What Is Annealing? Annealing is a heat treatment process used mostly to increase the ductility and reduce the hardness of a material. This change in hardness and ductility is a result of the reduction of dislocations in the crystal structure of the material being annealed. Annealing is often performed after a material has undergone a hardening or cold working process to prevent it from brittle failure or to make it more formable for subsequent operations. Why Is Metal Annealed? As mentioned above, annealing is used to reduce hardness and increase ductility. Changing these mechanical properties through annealing is significant for many reasons: • Annealing improves the formability of a material. Hard, brittle materials can be difficult to bend or press without creating a material fracture. Annealing helps eliminate this risk. • Annealing can also improve machinability. A material that is extremely brittle can cause excessive tool wear. Reducing the hardness of a material via annealing can reduce the wear on the tool being used. • Annealing removes residual stresses. Residual stresses can create cracks and other mechanical complications, and it is often best to eliminate them whenever possible. What Metals Can Be Annealed? To perform an annealing process, a material that can be altered by heat treatment must be used. Examples include many types of steel and cast iron. Some types of aluminum, copper, brass and other materials may also respond to an annealing process. The Annealing Process There are three main stages to an annealing process. 1. Recovery stage. 2. Recrystallization stage 3. Grain growth stage Recovery Stage During the recovery stage, a furnace or other type of heating device is used to raise the material to a temperature where its internal stresses are relieved. Recrystallization Stage During the recrystallization stage, the material is heated above its recrystallization temperature, but below its melting temperature. This causes new grains without pre-existing stresses to form. Grain Growth Stage During the grain growth, the new grains fully develop. This growth is controlled by allowing the material to cool at a specified rate. The result of completing these three stages is a material with more ductility and reduced hardness. Subsequent operations that can further alter mechanical properties are sometimes carried out after the annealing process. When Are Annealed Metals Used? Common applications for annealed metals include: • Work-hardened materials such as sheet metal that has undergone a stamping process or cold drawn bar stock. • Metal wire that has been drawn from one size to a smaller size may also undergo an annealing process. • Machining operations that create high amounts of heat or material displacement may also warrant an annealing process afterward. • Welded components can create residual stresses in the area of the material exposed to elevated temperatures; to recreate uniform physical properties, annealing is often used.

What is Quenching? Quenching is a type of metal heat treatment process. Quenching involves the rapid cooling of a metal to adjust the mechanical properties of its original state. To perform the quenching process, a metal is heated to a temperature greater than that of normal conditions, typically somewhere above its recrystallization temperature but below its melting temperature. The metal may be held at this temperature for a set time in order for the heat to “soak” the material. Once the metal has been held at the desired temperature, it is quenched in a medium until it returns to room temperature. The metal also may be quenched for an extended period of time so that the coolness from the quenching process is distributed throughout the thickness of the material. Quenching Media There are a variety of quenching media available that can perform the quenching process. Each media has its own unique quenching properties. Considerations for the type of media use include quenching speed, quenching media environmental concerns, quenching media replacement, and quenching media cost. Here are the main types of quenching media: o Air o Oil o Water o Brine Air Air is a popular quenching media used to cool metals for quenching. Affordability is one of the main benefits of air; its affordability is a result of its profusion on earth. In fact, any material that is heated and then allowed to cool to room temperature simply by being left alone is considered to have been air quenched. Air quenching is also more intentionally performed when it is compressed and forced around the metal being quenched. This cools the part more rapidly than still air, although even compressed air may still cool many metals too slowly to alter the mechanical properties. Oil Oil is able to quench heated metals much more rapidly than compressed air. To quench with oil, a heated part is lowered into a tank that is filled with some type of oil. The oil can also be flushed through the part. Different types of oil are often used depending on the application because of their varying cooling rates and flash points. Water Water is able to quench heated metals rapidly as well. It can cool a metal even faster than oil. In a fashion similar to oil quenching, a tank is filled with water and the heated metal is submerged in it. It can also be flushed through a part. One benefit of water is that flammability of the media is not a concern. Brine Brine is a mixture of water and salt. Brine cools faster than air, water, and oil. The reason for this that the salt and water mixture discourages the formation of air globules when it is placed in contact with a heated metal. This means that more of the surface area of the metal will be covered with the liquid, as opposed to air bubbles. Quench Hardening Steel Steel deserves a special mention when the quenching process is being discussed because its mechanical properties are very sensitive to quenching. Through a quenching process known as quench hardening, steel is raised to a temperature above its recrystallization temperature and rapidly cooled via the quenching process. The rapid quenching changes the crystal structure of the steel, compared with a slow cooling. Depending on the carbon content and alloying elements of the steel, it can get left with a harder, more brittle microstructure, such as martensite or bainite, when it undergoes the quench hardening process. These microstructures result in increased strength and hardness for the steel. However, they do leave the steel vulnerable to cracking and with a large reduction in ductility. For this reason, some steels are annealed or normalized following the quench hardening process.

