November 3, 2022
In order to improve and enhance some properties of steel and make it obtain some special properties deliberately added elements in the smelting process called alloying elements. Common alloying elements are chromium, nickel, molybdenum, tungsten, vanadium, titanium, niobium, zirconium, cobalt, silicon, manganese, aluminum, copper, boron, rare earth and so on. Phosphorus, sulfur, and nitrogen also act as alloys in some cases.
(1) Chromium (Cr)
Chromium can increase the hardenability of steel and has a secondary hardening effect, can improve the hardness and wear resistance of carbon steel without making the steel brittle. When the content exceeds 12%, the steel has good oxidation resistance and corrosion resistance at high temperature, and also increases the thermal strength of the steel. Chromium is the main alloying element of stainless steel acid resistant steel and heat resistant steel.
Chromium can improve the strength and hardness of carbon steel in rolling state and reduce the elongation and shrinkage of cross-section. When the chromium content exceeds 15%, the strength and hardness will decrease, and the elongation and section shrinkage will increase accordingly. The parts containing chromium steel are easy to obtain higher surface machining quality by grinding.
The main role of chromium in the tempered structure is to improve the hardenability, so that the steel after quenching and tempering has better comprehensive mechanical properties, in the carburized steel can also form chromium carbide, so as to improve the wear resistance of the material surface.
Spring steel containing chromium is not easy to decarbonize during heat treatment. Chromium can improve the wear resistance, hardness and hardness of tool steel, and has good tempering stability. Chromium can improve the oxidation resistance, resistance and strength of electrothermal alloys.
(2) Nickel
Nickel strengthens ferrite and refines pearlite in steel, the overall effect is to increase strength, but has no significant effect on plasticity. Generally speaking, a certain amount of nickel can improve the strength of the steel but not significantly reduce the toughness of the low carbon steel when it is rolled, normalized or annealed without tempering treatment. According to statistics, every 1% increase in nickel can improve the strength of 29.4Pa. With the increase of nickel content, the yield of steel increases faster than the tensile strength, so the ratio of nickel-containing steel is higher than that of ordinary carbon steel. Nickel can improve the strength of steel, but the damage to the toughness, plasticity and other process properties of steel is less than other alloying elements. For medium carbon steel, the pearlite becomes thinner because nickel reduces the pearlite transition temperature. Because nickel reduces the carbon content at the eutectoid point, there are more pearlite than carbon steel with the same carbon content, which makes the pearlite ferritic steel with nickel have higher strength than 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 improve the resistance of steel to fatigue and reduce the sensitivity of steel to notch. Nickel reduces the brittle transition temperature of low temperature steel, which is very important for low temperature steel. Steel with 3.5% nickel can be used at -100℃, and steel with 9% nickel can be used at -196℃. Nickel does not increase the resistance of steel to creep, so it is generally not used as a strengthening element of hot strength steel.
The linear expansion coefficient of Fe-Ni alloys with high nickel content changes significantly with the increase or decrease of nickel content. Using this property, precision alloys and bimetallic materials with very low or certain linear expansion coefficient can be designed and produced.
In addition, nickel added to steel is not only acid resistant, but also alkali resistant, corrosion resistant to atmosphere and salt, nickel is one of the important elements in stainless acid resistant steel.
(3) Molybdenum (Mo)
Molybdenum in steel can improve hardenability and thermal strength, prevent tempering brittleness, increase remanence and coercivity and corrosion resistance in some media.
In tempered steel, molybdenum can make parts with larger sections quench deeply, quench through, improve the resistance to fire or tempering stability of steel, so that parts can be tempered at higher temperatures, so as to eliminate (or reduce) residual stress more effectively, improve plasticity.
In addition to the above effects in carburized steel, molybdenum can also reduce the tendency of carbides to form a continuous network on the grain boundary in the carburizing layer, reduce the residual austenite in the carburizing layer, and relatively increase the wear resistance of the surface layer.
In the forging die, molybdenum can also maintain the steel has a relatively stable hardness, increase the deformation. Resistance to cracking and wear.
In stainless acid-resistant steel, molybdenum can further improve the corrosion resistance of organic acids (such as formic acid, acetic acid, oxalic acid, etc.), hydrogen peroxide, sulfuric acid, sulfite, sulfate, acid dyes, bleaching powder, etc. In particular, the addition of molybdenum prevents the point corrosion tendency caused by the presence of chloride ions.
W12Cr4V4Mo high speed steel containing about 1% molybdenum has wear resistance, tempered hardness and red hardness.
(4) Tungsten (W)
In addition to forming carbides in steel, tungsten is partially dissolved into iron to form solid solution. Its action is similar to molybdenum, according to the mass fraction calculation, the general effect is not as significant as molybdenum. The main sample of tungsten in steel is to increase tempering stability, red hardness, thermal strength and wear resistance due to the formation of carbides. Therefore, it is mainly used in tool steel, such as high speed steel, hot forging die steel and so on.
Tungsten forms refractory carbides in high quality spring steel. When tempering at higher temperature, the accumulation process of carbides can be eased and the high temperature strength can be maintained. Tungsten also reduces the sensitivity of steel to overheating, increases hardenability and increases hardness. After hot rolling, the 65SiMnWA spring steel has high hardness after air cooling. The spring steel with a section of 50mm2 can be quenched in oil, and it can be used as an important spring to withstand large loads, heat resistance (no more than 350℃) and impact. 30W4Cr2VA high strength heat resistant spring steel, with large hardenability, 1050 ~ 1100℃ quenching, 550 ~ 650℃ tempering tensile strength of 1470 ~ 1666Pa. It is mainly used in the manufacture of springs under high temperature (not more than 500℃) conditions.
