NANOMECHANICAL TESTING OF HYDROGEN EFFECTS ON SUPER DUPLEX STAINLESS STEEL

Transcription

1 The 3 d Intenational Confeene on DIAGNOSIS AND PREDICTION IN MECHANICAL ENGINEERING SYSTEMS DIPRE 1 NANOMECHANICAL TESTING OF HYDROGEN EFFECTS ON SUPER DUPLEX STAINLESS STEEL Adina BASA 1, Afooz BARNOUSH, Chistian THAULOW 1 1. NTNU, Depatment of Engineeing Design and Mateials, NO-7491 Tondheim, NORWAY,. Saaland Univesity, Depatment of Mateial Siene, P.O. Box , D/66041 Saabueken, GERMANY adina.basa@ntnu.no, a.banoush@matsi.uni-sb.de ABSTRACT The effet of hydogen on mehanial popeties of supe duplex stainless steel is examined using in situ eletohemial nanoindentation (ECNI) tests. Within the ECNI whih is a nanoindente ombined with an eletohemial setup, nanoindentation an be made on a sufae that is immesed in eletolyte and in situ eletohemially haged with hydogen. In situ eletohemial nanoindentation testing aptues the hange in the onset of plastiity (pop-in load level) as well as the hange in hadness due to absoption of atomi hydogen. Keywods: Nanoindentation, Hydogen, Supe Duplex Stainless Steel, Pop-in. 1. INTRODUCTION Ove the last yeas the inteation of steels with hydogen has led to many inidents, sometimes ausing atastophi failues. The main soues fo hydogen ae oosion fom aqueous solutions, athodi potetion and ontaminants in the melting and welding poesses. Diffeent mehanisms fo hydogen have been poposed, but the most established ones ae Hydogen Enhaned Deohesion (HEDE) and Hydogen Enhaned Loal Plastiity (HELP). The HEDE mehanism (bittle fatue) suggests that hydogen aumulated within the atomi lattie edues ohesive bonding stength and it was fist poposed by Toiano [1]. The HELP mehanism (dutile fatue) poposes that atomi hydogen enhanes the mobility of disloations ausing a loweing in shea stength and it was desibed fo the fist time by Binbaum and Sofonis []. Supe Duplex Stainless Steel (SDSS) is a widely used mateial in offshoe appliations due to its high stength and toughness and exellent oosion esistane [3] given by the two phases, austenite (γ) and feite (α). Non-magneti dutile austenite has a Faed-Cented Cubi (FCC) stutue and it is ating like a ak stoppe while the feite is moe bittle due to its Body-Cented Cubi (BCC) stutue. Despite these stong points, SDSS has often failed due to hydogen eleased duing athodi potetion leading to hydogen embittlement. The main diffusion mehanism fo hydogen in steel is lattie diffusion by intestitial jumps. The highe paking density of austenite (0.74 ompaed to 0.68 of feite) [4] and its lose paked lattie stutue gives a highe solubility of hydogen atoms and a lowe diffusion ate, while feite is haateized by a highe diffusion ate and a lowe solubility due to its open lattie stutue. Although the onentation of hydogen may be highe in austenite, the feite is pone to ak popagation at lowe hydogen onentation [5]. Even though many expeimental studies [6, 7] and simulations [8, 9] of hydogen effet on SDSS have been done, a moe omplete undestanding of the mio-mehanism is needed. A stating point fo that is to povide a bette haateization of hydogen effet on eah individual phase and this an be done by nanoindentation in ombination with an eletohemial setup desibed below. Nanoindentation is a widely used method [10,11] fo studies of miomehanial popeties of mateials. Reently Banoush et al. [1, 13] used the nanoindente in ombination with an eletohemial setup to investigate the effet of hydogen on The 3 d Intenational Confeene on 1

