**4. Experimental section**

44 Heat Treatment – Conventional and Novel Applications

**Figure 3.** The principle of measuring ovality of outer shape of the outer ring of a roller bearing. 1 - dial

240 250 260 270 280 290 300 310 320 330 340 350 360 0.17 0.17 0.18 0.19 0.2 0.19 0.18 0.17 0.17 0.16 0.13 0.09 0.05 0.16 0.13 0.09 0.05 0.02 0.04 0.09 0.13 0.16 0.18 0.19 0.2 0.21 0.08 0.1 0.1 0.11 0.12 0.13 0.14 0.15 0.15 0.14 0.13 0.12 0.1

In principle, the customer required a bearing that would withstand higher axial loads without any major run-in and with reduced ring ovality due to the placement of the rotor in an axial piston hydroelectric generating set. Table 4 shows a comparison of basic modified parameters between the standard design (Standard) and the design required by the customer (Special). As mentioned before, this paper will discuss only the issue of reducing ovality in the inner ring. Finding a solution for such task is even more difficult because the bearing is a thin-walled bearing (AX series) that is much more sensitive to material deformations and ring ovality compared to other bearings with a more favourable ratio of

deviation meter, 2 - outer ring, 3 - moving measuring contact line, 4 - supporting parts

**Table 3.** Measured values of ovality - continuation

**3. Customer's requirements** 

ring thickness and width.

Material deformations and ring ovality are caused by internal tension generated during machining and heat treatment operations. To process a bearing ring by turning, it has to be fixed at three points. The fixing is done pneumatically. A deformation may occur due to poor fixing, or due to a failure to follow technological conditions, when more material is removed. When rings are grinded after heat treatment, similar undesired deformations occur, if technological conditions are not followed. Major deformations occur even during the heat treatment itself, i.e. when the rings are hardened, due to uneven heating and cooling. Deformation that appeared after heat treatment are then reproduced at subsequent grinding, worsening this effect even more.

The customer accepted only 0.003 mm of stricter ovality for the outer ring after grinding, compared to the standard prescribed value of 0.006 mm (see Table 4), which tightens the requirements by 50%. To achieve this final ovality of rings after grinding , then the ovality of rings after hardening can be no more than 0.1 mm, which is also a stricter value compared to the standard requirement of 0.2 mm.

Ovality is defined as the difference in diameters measured in one plane perpendicular to each other. This means that, for example, the maximum diameter Dmax is measured first – i.e. the maximum value is found when the ring is turned, then the ring is rotated by 90 ° and the second, minimum diameter Dmin is measured. The outer shape of the ring should be close to a circle, but in fact, the outer ring is elliptical in shape. Our aim is to keep this ovality as small as possible.

**Figure 4.** Definition of ovality

## **4.1. Hardening and tempering of bearing components**

Steel 100Cr6 with the following chemical composition (values in % by weight)2 was used to manufacture bearing rings and rolling elements: C=0.9-1.1; Mn = 0.3-0.5; Si = 0.15-0.35; Cr = 1.3- 1.65; P=max 0.027; S=max 0.03; Ni=max 0.3; Cu=max 0.25; Ni+Cu=max 0.5. Desired mechanical properties of roller bearings are obtained by hardening and tempering their components at low temperature. The required hardness of bearing components is achieved by hardening and tempering is used to reduce internal tension and fragility of the hardened bearing steel.

Deformation Reduction of Bearing Rings by Modification of Heat Treating 47

Internal tensions occur because various structures develop in various volumes and at different stages in terms of time and temperature due to the temperature gradient. Tensions arising from differences in temperature between the component surface and its core are referred to as thermal tensions. Structural tensions originate from the difference in specific

Temperature tensions can be affected by reducing or extending the process of heating, especially by preheating. Structural tensions depend on chemical composition of steel and

Dimensional changes in chrome bearing steel increase with a higher hardening temperature. Tempering will reduce the increase in volume, and the reduction is higher with a higher

In bearing rings, hardening and tempering influence their ovality. Ovality is related with volume changes only a little. It originates from the technology, resulting from an uneven

Cooling rate in the hardening process has an impact on volume change in components. The higher is the rate the higher is the deformation and the higher is also the difference in length between the states after hardening and tempering. The dimensional changes (Fig. 5) 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]. This is the result of permanent changes in instable structural stages of martensite and residual austenite. Therefore, stabilization of dimensions in hardened and tempered bearing components depends on the degree of super saturation of a solid solution - martensite and the residual austenite content, i.e. on the microstructure as well as on operating conditions, temperature, time and tensions.

distribution of internal tensions before hardening and uneven heating and cooling.

**Figure 5.** Effect of hardening temperature on change in length and hardness after hardening and tempering [8]: a – hardness after hardening; b – hardness after hardening and tempering 150°C; c – change in length after hardening; d - change in length after hardening and tempering 150°C

volumes of the initial austenite and martensite formed or in other stages.

course of the heat treatment.

hardening temperature.

The resulting mechanical properties are then determined primarily by the microstructural state, distribution of internal tensions before hardening, uniformity of heating to austenitizing temperature, austenitizing conditions and cooling down from the austenitizing temperature.

