There are various elements in alloy steel, what role does each element play, and what side effects are there? Let's take a look！
Chromium can increase the hardenability of steel and has a secondary hardening effect, which can improve the hardness and wear resistance of steel without making the steel brittle. When the content exceeds 12%, the steel has good high temperature oxidation resistance and oxidative medium 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.
Nickel can strengthen ferrite and refine 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. As the nickel content increases, the yield strength of the steel increases faster than the tensile strength. Therefore, the yield strength of nickel-containing steel is 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, since nickel reduces the transformation temperature of pearlite, the pearlite becomes finer; 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.
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, the addition of nickel to steel can not only resist acid, but also resist alkali, and has corrosion resistance to atmosphere and salt.
Molybdenum can improve hardenability and thermal strength in steel, prevent temper brittleness, increase remanence and coercivity and corrosion resistance in certain media.
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 mass fraction, the general effect is not as significant as that of molybdenum. The primary use of tungsten in steel is to increase temper stability, red hardness, thermal strength, and increased wear resistance due to carbide formation. Therefore, it is mainly used for tool steel, such as high-speed steel, hot forging die steel, etc.
Vanadium has a strong affinity for 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, reduce the overheating sensitivity of the steel, and improve the strength and toughness of the steel. When it dissolves 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.
Titanium has a strong affinity for nitrogen, oxygen, and carbon, and has a stronger affinity with sulfur than iron. Therefore, titanium is a good deoxidizer and an effective element for fixing nitrogen and oxygen. Although titanium is a strong carbide-forming element, it does not combine with other metal elements to form complex compounds. Titanium carbide has strong binding force, stability, and is not easy to decompose. Only when it is heated to above 1000 °C in steel, it will slowly dissolve into solid solution. Before being dissolved, the titanium carbide particles have the effect of preventing grain growth.
Niobium often coexists with buttons, 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, the grains are refined and the hardenability of the steel is reduced. 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.
Zirconium is a strong carbide former and its role in steel is similar to that of niobium, titanium 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. It is often used in the manufacture of ultra-high-strength steel and nickel-based high temperature used in gas engines and ballistic missile structures. alloy
Cobalt is mostly used in special steels and alloys. Cobalt-containing high-speed steel can maintain high hardness at high temperatures. Adding molybdenum to maraging steel at the same time can obtain ultra-high strength and good comprehensive mechanical properties. In addition, drill 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, and when added to carbon steel, it can improve the hardness, yield point and tensile strength of steel in the annealed or normalized state. decreased as the cobalt content increased. Due to its anti-oxidation properties, cobalt is used in heat-resistant steels and heat-resistant alloys. It also shows its unique role in cobalt-based alloy gas turbines
Silicon dissolved in ferrite and austenite can improve the hardness and strength of steel, its effect is second only to phosphorus, and stronger than manganese, nickel, chromium, tungsten, molybdenum and vanadium. 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 of steel, as well as fatigue strength and fatigue ratio.
When the silicon-containing steel is heated in an oxidizing atmosphere, a layer of SiO: film will be formed on the surface, thereby improving the oxidation resistance of the steel at high temperature.
Manganese is a good deoxidizer and desulfurizer. 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.
Aluminum is mainly used for deoxidation and grain refinement. In nitrided steel, it promotes the formation of a hard, corrosion-resistant nitrided layer. Aluminum can inhibit the aging of low carbon steel and improve the toughness of steel at low temperature. When the content is high, the oxidation resistance of steel and the corrosion resistance in oxidizing acid and HS gas can be improved, and the electrical and magnetic properties of steel can be improved.
The prominent role of copper in steel is to improve the atmospheric corrosion resistance of ordinary low-alloy steel, especially when used in combination with phosphorus, adding copper can also improve the strength and yield ratio of steel without adversely affecting the welding performance.
The main function of boron in steel is to increase the hardenability of steel, thereby saving other rarer metals such as nickel, chromium, molybdenum, etc. For this purpose, its 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. It should be noted that molybdenum can be replaced by boron, because 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.
Rare earth elements can improve the plasticity and impact toughness of forged steel, especially in cast steel. It also improves the creep resistance of heat-resistant steels, electrothermal alloys and superalloys.
Rare earth elements can also improve the oxidation 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.
Nitrogen can be partially dissolved in iron, and has the effect of solid solution strengthening and hardenability improvement, but it is not significant. Due to the precipitation of nitrides on the grain boundary, the high temperature strength of the product boundary 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. It has no significant effect on the corrosion resistance of steel, but after nitriding the surface of steel, it not only increases its hardness and wear resistance, but also significantly improves corrosion resistance. In mild steels, residual nitrogen causes age brittleness
Increasing the content of sulfur and manganese can improve the cutting performance of steel, and sulfur is added as a beneficial element in free-cutting steel. Sulfur segregates seriously in steel,it can reduce the plasticity of steel at high temperature, and is a harmful element, which exists in the form of FeS with a lower melting point. Sulphur should be strictly controlled. In order to prevent brittleness caused by sulphur, enough manganese should be added to form MnS with a higher melting point.
Phosphorus has strong solid solution strengthening and cold work hardening effects in steel. Adding it as an alloying element to low-alloy structural steel can improve its strength and atmospheric corrosion resistance of steel, but reduce its cold stamping performance. The combined use of phosphorus with sulfur and manganese can increase the cutting performance of steel and increase the surface quality of the workpiece. Phosphorus dissolves in ferrite, although it can improve the strength and hardness of steel, the biggest harm is serious segregation, increase temper brittleness, and significantly reduce the plasticity and toughness of steel, resulting in easy brittle cracking of steel during cold working, also known as "cold brittleness". brittle" phenomenon. Phosphorus also has an adverse effect on weldability. Phosphorus is a harmful element and should be strictly controlled.
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