Annealing of Steels
Annealing of Steels
Annealing is a process involving heating and cooling, normally applied for the softening of the steel. The term also refers to treatments intended to alter mechanical or physical properties and sometimes chemical properties, produce a definite micro-structure, or remove gases to make it more workable. The temperature of the operation and the rate of cooling depend upon the material being annealed and the purpose of the treatment.
The purpose of annealing, is to make a part which has a uniform micro-structure, which is soft, so that it can be formed or machined. Annealing can be performed at the steel plant, or the material received at the processing plant ready to be machined, or it can be done in-house to facilitate machining. There are several different types of annealing which can be performed. But in all cases, the primary reason for annealing is to soften the part and increase its ductility for forming or machining.
Annealing dates back hundreds of years, as evidenced by the word itself, which comes from the Middle English ‘anelen’, meaning to set on fire or kindle, as well as bake and temper. Middle English was spoken and written in England from 1150 CE (common era) until 1500 CE and is a descendant of Old English. While it is not known exactly who discovered annealing, the etymology shows that it was in practice at least 900 years ago.
Annealing process is a process which alters a material to increase its ductility and to make it more workable. In this process. the material is heated to above its critical temperature, maintained at a suitable temperature, and then cooled. Annealing can induce ductility, soften material, refine the micro-structure by making it homogeneous, and improve cold working properties.
Annealing is a heat treatment process in which a metal is heated above its recrystallization temperature, kept at that temperature for some time for homogenization, and then cooled slowly to develop an equilibrium structure. In the crystal structure of the steel being annealed, the number of dislocations is reduced which causes an increase in ductility and relief of residual stresses. Normally annealing is performed for materials which have undergone some cold work or hardening treatment.
Annealing is one of the most fundamental processes which is performed on steel. While annealing is a relatively simple heat treatment to perform, there are a number of factors which are to be carefully considered and controlled. Annealing of steel means heating it to a pre-determined temperature, holding it at that temperature for a certain period of time, and finally cooling at a very slow rate. In steels, annealing normally means a heat treatment with furnace cooling from the austenitizing range.
Annealing is the softening heat treatment. The main purposes of annealing treatment are (i) to eliminate internal residual stresses which have occurred during earlier processing steps, (ii) to improve the machinability of the steel, (iii) to increase ductility which in turn improves formability, (iv) to prevent deformation and cracking, (v) to reduce hardness and increase toughness, (vi) to improve magnetic properties and decrease electrical resistance, (vii) to refine the grain size, and (viii) to reduce the gaseous contents of the steel. Annealing becomes necessary after cold working of the material. Cold working can be cold-rolling or wire drawing.
The annealing process needs the material to be heated above its recrystallization temperature for a set quantity of time before cooling. In case of steel, the material is normally left to cool down to room temperature in still air. The heating process cause atoms to migrate in the crystal lattice and the number of dislocations reduces, which leads to the change in ductility and hardness. The heat-treated material recrystallizes as it cools. The crystal grain size and phase composition depend on the heating and cooling rates and these, in turn, determine the material properties.
Theory of annealing process
Annealing occurs by the diffusion of atoms within the steel material, so that the steel material progresses towards its equilibrium state. Heat increases the rate of diffusion by providing the energy needed to break bonds. The movement of atoms has the effect of redistributing and destroying the dislocations in the steel material. This alteration in dislocations allows steel material to deform more easily, so increases its ductility.
The quantity of process initiating Gibbs free energy in a deformed steel material is also reduced by the annealing process. This reduction of Gibbs free energy is termed also as stress relief. The relief of internal stresses is a thermodynamically spontaneous process. However, at room temperatures, it is a very slow process. The high temperature at which annealing occurs serve to accelerate this process.
The reaction which facilitates returning the cold worked steel material to its stress-free state has several reaction pathways, mostly involving the elimination of lattice vacancy gradients within the body of the steel material. The creation of lattice vacancies is governed by the Arrhenius equation, and the migration / diffusion of lattice vacancies are governed by Fick’s laws of diffusion.