Tungsten is the main element of alloy tool steel because its addition can significantly improve the wear resistance and machinability of steel.
(5) Vanadium (V)
Vanadium has a strong affinity with carbon, ammonia and oxygen, and forms stable compounds with them. Vanadium exists mainly in the form of carbide in steel. Its main function is to refine the structure and grain of steel, improve the strength and toughness of steel. When dissolved into solid solution at high temperature, increase hardenability; On the contrary, if in the form of carbide, reduce the hardenability. Vanadium increases tempering stability of hardened steel and produces secondary hardening effect. The vanadium content in steel, except high-speed tool steel, is generally no more than 0.5%.
In common low carbon alloy steel, vanadium can refine grain, increase the strength and yield ratio after normalizing, and improve the welding performance of steel at low temperature.
Vanadium in alloy structural steel due to the general heat treatment conditions will reduce the hardenability, so it is often used in structural steel and manganese, chromium, molybdenum, tungsten and other elements. Vanadium in tempered steel is mainly to improve the strength and yield ratio of steel, refine grain, pick up the sensitivity of overheating. In the carburized steel can refine the grain, the steel can be directly quenched after carburizing, without secondary quenching.
In spring steel and bearing steel, vanadium can improve the strength and yield ratio, especially the proportional limit and elastic limit, and reduce the decarbonization sensitivity during heat treatment, thus improving the surface quality. 5 chromium bearing steel containing vanadium, high carbonation dispersion, good performance.
Vanadium in tool steel refines grains, reduces overheating sensitivity, increases tempering stability and wear resistance, and thus extends 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 deoxidizing agent 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 binding force is strong, stable, not easy to decompose, in steel only heating to more than 1000℃ can slowly dissolve into the solid solution. Titanium carbide particles have the effect of preventing grain growth before dissolution. Since the affinity between titanium and carbon is much greater than that between chromium and carbon, titanium is commonly used in stainless steel to fix the carbon in order to eliminate chromium dilution at the grain boundary, so as to eliminate or reduce the intergranular corrosion of steel.
Titanium is also one of the strong ferrite forming elements, which strongly increases the A1 and A3 temperatures of steel. Titanium can improve plasticity and toughness in common low alloy steel. As titanium fixes nitrogen and sulfur and forms titanium carbide, the strength of the steel is increased. After normalizing the grain refinement, precipiting carbide can significantly improve the plasticity and impact toughness of steel. The alloy structural steel containing titanium has good mechanical properties and process properties, but the main disadvantage is that the hardenability is slightly poor.
In high chromium stainless steel usually need to add about 5 times the carbon content of titanium, not only can improve the corrosion resistance (mainly intergranular corrosion resistance) and toughness of steel; It can also prevent the grain growth tendency of steel at high temperature and improve the welding performance of steel.
(7) Niobium/coltan (Nb/Cb)
Niobium often coexists with coltan and tantalum, and their role in steel is similar. Niobium and tantalum are partially dissolved into the solid solution to strengthen the solid solution. The quenching ability of steel can be improved significantly when it is dissolved into austenite. However, in the form of carbide and oxide particles, it refines the grain and reduces the hardenability of the steel. It can increase the tempering stability of steel and has the effect of secondary hardening. Trace niobium can improve the strength of steel without affecting the plasticity or toughness. Because of the effect of refining grain, the impact toughness of steel can be improved and the brittleness transition temperature can be reduced. When the content is more than 8 times of carbon, almost all the carbon in the steel can be fixed, so that the steel has good hydrogen resistance. The intergranular corrosion of austenitic steel by oxidizing medium can be prevented. Due to carbon fixation and precipitation hardening, the high temperature properties of hot strength steels, such as creep strength, can be improved.
Niobium can improve the yield strength and impact toughness and reduce the brittleness transition temperature in common low-alloy steel used in construction. Increase hardenability in carburizing and tempering alloy structural steel. Improve the toughness and low temperature performance of steel. It can reduce the air hardening of low carbon martensitic heat-resistant stainless steel, avoid hardening and tempering brittleness, and improve the creep strength.
(8) Zirconium (Zr)
Zirconium is a strong carbide forming element, its role in steel is similar to niobium, tantalum, vanadium. Adding a small amount of zirconium has the effect of degassing, purifying and refining grain, which is beneficial to the low temperature performance of steel and improving the stamping performance. It is often used in the manufacture of ultra-high strength steel and nickel-based superalloy for gas engine and ballistic missile structure.
(9) Cobalt
Cobalt is mostly used in special steels and alloys. High speed steel containing cobalt has high high temperature hardness, and molybdenum can be added to martensitic aging steel at the same time to obtain ultra-high hardness and good comprehensive mechanical properties. In addition, cobalt is an important alloying element in hot strength steels and magnetic materials.
Cobalt reduces the hardenability of steel, so adding carbon steel alone will reduce the overall mechanical properties of tempered steel. Cobalt can strengthen ferrite, and when added to carbon steel, it can improve the hardness, yield point and tensile strength of steel in annealed or normalized state, and has an adverse effect on elongation and section shrinkage. Impact toughness decreases with the increase of cobalt content. Cobalt is used in heat resistant steels and alloys because of its oxidation resistance. Cobalt-based alloy gas turbines show its unique role.
