NANOMECHANICAL TESTING OF HYDROGEN EFFECTS ON SUPER DUPLEX STAINLESS STEEL

Transcription

1 Mehanial Testing and Diagnosis ISSN , 2012 (II), Volume 2, 5-14 NANOMECHANICAL TESTING OF HYDROGEN EFFECTS ON SUPER DUPLEX STAINLESS STEEL Adina BASA 1), Afrooz BARNOUSH 2), Christian THAULOW 1) 1) NTNU, Department of Engineering Design and Materials, Trondheim, NORWAY, 2) Saarland University, Department of Material Siene, Saarbrueken, GERMANY ABSTRACT The effet of hydrogen on the mehanial properties of the super duplex stainless steel is examined using in situ eletrohemial nanoindentation (ECNI) tests. Within the ECNI, whih is a nanoindenter ombined with an eletrohemial setup, the nanoindentation an be made on a surfae that is immersed in an eletrolyte and in situ eletrohemially harged with hydrogen. In situ eletrohemial nanoindentation testing aptures the hange in the onset of plastiity (pop-in load level) as well as the hange in the hardness due to the absorption of the atomi hydrogen. Keywords: Nanoindentation, hydrogen, duper duplex stainless steel, pop-in 1. INTRODUCTION Over the last years, the interation of steels with hydrogen has led to many inidents, sometimes ausing atastrophi failures. The main soures for hydrogen are the orrosion from the aqueous solutions, the athodi protetion and the ontaminants in the melting and welding proesses. Different mehanisms for hydrogen have been proposed, but the most established ones are Hydrogen Enhaned Deohesion (HEDE) and Hydrogen Enhaned Loal Plastiity (HELP). The HEDE mehanism (the brittle frature) suggests that the hydrogen aumulated within the atomi lattie redues the ohesive bonding strength and it was first proposed by Troiano [1]. The HELP mehanism (the dutile frature) proposes that the atomi hydrogen enhanes the mobility of the disloations ausing a lowering in the shear strength and it was desribed for the first time by Birnbaum and Sofronis [2]. The Super Duplex Stainless Steel (SDSS) is a widely used material in offshore appliations due to its high strength and toughness and exellent orrosion resistane [3] given by the two phases, austenite (γ) and ferrite (α). The non-magneti dutile austenite has a Faed-Centred Cubi (FCC) struture and it is ating like a rak stopper while the ferrite is more brittle due to its Body-Centred Cubi (BCC) struture. Despite these strong points, SDSS has often failed due to the hydrogen released during the athodi protetion leading to the hydrogen embrittlement. The main diffusion mehanism for hydrogen in

2 6 Mehanial Testing and Diagnosis, ISSN , 2012 (II), Volume 2, 5-14 steel is the lattie diffusion by the interstitial jumps. The higher paking density of the austenite (0.74 as ompared to 0.68 of the ferrite) [4] and its lose paked lattie struture gives a higher solubility of the hydrogen atoms and a lower diffusion rate, while the ferrite is haraterized by a higher diffusion rate and a lower solubility due to its open lattie struture. Although the onentration of hydrogen may be higher in the austenite, the ferrite is prone to the rak propagation at lower hydrogen onentration [5]. Even though many experimental studies [6, 7] and simulations [8, 9] of the hydrogen effet on SDSS have been done, a more omplete understanding of the miro-mehanism is needed. A starting point for that is to provide a better haraterization of the hydrogen effet on eah individual phase and this an be done by the nanoindentation in ombination with an eletrohemial setup desribed below. The nanoindentation is a widely used method [10,11] for studing the miromehanial properties of the materials. Reently, Barnoush et al. [12, 13] used the nanoindenter in ombination with an eletrohemial setup to investigate the effet of hydrogen on the miromehanial properties of different materials under hydrogen harging onditions. This is a promising method beause the immediate effet of hydrogen on the loal material miroonstituents an be observed. Another advantage of using in-situ ECNI is related to the testing time, whih is onsiderably lowered and limited to some hours while the nanoindentations are performed. Also, another big advantage is that the surfae quality remains the same during testing, while in the ase of ex-situ ECNI, either the surfae is damaged [14] by the hydrogen during the long time harging or the hydrogen diffuses out of the material while the sample is transferred in between the eletrohemial setup and the nanoindenter [15]. In-situ ECNI method is used in this paper and the authors would like to mention that the aim of this paper is primarily to desribe the experimental proedure and to present only some preliminary results. Fig. 1. The ross-setional area of a nanoindentation During a nanoindentation, an indenter tip is fored into the sample with a defined load. The maximum fore is kept onstant for a few seonds and then dereased, see Fig. 1. A load displaement urve is produed, see Fout! Verwijzingsbron niet gevonden., whih is used to alulate the hardness and the redued modulus of elastiity. Three-sided pyramidal tips are standard for the nanoindentations [16]. The standard three-sided tip is the Berkovih tip, whih has a total inluded angle from plane to edge of and a half angle, θ, of 65.35, see Fig. 3. The hardness is alulated aording to: Pmax H (1) A where P - the maximum applied load, A - the ontat area, alulated from the tip area max funtion, based on the ontat depth, h.

