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Tracking hydrogen embrittlement using short fatigue crack behavior of metals
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Procedia Structural Integrity 13 00 (2018) 1427–1432 Structural Integrity Procedia (2016) 000–000
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Tracking hydrogen embrittlement using short fatigue crack behavior Tracking hydrogen embrittlement using short fatigue crack behavior XV Portuguese Conference on Fracture, of PCFmetals 2016, 10-12 February 2016, Paço de Arcos, Portugal of metals Vishal Singh, Rajwinder Singh,of Amanjot Dhiraj K. Mahajan* Thermo-mechanical modeling a highSingh, pressure turbine blade of an Vishal Singh, Rajwinder Singh, Amanjot Singh, Dhiraj K. Mahajan* Ropar Mechanics of Materials Laboratory, Department of Mechanical Engineering, Indian Institute of Technology Ropar, Rupnagar, Punjab, airplane gas turbine engine Ropar Mechanics of Materials Laboratory, Department of Mechanical Indian Institute of Technology Ropar, Rupnagar, Punjab, India,Engineering, 140001 India, 140001
P. Brandãoa, V. Infanteb, A.M. Deusc*
Abstract a Abstract Department of Mechanical Engineering, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1, 1049-001 Lisboa, Understanding hydrogen embrittlement phenomenon that leads Portugal to deterioration of mechanical properties of metallic components is b IDMEC, Department ofembrittlement Mechanical Engineering, Instituto Superior Universidade de Lisboa, Rovisco Pais, 1049-001 Lisboa, Understanding hydrogen phenomenon that leads to Técnico, deterioration of the mechanical properties of on metallic components is vital for applications involving hydrogen environment. Among these, understanding influence ofAv. hydrogen the1,fatigue behaviour Portugal vital for applications involving hydrogen environment. Among these, understanding the influence of hydrogen on the fatigue behaviour of metals is of great interest. Total fatigue life of a material can be divided into fatigue crack initiation and fatigue crack growth phase. c CeFEMA, of Mechanical Engineering, Instituto can Superior Técnico, Universidade de Lisboa, Av.and Rovisco Pais, 1, 1049-001phase. Lisboa, of metals is ofDepartment great interest. fatigue of athe material beof divided into fatigue crack While fatigue crack initiationTotal can be linkedlife with propagation short fatigue cracks, the initiation size of whichfatigue is of thecrack ordergrowth of grain size Portugal While fatigue crack initiation can be linked with the propagation of short fatigue cracks, the size of which is of the order of grain size (few tens of microns), that are generally not detectable by conventional crack detection techniques applicable for the long fatigue crack (few tens of microns), that are generally detectableExtensive by conventional crack detectionontechniques for the long crack fatiguegrowth crack growth behaviour using conventional CTnot specimens. literature is available hydrogenapplicable effect on long fatigue growth behaviour using CT specimens. Extensive is available on hydrogen long(∆𝐾𝐾 fatigue crack growth behaviour of metals thatconventional leads to the change in crack growth rateliterature and the threshold stress intensity effect factor on range ). However, it is Abstractof metals that leads to the change in crack growth rate and the threshold stress intensity factor range (∆𝐾𝐾�� ). However, it is behaviour the short fatigue crack growth behaviour that provides the fundamental understanding and correlation of the metallic �� microstructure the fatigue crack growthphenomenon. behaviour that provides fundamental understanding and correlation of the from metallic withshort hydrogen embrittlement Short fatiguethe crack growth behaviour is characteristically different longmicrostructure crack growth During their operation, modern aircraft components arebehaviour subjected to increasingly demanding operating with hydrogen embrittlement phenomenon. fatigue crack growth different from behaviour showing high propagation rate Short at engine much lower values than thresholdis characteristically stress intensity factor range aslong wellcrack as conditions, a growth strong especially the high pressure turbine (HPT) blades. Such conditions cause these parts to undergo different types of time-dependent behaviour showing high propagation rate at much lower values than threshold stress intensity factor range as well as aa strong dependency on the microstructural features such as grain boundaries, phase boundaries, and inclusions. To this end, novel degradation, one which is creep.features Ainvestigate model using finite element (FEM) was developed, inmaterials order beend, able atoin-situ predict dependency on theofmicrostructural such as the grain boundaries, phase boundaries, and charged inclusions. To to this novel experimental framework is developed to the short fatigue crackmethod behaviour of hydrogen involving the creep behaviour of HPT blades. Flight data records (FDR) for a specific aircraft, provided by a commercial aviation experimental framework is developed to investigate the short fatigue crack behaviour of hydrogen charged materials involving in-situ observation of propagating short cracks coupled with image processing to obtain their da/dN vs 𝑎𝑎 curves. Various metallic materials company, used stainless to short obtaincracks thermal and mechanical data for three different flight cycles. In order to alloy create the materials 3D observation propagating coupled with to obtain da/dNGrade vs 𝑎𝑎 curves. Various metallic ranging fromofwere austenitic steel (AISI 316L) to image reactorprocessing pressure vessel steeltheir (SA508 3 Class I low steel) and model line needed for the analysis, a studied HPT was scanned, and itssteel chemical andI low material were ranging from austenitic steel (AISIblade 316L) to reactor pressure vessel (SA508composition Grade 3 Class alloy properties steel) and line pipe steels (API 5LFEM X65stainless & X80) are in thisscrap work. obtained. The 5L data that&was wasinfed the FEM model and different simulations were run, first with a simplified 3D pipe steels (API X65 X80)gathered are studied thisinto work. block shape, in order better B.V. establish the model, and then with the real 3D mesh obtained from the blade scrap. The © rectangular 2018 The Authors. Published by to Elsevier © 2018 The Authors. Published by Elsevier B.V. overall expected behaviour in terms of displacement was observed, in particular at the trailing edge of the blade. Therefore such a © 2018 The Authors. Published by Elsevier B.V. Peer-reviewunder under responsibility of ECF22 the ECF22 organizers. Peer-review responsibility of the organizers. model can under be useful in the goal ofthe predicting turbine blade life, given a set of FDR data. Peer-review responsibility of ECF22 organizers. Keywords: Hydrogen embrittlement, short crack, fatigue, 316L, SA 508, X65, X80 © 2016 Hydrogen The Authors. Publishedshort by Elsevier B.V. 316L, SA 508, X65, X80 Keywords: embrittlement, crack, fatigue,
Peer-review under responsibility of the Scientific Committee of PCF 2016. Keywords: High Pressure Turbine Blade; Creep; Finite Element Method; 3D Model; Simulation.
