5. Heat treatment processes, annealng, normalising, hardening, Tempering

The speaker discusses the topic of heat treatment in material science. They mention four types of heat treatments: annealing, normalizing, hardening, and case hardening. They explain the processes and objectives of each treatment. They also mention different quenching mediums and the effects of cooling rates. Additionally, they discuss the concept of case hardening and its applications. They briefly mention carburizing as a process in case hardening. So, I am back and yesterday what we were doing in material science was we looked at some phase diagrams and some basics right and now what I will discuss about is heat treatment. Heat treatment is a whole chapter which is like a kind of easy and scoring. So we will see what it is and from the formula book I will be reading the theory I will be doing this but after sometime if I don't get enough then I will open my notes. So let us start, begin with it. In heat treatment first is annealing, then normalizing, then hardening, then case hardening. So four kind of heat treatments are there. In annealing quenching medium is furnace, in full annealing ductility increases and toughness increases. So what actually happens in annealing is annealing we don't just quench it to room temperature or like that we keep it in a furnace and cooling rate is not very high so that is why only stresses are relieved and all stresses are relieved so ductility will be increasing and toughness will be increasing. Process annealing stress relieving, spheroidized annealing machinability increases because if spheroids will be forming then if spheroids will be forming we have seen those spheroid does cast iron so this machinability will be increasing because of those carbon pockets of spheroids will be inside. Then diffusion annealing to harmonize the chemistry of material if somewhere more like the dosing is more at one place but if we will keep it at some heat so it will diffuse and it will go to the other zones and make it homogenized so that is four kind of annealing processes are there full annealing, process annealing, spheroidized annealing, diffusion annealing and one more thing I need to open my notes because I don't see that this will be able to quench my thirst because this is not fully explained this is just a short very very short but good enough to give us a gist of what we are looking for okay my notes material science heat treatment here now I am here with the heat treatment okay oh we just came to this annealing, normalizing, hardening, tempering one was tempering but what this formula mentions is case hardening so case hardening and hardening I think hardening will come under case hardening tempering is the process which okay I will see so I will go through the notes first quickly because I don't have much time if I go through the notes then it will be great so in heat treatment what happens annealing is the process so full annealing 200 degree centigrade per hour as we make a nodular cast iron okay but here we are talking about steel and the objective of full annealing is to increase ductility and toughness or reducing hardness and brittleness coarse grain will be forming of course they'll have a lot of time so grains will be fusing two grains will be fusing and making one grain hypo eutectoid steels are heated 50 degree centigrade above the upper critical temperature and hyper eutectoid steels are heated at 50 degree centigrade above the lower critical temperature where like some gamma will be there and Fe3C will be there so austenite should be there for this so this I okay this is a good information that in case of full annealing if it is hyper eutectoid it should be gamma and it should be above this the upper critical temperature okay upper critical temperature is this layer like from 910 degree centigrade to 725 degree centigrade okay now if it is about lower critical temperature is the 725 degree centigrade temperature itself so for that that is for hyper eutectoids okay after the after keeping the sample at the temperature for a certain period of time the sample is cool slowly in the furnace ductility of course is of course this okay process annealing so what is process annealing in process annealing point the objective of the process annealing is to release the internal stresses dislocation pile ups at grain boundaries yes so material is heated to recrystallization temperature then cooled slowly in the furnace due to formation of newer crystals dislocation forest will die out and the process meant only for low carbon steels because medium and high carbon steels are brittle and so excessive cold working leads to fracture so there we cannot even do the this thing process annealing this not process annealing we cannot even like hammer them so much they are already brittle we cannot do more cold work on them but low carbon steels we can do cold work so there this these stress stresses will be there okay dislocation forest that is like C kind of curve know which the dislocations go inside back forest becomes and so on we will discuss about this some other time so this is process annealing in the process annealing 0.5 Tm what is 0.5 Tm is of for iron it will be 630 degree centigrade so we have to go for 630 degree centigrade and come back slowly. Spheroidized annealing is again 20 degree centigrade per hour it increases machinability this is like literally very low low cooling rates are there as we have seen in spheroidized cast iron okay it increases machinability it is for medium and high carbon steels because they are not frictile and they are most liberated so machinability also will be less how they are hard actually so we have to remove that so now lower critical at lower critical temperature we do all this thing for medium and high carbon steel only the objective