, i.e., the crystallographic axes of the ferrite grains are oriented in a preferred rather than a random orientation.

– Spheroidization: to produce steel in as fine a state as possible, spheroidization is usually carried out by heating just above or below the eutectoid temperature of 1341°F (727°C) and holding it at that temperature for an extended period. This process breaks down the layered perlite into tiny beads of cementite in a continuous ferrite matrix. In order to achieve a very uniform dispersion of the cementite beads, the starting microstructure is usually martensite. This is because carbon is more evenly distributed in martensite than in layered perlite. Cementite plates must first dissolve and then distribute carbon in the form of beads, while cementite beads can come directly from martensite.

– Annealing: Residual stress steel products can be heated to a eutectoid temperature near 1341°F (727°C). Stress relieving will be achieved by annealing at this temperature.

– Hardening: is the process of cooling high-carbon steel very quickly after heating, thereby „freezing” the steel particles into a tough martensitic form, making the metal even harder. Any steel has a balance between hardness and flexibility; the harder it becomes, the less durable or impact-resistant it will be. To produce bainitic and martensitic compositions with higher strength, the steel must be heated to the austenitic phase field and quenched quickly by quenching in oil or water. This process produces high-strength low-alloy steel (HSLA), which is then quenched. It should be noted that micro-alloying additives, such as Nb, V, and Ti, can also produce HSLA steels. These micro-alloyed steels gain strength through thermomechanical processing rather than 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 stress on the metal caused by the hardening process, reducing the hardness of the metal while better withstanding impact without cracking. Hardening and tempering are applied to various steels to achieve the desired combination of strength and toughness. Machining and heat treatment are often combined in so-called thermomechanical treatments to achieve better properties and more efficient processing of materials. These processes are common to high-alloy special steels, superalloys, and titanium alloys.

Aluminum alloys, heat treatment

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

– Total Annealing (O tempering): provides the softest yet most ductile and machinable conditions for formable and non-heat treatable alloys. Reinforcement in cold work is reduced or eliminated by heat treatment at temperatures ranging from about 250°C to 450°C for seconds to an hour. The exact time and temperature depend on the amount of previous cold work and the concentration of the solute. The annealing treatment dissolves small hardened deposits to eliminate the effects of precipitation hardening. To avoid oxidation and grain growth, the annealing temperature should not exceed 415°C. Heating and cooling rates must be controlled to prevent precipitation hardening/softening in heat-treated alloys. For all alloys, relatively slow cooling is recommended to minimize distortion. However, for heat-treatable alloys, slow cooling may result in the formation of coarse-grained deposits. 

– Partial annealing: Intermediate mechanical properties: annealing (H2 tempering) of cold-worked alloys unsuitable for heat treatment is referred to as partial or reverse annealing. During this process, the material undergoes regeneration, partial recrystallization, or complete recrystallization. Partial annealing provides better bendability and formability than alloys with similar H1 tempering strengths. Strict temperature control is essential to achieve a uniform and consistent mechanical properties.

Heat treatment process is not standard. However, it can ensure maximum dimensional stability during high-temperature, high-strength operations. When used, treatment at 315-345°C for 2-4 hours is required to ensure optimum mitigation of residual stresses and phase precipitation formed by excess solute remaining in solid solution during casting. As seen above, many types of annealing of iron alloys and other metals exist. Stabilizing annealing, recrystallizing annealing, and other techniques will improve the mechanical properties of steel.