There are three categories of hardness testing standards for pit furnaces, namely Vickers, Rockwell and surface Rockwell hardness values. According to the heat treatment method, the hardness test methods of surface quenching and tempering heat treatment furnaces and chemical heat treatment furnaces are also different.
The main technical parameters of chemical heat treatment workpieces are hardened layer depth and surface hardness. The hardened layer depth should still be tested with a Vickers hardness tester. Test the distance from the workpiece surface to the point where the hardness drops to 50HRC. This is the effective hardening depth.
Chemical heat treatment is to infiltrate the surface of the workpiece with atoms of one or several chemical elements, thereby changing the chemical composition, structure and properties of the workpiece surface. After quenching and low-temperature tempering, the workpiece surface has high hardness, wear resistance and contact fatigue strength, while the core of the workpiece has high toughness.
The surface hardness test of the workpiece of the pit furnace chemical heat treatment is similar to the hardness test of the workpiece of the surface quenching heat treatment. Both can be tested by Vickers hardness tester, surface Rockwell hardness tester or Rockwell hardness tester. However, the thickness of the nitriding thickness is thinner, generally not more than 0.7mm, so the Rockwell hardness tester can no longer be used.
If the local hardness requirements of the parts are high, local quenching heat treatment can be carried out by induction heating and other methods. Such parts usually need to mark the location and local hardness value of the local quenching heat treatment on the drawings. The hardness test of the parts should be carried out in the designated area. The hardness testing instrument can use Rockwell hardness tester to test the HRC hardness value. If the heat treatment hardening layer is shallow, the surface Rockwell hardness tester can be used to test the HRN hardness value.
The surface quenching and tempering heat treatment of the pit furnace is usually carried out by induction heating or flame heating. The main technical parameters are surface hardness, local hardness and effective hardening layer depth. The hardness test can be carried out by Vickers hardness tester, Rockwell or surface Rockwell hardness tester. The selection of test force (scale) is related to the effective hardening layer depth and the surface hardness of the workpiece. There are three types of hardness testers involved here.
The Vickers hardness tester is an important means of testing the surface hardness of heat-treated workpieces. It can use a test force of 0.5 to 100 kg to test the surface hardened layer as thin as 0.05 mm. It has the highest accuracy and can distinguish the slight difference in the surface hardness of the workpiece. In addition, the effective hardened layer depth must also be detected by the Vickers hardness tester. Therefore, it is necessary to equip a Vickers hardness tester for units that carry out surface heat treatment or use a large number of surface heat-treated workpieces.
The surface Rockwell hardness tester is also very suitable for testing the hardness of surface quenched workpieces. The surface Rockwell hardness tester has three scales to choose from. It can test various surface hardened workpieces with an effective hardening depth of more than 0.1 mm. Although the accuracy of the surface Rockwell hardness tester is not as high as that of the Vickers hardness tester, it can meet the requirements as a means of quality management and qualified inspection of heat treatment plants. Moreover, it has the characteristics of simple operation, easy use, low price, rapid measurement, and direct reading of hardness values. The surface Rockwell hardness tester can be used to quickly and non-destructively test batches of surface heat-treated workpieces one by one. This is of great significance to metal processing and machinery manufacturing factories.
When the surface heat treatment hardening layer of the pit furnace is thick, the Rockwell hardness tester can also be used. When the hardening layer thickness is 0.4-0.8mm, the HRA scale can be used, and when the hardening layer thickness exceeds 0.8mm, the HRC scale can be used.
The three hardness values of Vickers, Rockwell and surface Rockwell can be easily converted to each other and converted into hardness values required by standards, drawings or users. The corresponding conversion tables are given in the international standard ISO, the American standard ASTM and the Chinese standard GB/T.
