18th World Conference on Nondestructive Testing, 16-20 April 2012, Durban, South Africa

Evaluation of Material Degradation in Steam Pipelines

Yonka P. IVANOVA1, Todor A. PARTALIN2, Bojana M.TABAKOVA3

1Institute of Mechanics - Bulgarian Academy of Sciences, Lab. NDT, bl.4, Acad.G.Bonchev Str.

1113 Sofia, Bulgaria, e-mail: yonka@imbm.bas.bg 2Sofia University “St.Kliment Ohridski”, Faculty of Mathematics and Informatics,

James Baucher Bul. 5, Sofia, Bulgaria, Phone:+359 2 8161562, e-mail: topart@fmi.uni-sofia.bg. 3Technical university-Sofia, Bulgaria, Phone:+359 2 9653697, e-mail: tabakova@tu-sofia.bg

Abstract Evaluation of the degradation of structure state of pipelines is an important problem in materials science and

industry. The nondestructive assessment of the damages that occurs in components at high stresses and

temperature is the only approach during service time for monitoring the state and estimation the residual life.

In the paper the investigations of material degradation are carried out by classical metallographic analysis as well

as by non-destructive ultrasonic method. Various procedures are developed and applied for material

characterization. The results are compared in order to find the more suitable technique for in-service evaluation

of pipeline status. The methods of data digital processing are applied with the purpose of obtaining useful

informative parameters.

Keywords: material degradation, pipelines, ultrasonic testing

1. Introduction

The equipments in power plants are working under continuously hard conditions. Under high

temperatures and pressures the microstructure in pipelines changes as a result of creep,

corrosion, carbide phase changes and appearance of large number of micro-defects. The

exploitation of the pipelines carries risks of failure and damage because of degradation of

structure and lowering the mechanical properties of the materials. The localization of the

damages may be of use in case of repair works. That is why the state of metal is the object of

incoming and periodical testing and monitoring through service. The aim of such activities is

estimation the degree of damages and prediction the residual life of pipelines in order to avoid

the failure in power stations.

At the present time, according to Technical Conditions TU 14-3-460-2003 the assessment of

the materials is carried out by metallographic analysis and mechanical testing. The application

of non-destructive methods is limited, because of insufficient accuracy and can be expanded if

the test results became more reliable. The possibilities for structure estimation by non-

destructive testing methods are presented in [1-10]. In [10] degradation of microstructure in

pipelines is evaluated by ultrasonic spectral analysis.

This paper presents the results of ultrasonic investigations by immersion pulse echo

method of pipe elements with different degree of degradation after long service in thermo-

electric power station.

2. Test samples

The test samples are cut from the pipes used in boiler, economizer and convective steam super

heater. The pipes were subjected to the different conditions, such as pressure (P), temperature

(T), working time (D) and cycles. The working conditions are given in Table 1. The samples

1 and 5 are reference tubes that are not exploited. (See table 1). The chemical content of steels

are: steel 20 C-0.17-0.24 %, Si-0.17-0.37; Mn-0.35-0.65%, Cr<0.25%, Ni<0.25%, S<0.0125,

P<0.03%, Cu<0.3%; steel 12H1MF C-0.08-0.15 %, Si-0.17-0.37%; Mn-0.4-0.7%, Cr -0.9-1.2%, Mo-

0.25-0.35%, V-0.15-0.3%, Ni<0.25%, S<0.0125, P<0.03%, Cu<0.3%.

Table 1. Operating conditions of tube elements in thermo-electric power station

� Steel T o C P, MPa Working

Duration,

hours

Working cycles

1 20 20 0 0 0

2 20 345 16 175730 522

3 20 345 16 175 730 522

4 20 345 16 25 117 150

5 12H1MF 20 0 0 0

6 12H1MF 514 2.5 175730 522

3. Metallographic and mechanical investigations

The procedures of metallographic and mechanical testing for metal state determination of tube

elements from heated surfaces of economizer, intermediate steam over-heaters and convective

steam over-heaters were presented in details in [1,2].