With annealing hardness decreases and ductility increases, since dislocations are eliminated and the steel material’s crystal lattice is altered. On heating to a specific temperature, atoms migrate within the lattice and the adjusted grain improves the mechanical properties.
Hot or cold working of the pieces of steel following annealing alters the material structure once more, so further heat treatments can be needed to attain the desired properties. However, with knowledge of steel composition and phase diagram, heat treating can soften steels and prepare them for further working such as forming, shaping, and stamping, as well as preventing brittle failure.
An annealing furnace works by heating a steel material above the recrystallization temperature and then cooling the material once it has been held at the desired temperature for a suitable length of time. The material recrystallizes as it cools once the heating process has caused atom movement to redistribute and eradicate dislocations in the workpiece.
The annealing heat treatment process consists of three stages namely (i) recovery stage, (ii) recrystallization stage, and (iii) grain growth stage.
In the recovery stage of the annealing process, the steel is heated in a furnace and its temperature is raised to such a point where the internal stresses are relieved. In the recrystallization stage, the steel is heated above the recrystallization temperature but below its melting point. It causes new grains to form without any residual stresses. Recrystallization is defined as a process where the deformed grains in a metallic structure are replaced by a new set of defect-free grains at a certain specific temperature range known as the recrystallization temperature. In the grain growth stage of the annealing process, the material is cooled at a pre-decided specific cooling rate for new grains to fully develop. After this, the material is more workable.
The annealing process is applicable for most of the common materials like steel, cast iron, and non-ferrous metals and alloys. However, the material selected are to be such that its properties can be altered by heat treatment. The majority of the annealing process is done over the steel. All work-hardened steel materials like sheet metals, wires, and welding processes creating high residual stresses are to undergo the annealing of steel processes to recover their properties.
The temperature to which the steel is to be heated and the holding time are determined considering different factors like chemical composition of the steel (% carbon content), shape and size of steel component (thickness and croos-section), and (iii) final properties desired. For hypo-eutectoid steels (steels with a carbon content of less than 0.8 %), full annealing consists of heating to 90 deg C to 180 deg C above the Ac3 temperature, and for hyper-eutectoid steels (steels with a carbon content of more than 0.8 %), heating above the A1 temperature, followed by very slow cooling.
Annealing of steel serves several purposes. As an example, steel wire is annealed to improve its ductility and to relieve internal stress created by drawing, cold forming, or uneven cooling after hot rolling. Annealing also help to refine the grain size. Annealing also affects micro-structure. On heating, low-carbon (less than 0.03 %) steels form ultra-fine particles of austenite as they reach, then exceed, the lower critical temperature (Ac1). As the temperature rises, excess ferrite continues to dissolve, finally disappearing at the upper critical point (Ac3). As the temperature continues to rise, there is increase in the grain size. The properties achieved as a result of annealing depend on the quantity of carbon present, the coarseness of the ferrite and pearlite, and their relative distribution throughout the matrix. These factors are influenced by (i) the size of the austenite grains, the smaller is their size, the better is the distribution of the ferrite and pearlite, (ii) the rate of cooling through the critical range, and (iii) time at temperature, which is necessary for carbon to uniformly distribute in austenite.
On slow cooling through the critical range, ferrite formation begins at the austenite grain boundaries. Large, rounded ferrite particles are formed, evenly distributed among the (relatively) coarse pearlite. With a higher rate of cooling, a network structure of small ferrite grains is produced with fine pearlite distributed in the centre of these grains. Since annealing cycles are performed in and around the critical temperatures, it is important to remember that, as far as transformation is concerned, the cooling practice is critical. The rate with which the steel passes through this range determines the micro-structure, hardness, and other properties of the transformed product. A very slow rate of cooling results in the softer micro-structure, which is spheroidic. A faster rate results in lamellar pearlite of varying degrees of coarseness and hardness. If the rate of cooling is too rapid, formation of the soft products of transformation are suppressed and the harder constituents i.e., bainite and martensite, are formed. These latter micro-structures are undesirable in the annealed structure. Fig 1 shows representative micro-structures of some annealed carbon steels.
