**2. Material for research**

The material for study was Cr – Mo – V low-alloy **L21HMF** cast steel (designation according to Polish Standards) with its chemical composition given in Table 1. Test pieces for investigation were taken from an inner cylinder of a steam turbine serviced for around 186 000 hours at the temperature of 540 oC and pressure of 13.5MPa.


**Table 1.** Chemical composition of the L21HMF cast steel, % mass.

## **2.1. Microstructure and properties of the examined cast steel after service**

In the post-operating condition the L21HMF cast steel was characterized by a degraded ferritic-pearlitic microstructure (Fig. 4). The dominant phase in the microstructure after operation was quasi-polygonal ferrite. The size of ferrite grain in the cast steel was diverse and ranged from 88.4 to 31.2m, which corresponds to the grain size grade: 4 7, according to ASTM standard scale.

**Figure 4.** The microstructure of L21HMF cast steel after service

Long-term service of Cr – Mo – V cast steel contributed to the changes in microstructure, including:

 preferential carbide precipitation of M23C6 carbides on ferrite grain boundaries. In some areas the number of carbides precipitated on boundaries was so large that they formed the so-called "continuous grid" of precipitates;

 the process of degradation of pearlite grains consisting in fragmentation, spheroidization and coagulation of pearlitic carbides. Performed identifications have revealed the occurrence of the M3C and M7C3 type of precipitations in those areas (Fig. 5);

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

24 Heat Treatment – Conventional and Novel Applications

**2. Material for research** 

to ASTM standard scale.

including:

lower than the minimum ones expected in the new castings.

186 000 hours at the temperature of 540 oC and pressure of 13.5MPa.

**Table 1.** Chemical composition of the L21HMF cast steel, % mass.

**Figure 4.** The microstructure of L21HMF cast steel after service

the so-called "continuous grid" of precipitates;

**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

The material for study was Cr – Mo – V low-alloy **L21HMF** cast steel (designation according to Polish Standards) with its chemical composition given in Table 1. Test pieces for investigation were taken from an inner cylinder of a steam turbine serviced for around

**C Mn Si P S Cr Mo V**  0.19 0.74 0.30 0.017 0.014 1.05 0.56 0.28

In the post-operating condition the L21HMF cast steel was characterized by a degraded ferritic-pearlitic microstructure (Fig. 4). The dominant phase in the microstructure after operation was quasi-polygonal ferrite. The size of ferrite grain in the cast steel was diverse and ranged from 88.4 to 31.2m, which corresponds to the grain size grade: 4 7, according

Long-term service of Cr – Mo – V cast steel contributed to the changes in microstructure,

 preferential carbide precipitation of M23C6 carbides on ferrite grain boundaries. In some areas the number of carbides precipitated on boundaries was so large that they formed

**2.1. Microstructure and properties of the examined cast steel after service** 

 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.

Regenerative Heat Treatment of Low Alloy Cast Steel 27

**Figure 7.** Transcrystalline ductile fracture with areas of microductility and secondary cracks

– 15 oC and times of holding at the austenitizing temperature: 3 and 5 hours.

**Max. diameter of grain, m** 

**Heat treatment parameters, oC/h** 

**Amount n** 

**Min. diameter of grain, m** 

**2.2. Influence of austenitizing parameters on the size of prior austenite grain** 

Influence of austenitization parameters on the prior austenite grain size has been described in a quantitative way using chosen stereological and statistical parameters, such as: mean diameter and mean area of grain, and also the coefficient of variation of grain size *ν* was calculated. The *ν* coefficient is characterized by the inhomogeneity of grain sizes: the more heterogeneous grains in terms of size within the casting, the higher the values of variation coefficient. The tests were run for the austenitizing temperature range of - 910 970 oC with the "measurement step"

The character of austenite grain distributions was determined using the λ - Kolmogorov test of goodness of fit with normal distribution for logarithmed values (Fig. 8). The assumed significance level was = 0.01, with its limiting statistics value amounting to 1.63. Selected logarithm-normal layouts of mean diameters and mean surface areas of former austenite grains for austenitization option of 925 oC and holding time 3 hours, are shown in Fig. 8. Obtained results of the tests are presented in Table 3 and 4 and graphically shown in Fig. 9 ÷ 11.