of spheroidized annealing is to increase the machinability of medium and high carbon steel these materials are heated close to lower critical temperature and then cooled extremely slowly in the furnace due to formation spheroids machinability will improve now the fourth one diffusion annealing or homogenizing till now we were talking about annealing see four heat treatment processes are there in that first is annealing and annealing we see full annealing then process annealing then spheroidized annealing then this last diffusion annealing or homogenizing so up to around 50 degree centigrade we'll heat it up the process is used to homogenize the chemical composition especially after welding process after alloy content increases material is heated to very high temperature and then cooled slowly in the furnace high temperature are selected to enable the diffusion phenomena more and more okay simply about that now I go up to normalizing so what normalizing has in normalizing is above where like last time we talked about upper critical temperature lower critical temperature so in this we'll be on that like how should I say those two curves are there now over 720 eutectoid points is there so on at eutectoid points gamma wherever gamma will be there which is over the curves so above that temperature will take okay wherever austenite is stable from there we will start and normalize so in normalizing what we do steel is heated to a temperature at which austenite is in the stable condition from this temperature material is air quenched since the surface experiences a fast cooling rate there will be a fine grain structure and towards center grain will be coarser and coarser normalizing produces hard surface tough core and the microstructure in any engineering application because we don't want scratches to form but also we don't want this thing to break over and impact so that is the reason it is called a final heat treatment process okay and there is another reason why it is called final heat treatment process because hardening and tempering are meant for selected materials so hardening and tempering we cannot do for everything but normalizing we used to do for everything so this was the normalizing then I come to hardening now in hardening steels are heated to a temperature at which austenite is in stable condition from the temperature material is cooled at a rate equal to or greater than critical cooling rate to produce a martensite the objective of hardening is to produce martensite hardening now get the gist hardening is for producing a martensite okay hardening is only to produce martensite that is very very very important benzene medium oil bath can be there for alloys oil bath will be there otherwise water water plus any much salt brine solution 30% salt is there quenching medium what is the quenching medium all the water is inexpensive but as soon as a sample is quenched in it a thin layer of vapor blanket forms all around the specimen and this decreases the cooling rate of course and that what is that lead and lead and effect some some effect is there just I don't just slipped we'll talk about it now addition of salt suppresses the formation of vapor blanket and hence increases the cooling rate right okay brine solution provides the fastest cooling but uniform cooling will be provided by oil bath with the cooling rate whose cooling rates are low so oil baths are used to quench alloys so quenching is mainly for very fast rapid cooling if rapid cooling will be there then it will be called hardening martensite a simple now let's say go for case hardening what is case hardening second it is a secondary to heat treatment process only the eutectoid composition produces 100% martensite and the moment we deviate from the eutectoid composition there will be pro eutectoid phases and these phases do not contribute to the formation of martensite so low carbon steels and very high carbon steels do not respond to heat treatment processes but in the most engineering application we use low carbon steels and their strength is increased by case hardening case hardening is something which comes into picture when we are not able to harden them just fast cooling rate so what do we do we do this thing carburizing is the process and nitriding is a process nitriding cyaniding carbonitriding flame hardening induction so these four processes are there okay now let me talk about this carburizing carburizing is done at 950 degree centigrade and carburizing is a gain of four types three types of carburizing is there the one is pack carburizing in which we will put the material we have in the carburized carburizing mixture and we'll just burn it I think at 950 degree centigrade 50% charcoal will be there 20% BaCO3 barium carbonate will be there and then calcium carbonate those other things will be there like that some things will be there and we'll just burn it and liquid carburizing so in liquid carburizing what will be there again the temperature we have to remember the carburizing will happen at 950 degree centigrade 950 degree centigrade does it do we know about this temperature actually 910 degree centigrade is the temperature after which alpha transforms to gamma austenite at full iron if full iron content will be there that's okay let's talk about this thing diluted and diluted sodium nitride cyanide will be there if sodium cyanide in the sodium cyanide we will keep the sample and but then we'll try to carburize it then comes up gas carburizing in gas carburizing from one end we will supply CH4 methane and so and samples will be kept inside and we'll be burning it and something like this will happen and samples will be carburized so what actually now I'll repeat it carburizing is the process of diffusing carbon into mild steel mild steel why we are saying mild steel okay mild steel specimen since the