Journal of Alloys and Compounds 680 (2016) 6e7
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Letter
Comments on ‘microstructural evolution of hydroformed inconel 625 bellows’ a b s t r a c t Keywords: Alloy 625 Fatigue properties g0 0 precipitation Secondary carbides
In a recent paper, E. Pavithra and V.S. Senthil Kumar [Journal of Alloys and Compounds 669 (2016) 199] studied the microstructure evolutions during fatigue of hydroformed bellows made of Alloy 625. The authors made several shortcuts in their analysis of microstructure evolutions after two fatigue tests performed at room temperature and at 650 C. Moreover, the conclusions of the paper are not supported by the very poor SEM observations provided in this paper. The aim of the present comments is to reconsider these results based on the available literature on the microstructure evolutions and fatigue properties of Alloy 625. © 2016 Elsevier B.V. All rights reserved.
E. Pavithra and V.S. Senthil Kumar studied the fatigue properties of Alloy 625 bellows expansion joints at room temperature and at 650 C and they are probably the first ones to perform such tests [1]. However, contrary to what is claimed in the prime novelty statement of this paper, several other contributions studied the fatigue properties of Alloy 625 at elevated temperatures [2e9], references which could have been considered in this paper. More specifically, in a recent publication, L. Mataveli Suave & al. investigated the low cycle fatigue (LCF) durability of Alloy 625 at 650 C and 750 C, as well as microstructure evolutions during LCF loading and the way the g00 and d precipitation kinetics are affected by the fatigue loading [4]. As a first main deficiency of the Pavithra and Senthil Kumar paper, no information at all is given on the hydroforming process and no information is given on the thermal treatment applied after hydroforming. Hence, the hardness increase observed in Fig. 3 cannot be linked to either a strain hardening mechanism introduced by the hydroforming process or to the precipitation of intermetallic phases, as observed by Mataveli Suave & al [10,11]. Moreover, no indication at all is given on the chemistry of the alloy. Considering these three omissions, the authors are only speculating on the possible microstructure evolutions occurring in the tested Alloy 625 bellows. Their analyses are based on very poor scanning electron microscope (SEM) observations supposed to show that the bellows are strengthened by the Ni3Nb g00 phase (DO22 body centered tetragonal structure) after fatigue tests or after hydroforming. With such SEM observations using 750 or 1500 times magnifications, the g00 particles cannot be resolved. Indeed, g00 precipitates can only be characterized using transmission electron microscopy (TEM) observations [12,13], or, alternatively, high resolution SEM observations with magnifications greater than 25,000 times, as shown in Fig. 1, in the case of Alloy 625 specimens http://dx.doi.org/10.1016/j.jallcom.2016.04.078 0925-8388/© 2016 Elsevier B.V. All rights reserved.
heat treated for two different thermal exposures at 650 C. Hence, according to our results [4,11], we agree with the fourth conclusion stating that the g00 particles are likely to precipitate in the bellow tested in fatigue at 650 C for 34,683 cycles (i.e. nearly 13 h at 650 C, neglecting the soak time before the fatigue test which is not given) but SEM observations provided by Pavithra and Senthil Kumar do not support such a conclusion and no reliable reference is provided to support it. Also, we do not want to further comment on the carbide precipitation claimed after fatigue testing at 650 C since almost no information is provided on the location where the EDS scan presented in Fig. 7b has been performed (point scan at grain boundaries?). The interested reader is referred to one of our recent publications to have a better overview on the evolution of secondary carbides (both M6C and M23C6) at grain boundaries in Alloy 625 at 650 C (see Figs 8 and 9 of reference [11]), as well as typical chemistries of these particles. As a second deficiency of this paper, the sentence “The density of the g00 is higher at twin/grain boundaries while the carbides are randomly distributed” in the introduction section (end of fourth paragraph) is totally wrong. First of all, in the specific literature devoted to Alloy 625, secondary carbides are mainly located at grain boundaries and g00 particles, when present, are almost homogeneously dispersed inside the grains [2,4,11,13,15e20]. Moreover, the two references used at the end of this sentence (Refs. 26 and 27) do not support this claim at all and Ref. 27 is about a g0 strengthened alloy, not a g00 -strengthened alloy like Alloy 625. As a third main flaw of the paper, the authors claimed that recrystallization and grain growth are occurring during fatigue testing at 650 C. To our best knowledge, this is impossible. This temperature is too low to either entail recrystallization or grain growth, especially at such a temperature favoring secondary carbides precipitation which act as grain boundary pinning particles.
Letter / Journal of Alloys and Compounds 680 (2016) 6e7
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Fig. 1. High resolution SEM observations of g0 0 precipitates in Alloy 625 after thermal exposures of 50 h (a) and 1000 h (b) at 650 C [14].