Micro-structural analysis include the determination of the type and grade of non-metal

includes, the size of the grains, the composition and characteristic features of the structure, as

well as the state of external and internal surface of the tubes concerning the corrosive damage,

cracks and other defects. The estimation of the separate micro structural characteristics was

done according standards and normative documents.

It is found irregular oxidative layer with thickness of 0,4mm on the whole external surface of

all samples. It is determined a decrease of carbons along the internal and the external sides of

the heated parts of the tube samples.

On the heated parts of pipelines it is observed an irregular corrosion of the external surface of

samples from 12H1MF [1] and an intergranular corrosion on the internal surface of all tube

samples.

3.1 Type and grade of non-metal inclusions

The existence of non-metal inclusions in all samples, which in this case are dot-like oxides

(up to grade 3 – grade 2 predominates) and sulfides (up to grade 3) are within the norms (3.5

grades and 3 grades, respectively) for the examined steels, according TU 14-3-460-2003.

3.2 Microstructure and grain size

Under long-time operation at high pressures and temperatures, the microstructure of metals of

steam pipelines in thermal power stations is changed. Degradation of structure is expressed by

coagulation of grains and carbides, grain growth, and arrangement of pearlite in strips and

appearance of microdefects (micropores).

The results of the metallographic study of tested samples show inhomogeneity�and anisotropy

of the structure in the longitudinal and transversal directions. Microstructure consists of ferrite

and pearlite (15-20%) with spheroidization of the pearlite phases. The structure of the steel

12H1MF is evaluated as a grade 6 [1]. These results are unacceptable according normative

documents [2]. Microstructure of tube �2,3 consists of different grains from 7 to 9 scale

grade [1]. Figures 1 show typical photographs in the transversal sections (external, middle,

internal parts) of the tube elements obtained in heated and unheated parts (sample �2, �3). It

can be observed the consolidation and augmentation of the grains.

heated part �100

External side Middle Internal side

unheated part �100

External side Middle Internal side

Figure 1. Photographs of tube elements �2 in transversal sections

Some of samples have an exclusive arrangement of pearlite in strips. Micrographs of

microstructures for different points of sample �3 are showed in Figure 3. That kind of

inhomogeneities (strips of pearlite phases) can be estimated as a scale 3, line B, grade 5

according to documents [1,2].

heated part �100

External side Middle Internal side

unheated part �100

.

External side Middle Internal side

Figure 2. Photographs of tube elements �3 in longitudinal sections

heated part �100

External side Middle Internal side

unheated part �100

External side Middle Internal side

Figure 3. Photographs of tube elements �6 (steel 12H1MF) in transversal sections

Microstructure of pipe � 6 (steel 12H1MF) is shown on Figure 3 transversal and on Figure 4

longitudinal sections. Tube samples have grained structure from 6 to 8 grades with prevalent

grade 8; the consolidation of the grains in the inner and outer surface of the pipes is quite

visible. There are coagulated carbides in the border of the grains. As a result of that, some of

the margins are thickened. Some of them contain chain-bounded carbides. These features are

precondition for decreased plastic quality of the steels.

heated part unheated part

External side

Middle

Internal side

Figure 4 . Photographs of tube elements �6 (steel 12H1MF) in longitudinal sections

The investigation in [1] shows that tube samples � 2, 3, 4, 6 are in the phase of metal fragility

and low plasticity. The results of mechanical testing of samples at room temperatures (20°C)

and high temperatures (345, 345, 514 oC) show that the values of tensile and yield stress are

close to the minimal allowed values [1,2].

4. Ultrasonic Study

The experimental setup for ultrasonic study of pipelines is shown in Figure 5, where 1 is an

immersion tank full with alcohol -water solution, 2 - test object, 3 – immersion transducer, 4-

US box with pulser and receiver, 5- computer. The ultrasonic system is composed of

ultrasonic US box consisted of pulser / receiver and computer.