Fig 1 Representative micro-structures of annealed carbon steels
The high temperature of annealing can result in oxidation of the surface of the steel material resulting in formation of scale. If scale is to be avoided then annealing is carried out in a protective atmosphere such as a mixture of carbon mono oxide gas, hydrogen gas, and nitrogen gas, or a mixture of hydrogen gas and nitrogen gas.
Depending on the temperature of annealing treatment, phase transformation, and purpose, the annealing of steel can be classified into various groups.
Types of annealing of steel based on annealing temperature – Considering the steel annealing temperature, the annealing process is categorized into three types namely (i) full annealing, (ii) partial annealing, and (iii) sub-critical annealing. Fig 2 shows the temperatures at which steel is heated for these three annealing processes.
Fig 2 Types of annealing based on annealing temperatures
In the full annealing process, steel is heated above the upper critical temperature (Ac3) and then cooled very slowly. In the case of partial annealing, steel is heated to a temperature which lies between upper (Ac3 or Acm) and lower (Ac1) critical temperatures and then cooled slowly. Partial annealing is also known as inter-critical annealing or incomplete annealing. For the sub-critical annealing process, the steel is heated below the lower critical temperature (Ac1). No phase change takes place in sub-critical annealing and hence only recrystallization, softening, recovery, and grain growth occur.
In sub-critical annealing, a degree of is to be exercised in selecting the annealing temperature since the rate of heating also has an effect. For rapid heating rates, the Ac1 temperature is to be higher than with slow heating rates. If Ac1 temperature has been incorrectly determined and the real value is exceeded, then austenite gets formed. This austenite is going to or is not going to transform upon subsequent cooling from the annealing temperature, resulting in unwanted retained austenite, untempered martensite, or both. When sub-critical (process) annealing large loads, it is expedient to use a furnace temperature which is somewhat higher than Ac1 temperature to speed up the process. Such a method, however, demands close control to prevent any of the charge from becoming austenitized. Prolonged annealing induces higher ductility at the expense of strength. A serious embrittlement problem can arise after prolonged treatment. With severe forming operations, cracks are liable to occur
Based on the specific purposes of annealing of steel, the annealing process is of several types. These are described below. Fig 3 shows heating temperature ranges for some important types of annealing.
Fig 3 Heating temperature for different annealing processes
Full annealing of steel – Almost all castings, forgings, and rolled stocks are provided with full annealing treatment to get improved mechanical properties. Full annealing involves heating to a temperature above Ac3 temperature. At this temperature, the steel is completely austenitic and the method and rate of cooling the austenite are very important for proper micro-structure and other related properties. In full annealing, it is essential to know the critical temperatures on heating and cooling from the Isothermal Transformation (IT) diagram and the Continuous Cooling Transformation (CCT) diagram.
Full annealing is a softening process in which a steel is heated to a temperature above the transformation range and, after being held for a sufficient time at this temperature, is cooled slowly to a temperature below the transformation range. The steel is normally allowed to cool slowly in the furnace, although it can be removed and cooled in some medium such as mica, lime, or ashes, which ensures a slow rate of cooling. Since the transformation temperatures are affected by the carbon content, it is apparent that the higher carbon steels can be fully annealed at lower temperatures than the lower carbon steels. The temperature range normally used for full annealing is 15 deg C to 25 deg C above Ac3 temperature (the upper critical), as shown in Fig 4. The micro-structures of the hypoeutectoid steels which result after full annealing are quite similar to those shown for the equilibrium conditions (Fig 1b and Fig 1c). Eutectoid and hyper-eutectoid steels frequently spheroidize partially or completely on full annealing.
