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The role of various alloying elements in steel

The elements that are intentionally added during the smelting process in order to improve and improve certain properties of the steel and to achieve certain special properties are called alloying elements. Commonly used alloying elements are chromium, nickel, molybdenum, tungsten, vanadium, titanium, niobium, zirconium, cobalt, silicon, manganese, aluminum, copper, boron and rare earth. Phosphorus, sulfur, nitrogen, etc. also function as alloys in some cases.

(1) Cr Chromium can increase the hardenability of steel and have secondary hardening effect, which can improve the hardness and wear resistance of carbon steel without making the steel brittle. When the content exceeds 12%, the steel has good high-temperature oxidation resistance and oxidation resistance corrosion resistance, and also increases the heat strength of the steel. Chromium is the main alloying element of stainless steel acid-resistant steel and heat-resistant steel.

Chromium can increase the strength and hardness of carbon steel in rolling state, and reduce elongation and reduction of area. When the chromium content exceeds 15%, the strength and hardness will decrease, and the elongation and the area shrinkage rate will correspondingly increase. Parts with chrome steel are easily ground to achieve high surface finish quality.

The main role of chromium in the quenching and tempering structure is to improve the hardenability, so that the steel has good comprehensive mechanical properties after quenching and tempering. In the carburized steel, chromium-containing carbides can also be formed, thereby improving the surface resistance of the material. Grinding. Chromium-containing spring steel is not easily decarburized during heat treatment. Chromium can improve the wear resistance, hardness and red hardness of tool steel, and has good tempering stability. In electrothermal alloys, chromium increases the oxidation resistance, electrical resistance and strength of the alloy.

(2) Ni Nickel strengthens ferrite in steel and refines pearlite. The overall effect is to increase strength and have no significant effect on plasticity. In general, for low carbon steels that are used in rolling, normalizing or annealed conditions without quenching and tempering, a certain amount of nickel can increase the strength of the steel without significantly reducing its toughness. According to statistics, each increase of 1% nickel can increase the strength by 29.4Pa. As the nickel content increases, the yield of steel increases faster than the tensile strength, so the ratio of nickel-containing steel can be higher than that of ordinary carbon steel. Nickel, while increasing the strength of steel, has less impact on the toughness, plasticity, and other process properties of steel than other alloying elements. For medium carbon steel, since the pearl reduces the pearlite transformation temperature, the pearlite is made finer; and since nickel reduces the carbon content of the eutectoid point, the pearlite has a larger number of pearlite than the carbon content of the same carbon content. The strength of the nickel-containing pearlitic ferritic steel is higher than that of the carbon steel of the same carbon content. On the other hand, if the strength of the steel is the same, the carbon content of the nickel-containing steel can be appropriately lowered, so that the toughness and plasticity of the steel can be improved. Nickel can increase the resistance of steel to fatigue and reduce the sensitivity of steel to the gap. Nickel reduces the low temperature brittle transition temperature of steel, which is of great importance for low temperature steels. 3.5% nickel-containing steel can be used at -100 °C, and nickel-containing 9% steel can work at -196 °C. Nickel does not increase the resistance of steel to creep, so it is generally not used as a strengthening element for heat-strength steel.

The iron-nickel alloy with high nickel content has a linear expansion coefficient which changes significantly with the increase or decrease of nickel content. With this property, it is possible to design and produce precision alloys and bimetal materials with extremely low or constant linear expansion coefficients.

In addition, nickel is added to steel not only to resist acid, but also to alkali, and has resistance to the atmosphere and salt. Nickel is one of the important elements in stainless acid-resistant steel.

(3) Mo Molybdenum improves hardenability and heat strength in steel, prevents temper brittleness, increases remanence and coercivity, and resists in certain media.

In quenched and tempered steel, molybdenum can deepen and harden the parts of larger sections, improve the tempering resistance or tempering stability of the steel, so that the parts can be tempered at higher temperatures, thus eliminating more effectively ( Or reduce) residual stress and improve plasticity.

In addition to the above-mentioned effects, molybdenum in carburized steel can also reduce the tendency of carbides to form a continuous network on the grain boundaries in the carburized layer, reduce the retained austenite in the carburized layer, and relatively increase the surface layer. Wear resistance.

In the forging die steel, molybdenum can also maintain the steel with a relatively stable hardness and increase the deformation. Resistance to cracking and abrasion.

