 Welcome back friends. In the previous class, we have seen the casting defects and its classifications. We have seen that casting defects are broadly classified into 6 categories. One is defects due to evolution of gases, second one defects due to the pouring of the melt, next one defects due to metallurgical factors, defects caused by the moulding material, defects caused due to other factors and finally defects due to shrinkage. In the previous class, we have learnt about defects due to evolution of gases and defects due to pouring of the melt. Now in this class, let us learn about defects due to metallurgical factors. Now under this metallurgical factors, there is a defect called hot tear. What is this hot tear? This is the typical appearance of a hot tear. This is the casting. Now this is the pouring cup. The molten metal is poured through the pouring cup and it passes through the sprue and this is the runner and it enters into the cavity and here there is a core, a very hard core is there. Now during final stage of the solidification, a crack will be developed that happens during the final stage of solidification. This crack is known as the hot tear. Now what are the reasons for hot tear? One is differential contraction of the casting during solidification means this is a ring like casting, ring type of casting and there is a hollow cavity inside. To get this hollow cavity inside, we are placing a core mostly this will be a metallic core. Now as the what is a contraction is going on, the casting wants to undergo shrinkage and its size becomes smaller whereas there is a core inside it what is a objects the contraction that is how this crack can takes place, this is the hot tear. There is another reason why this hot tear can occur. The casting could not undergo shrinkage freely during solidification due to casting design. Here we can see another casting you see the casting has got three different sections and here one section and here one section and here one section. Now what happens is the middle section undergoes freezing early and it wants to contract whereas the two members they are objecting the contraction. That is how this middle portion could not undergo shrinkage freely because of the neighboring elements that is how a crack can takes place here. So this is the hot tear. The third reason is chemical composition in the cast iron and the steels sulphur is present very small proportion of sulphur may be 0.06 percent of sulphur is present in cast iron and in most of the steels. This sulphur enables missionability at the same time if the proportion of the sulphur is not controlled properly this can induce cracking in the casting. So chemical composition is another factor responsible for hot tear. Next reason is the long freezing range and decreased quantity of eutectic. So this is another reason which influences hot tearing. Long freezing range means what? There will be generally weak cast alloys means two or more metals will be there in the melt. For example let us take aluminium and 10 percent tin. Aluminium freezes at 660 degree centigrade whereas tin freezes at 232 degree centigrade. Now what happens because of this what say difference in the freezing points the freezing range is very longer. For example you consider let us consider say 4 types of compositions aluminium 10 percent tin, aluminium 20 percent tin, aluminium 30 percent tin and aluminium 40 percent tin. Now let us take aluminium 10 percent tin and we pour it about to say 700 degree centigrade and say aluminium starts somewhere about say 600 degree centigrade it starts freezing. Now what happens? The tin will be there. As the aluminium freezes the tin will be it will be filling the gap that happens in the case of the aluminium 40 percent tin case there will be more tin will be there. So this tin because of this what say reason the tin will be replacing the gap. So that is how the long freezing range is responsible for the hot tearing. How to prevent hot tearing? One is use exothermic pads. Now here is the one portion which is what say undergoing shrinkage very quickly we place a exothermic pad. So that its freezing and its cooling will be delayed that is how we can prevent hot tearing. Next one control the composition minimize the sulphur content in the liquid metal. We have seen that the sulphur enables for us and it improves the machinability at the same time if the sulphur content is more it induces hot tearing that is why we have to carefully control the sulphur content. Next one use grain refiners when we use the grain refiners the grain structure of the cast component becomes finer and finer. When the grain structure becomes finer the possibility of hot tearing will be lesser. So there are different types of grain refiners are there aluminium 8 boron, aluminium 3 boron, aluminium 3 titanium and 0.15 carbon, aluminium 5 titanium 1 boron and many more grain refiners are there. We have to choose an appropriate grain refiner so that we can refine the cast structure and which ultimately reduces hot tearing. Next one exploit the benefits of residual and alloying elements. What are the residual elements? If we take the what is a pure commercial aluminium do not think that the aluminium is 100% aluminium in that there will be small amount of iron will be there which comes what is a without or knowledge about say 0.5% iron this is known as the residual element it is an example. Now sometimes these kind of elements help us to prevent hot tearing. Now in a research it was found that iron which is the residual element in commercial aluminium can reduce and even prevent hot tearing in the aluminium castings. For example, let us consider this casting so this is the casting obtained with the addition of 1% tin castings freezing point is say aluminium freezing point is 660 degree centigrade and the tins freezing point is 232 degree centigrade. Now because of the long freezing range and here we can see everywhere there are