Investigation of Micro-structure and Creep Life Analysis of Centrifugally Cast Fe Cr Ni Alloy Reformer Tubes

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1 Manufacturing Science and Technology 3(4): , 2015 DOI: /mst Investigation of Micro-structure and Creep Life Analysis of Centrifugally Cast Fe Cr Ni Alloy Reformer Tubes Amitava Ghatak *, P.S. Robi Department of Mechanical Engineering, Indian Institute of Technology Guwahati, India Copyright 2015 by authors, all rights reserved. Authors agree that this article remains permanently open access under the terms of the Creative Commons Attribution License 4.0 International License Abstract Reformer furnace tubes are designed for exposure to high temperatures and pressures for prolonged time. HP grade steels which are cast austenitic Fe Cr Ni alloys are used as reformer tubes for service temperatures in the range of 950 C to 1000 C. This paper reports an investigation on the results of analysis of service exposed HP40Nb steel micro-alloyed with Ti. Microstructural investigation of the service exposed tube revealed carbides rich in Cr, Nb and Ti at grain boundary regions which are typical features of the cast material. No evidence of creep deformation was observed in this material indicating absence of degradation in structure and properties. Accelerated stress rupture tests at temperatures in the range of 850 C C and stress in the range of 47 MPa - 80 MPa on samples machined from tubes exposed for 11 years at 650 C was carried out to assess the life of the tube for various combinations of temperature and stress by Larson-Miller parameter. Observation of the fractured samples after accelerated stress rupture tests revealed nucleation and growth of voids at Cr rich phases as the main mechanism of damage at elevated temperatures. This paper also discusses the need for using the Larson-Miller constant (C L-M ) as a function of stress rather than considering it as a constant value. Keywords Reformer Tube, HP40Nb, Larson-Miller Parameter, Rupture Test 1. Introduction Reformer tubes are widely used in petroleum and chemical industries for production of hydrogen from natural gas [1,2,3]. Reformer tubes are typically made of centrifugally cast microalloyed HP40Nb heat resistant alloys. At higher temperatures and stresses, high Cr and Ni content in the alloy facilitate corrosion resistance and creep resistance respectively [4]. Design engineers found that these tubes made of HP40Nb microalloyed steel can work fora long period of service life at temperatures up to 1000 C and internal pressures of bar. The degradation of microstructure and properties during service many a times results in premature failure of these tubes. This results in forced shutdown of the plant accompanied by severe economic loss. In the recent years, remaining life assessment of the materials has been considered as an important factor by design engineers for the safety of these tubes. Various creep rupture techniques, such as Larson-Miller method [5], Orr-Sherby-Dorn method [6], Manson-Haferd method [7], Manson-Succop method [8] have been suggested by researchers to assess the remaining life of a material. These techniques represent the master curves which involve stress (σ), temperature (T) and rupture time (t R ). Among these parametric techniques, Larson-Miller parameter (P L-M ) is the most commonly used method to determine creep life [9,10,11,12,13]. P L-M is expressed by the relationship: P L-M (σ) = T (log(t R ) + C L-M ) 10-3 (1) where C L-M is the Larson-Miller constant. The C L-M value is obtained as the intersecting point of iso-stress lines on log(t R ) axis of log(t R ) vs. (1/T) plot. It is observed that in most of the cases, the value of C L-M is considered as a constant. Feng et al. proposed a Z-parameter which represents the degree of the deviation to the master curve due to scattered experimental data [14]. C L-M depends on the chemical composition, initial microstructure, grain size [15] and cold-work [16] imparted. The scatter of experimental data from master curve may be due to C L-M value which leads to the assumption of a constant significant value for the prediction of life of the component. The present investigation aims to study the microstructural degradation and assess remaining life of microalloyed HP40Nb austenitic stainless steel reformer tube material. In this study, the authors have attempted to find a method to reduce the scatter of experimental data from master curve by considering C L-M as a dependent parameter rather than a constant value. 2. Experimental Procedure The material for the investigation was centrifugally cast reformer tube obtained from Numaligarh Refineries Limited, India having inside diameter of 106 mm and thickness 15.3 mm. The supplied tube was service exposed to 650 C for 11 years. The composition of the alloy analyzed by optical emission spectrometer was found to be 0.4 C, 1.3 Si, 0.037