percentage of carbon surface increases so upon punching you may get martensite also even if we are not getting martensite carbon is going to the interstitial side and producing strain field this increases the hardness and strength of the surface although pack carburizing is cheap and easy to perform but the process takes long time cannot be automated and we don't get quality cases right pack carburizing we are just like burning it with the coal and of course it won't be good and it cannot be automated quality won't be there liquid carburizing process produces very good cases but both but both is not only expensive but only highly poisonous because we are talking about sodium cyanide okay okay next in gas carburizing thickness of case can be controlled and also process can be automated so we'll be using gas carburizing I will repeat about carburizing carburizing is a process which is a secondary treatment process when I am not able to harden the surface used directly because I don't have this thing well what I do in heat treatment is that low carbon steels if medium carbon steel will be there martensite will be and we will be able to form martensite but if so low carbon steel respond to heat treatment okay I was searching for this thing that whether if I would do this carburizing that is again I think it is for mild carbon steel only now next let us go for nitriding nitriding is done at 650 degrees centigrade same as gas carburizing we pass ammonia ammonia from one end and it will be burning inside and something will happen and we can control the thickness and quality okay carbon nitriding carbon nitriding is just supplying gas which is methane and also this methane plus ammonia both will supply so then it will become carbon nitriding cyanide we will put cyanides not diluted solution but the cyanides consider this solution 30% cyanide will be there and the temperature will be 850 so remember carburizing 950 degrees centigrade nitriding 650 degrees centigrade what is the in between 750 so we'll but we want to keep it more just remember yes 850 degrees centigrade is for if both are involved I think a hardening strength of nitrogen will be more than carbon cyanide and then carbon nitriding is a process of diffusing nitrogen into mild steel specimen it produces hardened strong stronger pieces as compared to carburizing since the specimen will be brittle so after nitriding the specimen sometimes need for further treatment cyanide being is a process of diffusing both carbon nitrogen and mild steel specimen by dipping it in concentrated sodium cyanide now if I talk about one thing that this thing what I was saying first of all let me complete it carbon nitriding is a process of diffusing carbon nitride into the mild steel specimen by keeping in the atmosphere of NH3 and some hydrocarbon gas the advantage of cyanide is we don't require a heat treatment okay so what I was talking in carbon nitride and this nitrogen when they go inside they will create the strain fields inside so that is how they are increasing the hardness now flame hardening what is the flame hardening mild steel and high carbon steel they are not heat treated first of all okay for medium carbon steels again for medium carbon steels flame will be there then we will be sprinkling water also so flame water flame water so what will happen if this heated and then quenches quickly so if quickly quenches the cooling rate is far faster than critical cooling rate and it will form martensite for sure and for camshafts we use flame hardening flame is ox if it is oxidized it will be that 3400 degrees centigrade if it is carburized then it will be around 3000 degrees centigrade now this thing is there now let's see the process is the flame hardening process is used to case harden cam camshafts and oxidized films are used for the purpose it is because these flames are hottest after heating the specimen and material is metallic binds to the water to produce a thin layer of martensite over it process doesn't produce quality cases and also thin sections cannot be case hardened because due to development of thermal stresses material will for sure buckle okay now I talk about induction hardening what happens in induction hardening some connecting rod will be there and the oil bath will be quenching as soon as after like heating it up in a coil induction coil we will just simply drop it into an oil bath and there will keep it so it is the fastest method of case hardening the specimen connecting rods are case hardened by and this technique low carbon steel cannot be case hardened advantage of induction hardening is heating can be highly localized and also very fast okay that is there because wherever the coil will be there only there heating will happen specimen are passed through induction coil so heat will develop only on surface you are getting me heat will develop only on surface specimens are passed through induction coil heat will develop only on the surface these specimens are then quenched into an oil bath simple good thing is that only surface is getting heated now tempering objective is like now you have seen normalizing then hardening and now we have come up to the fourth heat treatment process that is tempering what is tempering objective of tempering is after hardening the material is very brittle so the purpose of tempering is to reduce brittleness so let us reduce some of the brittle so that it doesn't just crack so some methods are there let us see each one by one so sorbite is the high temperature tempering just give me a second okay now I am here at the tempering so what is tempering specimen are passed through induction coil so heat will develop only on the surface these specimens are then quenched into an oil bath take care that we are talking about sorry induction and now let me go through this tempering process what