* Corresponding author. Tel.: +91-7814252244 * Corresponding Tel.: +91-7814252244 E-mail address:author. [email protected] E-mail address: [email protected] 2452-3216 © 2018 The Authors. Published by Elsevier B.V. 2452-3216 © 2018 Authors. Published Elsevier B.V. Peer-review underThe responsibility of theby ECF22 organizers. Peer-review underauthor. responsibility the ECF22 organizers. * Corresponding Tel.: +351of 218419991. E-mail address: [email protected]
2452-3216 © 2016 The Authors. Published by Elsevier B.V.
Peer-review under responsibility of the Scientific Committee of PCF 2016. 2452-3216 2018 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the ECF22 organizers. 10.1016/j.prostr.2018.12.296
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Vishal Singh et al. / Procedia Structural Integrity 13 (2018) 1427–1432 Author name / Structural Integrity Procedia 00 (2018) 000–000
1. Introduction Hydrogen has been identified as one of the cleanest energy resources capable of reducing our dependency on fossil fuels. Many energy applications are already being explored to replace fossil fuels with hydrogen-based energy solutions (Kanezaki et al. (2008)). Despite knowing the effectiveness of hydrogen energy, its use on larger scale (that requires hydrogen storage and transportation) is restricted due to the well-known phenomenon of hydrogen embrittlement (HE). HE is an insidious failure mode that causes loss of structural integrity of steels (Murakami and Matsuoka (2010)). Eextensive research on the effect of hydrogen on steel performance is available describing the hydrogen embrittlement behavior of steels based on slow strain rate test (SSRT), linearly increasing stress test (LIST), fracture toughness testing and long fatigue crack growth testing (Chatzidouros et al. (2014); Venezuela et al. (2015); Tsay et al. (2008)). For designing steel components capable of delivering satisfactory performance over the years under hydrogen environment, reliable fatigue data of steels under hydrogen environment is most desired. Total fatigue life of a material can be divided into fatigue crack initiation and long fatigue crack growth phase. Literature available on the effect of hydrogen on fatigue behavior of steels generally discusses their long crack propagation behavior that leads to the change in crack growth rate and the threshold stress intensity factor range (∆𝐾𝐾�� ) (Tsay et al. (2008); Ogawa et al. 2018). However, major portion of fatigue life is consumed in crack initiation phase that includes fatigue crack nucleation and short fatigue crack propagation. The size of short fatigue cracks is of the order of grain size (few tens of microns), that are generally not detectable by conventional crack detection techniques applicable for the long fatigue crack growth behaviour (Krupp, (2007)). Short fatigue crack growth behaviour is characteristically different from long crack growth behaviour showing high propagation rate at much lower values than threshold stress intensity factor range as well as strong dependency on the microstructural features such as grain boundaries, phase boundaries, and inclusions. The short fatigue crack growth behaviour can provide fundamental understanding and correlation of the metallic microstructure with hydrogen embrittlement phenomenon. Effective microstructural engineering capable to arrest the propagation of these short fatigue cracks in steels under hydrogen environment needs detailed investigation regarding their interaction with various microstructural features such as phase/grain boundaries, grain orientation, inclusions etc. (Obrtlık et al. (1997); Mikulich et al. (2006); Mazánová et al. (2018)). Such investigations can help to improve the choice of steels and can open new trends toward the material engineering of steels with enhanced fatigue performance under hydrogen environment. Hence, a framework capable to explore the role of microstructural features on short fatigue crack propagation under hydrogen environment is desired. To this end, a novel experimental framework is developed to investigate the short fatigue crack behaviour of hydrogen charged steels involving in-situ observation of propagating short cracks coupled with image processing to obtain their da/dN vs 𝑎𝑎 curves. Microstructural dependent behaviour of short fatigue cracks elucidated in the present work will help to improve the material design for better performance under hydrogen environment. SA508 Grade 3 Class I low alloy steel applicable in nuclear reactor pressure vessel; two API line pipe steel grades X65 and X80, and a steel versatile in applications from fuel cell, nuclear power plant to line pipes for chemical, petrochemical i.e. austenitic stainless-steel grade 316L were studied in the present work. This paper consists of four sections. Following this brief introduction, Section 2 describes the experimental procedure. Results and discussions are provided in Section 3 followed by conclusions in Section 4. 2. Experimental procedure Chemical composition of investigated steels is presented in Table 1. To investigate the role of hydrogen on microstructural interaction of short fatigue crack growth, fatigue specimens of investigated steels were electrochemically charged by exposing to an electrolyte containing 1 N H2SO4 solution + 1.4 g/L Thiourea (charging promoter) under current density of 20 mA/cm2 for 4 hours. Specimen as cathode and platinum mesh as anode were used during hydrogen charging. In-situ investigations of short fatigue crack growth on single edge notch tension (SENT) specimen (with an initial notch size of ~70 µm for 316L and SA508 steel, and notch size ~50 µm for X65 and X80 steel) was conducted using digital microscope. Fatigue experimentations were conducted with R ratio 0.1 at frequency 35 Hz.
