 We shall continue our discussion on intra granular corrosion of metals and alloys. We completedtwo lectures on intra granular corrosion of metals and alloys. If I can summarizewhat we have seen so far, we are I suppose clear on the definition of of intra granular corrosion right. I think people should be very clear that it is happening at the grain boundary selective attack taking place. Now, we said that among all the metals and alloys stainless steel suffer extensive intra granular corrosion especially when they are being welded when they are not properly heat treated actually. So, in order to understand the intra granular corrosion of stainless steels, I gave very brief introduction to classification of of stainless steels and the role of various alloying elements how they affect the corrosion performance right we saw that. Then we went on to discuss the mechanism or you may call it as a theory of intra granular corrosion of stainless steels. Please notice that the the intra granular corrosion mechanism may not be the same for all metals and alloys. So,we focused on what kind of mechanism operating in the intra granular corrosion of stainless steels and very specifically towards the austenitic grade stainless steel ok. And if you recall the major problem is the chromium depletion because of the formation of the chromium carbide. And that occurs in the selective temperatures in the range let us say about 450 to about 800 to be degree Celsius it occurs. But more importantly here it is just not the formation of chromium carbide and the depletion of chromium as a result of the formation of the chromium carbide. The chromium carbide formation is heterogeneous nucleation it forms at the grain boundaries. So, it is heterogeneous nucleation at the grain boundaries and so, the depletion of chromium is centered around the grain boundary area. And so, the stainless steel at that area loses the the stainless steel capacity to repassivate to passivate. So, there is a selective attack taking place. The sensitization process we said was dependent on what dependent of course, it is it is it is the chromium carbide formation. But what are the governing relationship there? The governing relationship there are the temperature, the time, the holding or if you are going to cool it the cooling rate. If you are going to look at cooling it is going to be in the cooling rate. These are the essential relationship that happens in the stainless steel. In a sense I can have a very high carbon containing stainless steel I can still avoid sensitization provided I cool it fast enough to avoid chromium carbide formation that may not be practical, but ok in all cases. But you will see later it is possible in some cases with high carbon content I can avoid sensitization. That means, the factors that govern are the kinetics ok. The kinetics are temperature, time, cooling rate. These are the factors that actually control to what extent the sensitization can happen or if at all it can happen in a given thermal process. Then we went on to discuss the factors affecting intra granular corrosion from the point of view of the alloy from the point of view of the metallurgy of the stainless steels started looking at it. And we said that of course, carbon content is a primary one you know you need to reduce carbon content. By no way you can simply increase chromium content and let the carbon content remain the same and avoid the sensitization process ok. We saw that you know we discussed that in detail. In addition to carbon content there are other alloying elements can marginally influence the this sensitization process. Some of them are beneficial some of them are detrimental ok. We saw for example, nitrogen you know is beneficial. Chromium is beneficial ok, tantalum niobium are beneficial whereas, nickel is not good. So, you just look at the relationship actually. But one thing I forgot to tell there was that the nitrogen is beneficial over a range of concentration range of composition. Somewhere I would say below 0.16 weight percent. If I keep adding more nitrogen to stainless steel what could happen? It can also form chromium nitride right. It can form CR2 N, it can form CR2 N. That also you see here 1 nitrogen captures to chromium it can happen. But within the limited extent it is it is it can be beneficial. Then we looked at the the the grains right role of grains. After all the sensitization occurs at the grain boundary area. So, the nature of the grain boundaries affects the corrosion. So, we said that the energy of the grain boundary which means you know the energy of the grain boundary dictated by whether it is going to be low angle or high angle grain boundary. This has got low energy. So, less IGC or no IGC you can say equal to high energy you have high IGC occurs. I do not say high IGC because you know I mean most of the material that you encounter alloys and all have high angle grain boundaries only where. So, we we did not of course, go in detail about the definition of high angle low angle. But I just say that twin boundaries are supposed to be low angle grain boundaries. So, if you have twins in the in the austenitic stainless steel the twin boundaries will resist the the the formation of the chromium carbide and so, the sensitization