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Prediction of in-service microstructural degradation of A106steel using eddy current technique

Muhammad Mansoor⁎, Noveed EjazMetallurgy Division, Institute of Industrial Control Systems, PO Box. 1398, Islamabad, Pakistan

A R T I C L E D A T A

⁎ Corresponding author.E-mail address: malik01677@yahoo.com (

1044-5803/$ – see front matter © 2009 Elsevidoi:10.1016/j.matchar.2009.09.011

A B S T R A C T

Article history:Received 24 June 2009Received in revised form13 August 2009Accepted 16 September 2009

In-service microstructural degradation of ASTM A106 steel occurs by the spheroidising andcoarsening of pearlite. The specimens of the A106 have been exposed at 710 °C for differentdurations to simulate service conditions. The resultant pearlite coarseningwas evaluated bymeasuring eddy current inductive reactance and resistance; phase angles were measuredusing these values and compared with results of optical microscopy, image analysis andresistivity measurements. It was observed that the eddy current phase angle has a linearrelation with pearlite variations. It can further be used for in-situ eddy current evaluation ofmicrostructural degradation of pipes or steel vessels. A mathematical model has beenproposed to assist such field investigations.

© 2009 Elsevier Inc. All rights reserved.

Keywords:ASTM A106 steelPearlite coarseningEddy current testingMathematical model

1. Introduction

ASTM A106 seamless carbon steel pipes and sheets find wideapplications in petroleum, chemical and petrochemical indus-tries due to enduringperformance in extreme service conditions.Stable mechanical properties at static loads and elevatedtemperatures (<500 °C) with a relative lower manufacturingcosts make them a usual choice for designers [1]. A106 is a plaincarbon steel with pearlite phase in ferritic matrix. Typically, thesteel is extruded to seamless pipes and drawn to sheets forindustrial applications. Prolonged exposure to the serviceconditions may cause degradation of the microstructuralfeatures of the steel (i.e. coarsening and depletion of pearlite —spheroidising), resulting in diminution of mechanical propertiesand consequently, a potential threat to the serviceability.

In the oil and gas field, to assure further serviceability, it is ausual practice to evaluate thepearlitedegradationbymicroscopicexamination of the steel (A106) during annual plant inspection.Theexaminationrequires thesurface tobepolishedandprepared

M. Mansoor).

er Inc. All rights reserved

for etching, subsequently; the etched surface is replicated formicroscopy. This is a comparatively laboriousmethod due to thesurfacepreparation especially at difficult test location in the field.Moreover, surface replication technique has its native limitationsin surface profile, area coverage and optical resolution.

Evaluation of microstructures, mechanical properties,deformation, crack initiation and growth by non-destructiveevaluation (NDE) techniques plays a vital role in industrialapplications because of the growing awareness of the benefitsthat can be derived by using NDE techniques for assessing theintegral performance of the structures. Fracture mechanicsbased analysis of component integrity requires quantitativecharacterization of microstructural features. Any alteration inthe microstructure, which reduces the life or performance,should be predicted sufficiently in advance in order to ensuresafe, reliable and economic operation of the components. Thisprediction is possible with NDE techniques as the interactionof somenon-destructive probing energywith thematerials alsodepends on the sub structural/micro structural features [2].

.

Fig. 1 – Effect of pearlite and ferrite on the magneticcharacteristics of ferromagnetic steel [4].

Fig. 2 – Calibration curves of the eddy current equipment forvarious standard materials.

Fig. 3 – Etched microstructure of the A106 steel revealing variatiotreatment at 710 °C.

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1.1. Eddy current— a physical approach

Eddy current testing may allow evaluation of microstructuralchanges in ferromagnetic material [3] such as carbon steel.Any change in pearlite phase or percentage may affectmagnetic and electric characteristics of the material. Fuller[4] investigated the magnetic behaviour of pure ferritic andpearlitic phases being soft and hard, respectively. The Ferriticphase, beingmagnetically soft, exhibits feeble coercivity whencompared with the pearlitic phase (Fig. 1).

In principle, the technique is based on the analysis ofchanges in the impedance of one ormore coils placed near thework piece to be tested. Typical testing configurations mayconsist of ferrite or air core probes which are placed above aplanar (or at least locally planar) surface of the work piece andwhich are operated in a time-harmonic regime [2,5–7]. Earlier,many of the researchers have investigated the effect ofresidual stresses [7–13], while some researchers have focusedtheir efforts to study the response of NDE with respect tomicrostructural changes [14–19] in various materials.

In the present work, eddy current response has beenstudied with respect to the microstructural degradation ofA106 steel; prolonged exposures were simulated by laboratoryheat treatments. The specimenswere first homogenized to geta reference microstructure and then treated at variouscombinations of time and temperature to get a gradualcoarsening of pearlite phase. Subsequently, the specimenswere subjected to microscopy, resistivity measurement, andeddy current testing. The results were interpreted in terms ofeddy current evaluation of microstructural degradation of thesteel. The interpretation can further be used to evaluate in-service A106 steel structures as well.

2. Experimental

Specimens with 20×20 mm dimensions were sectioned froman A106 seamless pipe. The nominal composition of the pipematerial was 0.2%C–1%Mn and balance Fe. The specimenswere heated to 910 °C for 1 h followed by furnace cooling. As aresult, a fine and homogeneous distribution of pearlite in

n in the pearlite content in the ferrite matrix after heat

Fig. 4 – SEM micrographs of the specimens treated at 710 °Cfor various durations. Coarsening/spheroidising of pearlite isevident with increasing treatment time — magnification5000×.