Vishal Singh et al. / Procedia Structural Integrity 13 (2018) 1427–1432 Author name / Structural Integrity Procedia 00 (2018) 000–000
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Table 1. Chemical composition of investigated steels Element (wt. %) 316L SA508 X65 X80
C
S
P
Mn
Si
Cr
Ni
Mo
Cu
Al
V
Nb
Fe
0.019 0.19 0.054 0.04
0.016 0.002 0.001 0.001
0.026 0.018 0.012 0.012
1.629 1.3 1.498 1.8
0.328 0.23 0.230 0.24
16.74 0.17 -0.05
10.26 0.7 -0.19
2.022 0.44 0.086 0.18
-0.13 -0.18
-0.02 0.029 --
--0.0355 0.018
--0.0572 0.482
Balance
The tests were interrupted at regular intervals (in terms of number of cycles) to take the images of the crack path in front of the notch. The detailed experimental procedure used for short fatigue crack propagation can be found elsewhere (Singh et al. (2018)). 3. Results and discussion Fig. 1 presents the microstructures of all the steels investigated in the present work. AISI 316L austenitic steel was characterized with an average grain size of ~50 µm. SA508 Grade 3 Class I steel was characterized as upper bainitic microstructure with an average grain size of ~20 um. API 5L X65 steel was composed of ferrite, pearlite and some bainite, with average grain size of ~10 um. API 5L X80 steel was containing ferrite and bainite with average grain size of ~6 um. Small fraction of martensite/austenite (M/A) islands and stringers were also obtained in SA508, X65 and X80 steels samples. Tensile characteristics of all the materials with and without hydrogen charging are presented in Table 2. Table 2. Tensile properties of investigated steels with and without hydrogen environment Material 316L SA508 X65 X80
YS, MPa 315 523 523 625
Uncharged UTS, MPa 541 663 610 665
Elongation, % 86 28 25 22
YS, MPa 303 540 552 650
(a)
(b)
(c)
(d)
Hydrogen charged UTS, MPa Elongation, % 567 81 642 22 601 21 670 12
Fig. 1. (a) Optical image of 316L and SEM images of (b) SA508, (c) X65, and (d) X80 steels
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3.1. Effect of hydrogen on short fatigue crack growth in 316L steel Austenitic stainless steels are widely applicable in line pipes for hydrocarbons, chemicals and nuclear power industries. The hydrogen-based degradation of these steels is well accepted (Kanezaki et al. (2008)). Fig. 2a presents the variation of crack length (𝑎𝑎) with number of cycles (𝑁𝑁) for uncharged and hydrogen charged 316L steel obtained after fatigue loading at constant stress range (∆𝜎𝜎) of 270 MPa. Early initiation followed by accelerated crack growth in hydrogen charged specimen with increase in number of cycles can be observed in Fig. 2a. Shorter plateau regions (in 𝑎𝑎 vs 𝑁𝑁 curve, Fig. 2a) in hydrogen charged specimen than in uncharged 316L, confirmed lesser hindrances to short fatigue crack propagation from the microstructural features under hydrogen environment. Fig. 2b presents a comparison of short fatigue crack growth rate (𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑) as a function of crack length (𝑎𝑎) for hydrogen charged and uncharged 316L steel. The deceleration in the 𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑 curves correspond to the grain boundaries resisting the short cracks to propagate. Similar deceleration was provided by the grain boundaries for both hydrogen charged and uncharged specimens. However, the crack growth rate within the grain appears to be much faster in hydrogen charged specimen compared to uncharged specimen. This may be due to crack tip localized enhanced slip activities that resulted in an increased crack growth under hydrogen environment.
Fig. 2. Variation of (a) short fatigue crack length ‘𝑎𝑎𝑎 with numbers of cycles ‘𝑁𝑁𝑁 and (b) short crack growth rate ′𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑 with crack length ‘𝑎𝑎𝑎 of AISI 316L steel at ∆𝜎𝜎 = 270 MPa
For the uncharged specimen, slip bands were seen in multiple grains around the crack tip as well as away from the crack tip, as shown in Fig. 3a. On the other hand, for the hydrogen charged specimen, slip bands appeared in the vicinity of the crack tip with negligible slip bands formation away from crack tip. Some microcracks were also observed away from the crack tip for this specimen, as shown in Fig. 3b. For an uncharged specimen, the activation of slip systems led to the formation of slip bands in most of the grains. However, for the case of hydrogen charged specimen, hydrogen led to the pinning of dislocations hindering the slip activity. For this case, the stresses got relieved by microcracks generation away from crack tip. Due to localized slip activity, the crack propagation for this case appears to be more planar compared to the uncharged specimen.
Fig. 3. Optical micrographs of crack path ahead of notch in (a) uncharged and (b) hydrogen charged 316L specimens
Vishal Singh et al. / Procedia Structural Integrity 13 (2018) 1427–1432 Author name / Structural Integrity Procedia 00 (2018) 000–000
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3.2. Effect of hydrogen on short fatigue crack growth in SA508 Gr. 3 Cl. 1 low alloy steel Fig. 4 presents the comparison of short fatigue behavior of hydrogen charged and uncharged SA508 Gr. 3 Cl. 1 low alloy steel. Short fatigue crack growth investigations were conducted at stress range (∆σ) of 475 MPa. From Fig. 4a and b, it was concluded that the crack growth in SA508 hydrogen charged specimen was significantly higher than that of uncharged specimen.
