we will continue with the lecture now on steel and other metals and on this slide i have the gateway arch of saint louis in the us and this is one of the largest man made monuments made out of stainless steel on carbon steel it is hollow on the inside and this is supposed to represent the way to the far east to the far west in the united states so let us start talking about iron and steel probably the most popular metals used in civil engineering applications iron has been used for many many years iron has been used for many centuries the early use of iron involved heating the ore to lower than melting temperature because it was difficult to raise it to higher temperatures this resulted in lumps porus lumps (())(01:36) metal combined with glassy slag and this was made into pieces that could be used articles that could be used by hammering at high temperature this hammering process squeezed out the slag after melting it and the product was called wrought iron this has a low carbon content in the order of 0.05-01.5% later came in the technology of blast furnaces in the 18th century iron began to be melted in blast furnaces and in this process it could acquire up to about 4% carbon the resulting product was however brittle but could be cast easily into molds so it was used for molding different elements that were used in engineering whenever we have iron with more than 1.7% carbon we call it cast iron that is more ready for casting applications than other hot working processes at higher temperatures we have tough iron or steel being produced the carbon contents are lowered because the carbon is burnt off steel generally contains up to about 1.5% of carbon structural steels have less carbon content 025% or lower let us look at briefly the phase diagram of iron we looked at this when we looked at phase diagrams at equilibrium at normal temperature iron exists as a bcc structure ferrite the alpha phase on heating to 910 degree celsius the bcc alpha changes to fcc structure we get austenite represented by gamma further heating to 1410 degree celsius the austenite becomes a bcc delta phase and above 1536 degree celsius iron melts at very high pressures say 15 gpa and above the bcc iron transforms to a hcp phase represented by epsilon but we are more concerned with what happens between ferrite and austenite in the development of the microstructure and how the material will behave as a consequence of the microstructure formed so here we see the phase diagram that we have looked at before this is the austenite phase this region is where the austenite is stable and here we have ferrite being stable okay and this change in phase occurs at 910 degrees beyond about 1535 degrees there is melting and we have the molten iron and this is where that delta phase is stable and here we have the epsilon phase

a more complicated diagram is the phase diagram of iron and carbon but this is however very important to understand the microstructure of steels and this is also very important in the metallurgy processes involved in making steel so let us see quickly what is important for us we have a region where we have only liquid that is above melting this is the eutectic point between the liquid part and combination of austenite and iron carbide at very high carbon contents we have iron carbide which could occur as a single phase between each single phase region if you remember we have a 2 phase region we talked previously of the 1 to 1 rule so that is what you see here between the liquid alloy phase and the austenite region we have a liquid in austenite region so this is the 1 to 1 regions similarly we have a small region here where we can have only ferrite that is only iron very little carbon or almost no carbon and between this region and the austenite region we have a region where both austenite and ferrite occur in normal temperatures we have ferrite and iron carbide occurring here in this region this is a significantly important eutectic point when austenite changes into ferrite and iron carbide below this percentage of carbon we get hypoeutectoid steels above this carbon content we have hypereutectoid steels that is more than the eutectic composition we had discussed what was cast iron cast iron is a material that is brittle is used for casting not for hot working white cast iron is very hard and brittle it is not used for any structural purposes grey cast iron can be machined it has carbon presented as graphite flakes this makes the material soft suitable for machining then we have ductile iron which has good strength toughness and ductility it is used for tunnel linings and other mass applications some limitations of cast iron come up in the joining processes cast iron is difficult to weld it is not impossible but it is difficult to weld due to the brittleness of the cast iron brazing is better if you remember brazing is where we introduced second metal to connect the pieces of the cast iron without taking the cast iron to melting temperature cast iron has a compressive strength is of about 560 mpa a tensile strength of about 140 mpa you see the difference caused due to the brittle nature between the tensile and the compressive strengths the melting temperature is about 1200 degree centigrade which can be achieved even in small factories so you have a proliferation of factories where we can have elements cast with cast iron common application that you see are in the making of grills and other decorative elements using cast iron these are pictures of pearlitic grey cast iron with the percentage given in the top is about 34% of carbon low phosphorus this is called high-grade cast iron the grainy structure this is a zoomed image of this this zebra type structure that you have is pearlite the matrix is pearlite and this part is the graphite flakes so these are defects through which the crack can run it is a very soft layer of graphite if you remember has not