Figure 5.Experimental setup for ultrasonic study

Ultrasonic waves are excited in the samples by piezoelectric transducer with a central

frequency of 10 MHz. The transducer diameter is 8 mm. To obtain and record ultrasonic

signals a personal computer with LabView software is used. The typical sampling frequency

used for the Echo is 160MHz with a 12 bit resolution. During the ultrasonic investigation the

pipe samples are rotated by automatic scanning system with step of 15 degree for A-scan and

continuously for B-scan. Thus ultrasonic signals are obtained over the all perimeter of the

pipe under the same conditions. The received ultrasonic echoes are complex signals for

material structure. The registered ultrasonic echoes are stored as an A and B-scan images and

processed. The samples are investigated with corrosion layer and also after removing it.

5.Results, processing and analysis

Figure 6 presents a waveform and signal of pipe �1. The first registered signal is the

reflected pulse from interface water-sample, the second one is the first back-wall echo (from

inner side of the pipe) and all next are result of reflections between those surfaces as shown

on Figure 6a. Between those echoes emerge back-scattered (structural) noise. The total

attenuation coefficients of the pipe samples are estimated by the imposing exponential decay

of the multiple back wall echoes (Figure 6b). The first three back wall echoes are selected for

further spectrum analysis. The frequency dependent attenuation coefficient is defined by the

ratio of the spectra of two consecutive echoes [10].

127 129 131 133 135 137

A,V

1st back w all echo

2nd back w all echon th back w all

echo

reflection w ater-sample

t,µs

127 129 131 133 135 137

A,V

t,µs

a. Ultrasonic signal, pipe �1 b. A-scan and exponential decay of echoes

Figure 6.

One of the main hypotheses�in the work is that the presence of structure irregularities in the

material leads to significant scattering. The received backscatter is best observed around first

back wall echo. Figure 7 presents a B-scan of the ultrasonic signals obtained from referent

sample �1. The values of attenuation of ultrasonic waves obtained in different points of the

pipe perimeter are given in Figure 8. Some of waveform echoes are shown in the figure. The

presented results concern cleaned sample. The attenuation coefficient varies around the mean

value.

Figure 7. B-scan, pipe �1 Figure 8. Attenuation coefficients at different

positions over the perimeter

After exploitation structure degrades as we can see in photographs (Figures 1-4). The pipes

working at high temperature give different results (Figures 8, 9) compared to sample �1. The

signals are strongly deformed. �-scan shows bottom echoes, signal deformation and acoustic

noise (backscattered signals). There are areas with very high attenuation of the signal. The

distance changes between echoes indicate a change of the wall thickness or the wave velocity

or both, but it is not possible to determine the contribution of the causes. There is backscatter

that is due to high degree arrangement of pearlte in layers (strips in sections) near external and

internal surfaces of the pipe.

Figure 9.B-scan, pipe �2 Figure 10.Attenuation coefficients

Figure 11. B-scan, pipe �3 Figure 12.Attenuation coefficients

Figure13. B-scan, pipe �4 Figure 14. Attenuation coefficients

The results for the sample �4 that has worked at non stationary regime are shown on Figures

13, 14. The structural noise is high, so are the attenuation and the unevenness.

The results in Figures 13 and 14 show deformation of echoes and a lot of backscatter and

acoustic noise. Similar results are obtained for the pipes of another high-temperature resistant

steel 12H1MF, but the backscatter is smaller.

Figure15. B-scan, pipe �6 Figure16. Attenuation coefficients over the perimeter

6.Ultrasonic backscatteringThe envelope of the echo waveform inherits the scattering processes that happen in the tube

materials. There exists a relation between the shape of the signal and the type of scatterers.

However it is difficult to estimate only one changed structural feature, because the influence

of the structural parameters is more complex. The scattering the ultrasonic wave is caused by

the presence of grains with irregular and coagulated shapes, distributed in different ways

along the sections, the appearance of arranged pearlite strips or micropores.

The cumulated variance A(k) is computed for every echo signals as defined in [11]:

�

�

=

=

−

−

=iN

n

k

n

)A)n(A(

)A)n(A(

)k(A

1

2

1

2

, �=

=iN

nt

)k(AN

A

1

1 (1)

where A is the average value of the echo signal determined over N points, )k(A is the

cumulated variance of signal and is connected to the area under the echo waveform and its

envelope. Figure 17 shows the evolution of the echoes and its respective cumulated variances

related to different measurement points 3, 4, 1, 9, 11 of pipe �3. It can be seen two signals

(a) or backscatter between them and lost of second echo(b-f). Our opinion is that the pictures

are visually rich, but they are not so informative.