Fig. 4. Variation of (a) short fatigue crack length ‘𝑎𝑎𝑎 with numbers of cycles ‘𝑁𝑁𝑁 and (b) short crack growth rate ′𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑 with crack length ‘𝑎𝑎𝑎 of SA508 sample at ∆𝜎𝜎 = 475 MPa
SEM analysis of uncharged SA508 specimen (not presented here) revealed that the prior austenite grain boundaries (PAGBs) offer the highest resistance to the short fatigue crack propagation. Uniformly distributed carbides were found to resist the crack growth more significantly after PAGBs. In hydrogen charged specimen, role of PAGBs toward the resistance to crack growth was negligible that can be noticed by comparatively lesser decelerations under hydrogen charging condition in Fig. 4b. Transgranular crack propagation was dominating in uncharged specimen. However, under hydrogen environment, crack was found to propagate both intergranular and transgranular. Intergranular crack propagation occurred along the PAGBs of large size prior austenite grains whereas transgranular crack propagation occurred through small size prior austenite grains. 3.3. Effect of hydrogen on short fatigue crack growth in X65 and X80 steels Fig. 5 presents the effect of hydrogen on short fatigue crack growth on API line pipe grade X65 and X80 steels. Fatigue experiments for both the steels were conducted at stress range (∆𝜎𝜎) of 400 MPa. In uncharged condition, early crack initiation and comparatively slower propagation in X65 than X80 steel was observed (see Fig. 5a). Under hydrogen environment both the steels resulted in similar crack propagation rate, however like uncharged conditions early crack initiation in X65 than X80 steel was observed. The decelerations in uncharged specimen of X65 and X80 (see Fig. 5b) were in corresponding to the microstructural features such as grain boundary, phase boundary and crack branching. These decelerations were almost negligible under hydrogen charged specimens, hence confirmed the lesser resistance to crack propagation in hydrogen environment. Crack was found to propagate both inter and transgranular under charged and uncharged condition in both the steels. Decohesion at the interface of M/A stringers aligned with the direction of crack propagation was found to facilitate crack propagation under hydrogen charged and uncharged condition in both the steels. In general, the hydrogen charged specimen of both the steels showed high crack growth rate and lesser hindrance to the short fatigue crack propagation. Due to refined microstructure (in terms of grain size) not all grain/phase boundaries were capable to offer resistance to the propagation even in uncharged conditions.
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Vishal Singh et al. / Procedia Structural Integrity 13 (2018) 1427–1432 Author name / Structural Integrity Procedia 00 (2018) 000–000
Fig.5. Variation of (a) short fatigue crack length ‘𝑎𝑎𝑎 with numbers of cycles ‘𝑁𝑁𝑁 and (b) short crack growth rate ′𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑 with crack length ‘𝑎𝑎𝑎 of X65 and X80 steels at ∆𝜎𝜎 = 400 MPa. ‘H’ indicates the hydrogen charged sample.
4. Conclusions
The experimental framework to investigate the effect of hydrogen on various steels in terms of their short fatigue crack propagation behavior is presented. Hydrogen was found to accelerate the short fatigue crack growth in all type of investigated steels. In 316L steel, localized dislocation activity in the vicinity of the crack tip for hydrogen charged specimen increased the crack propagation rate. However, similar hindrance was offered by the grain boundaries for hydrogen charged and uncharged specimens. In the case of SA508 steel, hindrance to the crack propagation offered by PAGBs were found to diminish under hydrogen environment. Refined microstructure (in X65 and X80 steels) along with hydrogen environment was found to accelerate the fatigue crack growth propagation to greater extent. Presence of M/A islands and stringers specially those aligned with the crack propagation direction were found to facilitate the crack propagation through interface decohesion under both hydrogen charged and uncharged conditions. High resolution microscopy of the propagating short fatigue cracks in steels with refined microstructures demands further investigations. Such work is currently under progress using EBSD and fatigue stage for SEM in our group and shall be reported in future. References Kanezaki, T., Narazaki, C., Mine, Y., Matsuoka, S., Murakami, Y., 2008. Effects of Hydrogen on Fatigue Crack Growth Behavior of Austenitic Stainless Steels. International Journal of Hydrogen Energy 33, 2604 – 2619. Murakami, Y., Matsuoka, S., 2010. Effect of Hydrogen on Fatigue Crack Growth of Metals. Engineering Fracture Mechanics 77, 1926–1940. Chatzidouros, E.V., Papazoglou, V.J., Pantelis, D.I., 2014. Hydrogen Effect on a Low Carbon Ferritic-Bainitic Pipeline Steel. International Journal of Hydrogen Energy 39, 18498–18505. Venezuela, J., Liu, Q., Zhang, M., Zhou, Q., Atrens, A., 2015. The Influence of Hydrogen on the Mechanical and Fracture Properties of Some Martensitic Advanced High Strength Steels Studied Using the Linearly Increasing Stress Test. Corrosion Science 99, 98–117. Tsay, L.W., Chen, J.J., Huang, J.C., 2008. Hydrogen-Assisted Fatigue Crack Growth of AISI 316L Stainless Steel Weld. Corrosion Science 50, 2973–2980. Ogawa, Y., Okazaki, S., Takakuwa, O., Matsunaga, H., 2018. The Roles of Internal and External Hydrogen in the Deformation and Fracture Processes at the Fatigue Crack Tip Zone of Metastable Austenitic Stainless Steels. Scripta Materialia 157, 95–99. Obrtlık, K., Polak, J., Hajek, M., Vasek, A., 1997. Short Fatigue Crack Behaviour in 316L Stainless Steel. International Journal of Fatigue 19, 471– 475. Mikulich, V., Blochwitz, C., Skrotzki, W., Tirschler, W., 2006. Influence of texture on the short fatigue crack growth in austenitic stainless steel. Materials Science 42, 514–526. Mazánová, V., Polák, J., 2018. Initiation and Growth of Short Fatigue Cracks in Austenitic Sanicro 25 Steel, Fatigue & Fracture of Engineering Materials & Structures 41, 1529–1545. Krupp, U., 2007. Fatigue Crack Propagation in Metals and Alloys: Microstructural Aspects and Modelling Concepts. First Edition. Wiley-VCH Singh, R., Singh, A., Arora, A., K., Singh, P.K., Mahajan, D.K., 2018. On the Transition of Short Cracks into Long Fatigue Cracks in Reactor Pressure Vessel Steels. MATEC Web Conf. 165, 13001.
