The chemical composition characteristics of high-speed steel rolls are high carbon content, large number and high content of alloying elements. Based on various research results, the components of commonly used high-speed steel rolls are summarized in Table 1.
Table 1: Composition of commonly used high-speed steel rolls (mass percentage)
1) Carbon (C)
Compared with ordinary rolls, high-speed steel rolls have the characteristics of high carbon content, usually w(C)=1.5%~2.8%, to ensure that a sufficient number of high-hardness and high-thermal-stability carbides are formed with strong carbide-forming elements. Such as MC, M2C, MC, M, C3, etc., and ensure a strong martensite matrix to improve the red hardness and wear resistance of the roll.
However, there are many products produced by the action of carbon and other elements and the composition is complex. If the carbon content is too low, the number of carbides formed will be small. The secondary hardening effect is not good, which will reduce the hardness and wear resistance of the roll.
If the carbon content is too high, exceeding the ratio required for the carbide-forming elements, a large number of reticulated eutectic carbides will be generated at the grain boundaries, which will reduce the plasticity and fracture toughness of the high-speed steel: and, the carbon content will be too high. The amount of residual austenite after quenching of high-speed steel increases, the number of tempering also increases accordingly, energy consumption increases, cost increases and it is easy to induce casting cracks, resulting in a decrease in the comprehensive mechanical properties of high-speed steel rolls. Therefore, the carbon content in high-speed steel rolls should maintain a “balanced carbon” ratio with strong carbide-forming elements.
(2) Tungsten (W)
Tungsten is the preferred element to improve the tempering stability and red hardness of high-speed steel. Tungsten is mainly used in high-speed steel as W2C (M2C type) and Fe3W3C, FeW2C (M, C type) exist in the form, which can improve the wear resistance of the roll. During high-temperature quenching, part of MC dissolves into austenite to improve the hardenability of high-speed steel; the other part of undissolved MC can effectively prevent the growth of austenite grains at high temperatures to improve wear resistance). During high-temperature tempering, M2C disperses and precipitates, resulting in a secondary hardening effect, which significantly improves the hot hardness, hardness, and wear resistance of high-speed steel rolls.
Adding an appropriate amount of tungsten can improve the distribution of carbides, but if the tungsten content is too high, the excess tungsten will be discharged into the remaining liquid phase during the solidification process, which will intensify the eutectic reaction, generating a large amount of M2C at the grain boundary, and reduce the high speed. Plasticity and fracture toughness of steel.
If the tungsten content is insufficient, the amount of M2C generated will be small, which will affect the hardness of the roll. Considering comprehensively, the content of the tungsten element in the high-speed steel roll should be below 5%.
(3) Molybdenum (Mo)
The effect of molybdenum element is similar to that of the tungsten element, which can improve the red hardness of high-speed steel. Molybdenum also forms MC-type carbides, which have the same lattice-type as M and C-type carbides formed by tungsten, and the lattice parameters are almost the same, but the density is lower. Usually, tungsten and molybdenum elements are converted into tungsten equivalents):
The higher the tungsten equivalent, the better the wear resistance; the lower the tungsten equivalent, the better the toughness.
The effect of molybdenum on the properties of high-speed steel rolls is also different from that of tungsten: the density of molybdenum is lower than that of tungsten, which can eliminate segregation to a certain extent; during tempering, dispersion hardening occurs, which will reduce the toughness of high-speed steel, but molybdenum Elements can prevent the precipitation of carbides along the grain boundaries and improve the toughness of high-speed steel.
If all molybdenum elements are used to replace tungsten elements, there will be the following problems: during tempering, the temperature at which carbides are precipitated from martensite in molybdenum-containing high-speed steels is lower than that of tungsten-containing high-speed steels, so the thermal stability decreases; during heat treatment, The decarburization tendency of molybdenum-containing high-speed steel is serious, and the grain size of molybdenum-containing high-speed steel is larger than that of tungsten-containing high-speed steel.
Therefore, in general, tungsten and molybdenum elements are mixed and added. The content of the tungsten element should be controlled at 3.0%~6.5%.