According to our experiments, grain growth is likely to occur at 900 C and above in a rolled þ solution treated sheet made of Alloy 625 and 1000 h of thermal exposure at 900 C are required to trigger grain growth [11]. Finally, and as a main weakness of the paper, only two fatigue tests have been performed, one at each temperature. How can microstructure evolutions be interpreted with such a reduced database? We have shown recently that microstructure evolutions in Alloy 625 during LCF loading at 650 C are very complex [4]. The precipitation kinetics of g00 and d phases have been shown to be faster under LCF loading compared to pure thermal exposure and they are very sensitive to the loading frequency. Hence, it is impossible to draw accurate conclusions on the microstructure evolutions in Alloy 625 bellows by just using two experiments. References [1] E. Pavithra, V.S. Senthil Kumar, J. Alloys Compd. 669 (2016) 199e204. [2] C.R. Conder, G.D. Smith, J.F. Radavich, in: E.A. Loria (Ed.), Superalloys 718, 625, 706 and Various Derivatives, TMS, Pittsburgh, PA, USA, 1997, pp. 447e458. [3] L. Mataveli Suave, D. Bertheau, J. Cormier, P. Villechaise, A. Soula, Z. Hervier, F. Hamon, J. Laigo, in: E. Ott (Ed.), 8th International Symposium on Superalloy 718 and Derivatives, TMS, Pittsburgh, PA, USA, 2014, pp. 281e295. [4] L. Mataveli Suave, J. Cormier, D. Bertheau, P. Villechaise, A. Soula, Z. Hervier, F. Hamon, Mater. Sci. Eng. A 650 (2016) 161e170. [5] H.L. Eiselstein, D.J. Tillack, in: E.A. Loria (Ed.), Superalloys 718, 625 and Various Derivatives, TMS, Pittsburgh, PA, USA, 1991, pp. 1e14. [6] J.F. Grubb, in: E.A. Loria (Ed.), Superalloys 718, 625, 706 and Various Derivatives, TMS, Pittsburgh, PA, USA, 1997, pp. 629e637. [7] G.D. Smith, D.H. Yates, in: E.A. Loria (Ed.), Superalloys 718, 625 and Various Derivatives, TMS, Pittsburgh, PA, USA, 1991, pp. 509e517. [8] L.E. Shoemaker, in: E.A. Loria (Ed.), Superalloys 718, 625, 706 and Derivatives, TMS, Pittsburgh, PA, USA, 2005, pp. 409e418. [9] P.W. Trester, J.L. Kaae, R. Gallix, J. Nucl. Mater. 133e134 (1985) 347e350.
[10] L. Mataveli Suave, D. Bertheau, J. Cormier, P. Villechaise, A. Soula, Z. Hervier, dou (Ed.), Eurosuperalloys 2014, Matec Web of ConferJ. Laigo, in: J.-Y. Gue ences, Presqu'île de Giens, France, 2014, p. 21001. [11] L. Mataveli Suave, J. Cormier, P. Villechaise, A. Soula, Z. Hervier, D. Bertheau, J. Laigo, Metallurgical Mater. Trans. 45A (2014) 2963e2982. [12] M. Sundararaman, P. Mukhopadhyay, S. Banerjee, Metall. Trans. 19A (1988) 454e465. [13] M. Sundararaman, P. Mukhopadhyay, S. Banerjee, in: E.A. Loria (Ed.), Superalloys 718, 625, 706 and Various Derivatives, TMS, Pittsburgh, PA, USA, 2001, pp. 367e378. [14] L. Mataveli Suave, Master Thesis, ISAE-ENSMA, Chasseneuil du Poitou, France, 2012. [15] F. Cortial, J.M. Corrieu, C. Vernot-Loier, Metallurgical Mater. Trans. 26A (1995) 1273e1286. [16] S. Floreen, G.E. Fuchs, W.J. Yang, in: E.A. Loria (Ed.), Superalloys 718, 625, 706 and Various Derivatives, TMS, Pittsburgh, PA, USA, 1994, pp. 13e37. [17] V. Shankar, K.B.S. Rao, S.L. Mannan, J. Nucl. Mater. 288 (2001) 222e232. [18] V. Shankar, M. Valsan, K.B.S. Rao, S.L. Mannan, Scr. Mater. 44 (2001) 2703e2711. [19] M. Sundararaman, P. Mukhopadhyay, S. Banerjee, in: E.A. Loria (Ed.), Superalloys 718, 625, 706 and Various Derivatives, TMS, Pittsburgh, PA, USA, 1997, pp. 367e378. [20] C. Thomas, P. Tait, Int. J. Pres. Ves. Pip. 59 (1994) 41e49.
Jonathan Cormier*, Lorena Mataveli Suave Institut Pprime, CNRS e Universit e de Poitiers e ENSMA, Physics and Mechanics of Materials Department, ISAE-ENSMA, UPR CNRS no 3346, 1 avenue Cl ement Ader, BP 40109, Futuroscope, 86961 Chasseneuil, France *
Corresponding author. E-mail address:
[email protected] (J. Cormier). 19 February 2016 Available online 13 April 2016
