On 2018-08-08 14:48:54
Testing method of tool hardness in heat treatment
The surface heat treatment is divided into two categories, one is the surface quenching heat treatment, the other is the chemical heat treatment. The hardness maintenance methods are as follows:
1. Surface quenching and annealing heat treatment
Surface quenching and tempering are usually carried out by induction heating or flame heating. The main technical parameters are surface hardness, local hardness and effective hardening layer depth. Vickers hardness tester, Rockwell or surface Rockwell hardness tester can be used for hardness test. The selection of test force (scale) is related to the effective depth of hardening layer and the surface hardness of workpiece. There are three types of hardness machines involved.
Vickers hardness tester is an important means to test the surface hardness of heat-treated workpieces. It can select 0.5-100kg test force to test the surface hardening layer as thin as 0.05mm. Its precision is high, and it can distinguish the tiny difference of surface hardness of heat-treated workpieces. In addition, the effective hardened layer depth should also be tested by the Vickers hardness tester. Therefore, it is necessary to equip a Vickers hardness tester for the units that conduct surface heat treatment processing or use a large number of surface heat treatment workpieces.
Surface Rockwell hardness tester is also very suitable for testing the hardness of surface hardened workpieces. There are three scales for surface Rockwell hardness tester. It can test all kinds of surface hardened workpieces with effective hardening depth more than 0.1mm. Although the accuracy of the surface Rockwell hardness tester is not as high as that of the Vickers hardness tester, it has been able to meet the requirements as a means of quality control and qualification inspection in the heat treatment plant. In addition, it has the characteristics of simple operation, convenient use, low price, rapid measurement and direct reading of hardness value. The surface Rockwell hardness tester can be used to quickly and non destructively test a batch of surface heat treated workpieces piece by piece. This is of great significance to metal processing and machinery manufacturing plants.
The Rockwell hardness tester can also be used when the hardening layer of surface heat treatment is thick. HRA scale can be used when the thickness of heat treatment hardening layer is 0.4-0.8mm, and HRC scale can be used when the thickness of hardening layer exceeds 0.8mm.
Vickers, Rockwell and surface Rockwell hardness values can be easily converted into scale, drawing or hardness values required by users. The corresponding conversion tables have been given in international standard ISO, American Standard ASTM and Chinese standard GB / T. These three conversion tables can be found in the technical data column of Shenyang Tianxing website.
2. Chemical heat treatment
Chemical heat treatment is to make the surface of the workpiece permeate with atoms of one or several chemical elements, so as to change the chemical composition, structure and function of the surface of the workpiece. After quenching and low temperature tempering, the surface of workpiece has high hardness, wear resistance and contact fatigue strength, while the core of workpiece has high strength and toughness.
The main technical parameters of the chemical heat treatment workpiece are the hardening depth and surface hardness. The depth of hardened layer is still to be measured by Vickers hardness tester. Check the interval from the surface of the workpiece to the point where the hardness drops to 50HRC. This is the effective hardening depth.
The surface hardness test of the chemical heat treatment workpiece is similar to that of the surface quenching heat treatment workpiece, which can be tested by Vickers hardness tester, surface Rockwell hardness tester or Rockwell hardness tester, but the thickness of the nitriding thickness is relatively thin, generally not more than 0.7mm, at this time, Rockwell hardness tester can no longer be used.
3. Local heat treatment
If the local hardness of parts is required to be high, induction heating and other methods can be used for local quenching heat treatment. Such parts usually need to mark the location of local quenching heat treatment and local hardness value on the drawing. The hardness test of parts shall be carried out in the designated area. Rockwell hardness tester can be used to test HRC hardness. If the hardening layer of heat treatment is shallow, surface Rockwell hardness tester can be used to test hrn hardness.