 clear session we have already discussed the purposes of annealing heat treatment. Now in this session we will extend the same discussion by studying types of annealing processes. I am Mr. P. P. Mitrabhutri, Associate Professor in Department of Mechanical Engineering at Valtran Institute of Technology, Solapur. The outcome of learning for today's session will be that is students will be able to apply proper type of annealing process for desired changes in properties to be brought about. For all we will discuss what are the various types of annealing. First annealing process is full annealing. Full annealing the name itself suggests that it will solve most of our purposes that is it will refine the grain structure, it will restore the ductility in cold working components, it will fully relieve the stresses after manufacturing, it will restore electrical and magnetic properties, it will soften the steel as well it will prepare the structure for hardening except improving machinability of hyper eutectoid steel and tool steel and except homogenization all other purposes can be solved by full annealing that is why this treatment is called as full annealing treatment and full annealing treatment is always carried out by heating the steel 25 to 30 degrees above a 3 temperature that is if we plot a graph of temperature versus time we can show the treatment full annealing treatment like this heating holding and cooling in furnace very slowly slow heating and holding sufficient holding is very essential to get most desired structure and thereby desired properties. Now next annealing is annealing to restore the ductility of cold working products that is recrystallization annealing in this case crystal structure is restored stresses are relieved in cold working components and further processing can be done and this annealing is a subcritical annealing and it can be shown in the same diagram like this this is recrystallization annealing this is full annealing so we have studied two types of annealing in this now third type of annealing will be stress relief annealing stress relief annealing is basically intended to relieve the stresses developed after manufacturing this process is basically intended for casting casting for forging as well as for welded structures this annealing is very useful and in this stress relief annealing which is also a subcritical annealing which is carried out at around 600 to 650 by heating the steel at a temperature of 600 to 650 degrees Celsius slowly heating it holding it at that temperature and cooling it slowly to room temperature now before moving to the next type of annealing I would like to pose a question to you that is what is the difference between recrystallization annealing and stress relief annealing now next annealing is recovery annealing recovery annealing is carried out at a temperature of 300 by heating the steel to a temperature of 300 to 350 at the most 400 degrees Celsius and cooling it holding it at that temperature for sufficient length of time and cooling it slowly to room temperature recovery annealing will recover electrical and magnetic properties of the steel but will not be able to relieve the stresses completely but 50 to 60 percent of the stresses are relieved now recovery annealing is the best substitute for stress relief annealing in some cases because many of the welded structures or many of the castings are full of stresses and if we allow the stresses to remain as it is that those will cause problems in functioning but if we relieve the stresses totally steel will get softened and we require to retain the hardness under such circumstances we may carry out recovery annealing by which we can relieve this so really reduce the stress level to a minimum without affecting mechanical properties and that is the most important aspect of recovery annealing now next annealing process is important in improving machinability of hyper eutectoid steel and tool steel that is paralyzing annealing in hyper eutectoid steels and in alloy tool steels which are used to manufacture dies and tools used in various processes like forging extrusion we find that machinability of hyper eutectoid steel and tool steel is very poor we are unable to machine these to the required tolerances for that reason we require to make it machinable and for that we have to sporadize the pearlite present in hyper eutectoid steel and tool steel and we have to break the cementite network and carbide network present around pearlite grain boundaries and that is achieved by spheroidizing annealing and the spheroizing annealing is carried out by heating the steel 30 degrees above A1 temperature and holding it there for sufficient length of time and cooling slowly to room temperature we can show this treatment on temperature versus time graph like this now next annealing is homogenization annealing homogenization annealing is carried out to remove the chemical in homogeneity developed in the structure during manufacture and to remove the chemical in homogeneity developed during in the structure during manufacture this homogenization annealing is carried out by heating the steel 100 to 150 degrees above A3 temperature holding it there for sufficiently long period of time and then cooling it very slowly to room temperature this is very essential for homogenization this homogenization annealing is also known as diffusion annealing here diffusion of alloying elements from one location to another location is most important for diffusion we require to have very high temperature as also we require to give sufficient time and by holding at very high temperature at around 1050 or 1100 degree Celsius we can carry out homogenization of the structure but in homogenization after homogenization is achieved we invite another trouble that is our structure is homogenized but due to prolonged holding at very high temperature we have very coarse grained structure developed in the steel and this coarse grained structure will hamper many of the properties of the steel so after homogenization annealing it is always mandatory to carry out full annealing so that we can have refinement in grain size and structure so you might have learned that homogenization annealing can alone can never be a heat treatment suggested for alloy steel castings after homogenization annealing there will always be a full annealing treatment and along with this there is one more treatment called as isothermal annealing this isothermal annealing is carried out by heating the steel above a 3 temperature cooling it below a 1 temperature slowly and holding it at that temperature for sufficient length of time for a long period and then cooling it slowly but this isothermal annealing is not practically exercised in many of the industries many of the industries will exercise all the treatments that we have discussed now out of these three six treatments we find that our stress relief annealing recretation annealing and recovery annealing are the treatments which are carried out below a 1 temperature so these treatments can also be regarded as a subcritical annealing so today we have discussed most in metallurgy by Avner so hope you have understood the thing have a nice day thank you