In the stainless acid-resistant steel, molybdenum can further improve the corrosion resistance to organic acids (such as formic acid, acetic acid, oxalic acid, etc.) as well as hydrogen peroxide, sulfuric acid, sulfurous acid, sulfate, acid dyes, bleaching powders and the like. In particular, the addition of molybdenum prevents the tendency of pitting corrosion caused by the presence of chloride ions.

W12Cr4V4Mo high speed steel containing about 1% molybdenum has wear resistance, tempering hardness and red hardness.

(4) W Tungsten is partially dissolved in iron to form a solid solution in addition to carbide formation in steel. Its effect is similar to that of molybdenum. The general effect is not as significant as molybdenum by mass fraction. The main sample of tungsten in steel is to increase tempering stability, red hardness, heat strength and increased wear resistance due to the formation of carbides. Therefore, it is mainly used for tool steel, such as high speed steel, steel for hot forging die, and the like.

Tungsten forms refractory carbides in high-quality spring steel, which can alleviate the aggregation process of carbides and maintain high high-temperature strength when tempered at higher temperatures. Tungsten also reduces the heat sensitivity of steel, increases hardenability and increases hardness. 65SiMnWA spring steel has high hardness after air-cooling, and the spring steel with 50mm2 cross-section can be hardened in oil, which can be used as an important spring to withstand heavy load, heat resistance (not more than 350 °C) and impact. 30W4Cr2VA high-strength heat-resistant high-quality spring steel with large hardenability, quenching at 1050~1100°C, tensile strength after tempering at 550~650°C reaches 1470~1666MPa. It is mainly used to manufacture springs that are used at high temperatures (not more than 500 ° C).

Since the addition of tungsten can significantly improve the wear resistance and machinability of steel, tungsten is the main element of alloy tool steel.

(5) V Vanadium has a strong affinity with carbon, ammonia and oxygen to form a corresponding stable compound. Vanadium is mainly present in the form of carbides in steel. Its main role is to refine the structure and grain of steel and reduce the strength and toughness of steel. When dissolved in a solid solution at a high temperature, the hardenability is increased; conversely, when it exists in the form of a carbide, the hardenability is lowered. Vanadium increases the tempering stability of hardened steel and produces a secondary hardening effect. The vanadium content in steel is generally not more than 0.5% except for high speed tool steel.

Vanadium can refine grains in ordinary low carbon alloy steel, improve the strength and yield ratio and low temperature characteristics after normalizing, and improve the welding performance of steel.

Vanadium is often used in alloy structural steels in combination with elements such as manganese, chromium, molybdenum and tungsten because it reduces hardenability under general heat treatment conditions. In the quenched and tempered steel, vanadium mainly improves the strength and yield ratio of the steel, refines the grain and the superheat sensitivity of the crucible. In the carburized steel, because the grain can be refined, the steel can be directly quenched after carburizing without secondary quenching.

Vanadium improves strength and yield ratio in spring steel and bearing steel, in particular, increases the ratio limit and elastic limit, and reduces the decarburization sensitivity during heat treatment, thereby improving the surface quality. The five-chrome vanadium-containing bearing steel has high carbonization dispersion and good performance.

Vanadium refines grains in tool steel, reduces overheat sensitivity, increases tempering stability and wear resistance, and extends tool life.

(6) Ti Titanium has a strong affinity with nitrogen, oxygen and carbon, and its affinity with sulfur is stronger than that of iron. Therefore, it is a good deoxidizing deaerator and an effective element for fixing nitrogen and carbon. Although titanium is a strong carbide forming element, it does not combine with other elements to form a composite compound. Titanium carbide has strong binding force, is stable, and is not easy to decompose. It can be slowly dissolved into solid solution in steel only when heated to above 1000 °C. The titanium carbide particles have an effect of preventing grain growth before they are dissolved. Since the affinity between titanium and carbon is much greater than the affinity between chromium and carbon, titanium is commonly used in stainless steel to fix carbon therein to eliminate the depletion of chromium at the grain boundaries, thereby eliminating or reducing intergranular corrosion of the steel.

Titanium is also one of the strong ferrite forming elements, which strongly increases the temperature of steel A1 and A3. Titanium improves plasticity and toughness in ordinary low alloy steels. Since titanium fixes nitrogen and sulfur and forms titanium carbide, the strength of the steel is increased. After normalizing to refine the grain, precipitation of carbides can significantly improve the plasticity and impact toughness of the steel. Titanium-containing alloy structural steel has good mechanical properties and process properties. The main disadvantage is that the hardenability is slightly poor. .

