Annealing • Annealing is the process of heating a metal which is in a metastable of distorted structural state, to a temperature which will remove the instability or distortion and then cooling is so that the room temperature structure is stable and/or strain free. • Classification: -Full annealing -Bright annealing -Box annealing -Isothermal (cycle) annealing -Spheroidise annealing -Subcritical annealing Purpose of annealing • Inducing a completely stable structure • Refining and homogenizing the structure • Reducing hardness • Improving machinability • Producing desired microstructure • Removing residual stresses • Removing gases • Improving mechanical,physical,electrical and magnetic properties • Improving cold working, characteristics for facilitating further cold work Full annealing • The term full annealing is used for the treatment that involves heating a steel to its austenitic state before subjecting it to slow cooling. • PROCESS:The process consists of heating the steel to above A3 temperature for hypoeutectoid steels and above A1 temperature for hypereutectoid steels by 30-500C,holding at this temperature for a definite period and slow cooling to below A1 or to room temperature.To insure equalization of temperature throughout the cross section of the component and complete austenization, a holding period of at least 20minutes per cm of thickest section is necessary. • Hypereutectoid steels always annealed from above A1 temperature and never annealed from above Acm temperature because of the following reason: 1.)If slowly cooled from above Acm temperature, proeutectoid cementite separates along the grain boudaries of austenite and completely envelopes the austenite grains which transform to pearlite at A1 temperature.The microstructure of such a steel at room temperature shows a continuous network of cementitic areas all around the pearlite regions.Due to this the dislocations get blocked at cementitic regions and are not able to move from one pearlitic region to another.This increases the brittleness of steel, departing from the aim of annealing. 2.)Acm temperature is high and therefore, heating to above Acm results in more oxidation and decarburization of steel. 3.)Heating grain coarsening of austenite occurs above Acm temperature.This is because the last part of cementite dissolves at Acm and grain boundary migration occurs without any hinderance above Acm temperature.This leads to deterioration of mechanical properties because coarse grained structures are inferior to fine grained structure in respect of mechanical properties Bright annealing • Annealing of steel components is carried out using some protective medium to prevent oxidation and surface discolouration. • Such a type of annealing keeps the surface bright and hence is called bright annealing. • The surface protection is obtained by the use of an inert gas such as argon or nitrogen or by using reducing atmospheres. • A typical composition of a reducing gas is 15% H2,10% CO,5% CO2,1.5% CH4 and remainder N2. Box annealing • Here annealing is carried out in a sealed container under conditions that minimises oxidation.The components are packed with cast iron chips,charcoal or clean sand and annealed in a way similar to full annealing.It is also called black annealing, close annealing or pot annealing. Isothermal(cycle) annealing • In this process, the components are heated as for ordinary annealing and then cooled comparatively rapidly to a temperature below A1, held at this temperature for certain period of time to provide for complete austenite decompositon.This is followed by comparatively rapid cooling. • Advantage of isothermal annealing is that it reduces the time required for heat treatment of steel.It is especially true for alloy steels which must be cooled very slowly to obtain the required reduction in hardness. • Because of equalization of temperature,transformation occurs at the same time throughout the cross-section.This leads to more homogeneity in structure. • Isothermal annealing produces good results in treating relatively small charges of rolled stock or small forgings. • It shows improved machinability, improved surface finish after machining and less warping during subsequent hardening process. • Increase in surface finish is due to the formation of spheroidised structure. Sub-critical annealing • This process involves heating the steel to a temperature just below the A1 temperature,holding it at that temperature for a while and then allowing the material to cool in air.This is a faster rate of cooling than that employed with full annealing where the material is cooled in the furnace. • They are used after cold working of steels to relieve the internal stresses or to reduce the hardness or to refine the structure. • Stress-relief annealing: In this process, cold worked steel is heated to a temperature between 500 to 5500C which is below its recrystallization temperature for 1-2 hours and cooled room temperature in air.Due to this internal stresses are partly relieved without loss of strength and hardness i.e. without change of microstructure.It reduces the risk of distortion in machining, and also increases corrosion resistance.Since only low carbon steels can be cold worked,the process is applicable only for hypoeutectoid steels containing less than 0.4% carbon. • Recrystallization annealing:This done below A1 temperature i.e. at temperature between 6250 and 6750C.The cold worked ferrite recrystallizes and cementite tries to spheroidise during this annealing process.Not only internal stresses are eliminated but also steel becomes soft and ductile.Refinement in grain size is also possible by control of degree of cold work prior to annealing or by control of annealing temperature and time. • Process annealing(Intermediate annealing): In this method,cold worked metal is heated to above its recrystallization temperature.This is also accomplished by the formation of strain free equiaxed grains.This is given to metals to soften them during mechanical processing so as to continue the cold working process without cracking of metals.It may or may not involve full recrystallization of the cold worked metal.In principle,process annealing and recrystallization annealing are same and both the process involve formation of stress free equiaxed grains from strained and distorted cold worked grains. Spheroidise annealing • This process is performed by heating the steel slightly above the critical temperature A1(730 to 7700C) with subsequent holding at this temperature followed by slow cooling, at a rate of 25 to 300C/hr, to 6000C. • This heat treatment is given to high carbon and air hardening alloy steels to soften them and to increase machinability.The microstructure, typical of this heat treatment, shows globules of cementite or carbide in the matrix of ferrite.Hence this process is called as spheroidise annealing. Methods producing spheroidise structure 1.) Hardening and high temperature tempering:Due to tempering of hardened steels at 650-7000C for a long time,cementite globules are formed in the matrix of ferrite from martensite. Martensitecementite(globules)+ferrite 2.) Holding at just below A1:Due to holding for a long time just below the lower critical temperature, cementite from pearlite globularises.The process is very slow and requires more time for obtaining spheroidised structure.It can be accelerated by prior cold working of steel.However ,this may not be possible if the steel is brittle.(e.g. alloy steels and high carbon steels cannot be worked much in the annealed or normalised conditions without cracking) 3.) Thermal cycling around A1:Due to this thermal cycling in a narrow temperature interval around A1,lamellae from pearlite become spheroidal.During heating above A1 cementite or carbide try to dissolve and during cooling they try to form.This repeated action spheroidises the carbide particles. • Spheroidised structures are softer than annealed structures and have excellent machinability. • For plain carbon and high alloy structural steels,optimum machinability corresponds to 50% spheroidised and 50% lamellar carbides. • For high alloy steels of air hardening type, it corresponds to 100% spheroidised carbides. • The time of hardening is greatly reduced if the cementite is in fine spheroidised form and uniformly distributed because less time is required to dissolve spheroidised cementite than the lamellar cementite to obtain homogeneous austenite. • This reduces soaking time during hardening subsequently reducing the oxidation and decarburisation. • This is used in safety razors and needles to reduce decarburization and increase hardness. • Finally they have lower hardness and tensile strength and a correspondingly relative elongation and reduction of area than steel subject to normal annealing.