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Structural Integrity Procedia 00 (2018) 000–000 Available online www.sciencedirect.com Available online at at www.sciencedirect.com Structural Integrity Procedia 00 (2018) 000–000
ScienceDirect ScienceDirect
Procedia Structural Integrity 13 00 (2018) 1427–1432 Structural Integrity Procedia (2016) 000–000
www.elsevier.com/locate/procedia www.elsevier.com/locate/procedia
www.elsevier.com/locate/procedia
ECF22 - Loading and Environmental effects on Structural Integrity ECF22 - Loading and Environmental effects on Structural Integrity
Tracking hydrogen embrittlement using short fatigue crack behavior Tracking hydrogen embrittlement using short fatigue crack behavior XV Portuguese Conference on Fracture, of PCFmetals 2016, 10-12 February 2016, Paço de Arcos, Portugal of metals Vishal Singh, Rajwinder Singh,of Amanjot Dhiraj K. Mahajan* Thermo-mechanical modeling a highSingh, pressure turbine blade of an Vishal Singh, Rajwinder Singh, Amanjot Singh, Dhiraj K. Mahajan* Ropar Mechanics of Materials Laboratory, Department of Mechanical Engineering, Indian Institute of Technology Ropar, Rupnagar, Punjab, airplane gas turbine engine Ropar Mechanics of Materials Laboratory, Department of Mechanical Indian Institute of Technology Ropar, Rupnagar, Punjab, India,Engineering, 140001 India, 140001
P. Brandãoa, V. Infanteb, A.M. Deusc*
Abstract a Abstract Department of Mechanical Engineering, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1, 1049-001 Lisboa, Understanding hydrogen embrittlement phenomenon that leads Portugal to deterioration of mechanical properties of metallic components is b IDMEC, Department ofembrittlement Mechanical Engineering, Instituto Superior Universidade de Lisboa, Rovisco Pais, 1049-001 Lisboa, Understanding hydrogen phenomenon that leads to Técnico, deterioration of the mechanical properties of on metallic components is vital for applications involving hydrogen environment. Among these, understanding influence ofAv. hydrogen the1,fatigue behaviour Portugal vital for applications involving hydrogen environment. Among these, understanding the influence of hydrogen on the fatigue behaviour of metals is of great interest. Total fatigue life of a material can be divided into fatigue crack initiation and fatigue crack growth phase. c CeFEMA, of Mechanical Engineering, Instituto can Superior Técnico, Universidade de Lisboa, Av.and Rovisco Pais, 1, 1049-001phase. Lisboa, of metals is ofDepartment great interest. fatigue of athe material beof divided into fatigue crack While fatigue crack initiationTotal can be linkedlife with propagation short fatigue cracks, the initiation size of whichfatigue is of thecrack ordergrowth of grain size Portugal While fatigue crack initiation can be linked with the propagation of short fatigue cracks, the size of which is of the order of grain size (few tens of microns), that are generally not detectable by conventional crack detection techniques applicable for the long fatigue crack (few tens of microns), that are generally detectableExtensive by conventional crack detectionontechniques for the long crack fatiguegrowth crack growth behaviour using conventional CTnot specimens. literature is available hydrogenapplicable effect on long fatigue growth behaviour using CT specimens. Extensive is available on hydrogen long(∆𝐾𝐾 fatigue crack growth behaviour of metals thatconventional leads to the change in crack growth rateliterature and the threshold stress intensity effect factor on range ). However, it is Abstractof metals that leads to the change in crack growth rate and the threshold stress intensity factor range (∆𝐾𝐾�� ). However, it is behaviour the short fatigue crack growth behaviour that provides the fundamental understanding and correlation of the metallic �� microstructure the fatigue crack growthphenomenon. behaviour that provides fundamental understanding and correlation of the from metallic withshort hydrogen embrittlement Short fatiguethe crack growth behaviour is characteristically different longmicrostructure crack growth During their operation, modern aircraft components arebehaviour subjected to increasingly demanding operating with hydrogen embrittlement phenomenon. fatigue crack growth different from behaviour showing high propagation rate Short at engine much lower values than thresholdis characteristically stress intensity factor range aslong wellcrack as conditions, a growth strong especially the high pressure turbine (HPT) blades. Such conditions cause these parts to undergo different types of time-dependent behaviour showing high propagation rate at much lower values than threshold stress intensity factor range as well as aa strong dependency on the microstructural features such as grain boundaries, phase boundaries, and inclusions. To this end, novel degradation, one which is creep.features Ainvestigate model using finite element (FEM) was developed, inmaterials order beend, able atoin-situ predict dependency on theofmicrostructural such as the grain boundaries, phase boundaries, and charged inclusions. To to this novel experimental framework is developed to the short fatigue crackmethod behaviour of hydrogen involving the creep behaviour of HPT blades. Flight data records (FDR) for a specific aircraft, provided by a commercial aviation experimental framework is developed to investigate the short fatigue crack behaviour of hydrogen charged materials involving in-situ observation of propagating short cracks coupled with image processing to obtain their da/dN vs 𝑎𝑎 curves. Various metallic materials company, used stainless to short obtaincracks thermal and mechanical data for three different flight cycles. In order to alloy create the materials 3D observation propagating coupled with to obtain da/dNGrade vs 𝑎𝑎 curves. Various metallic ranging fromofwere austenitic steel (AISI 316L) to image reactorprocessing pressure vessel steeltheir (SA508 3 Class I low steel) and model line needed for the analysis, a studied HPT was scanned, and itssteel chemical andI low material were ranging from austenitic steel (AISIblade 316L) to reactor pressure vessel (SA508composition Grade 3 Class alloy properties steel) and line pipe steels (API 5LFEM X65stainless & X80) are in thisscrap work. obtained. The 5L data that&was wasinfed the FEM model and different simulations were run, first with a simplified 3D pipe steels (API X65 X80)gathered are studied thisinto work. block shape, in order better B.V. establish the model, and then with the real 3D mesh obtained from the blade scrap. The © rectangular 2018 The Authors. Published by to Elsevier © 2018 The Authors. Published by Elsevier B.V. overall expected behaviour in terms of displacement was observed, in particular at the trailing edge of the blade. Therefore such a © 2018 The Authors. Published by Elsevier B.V. Peer-reviewunder under responsibility of ECF22 the ECF22 organizers. Peer-review responsibility of the organizers. model can under be useful in the goal ofthe predicting turbine blade life, given a set of FDR data. Peer-review responsibility of ECF22 organizers. Keywords: Hydrogen embrittlement, short crack, fatigue, 316L, SA 508, X65, X80 © 2016 Hydrogen The Authors. Publishedshort by Elsevier B.V. 316L, SA 508, X65, X80 Keywords: embrittlement, crack, fatigue,
Peer-review under responsibility of the Scientific Committee of PCF 2016. Keywords: High Pressure Turbine Blade; Creep; Finite Element Method; 3D Model; Simulation.
