**1. Introduction**

Cylinder and valve chamber castings of large power steam turbines are usually made of low alloy Cr - Mo - V and Cr - Mo cast steels. Forming of the microstructure and mechanical properties of cast steels takes place through heat treatment, thus far mostly consisting of normalizing and tempering. As a result of such a treatment the cast steels of diverse wall thickness reveal microstructures from ferritic – pearlitic to bainitic – ferritic with various ferrite, pearlite and bainite amount.

Operation of the cast steels under creep conditions contributes to the occurrence of deformations, fractures and changes in the microstructure, decreasing their functional properties. The resistance to crack expressed by impact energy falls drastically. The value of impact energy of test pieces taken from cast steels after long-term service was considerably below the required level of 27J, frequently reaching the value of 6 - 10J (Fig. 1).

**Figure 1.** Impact energy of turbine cylinder cast steel in the post-operating condition

© 2012 Golański, 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 Golański, 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.

#### 22 Heat Treatment – Conventional and Novel Applications

Along with the fall of impact energy there is also a growth of nil ductility transition (NDT) temperature, frequently rising above 50 60 oC. Large decrease in crack resistance is usually accompanied by a slight decrease in the strength properties (Fig. 2).

Regenerative Heat Treatment of Low Alloy Cast Steel 23

**Figure 3.** Influence of phosphorus amount and microstructure on impact energy KV of the Cr – Mo – V

The revitalization process consists in heat treatment of turbine cylinders in order to regenerate the structure to the extent which allows the improvement of plastic properties (increase in impact energy, decrease in the nil ductility transition temperature). Regenerative heat treatment of castings applied in industry so far consists in normalizing/full annealing from the austenitizing temperature with the following high-temperature tempering/under annealing. The ferritic – pearlitic or ferritic – bainitic structure, obtained as a result of the above-mentioned heat treatment, provides the required impact energy of KV > 27J, however,

Modern hardening plants applying aqueous solutions of polymers as a cooling agent make the cooling of massive castings possible at programmed rate which provides an optimum

Regenerative heat treatment at costs not exceeding 40% of a new casting's price, allows obtaining functional properties (yield strength, impact energy, NDT temperature) comparable to the properties of new castings. Regenerated cylinder is fit for further

In order to achieve an improvement in mechanical properties of cast steels after long-term

grain refinement – leads to an increase in crack resistance, decreases the NDT

eliminating irreversible brittleness caused by phosphorus segregation to grain

 dissolving of carbides in austenite, especially the carbides precipitated on grain boundaries, in order to obtain the required strength properties in the regenerated

operation, the following changes in the degraded microstructure are necessary:

cast steel after long-term operation at the temperature of 535 oC

structure throughout the whole casting section.

operation at least for another 100 000 hours.

temperature and raises yield strength;

boundaries and interphase boundary: matrix/carbides; removal of the needle shaped ferrite (Widmannstätten's ferrite); removal of pearlite precipitated on ferrite grain boundaries;

microstructure (hardness and tensile strength).

with the strength properties being comparable to those after operation.

Unfavourable changes in mechanical properties of the castings are related to the changes in microstructure which occur during long term service at elevated temperatures, first and foremost to:

**Figure 2.** Changes in tensile strength (TS) and yield strength (YS) depending on the time of service

Lowering of impact energy as a result of long-term service depends largely on the as received microstructure of a cast steel. The impact energy decrease is the smallest in the case of tempered bainite microstructure or bainitic – ferritic microstructure, with ferrite amount not higher than 5% (Fig. 3). High impact energy of quenched and tempered cast steel, considerably higher than 100J, guarantees that during long-term service of steel castings with low phosphorus volume fraction ( 0.015% P), the impact energy will not fall below the minimum required value of 27J.

Similar tendency has been noticed in new low-alloy bainitic 7CrWVMoNb9 – 6 (P23) steel. Impact energy in the case of this cast steel, whose microstructure is of tempered bainite in the as-received condition, after around 10 years of operation at the temperature of 555 oC and pressure 4.2MPa, was on the level of 70 – 80 J/cm2.

Degradation of the microstructure of castings and the related gradual decreasing of mechanical properties, however, do not limit the possibility of their further operation, especially as in most of the examined castings there were no irreversible creep changes observed. One of the conditions for extending the time of safe operation for cast steels above the calculated service time is running the process of revitalization of the castings.

