MICROSTRUCTURE AND MECHANICAL PROPERTIES DEGRADATION...

J. Micheľ, M. Buršák, M. Vojtko: Microstructure and mechanical properties degradation of CrMo creep resistant steel operating under creep conditions

Materials Engineering - Materiálové inžinierstvo 18 (2011) 57-62

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MICROSTRUCTURE AND MECHANICAL

PROPERTIES DEGRADATION OF CrMo CREEP

RESISTANT STEEL OPERATING UNDER CREEP

CONDITIONS

Ján Micheľ1, Marián Buršák1,*, Marek Vojtko1

1Department of Materials Science, Faculty of Metallurgy, Technical University of Košice, Park Komenského 11, 043 58 Košice, Slovak Republic

*corresponding author: Tel.: +421 55 602 2776, Fax.: +421 55 602 2243, e-mail: [email protected]

Resume In this contribution microstructure degradation of a steam tube is analysed. The tube is made of CrMo creep resistant steel and was in service under creep conditions at temperature 530°C and calculated stress level in the tube wall 46.5 MPa. During service life in the steel gradual micro structure changes were observed, first pearlite spheroidization, precipitation, coagulation and precipitate coarsening. Despite the fact that there were evident changes in the micro structure the strength and deformation properties of the steel (Re, Rm, A5, Z), the resistance to brittle fracture and the creep strength limit, were near to unchanged after 2.1x105 hours in service. The steam tube is now in service more than 2.6x105 h.

Available online: http://fstroj.uniza.sk/PDF/2011/10-2011.pdf

Article info

Article history:

Received 13 July 2011 Accepted 27 July 2011 Online 28 July 2011

Keywords:

Mechanical properties Creep resistant steel Creep properties Degradation ISSN 1335-0803

1. Introduction

Big power plant production capacities were built using CrMo and CrMoV steel grades. Their safe service life means high investment and production utilization and savings. That is why, such a high attention is paid to the monitoring and surveillance of their service conditions.

During service life in creep conditions there is a gradual micro structure degradation of steel and this way some decrease of properties.

The microstructure of creep resistant CrMo, CrMoV steel grades in the initial state with the common heat treatment of the used products (normalizing annealing followed by tempering) is not really the equilibrium state of the steel. Any thermal or mechanical influences are changing the microstructure in the direction to a higher level of equilibrium.

Coagulation and coarsening of precipitates, carbides transformation, additional precipitation, and the evacuation of alloying elements from the matrix, are supposed to be the most detrimental processes [1,2,3,8,9]. Embrittlement, weakening of the micro structure and the final creep failure can be the result. The degree and intensity of creep degradation depend on both, in service conditions (temperature, stress, environment) and exposition time [2,3,4,6,7]. It is very important to study and know the time dependence of the performance of steel in service conditions and this way survey the possibilities of service life increase.

The aim of this contribution is to consider the extent of microstructure degradation, the influence on mechanical and brittle fracture properties and first of all on creep strength limit for the tested CrMo creep



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resistant steel. Service life up to 2.6x105 h or more was considered in the given conditions.

2. Material and methods

The experimental material was cut out from pieces of the steam tube Ø 335,6x41 mm. The tube was in service at temperature 530°C and calculated stress level in the tube wall 46.5 MPa for exposition times 1.02x105 h, 1.57x105 h, and 2.21x105 h. The steam tube was made of CrMo creep resistant steel (10CrMo9.10). Chemical composition of the tested steel is in Table 1.

Table 1

Chemical composition of the tested steel

[in weight %]

Material C Mn Si Cr Mo V

CrMo

Steel

0.11

0.13

0.46

0.48

0.24

0.27

2.06

2.19

0.96

1.02

0.005

0.02

Ranges of chemistry are given in Table 1. The different cut outs were from different parts of the steam tube showing slight differences in element contents.

From the tested steel tube test samples were machined in longitudinal direction for tensile tests, creep strength limit tests, hardness tests, and polished surfaces for microstructure evaluation.

Creep strength limit was determined by creep tests at 530°C. The stress values for the test were calculated to end tests with defined experimental time to fracture. Results were in the range from 5.102 to 5.104 hours.

After service life 2.5x105 and 2.6x105 h samples were cut out from the critical place in the steam tube. The shape of the cut outs was a spherical cap 0.6 mm high and Ø 8 mm in diameter. The samples were tested for changes in the microstructure and micro hardness HV0,05. Microstructure was analysed in polished surfaces from the original material as

well as on the samples cut out after the listed service life times by light microscope OLYMPUS and electron microscope JEOL JSM 7000 F.