Fig 4 Full annealing and process annealing range
IT diagrams predict the micro-structure after transformation, the temperature at which this transformation takes place and the time needed so their use allows closer control of the end product. It is only necessary to cool the steel to the temperature where the desired micro-structure forms, hold until transformation is complete, and then to cool in any convenient manner. The only precaution needed is to cool the steel to the desired transformation temperature at a rate which avoids underheating. CCT diagrams can also be useful to determine the rate of continuous cooling which results in a given product
Process annealing treatment – Cold worked steels normally tend to possess increased hardness and decrease ductility making it difficult to work further. Process annealing tends to improve these characteristics. Process annealing, also frequently termed as intermediate annealing, sub-critical annealing, stress-relief annealing, or in-process annealing. It is a heat treatment cycle which restores some of the ductility to the steel material during the process of cold working so that it can be worked further without breaking. This process is normally applied to cold-worked low carbon steels (up to around 0.25 % of carbon) to soften the steel sufficiently to allow further cold working.
The cold worked steel is normally heated close to, but below the Ac1 temperature (around 15 deg below Ac1 temperature) as shown in Fig 4. The steel is held there for a prolonged period of time. As a result, the existing cementite particles coalesce (ball up) and form spheroids. If the steel is not to be further cold-worked, but relief of internal stresses is desired, a lower range of temperature suffices (around 540 deg C), held there long enough to relieve stresses in the steel, and cooling by a convenient means. Rate of cooling is immaterial but normally it is furnace cooled. This type of annealing causes recrystallization and softening of the cold-worked ferrite grains, but normally does not affect the relatively small quantities of cold-worked pearlite. Typical structures of cold-worked is shown in Fig 1a, and process annealed low-carbon steel is shown in Fig 1b and Fig 5a.
For this type of annealing, a fine prior micro-structure such as martensite, bainite or fine pearlite is desirable. Coarse cementite in the prior structure is to be avoided since the large cementite particles do not coalesce as readily as the finer ones. Because of the comparatively long times needed to spheroidize at the sub-critical temperature, this procedure is seldom used in commercial practice. The recrystallization temperature of pure iron is in the region of 500 deg C. Hence, a higher temperature brings about rapid recrystallization.
Spheroidizing annealing – Spheroidizing is a process of heating steel which produces a rounded or globular form of carbide in a matrix of ferrite. Spheroidizing annealing results in carbide spheroids in a ferrite matrix. The degree of spheroidization varies according to heat-treatment temperature and holding time. It is done to get spheroid structures in steel for the improvement of the machinability and ductility. Spheroidizing annealing is frequently performed for those alloy steels and high carbon steels which are to be machined or cold formed in subsequent processes. The process improves the machinability of the steels by improving the internal structure of the steels. Spheroidizing annealing is beneficial when subsequent machining and / or hardening is needed, since the micro-structure consists of rounded cementite particles in a ferrite matrix.
Spheroidizing annealing is normally accomplished by prolonged heating at temperatures just below the Ac1 (Fig 5), but can be facilitated by alternately heating to temperatures just above the Ac1 and cooling to just below the Ac1. The final step, however, consists of holding at a temperature just below the critical temperature Ac1. The rate of cooling is immaterial after slowly cooling to around 540 deg C. The rate of spheroidization is affected by the initial structure. The finer the pearlite, the more readily spheroidization is accomplished. A martensitic structure is very amenable to spheroidization. This treatment is normally applied to the high carbon steels (0.6 % of carbon and higher). The purpose of the treatment is to improve machinability and it is also used to condition high-carbon steel for cold-drawing into wire.
