Usually we often discuss the properties of steel amongst ourselves, which chemical element affects it. In this article, we will discuss how some chemical elements affect steel’s mechanical properties and other. Generally, steel is an alloy of carbon and iron, it does contain many other elements, some of which are retained from the steel making process, others element are added to produce specific properties.
I discussing here about some chemical elements such as Carbon, Phosphorus, sulfur, Silicon, Manganese, Chromium, Aluminum, Molybdenum, Nickel and Vanadium etc.
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Carbon play most important role in steel. Generally carbon content is directly linked with strength and hardenability. When we increase the carbon content then tensile strength and hardness also increase, as well as response to hardenability (Heat treatment). When carbon increase, then decrease the ductility, forgeability and machinability. Also it is true for weldability properties of the steel means if the carbon content increase then weldability decrease respectively.
In case carbon content more than 0.22%, steel welding performance will get worse. Meanwhile, high carbon content steel also reduces the air corrosion resistance, and the high carbon steel in the open field is easy to rust.
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2. Manganese (Mn):
Steels usually contain minimum 0.30% manganese, however, amounts of up to 1.5% can be found in some carbon steels. In addition, the steel which contains 11-14% manganese have high-wear resistance and its high work hardening ability makes it applicable for various applications including armors.
It increases the hardenability and tensile strength but decreases ductility. The increased hardenability is due to the cooling rate which is decreased by manganese during the hardening process. Apart from the effect on hardening and strength it also fulfils the role of a mild deoxidiser.
During carburizing process, manganese also tends to increase the rate of carbon penetration in steel. Manganese is capable to form Manganese Sulphide (MnS) with sulphur, which is beneficial to machining. At the same time, it counters the brittleness from sulphur and is beneficial to the surface finish of carbon steel.
If manganese content increasing then corrosion resistance and welding performance reduces.
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Sulfur is usually an undesirable impurity in steel rather than an alloying element. Even sulfur is a harmful element, if content less than 0.05%, it would be accept in general applications.
Sulfur improves machinability but lowers transverse ductility and notched impact toughness and has fewer effects on the longitudinal mechanical properties.
However, the content of sulfur in free cutting or free machining steel can contain up to 0.35% sulfur. Free machining steel also known as resulfurized steel has improved machinability properties, but is not recommended for welding. Higher sulfur content will lower the weldability and increases the risk of welding cracks forming.
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Phosphorus is an undesirable impurity because of its embrittling effect. In high grade steel, the maximum amount of phasphorus between 0.03%- 0.05%, due to the fact that is detrimental. Up to 0.10% of phosphorus in low-alloy high-strength steels will increase the strength as well as improve the steel’s resistance against corrosion.
The possibility of brittlement increases when the content in hardened steel is too high. Even though the strength and hardness is improved, the ductility and toughness decreases.
Chromium is most powerful alloying element for steel. Chromium presents in small amounts in certain structural steels. It is primarily used to increase hardenability of steel and increase the corrosion resistance as well as the yield strength of the steel material.
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For that reason often occurs in combination with nickel and copper. Stainless steels may contain in excess of 12% chromium. The well-known “18-8” stainless steel contains 8 percent of nickel and 18 percent of chromium.
Generally in steel the percent of chromium exceeds 1.1% a surface layer is formed that helps protect the steel against oxidation.
Generally small amount of silicon are present in rolled steel, it used as deoxidizer. However, commonly 0.35 to 1.00% silicon present in steel castings. Silicon dissolves in iron and tends to strengthen it. Weld metal usually contains approximately 0.50% silicon as a deoxidizer in order to prevent defects and/or damages.
The strengthening factor is due to the silicon which dissolves in the iron. If silicon combines with tungsten, molybdenum, chromium and other composition then improve the corrosion resistance and anti-oxidation. So this combination of steel fit for producing the electrical heat-resistant steel.
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Molybdenum can also increase the toughness, hardenability and tensile strength of steel and also increase elevated temperature strength, improves corrosion resistance as well as increased creep strength. Molybdenum is a strong carbide former and is usually present in alloy steels in amounts less than 1%.
In austenitic stainless steels it improves pitting corrosion resistance. It is also used in high speed tool steel.
It is very strong deoxidizer, grain refining and crystalline microstructure and increases the steel grade’s toughness. It is use as an alloying element and use as nitride former. It forms aluminum oxide or alumina (Al2O3) and decreases the amount of oxygen in the steel during the production of killed steels.
Combined with chromium and silicon can improve the peeling resistance and high temperature resistance of steel.
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9. Tungsten (W):
Tungsten used with chromium, vanadium, molybdenum, or manganese to produce high speed steel used in cutting tools. It improves the red hardness and thermal strength significantly. Tungsten steel is said to be “red-hard” or hard enough to cut after it becomes red-hot. Tungsten with Carbon together forms tungsten carbide which has a high hardness and wear resistance.
When it used with other alloying elements it restricts grain growth, refines grain size, increases hardenability, fracture toughness, and resistance to shock loading. As soon as it exceeds 0.05% the hardenability decreases due to the forming of carbide.
The positive effect of the formation of carbide is that wear resistance is higher and the temperature strength is increased. In hardened and tempered steels the rate against which the grain grows during heat treatment is lower due to the presence of vanadium. Mostly vanadium used in high speed steel and hot forming steel.
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