Hardness is usually the basis for measuring the heat treatment quality and service performance of molds, and it is an important performance indicator for mold materials and finished molds. The stress state of hot work molds during operation is very complex; for example, hot work molds generally bear alternating stress in an alternating temperature field. Therefore, molds should prevent softening or plastic deformation and maintain their shape and dimensional accuracy unchanged over long-term working environments. Typically, the hardness of finished cold work molds is 59-60 HRC, while hot work molds are commonly used at around 48 HRC.
Wear resistance is an important performance indicator for finished molds. During the forming process, the metal moves relative to the mold cavity surface, causing wear on the cavity surface, leading to changes in mold dimensions, shape, precision, and surface roughness, resulting in failure. Wear resistance mainly depends on the heat treatment of the mold, especially surface heat treatment. The main basis for evaluating mold wear resistance is hardness.
Hardness testing mainly targets three situations: hardness inspection of mold steel materials, hardness inspection of semi-finished molds after heat treatment, and surface hardness testing of molds after surface heat treatment.
Steel mold materials are mainly forged steel plates, blocks, or bars, generally supplied in an annealed state. Some plastic mold steels are also provided in a pre-hardened (quenched and tempered) condition, allowing users to directly machine them into molds without subsequent heat treatment. Depending on the steel type, mold steels can be classified into carbon tool steel, alloy tool steel, and high-speed tool steel. Chinese standards specify factory hardness requirements for various mold steels and inspect the annealed hardness and quenched hardness of the steel. The Metalee Leeb hardness tester is widely used due to its fast testing speed and simple operation, and the measured values can be automatically converted to Brinell hardness values, thus gaining certain application.
Mold steel materials that have completed machining must undergo quenching and tempering treatment, and after fine grinding and polishing, they can be made into finished molds. Hardness testing of quenched and tempered molds is even more important because at this stage, material hardness is a very important quality indicator, largely determining the service life of the finished mold. Quenched and tempered mold materials need to be measured with a Rockwell hardness tester for HRC Rockwell hardness. Molded products require optimal comprehensive mechanical properties, not only sufficient hardness but also a certain toughness. Hardness and toughness are a pair of contradictions. To ensure reasonable toughness while achieving higher hardness, the optimal hardness value will be limited to a relatively small range, generally 2-4 HRC units.
In hardness testing of finished and semi-finished molds, there are great difficulties, and there is no more ideal solution. Only a few molds with small volume and weight can be moved to a bench Rockwell hardness tester for testing. For finished molds, the Leeb hardness tester is a solution, converting to HRC Rockwell hardness before testing semi-finished and Leeb hardness. Currently, the common method in the mold industry is to use a Leeb hardness tester to measure hardness, and the Metalee Leeb hardness tester is the most widely used in the mold industry.
As mentioned earlier, the reasonable hardness range for finished and semi-finished molds is relatively narrow, and the Leeb hardness tester cannot meet this precision requirement. However, this is also the current situation in the mold industry, with no better solution. For molds subjected to surface carburizing, nitriding, or laser quenching treatment, the surface hardness of the hardened molds needs to be tested. When the penetration layer thickness exceeds 0.8 mm, a Rockwell hardness tester can be used to directly measure HRC hardness. When the carburized layer thickness is 0.6-0.8 mm, the A scale of the Rockwell hardness tester can be used. The test force A = 60 kg (C scale test force value is 150 kg), which can press a shallower indentation on the mold surface without penetrating the hardened layer, and the hardness test is relatively accurate. HRA hardness values can be easily converted to HRC hardness values by looking up a table. If the thickness exceeds 0.1 mm, for thinner carburized or nitrided layers, a superficial Rockwell hardness tester can be used. The superficial Rockwell hardness tester only uses 15 kg, 30 kg, or 45 kg. For example, when the surface Rockwell hardness is 60 HRC, the superficial Rockwell hardness value is 90.6 HR15N, and the indentation depth equals (100-90.6) X 0.001 mm = 0.009 mm, which is almost difficult to distinguish. Therefore, the superficial Rockwell hardness tester can also inspect finished molds, and the obtained hardness values can also be converted to HRC hardness values.
For mold steel, hardness is the most important performance. To consider other properties such as toughness, the optimal hardness range is relatively narrow. Therefore, how to use the Metalee portable hardness tester to quickly and accurately test mold hardness is of great significance for mold manufacturing and user units. It can improve mold product quality, enhance mold manufacturing processes, and extend mold service life.
In fact, the hardness of mold processing materials is not poor; only in specific areas is the wrong material used. Therefore, designers must fully understand the properties of various different materials, carefully test materials, conduct thorough experiments on materials, and study the effects of various factors on the performance of molded products. Hopefully, the above content is useful to readers.