> **Diameter of grain, m**

910/3 976 2 29 11.34 6.36 1.54 1.63 0.945 925/3 969 2 30 9.84 5.34 1.36 1.63 0.834 940/3 954 2 31 10.16 6.22 1.56 1.63 0.957 955/3 964 2 38 14.08 9.34 1.36 1.63 0.834 970/3 2024 2 297 22.14 17.73 2.26 1.63 1.387 910/5 946 2 27 9.36 5.70 1.35 1.63 0.828 925/5 915 2 28 11.00 7.01 1.55 1.63 0.951 940/5 959 2 32 9.05 5.41 1.33 1.63 0.816 955/5 937 2 39 12.02 8.37 1.18 1.63 0.724 970/5 2034 2 324 23.67 18.31 1.43 1.63 0.877 **Table 3.** The results of measurements and calculations of the size of prior austenite grains for the cast steel

**Standard** 

**deviation emp =0.01**  .

**emp**

**α**

**λ λ 0 01**

**Figure 6.** Precipitation of "H - carbide" type in the cast steel


Mechanical properties of L21HMF cast steel after long-term service are shown in Table 2.

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

**Table 2.** Mechanical properties and microstructure of the L21HMF cast steels after service

Tensile strength and elongation of the examined cast steel after service were higher than the minimum values required for new castings, while the value of yield strength was lower than the minimum required by 15MPa. Hardness of the investigated cast steel after operation amounted to 156HV30.

A significant feature of the material proving its strain capacity, apart from elongation determined in the static test of tension, is the value of impact energy. Knowledge of this factor gives the possibility of assuming the right temperature for the hydraulic pressure tests used in industrial practice, as well as the right conditions of start-ups and shut-downs of a boiler, adjusted to the material state after long-term service. After operation the examined cast steel was characterized by low impact energy amounting to 10J, and the cracking of samples occurred through the transcrystalline fissile mechanism (typical for brittle fractures) with little energy absorbed due to the limited plastic strain preceding the decohesion. Fissile cracking requires little energy supply which is necessary for crack propagation, hence the low impact energy of the cast steel after service (Fig. 7). Low impact energy of the examined materials is related to the nil ductility temperature (brittle temperature). The fracture appearance transition temperature determined for the examined cast steel amounted to 65 oC.

**Figure 7.** Transcrystalline ductile fracture with areas of microductility and secondary cracks

26 Heat Treatment – Conventional and Novel Applications

**Figure 6.** Precipitation of "H - carbide" type in the cast steel

YS MPa

min.

Material TS

amounted to 156HV30.

oC.

Requirements of PN \*

MPa

500 670

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

Mechanical properties of L21HMF cast steel after long-term service are shown in Table 2.

KV

L21HMF 545 305 26 10 156 65 ferritic-pearlitic

Tensile strength and elongation of the examined cast steel after service were higher than the minimum values required for new castings, while the value of yield strength was lower than the minimum required by 15MPa. Hardness of the investigated cast steel after operation

A significant feature of the material proving its strain capacity, apart from elongation determined in the static test of tension, is the value of impact energy. Knowledge of this factor gives the possibility of assuming the right temperature for the hydraulic pressure tests used in industrial practice, as well as the right conditions of start-ups and shut-downs of a boiler, adjusted to the material state after long-term service. After operation the examined cast steel was characterized by low impact energy amounting to 10J, and the cracking of samples occurred through the transcrystalline fissile mechanism (typical for brittle fractures) with little energy absorbed due to the limited plastic strain preceding the decohesion. Fissile cracking requires little energy supply which is necessary for crack propagation, hence the low impact energy of the cast steel after service (Fig. 7). Low impact energy of the examined materials is related to the nil ductility temperature (brittle temperature). The fracture appearance transition temperature determined for the examined cast steel amounted to 65

J HV30 DBTT

140 197\*\* oC Microstructure

\_\_\_ \_\_\_

El. %

<sup>320</sup>min. 20 min. 27

**Table 2.** Mechanical properties and microstructure of the L21HMF cast steels after service