Fig 5 Spheroidize annealing range
The simplest method of spheroidizing is to use a sub-critical anneal as shown in Fig 5. The steel material is heated just below the lower critical temperature Ac1 and the temperature is maintained for around 8 hours before the steel material is allowed to cool down slowly. A more common commercial method consists of heating to a temperature in the range of 15 deg C to 25 deg C below Ac1, hold at this temperature, then increase the temperature setpoint between Ac1 and Ac3 and hold again. Following the second soak period, the temperature is decreased slowly. Another common method is to heat to a temperature of in the range of 15 deg C to 25 deg C below the Ac3, holding at one temperature and then increase it to slightly above Ac3 followed by slow (controlled) cooling. It is necessary in any of these practices that nuclei be present to ensure formation of spheroids. The nuclei can be undissolved cementite, carbon concentration gradients (in-homogeneous austenite) or, in some cases, non-metallic inclusions. If excessively long annealing times are used at relatively high temperatures, however, a very coarse and abnormal agglomeration of the cementite particles result. This condition is extremely undesirable.
The temperature used and the holding time for spheroidized annealing depends on the previous structure and chemical composition of the steel. A steel containing less than 0.3 % carbon is not suitable for spheroidizing, since the structure of low-carbon steels consists of ferrite with only a small quantity of pearlite.
Coarse pearlite coagulates at a slower rate in comparison to fine pearlite. For example, plain carbon and low alloy steels are soft-annealed at around 700 deg C for periods ranging from 4 hours to 6 hours. But the period increases with the increasing coarseness of pearlite.
The temperature of soft annealing is affected by alloying elements. The presence of nickel or manganese decreases the Ac1 temperature and, as a result, decreases the soft annealing temperature. But the presence of chromium and molybdenum raises the Ac1 temperature. Hence, for steels containing 4 % nickel, the soft annealing temperature is as low as 670 deg C. A lower temperature, of course, affects the process-time, extending it to 8 hours to 10 hours. On the contrary, high-speed steels, highly alloyed by tungsten, vanadium and molybdenum and further by chromium and vanadium, have to be soft-annealed at a temperature above 800 deg C. The presence of strong carbide forming elements increases the stability of carbides in steels. They, hence, reduce the coagulation and increase the time for annealing at the particular temperature considered.
The spheroidized condition is the true equilibrium state of the steel and is its softest condition. The spheroidized micro-structure also possesses good cold-forming characteristics. Normally, the larger the spheroids and the more distance between them, the higher is the ability of the steel to be cold formed. Fig 1d and Fig 6b shows spheroid microstructure.
Fig 6 Difference in annealed micro-structures
Diffusion annealing – The purpose of diffusion annealing is to eliminate dendritic segregation, eliminate regional segregation, homogenize the chemical composition, and modify columnar grains which appear during the crystallization of steels. During the process of diffusion annealing, the iron and carbide are diffused together. This process needs higher temperature, so the steel is heated above the upper critical temperature.
Diffusion annealing is normally carried out at a temperature of 1,100 deg C to 1,300 deg C and then the steel is held at this temperature for 10 hours to 20 hours, followed by cooling. Post application of diffusion annealing, full annealing, and normalizing are performed to improve the grains. Diffusion annealing is used to smooth out a difference in the content of alloying elements because of the inter-crystal liquation and chemically homogeneous steel is achieved. This process is applied to high-quality steel for controlling the segregation of alloy steel casting and ingots.
Isothermal annealing – Isothermal annealing is derived from the exact knowledge of temperature-time diagrams. This type of annealing is useful for softening steels for the subsequent machining operations. This treatment consists of austenitizing the steel at the normal annealing temperature (full annealing) and then cooling rapidly to the appropriate temperature below Ac1, normally around 50 deg C to 60 deg C below Ac1 (isothermal holding in pearlite range). This temperature is held for a pre-determined time (Fig 7), enabling the complete austenite decomposition to take place for producing a structure having optimum machinability.
After the transformation is complete, the steel is cooled in a furnace, or air-cooled, or rapidly cooled. There is no metallurgical reason for slow cooling during the change from the austenitizing to the transformation temperature, or after the transformation is completed. The hardness achieved after isothermal annealing depends on holding the steel below Ac1 temperature. The steel after austenitizing is held just below Ac1 temperature, it decomposes slowly. The product then can contain relatively coarse spheroid carbides or coarse lamellar pearlite depending on the austenitizing temperature. This product tends to be very soft.