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ferritic matrix was attained. For coarsening of pearlite, thespecimens were heated to 710 °C for various durations (2 to20 h). Five specimens were used for each experimentalcondition. These specimens were further subjected to opticalmicroscopy, scanning electron microscopy (SEM), resistivityand eddy current testing.

The optical microscope was used in differential interferencecontrast (DIC)mode tostudy thecoarseningofpearlitewith time–temperature exposures. An image analyser was used to quantify

Fig. 5 – (a and b) Micrograph of specimens heat treated at 710 °C fospecimens after image analyses, and (e and f) the corresponding

the pearlite. The imageswere taken at 500×with 10 fields of viewfor each specimen. The total area of the fields of viewwasaround2mm2. The specimenswere also subjected to SEM to see pearlitecoarsening phenomenon at higher magnification.

Resistivity of the specimens was measured using four-probe method. The voltage variation was recorded for eachspecimen between two points kept at constant current. Theresistivity was calculated using Eq. (1).

ρ = R ×A=L ð1Þwhere; A and L are the cross-sectional area and the length ofconducting path, and R is the resistance at given current andmeasured voltage.

Eddy current testing was carried out using an absoluteplanner probe with “Foerster-Defectoscpe” operating at fre-quency and sensitivity of 4 kHz and 20 dB, respectively.

During testing, the lift-off signal was phased to 135°. Theequipmentwas calibratedusing a ferrite standard specimen (i.e.ferromagnetic function) and then calibrated with standardspecimens of copper, aluminium, lead, austenitic stainlesssteel, and titanium for transitional behaviour of the equipmentresponse fromnon-ferromagnetic to ferromagnetic regime. Thecurves obtained during the standardization are shown in Fig. 2.The calibration was further extended to evaluate various heattreated specimens for eddy current response in terms ofinductive reactance (XL) and resistance (R).

3. Results and discussion

Optical microscopy of the heat treated specimens was carriedout; Fig. 3 shows the variations in the pearlite with treatmenttime at 710 °C. An increase in the pearlite coarsening/spher-oidising was observed with the time. It led to a diminution in

r 0 and 2 h, respectively, (c and d) drawing images of the samehistogram elaborating the grain size distribution.

Fig. 6 – The experimental andmathematical plots between ρ and P. Inset is the plot of mathematical residual data and P (%age).

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pearlite phase after 20 h treatment; cementite (Fe3C) wasobserved only at the grain boundaries, see Fig. 3. This mech-anism ismore vividly resolved in the highmagnificationmicro-graphs of the specimens taken using the SEM. The specimenwith maximum pearlite contents (i.e. 24%) was found to havecomparatively finer cementite (Fe3C) plate distribution. SEMstudies also revealed that the Fe3C plates turned into globularlike short staples during coarsening, see Fig. 4.

Fig. 5 shows the results of the image analyses for twospecimens to elaborate the pearlite variations with treatmenttime quantitatively. During image analysis, pearlite contentswere evaluated by optimizing all pragmatic analytical para-meters i.e. magnification, imaging contrast, threshold, andparticle counts. The drawing of optical micrographs afterimage analysis for percentage pearlite contents and histogramof the same elucidate pearlite distribution in the ferritematrix.A careful control of the analytical parameters made areproducible data with respect to themicrostructural features.Image analysis of other specimens exposed at 710 °C forvarious durations was analysed on the same pattern and thedata of the percentage pearlite was recorded.

Fig. 7 – The experimental and mathematical plots between θ and(degrees).

During four-probe resistivitymeasurements, itwas observedthat the pearlite percentage vs. resistivity have a logarithmicrelation. The relation was derived from the experimental data:

lnðPÞ = 3ρ−1:5 ð2Þ

where P is pearlite contents (percentage) and ρ is resistivity(10−7Ωcm).

Residual of the data was calculated by using Eq. (2) andplotted against pearlite contents. Plot of residual of themathematical values and P has a random distribution,depicting the existence of a rational relation between exper-imental and mathematical results [20], see Fig. 6.

Finally, test specimens were studied for eddy currentresponses. The eddy current apparatus was first calibrated,as described earlier, and then corresponding values of R andXL were measured for each specimen. Eq. (3) was used tocalculate phase angle (θ) from individual set of R and XL

values.

Tanθ =XL=R: ð3Þ

P. Inset is the plot of mathematical residual data and θ

Fig. 8 –Opticalmicrographs of the steel specimens showing gradual coarsening of pearlite phase. Peripheral indicators describevariation in treatment time, θ, and ρ with P.

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The plot between P and θwas a straight line with an inverseproportionality. From the experimental results, amathematicalexpression (Eq. (4)) was derived:

P = 30−0:4θ: ð4Þ

Residual of the data was calculated and compared with θ(see Fig. 7). This equation (Eq. (4)) indicates the fact that thesteel behaves more ferromagnetic with a decrease in P.

A complete picture of the variations in the experimentalphysical properties (ρ and θ) with the change in the pearlitepercentage is presented in Fig. 8. Due to good mathematicalrelations of the microstructural changes with ρ and θ, Eqs. (2)and (4) can be used to predict the microstructure withoutgoing through the optical microscopy, thus giving a non-destructive way in life assessment of the structures.

4. Conclusions

In eddy current testing of A106 steel, θ has a good mathemat-ical relationship with P (Eq. (4)). Residual plot of the fittingcurve for Eq. (4) portrays a reliable mathematical model.

1. By using the average values of XL and R after equipmentcalibration, θ can be calculated and compared with the θ vsP plot (Fig. 7), entailing pearlite percentages.

2. Eq. (4) can equally be used for the assessment of themicrostructural variations without going through themicroscopic observation; such evaluation has a goodreproducibility.

3. The experimental and mathematical results can be usedfor in-service prediction of microstructural degradation ofA106 steel.

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