The role of the hottest alloy elements in die stee

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As we all know, carbon steel, which accounts for about 80% of the total steel output, is a basic industrial steel. It has a wide variety, simple production and low price. It can obtain different mechanical properties after different heat treatment, so it has been widely used. However, carbon steel has low strength and hardenability, poor thermal hardness, low wear resistance, corrosion resistance and heat resistance. Moreover, the development of industry, especially the development of national defense, transportation, petroleum and chemical industry, has put forward higher requirements for materials, so the field of use is limited. In order to improve the mechanical properties, process properties or some special physical and chemical properties of carbon steels, some alloy elements are selectively added to the molten steel during smelting, such as manganese, silicon, chromium, nickel, aluminum, tungsten, vanadium, titanium, niobium, zirconium, rare earth elements, etc., which are collectively referred to as alloy steels. Die steel is a kind of alloy steel

I. limitations of the use of carbon steel

⑴ low hardenability

generally, water cooling is required for the quenching of carbon steel, and the maximum hardenability diameter of water quenching is 15 ~ 20mm. Therefore, when manufacturing large-size and complex parts, the uniformity of performance and geometric shape cannot be guaranteed

⑵ low strength and yield ratio

low strength makes engineering structures and equipment cumbersome. A3 steel σ S ≥ 240mpa, while low alloy structural steel 16Mn σ S≥360MPA。 Low yield strength ratio indicates low effective utilization of strength. 40 carbon steel σ S/ σ B is 0.43, while alloy steel 35crni3mo σ S/ σ B up to 0.74

⑶ poor tempering stability

due to poor tempering stability, the toughness of carbon steel is low when the tempering temperature should be lower in order to ensure high strength during quenching and tempering treatment; In order to ensure better toughness, the strength is low when the tempering temperature is higher, so it is difficult to improve the comprehensive mechanical properties of carbon steel

⑷ can not meet the requirements of some special properties

carbon steel is often poor in oxidation resistance, corrosion resistance, heat resistance, low temperature resistance, wear resistance and special electromagnetic properties, so it can not meet the special use requirements

in order to solve the above problems, alloy elements are specially added to carbon steel to make up for the above shortcomings

II. The role of alloy elements in die steel

the role of alloy elements in die steel is very complex, so far the understanding of it is not comprehensive

the following focuses on the analysis of the effect of alloy elements on iron and carbon, and the influence law on iron carbon phase diagram

1. The role of alloy elements with iron and carbon

when alloy elements are added to steel, they mainly form solid solutions with iron or carbides with carbon. A small amount of them exist in inclusions (such as

oxides, nitrides, sulfides and silicates). In high alloy steels, intermetallic compounds may also be formed

⑴ dissolved in iron

almost all alloy elements (except Pb) have a year-on-year increase of 23.6%, which can form alloy ferrite or alloy austenite with iron. According to alloy elements α? Fe or medium nickel products with the best performance price ratio in a short time γ? The role of Fe can be divided into two categories

① expansion γ Phase zone elements, also known as austenite stabilizing elements, are mainly Mn, Ni, Co, C, N, Cu, etc

they make the N point in the phase diagram drop and the G point rise, thus expanding γ Range of phases. When Ni, Mn and other elements are added to a certain amount, the G-point can be reduced below room temperature and α The phases disappear completely, and they are called complete enlargement γ Element of the zone. Other elements such as C, N and Cu are expanded γ Phase region, but it cannot be expanded to room temperature, so they are called partial expansion γ Element of the zone

② zoom out γ Phase region elements, also known as f stabilized elements, mainly include Cr, Mo, W, V, Ti, Al, Si, B, Nb, Zr, etc. They increase the G-point and decrease the N-point (except Cr, when the Cr content is less than 7%, the G-point decreases; when the Cr content is greater than 7%, the G-point rises rapidly), so as to reduce the γ The existence range of phase makes the stable region of F expand. When Cr, Mo, W, V, Ti, Al, Si and other elements exceed a certain content, point G coincides with point n, making γ When the phase zone is closed, the alloy is in single phase in the solid range α Phase states, which are called completely closed γ Element of the zone. Other elements, such as B, Nb, Zr, etc., although γ The temperature range of the phase region is reduced, but it cannot be closed, which is called partial reduction γ Element of the zone

among the above elements, only C, N and B form interstitial solid solution with iron, while others form displacement solid solution with iron

⑵ carbide forming

alloy elements can be divided into carbide forming elements and non carbide forming elements according to their affinity with carbon in steel

① common non carbide forming elements include: Ni, Co, Cu, Al, Si, N, B, etc. They do not form compounds with carbon, and are basically soluble in F and a except that intermetallic compounds can be formed in a few high alloy steels

② common carbide forming elements include Mn, Cr, Mo, W, V, Ti, Nb, Zr, etc. (arranged in the order of the stability of the formed carbides from weak to strong). They are all transition group elements on the left side of iron in the periodic table. The affinity between Mn and carbon is weak, a small part of Mn is dissolved in cementite, and most of Mn is dissolved in f or a. Cr, Mo, w with strong affinity to carbon basically form alloy cementite with iron when the content is low; When the content is high, new alloy carbides can be formed. The elements V, Ti, Nb, Zr with strong affinity to carbon almost all form special carbides. In addition, some strong carbide forming elements will be dissolved in f or a

alloy cementite is cementite after some iron atoms are replaced by carbide forming elements, such as (Fe, Cr) 3C, (Fe, Mn) 3C, etc. its crystal structure is the same as that of cementite, but it is slightly more stable than cementite, and its hardness is slightly higher, which is more beneficial to improve the wear resistance of steel

alloy carbides mn3c, Cr7C3, Cr23C6, fe3w3c, etc. have higher stability than alloy cementite, while special carbides Mo2C, W2C, VC, tic, etc. have the highest stability. The higher the stability of carbides, the higher the melting point and hardness, and the more difficult it is to dissolve in austenite when heated. Therefore, it has a great impact on the mechanical and technological properties of steel

2. Influence of alloy elements on iron carbon phase diagram

the influence of alloy elements on iron carbon phase diagram is similar to that on pure iron, but more complex. The influence is mainly divided into two aspects:

⑴ influence on the existence range of a and f

① expansion γ All the elements in the phase region enlarge the region of a in the iron carbon phase diagram, in which γ When the content of Ni or Mn in the zone is high, the steel can obtain single-phase a structure at room temperature, such as 1Cr18Ni9 high nickel a stainless steel and ZGMn13 high manganese wear-resistant steel

② zoom out γ All the elements in the phase region reduce the region where a exists in the iron carbon phase diagram, in which it is completely closed γ When the elements in the zone (such as Cr, Ti, Si, etc.) exceed a certain content, the steel can obtain single-phase f structure in a wide temperature range including room temperature, such as 1cr17ti high chromium f stainless steel

⑵ effect on critical points (s point and e point) of iron carbon phase diagram

① enlargement γ The elements in the phase region decrease the eutectoid transition temperature in the phase diagram of Fe-C alloy

② zoom out γ The elements in the phase region make it rise and cause the eutectoid reaction to proceed in two temperature ranges. At present, it is impossible to calculate an exact number. Alloy elements also affect the composition of eutectoid point and eutectic point. Almost all alloying elements reduce the carbon content at eutectoid point; The eutectic point also has a similar rule, especially the strong carbide forming element. The left shift of s point and e point changes the equilibrium structure of alloy steel (which can not be completely analyzed by iron carbon phase diagram). For example, 3Cr2W8V hot die steel containing 0.3%c is hypereutectoid steel, and the carbon content does not exceed

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