 3. Working in Department of Mechanical Engineering at Valchan Institute of Technology, Solapur. Today we are going to discuss next part of hardening and tempering treatment. Learning outcome for this session will also be same as the earlier session that is after studying hardening and tempering students will be able to design proper hardening and tempering heat treatment cycle for a component given to them. Hardening will give you hardness, tensile strength but it will make the steel brittle and this brittleness introduced is a main cause and after hardening your steel will always have toughness at its lowest level. Secondly, during hardening as our cooling rates are greater than critical cooling rates, there will be temperature and transformation stresses developed in this steel and these stresses are having tensile stresses on the surface and development of tensile stresses on surface will considerably hamper the fatigue life of the steel. So, we have to subject the hardened steel to tempering. Now, first of all what is tempering? Tempering is heating the steel below subcritical temperature, heating below subcritical temperature and holding it there for sufficient period of time and cooling it slowly so that we will achieve the required purposes. What are the purposes of tempering then? First of all to relieve the stresses developed during hardening is one of the major purpose of hardening sorry tempering. Second is to soften the steel and to get to obtain best possible combination of strength and hardness, strength hardness and toughness and this can only be achieved through hardening and tempering treatment. We will study it by one example in the later part of today's session. Next objective is to improve fatigue life. Fatigue life of the steel is improved by hardening and tempering and to reduce the brittleness of the steel are the purposes of tempering. Now, after studying these purposes, now I will pose one question before you that is tempering results in softening and relief of stresses annealing also does the same. So, please think over what is the difference between annealing and tempering. Now, we will go for types of tempering and tempering cycle. First of all we will study tempering cycle in that there are three stages heating. Second is holding and third is cooling in furnace that is a slow cooling. Slow cooling is very much essential because during cooling, stress shall not again get introduced in the steel which is to be tempered. Now, depending upon the temperature of heating, we can classify tempering treatment into three types that is types of tempering are low temperature tempering which is carried out around by heating the steel to 250 degree Celsius only. And by heating the steel only to this temperature and holding it at that temperature for sufficient length of time and cooling what we do, we reduce the hardness to a very little level very little hardness is reduced, very little strength is reduced, toughness improvement is very little and stress removal or stress relief is around 40 to 50 percent. This low temperature tempering is basically devised for the steels where we require very high hardness to be retained steels or components which are subjected to wear resistance in their service are subjected to low temperature tempering. Another aspect or another type of tempering is medium temperature tempering. This medium temperature tempering is generally carried out by heating the steel in the temperature range of 400 to 450 degree Celsius. In this case we achieve stress relief around 60 to 70 percent in some cases up to 75 percent. So, there will be no question of cracking the steel and most of the stresses are relieved. So, stress will not be a problem in the component. Secondly, due to medium temperature tempering we develop a benetic structure in the steel. This structure has a moderate hardness and moderate toughness moderate hardness moderate moderate strength and moderate toughness and when we desire to have good toughness along with moderate strength we generally go for medium temperature tempering especially the bearings are subjected to medium temperature tempering so that its hardness and wear resistance is also retained and toughness is also good fatigue life is also good. Then one more type is high temperature tempering and in high temperature tempering we generally heat the hardened steel in the temperature range of 600 to 650 degree Celsius. In this case hardness and strength are considerably reduced hardness and strength considerably reduced while toughness reaches to its highest level and when we require very high toughness with considerable strength and hardness along with best possible fatigue limit one should go for high temperature tempering after hardening treatment that is very essential. Now this tempering heat treatment cycle can be shown versus time temperature versus time graph like this this is A1 line we have to heat below A1 line during tempering this shows high temperature tempering this shows medium temperature tempering and this shows low temperature tempering so I will write temperature along y axis and time along x axis so hope you understood a tempering heat treatment cycle. Now we will take one example how change in properties take place during tempering suppose we have a steel having 0.55 carbon it will have certain set of properties in as received condition we will write down we will consider its properties in as received condition in hardened condition and if it is hardened and low temperature tempered what are its properties if it is hardened and medium temperature temperature tempered what will be its properties and if it is hardened and high temperature tempered what will be its properties so we will consider tensile strength hardness tensile strength and toughness and first case will be as received that is without any treatment suppose hardness is RC 12 that is Rockwell C scale hardness is 12 tensile strength is say 58 kg per mm square and toughness is 4.8 kg meter now if it is hardened what will be its properties its hardness will be RC 59 toughness will be sorry tensile strength will be 98 kg per mm square toughness will be 0.1 kg meter if it is hardened and low temperature tempered its hardness will be RC 52 tensile strength will be 90 kg per mm square but toughness will be again very low that is 0.5 kg meter if it is hardened and medium temperature tempered we find decrease in hardness to RC 40 decrease in tensile strength to 72 kg per mm square and toughness will rise considerably to 8 kg meter and if we carry out hardening and high temperature tempering we find that our hardness may be reduced to RC 32 tensile strength may be reduced to 65 kg per mm square but toughness will be 18 kg meter so this combination of strength and toughness which is rarely achievable can be achieved by hardening and tempering so this is if this hardening and tempering tempering treatment cycle is properly designed and implemented we can get drastic improvement in fatigue life and best possible combination of strength and toughness the book by Dr. Kodgire material science and metallurgy metallurgy for engineers by Roleson and introduction to physical metallurgy by Avner thank you