Degradation due to ageing in extraction steam piping of a nuclear power plant

Waseem et al / International Journal of Advanced and Applied Sciences, 3(7) 2016, Pages: 75‐80 76 2009), but this paper extends this very important to microstructural degradation. 2. Materials and methods The nuclear power plant under study has been in operation for 40 years. The pipe samples of extraction steam passage way were taken, which had been carrying steam from high pressure turbine to deaerator for twelve years. The location, from where pipe samples were obtained, is shown by a red arrow in Fig. 1. Fig. 1: Schematic diagram of extraction steam piping system The ageing effects on extraction steam piping’s material were investigated. The test sample (i.e. aged pipe of carbon steel SA 106B) having 21.34 mm outside diameter (OD) and 2.76 mm thickness was cut and subjected to thorough examination. A virgin pipe of same size, type and material was also subjected to the same analytical protocol for comparison of results. Metallographic examination was carried out using metallurgical microscope. Samples were prepared for metallography according to standard procedure as given in ASTM E3. Image software was used to determine the content of different phases. Radiographic examinations were carried out by Smart 225 KV X-ray machine according to ASME SEC-V guideline. Samples were exposed for 60 seconds. The image quality indicator (IQI) having 0.25 mm diameter was used as per ASTM 1A-6. Ultrasonic tests were performed to measure the reduction in thickness due to erosion according to ASTM E-18. DM4 DL ultrasonic thickness gauge was utilized. Hardness measurement was done in Rockwell Hardness Scale B according to ASTM E-18. Hardened steel sphere of 1/16” diameter was forced in the specimen surface by applying 100 Kg load using ZHR Rockwell Hardness Tester; 10 Kg minor load was also applied initially to hold the sample firmly by an initial penetration and to enhance the accuracy of results. 3. Results and discussion Aged, (c) Cut-off View of External Surface of Aged Pipe Showing Corrosion Pits at 100X magnification. Visual examination shows no disturbance in uniformity of virgin pipe, See Fig. 2(a). Whereas, aged pipe surface as shown in Fig. 2(b) shows corrosion, as it has been in sea shore environment which is highly corrosive. Corrosion has the potential to reduce a design life of the component by premature degradation (Ginzel, 2002). Examination of corroded surface under metallurgical microscope, as shown in Fig. 2(c), reveals formation of corrosion pits, maximum depth of pit is 0.07 mm, which causes non-uniform reduction in thickness and reduces the ultimate strength of the material (Sidharth, 2009). Fig. 2: SA 106B seamless pipe, (a) virgin, (b) Radiographic examination of virgin pipe exhibits no sign of erosion, as shown in Fig. 3(a). Whereas material removal is evident on the internal surface of aged pipe, see Fig. 3(b) which shows mechanical rubbing of exposed pip is supplied Ultrason determine t material rem thickness is to 12 years hoop stress the pressur Microstr bands of pea are typical Sikka, 2006) decarburize layer form temperature Rethwisch, poor resista 2006). Abra Wasee steam on i e (Callister a from middl ic testing he intensity oval. On av observed fro of service. which acts in e inside th ucture of new rlite and fer of the extru . Inner surfa d layer up to s during c after hot 2007) this a nce to imp sive action o m et al / Interna nternal sur nd Rethwisc e stage ext Fig. 3: has been of wall th erage; 10.46 m 2.77 mm Consequent radial direc e pipe and Fig. 4: Incr carbon ste rite, see Fig. ded materia ce layer of vi 0.1 mm. Th ooling of s extrusion nisotropic s act load (C f steam rem tional Journal of face of serv h, 2007). Ste raction of h Radiographs performed inning due % reduction to 2.48 mm d ly, the aver tion because depends up ease in hoop s el pipe exhib 5. These ban l (Misiolek a rgin pipe sho is decarburiz teel to ro (Callister a tructure sho aballero et oves the in Advanced and A 77 ice am igh pre 25 of (a) virgin pi to to in ue age of on thi psi red to Fig red tress due to re its ds nd ws ed om nd ws al., ner de Pro pe ba pip mo an Im car 75 pplied Sciences, ssure turbin 0 0F tempera pe and (b) age ckness of pi to 56.87 ps uces, the po increase in h . 