INTERGRANULAR STRESS CORROSION CRACK PROPAGATION IN SENSITISED AUSTENITIC STAINLESS STEEL (MICROSTRUCTURE MODELLING AND EXPERIMENTAL OBSERVATION)

Transcription

1 INTERGRANULAR STRESS CORROSION CRACK PROPAGATION IN SENSITISED AUSTENITIC STAINLESS STEEL (MICROSTRUCTURE MODELLING AND EXPERIMENTAL OBSERVATION) T.J. Mrrow*, A.P. Jivkov, P. Wood, D. Engelberg, L. Bbout, N. Anychor, N. Stevens, P.J. Withers, School o Mterils, University o Mnchester, UK nd R.C. Newmn,. Deprtment o Chemicl Engineering nd Applied Chemistry, University o Toronto, Cnd *Corresponding Author (Jmes.Mrrow@mnchester.c.uk) ABSTRACT Three dimensionl observtions o intergrnulr stress corrosion crck nucletion nd growth in sensitised ustenitic stinless steel provide evidence or the development o crck bridging ligments, cused by the resistnce o non-sensitised specil grin boundries. A simple grin bridging model, introduced to quntiy the eect o crck bridging on crck development, hs been ssessed vi stticlly loded room temperture tests, s well s high temperture/pressure wter utoclve studies. Thermo-mechnicl tretments hve been used to modiy the microstructure o type 34 ustenitic stinless steel. Grin reinement hs signiicnt beneicil eect on crck growth resistnce. A beneicil eect o the residul stresses rom surce mchining is observed nd compres well with the model prediction. Two nd three-dimensionl inite element models o intergrnulr crck propgtion hve been developed, which re consistent with the observed eects o microstructure. These models hve the potentil to be developed to describe the kinetics o short intergrnulr stress corrosion crck growth nd colescence. Experimentl Observtions Grin Boundry Design nd Control [] ims to improve the intergrnulr corrosion (IGC) nd intergrnulr stress corrosion crcking (IGSCC) resistnce o mterils. The grin boundry network is chrcterised by the crystllogrphic reltionships between djcent grins. Grin boundry engineering, chieved by therml nd thermo-mechnicl processing, is typiclly pplied to low nd medium stcking ult energy ce centred cubic (FCC) mterils, such s stinless steels, nd is typiclly used to increse the rction o low energy, specil, grin boundries. The grin orienttion popultions, nd hence the grin boundry network, re obtined by Electron Bcksctter Dirction (EBSD) techniques [2]. Crystllogrphic s well s sptil inormtion is recorded, nd is used to determine Grin Boundry Chrcter Distribution (GBCD), which is representtive o the microstructure. The GBCD is discrete popultion o grin boundry types, bsed on either their requencies or reltive length rctions. EBSD my lso be used to study the interction o stress corrosion crcks with microstructure (Figure ), in order to identiy resistnt nd susceptible boundries [3]. 5 µm Figure : EBSD nlysis o crcks obtined in high temperture utoclve conditions under sttic lod ) bcksctter electron imge b) EBSD dt or the sme region the grey levels indicte dierent grin orienttions, rom which grin boundry chrcter cn be derived. The crcks hve propgted rom the surce o the specimen, which is t the upper edge o the imge.