* Corresponding author. Tel.: +91-7814252244 * Corresponding Tel.: +91-7814252244 E-mail address:author. [email protected] E-mail address: [email protected] 2452-3216 © 2018 The Authors. Published by Elsevier B.V. 2452-3216 © 2018 Authors. Published Elsevier B.V. Peer-review underThe responsibility of theby ECF22 organizers. Peer-review underauthor. responsibility the ECF22 organizers. * Corresponding Tel.: +351of 218419991. E-mail address: [email protected]
2452-3216 © 2016 The Authors. Published by Elsevier B.V.
Peer-review under responsibility of the Scientific Committee of PCF 2016. 2452-3216 2018 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the ECF22 organizers. 10.1016/j.prostr.2018.12.296
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Vishal Singh et al. / Procedia Structural Integrity 13 (2018) 1427–1432 Author name / Structural Integrity Procedia 00 (2018) 000–000
1. Introduction Hydrogen has been identified as one of the cleanest energy resources capable of reducing our dependency on fossil fuels. Many energy applications are already being explored to replace fossil fuels with hydrogen-based energy solutions (Kanezaki et al. (2008)). Despite knowing the effectiveness of hydrogen energy, its use on larger scale (that requires hydrogen storage and transportation) is restricted due to the well-known phenomenon of hydrogen embrittlement (HE). HE is an insidious failure mode that causes loss of structural integrity of steels (Murakami and Matsuoka (2010)). Eextensive research on the effect of hydrogen on steel performance is available describing the hydrogen embrittlement behavior of steels based on slow strain rate test (SSRT), linearly increasing stress test (LIST), fracture toughness testing and long fatigue crack growth testing (Chatzidouros et al. (2014); Venezuela et al. (2015); Tsay et al. (2008)). For designing steel components capable of delivering satisfactory performance over the years under hydrogen environment, reliable fatigue data of steels under hydrogen environment is most desired. Total fatigue life of a material can be divided into fatigue crack initiation and long fatigue crack growth phase. Literature available on the effect of hydrogen on fatigue behavior of steels generally discusses their long crack propagation behavior that leads to the change in crack growth rate and the threshold stress intensity factor range (∆𝐾𝐾�� ) (Tsay et al. (2008); Ogawa et al. 2018). However, major portion of fatigue life is consumed in crack initiation phase that includes fatigue crack nucleation and short fatigue crack propagation. The size of short fatigue cracks is of the order of grain size (few tens of microns), that are generally not detectable by conventional crack detection techniques applicable for the long fatigue crack growth behaviour (Krupp, (2007)). Short fatigue crack growth behaviour is characteristically different from long crack growth behaviour showing high propagation rate at much lower values than threshold stress intensity factor range as well as strong dependency on the microstructural features such as grain boundaries, phase boundaries, and inclusions. The short fatigue crack growth behaviour can provide fundamental understanding and correlation of the metallic microstructure with hydrogen embrittlement phenomenon. Effective microstructural engineering capable to arrest the propagation of these short fatigue cracks in steels under hydrogen environment needs detailed investigation regarding their interaction with various microstructural features such as phase/grain boundaries, grain orientation, inclusions etc. (Obrtlık et al. (1997); Mikulich et al. (2006); Mazánová et al. (2018)). Such investigations can help to improve the choice of steels and can open new trends toward the material engineering of steels with enhanced fatigue performance under hydrogen environment. Hence, a framework capable to explore the role of microstructural features on short fatigue crack propagation under hydrogen environment is desired. To this end, a novel experimental framework is developed to investigate the short fatigue crack behaviour of hydrogen charged steels involving in-situ observation of propagating short cracks coupled with image processing to obtain their da/dN vs 𝑎𝑎 curves. Microstructural dependent behaviour of short fatigue cracks elucidated in the present work will help to improve the material design for better performance under hydrogen environment. SA508 Grade 3 Class I low alloy steel applicable in nuclear reactor pressure vessel; two API line pipe steel grades X65 and X80, and a steel versatile in applications from fuel cell, nuclear power plant to line pipes for chemical, petrochemical i.e. austenitic stainless-steel grade 316L were studied in the present work. This paper consists of four sections. Following this brief introduction, Section 2 describes the experimental procedure. Results and discussions are provided in Section 3 followed by conclusions in Section 4. 2. Experimental procedure Chemical composition of investigated steels is presented in Table 1. To investigate the role of hydrogen on microstructural interaction of short fatigue crack growth, fatigue specimens of investigated steels were electrochemically charged by exposing to an electrolyte containing 1 N H2SO4 solution + 1.4 g/L Thiourea (charging promoter) under current density of 20 mA/cm2 for 4 hours. Specimen as cathode and platinum mesh as anode were used during hydrogen charging. In-situ investigations of short fatigue crack growth on single edge notch tension (SENT) specimen (with an initial notch size of ~70 µm for 316L and SA508 steel, and notch size ~50 µm for X65 and X80 steel) was conducted using digital microscope. Fatigue experimentations were conducted with R ratio 0.1 at frequency 35 Hz.
