The hardness testing method and instrument selection of mold steel

Die steel is the main material of the mold industry. According to the different service conditions, environment and state of the mold, the mold steel should have different characteristics. In industrial production, the service life of the mold and the precision, quality and appearance performance of the finished parts are related to the design technology and manufacturing accuracy of the mold, as well as the accuracy of the machine tool and the manufacturing operation. The correct selection of mold materials and the correct implementation of the heat treatment process It is also very important. According to the data, the early failure of the mold is about 10% caused by improper material selection and internal defects of the material, and about 49% caused by improper heat treatment.

Hardness is an important performance index for mold materials and finished molds. The stress state of the mold at work is complicated. For example, a hot work mold is usually subjected to alternating stress under an alternating temperature field, so it should have a good ability to prevent the mold from turning into a softer or plastic state, and In a long-term working environment, the shape and dimensional accuracy of the mold remain unchanged. Generally, the hardness of the finished mold is usually 59 to 60HRC for cold work molds, and about 48HRC for hot work molds.

Wear resistance is also an important performance index of finished molds. When the part is formed, the metal and the mold cavity surface move relative to each other, and the cavity surface is worn out, so that the size, shape, accuracy and surface roughness of the mold change and become invalid. The wear resistance of the mold is determined by 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 hardness test of die steel is mainly aimed at three situations, namely, the hardness test of die steel materials, the test hardness of heat-treated semi-finished molds, and the surface hardness test of molds that require high wear resistance after heat treatment. The die steel in the supply state is mainly forged steel plates, steel blocks or steel bars, which are generally supplied in an annealed state. Certain plastic mold steels are also supplied in a pre-hardened state (quenched and tempered), and users can directly process them into molds without the need for subsequent heat treatment. Die steel can be classified into carbon tool steel, alloy tool steel and high-speed steel according to steel types. my country's GB standard stipulates factory hardness requirements for various die steels, requiring inspection of the annealing hardness and sample quenching hardness of the steel.

my country’s GB/T standard 1298-1986 "Technical Conditions for Carbon Tool Steels" stipulates that for steel supplied in annealed condition, for various grades such as T7, T8, T13, the hardness value should be less than 187 to 217HBS, and the quenching hardness of the sample should be greater than 62HRC. GB/T standard 1299-2000 "Alloy Tool Steel" stipulates that for different grades of steel supplied in annealed state, the hardness value should be less than a certain value or within a certain hardness range. For example: 9SiCr steel has a hardness of 197 to 241HBW, Cr12 steel has a hardness of 217 to 269HBW, and Cr5Mo1V steel has a hardness of ≤255HBW. According to different grades, the quenching hardness of the sample is generally between 53 and 64HRC. GB/T Standard 9943-1988 "Technical Conditions for High Speed ​​Tool Steel Bars" stipulates that the hardness of annealed steel should be less than 255 to 285HBS, and the hardness of quenched and tempered steel should be greater than 63 to 66HRC.

According to the requirements of the standard, the mold steel in the ex-factory state should use a Brinell hardness tester to test the hardness of HBS (steel ball indenter) or HBW (hardness alloy indenter), and as long as the material has sufficient size, 3000 kg force and Test conditions for 10mm balls. However, mold steel materials are usually large in size and cannot be tested directly on the Brinell hardness tester. The common method is to take a sample from the steel and test it on the Brinell hardness tester after proper processing. In a few occasions with low requirements, some people use simple portable Brinell hardness tester or hammer type Brinell hardness tester to test. However, the error of these instruments is too large, and the error given in the technical indicators is ± 8%, practical application The error in is often more than 10% and can only give a rough result.

In recent years, the Leeb hardness tester has been widely used. Its test is fast and convenient. The test value can be automatically converted into the Brinell hardness value, so it has been used to a certain extent. However, the Leeb hardness tester uses the principle of dynamic hardness testing. There are many factors that affect the accuracy of the instrument. The requirements for the surface finish of the material are also very high, and the error of the hardness value conversion is also large. Therefore, its accuracy is often higher than that of simple Portable Brinell hardness tester and hammer type Brinell hardness tester, but far lower than conventional Brinell and Rockwell hardness tester.