2 miomehanial popeties of diffeent mateials unde hydogen haging onditions. This is a pomising method beause the immediate effet of hydogen on loal mateial mioonstituents an be obseved. Anothe advantage of using in-situ ECNI is elated to testing time whih is onsideably loweed and limited to some hous while the nanoindentations ae pefomed. Also anothe big advantage is that the sufae quality emains the same duing testing, while in the ase of ex-situ ECNI the sufae eithe is damaged [14] by the hydogen duing long time haging o the hydogen diffuse out of mateial while the sample is tansfeed in between the eletohemial setup and the nanoindente [15]. Insitu ECNI method is used in this pape and the authos would like to mention that the aim of this pape is pimaily to desibe the expeimental poedue and only pesent peliminay esults. Duing a nanoindentation, an indente tip is foed into the sample with a defined load. Maximum foe is kept onstant fo a few seonds and then deeased, see Fig. 1. A load displaement uve is podued, see Fig., whih is used to alulate the hadness and the edued modulus of elastiity. Fig. 1. The oss-setional aea of a nanoindentation Fig.. Load displaement uve podued duing nanoindentation Thee-sided pyamidal tips ae the standad fo nanoindentations [16]. The standad thee-sided tip is the Bekovih tip, whih has a total inluded angle fom plane to edge of 14.3 and a half angle, θ of 65.35, see Fig. 3. Fig. 3. Thee-sided pyamidal Bekovih tip The hadness is alulated aoding to: P H (1) A P - imum applied load; A - ontat aea, alulated fom the tip aea funtion based on the ontat depth, h. Fo a pefet Bekovih indente [10]: A 4.5h P h S h - the imum displaement; () h (3) - geometi onstant equal to 0.75; S - the stiffness of the mateial. In eality an indente tip will neve be pefet so eah tip will be alibated aoding to its own tip aea funtion. The poedue involves pefoming a seies of 5 up to 100 indents of diffeent ontat depths (vaying the loading levels) on a fused quatz sample with a know edued modulus of 69.6 GPa. The ontat aea will be detemined fom measuing the stiffness based on the following equation: S A 4 E E - edued modulus of elastiity. (4) The ontat aea will be then plotted as a funtion of ontat depth and the points will be fitted to a sixth ode polynomial: The 3 d Intenational Confeene on

3 A C h C h C h C h 1/ 1/ / 8 1/ 16 C4h C5h C - equal to 4.5 fo a Bekovih tip; as: 0 (5) C 1 to C5 - uve fitting paametes. The edued modulus of elastiity is alulated S E (6) A The elation between the edued modulus and the elasti modulus is: 1 1- s 1- i - (7) E E E s i - the Poisson atio fo the sample, espetively indente; E - elasti modulus of the sample, espetively indente. Fo a standad diamond indente tip 1140GPa and i Ei. EQUIPMENT AND SAMPLE PREPARATION The expeiments wee pefomed with a Hysiton TiboSan TI-750 with Pefomeh ontolle in ombination with an eletohemial setup as shown in Fig. 4. A diamond Bekovih long tip designed speial fo testing inside eletolyte was used. The load funtion used is pesented in Fig. 5, whee the last segment of 1 seond holding time at 10% peak value was added fo dift oetion. Fig. 5. Load funtion As a ounte eletode fo the eletohemial setup a platinum wie was used while a Satuated Calomel Eletode (SCE) was the efeene eletode. The sample holde allows the sample to be oveed with eletolyte duing testing. A oase gained SDSS with a hemial omposition of 0.016% C, 0.46% Mn, 0.4% Si, 0.04% P, 0.001% S, 7.16% Ni, 5.% C, 3.76% Mo, 0.76% N, 0.05% Cu and a Pitting Resistane Equivalent Numbe (PRE N ) equal to has been investigated. The high pitting esistane is given by the high levels of homium, molybdenum and nitogen as is alulated aoding to: PRE % C 3.3 % Mo 16 % N (8) N The maosopi yield stength fo this mateial is 560 MPa and the tensile stength is 790 MPa. These exellent popeties ae given by the dual phase miostutue of austenite (γ) and feite (α), Fig. 6. Fig. 6. SEM image of oase gained supe duplex stainless steel Fig. 4. The eletohemial setup The stating point fo sample pepaation was ginding with silion-abide papes at gade 500, 1000 and 400 followed by mehanial polishing with wate based diamond suspension at 3 and 1 µm. The last step was eletopolishing to emove the wok hadened miosopi leye of mateial aused by The 3 d Intenational Confeene on 3