2 156 Investigation of Micro-structure and Creep Life Analysis of Centrifugally Cast Fe Cr Ni Alloy Reformer Tubes Mo, 23.6 Cr, 34.9 Ni, 0.8 Nb, Ti and balance Fe (in weight %). Flat tensile creep specimens of 25 mm gauge length, 6.5 mm width and 3 mm thick as shown in Fig. 1, were machined from the longitudinal direction of reformer tube by wire-cut electric discharge machining (EDM). Constant stress creep tests were carried out at three different stresses, viz, 47, 68 and 80 MPa, at temperature levels ranging between 850 C and 1050 C. During testing, the temperature of the specimen was controlled accurately within ±3 C. Rupture time of each test was noted for parametric study of the material. Figure 2. Optical micrograph of as-received reformer tube. Figure 1. Specimen geometry for creep rupture tests. All dimensions are in mm The longitudinal sections of the failed creep specimens were machined by wire-cut EDM. The samples were polished for microstructural investigation following standard procedure. A solution of 5 ml HNO 3, 10 ml HCl and 15 ml glycerol was used for etching the samples. Metallographic examinations were performed using upright optical microscope as well as in a LEO 1430VP model scanning electron microscope (SEM) in equipped with energy dispersive X-ray (EDX) analysis set up. 3. Results and Discussion Metallographic Observation The optical micrograph of the as-received HP40Nb microalloyed steel is shown in Fig. 2. The microstructure shows dendritic structure. Network of carbide precipitates at austenitic grain boundary regions are observed. No evidence of any microscopic defects was observed during investigation under microscope. Fig. 3 shows the SEM image carried out using a back scattered electron detector. EDX results (Fig. 4) show three different types of carbide precipitates: Cr-rich carbide (dark gray, phase-a), Nb-rich carbide (white, phase-b) and Nb-Ti-rich carbide (tiny white, phase-c) at austenitic grain boundaries. SEM micrograph of the longitudinal section of the specimen failed by creep deformation at 850 C and 47 MPa stress is shown in Fig. 5. Microstructural observation at low magnification (Fig. 5(a)) indicates presence of micro void at grain boundary regions. High magnification (Fig. 5(b)) observation reveals nucleation of micro voids (around 3-8 μm) in the vicinity of carbides at grain boundaries. The growth of these micro voids along the grain boundary region is also observed. The microscopic feature reveals the failure mechanism by nucleation of voids in the vicinity of carbides, its subsequent growth and coalescences. Figure 3. SEM backscatter electron image of as-received microalloyed HP40Nb steel with Cr-rich carbide as dark gray (phase-a), Nb-rich carbide appearing white (phase-b) and Nb-Ti-rich carbide as tiny white precipitates (phase-c) in an austenitic matrix. (a) (b)

3 Manufacturing Science and Technology 3(4): , C L-M in the range from The plot is found to be linear. The coefficient of determination (R 2 value) was obtained corresponding to each C L-M value. From Fig. 6, the best fit was obtained for a C L-M value of The log-log plot of σ vs. P L-M considering a C L-M value of 24.5 is shown in Fig. 7. (c) Figure 4. EDX spectrum of (a) phase A, (b) phase B and (c) phase C. Figure 6. Variation of coefficient of determination for linear fit with Eq. 1 as a function of C L-M. (a) (b) Figure 5. SEM image of the longitudinal section of the gauge length of specimen crept at 850 C/47 MPa: (a) low magnification showing microcracks along the grain boundaries and (b) grain boundary decohesion under higher magnification. 4. Life Assessment The range of test temperature ( C) was close to the design temperature ( C). The creep tests were carried at three stress levels, viz. 47, 68 and 80 MPa. The creep rupture data was analyzed by plotting σ vs. P L-M for Figure 7. Plot of stress against P L-M for the creep test using C L-M value as Table 1 shows the values of C L-M for different applied stresses obtained from the iso-stress plots of log(t R ) vs. (1/T). Larson and Miller [5]opined that the iso-stress lines should intersect at a single point on the log(t R ) axis and value of C L-M corresponds to the value of this intersection point on the log(t R ) axis. It is observed that for the given stress ranges, C L-M varies linearly with stress. Eq. 1 was therfore modified in the form