The method of heating affects resultant oxidation and surface decarburization. Local overheating and imperfect soaking must be avoided during heating, because they lead to cracks formed during cooling. Austenitizing conditions, i.e. austenitizing temperature and dwell at the austenitizing temperature, affect quality of hardened bearing components. Dwell time selected depends on the shape and material of the component, its heating method and baseline microstructure. The dwell at the austenitizing temperature has a lower effect than the temperature value.

The outcomes of hardening depend also on the speed of heat dissipation. For bearing steel, the cooling rate must be very high for a temperature range of approximately 650 °C and below. The cooling efficiency of different environments depends mainly on thermal conductivity, specific heat, evaporating heat, viscosity of the hardening environment and amount of dissolved gases. The cooling process in water is very fast and is used to harden bearing balls. Different ingredients are added into quenching water; some of which increase the cooling capacity and some of them slow it down.

Due to the lower cooling rate, thus a smaller temperature gradient between the surface and core of the component being hardened, it is more convenient to cool bearing components in mineral oils rather than in water. The most suitable medium for a common hardening environment in terms of cooling rate is J4 bearing oil that can achieve the maximum cooling rate of 65 °C/s at surface temperature of 550 °C. Cooling in an AS140 salt solution (a mixture of KNO3 NaNO3) is used to equalise the temperature at 150 °C between the surface and core of the component. After this cooling, the component continues to cool down in oil or is finally cooled down in water. Increased cooling rate causes higher susceptibility of the hardened component to develop cracks, resulting from the higher temperature difference between the surface and the core of the hardened component, creating internal tensions.

Hardening and tempering of rings is one of the most important operations in production of roller bearings and it should ensure dimensional stability in addition to the required hardness of 60 to 63 HRC. Dimensional stability is necessary for subsequent technological operations needed to achieve the correct geometry of a finished bearing and stability of these dimensions in long-term operation. When bearing steels are hardened, martensite or a structure with a specific volume different than the original martensite is formed.

Internal tensions occur because various structures develop in various volumes and at different stages in terms of time and temperature due to the temperature gradient. Tensions arising from differences in temperature between the component surface and its core are referred to as thermal tensions. Structural tensions originate from the difference in specific volumes of the initial austenite and martensite formed or in other stages.

46 Heat Treatment – Conventional and Novel Applications

effect than the temperature value.

the cooling capacity and some of them slow it down.

**4.1. Hardening and tempering of bearing components** 

Steel 100Cr6 with the following chemical composition (values in % by weight)2 was used to manufacture bearing rings and rolling elements: C=0.9-1.1; Mn = 0.3-0.5; Si = 0.15-0.35; Cr = 1.3- 1.65; P=max 0.027; S=max 0.03; Ni=max 0.3; Cu=max 0.25; Ni+Cu=max 0.5. Desired mechanical properties of roller bearings are obtained by hardening and tempering their components at low temperature. The required hardness of bearing components is achieved by hardening and

The resulting mechanical properties are then determined primarily by the microstructural state, distribution of internal tensions before hardening, uniformity of heating to austenitizing temperature, austenitizing conditions and cooling down from the austenitizing temperature.

The method of heating affects resultant oxidation and surface decarburization. Local overheating and imperfect soaking must be avoided during heating, because they lead to cracks formed during cooling. Austenitizing conditions, i.e. austenitizing temperature and dwell at the austenitizing temperature, affect quality of hardened bearing components. Dwell time selected depends on the shape and material of the component, its heating method and baseline microstructure. The dwell at the austenitizing temperature has a lower

The outcomes of hardening depend also on the speed of heat dissipation. For bearing steel, the cooling rate must be very high for a temperature range of approximately 650 °C and below. The cooling efficiency of different environments depends mainly on thermal conductivity, specific heat, evaporating heat, viscosity of the hardening environment and amount of dissolved gases. The cooling process in water is very fast and is used to harden bearing balls. Different ingredients are added into quenching water; some of which increase

Due to the lower cooling rate, thus a smaller temperature gradient between the surface and core of the component being hardened, it is more convenient to cool bearing components in mineral oils rather than in water. The most suitable medium for a common hardening environment in terms of cooling rate is J4 bearing oil that can achieve the maximum cooling rate of 65 °C/s at surface temperature of 550 °C. Cooling in an AS140 salt solution (a mixture of KNO3 NaNO3) is used to equalise the temperature at 150 °C between the surface and core of the component. After this cooling, the component continues to cool down in oil or is finally cooled down in water. Increased cooling rate causes higher susceptibility of the hardened component to develop cracks, resulting from the higher temperature difference between the surface and the core of the hardened component, creating internal tensions.

Hardening and tempering of rings is one of the most important operations in production of roller bearings and it should ensure dimensional stability in addition to the required hardness of 60 to 63 HRC. Dimensional stability is necessary for subsequent technological operations needed to achieve the correct geometry of a finished bearing and stability of these dimensions in long-term operation. When bearing steels are hardened, martensite or a

structure with a specific volume different than the original martensite is formed.

tempering is used to reduce internal tension and fragility of the hardened bearing steel.

