In order to improve and improve some properties of steel and make it obtain some special properties, the elements that are intentionally added in the smelting process are called alloying elements. Commonly used alloying elements are chromium, nickel, molybdenum, tungsten, vanadium, titanium, niobium, zirconium, cobalt, silicon, manganese, aluminum, copper, boron, rare earth, etc. Phosphorus, sulfur, nitrogen, etc. also act as alloys in some cases.
(1) Chromium (Cr)
Chromium can increase the hardenability of steel and has the effect of secondary hardening, 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 corrosion resistance, and also increases the thermal strength of the steel. Chromium is the main alloying element of stainless acid-resistant steel and heat-resistant steel.
Chromium can improve the strength and hardness of carbon steel in the rolled state, and reduce the elongation and reduction of area. When the chromium content exceeds 15%, the strength and hardness will decrease, and the elongation and reduction of area will increase accordingly. Chromium-containing steel parts are easy to obtain high surface finish quality by grinding.
The main function of chromium in the quenched and tempered structure is to improve the hardenability so that the steel has better 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. Abrasiveness.
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 can improve the oxidation resistance, resistance and strength of the alloy.
(2) Nickel (Ni)
Nickel strengthens ferrite and refines pearlite in steel. The overall effect is to increase the strength, and the effect on plasticity is not significant. Generally speaking, for low carbon steel used in rolled, normalized or annealed state without quenching and tempering treatment, a certain nickel content can increase the strength of the steel without significantly reducing its toughness. According to statistics, every 1% increase in nickel can increase the strength by about 29.4Pa. With the increase of nickel content, the yield degree of steel increases faster than the tensile strength, so the ratio of nickel-containing steel can be higher than that of ordinary carbon steel. While improving the strength of steel, nickel has less damage to the toughness, plasticity and other process properties of steel than other alloying elements. For medium carbon steel, because nickel reduces the pearlite transformation temperature, the pearlite becomes thinner; and because nickel reduces the carbon content of the eutectoid point, the number of pearlite is larger than that of carbon steel with the same carbon content. The strength of nickel-containing pearlitic ferritic steel is higher than that of carbon steel with the same carbon content. On the contrary, if the strength of the steel is the same, the carbon content of the nickel-containing steel can be appropriately reduced, 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 notch. Nickel reduces the low-temperature brittle transition temperature of steel, which is of great significance for low-temperature steel. Steel with 3.5% nickel can be used at -100℃, and steel with 9% nickel can work at -196℃. Nickel does not increase the resistance of steel to creep, so it is generally not used as a strengthening element for thermally strong steels.
The linear expansion coefficient of iron-nickel alloys with high nickel content changes significantly with the increase or decrease of nickel content. Using this characteristic, precision alloys and bimetallic materials with extremely low or certain linear expansion coefficients can be designed and produced.
In addition, nickel added to steel can not only resist acid, but also resist alkali, and has corrosion resistance to atmosphere and salt. Nickel is one of the important elements in stainless acid-resistant steel.
(3) Molybdenum (Mo)
Molybdenum can improve hardenability and thermal strength in steel, prevent temper brittleness, and increase remanence and coercivity and corrosion resistance in certain media.
In quenched and tempered steel, molybdenum can harden and harden parts with larger sections, improve the tempering resistance or tempering stability of the steel, and enable the parts to be tempered at a higher temperature, thereby more effectively eliminating ( or reducing) residual stress and increase plasticity.
In addition to the above functions, molybdenum in carburized steel can also reduce the tendency of carbides to form a continuous network on the grain boundary in the carburized layer, reduce the residual austenite in the carburized layer, and relatively increase the surface layer. wear resistance.
In the forging die, molybdenum can also maintain a relatively stable hardness of the steel and increase the resistance to deformation. Resistance to cracking and abrasion, etc.