3 Mehanial Testing and Diagnosis, ISSN , 2012 (II), Volume 2, Fig. 2. Load displaement urve produed during the nanoindentation Fig. 3. Three-sided pyramidal Berkovih tip For a perfet Berkovih indenter [10]: h h h A P S (2) max hmax (3) where max - the maximum displaement, - the geometri onstant equal to 0.75, S - the stiffness of the material. In reality, an indenter tip will never be perfet, so, eah tip will be alibrated aording to its own tip area funtion. The proedure involves performing a series of 25 up to 100 indents of different ontat depths (varying the loading levels) on a fused quartz sample with a know redued modulus of 69.6 GPa. The ontat area will be determined by measuring the stiffness based on the following equation: 2 S A (4) 4 Er where: E - the redued modulus of elastiity. r Then, the ontat area will be plotted as a funtion of the ontat depth and the points will be fitted to a sixth order polynomial: 2 1/ 2 1/ 4 A C0h C1h C2h C3h (5) 1/ 8 1/ 16 C h C h 4 5 where: C0 - equal to 24.5 for a Berkovih tip, C 1 to C5 - the urve fitting parameters. The redued modulus of elastiity is alulated as: S Er (6) 2 A The relation between the redued modulus and the elasti modulus is: s 1- i - (7) E E E r s i where: - the Poisson ratio for the sample, respetively, the indenter; E - the elasti modulus of the sample, respetively, the indenter. For a standard diamond indenter tip E 1140GPa and i i

4 8 Mehanial Testing and Diagnosis, ISSN , 2012 (II), Volume 2, EQUIPMENT AND SAMPLE PREPARATION The experiments were performed with a Hysitron TriboSan TI-750 with Performeh ontroller in ombination with an eletrohemial setup as shown in Fig. 4. A diamond Berkovih long tip, speially designed for testing inside the eletrolyte was used. The used load funtion is presented in Fig. 5, where the last segment of 1 seond holding time at 10% peak value was added for the drift orretion. Fig. 4. The eletrohemial setup Fig. 5. Load funtion As a ounter eletrode for the eletrohemial setup, a platinum wire was used while a Saturated Calomel Eletrode (SCE) was the referene eletrode. The sample holder allows the sample to be overed with the eletrolyte during testing. A oarse grained SDSS with a hemial omposition of 0.016% C, 0.46% Mn, 0.24% Si, 0.024% P, 0.001% S, 7.16% Ni, 25.22% Cr, 3.76% Mo, 0.276% N, 0.205% Cu and a Pitting Resistane Equivalent Number (PRE N ) equal to has been investigated. The high pitting resistane is given by the high levels of hromium, molybdenum and nitrogen, as is alulated aording to: PRE % Cr 3.3 % Mo 16 % N (8) N The marosopi yield strength for this material is 560 MPa and the tensile strength is 790 MPa. These exellent properties are given by the dual phase mirostruture of the austenite (γ) and ferrite (α), Fig. 6.

5 Mehanial Testing and Diagnosis, ISSN , 2012 (II), Volume 2, Fig. 6. SEM image of the oarse grained super duplex stainless steel The starting point for the sample preparation was the grinding with silion-arbide papers of grade 500, 1000 and 2400, followed by a mehanial polishing with a water based diamond suspension of 3 µm and 1 µm. The last step was the eletropolishing in order to remove the work hardened mirosopi leyer of the material aused by the mehanial polishing. Table 1 presents the eletrolyte and the used eletropolishing parameters [14]. Table 1. Parameters used for the eletropolishing Eletrolyte Pot., V Flow rate Time, s Temp., C Methanol/H 2 SO The surfae quality after the eletropolishing is presented in Fig. 7. The Average Roughness (RA) is less than 10 nm, as given by the nanoindenter software. Fig. 7. Topography image, sanned with TI-750 prior testing, where the ferrite is light and the austenite is dark 3. EXPERIMENTAL RESULTS Freshly eletropolished samples were tested first in air and then in a 0.05 M Na 2 SO 4 eletrolyte. Hydrogen started to form when a athodi potential of mv was applied.

