**5. Result and discussion**

50 Heat Treatment – Conventional and Novel Applications

**Table 6.** Heat Treatment Technological Procedure

**4.3. Optimised heat treatment and grinding mode** 

tempering using this process was not possible.

ensure lower levels of ovality from 0.2 to 0.1 mm:

4. Fine grinding of functional surfaces of the rings

1. Heat treatment (hardening and tempering) as in 4.2, Table 6

processing is 0.2 mm.

consisting of nitrogen.

**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

**Description of actions Hardening Tempering**

Zone temperature [°C] 840 ± 5 170 ± 5 Oil temperature [°C] 50 – 80 – Method of placement 1 row freely Variator 3 – 5 – Heating time [min] 60 – 80 155 ± 5 Output [pcs/hour] 1 080 1 080

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

The heat treatment method (Table 6) followed by grinding as described in 4.2 is not sufficient to achieve the required ovality of 0.1 mm needed by the customer for the special bearings. It was necessary to develop, technologically master and verify a different, additional method of heat treatment and subsequent grinding of the rings, which would guarantee lower internal tensions, deformations and ovality values, whereas the required hardness of the rings should remain unchanged. Tempering is known to be used to eliminate internal tensions. This had been already done after hardening (see Section 4.2, Table 6), but resultant values of ovality were inadequate. It is also known that hardness of tempered parts decreases when tempering is done as described in Table 6, so any further

Considering these facts and drawing from our experience, a procedure was suggested to

2. Pre-grinding of functional surfaces in bearing rings, but not to reach the final value;

3. Re-tempering of the rings using the 155 °C/65' process, i.e. under different technological conditions than in 4.2. The technological conditions – see Table 7.

only with a partial use of the total allowances for grinding – rough grinding

Device Name hardening furnace Ø 100 furnace PP017 / 50

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 [Perez et al., 2009].

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:

	- reduced friction and tear and wear
	- lower noise of the equipment when these bearings are used
	- lower operating temperature even when running the equipment in
	- minimum run-in time
	- possibility to set initial stretch more reliably
	- running smooth even in the first hours of operation
	- slight increase in dynamic load (due to optimised shape of ring and tapered roll raceways)

Deformation Reduction of Bearing Rings by Modification of Heat Treating 53

**Tested property Max. prescribed values Measured values** HRc hardness 60 – 63 62 Post-grinding heating none parts none parts Microstructure 3 – 6 5 Carbide mesh 5,3 5,2 Carbide streakiness - closed 6,2 6,0 Carbide streakiness - free 7,3 7,1 Sulphides – SS 1,3 1,2 Oxides – OA 3,3 3,1 Oxides – OS 6,2 6,0 Oxides – OG 8,3 8,1

When a verification series was manufactured, the bearings were tested for endurance using a ZT1 testing station. There are 20 bearings tested simultaneously; up to the fifth discarded bearing; 90% of the bearings must withstand 1 million revolutions. Basic dynamic load rating prescribed in the catalogue is assessed. The attained value was 201% compared to the catalogue.

The herein described method of achieving lower internal tensions with effect on reduced deformations and ring ovality made it possible to attain ovality of finished rings of 0.003 mm compared to the standard requirements on ovality of 0.006 mm. This result was achieved with a satisfactory assessment of chemical, metrological and metallographic analysis with a further effect on improved endurance of bearings. This manufacturing method has been tested several times since then and the desired effect was confirmed. Subsequently, this procedure has been applied also to other types of the so-called thin-walled bearings for others customers, in particular from the automotive industry, who demanded stricter ovality

Note: Compliance with and check of prescribed processing condition are essential for heat treatment. An appropriate way how to improve checks during the heat treatment is to use a computer-controlled hardening line. Obviously, this incurs higher input costs. According to information from the company where this optimised method of heat treatment was implemented, the system started to be used for a typical hardening line and the benefit in

The authors would like to thank in words the grant agency for supporting research work

values. The possibility to use this technology has been confirmed in all cases.

terms of quality exceeded their expectations.

Anton Panda, Jozef Jurko and Iveta Pandová *Technical University of Košice, Slovak Republic* 

and cofinancing the projects: VEGA #1/0047/2010.

**Table 9.** Metallographic analysis of 100Cr6 bearing material used

**6. Conclusion** 

**Author details** 

**Acknowledgement** 

**Figure 9.** Measuring ovality using the Talyrond 73 measuring device

Extract from a chemical composition protocol - chemical analysis of 100Cr6 used is shown in Table 8. The material corresponds to the prescribed values.


**Table 8.** Chemical composition of 100Cr6 bearing material in % by weight

Extract from a metallographic analysis protocol under DIN 17230, DIN 50602, SEP 1520, and corporate standard is shown in Table 9. The material corresponds to the prescribed values.


**Table 9.** Metallographic analysis of 100Cr6 bearing material used

When a verification series was manufactured, the bearings were tested for endurance using a ZT1 testing station. There are 20 bearings tested simultaneously; up to the fifth discarded bearing; 90% of the bearings must withstand 1 million revolutions. Basic dynamic load rating prescribed in the catalogue is assessed. The attained value was 201% compared to the catalogue.