process. In fact, these are important from the point of view of intergranular stress corrosion cracking. So, stress corrosion cracking we have not discussed so far. Stress corrosion cracking can be assisted by the sensitization of the grain boundary because the crack can travel along the grain boundary and make the metal more susceptible to cracking under tensile loading conditions that part we will see later. Now, we look at the grains and we look at the grains from the point of view of the energy of the grains. There one more thing that we need to look at from the the the grains coming continuing the grains is that the size of the grain size of the grains you call a grain size of an alloy right. Find out the grains better it is from mechanical you know properties point of view you can have high strength and toughness together actually right. So, what would happen? What do you think will happen if I reduce the grain size I keep the composition same the carbon content the same the chromium content the same even other alloying elements content to the same, but I reduce the grain size. So, what do you think will happen to sensitization increase? Why do you think it will increase? The grain boundaries becomes more it makes yeah one way it makes some sense. But look at other way around the chromium carbide formation occurs only along the grain boundary. There is a limited carbon right the carbon content is limited. So, you cannot form more chromium carbide than that is allowed in a given alloy that is dictated by the the carbon content right. So, assume that all the carbon is now converted to chromium carbide now I have alloy which has got smaller grain size and larger grain size so, what will happen understood or not? The volume fraction of chromium carbide formation does not depend upon the grain size it depends upon what depends upon the carbon content of the alloy right right. I want to say that the volume fraction of chromium carbide formation is the same, but the grain size of the two alloys are different one has higher one has a lower one. So, what will happen to sensitization now? So, suppose like that now look at visualize now I want you to think yeah, but then over a time period you cannot have more carbon you know I mean the carbon content is limited only right. If you are going to have infinite source of carbon then probably you will have more sensitization right. Assume that the chromium carbide formation volume of 23 C 6 is the same. Now, I am distributing this chromium carbide over here and over here. So, what happens? See if you look at here the overall grain boundary area right the grain boundary area in this area is more grain boundary area is is less am I right. If you if you calculate the grain boundary area the grain boundary area I do not know you you guys might have studied this ASTM grain size and and so, the grain boundary area and all right. So, if the if the number of grains are increasing in a given in a given volume the number of grains are increasing then the volume of grain boundary area or I would say volume I would say the grain boundary area is more. So, I distribute this chromium carbide here and here the distribution becomes more dense here. So, this could be less continuous less continuous distribution of chromium carbide here happen more more continuous I would say continuous distribution of chromium carbide. So, you will see that the grain boundaries are so well closely packed with the chromium carbide here it is not packed with the chromium carbide because same chromium carbide has to be distributed over a larger area right. So, where is the attack here? Attack is more here, attack is less here. So, lower the grain size is beneficial from the point of view of sensitization. But please notice it is not a overriding factor you know you I cannot keep the carbon content at 0.08 and lower the carbon content I mean I mean keep the carbon content at 0.08 and then lower the grain size yeah it can have some effect, but it is not going to have overriding effect right. Please understand that ok, but it does affect the sensitization because of this particular factor. So, the depletion of chromium is more in the grain boundary areas of the larger grains as compared to the depletion of chromium over here ok. So, you find the smaller size are better from sensitization point of view as compared to the larger larger grain grain size in a in an alloy. I hope you you get this idea right or still anybody has a problem. The next point that I would like to talk about is role of crystal structure. We saw earlier we classify stainless steel as austenitic, ferritic, martensitic, duplex and precipitation order of the stainless steels. By nature the precipitation order of the stainless steels do not have very high carbon content. The carbon content is very low the strength of that stainless steel comes from where comes from the alloying elements right like aluminum and copper they form precipitates they give rise to strength. So, the carbon content of that is very low. So, the the remaining ones we should look at how these class of stainless steels perform with respect to sensitization. Let us take the ferritic grade. Let us look at the ferritic grade now and see how we if you take a an