6 10 Mehanial Testing and Diagnosis, ISSN , 2012 (II), Volume 2, 5-14 Both the austenite and the ferrite phases were tested with a maximum load of 4000 µn and a loading rate of 2000 µn/s. Fig. 8 and Fig. 9 present the effet of hydrogen on the hardness and on the redued modulus of elastiity, respetively. Fig. 8. The hydrogen effet on the sample hardness Fig. 9. The hydrogen effet on the redued modulus of elastiity The effet of hydrogen on the hardness is larger for the austenite than for the ferrite, the differenes in hardness, before and after hydrogen harging, being of about 1 GPa for the austenite and of 0.6 GPa for the ferrite. A higher inrease in the hardness for the austenite is due to a higher solubility of hydrogen in the austenite as omparing to the ferrite. From Fig. 9, we an disregard the effet of hydrogen sine the inrease of the redued modulus of elastiity is within the measurement error and it is not sure that is due to the presene of hydrogen. Another effet of hydrogen evolved during the athodi potential is on the onset of plastiity. Fig. 11 presents typial load displaement urves and the differene between the pop-in load levels, when the nanoindents are made with and without hydrogen. Sine more than 30 nanoindentations were made for eah phase and eah testing ondition in order to have repeatability, the pop-in event frequenies are presented in Fig. 10. A dereasing in pop-in load level was observed for both the austenite and the ferrite phases. The elasti part of the load displaement urves presented in Fig. 11 an be fitted to the Hertzian equation: 1.5 P 1.33E h R (9) r

7 Mehanial Testing and Diagnosis, ISSN , 2012 (II), Volume 2, where: P - the applied load, h - the indentation depth, R - the radius of the indenter tip, Er - redued modulus of the sample, given by the Eq. (7). Fig. 10. Pop-in event frequenies for (a) austenite, (b) ferrite Fig. 11. The redution in the onset of plastiity (pop-in load level) due to hydrogen

8 12 Mehanial Testing and Diagnosis, ISSN , 2012 (II), Volume 2, 5-14 During a nanoindentation, the indenter tip will approah the sample with a ertain veloity and as soon as the ontat is established, the initial elasti loading begins until the first disloation nuleation (or pop-in) ours. The stress field underneath the indenter tip during the elasti deformation is desribed via ontinuum mehanis, assuming that the indenter tip is spherial [17]. When this assumption is made, the maximum shear stress under the indenter tip is: 2 6E 3 r P max 0.31 (10) 3 2 R Aording to ontinuum mehanis, the maximum shear stress ours at a distane of approximately 0.48 times the ontat radius, r, diretly below the enter axis of the ontat, between the sample and the indenter tip [17]: z 0.48 r (11) max If the ontat radius between the indenter tip and the sample is [17]: R 6E P 2 3 r 3 2 2Er R r replaing Eq. (12) in Eq. (11), the position of the maximum shear stress, z τ(max) an be alulated as: z 0.48 r PR (12) 3 max (13) 4Er The maximum shear stress ating at the z max is responsible for the homogeneous disloation nuleation in the volume below the surfae and is in the order of the theoretial strength of a defet free material, aording to the Frenkel model [18]: G G th (14) 2 10 where G is the shear modulus. Now, we an equate the measured maximum shear stress during pop-in to the theoretial strength of the austenite or the ferrite phase in Eq. (14). This relates any hange in the pop-in load to a hange in the shear modulus of the given phase in SDSS as a result of hydrogen. In other words, hydrogen not only failitates the disloation nuleation, but also redues the lattie ohesion [19]. A deeper analysis of the pop-in behavior was made using a pop-in finder program developed by Barnoush [14]. The program is using the load displaement urves imported from the Hysitron software and is finding the pop-in from the analysis of the regions where the displaement is onstant. Further, based on the Hertzian fit, Eq. (9) the tip radius is found and used for alulating the position of the maximum shear stress and its value, Eq. (10) to (13). Fig. 12 presents the derease in both pop-in width and pop-in load for the austenite and the ferrite, in the presene of hydrogen. The pop-in i.e. homogeneous disloation nuleation in the austenite requires higher load levels than the ferrite, but the width of the pop-in is smaller. The pop-in width in the austenite is redued to half in the presene of hydrogen while for the ferrite, the dereasing is around one third.