very high strength between the sheets and the crack can run very easily through the graphite so this becomes an inherent defect in the cast iron on the other hand in ductile iron what happens is magnesium is introduced small quality 0.03% by weight and the addition of the magnesium changes the microstructure the graphite instead of being flakes now becomes nodules these pieces are the carbon the graphite now becomes these nodules so you do not have large defects through which the crack can run and you have these isolated pieces of graphite distributed in the pearlite structure the pearlite continues to exist while this is the grainy pearlite layers that you see on the outside this gives rise to higher tensile strength and ductility and we can use the material better than we can for in the case of low phosphorus cast iron steel is something that we use more steel is obtained by decreasing the carbon content by controlled oxidation we burn off the carbon so the carbon content decreases and we do not have the problems that we saw in the case of cast iron excess oxygen is removed by incorporating manganese and steel manganese and silicon excess oxygen is removed by incorporating manganese and silicon manganese also combines with the sulphur that could be present as an impurity and that can be harmful to the steel you will see later on that when we talk about the chemical composition we always have upper limits on the amount of sulphur that is there in the steel steel can be classified as follows we have mild steel or low carbon steel with carbon content of up to 025% medium carbon steel or carbon steel remember when i talked about the gateway arch of saint louis the inside is made of carbon steel the outside is made out of stainless steel which gives that shimmering look and does not need any painting or anything else so medium carbon steel or just carbon steel has a carbon content of 0.3 to 0.6% high carbon steel stronger steel has a carbon content of more than 06% when we have manganese and silicon we call it alloy steel stainless steel which we are used to very much in household applications like cutlery cooking vessels and so on are made out of stainless steel we see more and more applications of stainless steel in civil engineering applications we saw again the example of the gateway arch where the outside is covered with stainless we have a lot of stainless steel railings we have stainless cladding being used more and more so this is a metal where chromium and nickel have been added such that the oxide layer that forms is stable and shiny so we do not have to paint the surface we do not have to worry about the maintenance or loss of material due to corrosion from the phase diagram we saw that there were 2 important phases involved in steel ferrite and iron carbide when these combine in structural steel in thin layers 0.5 micron layers alternating ferrite and iron carbide we have a structure called pearlite this is a eutectic structure we have alternate layers of ferrite and iron carbide the overall composition of the pearlite is about 0.8% carbon above this there cannot be any more carbon accommodated in the pearlite with the result that you have the graphite nodules or you have the graphite flakes that we saw in grey cast iron when the carbon content is less we have some regions which are pearlite and the other is the alpha phase of the ferrite so in the case of low carbon content this would be how the microstructure looks the light colored regions are ferrite the alpha phase and then you have the darker shaded part which is the pearlite so you see pearlite here embedded in within the grains of ferrite

during the formation of pearlite what happens is at a higher temperature we have austenite you remember from the phase diagram at higher temperatures we had austenite as the austenite cools a eutectoid structures forms where you have these layers of ferrite and iron carbide alternate layers forming as the material cools and the pearlite now grows into the austenite completely transforming the austenite we have carbon going away from the ferrite into the iron carbide so we have a diffusion of carbon atoms going from the ferrite to the iron carbide and this now grows this structure starts to grow so we have the austenite iron becoming ferrite iron plus iron carbon this is the pearlite reaction and this gives rise to this layered structure which looks like what we see here in this graph we have these layers giving the zebra stripes in the microstructure that is pearlite now pearlite is very important or the content of pearlite is very important because we see that as the carbon content of the pearlite increases we mentioned before that maximum carbon content of pearlite will be about 08% we find that as the carbon content increases and reaches this 08 we have important repercussions in terms of the mechanical properties we have tensile strength and hardness increase so as the carbon content increases in the pearlite the