cracks are there. Now what happened is iron was added intentionally right iron was added and 1% of tin you can see and 3% of the grain refiner and 6% of aluminium 30 iron most alloy. Now the cracking has completely gone there is no hot cracking with the addition of iron in the aluminium melt hot tearing was totally prevented. Why it was prevented this is the SEM examination if we see this so here are the grains and here we can see a leaf like structure so this is the aluminium tin iron rich phase. So this has frozen much earlier than the remaining grains. So this has become hard and strong and it was able to catch the neighboring grains that is how the residual element like iron has prevented hot tearing in the aluminium tin casting and here we can see a magnified view. So here we can see that leaf like structure has become hard and strong much before the neighboring grains and it is catching the neighboring grains that is how the hot tearing is prevented. Now effects of alloying elements on hot tearing nickel is one of the common alloying elements in aluminium castings. Now here nickel is added intentionally we can see here casting obtained with the addition of 1% in 2% grain refiner and 15% aluminium 6 nickel most alloy again the hot tearing has completely gone hot tearing was completely prevented with the addition of nickel which is the common alloying element in aluminium. So the SCM was conducted here we can see these are the neighboring grains these grains are captured by the some what say here this region we can see this is the aluminium tin nickel rich phase this rich phase was interlocking the neighboring grains again this aluminium tin nickel rich phase has frozen much earlier than the neighboring grains that is how hot tearing in the in this casting was prevented. Now let us see the defects caused by the moulding material moulding material means the moulding sand in the moulding sand something is wrong that is how these defects are occurring under this we have scab metal penetration flash runout lug first let us see the scab. Now what is a scab so this is the casting you can see this is the typical appearance of scab what is this liquid metal flows beneath the mould surface and mixes with moulding sand the moulding sand is loosely compacted around the what say pattern maybe the moisture content is not enough maybe the clay content is not enough that is how the molten metal is able to take away certain sand grains and molten metal and sand grains are mixed together and they are occupying a place above the casting this becomes a ceramic and it is very difficult to machine it and to remove it so this is the scab. Now what are the reasons what are the factors responsible for scab low moisture content in the moulding sand when the moisture content is below 3 percent this can happen or insufficient clay when the clay is not sufficient that time also this defect can happen. Now how to prevent this defect proper moisture and clay contents are to be taken that is how we can prevent this defect scab next one let us see the metal penetration. Now you can see here this is the casting this is the casting but this is not part of the casting here we can see unwanted projection above the casting in fact the molten metal has penetrated into the mould unnecessarily and it is causing unnecessary projection. So this is known as metal penetration what are the factors responsible for metal penetration larger sand grains the mould is made up of sand grains or the moulding sand which contains the base sand clay water and the additives the base sand it contains the sand grains if the sand grains are too larger between the neighboring sand grains there will be gap and the molten metal can penetrate into the gap and cause this kind of projection. So when the sand grains are very larger this penetration defect can occur there is another reason insufficient compaction of the sand if the moulding sand is not compacted around the pattern sufficiently then also this defect can occur. What are the remedial measures use fine sand grains it does not mean that the sand should be very fine then in such a case the permeability will be very poor it should not be too fine the sand should not be too coarser. Next one reduce casting temperature when the casting temperature is very high it can penetrate. Next one apply sufficient compaction of the mould next one use additives in the moulding sand we have seen that the ingredients of the grains and or the base sand clay additives and the moisture now these additives they help us to prevent the metal penetration defect. Now let us see how it is possible now cold dust is one of the additives now when we mix the cold dust with the moulding sand what happens it produces carbon monoxide. Now here we can see these are the sand grains now this is the liquid metal now there is a definitely there is a gap between the neighboring sand grains the molten metal can penetrate this way and it was about to penetrate into the gap of the neighboring sand grains. But before the molten metal can penetrate into the this gap this carbon monoxide which is generated due to the mixing of cold dust is pushing the molten metal in the opposite direction that is how this penetration defect can be prevented and minimized. Next one let us see the flash now here we can see this is the casting now only this is the casting only this is the casting but here we can see this is the unwanted material or unwanted flash here we can see unwanted flash and here we can see unwanted flash here we can see unwanted flash means molten metal flows into the gap between cope and drag the mould is made up of cope and drag and these two are separated by the parting line along the parting line sometimes if there is a small gap the molten metal penetrates into that small gap that is how this flash will occur causes sand is not properly compacted along the parting line small gap exists between cope and drag now what are the