3. Results and discussion

During service life there were in the steel gradual micro structure changes (degradation of the initial microstructure). It was confirmed by the microstructure analyses of the tested steel. In Figure 1 to Figure 3 are documented the microstructures starting from the initial state up to service lives times 102000 h, 157000 h and 260000 h.

Based on metallography a conclusion can be made the initial micro structure is ferrite-pearlite, in condition after normalization annealing and tempering (Figure 1).

Fig. 1. Microstructure of tested steel, initial state,

REM

By this annealing and tempering the pearlite is spheroidized, making the microstructure more stable.

After 102000 h in service, the micro structure has changed. It is still ferrite-pearlite, but by the long time exposition to high temperature the pearlite was spheroidized more (Figure 2).

Pearlite spheroidization continued with the growth of service life time. For 157000 h exposition the pearlite was completely spheroidized.



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Fig. 2. Microstructure of tested steel after 102000 h

service, magnification 400 x

Microhardness in the pearlite grains was 168 HV0,05 and in ferrite 157 HV0,05. The process heading towards the equilibrium state continued and the microstructure had changed to a ferrite-carbide mixture. In the ferrite matrix there were only scattered carbides after 260000 h, as documented in Figure 3.

Fig. 3. Microstructure of tested steel after 260000 h

service time, REM

In grain boundaries coarse carbide particles were segregated, forming network like patterns in some localities.

A more detailed microstructure and phase analysis was completed by the means of electron microscopy (TEM and REM).

In Figure 4 the initial state microstructure is documented in more detail. In the ferrite fine precipitates are shown and the pearlite grains are spheroidized partially.

The precipitates in the ferrite are prevailingly carbide particles based on Mo (Mo2C) and Cr (Cr3C7) in elongated stick like

forms. Also complex carbide particles were found based on (Fe,Cr,Mo)C and they were globular. The carbide particles in cementite were characterized as cementite with alloying elements (Fe,Cr,Mo)C. Similar particles were in the grain boundaries, too. Chemical composition of particles analysed by EDX in locations marked in Figure 4 is given in Table 2.

Fig. 4. Sub microstructure of the tested steel - initial

state, with EDX analysis spots locations marked

[REM]

Table 2

Chemical composition of particles found in the

matrix – initial state [in weight %]

Element Spectrum

13

Spectrum

12 Spectrum 5

C

Cr

Fe

Mo

4.54

2.0

91.83

1.64

4.9

3.19

91.08

0.83

6.27

8.92

79.9

4.91

Sub microstructure of the tested steel after service time 102000 h is documented in Figure 5. The number of precipitates in the ferrite matrix was increased if compared to the initial state. In majority they are Molybdenum Mo(Mo2C) and Chromium carbides Cr(Cr7C3), but carbides of a number of other elements were found, too. They can be characterized as alloyed cementite. In grain boundaries in larger numbers an different sizes were precipitated particles first based on Cr, though there were others, too. The characteristic features of the sub microstructure



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did not change too much after 221000 h in service. See Figure 6.

Fig. 5. Sub microstructure of tested steel after

102000 h in service, TEM

Fig. 6. Sub microstructure of tested steel after

221000 h service, TEM

Selective electron microscope diffraction identified in the matrix particles (precipitates) of the kind M7C3 and M2C. Complex and larger carbide particles were identified first in grain boundaries. The micro and sub micro structure after 260000 h is documented in Figures 7, 8.

The micro structure was ferrite and carbides. Inside in the grains as well as in the grain boundaries carbide particles precipitated. In a number of places they were in the form of network like shapes prevailingly in the grain boundaries. The EDX analysis showed chemical composition of the particles. There were prevailingly particles containing complex alloying elements (Fe,Cr,Mo)C and (Fe,Cr,Mo,Mn)C with variable alloying elements contents. In Figure 8 particles analysed by EDX in the grain boundaries and in Table 3

the measured chemical compositions are showed.

Fig. 7. Sub microstructure of the tested steel after

260000 h service, REM

Fig. 8. Tested steel sub microstructure after service

life 260000 h with EDX analysis spots locations

marked, REM

Table 3

Chemical composition of the particles in the tested

steel after 260000 h in service [in weight %]

Element Spectrum 1 Spectrum 2 Spectrum 3

C

Cr

Fe

Mo

9.07

16.02

71.04

3.87

8.41

9.12

79.78

2.69

12.22

10.65

74.62

2.51

The EDX chemical analysis showed decreased Cr content (about 0.6 %) in the matrix, compared to the initial state chemical composition. Mo and Mn were not found in the matrix at all.