2 The interction between propgting intergrnulr stress corrosion crck nd the microstructure hs been investigted by in-situ three dimensionl observtions o intergrnulr stress corrosion crcking, using the ID9 X-ry microtomogrphy bem line t the Europen Synchrotron Rdition Fcility (ESRF), Frnce [4]. An nneled nd ully sensitised type 32 stinless steel wire (4 µm dimeter) ws tested in.5 M potssium tetrthionte (K 2S 4O 6) (ph 2). The progressive ilure o crck bridging ligments ws observed (Figure 2). Correltion o the tomogrphy nlysis with rctogrphy conirmed tht the ligments produced ductile etures, which hd morphology tht ws consistent with non-sensitised grin boundries [4]. Such boundries re commonly ound to hve high degree o coincidence between the crystl lttice orienttions o the grins, which my be described using the coincidence site lttice (CSL) model. Twin boundries (Σ3) nd some low CSL boundries with Σ 29 re ound to be generlly resistnt to intergrnulr corrosion nd stress corrosion crcking, lthough this my vry with environment [3]. These observtions show tht non-sensitised boundries re resistnt to stress corrosion crcking, nd the crck propgtes round the obstcle ormed by these ligments. The ductile ilure o the ligments is expected to shield the crck tip rom the pplied stress. The shielding mechnism is nlogous to ibre composite mechnics. 4 m 4 m Figure 2: Tomogrphic dt obtined t the sme loction inside the smple t successive periods ( nd b) during stress corrosion crcking o sensitised type 32 stinless steel wire. Progressive ilure o crck bridging ligments is observed. The eects o thermo-mechnicl processing on the stress corrosion crcking resistnce o sensitised type 34 ustenitic stinless steel hs thereore been studied [3-5], bsed on the proposition tht resistnce to stress corrosion crcking develops rom crck bridging, which is consequence o grin boundry triple junctions with one or more resistnt grin boundries (i.e. non-sensitised boundries). Crck propgtion depends on the requency o resistnt triple junctions encountered by the crck tip, thus both grin size nd the grin boundry chrcter distribution re predicted to be signiicnt prmeters. Grin boundry engineering to increse the contribution rom crck bridging ligments through n increse in the requency o nneling twins, or exmple, my be expected to improve stress corrosion crcking resistnce. Crck propgtion tests hve been perormed to evlute the popultion o short stress corrosion crcks tht re developed under sttic stress. Under such conditions, equilibrium cn develop between the pplied stress nd the shielding stresses rising rom crck bridging. Severl microstructures o ully sensitised type 34 ustenitic stinless steel were exmined using room temperture low ph thionte solution nd high temperture utoclve environment which gve conditions relevnt to high purity wter under oxidising conditions. Detils o the tests re published elsewhere [4]. The crck depths re observed to hve lognorml popultion, nd hence extreme vlue sttistics, in the orm o Gumbel distributions, were pplied to evlute the crck popultion. This method llows the extremes o lrge crck popultion to be described by mesuring the deepest crcks in the smple cross-section. Comprisons re mde t equivlent vlues o the reduced prmeter, y [4]. Typicl results re given in Figure 3, which illustrtes signiicnt inluence o microstructure on crck propgtion. Incresing the pplied tensile stress hs lso been shown to increse the mximum crck length in the thionte solution, independent o test durtion [4]. The 3%/9/3 microstructure hs reduced grin size ( µm) nd lower rction (27%) o twin nd low CSL boundries compred to the sreceived microstructure (27 um nd 35%). It ws cold worked by 3% nd nneled t 9 C or 3 minutes. Twin boundries (Σ3) were not counted in the grin size mesurements. Including twins gives verge grin size o µm nd 2 µm or the 3%/9/3 nd s-received microstructures respectively. Crck propgtion rtes were signiicntly slower in the utoclve environment, nd the crcks here my not hve reched equilibrium in the test durtion. Both microstructures were ully sensitised nd the dominnt eects ppers to be the reinement o the grin size. 3 4 m

3 y = -ln(-ln(rnk/n+)) y = -ln(-ln(rnk/n+)) ECF6 ) %/9/3 3 Hours - 3%/9/3 44 hours As-Received 44 hours - As-Received 3 Hours Crck length [µm] b) Crck Length [µm] Figure 3: Crck popultion dt, expressed s the reduced vrint (y) s unction o the observed crck length or the 3%/9/3 nd s-received microstructures tested in ). M potssium tetrthionte solution, cidiied with diluted sulphuric cid to ph 2.5, under n pplied stress o MP or 44 hours., b). M N 2SO 4 electrolyte t 25 C nd 4 br, 5 mv bove rest potentil, under n pplied stress o 244 MP or 3 hours. Dshed lines re the 95% conidence limits or the extreme vle distribution itted to the dt. Simple Anlyticl Model An nlyticl model hs been developed to predict the eects o crck bridging grin boundries [4, 5]. The objective ws to quntiy the reltive eects o grin size nd rction o resistnt grin boundries, nd to include the eects o pplied stress. The probbility, P, o crck bridge ormtion t triple junction is expressed by eqution (). The expression chosen or P is similr tht used in percoltion-like models [6, 7], nd gives pproximtely the rtio o the number o triple junctions with potentil to orm crck bridges to the totl number vilble to the crck tip. Here, TJ(n-CSL) is the requency o occurrence o triple junctions with n resistnt boundries (i.e. non-sensitised low CSL, Σ 29) nd nd b re geometricl ctors (ssumed to be nd.5 respectively), which my ccount or unvourbly oriented sensitised boundries t the triple junction. The rtio o TJ(-CSL) to TJ(2-CSL) does not vry signiicntly over wide rnge o microstructures, thus the ctul vlues o nd b re not criticl in the reltive rnking o microstructures. EBSD does not relibly chrcterise ll triple junctions due to resolution limittions nd dt sctter, lthough dt which is suiciently representtive to evlute microstructures cn be obtined when suicient number o grins is chrcterised. P TJ 2CSL b TJ CSL TJ 3CSL The probbility, X, tht the crck tip will encounter resistnt triple junction s it propgtes distnce L my be expressed using eqution (2). The distnce, over which there is criticl probbility o bridge ormtion, sy L 99% or X=99%, will be relted to the verge distnce between crck bridging ligments on the rcture surce. Since there is probbility tht bridging my not develop or short stress corrosion crck nucleted t surce until the crck length exceeds L 99%, the verge crck bridging stress, b, will vry with length () or short crcks, rising to mximum vlue bmx with orm such s eqution (3). The crck bridging stress cts to shield the crck tip rom the pplied stress. L X 2 P L D () (2) 99% b b mx or L99%, b or L99% (3) The mgnitude o the crck bridging stress induced by the ligments is diicult to determine using simple model, since with incresing crck opening displcement reltive to the ligment size, the ligments my deorm elsticlly, yield or rupture. A constnt mximum crck bridging stress, bmx, is currently ssumed or ll microstructures. I this is suicient to rrest or signiicntly retrd crck propgtion, then

