Annealing of Steels
Annealing of Steels
Annealing is a process of heat treatment that alters the properties of steel to increase its ductility and to make it more workable. It involves heating the steel to slightly above its critical temperature, soaking at that temperature for a time sufficient to allow the necessary changes to occur and then cooling at a predetermined rate (Fig 1)
Fig 1 Heating range of different annealing processes
There are two main reasons for annealing. The first is to soften the steel material and remove the stresses. The second is to homogenize the structure of the steel material. By the process of annealing the properties of steel material are enhanced to meet machinability requirements. Annealing process induces ductility, improves toughness, softens the steel, relieves internal stresses, refines the structure by making it homogeneous, and improves cold working properties. Annealing also prepares the steel for further heat treatment.
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 amount 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 that facilitates returning the cold worked steel material to its stress free state has many 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, because 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.
The three stages of the annealing process that proceed as the temperature of the steel material is increased are namely (i) recovery, (ii) recrystallization, and (iii) grain growth. Recovery results in softening of the steel material through removal of primarily linear defects called dislocations and the internal stresses they cause. Recovery occurs at the lower temperature stage of all annealing process and before the appearance of new strain free grains. The grain size and shape do not change during this stage. During the second stage of recrystallization new strain free grains nucleate and grow to replace those deformed by internal stresses. If annealing is allowed to continue once recrystallization has completed, then grain growth (the third stage) occurs. In grain growth, the microstructure starts to coarsen and may cause the steel material to lose a substantial part of its original strength. This can however be regained with hardening.
The high temperature of annealing may 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 CO, H2, and N2 gas or a mixture of hydrogen and nitrogen.
Types of annealing
There are different types of annealing processes. These processes are described below.
- Full annealing – The process involves slowly heating the steel material (carbon less than 0.9 %) to a temperature which is 30 to 50 deg C above the upper critical temperature point of steel as indicated by the Fe – Fe3C equilibrium diagram. For higher carbon steels the temperature is 50 deg C above the lower critical temperature. It is held at this temperature for sufficient period of time for soaking to allow the microstructure of the complete steel material to transform. The steel material is then allowed to slowly cool down inside the furnace to room temperature without any forced means of cooling. Hot worked steels, forgings, and castings made from medium and high carbon steels need full annealing.
- Process annealing – Cold worked steels normally tend to posses increased hardness and decrease ductility making it difficult to work further. Process annealing tends to improve these characteristics. Process annealing, also called intermediate annealing, subcritical annealing, or in-process annealing, is a heat treatment cycle that 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 mainly suited for low carbon steels. The steel material is heated up to a temperature just below the lower critical temperature of steel and held there long enough to relieve stresses in the metal. The piece is then furnace cooled. It can then be subjected to additional cold working.
- 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. 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. This process involves heating the steel materials to a temperature of around 600 – 650 deg C. The temperature is maintained constantly for a few hours and then the steel materials are allowed to cool down slowly in still air.
- Spheroidize annealing – Spheroidizing is a process of heating and cooling steel material that produces a rounded or globular form of carbide in a matrix of ferrite. Spheroidizing annealing process is used for those high carbon steels and alloy 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. This can be done by three methods. In the first method the steel material is heated just below the lower critical temperature of about 700 – 750 deg C and the temperature is maintained for about 8 hours before the steel material is allowed to cool down slowly. In the second method heating and cooling of the steel material is done alternatively between temperatures just above and below the lower critical temperature and then the material is cooled slowly. In the third method which is for tool and alloy steels, the steel material is heated to a temperature around 750 – 800 deg C and held at that temperature for several hours before slow cooling.
- Isothermal annealing – This is a process where is steel material is heated above the upper critical temperature. This causes the structure of the steel material to be converted rapidly into austenite structure. The steel material is then cooled to a temperature below the lower critical temperature of around 600 to 700 deg C. This cooling is done by means of forced cooling. The temperature is then maintained constant for a specified period of time in order to produce a homogenous structure in the steel material before cooling. The process is usually employed for low carbon and alloy steels to improve their machinability.