3

1

4

1

7 9

11

a b c d f

Figure 17.Echoes (in blue) and respective cumulated variances (in magenta)

0.01

0.1

1

1 10 100f,MHz

Np/mm

αααα

�1

�2

0.01

0.1

1

1 10 100

�5

�6

αααα

Np/mm

f,MHz

Figure 18. Attenuation versus frequency, pipes �1, �2, �5, �6

The attenuation coefficient is defined as the ratio of spectra ( 1)f(S , 2)f(S ) of ultrasonic signals.

The procedures is presented in details in [10]

d.

)f(S

)f(Sln)f(

2

1

2

1

���

����

�=αααα (2)

Figure 18 presents the experimental results for attenuation coefficients of tube elements �2

(steel 20) and �6 (12H1MF). There are obtained different dependencies of frequency

attenuation on perimeter of pipes during one scanning motion and compared to the attenuation

of reference samples �1 and �5. Obviously the inhomogeneous structure, grain coagulations

and pearlite strips influence on the attenuation values.

Conclusion

The investigations of microstructure changes in pipes were carried out. The immersion system

with automatic rotating and B-scan registration is found to be suitable approach. The

degradation of microstructure proved by metallographic study can be registered by

ultrasound. It is difficult to estimate and separate the influence of the various structural

parameters on ultrasonic backscatters and signals. Though it was confirmed that the high

degree of “banded structure” (pearlites in strips) in low carbon steel raises large backscatter

and lack of bottom echoes. The number of backscatter in high resistant steel is smaller.

Probably the higher attenuation of ultrasonic waves in 12H1MF is mostly due to the

coarsened structure and lesser to “banded structure”. Further investigation by ultrasonic shear

and surface waves will clarify and specify the criteria for structure state assessment in situ of

pipelines in power stations. Acknowledgement

The research was performed as a part of Project “Research on fatigue of ferromagnetic materielas by

ultrasound and Barkhausen noise” with the Sofia University “St. Kliment Ohridski”.

References

1. Tabakova B. Technical report “Estimation of the metal state and residual life assessment of

the tube elements in “ENEL operations Bulgaria AD” 2010-2011, Technical University-Sofia.

2. Technical Conditions TU 14-3-460-2003

3. Tabakova B., A. Mihaylov, Determination of Metal and Resourses State in Tube Elements of

Thermo-elektric Power Plants.11th International Metallurgy & Materials Congress, 05-07

June 2002, Istanbul, Turkey.

4. Ivanova Y, T. Partalin, B. Tabakova, Non-Destructive Ultrasonic Investigation of the

Structure State of Steam Pipelines, Russian Journal of Nondestructive Testing, 2011, Vol. 47,

No. 1,. pp.57–64, (2001)

5. Permikin V.S. About diagnostics of heat-resistant steel creep by ultrasonic wave velocity

changes using non-destructive energy equipment control, Defektoskopia, 2001, � 2,

February, 2004. (in Russian)

6. Permikin V.S. ,Perov D. V. ,Rinkevich A. B. , Acoustic Noise in 12KhMF-Grade Steel

Containing Micropores, Defektoskopia , Volume 40, Number 2 / February, 2004. (in Russian)

7. Artamonov V.V., Artamonov, V.P., Nondestructive Testing of Metal Microstructure in Heat

Power Equipment, Russian J. of Nondestructive Testing, 2002, vol. 38, no. 2, p. 105.

8. Papadakis �., Ultrasonic attenuation in polycrystalline media, Phizicheskaja akustika, Meson,

�., �ir, pp.317-381 (in Russian).

9. Truell R. and all, Ultrasonic methods in physique of the solid, �., �ir,1972. (in Russian).

10. Partalin T., Y. Ivanova, Al. Popov, Modeling of acoustic wave attenuation in polycrystalline

structure, Journal of Mat. Science and Technology, Vol.6, � 4, 1998.

11. Ghislain J. Retaureau Detection of surface corrosion by ultrasonic backscattering, Thesis,

Georgia Institute of Technology August, 2006