What is Annealing?
Annealing is described in metallurgy or metal science as the process of heat treatment that is done to alter the physical and sometimes the chemical properties of the substance to reduce its hardness and make it workable. Annealing improves ductility by reducing the hardness of the substance. On the other hand process annealing is the process of softening the steel by heating it to a temperature that is near but less than its transformation temperature and then is cooled slowly. In order to restore the ductility lost of a piece of metal, the process annealing is used intermittently for the metal by repeated hammering or other working. Full annealing involves the heating of the material over its recrystallization point and then maintaining a temperature for a suitable amount of time and then cooling off the material slowly.
In the process of annealing, the atoms start migrating into the crystal lattice and thus the number of dislocations decreases and hence it reduces the hardness and improves the ductility. As the heat is slowly taken out, it again recrystallizes. In annealing, the heating rate and the cooling rate determines the crystal grain size and phase composition of many alloys including steel that is the main deciding factor of the properties of the materials. Sometimes further heat is required after the hot working and the cold working to further achieve the properties required because the structure of the material changes after the later working is imposed. If you have a phase diagram and a good understanding of the composition, you can change the material's hard and brittle property to soft and ductile.
After annealing, the ferrous alloys, such as steel, are cooled slowly for a prolonged period of time in still air till it does not come back to room temperature. But for the other metals like silver, copper and brass, are cooled very quickly either in the air or quenched into water post-annealing. In such a manner, the metals are first softened to put it to further work like stamping, forming and shaping.
Simulated annealing is a general and effective method of stimulation. Thus for obtaining the global optimum of a given function, it is considered as the probabilistic technique. Thus for optimization of a problem, it is a high-level procedure for the global optimization of a larger space. It is often used for searching spaces that are discrete in nature. In places where the global space optimization for a larger scale is required than a specific local optimization for a fixed period of time, simulated annealing is preferred over exact algorithms such as gradient descent or branch and bounds.
Annealing in metallurgy is always referred to as a process of heating and then controlled cooling to alter the physical and chemical properties of a material to make it applicable to certain needs. Both are the features of the material that is being annealed and depends on the thermodynamics free energy or Gibbs free energy. Thermodynamics free energy and Gibbs energy, both are affected by the heating and the cooling process of the materials. Thus simulated annealing is used for very complex computational optimization problems where exact algorithms fail although it only provides an approximation solution to the global minimum. But this solution is working well and is considered enough for many practical problems. The problems that are solved by SA are currently accumulated and formulated with the help of an objective function of many random variables that are the subject of several constraints. Thus in real life, the constraints of the SA can be subjected to penalty for being a part of the objective function.
Thus in the algorithm of the SA slow cooling is subjected to the slow decrease in the probability of acquiring the worst solution as the solution space is explored. As because here in the algorithm of SA, accepting the worst solution means looking extensively for the search results of the global optimal solutions. Thus in general the simulated annealing works as follows. As the temperature decreases from positive towards zero, the algorithm chooses a solution that is near the current solution at each step. Then the quantity of the solution that is selected by the algorithm is calculated and moved to it according to the temperature-dependent probabilities. The probabilities are to select the better or the worst solutions which remain at 1 or decrease to zero during the search. This stimulation can be performed in two ways, either by the stochastic sampling method or by the chemical kinetics method for density functions.
Types of Annealing
The commonly used methods of Annealing are divided into two major categories, namely, annealing involving phase change crystallization above critical temperature and annealing below the critical temperature.
1. Annealing above the Critical Temperature.
2. Annealing below the Critical Temperature
Thus the six Major Types of Annealing Process are as Follows:
Complete Annealing: In Complete Annealing, the steel is heated at 32 to 50-degree centigrade which is above the critical temperature of the steel and then the temperature is maintained for a specific period of time and then the heat is preserved for a period of time after it is cooled slowly. The rate of cooling is about 10 degrees centigrade per hour. The material is allowed to cool slowly inside the furnace without any forced cooling. After completing the annealing, the steel is used for casting and forging which is made up of medium to high carbon steel. The low carbon steel has low hardness and therefore it cannot be matched for machining. The carbon steel is precipitated in a mesh along the grain boundaries after the completion of the annealing process. By this process, the hardness, plasticity, strength and toughness of the steel is significantly reduced.
Diffusion Annealing: As because in this process the iron and carbon diffuses with each other their food this process is referred to as diffusion annealing process. The steel here is heated above the upper critical temperature because the diffusion requires a higher temperature. Thus, the temperature here is about 1000 to 1200°C. The heat preservation time in this process is about 10 to 15 hours. After the diffusion, the annealing process is completed, the complete annealing process followed by normalizing is done to refine tissues. High-quality steel and segregation of series alloy Steel casting and ingots are done through this process.
Incomplete Annealing: The steel in this process is heated to the upper critical temperature. After thermal insulation, the heat treatment process is done by slow cooling.
Isothermal Annealing: As with other annealing processes in isothermal processing the steel is heated above the upper critical temperature. After doing so the steel rapidly converts into an austenite structure. This steel is then cooled after that it is below the lower critical temperature which is 600 to 700 degrees Celsius. Here the cooling is done by the forced method. The temperature is then maintained for a specific period of time in order to generate the homogeneous structure of the material. The isothermal annealing process is applied to the low carbon and alloy steel so as to improve the machinability of the grains.
Spheroidizing Annealing: Spheroidizing annealing process is done for the high carbon and alloy steel in order to improve their machinability. In this annealing process, the steel is heated to a temperature that is below A1 temperature and then the temperature is maintained for some time before it is slowly cooled. The time required for holding on to the temperature is about 15 to 25 hours. This is mainly applied to eutectoid steel and hypereutectoid steel such as carbon tool steel, alloy tool steel, bearing steel etc.
Stress Relief Annealing: In this process, the metal is heated to a lower temperature of about 650 degrees and then the temperature is maintained for some time so as to remove the internal stress of the metal. Then it is subjected to slow cooling which is uneven cooling; the large casting and welding structure contains internal stress that is mainly caused during their manufacturing. There is no phase transformation during the stress relief annealing process.
FAQs on Annealing
1. What is the major purpose of the annealing process?
2. What is the difference between tempering and annealing?