austenitic grade and ferritic grade which one will suffer more or what is the tolerance of carbon when it comes to ferritic stainless steels? So, for more right. So, what is the solubility of carbon in ferrite as compared to austenite? So, ferrite is dissolves very low amount of carbon right. So, it cannot saturate more. So, that means, when you are going to heat it it is much easier for the carbon to precipitate into chromium carbide in the ferritic stainless steel as compared to the austenite stainless steels. So, the solubility level of because you know you know isn't it for FCC structure the solubility of carbon is always more as compared to a BCC structure of steel right. So, you find that ok. So, that means, here it is a poor carbon solubility. So, in this case in this case even nitrogen is detrimental. So, we can say that carbon plus nitrogen has to be less than 100 ppm. So, what sometimes people do is they add some titanium, some stabilizes titanium is added to stabilize. Of course, modern ferritic stainless steels have very low carbon I mean there is no problem, but even then you can add some titanium because it is easy. But there is always a problem in the low carbon varieties right. When you weld you can you can pick up carbon from the surrounding suppose you have some grease or something right you weld it the grease gets converted into carbon on on on when you when you strike arc and carbon gets into the material right. So, ferritic stainless steels are more problematic compared to the the austenitic gray stainless steels. Austenitic we have discussed very extensively before right here the the carbon content can be less than 0.038 percent because the solubility of carbon in the austenite is better as compared to this. Let us take the case of martensitic. Martensitic stainless steels are generally containing higher carbon content right. Why need a higher carbon content? Yeah it forms a modern side and secondly that is what gives hardness also right. So, in order to form you know modern side you need to have reasonable carbon content without that you cannot form. So, the carbon content of martensitic stainless steels are higher compared to austenitic grade of course, ferritic grade actually, but even then they are less susceptible they have higher carbon right. In fact, highest carbon in among all the stainless steels they are less susceptible to IGC you know tell me why. So, modern side stainless steels are used in what condition generally or modern side steels generally what conditions do you do use it in fully modern side condition? What do you do that? You temper it you know if you do not temper it then what happens? Then becomes so brittle you know the toughness is so bad. So, when a temper is what happen? They form what? They form carbides right. So, they form carbides. In fact, the modern side is stainless steel also form carbides actually without that you cannot temper it right. So, when you are going to temper it you form carbides you can form chromium carbide you can form iron carbide, but preferentially it forms it forms chromium carbide because chromium carbide is more stable as compared to the iron carbide. But even then I say it is less susceptible to IGC as compared to austenitic grade why do I say that? Have you have you heard of tempering martensitic steels? Forget about stainless steel for a moment right tempering of martensitic steels anybody have seen the microsex of temper tempered martensite? Tempered martensite microsex is anybody seen? No? It is sad. Then you cannot get an idea about what it is actually right. What do you when you take a martensitic steel and you solutionize it take it to austenitic region and then quench it you form a martensite. When you temper it how does the carbide form? The carbon forms within the martensite isn't it? What is the martensitic? What is the martensitic phase? It is actually it is a BCT right martensite is a it is a BCT structure we saw it before right and this is a FCC structure right and this of course is a BCC structure. So, it is it is total lattice. So, so much of carbon is saturated now it is not martensite is single phase. So, when you heat it it forms carbide, but what does the carbide form? The carbides form within the martensite martensite is given by some called alpha prime. So, you have more or less more or less uniform nucleation it is not right term to use, but I will use it. So, within so, the grain boundary precipitation of chromium carbide is very less here. So, the chromium carbide occurs all through ok. Now, please understand IGC occurs only when the chromium depletion occurs in the grain boundary area that will happen only when the chromium carbide forms at the grain boundary area. If forms everywhere IGC does not occur of course, the chromium carbide formation by itself can bring down the passivity you all we know that right because the matrix chromium content comes down passivity is lost I am sure. But the issue here is we are talking about the IGC we are not talking about the overall loss in the passivity. In fact, the martensitic stainless steels when they temperate they temper optimally so, that you do not lose too much of passivation. If you do not temper it it is brittle. So, they temperate optimally in order to have