At the transformation temperature, the austenite normally decomposes more rapidly, and the resultant product is harder, more lamellar, and less coarse than the product from just below Ac1 temperature. Case-hardening alloy steels are normally subjected to isothermal annealing. After carburizing, the steels at 900 deg C to 930 deg C, are held at 630 deg C to 680 deg C for 2 hours to 4 hours for the completion of austenite transformation and then cooled. The structure achieved consists of ferrite and pearlite, suitable for most machining operations. Normally, isothermal holding is extended beyond the end of transformation for 1 hour to 2 hours. So, additional improvement of machinability is obtained as a result of partial spheroidization of pearlitic cementite.
Isothermal annealing is best suited for applications in which full advantage can be taken of the rapid cooling to the transformation temperature. The cooling is continued from this temperature to the room temperature. This process improves machinability and a better surface finish during machining. The heat treatment cycle in isothermal annealing is shown in Fig 7.
Fig 7 Heat treatment cycle in isothermal annealing
Recrystallization annealing – Recrystallization is defined as the process in which grains of a crystal structure come in new structure or new crystal shape. Recrystallization is a process by which deformed grains are replaced by a new set of undeformed grains which nucleate and grow until the original grains have been entirely consumed.
Recrystallization annealing is also known as process annealing. It consists of heating the steel below Ac1 temperature and holding it at this temperature for sufficient time and then cooling it. It is an annealing process applied to cold-worked steel to achieve nucleation and growth of new grains without phase change. This heat treatment removes the results of the heavy plastic deformation of highly shaped cold formed parts. It is normally an intermediate operation for processing the steel further. Recrystallization annealing treatment is widely given for steel fabricated by extrusion, stamping, upsetting, and drawing.
Recrystallization annealing is an annealing process which is applied to cold-worked steel to achieve nucleation and growth of new grains without phase change. This heat treatment removes the results of the heavy plastic deformation of highly shaped cold formed parts. The annealing is effective when applied to hardened or cold-worked steels, which recrystallize the structure to form new ferrite grains.
Recrystallization annealing is normally accompanied by a reduction in the strength and hardness of a steel material and a simultaneous increase in its ductility. The temperature at which the steel is to be heated depends on the quantity of prior deformation, chemical composition, holding time, and initial grain size. Normally, alloy steels and high carbon steels need a higher temperature.
Recrystallization annealing process can be introduced as a deliberate step in steel processing or can be an undesirable by-product of another processing step. The most important industrial uses of recrystallization annealing are the softening of steels previously hardened by cold work, which have lost their ductility, and the control of the grain structure in the final product.
Stress relief annealing – Steel materials in the form of large steel castings, welded steel structures or cold formed parts tend to possess internal stresses caused mainly during their manufacture and uneven cooling. This internal stress causes brittleness at isolated locations in the steel materials, which can lead to sudden breakage or failure of the material. The main objective of the stress relief annealing is to remove the internal stresses produced in the steel material because of (i) plastic deformation, (ii) non-uniform cooling, and (iii) phase transformation.
Stress relief annealing process is used to ensure there is reduced risk of distortion of the work piece during machining, welding, or further heat treatment cycles. Stress relief annealing process involves heating the steel material to a temperature of around 600 C to 650 deg C. The temperature is maintained constantly for a few hours and then the steel material is allowed to cool down slowly in still air. No phase transformation takes place during stress relief annealing.