Micro structure analysis with phase identification confirmed the tested steel micro structure degradation after long time service in creep conditions. Pearlite disintegration,



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secondary precipitation, coagulation took place, and grain coarsening, changes of the particles chemical composition and alloying elements depletion was observed. After 2.6x105 h in service was the micro structure of the tested steel a mixture of ferrite with carbides, with coarser particles in the grain boundaries forming sometimes network patterns. We consider it extremely important, that electron microscopy had not given any sign of creep crack initiations.

The tensile test results, and hardness test results HV for the original material and after service life at temperature 530°C and stress level 46.5 MPa are plotted in Figure 9.

The strength values Re and Rm after 2.5x105 h and 2.6x105 h in service are from reference [5]. It can be declared, after service life as long as 2.6x105 h the changes or degradation of mechanical properties were not strong. Here are documented (Re, Rm,) and micro hardness HV (HV0,05) only.

For steam tube design the most important value for the load bearing calculation is the creep strength limit RTm.

Fig. 9. Influence of service life t on yield point Re,

strength Rm and hardness HV. Service conditions

530°C and stress level 46.5 MPa

In Table 4 are the determined creep strength limit values RTm for the tested steel for initial state as well as after the listed service life times. Test results confirmed that the creep strength limit values did not change significantly with the service life time, and they are in good agreement with strength data form tensile tests

(Figure 9). The creep strength limit decrease was in the range from 3 to 6%.

Table 4

Creep strength limit RTm at 530°C initial state and

three in service life times for tested CrMo steel

Service life

[h x 103] 0 102 157 221

RTm104/530 128 120 121 128

RTm105/530 86 81 81 84

Results showed gradual microstructure degradation of the tested steel during the long time exposition in creep conditions. Pearlite decomposition, secondary precipitation, coagulation and coarsening of particles, changes in the chemical composition of carbide particles, the last ones based in majority on (Fe,Cr,Mo)C and depletion of alloying elements from the matrix were observed.

The experiments confirmed changes of microstructure in the creep resistant CrMo steel at 530°C and stress 46.5 MPa in service during 260000 h. However, there were neither significant changes in common mechanical properties nor that of the creep strength limit. It is confirmed, too, by the known dependence of the creep strength limit RTm on the yield point Re. As results showed the yield point changed a little only after 220000 h or 260000 h, also a slight change could be supposed for the RTm105/530. It was confirmed by the experiments.

The yield point Re can be considered to be the macro characteristic of the microstructure. The relation between Re and the micro structure can be described by parametric equations [1,10]. Changes of microstructure during service in creep conditions are changing the contributions of precipitation and dislocation strengthening, what is decisive for the value of Re. The precipitation strengthening contribution is increased by secondary precipitation and it is decreased by the coarsening of precipitates. In the case of no significant variations of Re, it is supposed the opposite influences of the two



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processes are near to similar and they form a kind of equilibrium.

On the other hand in our experiments as is shown in Figure 9 experimental results confirmed some less decisive decrease of the yield point mean value Re (about 5%) and little more of the ultimate tensile stress Rm (about 8%) after 2.2x105 h at 530°C. It is in close correlation to the extent of coagulation and coarsening of dispersed phases. In proportion to that it was possible to suppose the decrease of the value of RTm after 2.2x105 h in service. It was supposed that after 2.6x105 h in service the fatigue strength limit to be RTm105/530 = 80.8 MPa.

Taking into account the experimental results and the described analysis it has been recommended to continue the power plant operation up to 2.8x105 h.

3. Conclusion

The aim of this contribution was experimental service life verification and safety analysis of a CrMo steel grade steam tube in creep conditions. Influence of service conditions on properties was evaluated. The steam tube working at high temperature 530ºC and stress level in the tube wall 46.5 MPa was tested after service life times 1.01x105, 1.57x105, 2.21x105, 2.5.x105 and 2.6x105 h. The following has been shown by the experiments and analyses:

The morphology of the steel micro structure has changed with the time in service. At the first 1.01x105 h of service predominantly the pearlite spheroidization and secondary precipitation took place. The initial micro structure was gradually transformed into a ferrite carbides mixture. After 2.21x105 h in service the coagulation and carbide particles coarsening was more intensive. More coarse carbide particles were found in the grain boundaries.

- The carbide particles with complex alloying elements were prevailing (FeCrMo)C and (FeCrMoMn)C. The elements content in the particles has changed.

- Up to the service life time 2.6x105 h no creep crack initiations were observed by electron microscopy. In consequence, even after this time of operation the creep is in the second predictable linear stage.

- Changes in the following mechanical and creep properties were not significant after 2.21x105 h in service: yield point, ultimate tensile strength, ductility, reduction of area, and creep strength limits RTm104/530, and RTm105/530.

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