4 mehanial polishing. Table 1 pesents the eletolyte and the eletopolishing paametes used [14]. Table 1. Paametes used fo eletopolishing Eletolyte Pot., V Flow ate Time, s Temp., C Methanol/H SO The sufae quality afte eletopolishing is pesented in Fig. 7. The Aveage Roughness (RA) is less than 10 nm as given by the nanoindente softwae. Fig. 7. Topogaphy image sanned with TI-750 pio testing, whee feite is light and austenite is dak 3. EXPERIMENTAL RESULTS Feshly eletopolished samples wee tested fist in ai and then in a 0.05 M Na SO 4 eletolyte. Hydogen stated to fom when a athodi potential of mv was applied. Both austenite and feite phases wee tested with a imum load of 4000 µn and a loading ate of 000 µn/s. Fig. 8 and Fig. 9 pesent the effet of hydogen on hadness and on edued modulus of elastiity espetively. Fig. 9. The hydogen effet on edued modulus of elastiity The effet of hydogen on hadness is lage fo austenite than fo feite, the diffeenes in hadness befoe and afte hydogen haging being of about 1 GPa fo austenite and 0.6 GPa fo feite. A highe inease in hadness fo austenite is due to a highe solubility of hydogen in austenite ompaing with feite. Fom Fig. 9 we an disegad the effet of hydogen sine the inease of edued modulus of elastiity is within the measuement eo and is not sue that is due to the pesene of hydogen. Anothe effet of hydogen evolved duing athodi potential is on the onset of plastiity. Fig. 10 pesents typial load displaement uves and the diffeene between the pop-in load level when the nanoindents ae made with and without hydogen. Sine moe than 30 nanoindentations wee made fo eah phase and eah testing ondition in ode to have epeatability, the pop-in event fequenies ae pesented in Fig. 11. A deeasing in pop-in load level was obseved fo both austenite and feite phases. The elasti pat of the load displaement uves pesented in Fig. 10 an be fitted to the Hetzian equation: 1.5 P 1.33E h R (9) P - the applied load; h - the indentation depth; R - the adius of the indente tip; E - edued modulus of the sample, given by the Eq.(7). Fig. 8. The hydogen effet on hadness 4 The 3 d Intenational Confeene on

5 The 3 d Intenational Confeene on DIAGNOSIS AND PREDICTION IN MECHANICAL ENGINEERING SYSTEMS DIPRE 1 Fig. 10. The edution in the onset of plastiity (pop-in load level) due to hydogen Fig. 11. Pop-in event fequenies fo (a) austenite, (b) feite Duing a nanoindentation the indente tip will appoah the sample with a etain veloity and as soon as the ontat is established the initial elasti loading begins until the fist disloation nuleation (o pop-in) ous. The stess field undeneath the indente tip duing elasti defomation is desibed via ontinuum mehanis, assuming that the indente The 3 d Intenational Confeene on tip is spheial [17]. When this assumption is made, the imum shea stess unde the indente tip is: 6E P (10) R Aoding to ontinuum mehanis the imum shea stess ou at a distane of appoximately 0.48 times the ontat adius,, dietly below the ente axis of ontat between the sample and the indente tip [17]: z 0.48 (11) If the ontat adius between the indente tip and the sample is [17]: R 6E P 3 3 E R (1) Replaing Eq. (1) in Eq. (11) the position of the imum shea stess, z τ() an be alulated as: z PR 3 (13) 4E The imum shea stess ating at the z is esponsible fo homogeneous disloation nuleation in volume below the sufae and is in the ode of the theoetial stength of a defet fee mateial aoding to the Fenkel model [18]: G G th (14) 10 G - the shea modulus. 5

6 Now we an equate the measued imum shea stess duing pop-in to the theoetial stength of the austenite o feite phase in Eq. (14). This elates any hange in the pop-in load to a hange in the shea modulus of the given phase in SDSS as a esult of hydogen. In othe wods, hydogen not only failitates disloation nuleation but also edues the lattie ohesion [19]. A deepe analysis of the pop-in behavio was made using a pop-in finde pogam developed by Banoush [14]. The pogam is using the load displaement uves impoted fom the Hysiton softwae and is finding the pop-in fom analysis the egions whee the displaement is onstant. Futhe, based on the Hetzian fit, Eq. (9) the tip adius is found and used fo alulating the position of the imum shea stess and its value, Eq. (10) to (13). Fig. 1 pesents the deease in both pop-in width and pop-in load fo austenite and feite in pesene of hydogen. The pop-in i.e. homogeneous disloation nuleation in austenite equies highe load levels than feite but the width of the pop-in is smalle. The pop-in width in austenite is edued to half in the pesene of hydogen while fo feite the deeasing is aound one thid. Table pesents a summay of the nanomehanial popeties of the SDSS extated fom load-displaement uves and fom the pop-in finde pogam. The imum shea stesses ae highe when hydogen is pesent in the mateial and lose to the sufae. It should be mentioned that in the SDSS in ode to have both phases in quasi equilibium with eah othe it is neessay to quenh them fom about 1100 C down to the oom tempeatue. This esults in the fomation of vey high tensile stesses in the austenite and ompessive stesses in feite [0]. This is vey impotant to onside the effet of these loal high esidual stesses on hydogen uptake and its effet on the measued nanomehanial popeties. Theefoe, futue woks ae planned to study the effet of hydogen on nanomehanial esponse of the austenite and feite phases in ombination with these esidual stesses. 4. CONCLUSIONS AND FURTHER WORK The inease in hadness and the deease in the pop-in load level of both austenite and feite phases wee obseved due to hydogen evolution afte athodi potential was applied to the sample. While the deease in the pop-in an be elated to the hydogen effet on the inteatomi potential and ohesion the effet of hydogen on hadness is in ageement with hydogen pinning effet on disloations. Supe Duplex Stainless Steel is a vey omplex mateial as well as is hydogen embittlement miomehanism and futhe investigations have to be made. Futhe wok will fous on hydogen effet on gains with diffeent ystallogaphi oientations. Also diffeent loading ates will be onsideed. Fig. 1. Pop-in width and pop-in load fom the pop-in analysis Load displaement uves H (GPa) E (GPa) S (µn/nm) Table. Summay of nanomehanial popeties of SDSS Pop-in analysis Stat Maximum shea Stat depth Width stess load (nm) (nm) z τ() τ (µn) (nm) Austenite_ai 5, ,8 9,3 31,1,90 Austenite_hydogen 6, ,5 4,0 6,5 3,74 Feite_ai 4, , 11 7,7 3,40 Feite_hydogen 5, ,9 8,0 3,4 3,43 (GPa) 6 The 3 d Intenational Confeene on