material becomes stronger and harder however it also becomes more brittle we have elongation decreasing okay tensile strength can range up to 900 mpa but elongation can decrease significantly as the carbon content increases in the pearlite and when you have excess carbon it can become even worse so in a steel at high carbon contents the properties of the steel are dominated by those of pearlite that means we have high hardness high strength but poor ductility and toughness the material cannot elongate very much it is hard it is strong but when you need it to elongate a lot it will break at low carbon contents we have a domination by the metallic ferrite the alpha phase and here we have the strength another properties depending on the grain size on work hardening as the grain size decreases the yield strength increases why remember when we talked about dislocation movement we said that the grain boundaries are hindrances barriers to dislocation movement so when we have smaller grains we have more barriers the dislocation cannot move so easily so you need more energy and we need more stress to drive the dislocation so the yield strength increases also we find that as the grain size decreases the ductile to brittle transition temperature also decreases that is we have to go to lower temperatures in a small grain material for the material to become brittle so it is more stable under lower temperatures we talked about cold working and cold rolled steel sections are becoming more and more popular in applications like housing replacing traditional materials like timber we can have small light weight sections that are produced from cold rolled steel with very low carbon content these are light to transport and can be manipulated easily and made into structures and frame works the strength is derived from work hardening that is the purpose of the cold rolling you get work hardening of the ferrite and therefore the strength is quite high and from the cold rolling angular sections can be made square sections and so on and these can be put together to form a frame work very easily however we

cannot use welding to join these elements welding will locally anneal the material with consequent changes in properties if you remember when we talked about strain hardening we said we said that annealing which is the application of temperature will reverse the strain hardening it will go back to having a lower strength which we do not want in this particular application so what welding will do is when we took put 2 pieces of cold roll steel together and weld at the joint we will have annealing occurring the strength will go down and this will be weaker than the remaining part of the frame work and you could have failure which we do not want therefore such sections cold rolled sections are joined mechanically through riveting bolting crimping and so on you can also have screws that can be used for joining cold roll sections this is an example of a frame structure under construction and the elements that you see the vertical elements and even the diagonal struts and the roofing elements are all made out of sections of cold rolled steel and you see what i said this could have been traditionally done with timber and now cold rolled steel is replacing this and this could lead to fast construction and efficient construction there is an interesting type of steel called cor-ten steel it is a special steel containing a small quantity of cooper and it is used for making sheets and other sections when exposed to rain what the copper content dose is it gives it a rusted surface it forms a hard adherent protective oxide layer of an attractive brown colour if you like brown then it looks attractive it is used for cladding for bridges buildings lamp posts sculptures and other exterior applications i do not think it is very common in india but in europe and in the united states it has been used in several architectural applications like this building in chicago from the 1969 we have the surface with the cor-ten steel and you see here how the steel looks like when you look close up they have you have this rusted copper look but this rust now does not flake and fall off due to the presence of cooper this oxide now sticks to the surface and gives it a pleasing look if you like the brown rusted look let us look at heat treatment of steel this is also important to us for several applications when the carbon content in steel is higher than about 0.3% the properties of the steel can be varied through heat treatment generally this is done by fast cooling from high temperature or quenching quenching is where the element at a high temperature is suddenly dipped in cold water followed by reheating not exceeding 650 degree celsius this is called tempering so the process of quenching and tempering produces a microstructure and a gradient in the structure of the material that can help in certain properties the fast cooling produces a hard brittle microstructure called martensite this is not of very high use except say in tools and cutlery where you need a very hard surface without having much wear but you do not need a lot of ductility in it so tool bits knives and so on can be made of martensite where you have an edge or a point which is very hard and does not break very easily but there is not much of ductility requirement in the application nevertheless upon reheating upon tempering the carbon of the martensite precipitates as tiny particles of carbide through the matrix you have iron carbide precipitating outer the martensite and this makes the material now be softer and more ductile the martensite becomes softer and more ductile due to tempering

