 I am Mr. P. P. Mitragutri, Associate Professor in Mechanical Engineering Department of Valkchang Institute of Technology, Sholapur. In the earlier session, we have studied HSS Tool Steel, SCSC Steel and HSS Steel. We are going to study some more varieties of special alloy and tool steels in this session. And outcome of the session will again be students will be able to select correct steel for given application. Now in this session, we are going to study maraging steels, water hardening steel, oil hardening steels and air hardening steels. Now in this, maraging steel is one of the most important variety of steel because this is heavily alloyed steel and very costly steel containing very little amount of carbon. Generally carbon content is 0.03 maximum and this steel is mostly used in sheet and plate form in fabrication industry and this steel has excellent forming properties and fabrication properties because of lower carbon content and heavy structures, heavy vessels can be cold and hot formed and welded from these steels. What is the typical composition of maraging steel? You find you observe that carbon content 0.03 percent, heavy nickel 18 to 25 percent, molybdenum amount of molybdenum 3 to 5 percent means this steel is very costly, cobalt 3 to 8 percent. This is another steel containing cobalt. There are very few varieties of steels containing cobalt while titanium is also present in amount of 0.5 to 1.5 percent. Generally in most of the steel, titanium is present below 0.5 percent but in this steel it can be up to 1.5 percent means this is very very very costly steel. No doubt it is containing nickel which is costlier element, molybdenum which is very costly element, titanium very very costly element and cobalt is also costly. This steel is hardened conventionally first of all to form martensite but martensite obtained is soft because its carbon content is low and we already know that hardness of martensite is directly proportional to carbon content. So due to low carbon content of the steel martensite obtained is soft but this martensite soft martensite allows us cold working of alloy to desired shape because martensite even though it is hard it is supersaturated solid solution and solid solution can be cold work. So it can be cold work to desired shape after cold working this martensite is subjected to artificial aging at around 500 degree Celsius and due to this aging strain induced precipitation occurs in this steel and due to precipitation of Ni3TiAl and Ni3MO phases this steel is having very high hardness and wear resistance and due to presence of nickel it has very good toughness. In these steels aging of martensite is taking place and due to formation of that Ni3TiAl and Ni3MO phases their tensile strength may be as high as 210 kg per mm square mind well. These steels have excellent fracture toughness that is shock resistance due to presence of nickel in very very high amount and they can retain their tensile strength and wear resistance even up to the temperature of 350 to 400 degree Celsius very easily. Tension of tensile strength and wear resistance to high temperature is one of the greatest property of these steels no doubt this steel is costly but it has excellent formability it has very good weldability too and where it finds its application in the manufacture of rocket casings in the manufacture of air frame and engine components this steel is widely used in the manufacture of heavy pressure vessels and manufacture of injection molds, extrusion rams, landing mats for aircrafts are manufactured from these steels and these steels are heavily fabricated steels this is one of the important variety of stainless steels. Now we will come to next varieties of alloy steels that is water hardening, air hardening and oil hardening steel first of all we will discuss about water hardening steels. Water hardening steels are essentially plain carbon steels with high carbon content that is their carbon content is above 0.6 and up to 1.4 percent essentially these are plain carbon steels and if we require higher shock resistance and impact strength carbon content is between 0.6 to 0.8 if we require higher hardness higher wear resistance carbon content is above 0.8 if for maintenance of shape cutting edges and high wear resistance are more important that is already stated. Then these are plain carbon steels and they have poor hence they have poor hardenability because plain carbon steels are known for their poor hardenability they can only be water hardened if we try to oil harden them they will not get hardened to martensitic structure therefore only severe cooling that is water cooling can harden these steels. These steels are used for in applications such as manufacture of woodworking tools like firmer chisel, mortise chisel and other thing. Then drills, forming tools, regular hammers, chisels, shear blades, knives, low cut razors can be low cost razors can be manufactured from water hardening steels. Then these steels have a tendency to distort during quenching as they do not have secondary hardening they have poor resistance to oxidation but one more advantage point of these steels is these steels are very cheap so they find applications in like woodwork manufacture of woodworking tools, woodworking drills, drills used for woodworking drills used on soft metals, remers used on soft metals, taps used on soft metals can be manufactured of this water hardening steel. Then comes oil hardening steels no doubt oil hardening steels are supposed to have better hardenability than water hardening steels. These steel contains small amount of alloying elements such as chromium, tungsten, molybdenum, manganese, vanadium along with carbon higher than 0.8 percent. They have better hardenability and can be oil hardened. No doubt as they contain alloying elements they are expensive as compared to water hardening steel but they are not very expensive and secondly tendency to distort during quenching is totally eliminated because we can use less severe cooling medium that is oil which has intensity of 30 percent as compared to water cooling. Now typical compositions of what oil hardening steels they are designated as O2O3O4O9. You can observe O2 contains 0.9 percent carbon and 1.6 percent manganese while O9 contains 1 percent carbon, 0.75 percent tungsten and 0.2 percent vanadium. What are the applications of these steels? These steels are generally used for manufacture of gauges used in measurement, cutting tools, master tools, blanking and forming dies for mild steel and some other materials and some of the shear blades can also be manufactured from oil hardening variety of tool steels. We are coming to the last variety of steel that is air hardening steels. These air hardening steels generally when we cool any steel by after heating it above a 3 temperature in air we call that process as normalizing process because air cooling is much slower as compared to oil cooling and it is very much slower as compared to water cooling but some of the steels can be air hardened even by air cooling. So, there is no question of distortion. They have high wear resistance and retain hardness can retain hardness up to 100 degree Celsius. Total content of alloying element exceeds 5 percent. Typical composition has been given carbon 1 percent, manganese 3 percent, chromium 1 percent, molybdenum 1 percent. Applications are thread rolling dies, slitting dies, intricate die shapes, punches, gauges. Now before concluding I will I want to ask one question what is the difference between water hardening and air hardening steel and I will suggest you to read material science and metallurgy by Dr. Kudgire engineering metallurgy 1 and 2 by Ghulap and Kulkarni introduction to engineering materials by B.K. Agarwal. Thank you.