Vishal Singh et al. / Procedia Structural Integrity 13 (2018) 1427–1432 Author name / Structural Integrity Procedia 00 (2018) 000–000
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Table 1. Chemical composition of investigated steels Element (wt. %) 316L SA508 X65 X80
C
S
P
Mn
Si
Cr
Ni
Mo
Cu
Al
V
Nb
Fe
0.019 0.19 0.054 0.04
0.016 0.002 0.001 0.001
0.026 0.018 0.012 0.012
1.629 1.3 1.498 1.8
0.328 0.23 0.230 0.24
16.74 0.17 -0.05
10.26 0.7 -0.19
2.022 0.44 0.086 0.18
-0.13 -0.18
-0.02 0.029 --
--0.0355 0.018
--0.0572 0.482
Balance
The tests were interrupted at regular intervals (in terms of number of cycles) to take the images of the crack path in front of the notch. The detailed experimental procedure used for short fatigue crack propagation can be found elsewhere (Singh et al. (2018)). 3. Results and discussion Fig. 1 presents the microstructures of all the steels investigated in the present work. AISI 316L austenitic steel was characterized with an average grain size of ~50 µm. SA508 Grade 3 Class I steel was characterized as upper bainitic microstructure with an average grain size of ~20 um. API 5L X65 steel was composed of ferrite, pearlite and some bainite, with average grain size of ~10 um. API 5L X80 steel was containing ferrite and bainite with average grain size of ~6 um. Small fraction of martensite/austenite (M/A) islands and stringers were also obtained in SA508, X65 and X80 steels samples. Tensile characteristics of all the materials with and without hydrogen charging are presented in Table 2. Table 2. Tensile properties of investigated steels with and without hydrogen environment Material 316L SA508 X65 X80
YS, MPa 315 523 523 625
Uncharged UTS, MPa 541 663 610 665
Elongation, % 86 28 25 22
YS, MPa 303 540 552 650
(a)
(b)
(c)
(d)
Hydrogen charged UTS, MPa Elongation, % 567 81 642 22 601 21 670 12
Fig. 1. (a) Optical image of 316L and SEM images of (b) SA508, (c) X65, and (d) X80 steels
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Vishal Singh et al. / Procedia Structural Integrity 13 (2018) 1427–1432 Author name / Structural Integrity Procedia 00 (2018) 000–000
3.1. Effect of hydrogen on short fatigue crack growth in 316L steel Austenitic stainless steels are widely applicable in line pipes for hydrocarbons, chemicals and nuclear power industries. The hydrogen-based degradation of these steels is well accepted (Kanezaki et al. (2008)). Fig. 2a presents the variation of crack length (𝑎𝑎) with number of cycles (𝑁𝑁) for uncharged and hydrogen charged 316L steel obtained after fatigue loading at constant stress range (∆𝜎𝜎) of 270 MPa. Early initiation followed by accelerated crack growth in hydrogen charged specimen with increase in number of cycles can be observed in Fig. 2a. Shorter plateau regions (in 𝑎𝑎 vs 𝑁𝑁 curve, Fig. 2a) in hydrogen charged specimen than in uncharged 316L, confirmed lesser hindrances to short fatigue crack propagation from the microstructural features under hydrogen environment. Fig. 2b presents a comparison of short fatigue crack growth rate (𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑) as a function of crack length (𝑎𝑎) for hydrogen charged and uncharged 316L steel. The deceleration in the 𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑 curves correspond to the grain boundaries resisting the short cracks to propagate. Similar deceleration was provided by the grain boundaries for both hydrogen charged and uncharged specimens. However, the crack growth rate within the grain appears to be much faster in hydrogen charged specimen compared to uncharged specimen. This may be due to crack tip localized enhanced slip activities that resulted in an increased crack growth under hydrogen environment.
Fig. 2. Variation of (a) short fatigue crack length ‘𝑎𝑎𝑎 with numbers of cycles ‘𝑁𝑁𝑁 and (b) short crack growth rate ′𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑 with crack length ‘𝑎𝑎𝑎 of AISI 316L steel at ∆𝜎𝜎 = 270 MPa
For the uncharged specimen, slip bands were seen in multiple grains around the crack tip as well as away from the crack tip, as shown in Fig. 3a. On the other hand, for the hydrogen charged specimen, slip bands appeared in the vicinity of the crack tip with negligible slip bands formation away from crack tip. Some microcracks were also observed away from the crack tip for this specimen, as shown in Fig. 3b. For an uncharged specimen, the activation of slip systems led to the formation of slip bands in most of the grains. However, for the case of hydrogen charged specimen, hydrogen led to the pinning of dislocations hindering the slip activity. For this case, the stresses got relieved by microcracks generation away from crack tip. Due to localized slip activity, the crack propagation for this case appears to be more planar compared to the uncharged specimen.
Fig. 3. Optical micrographs of crack path ahead of notch in (a) uncharged and (b) hydrogen charged 316L specimens
Vishal Singh et al. / Procedia Structural Integrity 13 (2018) 1427–1432 Author name / Structural Integrity Procedia 00 (2018) 000–000
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3.2. Effect of hydrogen on short fatigue crack growth in SA508 Gr. 3 Cl. 1 low alloy steel Fig. 4 presents the comparison of short fatigue behavior of hydrogen charged and uncharged SA508 Gr. 3 Cl. 1 low alloy steel. Short fatigue crack growth investigations were conducted at stress range (∆σ) of 475 MPa. From Fig. 4a and b, it was concluded that the crack growth in SA508 hydrogen charged specimen was significantly higher than that of uncharged specimen.