stainless steels are those alloys which contain at least 12% of chromium other alloying elements in stainless steel are nickel molybdenum that could also be present they are 3 basic types of stainless steel martensitic we have discussed this in the previous slide it contains 13% chromium very hard heat treatable ferritic it contains 13% chromium low carbon content it is ductile medium strength not heat treatable austenitic with 18% chromium 8% nickel again with ductility and higher strength and not heat treatable so these are 3 basic types of stainless steel all of these of a good resistance to corrosion as long as the passive layer passive oxide film is maintained we should not think that stainless steel will never corrode stainless steel eventually can corrode it has an oxide layer that prevents further corrosion it has an oxide layer that gives a shiny surface but it is not that it is completely free from any oxidation let us now move on to looking at reinforcing bars for concrete structures this is a major application of steel in construction almost all concrete needs reinforcement to take care of the weak tensile properties of concrete and the combined reinforced concrete is what is used for frameworks and other structural components and structures in civil engineering the common types of reinforcement bars are mild steel bars ctd bars and tmt bars the former 2 are being used less and less in india plain and ribbed hot rolled mild steel bars were used a lot in say the 60s and 70s almost all construction was in india was with mild steel bars the ribs were made in the hot rolling process to improve the mechanical bond and griped the steel within the concrete in probably the 80s there was a big change to using what are called ctd bars standing for cold twisted deformed bars here work hardening was used to pull and twist hot rolled bars this pulling and twisting also included some formation of corrugations on the surface through ribs the twisting and the pulling increase the yield strength by work hardening so the strength was higher and higher strength was used in the design unfortunately the work hardening also gave rise to residual stresses which increased the corrosion the corrosion resistant decreased its material due to work hardening if you remember from previous discussions we said that there is a change in the microstructure there are residual stresses from the grain boundaries have higher energy and therefore to stabilize to lower their energy they can react with chemicals from outside so this is what happens in work hardening we have a lot of residual stresses build up of energy on the grain surfaces and when there is a possibility of moisture or chlorides entering the concrete and reaching the steel we have corrosion occurring quite fast so ctd bars are prone to corrosion and therefore they are being phased out gradually from the civil engineering construction sector what is now more common is what we call tmt bars or thermo-mechanically treated bars bars with hard high strength surfaces but a ductile core there are also other bars called corrosion resistant tmt bars where a small amount of copper and chromium and a higher than usual percentage of phosphorus is used and this increases the corrosion resistance remember

that chromium and copper when they are used to alloy steel increase the corrosion resistance so there are tmt bars with small amounts of copper and chromium and little more than usual percentage of phosphorus other than that we can have very special cases galvanized bars where there is a treatment done on the surface that prevents further corrosion or epoxy-coated bars like what we talked in cathodic protection bars that are coated such that water and oxygen do not reach the bars or in an extreme case we can even have stainless steel bars instead of the normal steel bars used as reinforcement currently the common grades of steel used as reinforcement in india are the fe 415 fe 500 and fe 500d grade fe 250 mild steel is also available but only used as secondary reinforcement the number indicates the yield stress that is used in the design and when we have a d following the number that means that there is a higher ductility requirement there are a large number of grade specified in the norms but these i think are the most common reinforcement grades that are used in india the chemical composition is regulated by the highest standard 1786 where we have here again for these common steels the limits of some of the constituents like carbon sulphur and phosphorus and you see the higher the grade we have a stricter requirement on the amount of carbon sulphur and phosphorus that can be present in terms of the mechanical properties these are the values that we need the properties that are checked normally for qualifying this steel reinforcement is the 0.2% proof stress or the yield stress you remember we had introduced the concept of 0.2% proof stress couple of lectures back when we do not have a well-defined yield point what we do is we draw a line with the same slope as the initial part of the stressed in diagram at an offset of 02% strain the point where this cuts the stress strain diagram is called the 0.2% proof stress and is taken as