good passivation at the same time good toughness actually. So, there is optimization process. So, coming back to the point here the martensitic stainless steels are not prone to IGC primarily because of the fact that the chromium carbides form within the lot within the martensites ok. And so, the depletion of chromium the the grain boundary grain boundaries or large boundaries whatever you can call it ok, acicular boundaries whatever you can call it is less. But of course, we keep on aging for long time they also can undergo intragonal corrosion, but generally they are not prone to intragonal corrosion. So, please understand the concept of intragonal corrosion in a proper manner actually ok. So, it is not the chromium carbide formation alone is responsible the chromium carbide formation of the grain boundary. And the consequence depletion of chromium the grain boundary is responsible for the intragonal corrosion of of stainless steels. Why does chromium carbide precipitate inside martensite? It is a good question ok. Yeah, then I think then you have to go into the the metallurgy aspect of what is called a nucleation and growth. See martensite when what is the martensite? The martensite consists of a lot of dislocations most of you know about it that is that is the reason why it gives you higher hardness higher strength. So, when you have lot of dislocations within the martensite when you do annealing or aging process or tempering process what happens now? The precipitations occur on the dislocations. The dislocations are large in number within the martensite. So, they preferentially precipitate along the dislocation that is why when I say uniform corrosion I say within quartz it is not really that uniform corrosion. I am sorry when I say it is uniform nucleation I do not mean in true sense of it ok. The true sense means you should not happen at the dislocation also. But because you have lot of dislocations martensite and so, they are very well uniformly distributed in the martensite. So, you have lots of chromic orbit being precipitated there only ok. So, so that is what happens in the case of a martensitic stainless steels. Any other question? Let us get into the next class of stainless steel duplex stainless steels. The duplex grade we need to talk about we say it is a mixture of alpha and gamma each of them in 50 volume percent each it is more resistant than ferritic and austenitic grade. So, very interesting thing actually right. Ferritic stainless steels are more prone to sensitization austenitic stainless steels also prone to sensitization. But you have two phases in the alloy they are better than both of them actually ok. And the reason is not very difficult to understand actually ok. Let me draw some diagram. So, this is an alpha phase and this is the gamma phase in a duplex stainless steel. The precipitation of the chromic orbit occurs here the grain mood I just take only one one precipitate here it occurs very interesting thing happened. Chromium diffusion in alpha is much faster than in gamma. Carbon diffusion and the solubility both are high in gamma phase look at how the cooperation takes place. So, you have here more carbon isn't it because the solubility of carbon is more here. If you take if you take alpha phase and the gamma phase in a given alloy can you guess which would have higher higher chromium content with the alpha or the gamma? Is chromium is what stabilized? Is it gamma stabilizer or alpha stabilizer I mean alpha stabilizer also. So, come on here you should be able to tell right. So, it is a chromium is it has got higher chromium level here isn't it. Partitioning taking place in that you will have more chromium it may even have more molybdenum also anyway that is not important for us. So, you will have more chromium here and the diffusion of chromium is higher in alpha higher carbon. Now, what happens now? The carbon diffuses here fast and chromium diffuses fast here. So, what happens? Now, if it happens now, now what happens? As a consequence chromium depletion is minimal on both the sides because when you have faster diffusion then what happens? Then you do not get a strong concentration gradient right quite fast and there is no need for the chromium in the austenic side to diffuse because when the carbon goes over here the chromium moves from alpha to form the chromium carbide right. So, that means, the grime boundaries are not going to get depleted so much as it happens in the ferrite or it happens in the austenite right. So, that means, what happens? So, it is minimal and so IGC is minimal. So, duplex stainless steels are generally more resistance to IGC as compared to austenite grade and ferrite grade. Of course, modern stainless steel the issue does not come into picture because they they they nucleate in the in the grains of course, there the the corrosion resistance of modern stainless steel falls because of the tempering process. So, this is about the castle structure you have seen how they affect the the sensitization of of the stainless steels ok. So, this should be a known to you any questions here right ok. So, let us let us move forward effect of cold work. If at a cold work when you do a cold working what happens to the material or dislocations right. So, cold