Homogenization annealing – It is an annealing method which is used at the steel plants. This annealing process is somewhat specialized, in that the purpose is to level out segregation in steel ingots or continuously cast strip. Very high temperatures and very long times are used to allow variations in chemistry because of segregation to level out. This segregation is the cause of different hardenability at the ends of a coil. Once ready to be cooled, the ingots or coils are removed from the furnace, and allowed to air-cool. Since air cooling is relatively uncontrolled, variations in grain size and microstructure can occur. This frequently explains the different performance of one plant over the other during forming, even while meeting the procurement specifications
Homogenization annealing process is carried out in the temperature range 1,100 deg C to 1,200 deg C. The diffusion which takes place at this temperature equalizes the composition of steels. This process is adopted to alloy steel ingots which, when solidified after pouring, do not have a homogeneous structure. This lack of structural homogeneity is removed to a large extent during forging and rolling of steel ingots. Where homogeneity has not been corrected completely, the structural homogeneity is to be corrected by homogenizing or diffusional annealing treatment. In this case, the phenomenon of diffusion brings about a uniform concentration of primary grains.
The homogenizing treatment is carried out for several hours at temperatures between 1,150 deg C to 1,200 deg C. After the treatment, the charge is cooled to 800 deg C to 850 deg C, and then further cooled in air. After this treatment, the steel can undergo either normalizing or annealing to refine the over-heated structure. This treatment is applied only in very special cases, since the treatment cost is very high. Fig 8 shows homogenization annealing process.
Fig 8 Homogenization and intermediate annealing processes
Intermediate annealing – Intermediate annealing treatment is carried out after case-hardening in order to carry out further machining such as turning, drilling, and milling, etc. It consists of holding components below the Ac1 temperature, i.e., around 630 deg C to 680 deg C, for 4 hours to 6 hours, followed by slow cooling (Fig 8). The object of this treatment is the same as that of spheroidizing, i.e., improved machinability through the formation of globular cementite.
Incomplete annealing – In process of incomplete annealing, the steel is heated to around upper basic temperature. Both the hypo-eutectic steel or hyper-eutectic steel are given this treatment. The heat treatment process is obtained by slow cooling after thermal insulation. It is largely performed to get spherical pearlite for the hyper-eutectic steel, to remove the internal stress, to decrease the hardness, and to increase the machinability.
Bright annealing – Bright annealing treatment is carried out to achieve a bright surface free from oxides. Protection from oxidation during heat treatment is normally achieved by blanketing the charge with a suitable protective gas atmosphere. The atmosphere gases, however, are to be such as to avoid undesirable effects on the treated steel such as sulphidization, embrittlement, and decarburization, etc. Bright annealing is done in a variety of ways on a wide range of steel materials. The material can be in the form of wire, strip, sheet, tube pressing, etc. The selection of gases used for bright annealing depends on the type of steel. The gases used are pure nitrogen, cracked ammonia, and pure hydrogen, etc.
Solution annealing – Solution annealing (also referred to as solution treating) is a common heat-treatment process for stainless steels. The purpose of solution annealing is to dissolve any precipitates present in the material, and transform the material at the solution annealing temperature into a single-phase structure. At the end of the solution annealing process, the material is rapidly quenched down to room temperature to avoid any precipitation from occurring during cooling through lower temperature ranges. The single-phase solution annealed material is in a soft state after treatment.
The solution annealing treatment is needed prior to age hardening / precipitation hardening. The single-phase microstructure created during solution annealing is needed prior to age hardening, so that only the precipitates formed during age hardening are present in the final product. The composition, size, and quantities of these precipitates formed during aging determine the final product’s hardness, strength, and mechanical properties after aging. It is critical that the structure be properly solution annealed prior to aging in order to meet all of these requirements.
Advantages and disadvantages of annealing
The main advantages of annealing lies in that the process improves the workability of the steel material, increasing toughness, reducing hardness, and increasing the ductility and machinability of the steel material. The heating and cooling process also reduces the brittleness of steels while improving their magnetic properties and electrical conductivity.
The main disadvantage of annealing is that it can be a time-consuming procedure, depending on which steel material is being annealed. Steel materials with high temperature requirements can take a long time to cool sufficiently, especially if they are being left to cool naturally inside an annealing furnace.