德语
1. Working conditions and performance requirements of hot extrusion dies and medium and small machine forging dies
When the hot extrusion die is working, it is subject to compressive stress, bending stress and tensile stress of mold release. The impact load is smaller than that of the hammer forging die, but the time it takes to contact with the hot metal is longer than that of the hammer forging die, and the working temperature is higher than that of the hammer forging die, and the temperature rise is also different when extruding different metals, and the maximum can be reached.800-85090. The thermal stress caused by the rapid cold and rapid heat is also greater than that of the hammer forging die, and the friction is more severe. The main failure form of hot extrusion die is excessive plastic deformation of the mold cavity, fatigue damage, thermal wear and surface oxidation corrosion caused by repeated heating and cooling.
The working conditions of medium and small machine forging dies are similar to those of hot extrusion dies, the difference is that the impact load is larger than that of hot extrusion dies. Therefore, the failure form of medium and small machine forging dies is also similar to that of hot extrusion dies. Therefore, it is required that the hot extrusion die and medium and small machine forging dies have higher heat fatigue resistance, thermal stability and good wear resistance than the hammer forging dies, as well as higher high temperature strength and sufficient toughness than the hammer forging dies.
2. Hot extrusion die and steel for medium and small machine forging dies
Commonly used hot extrusion molds and steels for medium and small machine forging dies are tungsten hot-work mold steel and chromium hot-work mold steel, as well as new hot-work mold steel such as chromium key system, tungsten aluminum system, chromium platinum tungsten system, and matrix steel.
(1) The representative steel type of tungsten hot-working mold steel is the traditional 3Cr2W8V steel. Due to its poor heat fatigue resistance, its application in hot extrusion die will gradually decrease, but it is used in die casting die casting die, so it is introduced in the steel for die casting die.
(2) Chromium-based hot-work mold steel Representative steels of chromium-based hot-work mold steel are 4Cr5MoSiV, 4Cr5MoSiV1 and 4Cr5 W2VSi. The first two are equivalent to the H11 and H13 steels in the United States, and 4Cr5W2VSi evolved from 4Cr5MoSiV steel, from. w 2 2% instead of two Mo=1%O. The mass fraction of these three steels is about 5%, which is medium carbon and medium chromium steel.
The common characteristics of this type of steel are:
① Due to its high chromium content, it has high hardenability. For example, 4Cr5MoSiV1 steel parts with a thickness of 150mm can be oil-cooled and quenched. Moreover, the supercooled austenite of this type of steel has high stability between 400 and 600°C, and can be heat-insulated for a long time without transformation, so it is suitable for grading quenching.
② Thermal fatigue resistance is better, because chromium and silicon improve the oxidation resistance of steel, so chromium-based steel can better adapt to the working conditions of emergency cold and emergency heat.
③The tempering stability is high, such as the quenching hardness of 4Cr5MoSiV and 4Cr5W2VSi steels reach the maximum value when quenched at around 1070℃ and 1200℃ respectively. Then temper, its hardness remains almost unchanged as the tempering temperature increases, and a secondary hardening peak occurs at 500-550°C, and then it decreases rapidly.
④ Compared with tungsten-based hot-working mold steel, it has higher toughness, but insufficient high-temperature strength, slightly poor heat resistance, and the working temperature generally does not exceed 6501.
⑤The types and quantities of carbonized species contained in this type of steel are roughly the same, so the superheating sensitivity is roughly the same.
⑥ The chromium-based hot-working mold steel has high thermoplasticity, low deformation resistance, and a small tendency to crack in forging, but the forging temperature range is slightly narrow, so the forging temperature must be strictly controlled.
(3) Chromium-keyed steel and chromium-tungsten-keyed steel such steel include 4Cr3Mo3SiV (H10), 3Cr3Mo3VNb (HM3), 3Cr3Mo3W2V (HMI), 5Cr4W5Mo2V (RM2), 4Cr3Mo3 W4 VNb (GR), etc.
The following briefly introduces the performance characteristics and applications of some steel types
1) 3Cr3 Mo3 VNb(HM3) steel: This steel has a low carbon content and a small amount of keys added, so it has high heat fatigue resistance and toughness, good tempering stability, and excellent other process performance. Suitable for the manufacture of highly water-cooled press forming dies, roller forging dies, small hammer forging dies, etc., its working life is significantly higher than that of molds made of 5CrNiMo, 4Cr5W2VSi, 3Cr2W8V steel.
2) 5Cr4W5Mo2V (RM2) steel: The mass fraction of this steel is about 0.5%, the total mass fraction of alloy elements is 12%, and the state of use contains more carbides, including M6C. Therefore, this steel has high tempering resistance and thermal stability, and its thermal stability can reach 700cC at a hardness of 50HRC, and its wear resistance is also good. Suitable for making small section hot extrusion, high-speed forging dies and roller forging dies.