In stainless acid-resistant steel, molybdenum can further improve the corrosion resistance to organic acids (such as formic acid, acetic acid, oxalic acid, etc.) and hydrogen peroxide, sulfuric acid, sulfurous acid, sulfate, acid dyes, bleaching powder liquid, etc. Especially due to the addition of molybdenum, the tendency to pitting corrosion caused by the presence of chloride ions is prevented.
W12Cr4V4Mo high-speed steel containing about 1% molybdenum has wear resistance, tempering hardness and red hardness.
(4) Tungsten (W)
In addition to forming carbides in steel, tungsten partially dissolves into iron to form a solid solution. Its effect is similar to that of molybdenum. Calculated by a mass fraction, the general effect is not as significant as that of molybdenum. The main pattern of tungsten in steel is to increase tempering stability, red hardness, thermal 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 dies, etc.
Tungsten forms refractory carbides in high-quality spring steel. When tempering at higher temperatures, it can ease the aggregation process of carbides and maintain high high-temperature strength. Tungsten can also reduce the thermal sensitivity of steel, increase hardenability and increase hardness. 65SiMnWA spring steel has high hardness after air cooling after hot rolling. Spring steel with a cross-section of 50mm2 can be hardened in oil, and can be used as an important spring that can withstand large loads, heat resistance (not more than 350 ℃) and shock. 30W4Cr2VA high-strength, heat-resistant and high-quality spring steel has great hardenability, is quenched at 1050-1100℃, and has a tensile strength of 1470-1666Pa after tempering at 550-650℃. It is mainly used to manufacture springs used under high temperatures (not more than 500℃).
Due to the addition of tungsten, the wear resistance and machinability of steel can be significantly improved, so tungsten is the main element of alloy tool steel.
(5) Vanadium (V)
Vanadium has a strong affinity with carbon, ammonia and oxygen, and forms corresponding stable compounds with it. Vanadium exists mainly in the form of carbides in steel. Its main function is to refine the structure and grain of the steel and reduce its strength and toughness of the steel. When it is dissolved into a solid solution at high temperature, it increases the hardenability; on the contrary, when it exists in the form of carbide, it reduces the hardenability. Vanadium increases the tempering stability of hardened steel and produces a secondary hardening effect. The vanadium content in steel, except for high-speed tool steel, is generally not more than 0.5%.
Vanadium can refine grains in ordinary low-carbon alloy steel, improve the strength and yield ratio after normalizing and low-temperature characteristics, and improve the welding performance of steel.
Vanadium in alloy structural steel is often used in combination with elements such as manganese, chromium, molybdenum and tungsten in structural steel because it will reduce the hardenability under general heat treatment conditions. Vanadium is mainly used in quenched and tempered steel to improve the strength and yield ratio of steel, refined grains, and pick up overheating sensitivity. In the case of carburizing steel, the grain can be refined, so that the steel can be directly quenched after carburizing without secondary quenching.
Vanadium in spring steel and bearing steel can improve the strength and yield ratio, especially the proportional limit and elastic limit, and reduce the sensitivity of decarburization during heat treatment, thereby improving the surface quality. The bearing steel containing pentachrome and vanadium has high carbonization dispersion and good performance.
Vanadium refines grains in tool steels, reduces overheating sensitivity, and increases tempering stability and wear resistance, thereby extending tool life.
(6) Titanium (Ti)
Titanium has a strong affinity with nitrogen, oxygen, and carbon, and has a stronger affinity with sulfur than iron. Therefore, it is a good deoxidizer 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 complex compounds. Titanium carbide has a strong binding force, and stability, and is not easy to decompose. Only when it is heated to above 1000 °C in steel can it slowly dissolve into solid solution. Before being dissolved, the titanium carbide particles have the effect of preventing grain growth. Since the affinity between titanium and carbon is much greater than that between chromium and carbon, titanium is often used to fix carbon in stainless steel to eliminate the depletion of chromium at the grain boundary, thereby eliminating or reducing intergranular corrosion of steel.