remedial measures moulding sand should be levelled properly along the parting line after the compaction is over it must be levelled properly there should not be any depression along the parting line neither on the drag nor on the cope then only we can prevent this flash defect. Next one let us see the run out now we can see this is the casting and here we can see this is an unwanted projection and this is an unwanted projection this is similar to the previous defect which is known as the flash but it is not the same as the flash defect there is a difference in the previous case the flash occurred due to gap between cope and drag because there was unwanted unnecessary gap between cope and drag molten metal has penetrated into that gap that is how the flash has occurred but here even if there is no gap between cope and drag this run out can happen so that way it is different from flash then why does it occur hydrostatic pressure of the liquid metal lifts the cope the liquid metal has got hydrostatic pressure as we keep pouring the molten metal inside the mould because of the hydrostatic pressure of the liquid metal the cope can lift up that time again there is a gap between cope and drag that time the molten metal flows between this gap that is the run out so that way it is different from flash right cope rises during pouring of the metal due to hydrostatic pressure of the liquid metal sometimes even the insufficient weight of the cope can be another reason for run out now how to prevent this defect run out this is the remedial measure place some weight over the cope before pouring of the molten metal right so this is the drag this is the cope and here we have placed some weights here you see because of these weights the cope cannot lift up due to the hydrostatic pressure of the molten metal fine when we place weights on the cope it cannot raise up and this run out defect cannot occur that is how we can prevent the run out defect next one let us see the lug now here you can this is the typical appearance of a lug here we can see this is the casting this is not part of the casting this is not part of the casting means metal solidifies in unwanted cavities surrounding the mould somehow some unwanted cavities have occurred have developed around the mould molten metal flows even into these unwanted cavities and forms unwanted projections this is the lug causes some portions of the cast contour are broken off in the mould area after withdrawal of the pattern some portions of the what say cast contour are broken may be due to what say poor handling of the mould or it is a mechanical breakage of the mould cavity in such times this lug can happen remedial measures check for pressure points and broken off edges before pouring after withdrawal of the pattern we have to check the cavity whether it is perfect or not whether it is broken somewhere due to what say mishandling of the mould or due to mechanical breakage if there is any unwanted cavities we need to check if that be the case we have to repair the mould cavity then only we have to pour the molten metal and that is how we can prevent this defect lug next one defects cause due to other factors under this we have mismatch and hot cracks first let us see the mismatch now we can see here this is a casting now here we can see this portion was what say compacted in rack and this portion was compacted in cope now there is a shift between these two portions and this defect is known as mismatch means mismatching of top and bottom parts of the mould or the casting now what are the reasons for mismatch again we can see another casting this is the casting again we can see there is a mismatch causes misalignment of moulding boxes now the cope will be there and drag will be there now after withdrawal of the pattern again we place the cope over the drag and we place the dowtile pins for the proper alignment sometimes what can happens due to repeated use of these dowtile pins they may wear out and the dimensions becomes smaller and that is how there can be some displacement between the cope and drag or there can be misalignment between cope and drag remedial measures ensure proper alignment of the moulding boxes while we are placing the cope box over the drag box we have to ensure that it is perfectly resting over the drag next one replace worn out dowtile pins by new ones there should not be worn out if that be the case even if you place the dowtile pins there can always be a shift or there can always be a mismatch between the drag and cope next one hot cracks this is another what say defect which comes under the defects caused due to other factors now previously we have seen a similar defect under the metallurgical factors that was the hot tearing or hot air now this is hot crack so these two are different now you can see here this is the typical appearance of hot cracking now this is a big casting now here there is a crack and here there is a crack the hot tearing defect occurred during the final stage of the solidification whereas this defect hot cracking occurs after the solidification is completely over during the solid cooling this can occur that way hot tearing and hot cracking are different it is different from hot tearing that we have already seen now hot cracks may occur due to the following reasons right or in the following cases during cooling in the mould after the solidification is completely over when it is cooling down to the room temperature that time this defect can occur during cooling inside the mould or during knockout after the casting is completely solidified we break the mould and take the casting outside this process is known as the knockout that time this can happen during cooling after knockout may not be after during what say knockout after knockout may be the casting will be moved from one place to another place that time this defect can occur means the hot cracking or during a heat treatment cycle after the what say solidification