the yield stress so what we say is that a fe 415 steel should have at least 415 mpa 415 mpa yield strength 500 should have 500 500d should also have 500 another important requirement is the elongation elongation is measured on a gauge length of 56 times a square root of a where a is the cross sectional area of the test element and we say that this elongation percentage should be minimum for these steels for 415 it should be at least 14.5% for fe 500 it should be 12% for fe 500d we have an additional requirement of ductility so the elongation should be higher than in 500 say 16% if the 500d is what should be used in seismic applications where we have earthquake prone regions and we are putting we are doing structures where reinforce concrete there 500d should be used and not 500 because you see that the elongation is even less than in 415 in terms of tensile strength that is the failure strength we have these limits it is related to the proof stress at least a little bit more than the proof stress so there should be an increase in the stress strain diagram and not failure at the proof strength we want a little bit of extra strength to guarantee against sudden failure and the limiting values are also given for 415 failure should not occur at a stress less than 485 mpa for 500 and 500d it is 545 mpa and 565 mpa respectively so you find that though both of these 500 and 500d have the same yield stress requirement in terms of elongation and failure there is a higher requirement and that is what makes it more adapted to seismic detailing and design for earthquake prone regions and in our code

we have other steels define again with and without the d 550 and so on which can be used when they are available and when they are required now we have talked about tmt bars or themomechanically treated bars these are also called q&t bars or quenched and tempered bars because the process is basically quenching and tempering so the process is now given in this graph we have temperature on the y axis and time on the x axis if we have a steel that is let to cool by itself this would be the normal cooling of the rod okay however when we want to quench and temper what we do is in very first time say about 1 second the steel is brought from at least the surface of the steel is brought from 1000 degree celsius to about 200 by putting it in cold water so this is the quenching that is done very fast drop in temperature of the surface and then it is allowed to regain some of the temperature and then it follows what would be the normal cooling so the core now is following this evolution of temperature and time the core has a different evolution in temperature than the surface the surface is what is undergoing at transformation to martensite and becoming tempered so the surface has a harder surface harder properties the surface is harder whereas the core is more ductile so we have a change in the microstructure over the section of the rod so after hot rolling to the desired size and shape the low carbon steel bars which are used in making tmt or q&t bars are quenched suddenly put in cold water the quenching now converts the surface layer to hard martensite while the core remains as austenite it is still at high temperature as the bar cools heat flows from the core to the surface which is now cooler turning it into tempered martensite the martensite on the surface becomes a tempered martensite due to the precipitation of the carbide the core transforms to a ductile ferrite pearlite that we have seen before so this is a diagram which shows what happens to the section in terms of the microstructure initially there is hot rolling this would be the circular section of the reinforcement bar with the rib this is to create bonding mechanical bonding both the surface and the core are austenite the temperatures high enough that you have the gamma phase austenite then the steel is put in the quenching box very fast cooling of the surface the surface becomes martensite hard martensite the interior is still austenite then tempering is allowed to happen there is a heat flow from the core to the surface this tempers the martensite there is a precipitation of the carbide and you get a better martensite layer inside is still austenite then complete cooling occurs the temperature goes to normal temperatures in a cooling bed you have tempered martensite on the surface and inside in the core you have a ductile combination of ferrite and pearlite and this is from a website of one of the manufacturers of such bars which shows that the depth of the martensite is sizeable you have this hard layer represented here in dark blue and the lighter coloured core here shows the extent of the ferrite and pearlite giving the ductile core so this is how tmt bars or q&t bars are made and that is the reason why they have better properties and can be used in construction as reinforcement so we will stop here with this part of lecture 12 we will continue by looking at other metals until now we have talked a lot about iron and steel since they have more importance

as construction materials but there are other materials which are used to some extent for example aluminum is becoming more and more popular in structural elements framing doors and window frames are very often made of aluminum now copper titanium have also been used so we will look at how these materials are used and why they are used in the last part of this lecture on metals thank you