4 microstructures with the gretest resistnce to short stress corrosion crck propgtion re predicted to be those in which the bridging stress develops more rpidly with physicl crck length. Such microstructures will hve smller grin size nd higher P ctors. A mximum bridging zone size, which is limited by the ilure strin o the ligments nd the crck opening ngle, cn be simply included in the model. This would cuse loss o shielding with incresing crck length. It hs not been implemented here, nd the bridging zone size is ssumed to be the totl crck length. The consequences o crck bridging cn be illustrted using simple liner elstic rcture mechnics model [4, 5]. The criticl threshold stress, th, is the pplied stress necessry to increse the net crck tip stress intensity ctor bove K SCC (eqution 4), which is ssumed to be smll. This represents stress corrosion criteri or plstic strin t the crck tip. A semi-circulr crck, rdius, is ssumed. 2 ( K sh K ISCC ) th where K 2 sh b (4) This model is closely relted to percoltion-like models or intergrnulr stress corrosion crcking, in terms o the reltive eects o microstructure prmeters. However, it incorportes the shielding rom crck bridging, includes the eects o pplied stress mgnitude, nd cn be pplied to short crcks in residul stress grdients. Residul Stress The model cn be used to interpret the observed eects o surce residul stress on crck behviour. The predicted vrition o threshold stress with crck length is shown in Figure 4 or the s-received microstructure, with no residul stress. The mximum bridging stress hs been chosen to be 25 MP to it the mximum observed crck length t n pplied stress o MP. At 2 MP, the smples exhibited unstble stress corrosion crck propgtion. The crcks tht developed in typicl residul stress ield induced by surce milling re shown in Figure 5. The smple ws tested under n pplied stress o MP. The residul stress ws mesured by X-ry dirction nd successive removl o the surce through electropolishing [e.g 8] The closure stress, σ cl, which results rom the residul stress ield, ws obtined by integrting the residul stress, (RS) (Figure 5b) over the crck length (eqution 5). cl RS d (5) This hs been dded to the crck bridging stress to predict the vrition o crck propgtion threshold stress with crck length, using the sme microstructure prmeters s the stress-ree surce (Figure 4). A signiicnt eect o the sub-surce compressive stress is pprent, nd both reduction in crck length nd n increse in the threshold stress or unstble crck nucletion re predicted.. The mximum crck length observed t MP is compred with the prediction, with good, perhps ortuitous, greement. The nlyticl model is thereore consistent with the observed eect o surce residul stress on stress corrosion crck behviour. Finite Element Modelling The simple nlyticl model mkes number o signiicnt ssumptions, including the use o liner elstic rcture mechnics, microstructure independent mximum bridging stress nd rte o bridge development determined by the verge seprtion o bridges. Two-dimensionl [9] nd three-dimensionl [4, ] inite element intergrnulr crck propgtion models hve thereore been developed, to veriy nd quntiy the mechnicl eect o crck bridging ligments on short crck development nd interction. The im is to ultimtely provide tool or 3D theoreticl investigtions o the interction between multiple crcks nd the inluence o residul stress grdients. The models ssume criticl crck tip strin or crck propgtion, which depends on the reltive degree o sensitistion. Binry resistnt/susceptible behviour is currently ssumed to be representtive o ully sensitised microstructures. Typicl dt or the 2D model is shown in Figure 6, which illustrtes the eects o susceptible boundry rction nd grin size on the crck driving orce. A similr eect o susceptible boundry rction is observed in three-dimensions (Figure 7), lthough the predicted degree o shielding is smller or the sme grin boundry properties. An eect o grin size, comprble to tht observed in 2D, is lso expected in 3D. Veriiction o this is currently underwy, s it requires substntil computtionl eort. These results conirm the mechnicl shielding eect o crck bridging, nd the model hs thereore been used to investigte residul stress eects. Typicl dt is given in Figure 8, or crcks propgting in