Fig. 4. Variation of (a) short fatigue crack length ‘𝑎𝑎𝑎 with numbers of cycles ‘𝑁𝑁𝑁 and (b) short crack growth rate ′𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑 with crack length ‘𝑎𝑎𝑎 of SA508 sample at ∆𝜎𝜎 = 475 MPa
SEM analysis of uncharged SA508 specimen (not presented here) revealed that the prior austenite grain boundaries (PAGBs) offer the highest resistance to the short fatigue crack propagation. Uniformly distributed carbides were found to resist the crack growth more significantly after PAGBs. In hydrogen charged specimen, role of PAGBs toward the resistance to crack growth was negligible that can be noticed by comparatively lesser decelerations under hydrogen charging condition in Fig. 4b. Transgranular crack propagation was dominating in uncharged specimen. However, under hydrogen environment, crack was found to propagate both intergranular and transgranular. Intergranular crack propagation occurred along the PAGBs of large size prior austenite grains whereas transgranular crack propagation occurred through small size prior austenite grains. 3.3. Effect of hydrogen on short fatigue crack growth in X65 and X80 steels Fig. 5 presents the effect of hydrogen on short fatigue crack growth on API line pipe grade X65 and X80 steels. Fatigue experiments for both the steels were conducted at stress range (∆𝜎𝜎) of 400 MPa. In uncharged condition, early crack initiation and comparatively slower propagation in X65 than X80 steel was observed (see Fig. 5a). Under hydrogen environment both the steels resulted in similar crack propagation rate, however like uncharged conditions early crack initiation in X65 than X80 steel was observed. The decelerations in uncharged specimen of X65 and X80 (see Fig. 5b) were in corresponding to the microstructural features such as grain boundary, phase boundary and crack branching. These decelerations were almost negligible under hydrogen charged specimens, hence confirmed the lesser resistance to crack propagation in hydrogen environment. Crack was found to propagate both inter and transgranular under charged and uncharged condition in both the steels. Decohesion at the interface of M/A stringers aligned with the direction of crack propagation was found to facilitate crack propagation under hydrogen charged and uncharged condition in both the steels. In general, the hydrogen charged specimen of both the steels showed high crack growth rate and lesser hindrance to the short fatigue crack propagation. Due to refined microstructure (in terms of grain size) not all grain/phase boundaries were capable to offer resistance to the propagation even in uncharged conditions.
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Vishal Singh et al. / Procedia Structural Integrity 13 (2018) 1427–1432 Author name / Structural Integrity Procedia 00 (2018) 000–000
Fig.5. Variation of (a) short fatigue crack length ‘𝑎𝑎𝑎 with numbers of cycles ‘𝑁𝑁𝑁 and (b) short crack growth rate ′𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑 with crack length ‘𝑎𝑎𝑎 of X65 and X80 steels at ∆𝜎𝜎 = 400 MPa. ‘H’ indicates the hydrogen charged sample.
4. Conclusions
The experimental framework to investigate the effect of hydrogen on various steels in terms of their short fatigue crack propagation behavior is presented. Hydrogen was found to accelerate the short fatigue crack growth in all type of investigated steels. In 316L steel, localized dislocation activity in the vicinity of the crack tip for hydrogen charged specimen increased the crack propagation rate. However, similar hindrance was offered by the grain boundaries for hydrogen charged and uncharged specimens. In the case of SA508 steel, hindrance to the crack propagation offered by PAGBs were found to diminish under hydrogen environment. Refined microstructure (in X65 and X80 steels) along with hydrogen environment was found to accelerate the fatigue crack growth propagation to greater extent. Presence of M/A islands and stringers specially those aligned with the crack propagation direction were found to facilitate the crack propagation through interface decohesion under both hydrogen charged and uncharged conditions. High resolution microscopy of the propagating short fatigue cracks in steels with refined microstructures demands further investigations. Such work is currently under progress using EBSD and fatigue stage for SEM in our group and shall be reported in future. References Kanezaki, T., Narazaki, C., Mine, Y., Matsuoka, S., Murakami, Y., 2008. Effects of Hydrogen on Fatigue Crack Growth Behavior of Austenitic Stainless Steels. International Journal of Hydrogen Energy 33, 2604 – 2619. Murakami, Y., Matsuoka, S., 2010. Effect of Hydrogen on Fatigue Crack Growth of Metals. Engineering Fracture Mechanics 77, 1926–1940. Chatzidouros, E.V., Papazoglou, V.J., Pantelis, D.I., 2014. Hydrogen Effect on a Low Carbon Ferritic-Bainitic Pipeline Steel. International Journal of Hydrogen Energy 39, 18498–18505. Venezuela, J., Liu, Q., Zhang, M., Zhou, Q., Atrens, A., 2015. The Influence of Hydrogen on the Mechanical and Fracture Properties of Some Martensitic Advanced High Strength Steels Studied Using the Linearly Increasing Stress Test. Corrosion Science 99, 98–117. Tsay, L.W., Chen, J.J., Huang, J.C., 2008. Hydrogen-Assisted Fatigue Crack Growth of AISI 316L Stainless Steel Weld. Corrosion Science 50, 2973–2980. Ogawa, Y., Okazaki, S., Takakuwa, O., Matsunaga, H., 2018. The Roles of Internal and External Hydrogen in the Deformation and Fracture Processes at the Fatigue Crack Tip Zone of Metastable Austenitic Stainless Steels. Scripta Materialia 157, 95–99. Obrtlık, K., Polak, J., Hajek, M., Vasek, A., 1997. Short Fatigue Crack Behaviour in 316L Stainless Steel. International Journal of Fatigue 19, 471– 475. Mikulich, V., Blochwitz, C., Skrotzki, W., Tirschler, W., 2006. Influence of texture on the short fatigue crack growth in austenitic stainless steel. Materials Science 42, 514–526. Mazánová, V., Polák, J., 2018. Initiation and Growth of Short Fatigue Cracks in Austenitic Sanicro 25 Steel, Fatigue & Fracture of Engineering Materials & Structures 41, 1529–1545. Krupp, U., 2007. Fatigue Crack Propagation in Metals and Alloys: Microstructural Aspects and Modelling Concepts. First Edition. Wiley-VCH Singh, R., Singh, A., Arora, A., K., Singh, P.K., Mahajan, D.K., 2018. On the Transition of Short Cracks into Long Fatigue Cracks in Reactor Pressure Vessel Steels. MATEC Web Conf. 165, 13001.