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Die steel--machine knife (b)
- Jul 09, 2018 -

Hot working die is divided into hammer forging, die forging, extrusion and die casting, including hot forging die, press forging die, stamping die, hot extrusion die and metal die casting die. In addition to the great mechanical stress, the thermal deformation mold must bear repeated heating and cooling, which causes great thermal stress. Hot-work die steel in addition to should have high hardness, strength and red hardness, abrasion resistance and toughness, but should also have good high temperature strength, thermal fatigue resistance, heat conductivity and corrosion resistance, and also has high hardenability, to ensure the mechanical properties of the whole section with a consistent. For die casting steel, the surface layer should be repeatedly heated and cooled without cracks, and withstand the impact and erosion of liquid metal flow. This kind of steel is generally belongs to the medium carbon alloy steel, carbon mass fraction was 0.30% ~ 0.60%, belong to hypoeutectoid steel, there is also a part of steel due to join more alloy elements, such as tungsten, molybdenum, vanadium, etc.) and become a eutectoid or hypereutectoid steel. The steel commonly used are chrome manganese steel, chromium nickel steel, chrome tungsten steel and so on.


[application]: the hot-working mould is used for the formal processing of hot metal, such as hot-forging mould, die-casting mould, hot-extrusion die, etc. Hot-work die under higher impact load at work, at the same time the cavity surface with hot metal contact and friction, local temperature can reach 500 ℃ above, and will constantly repeated heating and cooling, often make the cavity surface due to thermal fatigue crack, therefore requires hot-work die steel at high temperature with high comprehensive mechanical properties and good thermal fatigue resistance. In addition, sufficient hardenability is required.


[chemical composition]: hot-working die steel is medium carbon, wC=0.3%~0.6%, so as to obtain comprehensive mechanical properties. The alloy elements include Cr, Mn, Ni, Mo, W, Si, etc. W and Mo improve the tempering resistance and prevent the tempering brittleness. Cr, W, Mo and Si also improve the heat resistance fatigue of steel.


[commonly used hot forging die steel]:5CrMnMo and 5CrNiMo are the most commonly used hot forging die steel, among which 5CrMnMo is commonly used to manufacture medium and small sized hot forging die, and 5CrNiMo is often used to manufacture large and medium-sized hot forging die. 3Cr2W8V or 4Cr5W2VSi steel shall be used for molds under static pressure (such as die casting, die extrusion, etc.).


[heat treatment]: after forging, the hot forging die is annealed to eliminate the forging stress and reduce the hardness, which is conducive to cutting and processing. The final heat treatment is quenching, high temperature (or medium temperature) tempering. After tempering, even tempered soxhlet body or tempered tot body can be obtained. The hardness is about 40HRC.

Plastic molds include thermoplastic molds and thermosetting plastic molds. Steel used in plastic mould should have certain strength, hardness, abrasion resistance, thermal stability and corrosion resistance. In addition, it also requires good manufacturability, such as smaller heat treatment, machining performance, corrosion resistance, good repair welding, grinding and polishing performance is good, good performance, high roughness, good thermal conductivity and working conditions have stable size and shape, etc. Generally, hot die steel can be used for injection molding or extrusion forming. Cold die steel can be used for thermosetting and high wear-resisting and high strength moulds.


[performance] : plastic mold of stress and wear and tear is small, the main failure forms of mould surface quality to drop, therefore should have the following properties: (1) the good processability, hard in advance higher hardness (28 ~ 35 HRC), convenient for machining or electrical discharge machining, easy to etching patterns, words and symbols; The surface of the mould reaches a high degree of mirror-surface after being polished (generally the Ra value is 0.1~0.012 joule m). Torque high hardness (hardness should exceed 45~55HRC after heat treatment), good abrasion resistance, sufficient strength and toughness; Low deformation of heat treatment (to ensure accuracy), good weldability (to facilitate mold welding and repair), etc.

[common plastic mold steel]: because plastic mold does not require high mechanical performance, so the material selection has greater flexibility. The brands, properties and USES of common plastic mould steel are shown in the following table. Steel grade are listed in the table for the domestic plastic die steel, at present there are still a lot of plastic mold used steel grade abroad (such as Japan, the United States, Germany, Sweden, etc.) or according to the improved steel grade steel grade production abroad.