How to measure the hardness of a workpiece after surface heat treatment

A metal heat treatment process that changes the mechanical properties of the surface layer by heating and cooling the surface of the steel part. Surface quenching is the main content of surface heat treatment. Its purpose is to obtain a high hardness surface layer and favorable internal stress distribution to improve the wear resistance and fatigue resistance of the workpiece.

Surface heat treatment testing methods are mainly divided into two categories, one is surface quenching and tempering heat treatment, and the other is chemical heat treatment. The specific hardness testing methods are as follows:

1. Surface quenching and tempering heat treatment

Surface quenching and tempering heat treatment is usually carried out by induction heating or flame heating. The main technical parameters are surface hardness, local hardness and effective hardened layer depth. Hardness testing can use Vickers hardness tester, Rockwell or surface Rockwell hardness tester can also be used. The choice of test force (scale) is related to the effective hardened layer depth and the surface hardness of the workpiece. There are three hardness testers involved here.

The Vickers hardness tester is an important method for testing the surface hardness of heat-treated workpieces. It can choose a test force of 0.5~100kg to test the surface hardening layer as thin as 0.05mm. It has the highest accuracy and can distinguish heat-treated workpieces. Small differences in surface hardness. In addition, the effective hardened layer depth must also be detected by a Vickers hardness tester. Therefore, it is necessary to equip a Vickers hardness tester for units that perform surface heat treatment or use a large number of surface heat treatment workpieces.

Surface Rockwell hardness tester is also very suitable for testing the hardness of surface quenched workpieces. There are three types of scales for the surface Rockwell hardness tester. It can test various surface hardened workpieces whose effective hardening depth exceeds 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 management and qualification inspection for heat treatment plants. Moreover, it also has the characteristics of simple operation, convenient use, low price, rapid measurement, and direct reading of the hardness value. The surface Rockwell hardness tester can be used to quickly and non-destructively detect a batch of surface heat-treated workpieces. This is of great significance to metal processing and machinery manufacturing plants.

When the surface heat treatment hardened layer is thick, Rockwell hardness tester can also be used. When the thickness of the heat-treated hardened layer is between 0.4 and 0.8mm, the HRA scale can be used, and when the thickness of the hardened layer exceeds 0.8mm, the HRC scale can be used.

The three hardness values u200bu200bof Vickers, Rockwell and Surface Rockwell can be easily converted into each other and converted into the hardness value required by standards, drawings or users. The corresponding conversion table has been given in the international standard ISO, the American standard ASTM and the Chinese standard GB/T. These three conversion tables can be found in the technical data column of the Shenyang Tianxing website.

2, chemical heat treatment

Chemical heat treatment is to make the surface of the workpiece infiltrate the atoms of one or several chemical elements, thereby changing the chemical composition, organization and performance of the surface of the workpiece. After quenching and low temperature tempering, the surface of the workpiece has high hardness, wear resistance and contact fatigue strength, and the core of the workpiece has high strength and toughness.

The main technical parameters of the chemical heat treatment workpiece are the depth of the hardened layer and the surface hardness. The depth of the hardened layer still needs to be tested with a Vickers hardness tester. Detect the distance from the surface of the workpiece to the point where the hardness drops to 50HRC. This is the effective hardening depth

The surface hardness testing of chemical heat treatment workpieces is similar to the hardness testing of surface quenching heat treatment workpieces. Both can be tested with Vickers hardness tester, surface Rockwell hardness tester or Rockwell hardness tester, but the thickness of nitriding is thinner. Generally not larger than 0.7mm, Rockwell hardness tester can no longer be used at this time.

3, local heat treatment

If the parts have higher local hardness requirements, they can be locally quenched and heat treated by induction heating. Such parts are usually marked with the location of the local quenching heat treatment and the local hardness value on the drawing. 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-treated hardened layer is shallow, the surface Rockwell hardness tester can be used to test the HRN hardness value.