(4) Chromium (Cr)
The chromium element in the high-speed steel roll mainly enhances the hardenability of the steel and has the effect of secondary hardening, which can improve the hardness and wear resistance of the high-carbon steel without making the steel brittle. Chromium and carbon form Cr23C6 carbides, which are almost completely dissolved in austenite during quenching and heating, increasing the stability of supercooled austenite and greatly improving the hardenability of steel. Chromium can also adjust the carbon balance of the matrix and improve the oxidation resistance, decarburization, and corrosion resistance of steel.
However, when the other components remain unchanged, the chromium content increases, the eutectic carbide increases, and the fine chromium-rich carbides in the matrix increase, which has little effect on the primary carbide composition, but too high a chromium content will form an unstable alloy during tempering. Carbide, reducing thermal stability and red hardness. Usually, the content of chromium in high-speed steel rolls is between 4% and 9%.
(5) Vanadium (V)
Vanadium is a strong carbide-forming element, mainly used to improve the hardness and wear resistance of steel. Vanadium can form carbide VC (or V4C3), which is very stable, extremely difficult to dissolve, has high hardness (greatly exceeding the hardness of W2C), and the particles are small and evenly distributed, so it has a great effect on improving the hardness and wear resistance of steel.
When tempering, VC is spherical or nearly spherical and dispersed on the matrix, which can improve the impact toughness, hardness, and wear resistance of high-speed steel. However, when the vanadium content is too high, relatively coarse primary VC will be precipitated in the liquid phase first, and the density of VC is low, so it is easy to segregate inward during centrifugal casting and weaken the effect of vanadium.
Therefore, considering the influence of vanadium element comprehensively, the vanadium content in high-speed steel is controlled at 2.5%~6%.
(6) Niobium (Nb)
The effect of the niobium element is similar to that of the vanadium element, but the niobium element has a stronger ability to form carbide than the vanadium element, and forms carbide NbC which is coarser than VC. Because the carbides of niobium are coarse, the content of niobium in high-speed steel should be controlled. However, the combined effect of niobium and vanadium can increase MC and reduce M and C3 in the tempering process, making the secondary hardening effect more significant. Usually, the content of niobium in high-speed steel rolls is 0~2%.
(7) Nickel (Ni)
Nickel can improve the mechanical properties of high-speed steel, especially the plasticity and toughness of high-speed steel. Under the condition that the matrix is martensite, it has little effect on the strength of high-speed steel rolls, and adding an appropriate amount of nickel can improve the roll. Anti-cracking and anti-flaking properties.
However, nickel will increase the amount of retained austenite in high-speed steel, resulting in a decrease in the hardness and red hardness of high-speed steel). Considering the influence of nickel elements on the performance of high-speed steel rolls, the nickel content is generally controlled within the range of 0.5% to 1%.
(8) Silicon (Si)
Silicon is a non-carbide-forming element, which can promote the decomposition of carbide M2C, which is unfavorable to the performance of high-speed steel, into MC and MC when reheating, so as to refine the carbide and improve the toughness of high-speed steel. Silicon also has a certain deoxidation effect.
However, when silicon dissolves in the martensite matrix, it will increase the brittleness of the matrix, and if the silicon content is too high, the working layer of the roll will easily crack and peel off. Therefore, the silicon content in high-speed steel rolls should be controlled at 0.2%~1.0%.
(9) Aluminum (Al)
The effect of aluminum and silicon is similar. Aluminum dissolves in the matrix, which can refine the grain and improve the tempering stability, hardness, and red hardness of high-speed steel. The aluminum element can also reduce the decomposition temperature of coarse columnar M2C carbides, which is beneficial to improve the toughness of high-speed steel rolls.
However, the aluminum element increases the decarburization sensitivity of the high-speed steel, and when the aluminum element content exceeds 2%, the quenching hardness of the high-speed steel will decrease sharply. Therefore, the aluminum element content in high-speed steel rolls should be controlled within the range of 0.1% to 0.6%.
(10) Manganese (Mn)
Manganese elements in the low content range can improve the strength, hardness, and wear resistance of high-speed steel, and with the increase of manganese content, it can reduce the martensitic transformation temperature and critical cooling rate, and improve the hardenability of high-speed steel. However, an increase in manganese content will lead to an increase in the amount of retained austenite, reducing the thermal stability and hardness of high-speed steel. Usually, the manganese content in high-speed steel rolls is 0.3%~1.0%.
