 we have studied what is annealing what is normalizing now today we are going to discuss one more type of treatment most important type of treatment that is hardening and tempering part one of that will be discussed today I am Mr. P. P. Mitravutri working in mechanical engineering department of Vulture technology solar poor outcome of this session will be students will be able to design proper hardening and tempering heat treatment cycle for a given component because designing of hardening and tempering cycle to get exact set of properties is very very difficult task is faced by industrial industrial personnel and then tempering hardening the word itself indicates that hardening is the treatment in which we harden the steel that is we transform the steel into martensitic structure by heating it above a three temperature first of all we get austenite and then austenite is transformed to martensite and this transformation of austenite to martensite takes place by cooling the steel at a rate greater than critical cooling rate and for cooling it at a rate greater than cooling critical cooling rate we have to under cool the austenite to martensitic starter temperature which is generally well below 250 degree Celsius sorry and due to that we have no other go than to carry out sudden transformation of austenite to martensite and in this transformation we transform FCC structure into BCT structure that is face centered cubic structure having C by a ratio of 1 is transformed to C by a ratio 1.04 so transformation of austenite to martensite is accompanied by volume change it is sudden transformation it doesn't involve nucleation and growth it differs from transformation of austenite to ferrite cement admixture that is either perlite or benite so now at this juncture of time before proceeding further I will pose a question before you that is whether transformation of austenite to martensite is diffusion type of transformation or diffusion less transformation and why if it is diffusion less transformation why it is so now as already stated austenite is transformed to martensite by cooling it at a rate greater than critical cooling rate so first of all we will discuss what is the temperature range for heating the steel for hardening we generally heat the steel for hardening by heating hypo eutectoid steel above a three temperature 40 to 50 degrees above a three temperature and hyper eutectoid steel 40 to 50 degrees above a one temperature now you may ask me why hyper eutectoid steel is not heated above a three temperature answer is simple above a one temperature in hyper eutectoid steel we have austenite and cementite cementite is equally hard to that of martensite so there is no point in wasting the time by heating the steel above a three temperature and after heat treatment hence or otherwise we are going to get martensite and cementite and we are going to get the same hardness so save to save the time and to avoid greater number of stresses it is always advised to heat hyper eutectoid steel 40 to 50 degrees above a one temperature only now in hardening when we are cooling the steel at a such high such a high cooling rate we have to we are not allowed to cool it in air normally we have to cool it in water or oil as a cooling medium and these will provide very high cooling rate when our cooling rate is very high it will introduce temperature stresses and as already stated when austenite is transforming to martensite C by a ratio is changing from 1 to 1.04 which will accompany it which will be accompanied by 4.6 percent volume change as a result of which there will be transformation stresses so after hardening we have total stresses equal to temperature stresses plus transformation stresses so stress level of component is very high and due to that our component may have a chance of distortion or cracking during hardening so due care has to be taken while heating the steel and while cooling and while cooling many a times we use fixtures and we apply proper cooling time to avoid distortion secondly when we are aiming to harden the steel after hardening we may get martensite but this martensite that we get is very hard it has high hardness of our C 60 Rockwell C scale 60 plus but at the same time it is brittle it has very good strength but its toughness is low and in most of the industrial components along with hardness and strength we require higher level of toughness and to get best combination of strength and toughness it is always necessary to subject hardened steel to a treatment called as tempering and this tempering will relieve the stresses as well as it will reduce the hardness but it will improve the toughness considerably but it will it is going to definitely going to reduce the hardness but reduction in hardness is much lesser as compared to increase in toughness for example if a particular steel after normalizing has a tensile strength of 52 kg per mm square and toughness of say 7.2 kg meter and our component is required to possess a tensile strength of 58 kg per mm square and shall have toughness of 12.4 kg meter if we harden the same steel and temper at proper temperature we may get tensile strength of 60 kg per mm square and toughness of 13.2 kg meter it means that we are able to achieve required level of hardness and required level of strength and required level of toughness by combining proper heat treatment cycle of hardening and tempering but for that judicious judgment of stresses developed during hardening is very essential it is very essential to select proper quenching media for certain steels if we employ water as a cooling medium our steel may get cracked and if we employ oil as a cooling medium our steel may not be able to develop required martensitic structure and required set of properties so proper selection of cooling medium proper cooling process and after that proper tempering cycle is important in getting required set of properties mind well best set of properties is always achieved best possible combination of strength and toughness can only be achieved after hardening and tempering no other treatment is comparable to this treatment in hardening treatment we have to carry out heating of steel in a proper condition our rate of heating must not be excessively high and shall not be too low and if our component has variable cross section generally preheating is preferred in such cases you preheat the steel to a temperature of 550 to 600 hold there for sufficient period of time and then heat it finally to the required temperature and then hold it for sufficient length of time and cool it in water or oil as per the specification of the steel and get the required set of properties for further reading i will recommend the book by Dr. V.D. Kodgire average publication that is material size and metallurgy metallurgy for engineers by E.C. Roleson and introduction to physical metallurgy by Avner T.M.H publication thank you have a nice time