 Hello everyone, welcome back to another session in dentistry and more. Today's topic in orthodontics is stainless steel. Stainless steel, the most commonly used wires in orthodontics introduced to the branch around 1950s. Since then, it has remained in clinical use mainly because of its excellent formability, its low cost, its ability to resist corrosion and the ease of joining. So let's learn about stainless steel. As we all know, steel is an alloy of iron containing carbon which is less than 0.2%. And when we add chromium to the steel that is around 11%, more than 11%, that alloy is referred to as stainless steel. So stainless steel has iron, carbon and chromium. Chromium will be more than 11%. So we have three major types of stainless steels classified based on their crystal structure arrangement, namely ferritic, then austenitic and martensitic. So these are three types of stainless steel. So the first one ferritic has a chromium in range of 1.5 to 27% and there is no nickel and carbon is around 0.2%. There is a maximum range. Then the austenitic which is also known as 18-8 steel which has 16 to 26% chromium. So this is 18 of this 18. Then nickel 7 to 22%. That is a range mostly it is 8 that is why this name 18-8 and carbon is around 0.25%. Then the third category martensitic which has chromium in range of 11.5 to 27% where the nickel component is comparatively less 0.2 to 5% whereas the carbon is in range of 0.15 to 1.2%. So how does it differ in structure? So the structure of these three compounds are like the ferrite one has body centered cubic lattice. So this is in BCC configuration that is body centered cubic lattice whereas the austenitic which has phase centered cubic lattice. So this is FCC structure and the martensite which has body centered tetragonal that is BCT body centered tetragonal. This is the arrangement within the crystal. So body centered cubic lattice, phase centered cubic lattice and martensitic is body centered tetragonal. So these are the difference between these three. So the ferritic stainless steel which has very little application in the industry. Martensite steel has high strength and hardness and they are used for making surgical and cutting instruments but the problem is corrosion resistance of martensite. The corrosion resistant is very less but it has a good property of strength and hardness. So it can be used for the surgical and cutting instrument but the austenitic one that is the most corrosion resistance and hence widely used in the industry because we cannot afford the corrosion because it is exposed to the oral environment, saliva, other fluids. So we need to have a good corrosion resistant properties because it is placed in the oral cavity for a longer period maybe one and a half years. So it is not like martensitic it has very less corrosion resistant. This is having the highest corrosion resistant that is why austenitic one is used for the orthodontic wire and commonly used in dentistry. Whereas this austenitic has basically two times one is I will write as one 302 and the second one is AISI 304. So these are the two different varieties. The AISI 302 has 18 percentage chromium this is 18 18 percentage chromium 8 percentage nickel then 0.15 percentage carbon okay 18 8.15 whereas the 304 group has 18 percentage chromium 8 percentage nickel but the carbon component is comparatively less that is 0.08 percentage so these are the two forms of a two different types of austenitic wires. So next we need to learn about the few properties of stainless steel that is very crucial properties of stainless steel. The first one is passivating effect passivating effect is nothing but the ability of the stainless steel to prevent the steel from tarnish and corrosion okay. So it is mainly provided by the chromium. Chromium should be at least 12 percentage to get the passivating effect. So this chromium forms a thin adherent transparent but tough and impervious oxide layer on the surface of this alloy when this layer forms when it is subjected to an oxidizing atmosphere okay. So when this stainless steel is introduced freshly to an atmosphere it should be very mild as a clean air there will be a layer forms on the surface that is an oxide layer forms on the surface. So this protective layer prevents further tarnish and corrosion by blocking the diffusion of oxygen to the underlying bulk alloy this is called passivating effect. So when there is enough chromium that has 12 percentage chromium and the freshly prepared or freshly made stainless steel brought to the atmospheric atmosphere which should be very clear okay. There will be a formation of oxide layer that gives the passivating effect that is to prevent the corrosion. So passivating effect will be there only if the chromium amount is minimum 12 percentage 12 percentage is the cutoff rate and if the oxide layer so this formed oxide layer is ruptured by any form like mechanical or chemical means the protection against corrosion that will result. So this oxide layer should not be lost if it lost there will not be any protection against corrosion or tarnish and loss of this oxide layer and it is subjected to corrosion and tarnish is known as sensitization sensitization is nothing but the loss of passivating effect of stainless steel or in other words we can say that the loss of oxide layer. So when there is heating of this stainless steel around 400 to 900 degree Celsius what happens is the chromium and carbon at the green boundaries reacts and forms chromium carbide. So there will be formation of chromium carbide at the green boundaries okay. So chromium and carbon reacts and forms as chromium carbide. So loss of chromium will be happening from the matrix of stainless steel which is the amount will be less than 12 percentage. So it losses the passivating effect and there will be reduction in corrosion resistance and it results in the weakening of the metal. So this process is known as sensitization that is nothing but the loss of corrosion resistance of the stainless steel. So how do we manage this problem that is sensitization or the loss of corrosion resistance. So prevention of sensitization can be done by stabilization. So it is all a connected passivating effect sensitization and stabilization. So stabilization is nothing but eliminate the chromium carbide precipitation by introduction of some other element which will react with carbon. So chromium carbide formation will be reduced. So what we are doing is we introduce titanium that is the most often used and added six times that of carbon. So formation of chromium carbide can be prevented okay. So stainless steel that has been modified in this manner is said to be stabilized. So we add titanium which is six times more than that of carbon because this carbon has more affinity towards titanium than chromium to form the carbide. Okay. So hope you understood the concept of passivating effect sensitization and stabilization. Now let's move on to the other properties of soldering welding and other heat treatment. And lastly guys we have started channel membership in dentistry and mode channel. So you can explore various exclusives for the channel members. So you can explore various options by clicking on the join button adjacent to subscribe button. So we have options of personal whatsapp help. So you can ask any doubts you will get answered through whatsapp text messages or voice messages. And we have one more option that is the PDF notes will be available. But as of now we have only the public health dentistry topic. The more topics will be applauded soon. 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