**3. Summary**

36 Heat Treatment – Conventional and Novel Applications

**42**

**162**

temperature of ( + ) annealing

**Mechanical properties**

character (Fig. 17b).

**Figure 16.** Change in the values of hardness and impact energy of the cast steel depending on the

**Temperature, o**

800 820 840 860

**42**

**164**

value of impact energy and hardness is presented in Fig.16.

a transcrystalline fissile mechanism with micro fields of ductile character.

For the L21HMF cast steel of ferritic – pearlitic microstructure it is required to apply ( + ) annealing (under annealing) instead of tempering which did not always provide the required impact energy. Applying under annealing causes: dissolution of carbides precipitated on grain boundaries during slow cooling from the temperature of austenitization, decrease of phosphorus segregation on ferrite grain boundaries and further reduction of austenite grain size. This allows to obtain the required strength properties and impact energy KV on the level ~ 40J. The influence of ( + ) annealing temperature on the

**41**

**C**

**161**

**45**

**165**

KV, J HV30

The performed heat treatment, apart from the changes in microstructure and properties of the examined cast steels, also caused a change in the mechanism of cracking (Fig. 17). In the cast steel of high-temperature tempered bainite structure, on the entire surface under the fracture, there was a transcrystalline ductile fracture initiated by fine-dispersion precipitates of carbides and sulfide inclusions (Fig. 17a). The characteristic feature of plastic cracking is its ability to absorb significant amounts of energy connected with plastic deformations preceding the decohesion. The cast steel of bainitic – ferritic structure was subject to decohesion through mixed mechanism. Directly under the notch, at a depth of about 1.0 ÷ 1.5 mm, cracking proceeded in plastic manner through transcrystalline ductile mechanism. Below the area of plastic strain, fissile cracking could be observed, running through

The cast steel with regenerated ferritic – pearlitic structure, obtained as a result of slow cooling and under annealing, was cracking through a mechanism similar to decohesion of the cast steel after service, i.e. transcrystalline fissile mechanism with micro fields of ductile The research performed on the L21HMF cast steel, taken from a steam turbine cylinder serviced for around 186 000 hours at the temperature of 540 oC, has revealed that long-term service contributed to: the processes of recovery and polygonization of ferrite grains, preferential precipitation of M23C6 carbides on grain boundaries and formation of "H – carbide" complexes near the boundary areas of ferrite grains. During long-term operation the strength properties were decreasing slowly – yet faster in the case of yield strength than tensile strength, and the impact energy decreased drastically below the required minimum level of 27J.

Changes in the microstructure and properties of the long-term serviced cast steel do not eliminate the possibilities of their further safe operation. Extending the safe operation time beyond the calculative time of 100 000 hours (with the target up to 200 250 000 hours) is possible thanks to regenerative heat treatment.

Performed research has proved that applying bainitic hardening instead of normalizing/full annealing, thus far applied in the castings, allows to achieve the best combination of high strength properties and very high impact energy. Moreover, the bainitic microstructure makes it possible to apply high temperatures of tempering, amounting to 710 730 oC. This allows increasing the stability of microstructure of long-term serviced cast steels without concern for reduction in the strength properties below the required minimum. High impact energy KV > 100J of the cast steel with high-tempered bainite structure guarantees that after long-term operation the impact energy will not drop below the minimum required level of 27J.

Applying normalizing for the castings allows to obtain bainitic – ferritic microstructure, which is characterized by similar strength properties as the cast steel with tempered bainitic microstructure, with the impact energy, however, being almost two times as low. What seems evident here, is the negative influence of ferrite in the microstructure on the impact strength.

Regenerative Heat Treatment of Low Alloy Cast Steel 39

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The ferritic – pearlitic microstructure, obtained as a result of slow cooling of the castings from the austenitizing temperature (full annealing), allows to obtain the strength properties comparable to those after service and impact energy on the level of 40J. After the process of full annealing it is recommended to apply the ( + ) annealing (under annealing) instead of the process of tempering, which makes it possible to obtain the required impact energy.

## **Author details**

Grzegorz Golański *Institute of Materials Engineering, Czestochowa University of Technology, Poland* 

## **4. References**


[12] Komai N., Masuyama F., Igarashi M., *10 – year experience with T23(2.25Cr – 1.6W) and T122 (12Cr – 0.4Mo – 2W) in a power boiler*, Transations of the ASME, 127, 2005, 190

38 Heat Treatment – Conventional and Novel Applications

strength.