work at a low level increases IGC because dislocations easily transport carbon actually right easily transport carbon. High level high level of cold work lowers IGC and you can anybody can guess why at high level IGC is getting reduced. Yeah you have lots of dislocations now in fact, they get entangled right. So, because they promote chromium carbon precipitation within the grains. Of course, you see overall passivation all this will come down you know we are talking please. Again do not try to club everything together we are only talking about IGC. I am not saying that cold working is better for overall performance of stainless steel that is not the the idea here ok. So, this is about the intergranular corrosion the theory the factors affecting the integral corrosion of stainless steels. We have seen so far and any of you have any questions please let me know. How does dislocation facilitate diffusion of carbon, but not chromium? Yeah see when you do the when you see what happens is when you do cold work and you anneal it at temperatures the dislocation gets annealed you know they go to the grain boundaries and all right. So, the dislocation move and carbon can go along with the dislocation actually. So, it it facilitates the movement of or diffusion of carbon to the grain boundary. So, of course, it is not going to facilitate that much of chromium because see and chromium I mean carbon is so small atom it get entangled with the dislocations right ok. So, let us now go to the main topic of weld decay this is the real problem in industry ok. What you have seen is science of that it is a real problem in industry. I am going to show you the one diagram ok one diagrams photo of the weldment ok get back to this here. You weld a stainless steel and it is what is what is this actually is a is a TIG welding and it is a butt welding right it is a butt joint right. What do you do in a TIG welding you strike arc right and use a filler wire you an arc is stuck between the metal and the electrode right. The temperature is so high it melts it melts the electrode either it is it is a consumer electrode for example, or use a filler wire it melts and then they deposit and they join that is how you do fusion welding you do that ok it is a thermal welding here. So, you have a TIG welding you have a mig welding you have laser welding your electron beam welding the brazing the several kind of metal joining techniques they do exist. In all these cases what is common is the joining process is due to the thermal process. They melt and the union between the two pieces they occur. So, stainless steels are being heated the molten things join them together. Now, what is very interesting you you look at this diagram here is the highest temperature this is the highest temperature and here the lowest temperatures the temperature starts decreasing from here to this onwards temperature decreases. Though the maximum temperature is here the sensitization does not occur at the fusion zone it occurs away from it which is called as a weld decay zone generally termed as a heat affected zone. Actually, the heat affected zone is is generally categorized as what that zone where there is no melting takes place, but there is some solid state transformation metallurgical transformation occurs. Over here the so called metallurgical transformation occurs only here not even here also it happens here it does not happen here of course, lower temperature nothing happens. Now, we need to understand why it happens here why not here why not here. Also, we need to understand why can it can move towards left can move towards right you show what happens that means, you need to understand clearly what governs the formation of the weld decay zone that is very important for us to do that. We have seen the science of it we have looked at the interrelation between the metallurgy of the alloy on the time and temperature cycles that you have seen that actually. So, with that we try to understand this particular phenomena of weld decay ok. Now, let us go back sensitization is temperature dependent and time dependent composition dependent can you make a statement like that ok. So, now, now let us see why should the weld decay occur at the heat affected zone though that is the that is the first discussion that you are going to make it. Let us take a schematic of this it is a plain view this ok. Now, when you weld when you weld if I keep a thermocouple here or if I keep a series of thermocouples here up to this point what I can follow? I can follow temperature rise and fall with respect to the time can you do that? So, when I start welding from one end to another end put a thermocouple there and and monitor the temperature with respect to time. If you do this very interesting thing can be observed I am going to give you very schematic of the the temperature variation with respect to time say at location A say fusion zone and this is the heat affected zone. Let us take the fusion zone the maximum temperature will be at the fusion zone right. So, what happens now? The temperature suits up something like that you know something like that goes actually right. What is this temperature? It is well above the melting point of the alloy that you are talking about right 1600 beyond