**2.4. Influence of heat treatment on the microstructure and properties of L21HMF cast steel**

Regenerative Heat Treatment of Low Alloy Cast Steel 33

well as inside and on the boundaries of prior austenite grain. Matrix after heat treatment was characterized by high dislocation density, however, some sparse polygonized areas were observed as well - showing lower density of dislocations (Fig. 14). Presence of the polygonized areas in the cast steel after heat treatment can be caused by the difference in chemical composition of particular grains resulting from a dendritic micro segregation or from the lack of austenite homogeneity during heat treatment. Differences of chemical

**Figure 14.** Microstructure of the L21HMF cast steel after heat treatment (bainitic hardening and high-

Normalizing and tempering allowed obtaining tempered bainitic – ferritic structure with

The observed microstructures after bainitic hardening and normalizing, apart from ferrite amount in the structure, differed in bainite morphology as well. After bainitic hardening only the "needle-shaped" form of bainite was observed, and it was morphologically similar to martensite, which indicates lower bainite presence in the structure (it can also be proved by the characteristic arrangement of carbides illustrated in Fig. 14). After normalization, however, the Cr – Mo – V cast steel microstructure showed the "feathery" bainite form, which indicates the presence of upper bainite. Apart from the "feathery" bainite also some

The identifications of precipitates performed by means of the extraction carbon replicas revealed in the investigated cast steel after heat treatment (in the tempered bainitic and bainitic – ferritic structure) the occurrence of the following carbide types: MC, M3C, M7C3

The study of mechanical properties at room temperature has shown that the structure of high-temperature tempered bainite provides the combination of high strength properties and impact energy. Tensile strength and yield strength after tempering exceeded the minimum requirements considerably, and similarly, the impact energy was several times

temperature tempering)

and M23C6.

around 20% of ferrite in the Cr – Mo – V cast steel.

single areas of "needle-shaped" bainite could be seen.

composition may cause local decrease in the temperature of recrystallization.

The L21HMF cast steel was subject to heat treatment consisting in three-hour austenitizing of test pieces at the temperature of 910 oC and the following cooling at the rate corresponding to the processes of: bainitic hardening, normalizing and full annealing. The test pieces, bainite-hardened and normalized, were then tempered in the temperature range of 690 730 oC and 690 720 oC, respectively. While the test pieces cooled slowly from the austenitizing temperature (fully annealed), were subject to ( + ) annealing (under annealing) at the temperatures of 780 860 oC. Examples of microstructure of the examined cast steel after heat treatment are illustrated in Fig. 13.

**Figure 13.** Microstructure of cast steel after: a) service; b) bainitic hardening and tempering; b) normalizing and tempering; c) full annealing and tempering

Bainitic hardening made it possible to obtain bainitic – ferritic microstructure in Cr – Mo – V cast steel. The amount of ferrite in the microstructure did not exceed 6%. In the tempered microstructure there were numerous precipitations of carbides on the lath boundaries, as well as inside and on the boundaries of prior austenite grain. Matrix after heat treatment was characterized by high dislocation density, however, some sparse polygonized areas were observed as well - showing lower density of dislocations (Fig. 14). Presence of the polygonized areas in the cast steel after heat treatment can be caused by the difference in chemical composition of particular grains resulting from a dendritic micro segregation or from the lack of austenite homogeneity during heat treatment. Differences of chemical composition may cause local decrease in the temperature of recrystallization.

32 Heat Treatment – Conventional and Novel Applications

cast steel after heat treatment are illustrated in Fig. 13.

