The degree of degradation of ferritic-pearlitic steels is assessed by describing the size, dispersion and distribution of carbides along the grain boundaries, grain growth and the occurrence of microcracks. The article reports the results of the microstructural assessment of primary steam pipeline elbows made of 13HMF steel. Digital image analysis of matrix replicas was performed. It included the classification of microstructures in terms of pearlite/bainite change classes (fragmentation of cementite plates, spheroidization, coagulation) and the evaluation of carbide precipitation processes and damage processes (presence of isolated oriented creep pores and microcracks). The perlite/ bainite areas were found to disappear completely after long-term operation

Barwicki, L.

Kasinska, J.

Malinowski, P.

Matusiewicz, P.

English

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845METALURGIJA 61 (2022) 3-4, 845-847J. KASIŃSKA, P. MATUSIEWICZ, P. MALINOWSKI, L. BARWICKIMATRIX REPLICA-BASED ASSESSMENT OF THE MICROSTRUCTURE OF PRIMARY STEAM PIPING ELBOWS AFTER LONG-TERM OPERATIONReceived – Primljeno: 2021- 11-05Accepted – Prihvaćeno: 2022-03-10Preliminary Note – Prethodno priopćenjeThe degree of degradation of ferritic-pearlitic steels is assessed by describing the size, dispersion and distribution of carbides along the grain boundaries, grain growth and the occurrence of microcracks. The article reports the results of the microstructural assessment of primary steam pipeline elbows made of 13HMF steel. Digital image analysis of matrix replicas was performed. It included the classification of microstructures in terms of pearlite/bainite change classes (fragmentation of cementite plates, spheroidization, coagulation) and the evaluation of carbide precipita-tion processes and damage processes (presence of isolated oriented creep pores and microcracks). The perlite/bainite areas were found to disappear completely after long-term operation.Keywords: 13HMF steel, piping elbows, damage processes, scanning electron microscopy (SEM), phaseISSN 0543-5846METABK 61(3-4) 845-847 (2022)UDC – UDK 669.15.26.28.292:621.181.021.004.64:539.26:536.425 = 111INTRODUCTIONCr-Mo-V steels are extensively used in power gen-eration and petrochemical industries primarily due to their good high-temperature corrosion and creep resist-ance. The production cost to benefit ratio is favourable and the Cr-Mo-V steel application range includes pres-sure components for boilers and pipelines.In spite of their superior strength properties, these steels degrade at elevated temperatures and in variable pressure service. To ensure safe operation, the power generation or petrochemical industries require that plant components are subject to periodic inspection.Currently, there is no standardised/calculation-based assessment of ferritic-perlitic steel degradation. The procedure used involves a descriptive assessment of the size and dispersion of precipitates, carbide release along grain boundaries, grain growth and the occurrence of micro-cracks [1-4]. The microstructure bainitic or fer-ro-bainitic steels may not show such significant chang-es while ageing [5,6], which makes degradation level assessment even more difficult. Here, the processes of coagulation of carbides containing alloying elements deplete the matrix of these elements. A reduction of the solid solution occurs and the evaluation of the material without knowledge of the primary microstructure is sig-nificantly hampered [7]. Several studies reported at-tempts made to apply computational methods of perlite assessment using image analysis and machine learning algorithms [8-11].MATERIALS AND EXPERIMENTAL METHODSAssessment of the microstructure degradation was performed on micrographs of replicas of bends/elbows of the primary steam pipelines made of 13HMF steel (external surfaces, elbow tensile zones and straight sec-tions). The elbows were in service for 245 000 hours at 540 ° C and a pressure of 13,8 MPa. The replicas fabri-cated using the matrix method were observed in a scan-ning electron microscope at magnifications x 1000 and x 2000. The assessment was performed in accordance with the guidelines of the Office of Technical Inspec-tion in force in the Polish supervision system for power units [12]. The class of demage for 13HMF steels ac-cording to [12] are summarized in Table 1. The grain size was measured on 21 photos of 13HMF steels repre-senting the microstructure classes assigned by an expert J. Kasińska (kasinska@tu.kielce.pl, Kielce University of Technology, Faculty of Mechatronics and Mechanical Engineering Kielce, Kielce, PolandP. Matusiewicz, AGH University of Science and Technology, Faculty of Metals Engineering and Industrial Computer Science, Krakow, PolandP. Malinowski, AGH University of Science and Technology, Faculty of Foundry Engineering, KrakowL. Barwicki, Enrem Połaniec Sp. z o.o. Połaniec,  PolandTable 1  Classification of internal damage in materials operating under creep conditions Class of damageType of damage0 a mixture of ferrite with bainite, incl. perlite1/2 coagulation of precipitates in bainite, numerous minor precipitates evenly spaced in ferrite, and few large precipitates at the boundaries of ferrite grains 3 almost complete disappearance of bainitic areas and chains of signicant size precipitates at the boundaries of ferrite grains 4 coagulated carbides in ferrite and chains of signicant size precipitates at the boundaries of ferrite grains 846  METALURGIJA 61 (2022) 3-4, 845-847. ASI S A et al.: MATRI  REPLI A BASED ASSESSMENT O  T E MI ROSTRU TUREMicrostructures corresponding to the classes in [12] are shown in Figure 1.RESULTS AND DISCUSSIONMicrostructureMatrix replicas were observed under a scanning electron microscope. According to the guidelines in [12], the main class of the structure involves the assess-ment of:• perlite/bainite change classes: 0; 