Titanium is also one of the strong ferrite forming elements, which strongly increases the A1 and A3 temperatures of the steel. Titanium can improve plasticity and toughness in ordinary low alloy steel. The strength of the steel is increased as titanium fixes nitrogen and sulfur and forms titanium carbide. After normalizing, the grains are refined, and the precipitation to form carbides can significantly improve the plasticity and impact toughness of the steel. Titanium-containing alloy structural steels have good mechanical properties and process properties. The main disadvantage is that the hardenability is slightly poor.
Titanium with a content of about 5 times carbon is usually added to high chromium stainless steel, which can not only improve the corrosion resistance (mainly anti-intergranular corrosion) and toughness of the steel, but also organize the grain growth tendency of the steel at high temperature and improve Weldability of steel.
(7) Niobium/Columbium (Nb/Cb)
Niobium and columbium often coexist with tantalum, and their roles in steel are similar. Niobium and tantalum partially dissolve into solid solution and play a role in solid solution strengthening. When dissolved in austenite, the hardenability of steel is significantly improved. However, in the form of carbides and oxide particles, it refines the grains and reduces the hardenability of the steel. It can increase the tempering stability of steel and has a secondary hardening effect. Trace amounts of niobium can increase the strength of steel without affecting its ductility or toughness. Due to the effect of grain refinement, it can improve the impact toughness of steel and reduce its brittle transition temperature. 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 austenitic steels, it can prevent intergranular corrosion of steel by oxidizing media. Due to fixed carbon and precipitation hardening, it can improve the high-temperature properties of thermal strength steel, such as creep strength.
Niobium can improve the yield strength and impact toughness of ordinary low alloy steel for construction, and reduce the brittle transition temperature, which is beneficial to welding performance. In carburizing and quenched and tempered alloy structural steel while increasing the hardenability. Improve toughness and low-temperature properties of steel. It can reduce the air hardenability of low carbon martensitic heat-resistant stainless steel, avoid hardening and tempering brittleness, and improve creep strength.
(8) Zirconium (Zr)
Zirconium is a strong carbide former, and its role in steel is similar to that of niobium, tantalum, and vanadium. Adding a small amount of zirconium has the effect of degassing, purifying and refining grains, which is beneficial to the low-temperature performance of steel and improves the stamping performance.
(9) Cobalt (Co)
Cobalt is mostly used in special steels and alloys. Cobalt-containing high-speed steel has high-temperature hardness. Adding molybdenum to maraging steel at the same time can obtain ultra-high hardness and good comprehensive mechanical properties. In addition, cobalt is also an important alloying element in thermally strong steels and magnetic materials.
Cobalt reduces the hardenability of steel, so adding it to carbon steel alone will reduce the comprehensive mechanical properties after quenching and tempering. Cobalt can strengthen ferrite. When added to carbon steel, it can improve the hardness, yield point and tensile strength of steel in the annealed or normalized state. decreased with increasing cobalt content. Due to its anti-oxidation properties, cobalt is used in heat-resistant steels and heat-resistant alloys. Cobalt-based alloy gas turbines show their unique role.
(10) Nitrogen (N)
Nitrogen can be partially used in iron, and it has the effect of solid solution strengthening and hardenability improvement, but it is not significant. Due to the precipitation of nitrides on the grain boundaries, the high-temperature strength of the grain boundaries can be improved, and the creep strength of the steel can be increased. Combined with other elements in steel, it has a precipitation hardening effect. The corrosion resistance of steel is not significant, but after nitriding the surface of steel, it not only increases its hardness and wear resistance but also significantly improves corrosion resistance. Residual nitrogen in mild steel can cause age brittleness.
Compared with carburizing, the parts after nitriding have: high hardness and wear resistance, high fatigue strength, high seizure resistance, high corrosion resistance, and the nitriding process is below the transformation temperature of the steel. (450-600℃), so the deformation is small and the volume is slightly expanded. The disadvantage is that the cycle is long (the nitriding time of general gas nitriding soil technology is as long as tens to 100h), the cost is high, the nitriding layer is thin (generally about 0.5mm) and brittle, and it cannot withstand too much contact stress and impact load.
