In this lesson, I'd like to talk about stainless steel briefly. So far, I think you already learned [about] many aspects of different kinds of carbon steel, which usually contain [a] very low amount of alloying elements in [the] steel. However, stainless steel is a little different. It contains high amount of alloying elements, especially chromium. And as you know, one of the weakest point of steel is rust formation in steel in an ambient environment. However, if you use stainless steel, you can avoid that kind of rust formation. Here in this figure, you can see the iron oxide formation if carbon steel is exposed to air atmosphere since the air contains around 20% oxygen. And they can react very easily with iron, and they make iron oxide. However, this oxide is very porous, and also, the color is usually red, so it is not so good for the usage of steel. However, in stainless steel, if oxygen meets with stainless steel, the chromium inside stainless steel reacts with oxygen very rapidly. And they make a very thin oxide film which is chromium oxide, and this is very dense. And usually the thicknesses is around from one to five nanometers. However, since they are very dense and very resistant to the environment, they can protect the metal underlying this film. So in order to be stainless steel, the steel must contain a minimum of 11 to 12 percent chromium, which is a little bit more expensive than carbon steel. That's why we have to pay a little more money to buy stainless steel. Stainless steel has more than 100 years of history. And at first, it was developed in France and Germany. However, it could not be commercialized due to difficulties of processing. In 1913, the first commercial ingot was made in England, and at that time the chemical composition was 0.24 percent carbon and around 13 percent chromium. However, since this carbon content is too high, it causes a great brittleness of stainless steel and also harmful effects on many properties of stainless steel. So they were not used popularly. But nowadays, since we can decrease the carbon content effectively by new technology, stainless steel is widely applied in our daily life. And the annual production of stainless steel is more than 40 million tons, and in 2015 the production was 41.5 million tons. And more than 50% of production was made in China, and for the last 40 years the annual growth rate of the stainless steel production was 5.27 percent, which is larger than carbon steel. Stainless steel is very widely used, and the first example I can say is kitchen items, because you see those every day in your daily life: a spoon, fork, or knife, which usually meets water environment, so it needs high corrosion resistance. And also bowls and pots as you can see. So, like in this figure kitchen items are an important area where stainless steel is used. And next one is construction, and if the roof of this building is made with stainless steel, you don't have to worry about maintenance because it does not rust forever, almost forever. And some other items like guardrails can also be made with stainless steel. Another application is vehicles. And here, the upper diagram shows stainless steel frames used in an electric vehicle, and also in the lower diagram, you can see the main frame of trains. And because they are running in the atmosphere. And in this case you don't even need any painting, and you can run these trains for very long time without any rusting. And one more important area for the application of stainless steel is power plants. Especially, a power plant is usually operated at very high temperature, and since stainless steel has very good properties from low temperatures to high temperatures up to 800 or sometimes 900 degree Celsius. So usually the boiler tubes, which run at high temperatures for a very long time are made with stainless steel. And one more area for stainless steel applications is the chemical and paper industries because they are used in extreme corrosive environment. So that's why we especially need to use the stainless steels in these industries. Now I want to talk about the production of stainless steel very briefly. We need raw materials like scrap, nickel, chromium, and other alloying elements. And if we have a raw material ready, then first we melt it using electric arc furnace. After that, we need some special techniques or special technology in order to reduce or refine the melt by steel making which is called AOD or VOD. After refining the steel, and then we make solid slabs. These slabs are sometimes hot rolled or cold rolled, and then finally after heat treatment we use these plates in our application. And as I said before, the decarburization is the most important process or technology in stainless steel making. So the introduction of AOD and VOD – actually, AOD is argon-oxygen decarburization and VOD is vacuum-oxygen decarburization. So by the introduction of these new technologies in the early 1970s, the production of stainless steel had a breakthrough and high quality stainless steel production was made possible. This decarburization is favored at high temperatures, low chromium activity and also at low CO pressures. And such conditions are met by these steel making processes like AOD and VOD. So at the end of AOD, we can obtain around 0.3 percent carbon and also 30 to 50 ppm of sulfur. And by applying this VOD process we can obtain very low content of carbon up to 0.04 percent, and also sulfur can be drastically reduced to as low as around 10 ppm. Since sulfur is a very harmful element in stainless steel, which makes the grain boundary cracking, we need to reduce sulfur effectively. So by the adoption of this VOD process, we can reduce simultaneously the carbon content and also the sulfur content. In stainless steel, we can categorize stainless steels into four families. And the main four types of stainless steel are called (firstly) ferritic, and (secondly) martensitic, and the third one is austenitic, and the final one is duplex stainless steel which contains two phases – ferritic and austenitic phases. Among these categories, austenitic stainless steel is most widely used. As I said in the former slide, stainless steels are categorized into four main types. And in this diagram, I expressed the four categories of stainless steel based on the nickel content and the chromium content. So, if stainless steel contains higher chromium and also higher nickel, then usually it is made for austenitic stainless steel. And the ferritic and martensitic stainless steel grades usually contain a high amount of chromium and also a very limited amount of nickel. And duplex stainless steel is in between. In this figure, stainless steel can be divided into two groups based on the chemical composition. Iron-chromium base and iron-chromium-nickel base. This iron-chromium is divided into ferritic and martensitic grades, and also this iron-chromium-nickel grade is usually divided into austenitic and duplex stainless steel. Sometimes we include this precipitation hardened grade. However, since this grade of stainless steel has a very limited application and is produced in very small quantities, usually we exclude this one from the main categories. And finally in this table, I just summarized the four main categories of stainless steel based on its microstructure and major alloy composition. Usually the ferritic and martensitic grades are expressed in numbers of four hundred series and the austenitic grades are expressed in three hundred or sometimes two hundred series, and duplex stainless steel is in another number. And here, you can see the main chemical composition or typical amount of the chromium and nickel, and also carbon in this table.