that ok. Let us take this let us take this zone here this is the fusion zone this is your weld decay zone and this is your base metal. Now, let us identify the the weld let us let us identify the sensitization temperature region that is between 450 and 450 to 850 degree Celsius right. So, you have between 450 somewhere in about let us say 600 or 70 you know it may be 1800 you know temperature is quite high temperature. So, between these two temperatures the alloy will form chromium carbide provided what provided you give enough I mean you forgetting that right. It may sensitize between 458 850 this temperature region provided you give enough time. If time is not sufficient then the guy is not going to give rise to chromium carbide precipitations. Now, let us look at the heat affected zone. Now, I am going to now look at sensitization I am going to draw a small temperature time sensitization diagram for you because you need to also know this right what is this temperature around 850 what is the temperature 450 degree Celsius. If I take it to this temperature and quench it very fast no sensitization right or I put to other way around if I can draw a nice thing look at this this is the maximum time that is allowed for the material to cool down right. When you hold it in the gamma this is what this is called gamma region right this is gamma region. This is the maximum time allowed to cool if you go beyond that what happens chromium carbide forms right. So, when you take it to this temperature if you are going to cool it faster than this that means, less time no problem slower than that you are getting into sensitization that means, the time that can be held in this temperature in fact, this region right or actually speaking you should go from here you know if you if you really look at it is a time and is a time am I right time is it starts from here and goes here right this is the time right. If you draw in fact, you know more appropriate way of talking about is what this is the delta t I hope you get it isn't it time taken starts from here right up to this temperature is no problem after the start. So, this is the this is the delta t critical temperature critical time required to avoid to avoid the why the sensitization right. So, this is delta t here now look at this look at this this curve I have started from here this is the time is started and time the ends is here this time it ends. So, this is the delta t that I have here assume that it forms the chromium carbide even chromium carbide forms when you rise the temperature above what happens to chromium carbide? Dissolve right. So, it dissolves. So, this time is not counted because when temperature goes up the chromium carbide dissolves now it starts counting from here and then count from here right. So, this is the delta t the fusion zone am I right where the material is held in that in the sensitization temperature of region am I right isn't it? It starts this temperature it ends here because below there is no problem. So, this is the time it has been kept in the temperature region of 450 to 850 right. If this time if this time delta fusion is going to be smaller than delta t critical what happens? This small t no oh yeah yeah I am sorry yeah thank you very much. If it is if it iswithin that then there will be no sensitization agreed. Now, let us take this case of e temperature zone you guys are ok. Now, let us take the case of e temperature zone it starts from here where does it end ends here delta t it says that let us follow or not follow ok. Now, if you look at here it becomes so obvious for you it becomes so obvious you would find at least delta t fusion is much smaller than delta t h is that can you make it you get it from this diagram. Now, what happens in the case of base metal yeah it it does not get into the region at all. So, there is no sensitization. So, the sensitization of the e temperature zone is quite understandable right because the time the e temperature zone resides in the sensitization region is good enough to form what to form chromic orbit precipitates to get sensitized. So, the e temperature zone is is prone to is prone to sensitization. So, this is the thing that you should be looking at. What is the significance of this delta t? It only says that when you weld it the fusion zone goes through a sensitization regime from 450 to 8, 850 for this much time ok. It may not sensitize even it sensitizes assume that if this delta t is much larger assume that this delta t is much larger even then what will happen because it goes through higher temperature that will that will get dissolved chromic orbit get dissolved. So, that means, this one anyway has no significance from the point of your welding right there is no significance. I am only trying to analyze the thermo cycles during a welding process ok, but otherwise this one does not have any significance at all because even it gets sensitized for argument sake if you make it they get dissolved when the chromic orbit you know I mean when the temperature exceeds the solubility of the chromic orbit precipitate. My word is taking in case of the heat of the kettles zone at the temperature 4 to 3 degrees Celsius yeah taking delta t from that point to that point that is yeah, but in case of