4 158 Investigation of Micro-structure and Creep Life Analysis of Centrifugally Cast Fe Cr Ni Alloy Reformer Tubes P L-M (σ) = T (log(t R ) + (m σ + A)) 10-3 (2) where m and A are constants obtained from C L-M vs. σ plot. The value of m and A were obtained as MPa -1 and , respectively. The σ vs. P L-M plot on a log-log scale for the reformer tube considering C L-M value as a function of stress is shown in Fig. 8. The plot of σ vs. P L-M is found to be linear with a R 2 value of 0.994, indicating a very good fit. It may be noted that the R 2 value for the σ vs. P L-M plot, considering C L-M as a constant, was only Comparision of Fig. 7 & 8 indicates a better fit can be obtained considering C L-M as a function of stress. Therefore, the P L-M considering C L-M value as a function of stress shows a satisfactory result for predicting the long-term creep rupture life of the reformer steel. P L-M values for hoop stresses of 10, 20 and 30 MPa for the service exposed reformer tube were determined by extrapolating the curves in Fig. 7 and Fig. 8 and is given in Table 2. Fig. 9 and Fig. 10 show the remaining life of the reformer tube using the following equations: t R = 10 ((1000 P /T)-25.4) L-M (3) and t R = 10 ((1000 P /T) (m σ + A)) L-M (4) Fig. 9 and Fig. 10 show the remaining life decreases with increase in tempetrature. At lower stress the material can work at higher temperature than higher stress for same remaining life. Comparison of remaining life of the reformer tube obtained by considering constant C L-M and C L-M as a function of stress in Eq. 1, at various service temperatures and stresses are given in Table 3. From Table 3, it is apparent that remaining life of the reformer tube is very sensitive to small difference in temperture and C L-M value, even if C L-M is considered as a constant. Table 1. C L-M values for different stresses, σ [MPa] Larson-Miller Constant, C L-M Table 2. P L-M values from Fig. 6 and Fig. 7 for different stresses. Larson-Miller Parameter, P L-M [MPa] Using C L-M as a Function of Using Constant C L-M Table 3. Remaining life of the reformer tube for different stress and temperature combinations. [MPa] Temperature [ C] Remaining life [years] Using Constant C L-M Using C L-M as a Function of Figure 9. Temperature predicted rupture life behavior for the service exposed steel using constant C L-M value. Figure 8. Plot of stress vs. P L-M for C L-M as a function of stress. Figure 10. Temperature predicted rupture life behavior for C L-M as a function of stress.

5 Manufacturing Science and Technology 3(4): , Conclusions The creep behaviour of service exposed microalloyed HP40Nb reformer steel was investigated in the study. The main results are summarized as follows: 1. The microstructure of the as-received material mainly consisted of Cr-rich carbide, Nb-rich carbide and Nb-Ti-rich carbide phases at the austenite grain boundaries. 2. Creep rupture data analyzed by Larson-Miller parameter revealed that C L-M is dependent on stress. 3. It is suggested that C L-M in the P L-M equation should be considered as a function of stress rather than considering as a constant value. 4. The creep deformation of the investigated material indicates that the failure is by initiation of micro-voids at the interface of Cr-rich carbides-matrix, its growth and subsequent coalescence. REFERENCES [1] A. Ghatak and P.S. Robi, in: Effect of temperature on the tensile properties of HP40Nb microalloyed reformer steel, edited by S. Thakur, M. Gupta. F.S. Chau and T.S. Srivatsan, Processing and Fabrication of Advanced Materials XXII (PFAM XXII), p. 534, Research Publishing (2013). [2] A.A. Wahab, M.V. Kral: Mater. Sci. Eng. A Vol. 412 (2005), p [3] C.E. Jaske: Issues in the Life Assessment of Reformer Tubes, Paper 05419, CORROSION/2005. NACE International, Houston (2005). [4] J. Joubert, W. St-Fleur, J. Sarthou, A. Steckmeyer and B. Fournier: Comput. Coupling Phase Diagr. Thermochem. Vol. 46 (2014), p. 55. [5] F.R. Larson and J. Miller: Trans. ASME Vol. 74 (1952), p [6] R.L. Orr, O.D. Sherby and J.E. Dorn: Trans. ASM Vol. 46 (1954), p [7] S.S. Manson and A.M. Haferd: NASA TN2890, Mar (1952). [8] S.S. Manson and G. Succop, in: -Rupture Properties of Inconel 700 and Correlation on the Basis of Several Time-Temperature Parameters, edited by ASTM, of Symposium on Metallic Materials for Service Temperatures Above 1600 F, ASTM-STP 174, ASTM, Philadephia, (1956). [9] A. Kim, K. Tunvir, S. Nahm and S. Cho: J. Mater. Process. Tech. Vol. 202 (2008), p [10] A.K. Ray, K. Diwakar, B.N. Prasad, Y.N. Tiwari, R.N. Ghosh and J.D. Whittenberger: Mater. Sci. Eng. A Vol (2007), p [11] J.E. Indacochea and R.A. Seshadri: Mater. Sci. Eng. A Vol (1997), p [12] A.K. Ray, S. Kumar, G. Krishna, M. Gunjan, B. Goswami and S.C. Bose: Mater. Sci. Eng. A Vol. 529 (2011), p [13] J. Swaminathan, K. Guguloth, M. Gunjan, P. Roy and R. Ghosh: Eng. Fail. Anal. Vol. 15 (2008), p [14] W. Feng, J. Zhao and L. Xing: J Press. Equip. Sys. Vol. 5 (2007), p. 20. [15] S. Wignarajah, I. Masumoto and T. Hara: ISIJ Int. Vol. 30 (1990), p. 58. [16] M. Vasudevan, S. Venkadesan, P.V. Sivaprasad and S.L. Mannan: J Nucl. Mater. Vol. 211 (1994), p. 251.