4 shows uction in wa duction in wa carburized long exposu arlite unifor nds of pearl e, see Fig. re resistant d ferrite, det ageJ, was 4 bon steel, w % ferrite du 3(7) 2016, Pages e to deaerat ture conditio d pipe pe, increases i. As the pipe ssibility of p oop stress ( the increase ll thickness o ll thickness layer in ag re to high mly through ite and ferri 6. Mixture o to impact l ermined by i 0% and 60 hich chang e to twelve : 75‐80 or at 15 psi p ns. up to 13 % degrades a ipe failure in Braverman in hoop st n the pipe. ed carbon temperature out the ma te are obser f ferrite an oad. Amoun mage analyz % respectiv es to 25% years of ser ressure and from 50.29 nd thickness creases due et al., 2005). ress due to steel pipe. distributes trix and no ved in aged d pearlite is t of pearlite ing software ely in new pearlite and vice in high Waseem et al / International Journal of Advanced and Applied Sciences, 3(7) 2016, Pages: 75‐80 78 temperature and pressure conditions. Reduction in pearlite occurs due to formation of graphite nodules, see Fig. 6 and Table 1. Decreasing amount of pearlite has deleterious effects as it decreases strength and hardness of steel (Gonzaga et al., 2009). Fig. 5: Nital (1-10% Nitric Acid in Ethanol) etched microstructure of new seamless SA 106B pipe at 100X Fig. 6: Nital etched microstructure of aged seamless SA 106B pipe at (a) 100X and (b) 400X. Table 1: Reduction in hardness due to ageing Rockwell Hardness Scale B (HRB) Pipe Surface Virgin Pipe Aged Pipe Reduction (%) Internal 68.5 54.5 20 External 64.5 64 0.7 Hardness of the pipe samples was measured on internal and external surfaces. Reduction in hardness of internal pipe surface is observed 20%. However, external surface experiences only 0.7% reduction in hardness. Reduction in hardness is 29 times more on internal layer of pipe material because this layer has been directly exposed to hot and pressurized steam. 4. Conclusion Age related degradations have been observed in extraction steam piping of a nuclear power plant. Corrosion pits, which cause non-uniform reduction in thickness and reduce ultimate strength, have been observed with maximum depth of 0.07mm. 8.3% reduction in wall thickness has been observed due to mechanical rubbing and impinging of hot and Waseem et al / International Journal of Advanced and Applied Sciences, 3(7) 2016, Pages: 75‐80 79 pressurized steam on internal surface. The hoop stress, acting in radial direction has been increased due to reduced wall thickness. Initially present bands of pearlite and ferrite have been eliminated due to prolong exposure to high temperature which changes the mechanical properties. Microstructure of pipe material has been transformed into uniform mixture of pearlite and ferrite. Graphite nodules have also been formed by detachment of carbon from iron in pearlite, which has reduced pearlite content from 40% to 25%. The reduction in pearlite is comparatively higher on internal surface layer of pipe as this layer faced hot and pressurized steam directly. The strength and hardness of the carbon steel SA 106B has been reduced due to this microstructural transformation. The need to frequent examination has been revealed by this study as the combine effect of microstructural degradation and pitting on internal surface of the piping may lead to fracture. Acknowledgement The authors are thankful to Karachi Nuclear Power Plant (KANUPP) for providing samples and Peoples Steels Mills, Karachi, NED University of Engineering and Technology, Karachi and National Center for Non-Destructive Testing, Islamabad, for providing experimental facilities. References Bai G, Lu S, Li D and Li Y (2016). Influences of niobium and solution treatment temperature on pitting corrosion behaviour of stabilised austenitic stainless steels. Corrosion Science, 108: 111-124. Braverman JI, DeGrassi G, Hofmayer C, MartinezGuridi G and Morante R (2005). Risk-Informed assessment of degraded buried piping systems in nuclear power