is over after the knockout is over we give heat treatment to improve the properties that time also this defect can occur. So, hot crack may occur due to uneven cooling conditions and also due to differential contraction these are the remedial measures one is use chills chills are the what say large steel blocks which are kept close to the mould cavity these absorb heat rapidly. So, when we place the chills the possibility of hot cracking comes down next one use fillets and avoid rough handling sometimes because the what say casting is handled roughly that time also this defect can occur. So, that is why avoid rough handling finally, we have another defects defect due to shrinkage under this we have only one defect called shrinkage cavity solidification causes a reduction in volume in almost all the metals it is a universal fact solidification causes reduction in the volume, but there is a an exceptional case also right cast iron with high carbon content what happens when it is undergoing what say solidification its volume will be increasing due to graphitization. Now here we can see this is the liquid metal and here we can see the liquid metal is poured and say this there is a reduction in volume due to liquid cooling reduction in level caused by the liquid contraction during cooling. Here we can see reduction in height and formation of the shrinkage cavity caused by the solidification shrinkage. Now the solidification is going on that is how there will be reduction in height and also there will be formation of the shrinkage cavity. Now this what say we have we have seen two types of shrinkages in the previous case there is liquid cooling in because of the liquid cooling there is a reduction in volume this will be compensated by the riser we always keep sufficient amount of liquid metal in the riser that is the primary function of the riser the primary function of the riser is to supply liquid metal to the casting during solidification and also during liquid cooling so that the volume will be same as the volume of the mould cavity. So this gap will be compensated by the liquid metal in the riser. Similarly here we are going to get a what say solidification shrinkage this is also is going to be compensated by the liquid metal in the riser that is fine because of the what say liquid cooling because of the solidification shrinkage this shrinkage defect can occur. But when this riser or the liquid metal in the riser is what say enough this defect can be minimized and here we can see there is a casting with shrinkage here we can see because there is no riser here for this casting no riser here we can see no riser that is how shrinkage defect is occurring causes insufficient rise of the riser we may use the riser but it must be large enough to supply liquid metal to the casting during liquid cooling and also during solidification shrinkage. Next one we may have the enough what say liquid metal in the riser but the position of the riser may not be proper it must be proper in the it must be positioned in the right place. Next one premature freezing of the liquid metal in the riser this is most unwanted situation the purpose of the riser is to feed liquid metal to the casting. So it must be the one which should free at the end whereas the casting must freeze before riser on the other hand due to some reasons due to the casting design or the riser design the liquid metal in the riser freezes first then the liquid metal in the casting then how the riser can supply liquid metal to the casting it cannot that is why premature freezing of the liquid metal in the riser can be another reason for the casting what say shrinkage defect the what say size of the riser may be enough and its position may be good but it is freezing in a premature way that is most unwanted situations. Next one abrupt changes in the casting design when the casting design right because of the abrupt changes the riser liquid metal in the riser may not supply to the required location in such a case also the shrinkage cavity can occur remedial measures one is design the riser sufficiently larger we have seen that the riser supplies liquid metal during liquid cooling and solidification shrinkage and the liquid metal in the riser must be large enough to supply the entire liquid shrinkage and solidification shrinkage. So, use of the riser is not enough it must be large enough to supply the entire liquid metal to the entire liquid cooling and solidification shrinkage. Next one ensure directional solidification we may have the sufficient what say riser which is large enough which is having sufficient liquid metal to what say supply the liquid what say cooling to compensate the liquid cooling and solidification shrinkage, but if the directional solidification is not there the riser may not supply or the efficiency of the riser may not be high. Now, let us see what is this directional solidification growth of partially solid and partially liquid zone from the outside to inside is known as directional solidification. Here we can see this is the what say casting and this is the riser. Now, the directional solidification means from outside to inside outside to inside. So, this directional solidification must be there was to achieve directional solidification locate risers away from the sections with lower volume to area ratios. So, freezing occurs first in these regions. Second one use chills at required locations. Next one modify the casting design such that it promotes directional solidification. Next one use blind riser. Next one use exothermic material on the top of riser. Next one cover the riser with insulating compound or the pad. So, these are the ways to achieve directional solidification. Now, let us discuss this one by one. First let us see the first reason locate the risers away from the sections with lower volume to area ratio. Lower volume to area ratio means what you can see here this volume to area ratio can be lower when this