Heat Treatment Classification and Hardness Testing Methods

Jul 27, 2023

FV bearing surface heat treatment is divided into two categories, one is surface quenching tempering heat treatment, and the other is chemical heat treatment. 

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

The main technical parameters of the chemical heat treatment bearing rings are the depth of the hardened layer and the surface hardness. The depth of the hardened layer is still tested by the Vickers hardness tester. This is the effective hardening depth. Measure the distance from the surface of the rings to the point at which the hardness drops to 50HRC. 

The surface hardness test of chemical heat treatment bearing rings is similar to the hardness test of surface quenching heat treatment rings, which can be tested by Vickers hardness tester, surface Rockwell hardness tester or Rockwell hardness tester, but the carburizing depth is thinner, generally not greater than 0.7mm, then the Rockwell hardness tester can no longer be used.

If the local hardness requirements of the bearing rings are high, can be used in induction heating and other ways for local hardening heat treatment, such rings are usually marked on the drawing of the local hardening heat treatment location and local hardness value. If the heat treatment hardening layer is shallow, the Rockwell hardness tester can be used to test the HRN hardness value. The hardness test of the part should be carried out in the specified area. Rockwell hardness tester can be used to test HRC hardness value. 

Vickers hardness tester is an important method to test the surface hardness of heat treatment rings, it can choose 0.5 ~ 100kg test force, test the surface hardening layer as thin as 0.05mm thick, which the accuracy is high, can distinguish the small difference in the surface hardness of the rings. In addition, the depth of the effective hardening layer is also measured by the Vickers hardness tester, so it is necessary to equip the Vickers hardness tester for units that perform surface heat treatment processing or use a large number of surface heat treatment rings.

The surface Rockwell hardness tester is also very suitable for testing the hardness of the surface hardened bearing rings, the surface Rockwell hardness tester has three scales to choose from. A wide range of surface hardened bearing rings with an effective hardening depth of more than 0.1mm can be tested. Although the precision of the surface Rockwell hardness tester is not as high as the Vickers hardness tester, it has been able to meet the requirements as a testing means for quality management and qualification inspection of heat treatment plants.

When the surface heat treatment hardening layer is thicker, a Rockwell hardness tester can also be used. When the thickness of the hardened layer is 0.4 ~ 0.8mm, the HRA scale can be used, and when the thickness of the hardened layer is more than 0.8mm, the HRC scale can be used.

The three hardness values of Vickers, Rockwell and surface Rockwell can be easily converted into the hardness values required by standards, drawings or users.

Please see the picture below is the four-row tapered roller bearing we supply to foreign steel mill customers, model: M268749DW-710-710CD. The hardness marked on our engineer's drawing is HRC59-63, the actual measured value is HRC 60.3, the material is TIMKEN3311, which is a carburized steel material, and the double-row tapered roller bearings and four-row cylindrical roller bearings we supply to foreign customers mainly use this material.