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  • 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

  • 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 (20C)

    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,s

    127 129 131 133 135 137

    A,V

    t,s

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

    Figure 6.

  • One of the main hypothesesin 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 sig

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