Temperature tensions can be affected by reducing or extending the process of heating, especially by preheating. Structural tensions depend on chemical composition of steel and course of the heat treatment.

Dimensional changes in chrome bearing steel increase with a higher hardening temperature. Tempering will reduce the increase in volume, and the reduction is higher with a higher hardening temperature.

In bearing rings, hardening and tempering influence their ovality. Ovality is related with volume changes only a little. It originates from the technology, resulting from an uneven distribution of internal tensions before hardening and uneven heating and cooling.

Cooling rate in the hardening process has an impact on volume change in components. The higher is the rate the higher is the deformation and the higher is also the difference in length between the states after hardening and tempering. The dimensional changes (Fig. 5) 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]. This is the result of permanent changes in instable structural stages of martensite and residual austenite. Therefore, stabilization of dimensions in hardened and tempered bearing components depends on the degree of super saturation of a solid solution - martensite and the residual austenite content, i.e. on the microstructure as well as on operating conditions, temperature, time and tensions.

**Figure 5.** Effect of hardening temperature on change in length and hardness after hardening and tempering [8]: a – hardness after hardening; b – hardness after hardening and tempering 150°C; c – change in length after hardening; d - change in length after hardening and tempering 150°C

48 Heat Treatment – Conventional and Novel Applications

With this heat treatment, we try to get a fine martensitic structure of components, as shown in Fig. 6 - microstructure of 100Cr6 steel; properly tempered; martensite and fine, evenly distributed carbides.

Deformation Reduction of Bearing Rings by Modification of Heat Treating 49

T6

**Temperature–exhaust gas oxygen sen. [°C]** 

Tank T7

**T1 T2 T3 T4 T5 T6 T7 t**  842 842 842 844 57 822 50 9:49 834 842 843 845 57 822 50 9:50 832 842 843 845 57 822 50 9:51 832 841 843 844 57 822 50 9:52 835 841 842 842 57 822 50 9:53 838 841 842 840 57 821 50 9:54 841 841 841 839 57 822 50 9:55 843 841 840 840 57 821 50 9:56 842 841 840 842 57 821 50 9:57 838 841 841 844 57 821 50 9:58 836 841 843 844 57 821 50 9:59 836 841 843 844 57 821 50 10:00 838 841 843 842 57 821 50 10:01 841 841 843 841 57 821 50 10:02 843 841 842 839 57 821 50 10:03 841 841 841 839 57 821 50 10:04 838 841 841 842 57 820 50 10:05 836 841 840 844 57 820 50 10:06 837 841 840 844 57 820 50 10:07 839 841 842 843 57 821 50 10:08 842 841 843 842 57 820 50 10:09

Hardening furnace T1-T4 Oxygen sensor Shielding

Harden.tank

T5

**Table 5.** Sample output from the computer-controlled hardening line with measuring data

**Temperature - tank [°C]** 

**Time [hour:min]** 

**Figure 8.** Diagram of the computer-controlled hardening line

**Temperature hardening tank [°C]** 

**Temperature – hardening furnace [°C]**

**Figure 6.** Microstructure of 100Cr6 bearing steel formed by martensite and fine carbides

To harden and temper bearing components, furnace equipment is used that can increase the level and quality of heat treatment and improve work productivity (Fig. 7) [Vasilko, 1998]. The company, where optimisation was implemented, uses a renovated hardening line, later fitted with computer control, and tempering is done on PP017/50 device (Fig. 8) [ZVL, 2008]. Furnace equipment can be operated also by the computer system, making easier the process of controlling and inspecting the heat treatment. Records on various parameters, such as temperature and time, are kept, making it possible to get back to them even after a longer period of time (Table 5) [ZVL, 2008].

**Figure 7.** Diagram of furnace equipment for hardening and tempering bearing components (1 hardening furnace, 2 - hardening tank, 3 - carrier, 4 - washing machine, 5 - tempering furnace).

**Figure 8.** Diagram of the computer-controlled hardening line

48 Heat Treatment – Conventional and Novel Applications

period of time (Table 5) [ZVL, 2008].

distributed carbides.

With this heat treatment, we try to get a fine martensitic structure of components, as shown in Fig. 6 - microstructure of 100Cr6 steel; properly tempered; martensite and fine, evenly

**Figure 6.** Microstructure of 100Cr6 bearing steel formed by martensite and fine carbides

To harden and temper bearing components, furnace equipment is used that can increase the level and quality of heat treatment and improve work productivity (Fig. 7) [Vasilko, 1998]. The company, where optimisation was implemented, uses a renovated hardening line, later fitted with computer control, and tempering is done on PP017/50 device (Fig. 8) [ZVL, 2008]. Furnace equipment can be operated also by the computer system, making easier the process of controlling and inspecting the heat treatment. Records on various parameters, such as temperature and time, are kept, making it possible to get back to them even after a longer

1 3 4 5

2

**Figure 7.** Diagram of furnace equipment for hardening and tempering bearing components (1 hardening furnace, 2 - hardening tank, 3 - carrier, 4 - washing machine, 5 - tempering furnace).


**Table 5.** Sample output from the computer-controlled hardening line with measuring data