Although the parts after nitriding have high hardness, high wear resistance and high fatigue strength, they are only a thin layer on the surface (chromium molybdenum aluminum steel at 500–540 ℃ for 35–65h, the nitriding layer depth is only 0.3 –0.65mm). There must be a strong and tough core tissue as a solid base for the nitriding layer, in order to play the maximum role of nitriding. In general, most of the nitrided parts work under the conditions of friction and complex dynamic loads, and the performance of both the surface and the core is very high.
Matters needing attention for nitriding parts
1) Preliminary heat treatment and tempering before nitriding – The nitriding workpiece should be quenched and tempered before nitriding to obtain a tempered sorbate structure. The tempering temperature of the quenching and tempering treatment is generally higher than the nitriding temperature.
2) Preliminary heat treatment before nitriding Stress relief treatment – Before nitriding, the internal stress generated in the machining process should be eliminated as much as possible to stabilize the size of the parts. The temperature for stress relief should be lower than the tempering temperature, the holding time should be longer than the tempering time, and then slowly cooled to room temperature. Parts with larger cross-sectional dimensions should not be normalized. Tool and die steel must be quenched and tempered, not annealed.
(11) Silicon (Si)
Silicon can dissolve in ferrite and austenite to improve the hardness and strength of steel, its role is second only to phosphorus and stronger than manganese, nickel, chromium, tungsten, molybdenum, vanadium and other elements. However, when the silicon content exceeds 3%, the plasticity and toughness of the steel will be significantly reduced. Silicon can improve the elastic limit, yield strength and yield ratio (σs/σb), and fatigue strength and fatigue ratio (σ-1/σb) of steel. This is because silicon or silicon-manganese steel can be used as spring steel.
Silicon can reduce the density, thermal conductivity and electrical conductivity of steel. It can promote the coarsening of ferrite grains and reduce coercivity. There is a tendency to reduce the anisotropy of the crystal, making the magnetization easy and reducing the magnetoresistance, which can be used to produce electrical steel, so the magnetoresistance loss of the silicon steel sheet is low. Silicon can improve the magnetic permeability of ferrite so that the steel sheet has a higher magnetic induction in a weaker magnetic field. But silicon reduces the magnetic induction of steel under strong magnetic fields. Silicon has strong deoxidizing power, thereby reducing the magnetic aging effect of iron.
When the silicon-containing steel is heated in an oxidizing atmosphere, a layer of SiO2 film will be formed on the surface, thereby improving the oxidation resistance of the steel at high temperatures.
Silicon can promote the growth of columnar crystals in cast steel and reduce plasticity. If the silicon steel cools quickly when heated, due to the low thermal conductivity, the temperature difference between the inside and outside of the steel is large, so it will break.
Silicon can reduce the weldability of steel. Because silicon has a stronger binding ability with oxygen than iron, it is easy to generate low-melting silicate during welding, which increases the fluidity of slag and molten metal, causes splashing, and affects welding quality. Silicon is a good deoxidizer. When deoxidizing with aluminum, adding a certain amount of silicon as appropriate can significantly improve the rate of deoxidation. There is a certain amount of residual silicon in steel, which is brought in as a raw material during iron and steel making. In boiling steel, silicon is limited to <0.07%, and when intentionally added, ferrosilicon is added during steelmaking.
(12) Manganese (Mn)
Manganese is a good deoxidizer and desulfurized. Steel generally contains a certain amount of manganese, which can eliminate or weaken the hot brittleness of steel caused by sulfur, thereby improving the hot workability of steel.
The solid solution formed by manganese and iron improves the hardness and strength of ferrite and austenite in steel; at the same time, it is an element formed by carbides, which enter into cementite to replace part of iron atoms. Manganese reduces the critical transformation temperature in steel. It plays the role of refining pearlite and indirectly improves the strength of pearlite steel. Manganese is second only to nickel in its ability to stabilize austenite and also strongly increases the hardenability of steel. Various alloys have been made of manganese with a content of not more than 2% and other elements.