**a b**

**cast steel** 

**2.4. Influence of heat treatment on the microstructure and properties of L21HMF** 

The L21HMF cast steel was subject to heat treatment consisting in three-hour austenitizing of test pieces at the temperature of 910 oC and the following cooling at the rate corresponding to the processes of: bainitic hardening, normalizing and full annealing. The test pieces, bainite-hardened and normalized, were then tempered in the temperature range of 690 730 oC and 690 720 oC, respectively. While the test pieces cooled slowly from the austenitizing temperature (fully annealed), were subject to ( + ) annealing (under annealing) at the temperatures of 780 860 oC. Examples of microstructure of the examined

**Figure 13.** Microstructure of cast steel after: a) service; b) bainitic hardening and tempering;

Bainitic hardening made it possible to obtain bainitic – ferritic microstructure in Cr – Mo – V cast steel. The amount of ferrite in the microstructure did not exceed 6%. In the tempered microstructure there were numerous precipitations of carbides on the lath boundaries, as

b) normalizing and tempering; c) full annealing and tempering

**Figure 14.** Microstructure of the L21HMF cast steel after heat treatment (bainitic hardening and hightemperature tempering)

Normalizing and tempering allowed obtaining tempered bainitic – ferritic structure with around 20% of ferrite in the Cr – Mo – V cast steel.

The observed microstructures after bainitic hardening and normalizing, apart from ferrite amount in the structure, differed in bainite morphology as well. After bainitic hardening only the "needle-shaped" form of bainite was observed, and it was morphologically similar to martensite, which indicates lower bainite presence in the structure (it can also be proved by the characteristic arrangement of carbides illustrated in Fig. 14). After normalization, however, the Cr – Mo – V cast steel microstructure showed the "feathery" bainite form, which indicates the presence of upper bainite. Apart from the "feathery" bainite also some single areas of "needle-shaped" bainite could be seen.

The identifications of precipitates performed by means of the extraction carbon replicas revealed in the investigated cast steel after heat treatment (in the tempered bainitic and bainitic – ferritic structure) the occurrence of the following carbide types: MC, M3C, M7C3 and M23C6.

The study of mechanical properties at room temperature has shown that the structure of high-temperature tempered bainite provides the combination of high strength properties and impact energy. Tensile strength and yield strength after tempering exceeded the minimum requirements considerably, and similarly, the impact energy was several times

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

higher than the required minimum of 27J for the new castings (Table 5, Fig. 15). Tempering of L21HMF cast steel with bainitic structure at the temperatures which are 10 and 20 oC higher than the maximum tempering temperature recommended by the standard, i.e. at 720 and 730 oC, caused an increase in impact energy by 8 and 35%, respectively, with the hardness decrease by 2 ÷ 5% in comparison with the tempering temperature of 710 oC (Fig. 15).

Regenerative Heat Treatment of Low Alloy Cast Steel 35

<sup>J</sup>HV30 Microstructure

of the cast steel with mixed bainitic – ferritic structure results from the presence of ferrite in the microstructure, which favours the fissile cracking, and from the presence of upper

Full annealing allows to obtain ferritic – pearlitic microstructure for the examined cast steel grade (Fig. 13d), with pearlite located mostly on ferrite grain boundaries. In pearlite the processes of fragmentation and spheroidization of carbides could be observed. The ferritic – pearlitic microstructure obtained as a result of repeated cooling from the austenitizing temperature was morphologically similar to the microstructure after long-

> El. %

after service 545 305 26 10 156 ferritic-pearlitic

KV

728 620 18 104 228 bainitic

721 594 17 62 220 bainitic-20%ferritic

558 336 27 26 153 ferritic-20%pearlitic

552 316 31 42 162 ferritic-20%pearlitic

550 324 28 42 164 ferritic-20%pearlitic

140 \*\* 197

\_\_\_

min. 27

bainite, characterized by greater brittleness than lower bainite.

TS MPa

> 500 670

min. 320

**Table 5.** Microstructure and properties of the L21HMF cast steel after heat treatment

min. 20

YS MPa

term service.

Heat treatment parameters

bainitic hardening + 720 oC/4h

> normalizing + 720 oC/4h

full annealing + 720 oC/4h

full annealing + 800 oC/4h

full annealing + 820 oC/4h

\*PN requirements

\*- PN - 89/ H - 83157 ; \*\* - hardness according to Brinell

Therefore, it can be concluded that for the cast steels of bainitic microstructure it is possible to apply higher temperatures of tempering compared to the ones recommended by the standards, without concern that the strength properties can go down below the required minimum. Apart form obtaining high impact energy with the required strength properties maintained, it also allows to achieve the microstructure of higher thermodynamic stability, which can guarantee slower process of its degradation.

