What are the standards and requirements for the hardness testing of mold materials in mold processing plants?

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Hardness is usually the basis for measuring the heat treatment quality and performance of molds, and hardness is an important performance indicator of mold materials and finished molds. The stress state of the hot mold during the working process is very complex. For example, the hot mold is generally subjected to alternating stress in an alternating temperature field. Therefore, the mold should prevent the mold from becoming soft or plastic, and maintain its shape and size in a long-term working environment. Precision remains unchanged. Usually, the hardness of the finished product and the cold mold are usually 59-60HRC, and the hot work mold is often used around 48HRC.

Wear resistance is an important performance indicator of the finished mold. During the molding process, the metal moves relative to the surface of the mold cavity, causing the cavity surface to wear, and the mold size, shape, precision and surface roughness to change and fail. Wear resistance mainly depends on the heat treatment of the mold, especially the surface heat treatment. The main basis for evaluating the wear resistance of the mold is the hardness.

The test hardness test is mainly aimed at three situations, namely the hardness test of the die steel material, the heat treatment hardness test of the semi-finished mold, and the surface hardness test after the heat treatment of the mold surface.

Steel mold materials are mainly forged steel plates, steel blocks or steel bars, which are generally supplied in an annealed state. Some plastic steel molds also provide pre-hardening (quenching and tempering treatment), and users can directly process them into molds without subsequent heat treatment. According to different steel types, die steel can be divided into carbon tool steel, alloy tool steel and high-speed tool steel. The Chinese standard specifies the factory hardness requirements of various die steels, and tests the annealing hardness and quenching hardness of the steel. Mattel Leeb hardness tester is widely used, with fast detection speed and simple operation, and the detection value can be automatically converted into Brinell hardness value, so it has been widely used.

The die steel material that has been machined must be quenched and tempered, and can be made into a finished die after fine grinding and polishing. It is more important to test the hardness of the quenched and tempered mold, because the hardness of the material is a very important quality index at this time, which determines the service life of the finished mold to a large extent. Quenched and tempered mold materials require a Rockwell hardness tester to measure HRC Rockwell hardness. Molded products are required to have the best comprehensive mechanical properties, not only sufficient hardness, but also certain toughness. Hardness and toughness are a pair of contradictions. In order to achieve reasonable toughness and make the mold have higher hardness, the most Optimal hardness values ​​will be limited to a fairly small range, typically 2-4 HRC units.

There are great difficulties in hardness testing of finished and semi-finished products. There is no more ideal solution. Only a small number of molds that are small in size and weight can be moved to a benchtop Rockwell hardness tester for testing. For finished products, the Leeb hardness tester is a solution to hardness testing semi-finished products and Leeb hardness before converting to HRC on Rockwell hardness. At present, the commonly used method in the mold industry is to use a Leeb hardness tester to measure hardness, and the Mattel Leeb hardness tester is the most widely used in the mold industry.

As mentioned above, the reasonable hardness range of finished and semi-finished molds is narrow, and the Leeb hardness tester cannot meet this precision requirement. But this is also the current status of the mold industry, and there is no better solution. For surface carburizing, nitriding or laser quenching treatment, the surface hardness of the mold with surface hardening treatment should be tested. When the thickness of the infiltrated layer exceeds 0.8mm, the hardness of HRC can be directly measured by a Rockwell hardness tester. When the thickness of the carburized layer is 0.6-0.8mm, the A scale of the Rockwell hardness tester can be used. The test pressure A=60kg (the C-level test force value is 150kg), which can press a shallow indentation on the surface of the mold, so that the hardened layer will not be penetrated, and the hardness test is relatively accurate. HRA hardness measurements are easily converted to HRC hardness values ​​by looking up the table. If the thickness exceeds 0.1mm, a surface Rockwell hardness tester can be used for thinner carburized or nitrided layers. Ground Rockwell hardness tester is only 15kg, 30kg or 45kg. For example, when a surface Rockwell hardness is 60HRC, the surface Rockwell hardness value is 90.6HR15N, and the indentation depth is equal to (100-90.6)X0.001mm=0.009mm, in this case, it is almost difficult to distinguish. Therefore, the surface Rockwell hardness tester can also test the finished product, and the obtained hardness value can also be converted into HRC hardness value.

For die steel, hardness is the most important property. To account for other properties, such as toughness, its optimum hardness range is narrow. Therefore, how to use the Mattel portable hardness tester to test the hardness of the mold quickly and accurately is of great significance to the mold manufacturing and use units. It can improve the quality of mold products, improve the mold manufacturing process, and prolong the service life of the mold.

In fact, the hardness of the mold processing material is not bad, only the wrong material is used in a specific area. Therefore, designers must fully understand the properties of various materials, and carefully test the materials, and carefully test the materials to study the influence of various factors on the performance of molded products. Hopefully the above will be useful to the reader.

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