 So friends, I am Mr. P. P. Mitragootri, Associate Professor in Mechanical Engineering Department of Valshchand Institute of Technology, Solapur. In earlier three sessions, we have studied regarding various varieties of stainless steels and their applications. Today, we are going to discuss some of the special alloy and tool steels. Outcome of this session will again be the same that is we will be able to learn to select correct variety of steel for given application. We are going to discuss HSLS steel, HCS steel and HSS tool steel. These are the three special varieties of steels and out of these three, HCS steel and HSS steels are called tool steels while HSLS steel is little different. So, we will begin with study of HSLS steel. Now, what is HSLS steel? H stands for high, S stands for strength, L stands for low, A stands for alloy and steels for steels. So, it is high strength low alloy steel and it is also called micro alloyed steel. Why micro alloyed steel? Because it is conventional mild steel or low carbon steel. You can say it is a dead mild steel containing less than 0.15 percent carbon. But this dead mild steel, if we have a steel containing less than 0.15 percent carbon, it will have very good ductility, but its strength is low. But if we add little amount of titanium or niobium or vanadium into these steels, how much amount? Little amount means how much amount? Around 0.5 percent. So, such steel is called as micro alloyed steels and due to that, this steel becomes superior in properties because titanium, niobium or vanadium are grain refining elements. They not only improve the strength, they not only improve hardness, but they also refine the grain structure and they dissolve in ferrite and they strengthen the ferrite and makes the steel having ultrafine grain structure. And due to that, these steels have good ductility, good weldability, good toughness due to low carbon, but due to presence of titanium, niobium and ultrafine grain structure, its tensile strength is very high, 50 to 80 kg per mu square. And therefore, these steels are supposed to have high strength to weight ratio and due to their high strength to weight ratio, we find their applications in the manufacture of component in automotive industry. This steel is a special variety of steel used in automotive industry to form manufacture the component firstly by forging and then machining them to the required dimensions. Then comes the next steel, which is widely used in industry to manufacture dies and tools used in press shop H C H C steel H stands for high, C stands for carbon, second H stands again for high, second C stands for chromium means this steel contains high carbon as well as high chromium. So, it is called as H C H C steel. So, how much carbon it contains? It contains 1.2 to 1.5 percent carbon. Chromium content is also around 12 percent to 13 percent along with chromium, we find that these steels will contain little amount of tungsten, molybdenum, vanadium as and when needed. What are the properties of these H C H C steel? No doubt due to their higher carbon, higher chromium and presence of little amount of tungsten, molybdenum or vanadium they have excellent hardenability and due to their very high hardenability, these steel shall never be hardened by water quenching. If you try to harden them by water quenching, you will find that your steel will get hardened, but your steel component which you have hardened will definitely get cracked into two pieces. So, it is always advisable to harden this steel by oil quenching only. So, this steel is specifically oil hardening steel mind well whenever you will be asked about name any one of the oil hardened steel, you should invariably say H C H C steel. Due to presence of complex alloy carbides in greater amount that is chromium carbides, these steels have higher hardness and higher wear resistance. These steels are definitely difficult to machine because it is basically hyper eutectoid steel containing cementite, it is containing chromium carbide, this cementite and chromium carbide is forming network around the grains of pearlite as a result of which this steel is difficult to machine. That is why this steel is to be supplied in ferro-ized condition and after machining it shall be again hardened and tempered, sorry oil hardened and tempered and it can maintain sufficient hardness even up to the temperature of 500 degree Celsius if needed. It shows less distortion during hardening and hence in some of the cases it is referred as non shrinking tool steel. Another name for SCSC tool steel, synonym for SCSC steel is non shrinking tool steel. Now, what are the applications of SCSC steel? This is cold work tool steels, dies for blanking, drawing, coining, thread rolling can be manufactured from SCSC steels. Shear blades are manufactured from SCSC steel, punches cold forming rolls are manufactured from SCSC steel. Some of the master gauges used in calibration of normal gauges can be manufactured from SCSC steel because of their non shrinking property. Now, before going to the next variety of stainless steel, sorry next variety of tool steel, I will pose one question before you, what is the difference between SCSC steel and martensitic stainless steel? A thought shall be given to this question. Now, we will come to the famous steel that is HSS tool steel, high speed steel. It is very commonly known as high speed steel. Nowadays high speed tools, high high speed steel tools are that way not used to the extent they were used 30, 40 years back. But at that time they brought revolution in the machining industry and high speed steel finds their application and are going to stay as long as machining process and tooling is required and what is their property? They can maintain high hardness and keen cutting age even up to the temperature of around 550 to 600 degree Celsius. Now, you presume that is we are carrying out cutting, turning of a cylindrical bar on a lathe machine with single point cutting tool made of HSS. You will find that that cutting tool point which is rubbing against the cylindrical bar will get heated to a red hot temperature of 550 to 600 degree Celsius. Presume that if it loses its hardness and keen cutting age will it be able to carry out cutting action? No, for that reason these steels require to retain their high hardness and wear resistance even at temperature of 550 to 600 degree Celsius and they shall maintain their hardness and they shall resist tempering that is they shall have property of secondary hardening. There are two types of HSS, one is tungsten high speed steels, another is bollif denim steels. Type one is tungsten high speed steel, type two is bollif denim high speed steel. Now, typical compositions have been given in tungsten high speed steel 18 percent tungsten, 4 percent chromium, 1 percent vanadium, 4 percent cobalt is there. While in bollif denim high speed steel tungsten is less, bollif denim is higher no doubt. Tungsten is costlier and tungsten steel will provide better properties than bollif denim steel. Tungsten steel will have better tool life as compared to bollif denim steel, but wherever possible one can go for type two steel which is little cheaper as compared to type one steel. Now, bollif denim tool steels are difficult to hit treat because of their tendency of oxidation. All these elements are strong carbide formers these alloy carbides increase red hardness wear resistance cutting ability while vanadium increases resistors to grain coarsening and cobalt permits cutting of hard and abrasive materials and we find their applications in manufacture of drill steps, remersed saws, milling cutters, lathe tools and punches. Hardening of these steels is carried out by heating these steels to 1100 to 1150 degree Celsius, tempering at 550 degree Celsius followed by oil quenching and due to this we find that secondary hardening takes place. For further reading related to these steels I will recommend material science and metallurgy by Dr. Kaurgire engineering metallurgy 1 and 2 by Kholap and Kulkarni introduction to engineering materials by B. K. Agarwal. Thank you.