The hottest powder metallurgy die steel

2022-10-15
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Powder metallurgy die steel

with the development of industry, the requirements for the stamping performance of cold working dies are becoming higher and higher. A simple punching die has a service life of 200000 times and no longer meets the requirements of low cost and high output standards. Using newly developed mold materials (such as powder metallurgy mold steel) can achieve a million times of mold life. From the aspect of die economy, the use of powder metallurgy (PM) die steel greatly reduces the single piece cost. Although the initial cost of the mold is high, users also benefit from extending the service life of the mold and reducing downtime. From the metallurgical point of view, the service life of powder metallurgy die steel produced by special smelting method is longer than that of conventional tool steel. Powder metallurgy die steel carbides are fine and evenly distributed. The type of carbide can be designed to form a harder carbide VC. The harder the carbide, the smaller the size and the more uniform the distribution, the longer the service life of the die will be

although powder metallurgy die steel has been more and more recognized, it is found through customer service that unexpected failure cases are still encountered when using powder metallurgy materials. These failures are usually not caused by material defects, but related to mold processing. As summarized in Figure 1, the cold working process of punching is affected by many factors. The correct adjustment of all parameters is very important, so that the mold can obtain reliable performance

Figure 1: factors affecting the service life of cold working steel dies

die steel materials

◆ basic principles for selecting cold working die steel

when selecting its samples, generally short gauge length (generally normal temperature samples), the two basic mechanical properties that should be considered first are hardness and toughness. The hardness (or strength) ensures that the cutting edge of the die is sufficient to cut off the processed materials, and the toughness ensures that the die will not collapse or crack during operation. The ideal state is the combination of high hardness and high toughness. However, due to the high alloy composition of cold working die steel, when the hardness increases, the toughness decreases sharply

Table 1 lists the brand and chemical composition of cold working die steel currently sold by Assab. Vanadis 4, ASP 23 and Vanadis l0 are powder metallurgy high alloy die steels. Table 1 grades and chemical compositions of cold working die steels currently sold by Assab Figure 2 lists the toughness comparison between powder metallurgy steel and conventional die steels AISI O1, D2, D3. It can be seen that compared with conventional die steel, P/m steel has little change in longitudinal and transverse toughness. Powder metallurgy steel shows higher toughness than conventional die steel, which also indicates that powder metallurgy steel can be used under higher hardness

Figure 2: comparison of toughness between powder metallurgy steel and conventional die steel

powder metallurgy steel has higher toughness. Committed to the research and discussion of experimental technology, and the utilization and implementation of new technology, it will undoubtedly contribute a lot to China's experimental machine and better isotropic performance. The reason is its unique steel production method. Powder metallurgy die steel is made of metal powder to ensure uniform distribution, fine and uniform size of carbides during molding. Figure 3 shows the microstructure of Vanadis 4. It can be seen that it has fine structure and small particle carbides

Figure 3: Microstructure of Vanadis 4 in hardened state

the manufacturing method of conventional die steel is that the liquid metal is cooled in the ingot mold, resulting in serious carbide segregation. Like carbides are broken into smaller sizes only in the subsequent forging or rolling process. These processes lead to greater changes in the toughness in the rolling direction and transverse direction, and the toughness is also lower than that of powder metallurgy steel

◆ die failure mechanism and die wear

common failures of cold working dies include wear, angle collapse, deformation, cracking and occlusion

it is easy to understand that after many punching cuts between the punch and the die, the cutting edges of the punch and the die will be rounded, which is called wear. Usually, the material cut by punching has redundant burrs, which is a sign of die wear. Punching is actually a low cycle fatigue process. The die processed by rough grinding or EDM will have the problem of angle collapse. The rough surface means many stress concentration points, which may be the location of crack initiation

if the die hardness is selected correctly, the deformation of the die rarely occurs. Mold cracking is usually due to low toughness. Die sticking occurs especially when punching soft materials such as aluminum or copper. The harm is that a small amount of soft material changes the gap between the punch and the die, which leads to surface galling

there are two types of wear: adhesive wear and abrasive wear. Adhesive wear occurs when machining soft materials, such as stainless steel or copper. The research shows that the adhesive wear is caused by many microcracks in the direction perpendicular to the punching movement. Figure 4A shows SEM photos of the circumference of the punch (AISI D2) after cutting stainless steel 90000 times. When the punch undergoes low cycle fatigue under cyclic loading, the micro cracks caused by adhesive wear will further develop into collapsed micro pits. Figure 4B shows the wear trace of a die (AISI D2 type) after punching a steel strip with a hardness of hrc46. It can be seen from this micrograph that abrasive wear causes friction flow lines along the direction of die movement