The Rockwell hardness tester can be used for the hardness testing of mold steel. The hardness of the mold steel in the supply state is between 187 and 285HBS. It can be tested with the HRB and HRC scales of the Rockwell hardness tester. After the test, the Rockwell hardness value is converted into the Brinell hardness value by looking up the table, which still has a fairly high accuracy. This Rockwell hardness tester also uses a C-frame structure. The opening size of the C-frame is 200mm and 500mm. As long as the thickness of the mold steel is less than this size, the test can be performed. The test accuracy of the instrument can reach 1.0HRC, which is converted into Brinell hardness. The error is about 5 to 7HBS. 

The finished mold steel material must be quenched and tempered, and then finely ground and polished to become a finished mold. The hardness test of the mold after quenching and tempering is more important, because the hardness of the material at this time is a very important quality index, which determines the service life of the finished mold to a large extent.
The quenched and tempered die steel material requires Rockwell hardness tester and HRC Rockwell hardness test. The finished mold steel needs to have sufficient hardness and a certain degree of toughness. Hardness and toughness are a pair of contradictions. In order to have reasonable toughness, the mold has a higher hardness, the best hardness value will be It is restricted to a relatively narrow range, usually only 2 to 3 HRC units.

The hardness test of semi-finished molds is a difficult problem to solve. There has never been an ideal solution. Only a few molds with a small volume and weight can be moved to the bench Rockwell hardness tester for testing. In most cases, the common practice is to make a process test piece under approximately the same process conditions, and use the hardness of the process test piece to represent the hardness of the mold. However, there are often large differences between the two. This method It's not ideal.

The Leeb hardness tester provides a solution for the hardness test of semi-finished molds. The Leeb hardness of the mold can be tested and then converted into HRC Rockwell hardness. Although the Leeb hardness tester has a relatively large error, it is still the most commonly used hardness test method in the mold industry. It is understood that the most widely used field of the Leeb hardness tester is the mold industry. As mentioned earlier, the reasonable hardness range of semi-finished molds is relatively narrow, and the Leeb hardness tester cannot meet such accuracy requirements. But this is the current status of the mold industry, and there is no better solution.

Some molds that require the core to have high toughness, and the surface does not have effective high hardness and wear resistance, need to be carburized or nitrided on the surface, and surface hardened molds need to test the surface hardness of the mold.

The carburized layer is usually thick. When the thickness of the carburized layer is greater than 0.8mm, the Rockwell hardness tester can be used directly to test the HRC hardness. When the thickness of the carburized layer is 0.5 to 0.8 mm, the A scale of the Rockwell hardness tester can be used. The test force of scale A is small, only 60 kg (the test force of scale C is 150 kg), which can make a shallow indentation on the surface of the mold, which will not penetrate the hardened layer and make the hardness test more accurate. The measured HRA hardness value can be easily converted into HRC hardness value by looking up the table.

When the carburized layer is thin, such as 0.2 to 0.6mm, the surface Rockwell hardness tester can be used. The test force of the surface Rockwell hardness tester is only 15 kg, 30 kg or 45 kg, which can make a shallower indentation on the surface of the mold. , The measured hardness value can also be converted into HRC hardness value.

The nitriding layer is usually thin. The surface Rockwell hardness tester can be used for the mold with the thickness of the nitriding layer greater than 0.2mm. The mold with the thickness less than 0.2mm can only use the process test piece. Make a test piece under approximately the same process conditions. The hardness of the test piece represents the hardness of the nitrided layer. The surface hardness of this test piece can be measured on a small load Vickers hardness tester.

Hardness is one of the most important properties of die steel. The heat treatment quality and use performance of the die are usually based on hardness as the basis for judgment. In order to take care of other properties such as toughness, the best range of mold hardness is a relatively narrow range, usually only 2 to 4 HRC units. Therefore, how to use portable instruments to quickly and accurately test the hardness of the mold on the spot is of great significance in the mold manufacturing and use units. It can improve the quality of the mold, the manufacturing level and prolong the service life of the mold.More about:Large Cross-sectional Pre-hardened 2738 Plastic Mould Steel