Effect of Various alloying elements in Steel/ Iron & Stainless Steel
Steel is primarily an alloy of iron and carbon and certain additional elements such as Manganese & silicon. Alloying here refer to the addition of other elements to achieve the desired mechanical (higher tensile, yield, toughness etc.), physical (Hardness, color etc.) and chemical properties (e.g. corrosion resistance).
Different alloying elements have their own effect on the properties of steel. In this post you will learn the most of the alloying elements, their effect on the properties of steel with their addition as well as their importance for a Welding, material, metallurgist and QA-QC Engineers.
The below table summarize the effects of alloying elements in steel. For a detailed explanation, continue through the post.
Carbon (C) effect on steel
Carbon is a strong austenitic stabilizer, increases the tensile strength of steels by increasing the amount of carbide present. Carbon increases the hardening capacity of the steel so that it may be effectively quenched and tempered. Carbon with its unique effects on steel provides allotropic transformation to the steel.
Carbon strongly decreases the toughness & corrosion resistance in ferritic steels. martensitic grades carbon increases hardness and strength, but decrease toughness. This effect is more when present as lamellar (layered) cementite in pearlite rather than round (globular/spheroidal) particles.
Silicon (Si) effect on steel
Silicon increase resistance to oxidation, both at high temperature and in strongly oxidizing solutions at lower temperatures. Silicon being a ferrite stabilizer promote ferritic microstructures. Silicon increases strength in steel along with primary function as deoxidizer. It moderate increase in hardening capacity.
Manganese (Mn) effect on steel
Manganese is added upto to 1.8 wt%. It combines with sulfur to form less harmful manganese sulfide inclusions in high sulfur steels thus prevent issues of hot cracking during welding. It Increases the steel’s strength but less than silicon. It help to increases the steel’s toughness to room temperature. Manganese considerably increases the steel’s hardening capacity.
Manganese is generally used to improve hot ductility. Its effect on the ferrite/austenite balance varies with temperature: at low temperature manganese is an austenite stabilizer, but at high temperatures it will stabilize ferrite. Manganese increases the solubility of nitrogen and is used to obtain high nitrogen contents in duplex and austenitic stainless steels. Manganese, as an austenite former, can also replace some of the nickel in stainless steel.
Nickel (Ni) effect on steel
The main reason for adding nickel is to promote an austenitic microstructure. Nickel generally increases ductility and toughness. It also reduces the corrosion rate in the active state and is therefore advantageous in acidic environments. In precipitation hardening steels nickel is also used to form the intermetallic compounds that are used to increase strength. In martensitic grades adding nickel, combined with reducing carbon content, improves weldability.
Nickel has Little effect on steel’s strength and hardening capacity but considerably improves its low temperature toughness by promoting stable austenitic even at room temperature. Nickel also increases the atmospheric corrosion resistance of the steel.
Chromium (Cr) effect on steel
This is the most important alloying element and it gives stainless steels their basic corrosion resistance. All stainless steels have a Cr content of at least 10.5% and the corrosion resistance increases the higher chromium content. Chromium promotes a ferritic microstructure.
Chromium has little effect on steel’s strength but increases the steel’s hardening capacity. It increases the steel’s resistance to scale/oxide formation when heated to elevated temperatures, thus a primary alloying element for high temperature material such as Cr-Mo steels. Also, it combines with carbon to form chromium carbides that are more stable than cementite, i.e., they do not break down with time at elevated temperature applications. Chromium helps to maintain the steel’s strength and reduces its flow (creep) at higher temperatures and for longer periods of time.
Molybdenum (Mo) effect on steel
Molybdenum significantly increases the resistance to both uniform and localized corrosion. It slightly increases mechanical strength and strongly promotes a ferritic microstructure. However, molybdenum also enhances the risk for the formation of secondary phases in ferritic, duplex, and austenitic steels. In martensitic steels it increases the hardness at higher tempering temperatures due to its effect on carbide precipitation.
Molybdenum Increases hardening capacity, slightly more than chromium. It forms more stable carbide than cementite and increases the steel’s resistance to deformation (creep) thus also an important alloying element for high temperature application steels such as Cr-Mo steels.