fusion zone yeah same weld thermal cycle cutting 4 to 3 degrees at 4 to 3 degrees is equal to 2 points. Yeah. But in that case we are taking only little bit. Good question ok, but I want somebody to answer yeah see here look at here, here look at this I start from here the temperature goes above it is still within the 850 right. So, what happens the alloy continues to transform continues to form chromic orbit right and till it reaches back here that means, all this thermal cycle the alloy is subject to heating which means chromic orbit will continue to form right. If it goes above this only what happens even there is a chromic orbit form they will start dissolving right. So, if this assume that this curve goes here then what happens I would again will take only the second half of it or put it other way around these portions you see here see this is the this is thermal cycle for this right this is for that. Now, I can have a series of thermo cycles starting from here to this many of them are in between things right can you can I can you visualize that what will be the thermal cycle for this will be somewhere here only this guy will go the time is more it is it is it is more than this ok, but then the guy has gone up. So, if I have another thermal cycle the guy again exceeded here right. So, the guy dissolves if it does not dissolve completely of course, then you can count it also I mean it is not there no order first rule, but that you need to make a calculation and see what happens at all ok. So, what we are assuming here is that when it goes up the chromic orbit dissolves actually dissolution of carbides are much easier than nucleation. If you look at generally dissolving is much easier does not require you know that that kind of under you know normally over heating is not required when you melt it you do not need a overheating when you cool it down you need a under cooling because nucleation growth is a problem, but apart from that ok you can able to calculate and see what will be happens in the in the in the in the in the thermal cycles actually yeah. Delta t delta t f is smaller than delta t c then the sensitization does not occur. If delta. If f is smaller than delta t critical. Yes, yes, yes, yes. But why. That is what no you see here you know the fusion zone it starts from here and ends here right this is the delta t fusion zone right if that and generally this time is very small right you see what all are trying to say is that assume that I am going to take 304 of 0.3 percent carbon ok failure wire suppose I take hypothetically do things they do not sensitize it may sensitize why the time take required to form chromium carbide decreases when the carbon content is increasing. So, when you are increasing the carbon content even the fusion zone also can form chromium carbide provided the carbon content is very large, but the carbon content of the fusion zone is almost equal to that of the base metal. So, it requires same amount of time for the formation of chromium carbide ok. So, we need to understand from the point of view of again temperature time cycles ok. So, chromium carbide formation the temperature time required is the same whether it is in the base metal whether it is in the in the heat amplitude zone where in the fusion zone I am not looking at other small variations like you know I am not looking at all complication arising out of nucleation growth and all. Generally, you would say that time temperature required is same. If that is about the case then the fusion zone by inherently will have less time in the in the in the sensation region as compared to the interpretable zone because interpretable zone never crosses the never crosses the the solubility regime of that actually right does not go up that is the reason why it is happening at all. Any questions? This is a good question people asked I think that also brings to clarity in in in terms of understanding what is happening in typical welding of stainless steel. Can we now move? So, what it means to us actually? It means to us that we understand the science of welding in addition to the science of sensitization. What is science of welding? How much heat input I give to the weldment? It all depends upon how much heat I deposit on the weldment but that decides the cooling rate right. Suppose I have a weld somebody welds like this this sheet somebody welds like this this is called as a narrow gap welding now somebody welds like this. The heat input required to fill here because you are going to deposit some amount of amount of molten metal here you deposit small amount of molten metal here the cooling cycle here and here are different. So, the way they sensitize are going to be different am I right. Similarly, the conductivity of this metal and this is different things will be different. So, we need to superimpose the sensitization kinetics with respect to the science of the welding process then only you can be able to understand how the weld decay occurs right. So, this probably start very well in the welding course I do not know how many of you have taken the welding course there you will go through much rigorous treatment of the welding and all the things we will not go through that. I will give you very minimal things to appreciate how