Annealing (Metallurgy)
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Annealing (metallurgy) Annealing, in metallurgy and materials science, is a heat atoms and the dissolution of carbides within the steel.[3] treatment that alters the strength and sometimes chemi- Hardness decreases and ductility increases, because dis- cal properties of a material to increase its ductility and locations are eliminated and the metal’s crystal lattice to make it more workable. It involves heating a mate- is altered. On heating to a specific temperature atoms rial to above its glass transition temperature, maintaining will migrate within the lattice and the adjusted grain can a suitable temperature, and then cooling. Annealing can change the mechanical properties. induce ductility, soften material, relieve internal stresses, refine the structure by making it homogeneous, and im- prove cold working properties. 1.1 Stages In the cases of copper, steel, silver, and brass, this pro- cess is performed by heating the material (generally until The three stages of the annealing process that proceed glowing) for a while and then slowly letting it cool to room as the temperature of the material is increased are: re- temperature in still air. Copper, silver[1] and brass can be covery, recrystallization, and grain growth. The first cooled slowly in air, or quickly by quenching in water, un- stage is recovery, and it results in softening of the metal like ferrous metals, such as steel, which must be cooled through removal of primarily linear defects called dis- slowly to anneal. In this fashion, the metal is softened and locations and the internal stresses they cause. Recov- prepared for further work—such as shaping, stamping, or ery occurs at the lower temperature stage of all anneal- forming. ing processes and before the appearance of new strain- free grains. The grain size and shape do not change.[4] The second stage is recrystallization, where new strain- 1 Thermodynamics free grains nucleate and grow to replace those deformed by internal stresses.[4] If annealing is allowed to continue once recrystallization has completed, then grain growth Annealing occurs by the diffusion of atoms within a solid (the third stage) occurs. In grain growth, the micro struc- material, so that the material progresses towards its equi- ture starts to coarsen and may cause the metal to lose a librium state. Heat increases the rate of diffusion by pro- substantial part of its original strength. This can however viding the energy needed to break bonds. The movement be regained with hardening. of atoms has the effect of redistributing and eradicating the dislocations in metals and (to a lesser extent) in ce- ramics. This alteration to existing dislocations allows a metal object to deform more easily, increasing its ductil- 2 Controlled atmospheres ity. The amount of process-initiating Gibbs free energy in a The high temperature of annealing may result in oxida- deformed metal is also reduced by the annealing process. tion of the metal’s surface, resulting in scale. If scale In practice and industry, this reduction of Gibbs free en- must be avoided, annealing is carried out in a special ergy is termed stress relief. atmosphere, such as with endothermic gas (a mixture of carbon monoxide, hydrogen gas, and nitrogen gas). An- The relief of internal stresses is a thermodynamically nealing is also done in forming gas, a mixture of hydrogen spontaneous process; however, at room temperatures, it and nitrogen. is a very slow process. The high temperatures at which annealing occurs serve to accelerate this process. The magnetic properties of mu-metal (Espey cores) are introduced by annealing the alloy in a hydrogen atmo- The reaction that facilitates returning the cold-worked sphere. metal to its stress-free state has many reaction pathways, mostly involving the elimination of lattice vacancy gradi- ents within the body of the metal. The creation of lattice vacancies is governed by the Arrhenius equation, and the 3 Setup and equipment migration/diffusion of lattice vacancies are governed by Fick’s laws of diffusion.[2] The decarburization mecha- Typically, large ovens are used for the annealing pro- nism can be described as three distinct events: the reac- cess. The inside of the oven is large enough to place the tion at the steel surface, the interstitial diffusion of carbon workpiece in a position to receive maximum exposure to 1 2 5 SPECIALIZED CYCLES the circulating heated air. For high volume process an- 5.2 Process annealing nealing, gas fired conveyor furnaces are often used. For large workpieces or high quantity parts, car-bottom fur- Process annealing, also called intermediate annealing, naces are used so workers can easily move the parts in subcritical annealing, or in-process annealing, is a heat and out. Once the annealing process is successfully com- treatment cycle that restores some of the ductility to a pleted, workpieces are sometimes left in the oven so the product during the process of cold working, so it can be