3) 4Cr3Mo2W4VTiNb (GR) steel: This steel is added with a small amount of nitogram to the tungsten key-based hot working mold steel to obtain high tempering stability and high thermal strength. Its thermal fatigue resistance, thermal stability, wear resistance and high temperature strength are significantly higher than 3Cr2W8V steel. The steel is quenched by oil 1160-1200℃, and is tempered twice at 630-600℃. Each time it is treated with 1 h, its hardness can reach 50-55HRC, its tensile strength can reach 188 OMPa, and its impact toughness is 17J/cm2. The steel has good hardenability and hot and cold workability, and is suitable for making hot forging molds such as hot emblem, fine forging, high-speed forging, etc.
4) Matrix steel There are multiple steel types in the matrix steel that can be used as steel for cold work molds and steel for hot work molds, such as 6W8Cr4VTi (LM1), 6Cr5Mo3W2VSiTi (LM2) and 6Cr4Mo3Ni2WV (CG-2), among which 5Cr4Mo3SiMnVAI (012A1) steel is mostly used in hot extrusion molds, such as bearing hot extrusion punches, transmission rod hot emblem dies, etc., and its service life is significantly improved compared with traditional hot work mold steel 3Cr2W8V.
3. Material selection of hot extrusion molds and medium and small machine forging dies
When selecting materials for hot extrusion mold processing, it should mainly be determined based on the type of metal to be extruded and its extrusion temperature. Secondly, factors such as extrusion ratio, extrusion speed and lubrication conditions should also be taken into consideration to increase the service life of the mold. surfaceFigure 3 shows the selection of materials for hot extrusion die processing. The material selection of medium and small machine forging dies mainly considers the type of forging material and production batch. Secondly, the impact of die size, deformation speed and lubrication conditions on die life must also be considered.
4. Heat treatment of hot extrusion dies and medium and small machine forging dies
The manufacturing process route of this type of mold is generally:Blanking*forging, pre-heat treatment*machining and forming, quenching, tempering and finishing.
The process characteristics of each thermal processing process are analyzed below.
(1) The steel used in hot extrusion dies and medium and small machine forging dies in the forging process is mostly high alloy steel, so the die blank needs to be well forged, especially the hot work die steel containing aluminum. Pay attention to the control of the forging heating temperature and holding time to avoid early failure of the die caused by severe decarburization.
(2)Preparatory heat treatment
1) Annealing. The annealing process of hot extrusion dies for medium and small machine forging dies mainly depends on correctly selecting the annealing temperature, maintaining sufficient holding time, and cooling at an appropriate cooling rate. In addition, in order to ensure good wear resistance, a certain amount of carbides needs to be retained after quenching. Since the shape of carbides has a great impact on the toughness of steel, attention should also be paid to the shape of carbides after annealing. It is generally desired to obtain round and fine carbides.
2) High temperature tempering. In order to improve the mechanical properties (especially fracture toughness) of the forged blank, post-forging quenching and tempering is often used to pretreat the blank. This heat treatment method is to heat the forged mold blank to high temperature for quenching and then temper it at high temperature.
After this treatment, the carbides can be evenly distributed and round and small in shape, which not only improves the properties of the steel, but also shortens the pretreatment cycle. The quenching heating temperature for quenching and tempering treatment can be determined according to different steel types, such as3Cr3Mo3 W2V steel is 120090, which is similar to conventional quenching temperature. High temperature tempering temperature is generally between 700 and 750°C.
3) Normalize after forging. For mold blanks with obvious intergranular chain carbides after forging, they must be eliminated by normalizing before spheroidizing annealing. Because this kind of chain carbide is difficult to eliminate by direct annealing.
4) Quenching and tempering For commonly used hot extrusion die steels and medium and small machine forging die steels, when selecting the quenching temperature, the main consideration is the size of the austenite grain size and the level of impact toughness. Secondly, the performance requirements of the working conditions, structural shape, and failure mode of the mold must also be considered.
For the selection of the annealing holding time, the main consideration is to complete the structural transformation and fully dissolve the carbon and alloy elements to ensure high tempering resistance and hot hardness. The quenching holding time coefficient is generally taken for a salt bath furnace.0.5-1 min/mm, the smaller the size, the larger the coefficient.
Hot extrusion die steel and steel for medium and small machine forging dies are high alloy steels with good hardenability. Oil cooling or air cooling can be used for quenching cooling. Isothermal quenching or graded quenching can also be used for molds that require small deformation. The correctness of the tempering process plays an important role in the failure mode of the mold. The principle of selecting the tempering temperature is to increase the hardness of the mold as much as possible without affecting the mold's ability to resist brittle fracture. This requires determining the tempering parameters according to the specific failure mode of the mold. The quenched mold should be tempered as soon as possible, especially the mold with complex shape. When the mold surface temperature is lower thanAt 80℃, tempering must be carried out. In order to avoid the generation of residual stress, tempering heating and cooling should be carried out slowly. Tempering is generally performed twice, and the tempering time can be calculated as 3min/mm, but should not be less than 2h. The second tempering temperature can be 10-20CC lower than the first time.
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