area is larger. Then what will happen? The solidification occurs first at these regions. Then slowly the solidification will be propagating to the other regions. That is why locate the riser away from the sections with lower volume to area ratios. Second reason is use chills at required locations. Now, these are the chills. Now, this is the casting where there is no chill. Now, this is the casting this is the riser. Now, yes because there is no what is a chill here we there can be porosity defect or here they can be shrinkage defect. Now, what we are doing is here we are placing the chill. Chills are the steel blocks which are placed close to the mould cavity. These chill blocks rapidly absorb heat. Now, there cannot be porosity and even shrinkage defect cannot occur. So, use of chills can help us to prevent the shrinkage defect and here we can see this is the casting this is the casting instead there is a shrinkage cavity is there you see. On the other hand here we are placing the chills and here is one chill and here is one chill. These chills are extracting heat. So, this portion will be solidifying somewhat quickly that is how this what is a shrinkage defect cannot occur in this case. Next one modify the casting design such that it promotes directional solidification. Now, you can see here different designs are there. So, this is the what is a actual shape of the casting this is not so good this design instead you modify like this may be here this is the shape of the casting. Here we can see an abrupt change is there abrupt change in the design let us modify like this. So, by modifying the castings design we can minimize the hot tearing or we can achieve the directional solidification. Next one modification again we can see here yes here is the abrupt change and here we are using fillets. So, this is poor design whereas with fillet it is better design or a good design. And here we can see this is the casting here there is no what is a deviation in this portion or there is no change, but here we are making a taper. What happens when we make taper here at the central portion there is minimum thickness the portion which has got the minimum thickness starts freezing slowly the freezing will be propagating to the other portions and finally the freezing will come towards the razor. So, by placing or by making taper or by modifying the casting design we can achieve directional solidification. Regional solidification means the what is a solidification has to propagate from the lower volume to area ratio to the other regions or to towards the razor. Next one use blind razor. Now this is the mount and this is the drag this is the cope and here we can see this is the open razor. Now the efficiency of the razor can come down because of a open razor what happens it is exposed to the atmosphere that is how it is what say freezes early on the other hand there is a blind razor is there. This blind razor is not exposed to the atmosphere it will be in liquid state for a long time. Then what happens wherever there is a shrinkage cavity this if the open razor fails to supply liquid metal the blind razor can definitely supply liquid metal. That is how by using the blind razor we can achieve directional solidification then we can minimize the shrinkage defect. Next one use exothermic material on the top of the razor. Now exothermic what say compounds are mixtures of metal oxides oxides of nickel cobalt copper manganese iron and aluminum they are placed on the top of the razor. Now a typical exothermic compound will be like this Fe2O3 plus aluminum. Now what happens when these two what say mixtures are what say mixed here we can see Fe2O3 and aluminum. Now there will be an exothermic reaction will be taking place and finally lot of heat will be generated. Now this exothermic mixture is placed on the top of the razor. Now when the razor is about to freeze what happens because of this exothermic reaction the material or the metal inside the razor will be in liquid state for a prolonged time. Till it supplies liquid metal to every portion which requires liquid metal that is how by using exothermic material we can achieve the directional solidification. Directional solidification means the solidification has to commence right where there is minimum thickness then slowly it has to propagate towards the razor means in the directional solidification razor should solidify at last. So by placing the exothermic material we can ensure that the razor will be solidifying finally. Cover the razor with insulating compound or pad here we can see this is the razor. Now in this case we are placing or we are covering the razor with insulating compound or insulating pad what happens what is our aim first of all to achieve the directional solidification the liquid metal in the razor must be in liquid state for prolonged time. Now it is open to the atmosphere here but or it is exposed to the mould wall again the mould wall what say absorbs heat rapidly. Now we are placing an insulating what say pad around the razor now this would not allow heat to dissipate from the razor to the mould wall that is how the liquid metal in the razor will be in liquid state for a long time then what happens this can supply it can supply the liquid metal to the portion where there will be shrinkage. Friends now in these two lectures we have seen the classification of the defects and we have seen that defects are classified into totally six types one is the defects due to the evolution of the gases. Next one defects due to the pouring of the melt pouring of the melt means molten metal or the molten alloy. Next one defects due to the metallurgical factors we have seen next one defects caused by the moulding material means the moulding sand. Next one we have seen defects caused due to the other factors finally we have seen the defects due to the shrinkage. With this we are completing the casting defects and we will meet in the next class. Thank you.