• 30.06.2022 11:23:34

What is Stainless Steel?

Stainless steel is described as an iron-carbon alloy containing a minimum of 10.5% chromium in metallurgy.

The chromium element is the basic component which protects the steel against corrosion, as it can be seen in the following figure. It gets its name from the fact that these steels do not stain, corrode and rust like other steels. This material is also referred to as corrosion resistant steel, especially in the aerospace industry, without detailing alloy types and grades. Today, it is very easy to reach stainless steels in many different and easily accessible qualities and surface properties which operate without any problems in harsh environmental conditions where it is applied throughout the life of the product. Even in our normal daily life, we see that these products are used widely, from cutlery to watches.

Stainless steel has a significant resistance to corrosion and oxidation in many natural and man-made environments. However choosing the right grade and type of stainless steel for each particular application is crucial. The initial step in quality selection starts with a detailed analysis and definition of all possible and existing working conditions that stainless steel will be exposed to during the design phase.

The high oxidation resistance at normal room temperature and weather conditions is achieved by way of adding chromium up to a minimum of 13% (by weight), and up to 30% in very harsh and difficult environmental conditions. When the chromium element in stainless steel is exposed to oxygen (meaning the oxygen in the normal atmosphere), it immediately forms a chromium-oxide (Cr2O3) passivation layer. This layer is too thin to be seen with the naked eye and protects the metal it covers by completely preventing the penetration of water or any gas, oxygen to the metal (stainless steel) which makes up the product. Moreover, if this layer is torn, opened or scratched for any reason, the opening that will occur occurs again very quickly as the layer renews itself. This phenomenon is referred to as passivation and is also seen in some other metals such as titanium.

The element nickel also contributes to the passivation property, like other elements such as molybdenum and vanadium, which are used in low proportions.

The stainless steel's resistance to corrosion and tarnishing makes it an ideal and indispensable material in a wide range of commercial applications, due to its low maintenance cost, lower cost compared to others, and attractive appearance. As much as there are over 150 stainless steel grades in total, 15 of them are widely used and well-known in the market. The stainless steels, like other steels with their relatives, may be used in many forms such as flat products, plates, rods, wires, pipes, shaped long products, cold and hot rolling methods and casting parts, in the food industry, medicine, surgical equipment, industrial and also it is widely used in many areas such as hardware, automotive, white goods, structures and building elements, buildings. One of the fields where stainless steels are used is also used in products that we never leave with us in our daily lives, such as jewellery and watches. The most common grade which is used in jewellery is 316L. Stainless steel does not oxidize and tarnish over time like silver. Moreover, because the density of stainless steel is slightly lighter than silver, it provides practicality to designers.

Stainless steel is 100% recyclable. 60% of the stainless steel used is produced by way of re-evaluating recycled materials such as stainless steels from end-of-life products and scraps from production processes.

Several corrosion resistant irons have survived from ancient times to the present day. The most famous and biggest example of this is the “Sewn Iron of Delhi” built by Kumara Gupta I in the 400s in Delhi, India. However unlike stainless steel, this iron monument gets its non-corrosiveness not from chromium, however from the high phosphorus it contains. Phosphorus, together with suitable local weather conditions, forms a protective surface layer consisting of iron-oxide and phosphorus on the surface of the monument and provides the resistance of iron against corrosion.

The resistance of iron-chromium alloys to corrosion was first observed by the French metallurgist Pierre Berthier in 1821. Since the technology at that time was not sufficient to process chrome and iron as it is today, it could not be used practically.

In the era of 1890s, the German Hans Goldschmidt invented the alumina-thermic method, which paved the way for carbonless chromium production. Since then, many studies have been carried out on stainless steel.

The beginnings of modern stainless steels started in 1913 when they were accidentally discovered by British metallurgist Harry Brearly while researching to improve rifle barrels. This was the starting point for stainless steel. In later studies, austenitic stainless steel (304), which is referred to as 18-8, was introduced to the market in the 1920s and a great success was achieved in its applications. Since the 1930s, stainless steel has taken an indispensable place in the industry and in our daily life. Continuing these developments with other alloying elements, the researchers developed the stainless steel grades we know today over time.

Duplex stainless steels were developed about 70 years ago for use in the sulphide paper industry. Duplex alloys were initially used to deal with the problem of corrosion caused by chloride bed coolants and other harsh chemical process fluids. With the development of gas and oil stations in the North Sea in the 1970s, a second generation of duplex stainless steel was developed. Thus, the addition of a certain amount of nitrogen increased the toughness, weldability, and chloride corrosion resistance. The most commonly used mixed composition called UNS S31803 duplex steel UNS S32205 was standardized in 1996.

During World War II, precipitation hardened stainless steels were discovered through research. The first example of these non-standard grades was stainless steel referred to as 17-7PH in 1948.