parts cool in a controllable way. While some workpieces worked further without breaking further heat treatment are left in the oven to cool in a controlled fashion, other cycles. materials and alloys are removed from the oven. Once The temperature range for process annealing ranges from removed from the oven, the workpieces are often quickly 260 °C (500 °F) to 760 °C (1400 °F), depending on the cooled off in a process known as quench hardening. Typi- alloy in question. cal methods of quench hardening materials involve media such as air, water, oil, or salt. Salt is used as a medium for quenching usually in the form of brine (salt water). Brine provides faster cooling rates than water. This is because 5.3 Full anneal when an object is quenched in water air bubbles form on the surface of the object reducing the surface area the water is in contact with. The salt in the brine reduces the formation of air bubbles on the object’s surface, meaning there is a larger surface area of the object in contact with the water, providing faster cooling rates. Quench harden- ing is generally applicable to some ferrous alloys, but not copper alloys. 4 Diffusion annealing of semicon- ductors Full annealing temperature ranges In the semiconductor industry, silicon wafers are an- A full anneal typically results in the second most duc- nealed, so that dopant atoms, usually boron, phosphorus tile state a metal can assume for metal alloy. It creates a or arsenic, can diffuse into substitutional positions in the new uniform microstructure with good dynamic proper- crystal lattice, resulting in drastic changes in the electrical ties. To perform a full anneal on steel for example, steel properties of the semiconducting material. is heated to 50°C above the austenic temperature and held for sufficient time to allow the material to fully form austenite or austenite-cementite grain structure. The ma- terial is then allowed to cool slowly so that the equilibrium microstructure is obtained. In some cases this means the 5 Specialized cycles material is allowed to air cool. In other cases the material is allowed to furnace cool. The details of the process de- pend on the type of metal and the precise alloy involved. 5.1 Normalization In any case the result is a more ductile material but a lower yield strength and a lower tensile strength. This process is Normalization is an annealing process applied to ferrous also called LP annealing for lamellar pearlite in the steel alloys to give the material a uniform fine grained structure industry as opposed to a process anneal, which does not and make it less brittle. It is used on steels of less than specify a microstructure and only has the goal of soften- 0.4% carbon to transform austenite into ferrite, pearlite ing the material. Often the material to be machined is and sorbite. It involves heating the steel to 20-50 Kelvin annealed, and then subject to further heat treatment to above its upper critical point. It is soaked for a short achieve the final desired properties. period at that temperature and then allowed to cool in air. Smaller grains form that produce a tougher, more ductile material. It eliminates columnar grains and den- 5.4 Short cycle anneal dritic segregation that sometimes occurs during casting. Normalizing improves machinability of a component and Short cycle annealing is used for turning normal ferrite provides dimensional stability if subjected to further heat into malleable ferrite. It consists of heating, cooling and treatment processes. then heating again from 4 to 8 hours. 3 6 Resistive heating 9 Further reading Resistive heating can be used to efficiently anneal copper • Thesis of Degree, Cable Manufacture and Tests wire; the heating system employs a controlled electrical of General Use and Energy. - Jorge Luis Pedraz short circuit. It can be advantageous because it does not (1994), UNI, Files, Peru. require a temperature-regulated furnace like other meth- • Dynamic annealing of the Copper wire by using ods of annealing. a Controlled Short circuit. = Jorge Luis Pedraz The process consists of two conductive pulleys (step pul- (1999), Peru: Lima, CONIMERA 1999, INTER- leys), which the wire passes across after it is drawn. CON 99, The two pulleys have an electrical potential across them, which causes the wire to form a short circuit. The Joule effect causes the temperature of the wire to rise to ap- 10 External links proximately 400 °C. This temperature is affected by the rotational speed of the pulleys, the ambient temperature, • Annealing with induction: Ameritherm offers an- and the voltage applied. Where t is the temperature of nealing overview and Application Notes the wire, K is a constant, V is the voltage applied, r is the number of rotations of the pulleys per minute, and tₐ is • Annealing:efunda - engineering fundamentals the ambient temperature: • Full Annealing:Material Science t = ((KV ²)/(r))+tₐ • The constant K depends on the diameter of the pulleys Annealing: Aluminum and Aircraft Metal Alloys and the resistivity of the copper.