**4.2. Heat treatment of standard bearing rings**

For standard bearings, the rings are hardened and tempered in accordance with the conditions specified in Table 6. Heat treatment is followed by grinding and super finishing of functional areas. The value of ovality of hardened and tempered rings achieved after this processing is 0.2 mm.

Deformation Reduction of Bearing Rings by Modification of Heat Treating 51

**Description of action Tempering** Device Name PP017 / 50 Zone temperature [°C] 155 ± 5 Method of placement freely Heating time [min] 65 ± 5 Output [pcs/hour] 1 080

Table 5 shows that two parameters have been modified for additional tempering. The first one is the tempering temperature, which is now lower, i.e. 155+5 °C. This is a substantial change that will ensure lower internal tensions and ovality reduced by 50% against the standard design. The second one is the heating time, which is 65+5 minutes. It is a sufficient time. However, if a longer time was preserved, the effect would be the same without any risk of lower hardness values. Tempering, referred to as "artificial ageing" is followed by hardness tests on selected pieces. The required hardness after tempering remains constant of

Ovality on finished rings was measured using the Talyrond 73 device, see Figure 9. The

Hardening and tempering has an impact on ovality in rings. The dimensional changes that occur after heat treatment are caused by the lack of stability of the microstructure of hardened and tempered bearing steel in the given operating conditions [Vasilko, 1998]. The aim of the heat treatment was to obtain a fine martensitic structure of components. A microstructure composed mainly from martensite is formed by hardening components made from bearing steel. The martensitic microstructure usually contains a low percentage of residual austenite in terms of volume Rate of heating adjusted to the hardening temperature, or inclusion of pre-heating, can affect the value and distribution of thermal tension in the bearing ring being heated. Structural tensions that arise during heat treatment by hardening and tempering are determined by the chemical concept of the steel used and heat treatment process parameters. Dimensional changes that occur during the tempering process may be a consequence of ε-carbide precipitation, decomposition of residual austenite, cementite precipitation, dislocation substructures welded together and re-distribution of residual tensions after mechanical processing

To ensure production requirements on special bearings for this customer (see Table 4) it was necessary to optimise both the dimensional parameters and the heat treatment method and subsequent machining in order to reduce internal tensions. Modified heat treatment mode and subsequent fine grinding made it possible to fulfil the requirement of lower ovality from 0.006 to 0.003 mm. The useful value of the bearings improved, too, with these

modifications in heat treatment and adjusted dimensional parameters:

**Table 7.** Technological procedure of additional tempering

measuring device ensures very high accuracy, better than 0.1 μm.

60 to 63 HRc.

**5. Result and discussion** 

[Perez et al., 2009].


**Table 6.** Heat Treatment Technological Procedure

Hardness tests are done on selected pieces after hardening and tempering. The required hardness after hardening is 63.5 to 65.5 HRc. The required hardness after tempering is 60 to 63 HRc. After tempering, the bearing rings are inspected for ovality and microstructure.

When bearing material is being heat treated, oxidation occurs at common heating. To prevent oxidation of the surface, the heat treatment is done in a controlled atmosphere consisting of nitrogen.