7 REFERENCES 1. Toiano, A.R., 1960, The ole of hydogen and othe intestitials in the mehanial behaviou of metals. Tans ASM; 5: Binbaum, H.K., Sofonis P., 1994, Hydogenenhaned loalized plastiity-a mehanism fo hydogenelated fatue. Mateials Siene and Engineeing: A, 176(1-): pp Nilsson, J.O., 199, Supe duplex stainless steels. Mateial Siene and Tehnology, 8(8). 4. Kauss, G., 005, Steels: poessing, stutue, and pefomane. 5. Zakozymski, T., Owzaek E., 00, Eletohemial investigation of hydogen absoption in a duplex stainless steel. Ata Mateialia, 50(10): pp Olta, R., C. Bouillot, Magnin T., 1996, Loalized hydogen aking in the austeniti phase of a duplex stainless steel, Sipta Mateiale, 35(9): pp Johnsen, R., Nyhus, B., Wästbeg, S., and G.O. Lauvstad, 007, New Impoved Method Fo His Testing Of Stainless Steels Unde Cathodi Potetion. Coosion, pape no Olden, V., Thaulow, C.,Johnsen, R., Østby, E., and T. Bestad, 009, Influene of hydogen fom athodi potetion on the fatue suseptibility of 5%C duplex stainless steel - Constant load SENT testing and FEmodelling using hydogen influened ohesive zone elements. Engineeing Fatue Mehanis, 76(7): pp Olden, V., Thaulow, C., Johnsen, R., and E. Ostby, 007, Cohesive zone modeling of hydogen-indued stess aking in 5% C duplex stainless steel. Sipta Mateialia, 57(7): pp Olive W.C. and Pha G.M., 199, An impoved tehnique fo detemining hadness and elasti modulus. J. Mate. Res., 7(6). 11. Fang, T., W. Chang, Tsai S., 005, Nanomehanial haateization of polyme using atomi foe miosopy and nanoindentation. Mioeletonis Jounal, 36(1): pp Banoush, A., Vehoff H., 006, Eletohemial nanoindentation: A new appoah to pobe hydogen/defomation inteation. Sipta Mateialia, 55(): pp Banoush, A., Vehoff H., 008, In situ eletohemial nanoindentation: A tehnique fo loal examination of hydogen embittlement. Coosion Siene, 50(1): pp Banoush, A., 008, Hydogen embittlement, evisited by in situ eletohemial nanoindentation. PhD Dissetation. 15. Øveland, M., 007, Hydogen indued stess aking in supe duplex stainless steel. Maste thesis. 16. Hysiton, 009, TI-750 Ubi Use Manual NRL-M (TiboSan 9.1). 17. Johnson, K.L., 1985, Contat Mehanis. Cambidge Pess, pp Fenkel, J., 196, Zu Theoie de Elastizitätsgenze und de Festigkeit kistallinishe Köpe, Zeitshift fü Physik A Hadons and Nulei, Spinge Belin / Heidelbeg, 37: pp Banoush, A., H. Vehoff, 010, Reent developments in the study of hydogen embittlement: Hydogen effet on disloation nuleation. Ata Mateialia, 58(16): p Banoush, A., M. Zamanzade, H. Vehoff, 010, Diet obsevation of hydogen-enhaned plastiity in supe duplex stainless steel by means of in situ eletohemial methods. Sipta Mateialia, 6(5): pp The 3 d Intenational Confeene on 7