9 Mehanial Testing and Diagnosis, ISSN , 2012 (II), Volume 2, Fig. 12. Pop-in width and pop-in load from the pop-in analysis Table 2 presents a summary of the nanomehanial properties of the SDSS, extrated from the load-displaement urves and from the pop-in finder program. The maximum shear stresses are higher when hydrogen is present in the material and loser to the surfae. It should be mentioned that in the SDSS, in order to have both phases in quasi equilibrium with eah other, it is neessary to quenh them from about 1100 C down to the room temperature. This results in the formation of very high tensile stresses in the austenite and ompressive stresses in the ferrite [20]. This is very important to onsider the effet of these loal high residual stresses on hydrogen uptake and its effet on the measured nanomehanial properties. Therefore, future works are planned to study the effet of hydrogen on the nanomehanial response of the austenite and the ferrite phases in ombination with these residual stresses. 4. CONCLUSIONS AND FURTHER WORK The inrease in the hardness and the derease in the pop-in load level of both the austenite and the ferrite phases were observed due to hydrogen evolution after the athodi potential was applied to the sample. While the derease in the pop-in an be related to the hydrogen effet on the interatomi potential, the ohesion the effet of hydrogen on hardness is in agreement with the hydrogen pinning effet on the disloations. Table 2. Summary of the nanomehanial properties of SDSS Load displaement Pop-in analysis urves H (GPa) Er (GPa) S (µn/nm) Start load (µn) Start depth (nm) Width (nm) Maximum shear stress z τ(max) (nm) (GPa) Austenite_air Austenite_hydrogen Ferrite_air Ferrite_hydrogen The Super Duplex Stainless Steel is a very omplex material as well as is the hydrogen embrittlement miromehanism and further investigations have to be made. Further work will fous on the hydrogen effet on the grains with different rystallographi orientations. Also, different loading rates will be onsidered. τ max

10 14 Mehanial Testing and Diagnosis, ISSN , 2012 (II), Volume 2, 5-14 REFERENCES 1. Troiano, A.R., 1960, The role of hydrogen and other interstitials in the mehanial behaviour of metals, Trans ASM, 52, pp Birnbaum H.K., Sofronis P., 1994, Hydrogen-enhaned loalized plastiity-a mehanism for hydrogen-related frature, Materials Siene and Engineering: A, 176(1-2), pp Nilsson J.O., 1992, Super duplex stainless steels, Material Siene and Tehnology, 8(8). 4. Krauss G., 2005, Steels: proessing, struture, and performane. 5. Zakrozymski T., Owzarek E., 2002, Eletrohemial investigation of hydrogen absorption in a duplex stainless steel, Ata Materialia, 50(10), pp Oltra R., Bouillot C., Magnin T., 1996, Loalized hydrogen raking in the austeniti phase of a duplex stainless steel, Sripta Materiale, 35(9), pp Johnsen R., Nyhus B., Wästberg S., Lauvstad G.O., 2007, New Improved Method For His Testing Of Stainless Steels Under Cathodi Protetion. Corrosion, paper no Olden V., Thaulow C., Johnsen, R., Østby E., Berstad T., 2009, Influene of hydrogen from athodi protetion on the frature suseptibility of 25%Cr duplex stainless steel - Constant load SENT testing and FE-modelling using hydrogen influened ohesive zone elements. Engineering Frature Mehanis, 76(7), pp Olden, V., Thaulow, C., Johnsen R., Ostby E., 2007, Cohesive zone modeling of hydrogenindued stress raking in 25% Cr duplex stainless steel. Sripta Materialia, 57(7), pp Oliver W.C., Pharr G.M., 1992, An improved tehnique for determining hardness and elasti modulus. J. Mater. Res., 7(6). 11. Fang, T., Chang W., Tsai S., 2005, Nanomehanial haraterization of polymer using atomi fore mirosopy and nanoindentation. Miroeletronis Journal, 36(1), pp Barnoush A., Vehoff H., 2006, Eletrohemial nanoindentation: A new approah to probe hydrogen/deformation interation, Sripta Materialia, 55(2), pp Barnoush A., Vehoff H., 2008, In situ eletrohemial nanoindentation: A tehnique for loal examination of hydrogen embrittlement, Corrosion Siene, 50(1), pp Barnoush A., 2008, Hydrogen embrittlement, revisited by in situ eletrohemial nanoindentation, PhD Dissertation. 15. Øverland M., 2007, Hydrogen indued stress raking in super duplex stainless steel, Master thesis. 16. Hysitron, 2009, TI-750 Ubi User Manual NRL-M-201 r (TriboSan 9.1). 17. Johnson K.L., 1985, Contat Mehanis. Cambridge Press, pp Frenkel, J., 1926, Zur Theorie der Elastizitätsgrenze und der Festigkeit kristallinisher Körper, Zeitshrift für Physik A Hadrons and Nulei, Springer Berlin / Heidelberg, 37: pp Barnoush A., Vehoff H., 2010, Reent developments in the study of hydrogen embrittlement: Hydrogen effet on disloation nuleation, Ata Materialia, 58(16), pp Barnoush A., Zamanzade M., Vehoff H., 2010, Diret observation of hydrogen-enhaned plastiity in super duplex stainless steel by means of in situ eletrohemial methods. Sripta Materialia, 62(5), pp