Vanadium (V) effect on steel
Vanadium forms carbides & nitrides & promotes ferrite in the microstructure. Vanadium is added for strength and toughness via grain refinement in as-rolled (control) as well as normalized steels. It helps by retaining higher hardness and strength after tempering in quenched and tempered steels. Also added in some steels meant for elevated temperature applications such as Cr-Mo-V steels for reactors. It increases the hardness of martensitic steels due to its effect on the type of carbide present. It also increases tempering resistance. It is only used in stainless steels that can be hardened.
Niobium (Nb) effect on steel
Niobium also known as Colombium in U.S. is a strong ferrite and carbide former. Like titanium, it promotes a ferritic structure. In austenitic steels it is added to improve the resistance to intergranular corrosion (stabilized grades), but it also enhances mechanical properties at high temperatures. In ferritic grades niobium and/or titanium is sometimes added to improve toughness and to minimize the risk for intergranular corrosion. In martensitic steels niobium lowers hardness and increases tempering resistance.
It is added for strength and toughness since a fine dispersion of niobium carbides promotes grain refinement. It also helps retain fine grain size in the heat affected zones of welds. Niobium is added in stainless steel as a stabilizing element (other stabilizing element is titanium) as it easily combine with carbon and prevent the formation of chromium carbide in stainless steel.
Copper (Cu) effect on steel
Copper is added to increase the corrosion resistance and steel strength. Copper promotes an austenitic microstructure. The effects of copper on toughness and hardening capacity are small. It increases the atmospheric corrosion resistance of the steel. Total amounts of copper added are small to prevent hot shortness in steel.
Boron (B) effect on steel
Boron added to relatively low carbon steels in very small amounts to increase the hardening capacity of steels meant to be quenched and tempered. Boron is a very strong strengthening agent when used in combination with molybdenum, titanium or vanadium.
Nitrogen (N) effect on steel
Nitrogen is a very strong austenite former that also significantly increases mechanical strength. It also increases resistance to localized corrosion, especially in combination with molybdenum. In ferritic stainless steels nitrogen strongly reduces toughness and corrosion resistance. In martensitic grades nitrogen increases both hardness and strength but reduces toughness.
Nitrogen is intentionally added only when other elements like vanadium are present so that vanadium nitrides can improve strength and help refine the grain size. Nitrogen being a strong austenitic stabilizer is added in austenitic stainless steel and duplex stainless steel.
Aluminum (Al) effect on steel
Aluminum is added in substantial amounts. Aluminum improves oxidation resistance and is used in certain heat-resistant grades for this purpose. In precipitation hardening steels, aluminum is used to form the intermetallic compounds that increase the strength in the aged condition.
Titanium (Ti) effect on steel
Titanium is a strong ferrite and carbide former, lowering the effective carbon content and promoting a ferritic structure in two ways. In austenitic steels with increased carbon content it is added to increase the resistance to intergranular corrosion (stabilized grades), but it also increases mechanical properties at high temperatures. In ferritic grades titanium is added to improve toughness, formability, and corrosion resistance. In martensitic steels titanium lowers the martensite hardness by combining with carbon and increases tempering resistance. In precipitation hardening steels, titanium is used to form the intermetallic compounds that are used to increase strength.
Titanium is an element that is primarily added to tie up Carbon, also known as carbide stabilization. This improves weldability because the carbon and titanium combination (titanium carbides) are stable and hard to dissolve in steel. This minimizes inter-granular corrosion occurrences.
Cobalt (Co) effect on steel
Cobalt is used in martensitic steels, where it increases hardness and tempering resistance, especially at higher temperatures. Cobalt is also used in hard facing materials due to high hardness. With Nuclear applications, Cobalt restrictions are necessary though as the element can become highly radioactive when exposed to radiation.
Sulfur (S) effect on steel
Sulfur is added to certain stainless steels to increase their machinability. At the levels present in these grades, sulfur slightly reduces corrosion resistance, ductility, weldability, and formability. Lower levels of sulfur can be added to decrease work hardening for improved formability. Slightly increased sulfur content also improves the weldability of steel.
Tungsten (W) effect on steel
tungsten increases hardness particularly at elevated temperatures due to stable carbides, refines grain size. Tungsten is added to special grades such as Alloy 686, Super Duplex grade 4501, which is highly corrosion resistance material.