**Author details** 

Grzegorz Golański

**4. References** 

seems evident here, is the negative influence of ferrite in the microstructure on the impact

The ferritic – pearlitic microstructure, obtained as a result of slow cooling of the castings from the austenitizing temperature (full annealing), allows to obtain the strength properties comparable to those after service and impact energy on the level of 40J. After the process of full annealing it is recommended to apply the ( + ) annealing (under annealing) instead of the process of tempering, which makes it possible to obtain the required impact energy.

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[2] Stachura S., *Changes of structure and mechanical properties in steels and cast steels utilised in* 

[3] Balyts'kyi O.I., Ripei I.V., Protsakh Kh. A., *Degradation of the cast elements of steam turbines of thermal power plants made of 20KhMFL steel in the course of long – term operation*,

[4] Dobosiewicz J., *Influence of operating conditions on the changes in mechanical properties of* 

[5] Stachura S., Kupczk J., Gucwa M., *Optimization of structure and properties of Cr – Mo and Cr – Mo – V cast steel intended for use at increased temperature*, Foundry Review, 54, 5, 2004, 402 [6] Stachura S., Trzeszczyński J., *The choice of the regenerative thermal treatment of the Cr – Mo* 

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[8] Islam M. A.,. Knott J. F, Bowen P.*, Kinetics of phosphorus segregation and its effect on low temperature fracture behaviour in 2.25Cr - 1Mo pressure vessel steel*, Materials Sc. and

[9] Molinie E., Piques R., Pineau A., *Behaviour of a 1Cr – 1Mo – 0.25V steel after long – term exposure – I. Charpy impact toughness and creep properties,* Fatique Fract. Eng. Mater.

[10] Stachura S., Golański G., *Metallographic and mechanical properties of steel and cast steel after long term service at elevated temperatures*, Report BZ – 202 – 1/01 unpublished

[11] Golański G., Stachura S., Kupczyk J., Kucharska – Gajda B., *Heat treatment of cast steel using normalization and intercritical annealing*, Arch. Foundry Eng., 7, 2007, 123

*Institute of Materials Engineering, Czestochowa University of Technology, Poland* 

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researched

	- [31] Bhadeshia H. K. D. H., *Bainite in steels*, 2nd edition, The University Press Cambridge, Cambridge, UK, 2001

**Chapter 3** 

© 2012 Panda et al., licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2012 Panda et al., licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

**Deformation Reduction of Bearing Rings** 

Heat treatment of bearing rings implies the risk of deformations caused by internal tension. In order to eliminate internal tension, hardening is followed by tempering. In general, tempering will remove the tension. However, this tempering is not sufficient for the socalled thin-walled rings AX series bearing rings to eliminate the tension. This article discusses how to effectively eliminate occurrence of tensions in thin-walled bearing rings made from 100Cr6 by optimising their heat treatment. Results have been verified by

Manufacture of roller bearings (figure 1) is a challenging production process. Even though its specific manufacturing operations are widely known and established, some operations in bearing manufacture must be performed within narrow tolerances ranging from only a few micrometres to comply with requirements of tolerance analysis done before the parts are manufactured to ensure that clients receive a quality product that influences safety of plant operation, therefore safety of people. The manufacturing comprises a number of operations needed to produce rings, rolling elements and cages. It includes hammering of forgings at the beginning, turning, heat treatment, cutting, forming, grinding, washing of parts, their description, assembling bearing components and packaging. A number of preventive, intraoperational and final inspections and dimensional, chemical, metallurgic, endurance and other tests are carried out during the manufacturing process [ZVL & ZKL, 1996; ZVL, 2008]. Customers assemble these products in common applications with standard requirements on bearings, but sometimes they have special requirements either for the bearing as a whole, or for any of its parts. In this case it is not sufficient to implement only standard methods and working practices; they have to be modified or optimised. One of such requirements was a request from one of great important American and Deutschland company to produce bearings needed to place a rotor for an axial piston hydroelectric generating set with an inclined plate for one of its tractors. This application has its particulars. It was necessary to

**by Modification of Heat Treating** 

Anton Panda, Jozef Jurko and Iveta Pandová

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/50217

**1. Introduction** 

experiments.

[32] PN - 89/ H – 83157 Cast steels for elevated temperature applications. Grades.

**Chapter 3** 