plants. NUREG/CR-6876, BNLNUREG-74000-2005. Brookhaven National Laboratory, Washington DC, U.S. Nuclear Regulatory Commission, USA. Caballero FG, García-Junceda A, Capdevila C and García de Andrés C (2006). Evolution of microstructural banding during the manufacturing process of dual phase steels. Materials transactions, 47(9): 2269-2276. Callister WD and Rethwisch DG (2007). Materials science and engineering: an introduction. John Wiley and Sons, Inc., 7th Edition, New York, USA: 7: 665-715 Deng B, Wang Z, Jiang Y, Wang H, Gao J and Li J (2009). Evaluation of localized corrosion in duplex stainless steel aged at 850 C with critical pitting temperature measurement. Electrochimica Acta, 54(10): 2790-2794. Furtado HC and May IL (2004). High temperature degradation in power plants and refineries. Materials Research, 7(1): 103-110. Ginzel RK and Kanters WA (2002). Pipeline corrosion and cracking and the associated calibration considerations for same side sizing applications. NDT. net, 7(07): 1435-4934. Gonzaga RA, Landa PM, Perez A and Villanueva P (2009). Mechanical properties dependency of the pearlite content of ductile irons. Journal of Achievements in Materials and Manufacturing Engineering, 33(2): 150-158. Hänninen H (2009). Material development in new reactor designs–Gen III and SCWR concept. 20th international conference on structural mechanics in reactor technology (SMiRT), Dipoli Congress Centre, Espoo, Finland. IAEA Nuclear Energy Series (2009). Integrity of reactor pressure vessels in nuclear power plants: Assessment of irradiation embrittlement effects in reactor pressure vessels steels. No. NP-T-3.11, IAEA Nuclear Energy Series, Vienna, Austria, http://wwwpub.iaea.org/MTCD/publications/PDF/Pub1382_ web.pdf. 


Introduction
* Industrialization have prompted the world to get more economic and reliable source of power i.e. Nuclear Power Plants. Currently installed nuclear power plants have been in operation for decades. The specific design life of older nuclear power plants was based only on fatigue life calculation, whereas age related material degradation was ignored (IAEA Nuclear Energy Series, 2009). The materials in nuclear power plants endure hard conditions (Umer et al., 2016;Waseem and Ryu, 2016) and degrade as a result of creep, corrosion, phase changes and emerging of micro-defects. The degraded material can initiate any failure by lowering mechanical properties (Ivanova et al., 2012). Carbon steel SA 106B is extensively utilized in high temperature piping of nuclear power plants. Such as seamless pipes of SA 106B are used in main steam and feed water systems. Steam outlet nozzle and condenser structure is also fabricated from the same steel grade  (Hänninen, 2009). It is a plain carbon steel; its structure is composed of pearlite phase in ferrite matrix. Extended exposure to high temperature and pressure conditions causes degradation in this steel, consequently threatens the safe operation by reducing strength (Mansoor and Ejaz, 2009). Replacement of such material is not considered feasible, therefore assessment of age related degradation becomes essential (Furtado and May, 2004). Material ageing has been a critical issue therefore much research work has been done regarding material ageing assessment of critical components of the nuclear power plants such as reactor pressure vessel, pressure tubes, steam generators, pressurizer, main steam piping and pipe line corrosion etc. Ageing assessment of piping has been a topic of particular interest for researchers, various studies have been published such as Ossai et al. (2016) observed increasing degradation due to prolong service of pipelines (Ossai et al., 2016), Bai et al. (2016) observed negligible effect of solution treatment on degradation of austenite steel used in piping of power plants (Bai et al., 2016) and Deng et al. (2009) found the effect of microstructure on ageing (Deng et al., 2009). To the best of our knowledge, the ageing analysis of piping is still limited to loss of material, general corrosion, crevice and pitting corrosion and wall thinning (U.S.NRC, 2009), but this paper extends this very important to microstructural degradation.