the welding processes can affect sensitization weld decay in addition to the alloy chemistry. Now, let us take the cases of types of welding ok I am I am I am going to now look at types of welding ok. Now, we know we can look at an arc welding we can look at laser welding we can look at electron beam welding look at the gas welding. How many of you have taken a welding course? Ok for you guys must be very simple actually it is not that difficult to do that ok. So, before we get into this let us look at some simple properties of this. Let us look at the heat input maybe in terms of joules per mm square or whatever right isn't it something like you give it right. Let us look at the heat input required to weld in the case of arc welding, laser welding, electron beam welding, gas welding. So, how do you consider that? Which one is the highest here in this? Heat input required to weld. I am not talking about the power of the welding I am not talking about power of the welding right. I am talking about the heat input required to weld this much area required to weld right as we are doing. So, if you are not confused bring this to right isn't it ok. So, gas is the highest which is the least here electron weld welding is the least it is the second least this is the third least it is not the right way of defining I should put number 1 2 3, but anyway I hope you can understand this better actually ok. So, these have a consequence like this. So, I want to weld this is a cross section actually you know it is a cross section maybe let us say about 3 millimeter I want to weld and of course, you can you want you can draw this this also right. So, I keep welding using many of these techniques. If I take a if I take if I take the so called heat temperature zone and I measure the thermal cycle schematic only not very easy right. Now, can you map these thermo cycles versus this yeah. The first one is the electron beam welding great, second is the laser, third is arc and forth. Now, you can now see that what is the role of the welding technique on sensitization of a stainless steel can you can you not ok. Now, I I want to let us say I have 0.02 weight percent carbon ok, I have 0.8 weight percent carbon 1 and 2. I have 0.02 weight percent carbon and I have 0.08 weight percent carbon. I welded this this one I did not get sensitization I welded this I got sensitization ok. I welded the first one in one of these techniques I found it is weld decay a sensitized. I use one of these techniques I welded this even then it does get sensitized can it happen? Good good answer. If so, you tell me the technique that led to no sensitization electron beam welding for it there you go right and I use this silicon welded a problem which only this is the technique that technique. So, there is no sense in science. So, so, you need to understand the sensitization in terms of that it is not in abstract terms. You translate the sensitization kinetics in terms of welding techniques then you can able to appreciate it and if I want to weld it. So, what happens essentially the cooling rate right. Suppose, I use a TIG welding I have 0.05 weight percent carbon I do not want sensitize is a 304 stainless steel 0.05 weight percent I do not want sensitize what I should do possible, but yeah of course, I mean this good answer I would say when you increase welding speed what happens now the Q goes up, but the problem is you need a minimum Q in order to not fuse otherwise it will be a improper welding right ok. So, that may not give us a good welding ok. Now, what else we can do? I can think of cooling it right if I can think of cooling the system right I think of cooling the system. So, there is cooling I mean so, that the rate of cooling becomes we need to look at it from for a point of view of that ok. There are so, many variations that you get there are people you know ok. So, this should be seen in that light so, that you can understand it better. So, it is not simply that you know you know look at in abstract sense you know you need to correlate the process parameters with respect to metallurgy and then see how you can able to control weld decay. So, that in practice we can do that you know same answer that you have been giving you know the people have found thick welding 0.03 weight percent carbon sometimes they found there is no weld decay and sometimes they formed weld decay why it is a manual welding the guy got so, tired he was not welding very fast he started slowing down the slowing down the the torch ok. Now, what happens more heated both got sensitizing problem. So, it does not guarantee you that no sensitization taking place at all ok. So, that is how you need to analyze the problems when you go to industry you know problems are complex, but what gives you the way analyze is the scientific understanding you find carbon is same no carbon why does it happen we need to understand why things are really happening the way they are happening now to find a solution to the problems. I will stop here ok and I hoped we will finish this IGC when I go on to meet in the at 530 right 530 I will finish off that actually ok. And meanwhile we have any questions you please let me know how we can you know clarify those those doubts actually ok. So, thank you very much.