**Figure 15.** Influence of the tempering temperature on hardness and impact energy of the L21HMF cast steel with bainitic structure

The cast steel of tempered mixed (bainitic – ferritic) microstructure was characterized by the strength properties on a similar level as the cast steels with bainitic microstructure. However, the crack resistance of those cast steels was almost two times as low compared to that of cast steels with bainitic microstructure (Table 6).

High impact energy of the cast steel with the microstructure of tempered bainite is a consequence of large total amount of grain boundaries (boundaries of bainite packets) and high ductility of the tempered microstructure of lower bainite. Whilst, lower impact energy of the cast steel with mixed bainitic – ferritic structure results from the presence of ferrite in the microstructure, which favours the fissile cracking, and from the presence of upper bainite, characterized by greater brittleness than lower bainite.

Full annealing allows to obtain ferritic – pearlitic microstructure for the examined cast steel grade (Fig. 13d), with pearlite located mostly on ferrite grain boundaries. In pearlite the processes of fragmentation and spheroidization of carbides could be observed. The ferritic – pearlitic microstructure obtained as a result of repeated cooling from the austenitizing temperature was morphologically similar to the microstructure after longterm service.


\*- PN - 89/ H - 83157 ; \*\* - hardness according to Brinell

34 Heat Treatment – Conventional and Novel Applications

which can guarantee slower process of its degradation.

**HV30**

that of cast steels with bainitic microstructure (Table 6).

(Fig. 15).

steel with bainitic structure

**HV30**

higher than the required minimum of 27J for the new castings (Table 5, Fig. 15). Tempering of L21HMF cast steel with bainitic structure at the temperatures which are 10 and 20 oC higher than the maximum tempering temperature recommended by the standard, i.e. at 720 and 730 oC, caused an increase in impact energy by 8 and 35%, respectively, with the hardness decrease by 2 ÷ 5% in comparison with the tempering temperature of 710 oC

Therefore, it can be concluded that for the cast steels of bainitic microstructure it is possible to apply higher temperatures of tempering compared to the ones recommended by the standards, without concern that the strength properties can go down below the required minimum. Apart form obtaining high impact energy with the required strength properties maintained, it also allows to achieve the microstructure of higher thermodynamic stability,

**Figure 15.** Influence of the tempering temperature on hardness and impact energy of the L21HMF cast

**690 700 710 720 730**

**C** 

**KV**

**Tempering temperature, o**

**60**

**70**

**80**

**90**

**100**

**110**

**KV, J**

**120**

**130**

**140**

The cast steel of tempered mixed (bainitic – ferritic) microstructure was characterized by the strength properties on a similar level as the cast steels with bainitic microstructure. However, the crack resistance of those cast steels was almost two times as low compared to

High impact energy of the cast steel with the microstructure of tempered bainite is a consequence of large total amount of grain boundaries (boundaries of bainite packets) and high ductility of the tempered microstructure of lower bainite. Whilst, lower impact energy

Regenerative Heat Treatment of Low Alloy Cast Steel 37

**Figure 17.** Cracking mechanism of cast steel: a) transcrystalline ductile for tempered bainitic

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

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

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

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

microstructure; b) transcrystalline fissile for ferritic – pearlitic microstructure

**a) b)**

**3. Summary** 

level of 27J.

27J.

possible thanks to regenerative heat treatment.

**Figure 16.** Change in the values of hardness and impact energy of the cast steel depending on the temperature of ( + ) annealing

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

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 a transcrystalline fissile mechanism with micro fields of ductile character.

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 character (Fig. 17b).

**Figure 17.** Cracking mechanism of cast steel: a) transcrystalline ductile for tempered bainitic microstructure; b) transcrystalline fissile for ferritic – pearlitic microstructure