(11) Cobalt (Co)
Cobalt itself does not form carbides. The principle of cobalt improving the hardness and red hardness of high-speed steel is different from that of tungsten, molybdenum, vanadium, and niobium. Cobalt is mostly dissolved in the matrix in high-speed steel, and only a small part is dissolved in carbide, which can improve the wear resistance of high-speed steel.
Increase the nucleation rate of precipitated MC and M2C during tempering, and slow down their aggregation growth rate. In addition, cobalt can increase the melting temperature of the grain boundary of high-speed steel, thereby increasing the quenching temperature of the steel and increasing the alloying degree in the austenite. These effects have effectively improved the heat resistance of high-speed steel.
However, when the cobalt content is too high, it will also reduce the toughness of high-speed steel and increase the tendency of decarburization. Considering comprehensively, the content of cobalt element in high-speed steel roll should be 1%~4%.
(12) Sulfur, phosphorus (S, P)
Trace impurities such as sulfur and phosphorus are brought in from the raw materials and are harmful elements. Sulfur and phosphorus generally gather in the grain boundary in the form of low-melting inclusions, which will cause cracks during the hot and cold cycles of the roll, reducing the strength and plasticity of the roll. Therefore, the content of sulfur and phosphorus in high-speed steel rolls should be strictly controlled below 0.05%.
Carbides in High-Speed Steel Rolls
Through the field observation of high-speed steel rolls, it is found that the main wear form of high-speed steel rolls is fatigue wear. The main performance is that the carbides in the working layer of the roll and the matrix fall off, resulting in an increase in surface roughness and failure of the roll. Therefore, carbides in high-speed steel are very important. Eutectic carbides in high-speed steel are generally distributed on grain boundaries. Although carbides can improve the hardness and wear resistance of the roll, they will also reduce the fracture toughness. Therefore, it is of great significance to recognize the type, morphology, and distribution of carbides in high-speed steel.
(1) Types of carbides
The formation of different types of carbides depends on the strong carbide-forming elements. The common types of carbides in high-speed steel rolls are as follows:
M6C-type carbides mainly composed of tungsten and molybdenum can generate Fe2(W, Mo)4C or Fe4(W, Mo)2C, which can dissolve a small number of elements such as chromium, vanadium, and cobalt;
The M23C6 type carbide with chromium, tungsten, and molybdenum as the main solution into a small amount of vanadium and iron forms (Cr, Fe, Mo, V)23C6;
MC-type carbides mainly composed of vanadium can dissolve a small amount of tungsten, molybdenum, chromium, and other elements to form VC.
M2C-type metastable carbides such as W2C and Mo2C also exist in high-speed steel rolls. After annealing, tempering, and other heat treatments, they are transformed into stable MC-type carbides. The reaction formula is:
And when heated, M23C6 carbide dissolves in a large amount above 900°C, and almost completely dissolves into austenite at 1100°C; MC carbide begins to dissolve in austenite when heated to 1000~1200°C; MC Type carbides begin to dissolve rapidly above 1150°C, and cannot be completely dissolved at 1325°C. Therefore, the quenching temperature of high-speed steel must be selected reasonably. M6C and MC carbides will not be completely dissolved, and undissolved carbides can hinder grain growth and refine the structure.
(2) Hardness of carbides
High-speed steel has high carbon content and a large number of alloy elements. During solidification and heat treatment, a large number of primary and secondary carbides with high hardness are formed. This is the main factor to improve the hardness and wear resistance of high-speed steel rolls.
(3) Morphology of carbides
The morphology of carbides in high-speed steels is affected by composition, cooling rate, alloying elements, etc., so carbides have different microstructures in different high-speed steels. Table 2 briefly introduces the morphology characteristics of the four kinds of carbides in high-speed steel.
Table 2: Morphology of carbides in high-speed steel rolls
|MC||V, Nb||Rod-shaped, exists alone, larger in size|
|Spherical or petal-shaped eutectic|
|M2C||W, Mo||Flake, fibrous, rod|
|M6C||W, Mo, Fe||Herringbone, thin lath|