 Hello friends, I am Mr. P. P. Mitragootri, Associate Professor in Mechanical Engineering Department of Valchand Institute of Technology, Solapur. Today we are going to discuss regarding introduction to stainless steels and what are types of stainless steels. And in this session, at the end of this session, you will be able to apply your knowledge in selection of proper variety of stainless steel for given component. Now what are stainless steels, why they are called stainless, what is their property which makes them stainless that is very important. All of we know in general that stainless steels have very good corrosion resistance or very high corrosion resistance. And this corrosion resistance is due to presence of chromium in these steels. It is the chromium which is very very important element which makes the steel stainless. Now when these steels are exposed to oxidizing environments, chromium gets rapidly oxidized and forms a thin film of hydrous chromium oxide on the surface. And this film is very thin, impermeable and impervious as a result of which corrosion resistance is improved and this film is passive self-healing in character. And due to this film corrosion resistance is enhanced while before going forward, I want to ask one question that is what is the main constituent of stainless steels apart from carbon. For the steel to be stainless, what is required minimum amount of chromium in solid solution form should be 13 percent at least. That is whenever we add any chromium to steel, chromium being strong carbide former, what happens it will combine with carbon to form complex chromium carbides along with iron carbide. And if any chromium remains after combining with carbon that remaining chromium goes into solid solution. And it is this chromium which goes into solid solution is very very very important in conferring stainless property to a steel. Now for that amount of chromium that combines with carbon in general whatever the chromium that we add out of that how much chromium combines with carbon that is given by one empirical formula. That is amount of chromium that combines with carbon is 17 times the amount of carbon for example I will give you one example over here. Suppose our carbon content of the steel is 0.2 percent then if we add some amount of chromium 5 or 6 percent of chromium say into that. 17 into 0.2 3.4 percent chromium out of 6 percent chromium that we add will go into formation of chromium carbides and remaining 2.6 percent will only go into solid solution. Therefore chromium in solid solution is calculated as total chromium minus 17 into percentage carbon hope you have understood this empirical formula and I have exemplified it. Then higher the chromium in solid solution more will be the corrosion resistance of the steel. Generally as already stated at least 13 percent chromium must be present in solid solution. Along with chromium there in many of the stainless steels we find presence of nickel some amount of titanium little amount of molybdenum and manganese invariably present in many of the stainless steels and no doubt chromium has to be present carbon has to be present. Now whether stainless steels only do have their highest corrosion resistance or excellent corrosion resistance no along with excellent corrosion resistance along with passive to corrosion attack stainless steels have excellent mechanical properties manufacturing properties and that is why we find their applications in engineering to a great extent corrosion resistance of stainless steel is an added advantage besides that stainless steels are known for their excellent forming characteristics means what is the application of high formability high formability means these stainless steels can be plastically deformed very easily means we can manufacture components of stainless steel by cold working similarly they have excellent mechanical properties at low and high temperature means components made of stainless steel can be employed or can be put to use at subzero temperatures as well as at high temperatures then scaling and oxidation resistance is excellent that has already been discussed then they have very good weldability and machinability means fabrication properties of stainless steels are excellent as well as machinability is very good and as they as already stated they have excellent mechanical properties at high temperature means their creep resistance is also good and along with that they have very good surface finish due to that we find their application to a great extent in engineering now stainless steels in general are classified into three groups group a martensitic stainless steel group b ferritic stainless steels group c austenitic stainless steels now first of all we will study group a steels that is martensitic stainless steels now in case of martensitic stainless steel amount of chromium in solid solution is little less than 13 percent that is given by the formula percentage c r minus 17 into percentage is little less than 13 percent as these are martensitic stainless steel martensite is formed after hardening means that these steels are hardened by heat treatment and after heat treatment they form martensite and hence we call these steels as martensitic stainless steels now we have to strike proper balance between chromium and carbon content of these steels to obtain required level of hardness wear resistance and corrosion resistance because in this case carbon content varies from 0.15 percent to 1 percent while chromium content is generally between 12 to 18 percent means in this case chromium to carbon ratio is higher these are magnetic in nature these have higher chromium to carbon ratio of this steel is very less as compared to group b and group c steels and as their carbon content is higher and chromium content is slightly lower we find that these steels have lesser corrosion resistance as compared to other types of stainless steels that is group b and group c stainless steels and these stainless steels are designated by american iron and steel institute in 400 series that is this is one of the series in which we can designate these steels what are the properties of martensitic stainless steel in general as we form martensite after hardening as steel as these steels are hardened no doubt these steels have excellent hardness and wear resistance their strength is very good they have very good hardening ability but their corrosion resistance is little lesser as compared to b and c steel varieties of steels but we find that due to their hardness and wear resistance we find their applications in the manufacture of springs, ball bearings, valves, reserves, blades, valves in chemical industry, surgical instruments and cutlery items are manufactured from martensitic stainless steel some of the typical compositions and AISI designation has been given in the table you can find three types of stainless steels that is what you say martensitic stainless steels if carbon content is lesser chromium content is lesser if carbon content is higher chromium content is higher and in 440 steel little amount of molybdenum is also present for further reference related to stainless steels and martensitic stainless steel I will suggest you to read material science and metallurgy by dr. k. kodgire engineering metallurgy 1 and 2 by gholab and kulkarni and introduction to engineering materials by bk agar