is tempering process case hardening material brittle so we want to reduce the brittleness so what do two types of this tempering processes are there one is sorbite and another is trussite so three kind of are there one high temperature tempering medium temperature tempering and third is low temperature tempering so what is tempering how do we do it let us see martensite is heated around and 600 degree centigrade and cooled slowly in the furnace that is high temperature tempering more diffusion will be there you know every 20 degree centigrade will double the diffusion so that is how submicroscopic cementite the Fe3C is the cementite yesterday we were talking about this now this Fe3C is cementite so cementite will be where cementite combines together and forms microscopic structure these structures will be tough and the micro structures are called sorbite okay somehow this will happen okay great now we talk about medium temperature tempering what is medium temperature tempering 350 to 500 degree centigrade we heat and after 300 to 350 to 500 degree centigrade since the temperature of tempering is low cementite will appear on a finer level and the microstructure is called trussite right the temperature of tempering low cementite will appear in a finer level and sorbite it was like it has all clubbed together and making it like increasing the toughness so trussite microstructure is used in springs spark produced don't produce spark if we let them go through this thing no no I don't know exactly how does it work but it is what it does okay now low temperature tempering is there we will go only till 250 degree centigrade and the low temperature tempering since the temperature is very low so diffusion cannot happen and only thermal stresses will be released so there will be no change in microstructure and application are in man in agriculture products tools in metrology manually we do it okay so and one more thing is there Wiltzman-Stein Pro Alpha is there or Pro FF3C is there in perlite so there is something something I am written I don't know so I will quickly revise it okay revision starts at 28 last two minutes will be for revision okay let's go for it in heat treatment heat treatment processes of four heat treatment processes are annealing normalizing hardening tempering in annealing so first let's see annealing is just to relieve the stresses or make material more more ductile normalizing is to make material this hard surface tough core okay hardening is to produce martensite okay on the surface to make it hardened and the tempering is if after this hardening there will be stresses there the material will become too brittle so to make it a little ductile we use tempering process okay so these four processes are there in the annealing process there are four annealing and in annealing first of all in annealing there will be upper critical temperature and lower critical temperature which is just the curve and the straight line of 725 degree centigrade and now in full full annealing will be there process annealing will be there spheroidize annealing will be there and diffusion annealing will be there so in full annealing simple first of all which carbon steel all these processes are mostly only low carbon steels spheroidize annealing is for high carbon process annealing is for low carbon okay and diffusion annealing is that doesn't even matter you just do it for annealing and then comes to normalizing in normalizing simple hard surface tough core but the temperature the range should be in the austenite stable austenite zone and then we can do it otherwise we cannot okay and then comes hardening in the hardening is run for only for a mild so medium carbon steels other steels won't be touched low carbon steel if we touch then this alpha phase will be coming up and otherwise the Fe3O3 phase will be coming up so that we don't want so hardening will be there in hardening quenching medium will be there and then there are four kinds of hardening carburizing nitriding carbon nitriding and the last one is cyaniding so carburizing is at 950 degree centigrade nitriding is at 650 degree centigrade and what's a carbon nitriding will be in between which is at about 850 degree centigrade okay that's all and carburizing itself back liquid gas gas carburizing is just remember gas carburizing only and nitriding it is again gas carbon nitriding both cases ammonia and methane and then cyaniding is liquid now let's talk about flame hardening flame hardening again okay six kind of are there flame hardening and induction induction only surface is hardened and very fast and we can do it whatsoever we want there is a formula X is equal to 5000 root rho by mu f f is frequency mu is magnetic permeability and rho is electrical resistivity so these are not these are from electrical so thickness of case oh somebody may ask I forgot this one the thickness of case in case of induction hardening X is equal thickness is equal to 5000 multiplied by root rho by mu f now the f is frequency mu is magnetic permeability and this rho is electrical resistivity I have to remember this thing and low carbon steel cannot be done okay now tempering tempering can be of high temperature medium temperature low temperature low temperature is just to relieve thermal stresses at 250 degree centigrade medium is for spark less things okay and this the formation of the cementite will be clubbing together but won't be able to club that but it will be at finer level only so this is called this is it is called true site okay 350 degree centigrade to 500 degree centigrade and the high temperature tempering that is at 500 to 600 degree centigrade and here all of the cementite will come and club together so that will increase the toughness and this is for bite thank you so we have done this okay I will talk to you later I gotta go thank you