Materials and methods
The nuclear power plant under study has been in operation for 40 years. The pipe samples of extraction steam passage way were taken, which had been carrying steam from high pressure turbine to deaerator for twelve years. The location, from where pipe samples were obtained, is shown by a red arrow in Fig. 1. The ageing effects on extraction steam piping's material were investigated. The test sample (i.e. aged pipe of carbon steel SA 106B) having 21.34 mm outside diameter (OD) and 2.76 mm thickness was cut and subjected to thorough examination. A virgin pipe of same size, type and material was also subjected to the same analytical protocol for comparison of results. Metallographic examination was carried out using metallurgical microscope. Samples were prepared for metallography according to standard procedure as given in ASTM E3. Image software was used to determine the content of different phases. Radiographic examinations were carried out by Smart 225 KV X-ray machine according to ASME SEC-V guideline. Samples were exposed for 60 seconds. The image quality indicator (IQI) having 0.25 mm diameter was used as per ASTM 1A-6. Ultrasonic tests were performed to measure the reduction in thickness due to erosion according to ASTM E-18. DM4 DL ultrasonic thickness gauge was utilized. Hardness measurement was done in Rockwell Hardness Scale B according to ASTM E-18. Hardened steel sphere of 1/16" diameter was forced in the specimen surface by applying 100 Kg load using ZHR Rockwell Hardness Tester; 10 Kg minor load was also applied initially to hold the sample firmly by an initial penetration and to enhance the accuracy of results.

Results and discussion
Aged, (c) Cut-off View of External Surface of Aged Pipe Showing Corrosion Pits at 100X magnification.
Visual examination shows no disturbance in uniformity of virgin pipe, See Fig. 2(a). Whereas, aged pipe surface as shown in Fig. 2(b) shows corrosion, as it has been in sea shore environment which is highly corrosive. Corrosion has the potential to reduce a design life of the component by premature degradation (Ginzel, 2002). Examination of corroded surface under metallurgical microscope, as shown in Fig. 2(c), reveals formation of corrosion pits, maximum depth of pit is 0.07 mm, which causes non-uniform reduction in thickness and reduces the ultimate strength of the material (Sidharth, 2009). Radiographic examination of virgin pipe exhibits no sign of erosion, as shown in Fig. 3(a). Whereas material removal is evident on the internal surface of aged pipe, see Fig. 3 (Gonzaga et al., 2009).  Hardness of the pipe samples was measured on internal and external surfaces. Reduction in hardness of internal pipe surface is observed 20%. However, external surface experiences only 0.7% reduction in hardness. Reduction in hardness is 29 times more on internal layer of pipe material because this layer has been directly exposed to hot and pressurized steam.

Conclusion
Age related degradations have been observed in extraction steam piping of a nuclear power plant. Corrosion pits, which cause non-uniform reduction in thickness and reduce ultimate strength, have been observed with maximum depth of 0.07mm. 8.3% reduction in wall thickness has been observed due to mechanical rubbing and impinging of hot and pressurized steam on internal surface. The hoop stress, acting in radial direction has been increased due to reduced wall thickness. Initially present bands of pearlite and ferrite have been eliminated due to prolong exposure to high temperature which changes the mechanical properties. Microstructure of pipe material has been transformed into uniform mixture of pearlite and ferrite. Graphite nodules have also been formed by detachment of carbon from iron in pearlite, which has reduced pearlite content from 40% to 25%. The reduction in pearlite is comparatively higher on internal surface layer of pipe as this layer faced hot and pressurized steam directly. The strength and hardness of the carbon steel SA 106B has been reduced due to this microstructural transformation. The need to frequent examination has been revealed by this study as the combine effect of microstructural degradation and pitting on internal surface of the piping may lead to fracture.