Alloying Elements of Stainless Steel and Their Applications |
Posted: July 30, 2018 |
It is a known fact that stainless steel is the most commonly used alloy in all constructional purposes across the entire world. Be it a threaded steel bar or any fastener, stainless steel is used for providing maximum corrosion resistance, technical functionality and much more. Even high quality stainless steel are now presently used for making access hardware friction hinges too. This is done as the alloy exhibits long and trouble-free working lives of the same.
Elements of Alloying in Stainless SteelStainless steel comprises of numerous alloying elements depending on the specific grade and composition. Although, one thing must noted that Iron is the main constituent here, with which these alloying elements are present in subsequent amounts. Let us explore them one by one and also discuss their applications at the same time. Carbon (C)Iron is alloyed with Carbon to produce steel. The presence of carbon increases the strength and hardness of steel. For instance, more the carbon content in a threaded bar made out of carbon steel, more harder it is. One thing must be noted that pure iron cannot be hardened simply with the help of heat treatment and thus requires carbon to strengthen and harden it. On the contrary, in Austenitic and Ferritic stainless steels, a high carbon content is undesirable. This holds true especially for welding due to the threat of carbide precipitation. Chromium (Cr)Chromium is added to steel to make it absolutely ‘stainless’ by increasing resistance to oxidation. Traditionally, stainless steel has a Chromium percentage of (11-12)% and the minimum amount is 10 %. This provides a measured degree of general corrosion resistance as compared to other types of steel that have a much lower percentage of Chromium. The corrosion resistance is due to the formation of a self-repairing thin passive layer of Chromium Oxide, that forms on the surface of the stainless steel.
Image Courtesy - Wikipedia Manganese (Mn)It is added to steel for enhancing the hot working properties,strength, hardness and toughness as well. Manganese is an Austenite forming element much alike Nickel. It has been used as a substitute of Nickel in the AISI 200 Series of Austenitic Stainless Steels. For example, AISI 202 is usedas a substitute for AISI 304. Nickel (Ni)Nickel is added in significant amount to Chromium stainless steel in considerably large amounts (almost 8 %) of for instilling the class of heat resisting and corrosion free steels. These are Austenitic stainless steel, typified by 18-8 (304/1.4301). The tendency of Nickel to form Austenite is responsible for incorporating great impact strength and high strength at both high and low temperatures. Nickel also acts as one of the major contributors in providing better resistance against corrosion and oxidation. Due to its presence, an anchor bolt, made of stainless steel, remains corrosion free for a long period of time. Molybdenum (Mo)When Molybdenum is added to chromium-nickel austenitic steels, it improves resistance to pitting and crevice corrosion. This is mainly seen in chloride and sulphur containing environments. Titanium (Ti)It is added to stainless steel for carbide stabilization when the material is about to be welded. It combines with carbon to form titanium carbides, that are quite stable and do not dissolve in steel. This tends to minimize the occurrence of intergranular corrosion. Adding approximately (0.25 / 0.60)% titanium causes the carbon to blend with titanium in preference to chromium. This prevents a tie-up of corrosion-resisting chromium as inter-granular carbides and the accompanying loss of corrosion resistivity at the grain boundaries. The usage of Titanium, however has decreased gradually over the years as the manufacturers or stainless steel makers have developed low carbon content stainless steels that are readily weldable without stabilization.
Nitrogen (N)Nitrogen enhances the austenitic stability of stainless steel. Yield strength and resistance to pitting corrosion are greatly improved upon addition of Nitrogen. Copper (Cu)It is generally present as a residual element in stainless steel. However it is added to alloys for instilling precipitation hardening properties and enhance corrosion resistance especially in sea water environments and sulphuric acid. Phosphorus (P)Phosphorus is usually added with sulphur to boost up the machinability. Its presence in Austenitic stainless steel increases strength as well. On the other hand, it has a serious negative impact on the corrosion resistance and causes the material to crack under welding as well.
Sulphur (S)It improves machinability when added in small amounts. However quite similar to Phosphorus, it also has a detrimental effect on corrosion resistance and weldability. Selenium (Se)It was used for boosting machinability many years back. Niobium / Columbium (Nb)Niobium is also added to stainless steel for stabilizing carbon and functions pretty much similarly like Titanium. It also has a positive effect on strengthening steel and alloys at high temperatures. SiIicon (Si)It is mainly used for used as a deoxidizing agent in the melting of steel. As a result of this, most steel types including stainless steel contains a small percentage of it. Cobalt (Co)It becomes highly radioactive when exposed to intense radiation of nuclear reactors. This is the reason why any stainless steel that is in nuclear service will have a Cobalt restriction which is usually approximately 0.2% maximum. This issue is focused upon seriously because a residual; amount of Cobalt remains in the Nickel that is used for making Austenitic stainless steels. Calcium (Ca)Small amounts are added for improving the machinability. Although Calcium does not exhibit the detrimental effects like that of Sulphur, Phosphorus and Selenium. ConclusionSo we have seen excluding one or two elements, the rest of the elements used in the alloy of stainless steel have their own functionalities and are present in subsequent amounts. This makes stainless steel useful in many number of ways.
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