Figure 4

A: SEM microscope of adhesive wear B: SEM microscope of abrasive wear

processing of die steel

dies can be processed and manufactured by means of grinding, turning, drilling, wire cutting, grinding, etc. The structure near the surface will change during processing. From the metallurgical point of view, high-density dislocations are generated near the machining and cutting surface, thus forming a layer of stress zone. In the subsequent quenching process, the machined surface tends to release these stresses, resulting in a large amount of deformation of the die. In order to minimize the distortion, it is a good method to de stress the processed mold before quenching

cold working dies are usually machined by wire cutting in hardened state. There are two typical wire cutting problems: 1) the surface of wire cutting die is too rough to obtain reliable die life; 2) The mold cracks during cutting

as we all know, rough wire cutting will lead to unstable die f) automatic calibration: the system can automatically realize the calibration of indication accuracy; With service life. Rough wire cutting with high current will produce brittle recast layers that may contain cracks. Wire cutting surface usually has tensile stress, which makes the die life shorter than expected

stress relief treatment must be carried out at a temperature about 20 ℃ lower than the final tempering temperature to release the tensile stress. Sometimes it is recommended to polish the wire cut surface before stress relief. However, due to the complex geometry, precision design parts are sometimes difficult to polish. Therefore, it is recommended to perform wire cutting at least three times. Usually, seven times of wire cutting is needed to achieve a reliable and consistent die life

one of the main problems in wire cutting of thicker die plates (>35mm) that have been quenched and tempered is that the die occasionally cracks. It is found that die cracking is usually related to tempering temperature, and high temperature tempering (>500 ℃) is better for reducing the stress of hardened die. When the die is cooled from the tempering temperature, high temperature tempering can also transform residual austenite into martensite. For wire cutting dies, it is important to minimize the residual austenite, because during cutting, the residual austenite is transformed into martensite due to local high heat input, resulting in cracking

for integrally quenched die steel, the stress state after quenching and tempering is central compression and surface tension. If this stress state is disturbed during cutting, the die may deform or crack

Figure 5: die wire cutting section after seven times of wire cutting (400X)

the heat treatment transformation of die steel is the general trend

cold working die steel is supplied in soft annealed state. In order to obtain strength, quenching is needed. In order to obtain good microstructure, quenching quality should be controlled. It should be kept in mind that microstructure determines the (physical, mechanical, chemical) properties of steel. Good microstructure obtained by heat treatment ensures good die life

◆ quenching heat treatment cycle of die steel

quenching of die steel includes heating and cooling of die components. In the process of heat treatment, the die must be heated to the austenitizing temperature (for example, the austenitizing temperature of AISI D2 is 1030 ℃), so that the matrix structure of the die steel changes from spheroidal pearlite to austenite. After rapid cooling (quenching), austenite is transformed into martensite. Ideally, a properly quenched high alloy cold working steel should have a carbide strengthened martensitic matrix structure

the holding time and temperature during austenitizing must be set correctly, and no carburization or decarburization is allowed. Quenching medium must be selected correctly, because too fast quenching may lead to cracking. It must be kept in mind that when austenite is transformed into martensite during quenching, the absolute volume will increase by more than 4%

mold failure is often related to incorrect or inappropriate heat treatment methods. A failure case worth mentioning here is the forming mold of the back shell of the watch. The die often breaks corners. The mold material calmax is a patented material of UDDEHOLM tooling ab. The working hardness of this material at 57hrc is high toughness. It can be seen that there is local decarburization on the surface of the die, which is the cause of the angle collapse

◆ deformation control of die steel

die steel will deform after quenching due to high temperature process. Considering the application field of precision die, the deformation should be minimized. If different heat treatment equipment is used, the effect will be very different. A trend of quenching die steel requiring minimum deformation is to use vacuum furnaces for high-pressure quenching

The general principle of deformation control is to heat the parts as slowly as possible and quench them as slowly as possible to minimize the thermal shock. From the point of view of microstructure, the die should be rapidly quenched to obtain a good microstructure. Therefore, we must find a balance between the speed of quenching and cooling

when quenching precision molds, it is required that the flatness of the mold cannot be changed after heat treatment. Typical plate size is 30 × one hundred and fifty × 300mm。 However, due to the pre processed fine holes on the mold before heat treatment, the mold appears to be bent after quenching. Once bending occurs, it should be straightened

practical experience shows that the plate-shaped die is straightened when the soft austenite in the steel transforms to martensite. Keep this rule in mind that there are two ways to avoid the bending of the flat die: one is to put the die in the fixture when the die is cooled from the austenitic temperature during quenching. This method can be carried out in an open furnace, such as a salt bath furnace. The mold shall be placed in the fixture when the temperature is at the MS point of the relevant material

Another method of straightening is to clamp the quenched mold during tempering. It is important to choose a suitable temperature. When cooled, the residual austenite will be transformed into bainite or martensite. During the transformation process, the tempering die will be limited by the contour of the fixture and the bending will be straightened. This method is especially suitable for ASP 23 material, because it has quite high residual austenite before tempering (30% residual austenite when quenching at 1170 ℃)

◆ surface treatment

typical surface diffusion treatment of cold working die steel is nitriding or nitrocarburizing. In this process, nitrogen (carbon in Carbonitriding) atoms diffuse into the steel matrix and form nitrides with alloy elements in the steel. Most alloy elements such as Cr, Mo and V are strong nitride and carbide forming elements. After a certain period of treatment, a layer of evenly distributed nitriding layer is formed, which will prolong the service life of the mold because it

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