22 Heat Treatment – Conventional and Novel Applications

morphology and dispersion of precipitates;

minimum required value of 27J.

and pressure 4.2MPa, was on the level of 70 – 80 J/cm2.

foremost to:

Along with the fall of impact energy there is also a growth of nil ductility transition (NDT) temperature, frequently rising above 50 60 oC. Large decrease in crack resistance is usually

Unfavourable changes in mechanical properties of the castings are related to the changes in microstructure which occur during long term service at elevated temperatures, first and

the preferential precipitation of carbides on grain boundaries, as well as changes in

segregation of phosphorus and other trace elements to grain boundaries and near

**Figure 2.** Changes in tensile strength (TS) and yield strength (YS) depending on the time of service

Lowering of impact energy as a result of long-term service depends largely on the as received microstructure of a cast steel. The impact energy decrease is the smallest in the case of tempered bainite microstructure or bainitic – ferritic microstructure, with ferrite amount not higher than 5% (Fig. 3). High impact energy of quenched and tempered cast steel, considerably higher than 100J, guarantees that during long-term service of steel castings with low phosphorus volume fraction ( 0.015% P), the impact energy will not fall below the

Similar tendency has been noticed in new low-alloy bainitic 7CrWVMoNb9 – 6 (P23) steel. Impact energy in the case of this cast steel, whose microstructure is of tempered bainite in the as-received condition, after around 10 years of operation at the temperature of 555 oC

Degradation of the microstructure of castings and the related gradual decreasing of mechanical properties, however, do not limit the possibility of their further operation, especially as in most of the examined castings there were no irreversible creep changes observed. One of the conditions for extending the time of safe operation for cast steels above the calculated service time is running the process of revitalization of the castings.

accompanied by a slight decrease in the strength properties (Fig. 2).

boundary areas; disintegration of pearlite or/and bainite areas.

**Figure 3.** Influence of phosphorus amount and microstructure on impact energy KV of the Cr – Mo – V cast steel after long-term operation at the temperature of 535 oC

The revitalization process consists in heat treatment of turbine cylinders in order to regenerate the structure to the extent which allows the improvement of plastic properties (increase in impact energy, decrease in the nil ductility transition temperature). Regenerative heat treatment of castings applied in industry so far consists in normalizing/full annealing from the austenitizing temperature with the following high-temperature tempering/under annealing. The ferritic – pearlitic or ferritic – bainitic structure, obtained as a result of the above-mentioned heat treatment, provides the required impact energy of KV > 27J, however, with the strength properties being comparable to those after operation.

Modern hardening plants applying aqueous solutions of polymers as a cooling agent make the cooling of massive castings possible at programmed rate which provides an optimum structure throughout the whole casting section.

Regenerative heat treatment at costs not exceeding 40% of a new casting's price, allows obtaining functional properties (yield strength, impact energy, NDT temperature) comparable to the properties of new castings. Regenerated cylinder is fit for further operation at least for another 100 000 hours.

In order to achieve an improvement in mechanical properties of cast steels after long-term operation, the following changes in the degraded microstructure are necessary:

**The research aim:** The aim of the performed research was to determine the influence of regenerative heat treatment on the microstructure and properties of Cr – Mo – V cast steel with its microstructure degraded by long-term service and mechanical properties being lower than the minimum ones expected in the new castings.

Regenerative Heat Treatment of Low Alloy Cast Steel 25

 the process of degradation of pearlite grains consisting in fragmentation, spheroidization and coagulation of pearlitic carbides. Performed identifications have revealed the

 precipitation of compound carbide complexes called "H – carbides". The compound complexes of precipitates are created by MC and M2C carbides, where the MC carbide is a "horizontal" precipitation, while M2C carbides are precipitations of "vertical" type (Fig. 6). This sort of compound precipitations is defined as "H – carbide". During longterm operation the MC carbide is enriched in molybdenum as a result of diffusion. The growth of molybdenum concentration in the interphase areas of MC/matrix makes it possible for the "needle-shaped" precipitations of M2C (rich in molybdenum) to nucleate on the interphase boundary: MC carbide/ferrite. These processes run more intensely in the border areas of grains, which results in the occurrence of precipitation free zones. The appearance of such zones may be the cause of slow reduction of the strength properties, the yield strength in particular, during long-term operation. The occurrence of this type of complexes results in a decay of fine-dispersion MC carbides which may lead to the fall of creep resistance in the serviced materials. A similar phenomenon can be seen at present in the new high-chromium cast steels for power industry, where the Z phase is being formed and developed at the expense of fine dispersion precipitates of

the MX type, which causes a drastic drop of creep resistance of these cast steels.

occurrence of the M3C and M7C3 type of precipitations in those areas (Fig. 5);

**Figure 5.** Morphology and type of carbides in pearlite grain

**M3C M7C3**