What is solution annealing? Solution annealing (also referred to as solution treating) is a common heat-treatment process for many different families of metals. Stainless steels, aluminium alloys, nickel-based super alloys, titanium alloys, and some copper-based alloys all may require solution annealing. Purpose of solution annealing? 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 will be in a soft state after treatment. Why is solution annealing required? The solution annealing treatment is required prior age hardening / precipitation hardening. The single phase microstructure created during solution annealing is required prior to age hardening, such that only the precipitates formed during age hardening will be present in the final product. The composition, size, and quantities of those precipitates formed during aging will determine the final product's hardness, strength, and mechanical properties after aging. It is critical that the structure be properly solution treated prior to aging in order to meet all of these requirements. How does the process work? The Solution Annealing temperature is dependent on the material. High process temperatures are usually applied to stainless steels and exotic material ranges at temperatures from 1000°C plus. Nonferrous materials are much lower at 500°C plus. Acier Alloy India pvt ltd has the facility to achieve high temperatures very quickly, or to apply programmed ramp rates should the component require it. Importantly we load onto Ni /Cr furnace trays (RA330) to provide good support at high temperatures, eliminating the risk of cross contamination from carbon steel. The design of the tray allows free movement of heat during the heating cycle and quenching during the quench cycle. Which materials can be treated? Solution Annealing can be applied to stainless steel, duplex, super duplex and bronze alloys. Certain tool steels can be air cooled to harden them up, especially ones with high cobalt content. Rapid air blast may be required if the section size is vast.

What is hardening? Hardening is a metallurgical metalworking process used to increase the hardness of a metal. The hardness of a metal is directly proportional to the uniaxial yield stress at the location of the imposed strain. A harder metal will have a higher resistance to plastic deformation than a less hard metal. Material hardening of metal is required for these major applications • Machine cutting tools (drill bits, taps, lathe tools) need be much harder than the material they are operating on in order to be effective. • Knife blades – a high hardness blade keeps a sharp edge. • Bearings – necessary to have a very hard surface that will withstand continued stresses. • Armour plating - High strength is extremely important both for bullet proof plates and for heavy duty containers for mining and construction. • Anti-fatigue - Martensitic case hardening can drastically improve the service life of mechanical components with repeated loading/unloading, such as axles and cogs. Result of hardening The use of this treatment will result in an improvement of the mechanical properties, as well as an increase in the level of hardness, producing a tougher, more durable item. Alloys are heated above the critical transformation temperature for the material, then cooled rapidly enough to cause the soft initial material to transform to a much harder, stronger structure. Alloys may be air cooled, or cooled by quenching in oil, water, or another liquid, depending upon the amount of alloying elements in the material. Hardened materials are usually tempered or stress relieved to improve their dimensional stability and toughness.

What is stress relieving? Stress relieving is carried out on metal products in order to minimise residual stresses in the structure thereby reducing the risk of dimensional changes during further manufacturing or final use of the component. Benefits of stress relieving Machining, and cutting, as well as plastic deformation will cause a build-up of stresses in a material. These stresses could cause unwanted dimension changes if released uncontrolled, for example during a subsequent heat treatment. To minimise stresses after machining and the risk for dimension changes the component can be stress relieved. Stress relieving is normally done after rough machining, but before final finishing such as polishing or grinding. Parts that have tight dimensional tolerances, and are going to be further processed, for example by nitro carburising, must be stress relieved. Welded structures can be made tension free by stress relieving. Application of stress relieving Stress relieving does not change the material’s structure and does not significantly affect its hardness. Hardened and tempered parts to be stress relieved must be treated at a temperature around 50°C below the temperature used for previous tempering to avoid an impact on the hardness. Stress relieving before nitro carburising should be executed at temperatures >600°C.Copper and brass components can also be stress relieved. For stainless steels a high temperature solution heat treatment is normally necessary. Process details The stress relieving temperature is normally between 550 and 650°C for steel parts. Soaking time is about one to two hours. After the soaking time the components should be cooled down slowly in the furnace or in air. A slow cooling speed is important to avoid tensions caused by temperature differences in the material, this especially important when stress relieving larger components. If necessary, stress relieving can be performed in a furnace with protective gas, to protect surfaces from oxidation. In extreme conditions vacuum furnaces can be used. The temperature for stress relieving copper parts is, depending on the alloy, 150-275°C and for brass components 250-500°C.

What is double tempering? Double tempering is simply a process whereby the steel is heated and then cooled twice in succession, not necessarily at the same temperature each time. A study published in the Journal of Nuclear Materials entitled “Effect of Twice Quenching and Tempering on the Mechanical Properties and Microstructures of SCRAM Steel for Fusion Application” found that twice-tempered steel’s strength depended mostly on the temperature of the second tempering, and that a decrease in temperature from 1,033 to 1,013 degrees Kelvin between the first and second tempering processes did increase the steel’s overall strength. Double tempered steel uses Steel is used in the fabrication of large construction materials, smaller tools or industrial applications, or simple mechanisms such as bedsprings. Many mattress companies brag that their mattress coils are composed of twice-tempered steel, making them stronger and less liable to sag over the years. The steel used for construction of tools in industrial metal, composite or ceramics production industries also benefits from double tempering, though at lower temperatures than required to temper it for nuclear applications, usually only between 450 and 540 degrees Celsius.