In high-chromium stainless steel, it is usually necessary to add about 5 times the carbon content of titanium, which not only improves the corrosion resistance of steel (mainly resistance to intergranular corrosion) and toughness; but also tends to improve the grain growth tendency of steel at high temperatures. Welding properties of steel.

(7) Nb/Cb. Part of the cerium and lanthanum are dissolved in the solid solution to effect solid solution strengthening. When the austenite is dissolved, the hardenability of the steel is remarkably improved. However, in the form of carbides and oxide particles, the grains are refined and the hardenability of the steel is lowered. It can increase the tempering stability of steel and has secondary hardening effect. Trace bismuth can increase the strength of the steel without affecting the ductility or toughness of the steel. Due to the effect of refining the grains, the impact toughness of the steel can be improved and the brittle transition temperature can be lowered. When the content is more than 8 times that of carbon, almost all the carbon in the steel can be fixed, so that the steel has good hydrogen resistance. In the austenitic steel, intergranular corrosion of the steel by the oxidizing medium can be prevented. Due to the fixed carbon and precipitation hardening, the high temperature properties of the heat-strength steel, such as creep strength, can be improved.

In the ordinary low-alloy steel for construction, it can improve the yield strength and impact toughness, and reduce the brittle transition temperature and beneficial weldability. In the carburizing and quenching and tempering alloy structural steel while increasing the hardenability. Improve the toughness and low temperature properties of steel. It can reduce the air hardenability of low carbon martensitic heat-resistant stainless steel, avoid hardening and temper brittleness, and improve creep strength.

(8) Zr Zirconium is a strong carbide forming element, and its role in steel is similar to that of lanthanum, cerium and vanadium. The addition of a small amount of zirconium has the functions of degassing, purifying and refining grains, which is beneficial to the low temperature performance of steel and improves the punching performance. It is commonly used in the manufacture of ultra high strength steel and nickel base superalloys for gas engine and ballistic missile structures.

(9) Co Cobalt is mostly used in special steels and alloys. Cobalt-containing high-speed steel has high high-temperature hardness. Ultra-high hardness and good comprehensive mechanical properties can be obtained by adding molybdenum to maraging steel. In addition, cobalt is also an important alloying element in heat-strength steels and magnetic materials.

Cobalt reduces the hardenability of steel. Therefore, the addition of carbon steel alone reduces the overall mechanical properties after quenching and tempering. Cobalt strengthens ferrite and is added to carbon steel. It improves the hardness, yield point and tensile strength of steel under annealing or normalizing conditions. It has an adverse effect on elongation and reduction of area. Impact toughness also follows. The cobalt content increases and decreases. Since cobalt has antioxidant properties, it is used in heat resistant steels and heat resistant alloys. The cobalt-based alloy gas turbine shows its unique role.

(10) Si silicon can be dissolved in ferrite and austenite to improve the hardness and strength of steel. Its effect is second only to phosphorus, and stronger than elements such as manganese, nickel, chromium, tungsten, molybdenum and vanadium. However, when the silicon content exceeds 3%, the plasticity and toughness of the steel are significantly reduced. Silicon can increase the elastic limit, yield strength and yield ratio (σs/σb) of steel, as well as fatigue strength and fatigue ratio (σ-1/σb). This is because silicon or silicon manganese steel can be used as a spring steel.

Silicon can reduce the density, thermal conductivity and electrical conductivity of steel. It can promote the coarsening of ferrite grains and reduce the coercive force. There is a tendency to reduce the anisotropy of the crystal, make the magnetization easy, and the magnetoresistance is reduced, which can be used to produce electrical steel, so the magnetic resistance loss of the silicon steel sheet is low. Silicon can increase the magnetic permeability of ferrite, so that the steel sheet has a higher magnetic induction strength under a weaker magnetic field. However, silicon reduces the magnetic induction strength of steel under strong magnetic fields. Silicon has a strong deoxidizing power, which reduces the magnetic aging effect of iron.

When the silicon-containing steel is heated in an oxidizing atmosphere, a SiO2 film is formed on the surface, thereby improving the oxidation resistance of the steel at a high temperature. Silicon promotes the growth of columnar crystals in cast steel and reduces plasticity. If the silicon steel cools faster when heated, the internal and external temperature difference of the steel is large due to the low thermal conductivity, and thus it is broken.

