Surface heat treatment is divided into two categories: one is surface quenching and tempering heat treatment, and the other is chemical heat treatment. Common hardness testing methods are as follows:
1. Surface quenching and tempering heat treatment
The surface quenching and tempering heat treatment of seamless steel pipes is usually carried out by induction heating or flame heating. The main technical parameters are surface hardness, local hardness and effective hardened layer depth. Vickers hardness tester can be used for hardness testing, Rockwell or surface Rockwell hardness tester can also be used. The selection of the test force (scale) is related to the depth of the effective hardened layer and the surface hardness of the straight seam welded pipe. There are three durometers involved here.
1) Vickers hardness tester
Vickers hardness tester is an important means to test the surface hardness of heat-treated seamless pipes. It can use a test force of 0.5-100kg to test the surface hardened layer as thin as 0.05mm thick. It has the highest accuracy and can distinguish the surface of heat-treated workpieces. Small differences in hardness. In addition, the depth of the effective hardened layer is also detected by a Vickers hardness tester, so it is necessary to have a Vickers hardness tester for units that carry out surface heat treatment processing or use a large number of surface heat treatment seamless pipes.
2) Surface Rockwell hardness tester
The surface Rockwell hardness tester is also very suitable for testing the hardness of surface quenched workpieces. There are three scales for the surface Rockwell hardness tester to choose from. Various case hardened straight seam welded pipes with effective hardening depths exceeding 0.1mm can be tested. 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 detection method for quality management and qualification inspection of heat treatment plants. Moreover, it also has the characteristics of simple operation, convenient use, low price, rapid measurement, and direct reading of hardness values. Using the surface Rockwell hardness tester, batches of surface heat-treated seamless pipes can be quickly and non-destructively tested piece by piece. This is of great significance for steel pipe processing and machinery manufacturing plants.
When the surface heat treatment hardened layer is thick, the Rockwell hardness tester can also be used. When the thickness of the heat treatment hardened layer is 0.4-0.8mm, 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 of Vickers, Rockwell and superficial Rockwell can be easily converted to each other and converted into standard, drawing or user-required hardness value. The corresponding conversion table has been given in the international standard ISO, American standard ASTM and Chinese standard GB/T.
2. Chemical heat treatment
Chemical heat treatment is to infiltrate the workpiece surface 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 and 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 should be detected by a Vickers hardness tester, and the distance from the surface of the workpiece to the point where the hardness drops to 50HRC is the effective hardened layer depth.
The surface hardness test of chemically heat treated workpieces is similar to that of surface quenched heat treated workpieces, and can be tested with Vickers hardness tester, surface Rockwell hardness tester or Rockwell hardness tester. Only the thickness of nitriding thickness is thinner, generally not. If it is larger than 0.7 mm, the Rockwell hardness tester cannot be used at this time.
3. Local heat treatment
If the local hardness of the parts is high, induction heating can be used for local quenching heat treatment. For such steel pipes, the location and local hardness value of the local quenching heat treatment are usually marked on the drawings. The hardness test of the steel pipe should be carried out in the designated area. The hardness testing instrument can use the 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.