Annealing heat treatment process changes the physical and sometimes also the chemical properties of a material to increase ductility and reduce the hardness to make it more workable. Annealing heat treatment process increases machinability of the metal alloy and makes it soft. Many a times annealing process is also referred to as Stress Relief heat treatment and normalizing material processing.
The annealing heat treatment process requires the material to be heated above its recrystallization temperature for a set amount of time before cooling in the furnace air. The cooling rate depends upon the types of metals being annealed. For example, ferrous metals such as steel are usually left to cool down to room temperature in still air while copper, silver and brass can either be slowly cooled in air or quickly quenched in water.
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.
Hot or cold working of the pieces of metal following annealing alters the material structure once more, so further heat treatments may be required to attain the desired properties.
However, with knowledge of material composition and phase diagram, heat treating can soften metals and prepare them for further working such as forming, shaping and stamping, as well as preventing brittle failure.
Annealing Heat Treatment process details
Table of Contents
Annealing is a generic term and can be further classified according to temperature and atmosphere.
For steels, subcritical annealing takes place at 538°C – 649°C, so there is no crystal structure change. Intermediate annealing is carried out at 649°C – 760°C, so there is some transformation to austenite and full annealing involves completely austenitizing the work at 816°C – 927°C.
Parts can be annealed in a vacuum or reducing atmosphere where a bright surface finish is needed. Annealing in air is employed where surface finish is not an important factor and an endothermic/neutral atmosphere may be used during annealing to control decarburization.
Heat Treatment – Types (Including Annealing), Process and Structures (Principles of Metallurgy)
How does an Annealing Furnace Work?
An annealing furnace works by heating a 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.
Annealing works in three stages – the recovery stage, recrystallization stage and the grain growth stage. These work as follows:
1. Recovery Stage
This stage is where the furnace or other heating device is used to raise the temperature of the material to such a point that the internal stresses are relieved.
2. Recrystallization Stage
Heating the material above its recrystallization temperature but below its melting point causes new grains to form without any residual stresses.
3. Grain Growth Stage
Cooling the material at a specific rate causes new grains to develop. After which the material will be more workable. Subsequent operations to alter mechanical properties can be carried out following annealing.
When is Annealing Heat Treatment Process done and Why is annealing process Important?
Annealing is done to soften the material to make it workable or increase it’s machinability. Also during cold working processes such as bending, cold forming or drawing, metals often accumulate stress and to remove stress, annealing or normalizing process is carried out. If the material becomes too hard it can make working impossible or result in cracking.
By heating the material above the recrystallization temperature, it is made more ductile and therefore ready to be worked once more. Annealing also removes stresses that can occur when welds solidify. Hot rolled steel is also shaped and formed by heating it above the recrystallization temperature. While steel and alloy steel annealing is common, other metals can also benefit from the process, such as aluminum, brass, and copper.
Metal fabricators use annealing to help create complex parts, keeping the material workable by returning them close to their pre-worked state. The process is important in maintaining ductility and reducing hardness after cold working. In addition, some metals are annealed to increase their electrical conductivity. Sometimes annealing is also referred to as Normalizing and Stress Relief but stress relief is often carried out post welding on metal structures.
Complete Annealing heat treatment process
The full annealing process consists of heating to the proper temperature and then cooling slowly, through the transformation range, in the furnace. The purpose of annealing is to produce a refined grain, to induce softness, improve electrical and magnetic properties, and sometimes to improve machinability.
Annealing involves several thermal cycles, classified on the basis of the maximum temperature reached and below are the few annealing process examples:
- • Subcritical Annealing: Heating below the lower critical temperature.
- • Intercritical Annealing: Heating above but below upper critical temperature for hypoeutectoid and for hypereutectoid steels.
- • Full Annealing: Heating above upper critical temperature.
Can Annealing be Used with Alloys?
Annealing can be carried out with alloys, with a partial or full anneal being the only methods used for non-heat treatable alloys. The exception to this is with the 5000 series alloys, which can be given low temperature stabilization treatments.
Alloys are annealed at temperatures of between 300-410°C, depending on the alloy, with heating times ranging from 0.5 to 3 hours, depending on the size of the workpiece and the type of alloy. Alloys need to be cooled at a maximum rate of 20°C per hour until the temperature is reduced to 290°C, after which the cooling rate is not important.
Advantages
The main advantages of annealing are in how the process improves the workability of a material, increasing toughness, reducing hardness and increasing the ductility and machinability of a metal.
The heating and cooling process also reduces the brittleness of metals while enhancing their magnetic properties and electrical conductivity.
Disadvantages
The main drawback with annealing is that it can be a time consuming procedure, depending on which materials are being annealed. 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.
Applications
Annealing is used across a variety of industries where metals need to be worked into complex structures or worked on several times. Tempering heat treatment is also required sometimes post hardening process.
What are Different Types of Annealing Furnaces that we manufacture?
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Car Bottom Furnace (Bogie Hearth Furnace manufacturer)
The ability to load with an overhead crane, forklift or other methods provides options that make this type of furnace a common choice. These car bottom furnaces can be designed to operate at low temperatures for tempering processes as well as higher temperature annealing and even higher temperature reheating or forging. The temperature range is available from 450°C upto 1200°C and batch load capacity from 150 kgs upto 20,000kgs (20MT).
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Vacuum Annealing Furnace manufacturer in India
Vacuum is annealing carried out at a sub-atmospheric pressure. Similar to bright annealing, it protects the metal from coming into contact with the standard atmospheric gases during the annealing process. However, while bright annealing substitutes in a controlled atmosphere, vacuum annealing removes the atmosphere altogether. Though gases must still be employed to help cool down the metals within set amounts of time.
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Pusher Furnaces manufacturer in India
Pusher furnaces come with a hydraulic push system that pushes the trays or baskets containing the charge inside the of the refractory hearth of the furnace. Maan Global Industries is the leading manufacturer of Pusher furnace in India, that custom designs pusher furnaces according the production rate of the customer and process requirements. Pusher furnaces are ideal for forging companies that require continuous billet re-heating.
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Pit Furnace for Annealing and Stress relief
Pit furnaces are widely used across various industries in heat treatment processes whether it’s annealing, vacuum annealing, hardening or tempering. Pit furnaces offer versatility in annealing process as different shapes and sizes of material can be annealed in a pit furnace. Material is loaded using an overhead crane in the furnace and due to cylindrical hearth, material is heated uniformly from all the sides. Pit furnaces are used for steel wire annealing purpose, coil annealing, pipe annealing and material which is long and heavy. Vacuum annealing can also be done in a Pit furnace efficiently. Temperature range 450°C upto 1200°C
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Bell Furnace for Annealing and Stress Relief
Bell furnaces are highly versatile due to their design and construction. Bell furnaces are used for vacuum annealing of large batches of material such as wires, coils and large structures. Bell furnace allows uniform heating, low power consumption due to it’s sealed design and can be used in reducing atmosphere as well.
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Roller Hearth Furnaces for Bright Annealing
Roller hearth bright annealing furnace is common in stainless steel tube industry. A roller hearth furnace allows steel tubes in batches to slide inside the tunnel of the furnace which has an inert atmosphere especially Nitrogen that prevents oxidation of the steel tubes while annealing at 1100°C and tubes come out bright in stainless color. Similarly, Mesh Belt furnace are also same as roller heart furnace and depending on the type of material to be annealing continuously, a roller or a mesh belt design is chosen for the furnace.
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