Silicon can reduce the weldability of steel. Because silicon is stronger than iron, it is easy to form low-melting silicate during welding, which increases the fluidity of slag and molten metal, causing splashing and affecting the quality of welding. Silicon is a good deoxidizer. When deoxidizing with aluminum, a certain amount of silicon is added as appropriate, which can significantly improve the deoxidation rate. Silicon has a certain residual in steel, which is brought into the raw material during ironmaking steelmaking. In boiling steel, silicon is limited to <0.07%. When intentionally added, a ferrosilicon alloy is added during steel making.

(11) Mn Manganese is a good deoxidizer and desulfurizer. Steel generally contains a certain amount of manganese, which can eliminate or reduce the hot brittleness of steel caused by sulfur, thereby improving the hot workability of steel.

a solid solution of manganese and iron, which increases the hardness and strength of ferrite and austenite in steel; at the same time it is an element of carbide formation, which replaces a part of iron atoms in cementite, and manganese reduces the critical transition temperature in steel. It plays the role of refining the pearlite and indirectly improving the strength of the pearlite steel. Manganese’s ability to stabilize austenite is second only to nickel, and it also strongly increases the hardenability of steel. Manganese having a content of not more than 2% has been used in combination with other elements to form a plurality of alloy steels.

Manganese is rich in resources and diverse in performance, and has been widely used, such as carbon structural steel with high manganese content and spring steel.

In high-carbon high-manganese wear-resistant steel, the manganese content can reach 10% to 14%, and it has good toughness after solution treatment. When it is deformed by impact, the surface layer will be strengthened by deformation and has high resistance. Grinding.

Manganese and sulfur form a higher melting point of MnS, which prevents hot brittleness due to FeS. Manganese has a tendency to increase the grain coarsening of steel and temper brittleness sensitivity. If the smelting and pouring and chilling are not properly cooled, it is easy to make the steel white spots.

(12) Al aluminum is mainly used to deoxidize and refine grains. In the nitriding steel, a hard and corrosion resistant nitriding layer is formed. Aluminum can suppress the aging of low carbon steel and improve the toughness of steel at low temperatures. When the content is high, the oxidation resistance of the steel and the corrosion resistance in the oxidizing acid and the H2S gas can be improved, and the electrical and magnetic properties of the steel can be improved. Aluminum has a large solid solution strengthening effect in steel, which improves the wear resistance, fatigue strength and core mechanical properties of carburized steel.

In the hard alloy, aluminum and nickel form compounds to improve the smelting strength. The aluminum-containing iron-chromium aluminum alloy has near-resistance characteristics and excellent oxidation resistance at high temperatures, and is suitable for electrosmelting alloy materials and chrome aluminum. Resistance wire.

When some steels are deoxidized, if the amount of aluminum is too much, the steel will have an abnormal structure and a tendency to promote graphitization of the steel. In ferritic and pearlitic steels, when the aluminum content is high, the high temperature strength and toughness are lowered, and some difficulties are brought to the smelting and casting.

(13) The prominent role of Cu copper in steel is to improve the atmospheric corrosion resistance of ordinary low alloy steel. Especially when combined with phosphorus, adding copper can also improve the strength and yield ratio of steel, but it is not harmful to the welding performance. influences. The steel (U-Cu) containing 0.20% to 0.50% of copper has a corrosion-resistant life of 2 to 5 times that of a general carbon steel rail in addition to wear resistance.

When the copper content exceeds 0.75%, the aging strengthening effect can be produced after solution treatment and aging. When the content is low, its effect is similar to that of nickel, but it is weak. When the content is high, it is unfavorable for thermal deformation processing, and causes copper brittleness during hot deformation processing. 2% to 3% of copper can resist corrosion resistance and stress corrosion corrosion of sulfuric acid, phosphoric acid and hydrochloric acid in austenitic stainless steel.

(14) B The main role of boron in steel is to increase the hardenability of steel, thereby saving other less expensive metals, and nickel, chromium, molybdenum and the like. For this purpose, the content is generally specified in the range of 0.001% to 0.005%. It can replace 1.6% nickel, 0.3% chromium or 0.2% molybdenum. Boron molybdenum should be noted that molybdenum can prevent or reduce temper brittleness, while boron has a slight tendency to promote temper brittleness, so it cannot be used. Boron completely replaces molybdenum.

