 Hello everyone, welcome back to dentistry and more we are continuing our stainless steel topic. So last session we completed various forms of stainless steel then the properties such as passivating effect, sensitization and stabilization. So this session is about soldering, welding, flex, antiflex and brazing. So these are few shock knots being frequently asked for university exam. So we will start with soldering. So soldering is nothing but a process of joining metals by use of a filler metal. So the filler metal will be usually having less fusion temperature than the metals being joined. Okay so if this has 450 degree Celsius fusion temperature the filler will be definitely having less than 450 and soldering is always done at less than 450 degree Celsius. But sometimes we need to go for a temperature more than 450 degree Celsius which is known as brazing. So the procedure where the fusion temperature of the filler metal is greater than 450 which is known as brazing. So soldering and brazing basically the same procedure but the difference in the fusion temperatures of the intermediary metals. So this is the intermediary metal. So basically it is one or more of the same procedure but only difference is the temperature of the metal being used for joining these two metal parts. So soldering involves the flow of the molten filler metal due to the capillary attraction between these two metallic parts. So dental solders, so we are moving on to the dental solders. Dental solders are alloy that are used in intermediary or a filler metal to join two or more metallic parts used in dentistry. They should have many ideal requisites such as it should have excellent tarnish and corrosion resistant in the oral environment then the fusion temperature should be at least 50 degree to 100 degree lesser than that of the metals being joined then it should have free flowing and it should have adequately wetting properties and also the strength should be the strength of this metal should be equal to the parent metals. That is the metal actually we are trying to join and the color also should be matching with the parent metals. So most dental solders are composed of gold, silver, copper, zinc, tin and nickel. So copper gives a yellow appearance. So substitution by nickel gives a more whiter color to the solder. Now we are moving to the flex. So soldering and bracing is done. Now we are moving to flex. So flex is nothing but flow to flow, it is a Latin word. So any good solder depends on how well the dental solder flows over the parts to be joined. If this is the metal we are joining and we are keeping a solder here. So how well this flows towards the metal being joined? So dental solders do not adequately flow or wet metallic parts because of the presence of an oxide layer. So there will be an oxide layer over the metal parts being joined. So this flex, so we are going to add a flex and the function of this flex is to remove the oxidized layer. So there will be oxidized layer. So this flex will be added and it will remove the oxidized layer which is present over the metals so that the solder metal will be flowing easily over the two metals being joined. So it increases the wettability, it increases the flow of the solder. That is the basic idea of flex. And also it removes the impurities and it prevents oxidation of the metals and also significantly reduces the melting point of the dental solder. So the commonly used flex in dentistry are one is Borax, Borax glass is one commonly used but it is used at 55 percentage then boric acid and silica. Boric acid is used at 35 percentage and we have silica. Silica is used around at 10 percentage and also we have fluoride flexes which containing boric acid and potassium fluoride in one is to one ratio. So that is all about flex. It is nothing but to help the solder to easily flow over the metals. So it increases the wettability and reduces the fusion temperature of the solder. Now let us move on to the antiflex. This is a material which is used to confine the flow of the molten solder over the metals being joined. The function is to confine the flow. It is doing the opposite of flex that is why it is known as antiflex. So the commonly used antiflexes are one is lead pencil then we have graphite lead pencil markings should mark using lead pencil then graphite, graphite lines and iron roguet. These are the antiflex it is to confine the flow how much should it flow over the metals. So that is antiflex. So we learned about soldering that was the first part soldering and flex and antiflex. So we have two types of soldering one is investment type. One is investment soldering and the second one is free hand soldering. The name itself gives a clue. Hand soldering is carried out whenever the area of contact between the metallic parts being joined is large. So we have two metal parts and the area is too large we need to keep an investment between this. So the procedure involves embedding of the metallic parts in an investment. So we keep it on investment and it will be joined. So investment soldering will be having an investment over which we keep this we embed this metallic parts and do the soldering whereas the free hand soldering is the most commonly used in the industry where it is without any investment just keep the two metallic parts together after adequate stabilization and do the soldering. Then we move on to the steps of soldering. So the fundamental difference between investment and free hand free hand is it will be more closer. So this will be more closer we don't need to put an investment between these. Investment is when there is a huge gap or the gap is more between the metals. So what are the steps in soldering? The first one is cleaning the surface. First we need to clean then assemble the parts to be joined then select the right solder and flex then selection of a proper joint then application of flex heating and introduction of solder and finally quenching. So the only part which is little confusing is the last part that is quenching. Quenching is the final step. It is the assembly is immediately quenched in water so as to limit the spread of heat so it is just dipping into water and finishing the process of soldering. So basically the solder is used to join the parts of the surrounding appliances and also to tighten the attachment to bands. Now we move on to the last part that is welding. So soldering we finished now we are into welding. Soldering is the same procedure that is two metal parts are being joined but without the addition of another metal. So only two metal parts there is no introduction of intermediary metal or solder. So the same A and B or two metals are being joined. So how does it possible? So these are heated to high enough temperature so that they join together by melting and flowing. So we will keep on heating and these two will be joined by the melting and flowing without any extra solder or extra intermediary metal that is welding. So we have cold welding and hot welding. Cold welding is done by hammering or pressure. An example of cold welding is the gold foil filling. So gold foil filling is just by hammering so it will be joined. Whereas the hot welding is using the heat of sufficient intensity to melt the metals. So we apply heat here and it will be joined. So heat source is usually an oxyacetylene flame or high electricity that is hot welding and cold welding. So basically two methods are followed one is fusion welding and pressure welding. So fusion and pressure. Fusion welding when where the parts are melted and joined but without pressure that is gas and laser whereas pressure welding where the parts are heated and then applying pressure but there is no melting okay no melting here. Such as spot welding. So spot welding is an example of pressure welding and fusion welding, gas welding and laser welding. So we can classify this welding into spot welding. We already learned pressure welding, laser welding. So spot welding the two clean metal surfaces to be welded are placed together under pressure then two ends of wire or band to be welded are placed between two copper electrodes of welder and pressed together. When switch is pressed large current passes through the wires and there will be combined heat and pressure which fuses the metal pieces at that point okay. So that is spot welding whereas the pressure welding this is known as cold welding and does not require heat. We seen in case of gold foil and laser welding is nothing but by using lasers. So what are the application of this spot welding mainly to weld the stainless steel strip for making bands then secure the attachment to the bands and attaching spring to a rigid bow wire or to bands. So these are the importance of spot welding and also all other weldings are used less frequently in dentistry. So we finished the soldering, welding, flex and antiflex so in this session. So stainless steel there are lots of question will be asked. So we have learned about the first one was our types of steel then the properties sensitization, stabilization, passivating effect then we have soldering, welding then we have flex, antiflex all our shock notes various shock notes been frequently asked in this chapter. So I am winding up stainless steel and lastly guys we have started channel membership in dentistry and more 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 and dentistry topic. The more topics will be uploaded soon and the last option is one to one interaction session. 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