1/2; 3; 4 (frag-mentation of cementite plates, spheroidization, coagulation;• carbide precipitation processes,• damage processes (presence of isolated creep pores, oriented pores, microcracks)Qualitative analysis of microstructure images on the matric replicas showed a significant degree of degrada-tion of the primary material with the ferritic-perlitic ini-tial microstructure. Perlite disappeared after 245 000 operating hours (Figure 2a, b). Numerous voids were observed at the grain boundaries and inside the grains, proving the occurrence of precipitation processes (Fig-ure 2a). The released carbides also formed “chains” of the precipitates along the grain boundaries (Figure 2b).Grain size measurementThe statistical analysis of the chord length measure-ment results for individual microstructure classes as-signed by the expert is given in Table 2. The limiting Figure 1  Microstructre patterns with classes assigned according to [12].Figure 2  Example of a 13HMF steel microstructure after long term in service, replicas, SEMTable 2.  Statistical parameters for chord lengths of the microstructure grainsMicrostructure class The number of  measured / chordsMean chord / µmMedian chord / µmStandard deviation / µmLimiting error / µmClass 1 373 12,70 10,29 9,10 0,92Class 2 236 14,71 11,95 9,32 1,19Class 3 234 14,86 12,88 9,51 1,22Class 4 417 14,63 12,89 9,31 0,89from class 1 to class 4. The length of the grain chords was measured using a semi-automatic linear method in SigmaScan Pro computer image analysis software. Mi-crostructure images used in the measurements were magnified 1000x and calibrated to obtain results in mi-crometers. On each image, six secant lines were ran-domly projected with a length equal to the long side of the image. Then, the intersections of the grain bounda-ries with the secants that cut off subsequent chords on the secant were indicated. The lengths of the chords were obtained automatically.847. ASI S A et al.: MATRI  REPLI A BASED ASSESSMENT O  T E MI ROSTRU TUREMETALURGIJA 61 (2022) 3-4, 845-847error was calculated as a half of the confidence interval for the mean. The relationship between the microstruc-ture class assigned by the expert and the grain size rep-resented by the mean chord length and the median chord length is shown in Figure 3. The mean chord length in-creases with the degree of microstructure degradation level represented by the assigned class. The largest grain size differences are between class 1 and class 2 and then decrease in subsequent classes.As in the case of the mean chord length, the median chord length increases but this upward trend is more pronounced. This may be due to the insensitivity of the median to extreme values.CONCLUSIONAssessment of steel degradation level based on ma-trix replicas is a challenging task that depends on the replica fabrication quality. Generally, it consists of a qualitative description of the microstructure and as such, it is subjective and relies on the inspector’s expe-rience. For this reason, it is advisable to show a correla-tion between the degree of degradation and stereologi-cal parameters. It was demonstrated that the median chord can be used to describe the microstructure grain size in terms of its degradation degree, especially in connection with other quantitative parameters such as geometric and shape coefficients describing the struc-ture (area, perimeter, shape factors, Feret diameters, etc.). Describing all classes of structures on replicas with stereological parameters, e.g. using an artificial neural network for a large database could provide a tool for more precise qualification of the serviceability of tested elements.REFERENCES[1] R. Molenda, R. Kuziak, Physical metallurgy foundations of DP steel plates. Structure and properties formation in the process of continuous annealing IMŻ 2 (2011), 29 – 41.[2] D. Hauserová, J. Dlouhý, Z. Nový, Accelerated spheroidi-sation of carbides in medium carbon steels, Inżynieria Powierzchni (2011) 2 , 28-32[3] A. Czarski, T. Skowronek, W. Osuch, Influence of orienta-tion relationship between ferrite and cementite in pearlite on stability of cementite plates. Archives of Foundry Engi-neering 33 (2007) 1, 41 – 49[4] Toft LH, Marsden RA. Structural processes in creep: spe-cial report no. 70. London: Iron & Steel Institute; 1961, 238-244.[5] Bhadeshia HKDH, Honeycombe RWK. Steels: Micro-structure and Properties. 3rd ed. Oxford: Butterworths-Heinemann; 2006.[6] Bhadeshia HKDH. Bainite in Steels. Cambridge: Cam-bridge University Press; 2001.[7] Bhadeshia HKDH. Bainite in Steels. Cambridge: Cam-bridge University Press; 2001.[8] N. Nutal, C.J Gommes, S. Blacher, P. Pouteau, J.P. Pirard, F. Boschini, K. Traina, R. Cloots, Image Analysis Of Pear-lite Spheroidization Based On The Morphological Cha-racterization Of Cementite Particles, Image Anal Stereol. 29 (2010); 91 – 98.[9] Lis, J. Wieczorek, P. Łyszczarz, P, Quantitative estimation of microstructure of ribbed bars from low carbon steel af-ter QTB process, Hutnik, Wiadomości Hutnicze, 77 (2010) 9, 494 – 497.[10] J. Kasińska, P. Matusiewicz, A. Czarski, L. Barwicki, As-sessment of pearlite degradation in power industry cast steel after long-term exploitation, Inżynieria Materiałowa 38 (2017) 5, 212 – 216.[11] P. Malinowski – Casting production management system. Metalurgija 60 (2021) 3-4, 451-453,[12] Guidelines of the Office of Technical Inspection, Rules for the diagnostics and service life assessment of boiler and pipeline components operating under creep conditions - Annex B, UDT, 2015, 1- 72.Figure 3  Microstructure grain size for 13HMF steel, represented by the mean chord and the median chord in the classes assigned by the expertNote: English translation by Nina Kacperczyk

Matrix replica-based assessment of the microstructure of primary steam piping elbows after long-term operation

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