Boron is added to the medium carbon carbon steel. Since the hardenability is improved, the properties of the steel with a thickness of 20 mm or more can be greatly improved after quenching and tempering. Therefore, 40B and 40MnB steel can be used instead of 40Cr, and 20CrMnTi carburized steel can be replaced by 20Mn2TiB steel. However, since the effect of boron decreases or even disappears with the increase of carbon content in the steel, in the selection of boron-containing carburized steel, it must be considered that after the carburization of the part, the hardenability of the carburized layer will be lower than that of the core. This feature of permeability.

Spring steel is generally required to be completely hardened, and usually the spring area is not large, and it is advantageous to use boron-containing steel. The effect of boron on high-silicon spring steel fluctuates greatly and is inconvenient to adopt.

Boron has a strong affinity for nitrogen and oxygen. The addition of 0.007% boron to the boiling steel can eliminate the aging of steel.

(15) RE Generally speaking, the rare earth element refers to a lanthanide element (15) with an atomic number from 57 to 71 in the periodic table, plus 17 elements of No. 21 and No. 39. They are close in nature and difficult to separate. Unseparated is called mixed rare earth, which is relatively cheap. Rare earth elements can improve the plasticity and impact toughness of wrought steel, especially in cast steel. It can improve the creep resistance of heat-resistant steel electrothermal alloys and high-temperature alloys. Rare earth elements can also improve the oxidation resistance and corrosion resistance of steel. The effect of oxidation resistance exceeds that of elements such as silicon, aluminum, and titanium. It can improve the fluidity of steel, reduce non-metallic inclusions, and make the steel structure dense and pure.

Adding appropriate rare earth elements to common low-alloy steels has good deoxidation and desulfurization, improves impact toughness (especially low temperature toughness), and improves anisotropic properties. The rare earth element increases the oxygen resistance of the alloy in the iron-chromium-aluminum alloy, maintains the fine grain of the steel at a high temperature, and increases the high-temperature strength, thereby significantly increasing the life of the electrothermal alloy.

(16) N Nitrogen is partially used in iron and has solid solution strengthening and hardenability improvement, but it is not significant. Since the nitride precipitates on the grain boundary, the high temperature strength of the grain boundary can be increased, and the creep strength of the steel is increased. It combines with other elements in steel and has precipitation hardening. The corrosion resistance of the steel is not significant, but after the surface of the steel is nitrided, it not only increases its hardness and wear resistance, but also significantly improves the corrosion resistance. Residual nitrogen in low carbon steel results in ageing brittleness.

(17)S Increasing the sulfur and manganese content can improve the machinability of steel. In free-cutting steel, sulfur is added as a beneficial element. Sulfur is severely segregated in steel. Deteriorating the quality of steel, reducing the plasticity of steel at high temperatures, is a harmful element, which exists in the form of FeS with a lower melting point. The melting point of FeS alone is only 1190 ° C, and the eutectic temperature of forming a eutectic with iron in steel is lower, only 988 ° C. When the steel solidifies, the iron sulfide is concentrated at the primary grain boundary. When the steel is rolled at 1,100 to 1,200 ° C, the FeS on the grain boundary will melt, which greatly weakens the bonding force between the grains, resulting in hot brittleness of the steel, so the sulfur should be strictly controlled. Generally controlled at 0.020% to 0.050%. In order to prevent brittleness due to sulfur, sufficient manganese should be added to form a higher melting point of MnS. If the flow rate in the steel is too high, pores and looseness will form in the weld metal due to the generation of SO2 during welding.

(18) P Phosphorus has strong solid solution strengthening and cold work hardening in steel. As an alloying element added to the low-alloy structural steel, it can improve its strength and the atmospheric corrosion resistance of steel, but reduce its cold stamping performance. Phosphorus combined with sulfur and manganese can increase the cutting performance of steel, increase the surface quality of the machined parts, and be used for free-cutting steel, so the phosphorus content of the free-cutting steel is also relatively high. Phosphorus is used for ferrite. Although it can improve the strength and hardness of steel, the biggest harm is that segregation is serious, temper brittleness is increased, and the plasticity and toughness of steel are significantly increased, which makes the steel easy to be brittle during cold working. Brittle phenomenon. Phosphorus also has an adverse effect on weldability. Phosphorus is a harmful element and should be strictly controlled. The general content is not more than 0.03% to 0.04%.
Source: China Pipe Fittings Manufacturer – Yaang Pipe Industry Co., Limited (www.yaang.com)

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