5 Stress (MP) Stress (MP) ECF6 similr stress ield to Figure 5. The behviour o typicl crck developed without residul stress, nd the crck popultion developed with residul stress re compred. The 2D model with residul stress shows signiicnt surce crcking, which rrely penetrtes the sub-surce compressive stress region. This is consistent with the observed behviour (Figure 5), nd implies tht the eects o stress corrosion crck propgtion in residully stressed components my be predicted with pproprite tuning o the model prmeters. Applied Stress Threshold Stress (Surce Residul Stress) Threshold Stress (No Residul Stress) Mximum Crck Length (Surce Residul Stress) Mximum Crck Length (No Residul Stress) Crck Length (um) Figure 4: The predicted vrition o crck propgtion threshold stress with crck length or s-received microstructure with nd without surce residul stress. The mximum crck length observed t n pplied stress o MP is shown. The mximum bridging stress hs been ssumed to be 25 MP. μm Depth (um) Figure 5: Eect o residul stress on crck propgtion. ) crck development in s-received microstructure, ully sensitised, tested in. M potssium tetrthionte solution, cidiied with diluted sulphuric cid to ph 2.5, t MP or 44 hours. b) the mesured residul stress ield perpendiculr to the crck plne. A nickel lyer ws deposited ter testing or edge retention during metllogrphic sectioning.

6 Figure 6: Two dimensionl model or the development o n intergrnulr stress corrosion crck. The ilure strin o susceptible boundries is % o the ilure strin o resistnt boundries. Comprison is mde with non-bridged theoreticl crck. Predictions re shown or ) the eects o dierent rctions () o susceptible grin boundries on the development o the normlised crck tip stress intensity ctor (K) nd crck shielding (K sh) with pplied stress (σ ) or microstructure with grin size (D) s the crck length increses (W=5D), b) the eect on grin size (D) on the normlised crck driving orce with incresing crck size or 5% susceptible boundries (d= 4µm, W=5d). Figure 7: Three-dimensionl model or the development o n intergrnulr stress corrosion crck. The boundry properties re s in Figure 6. Predictions or ) crck tip stress intensity ctor or microstructure with 6% susceptible boundries, b) eect o the rction o susceptible boundries on the mgnitude o crck shielding (K sh) or equivlent crck size (A=4 D 2 ). Figure 8: Typicl results or the development o crcks using the 2D model with crck bridge development or ) pplied tensile stress, b) pplied tensile stress with sub-surce compressive residul stress.

7 Summry nd Conclusion The propgtion o short intergrnulr stress corrosion crcks is inluenced by microstructure nd residul stress. A simple nlyticl model hs been proposed which is consistent with the observed behviour. Two nd three-dimensionl inite element models or crck behviour hve lso been developed, guided by high resolution observtions o the mechnism o crck propgtion. These models re being vlidted using test specimens with well chrcterized microstructures nd surce residul stress distributions. Acknowledgments The uthors re grteul to EPSRC, Rolls Royce Plc, BNFL nd Serco Assurnce or their independent support o the reserch described in this pper. The opinions expressed re those o the uthors. Reerences. T. Wtnbe, An pproch to grin-boundry design or strong nd ductile polycrystls, Res. Mechnic, (984), F.J. Humphreys, Review - Grin nd subgrin chrcteristion by electron bcksctter dirction, Journl o Mterils Science, (2), 36, D.L. Engelberg, T.J. Mrrow, L. Bbout nd R.C. Newmn, Grin Boundry Engineering or Crck Bridging: Intergrnulr Corrosion nd Stress Corrosion Crck Pth Dependencies, 6 th Interntionl Corrosion Conerence, (25), Beijing, Chin. 4. T.J. Mrrow, L. Bbout, A.P. Jivkov, P. Wood, D. Engelberg, N. Stevens, P.J. Withers, Three dimensionl observtions nd modelling o intergrnulr stress corrosion crcking in ustenitic stinless steel, E-MRS Spring Meeting, Strsbourg, Frnce, (25). 5. D.L.Engelberg, T.J.Mrrow, R.C.Newmn, L.Bbout, A new model or IGSCC propgtion, Second Interntionl Conerence on Environment- Induced Crcking o Metls, EICM-2, (24), Bn, Cnd 6. G. Plumbo, P.J. King, K.T. Aust, U. Erb, P.C. Lichtenberger, Grin-boundry design nd control or intergrnulr stress-corrosion resistnce, Script Metll. Mter. 25 (99) V.Y. Gertsmn, K. Tngri, Modelling o intergrnulr dmge propgtion, Act Mter. 45 (997) M. Kurod, T.J. Mrrow nd A.H. Sherry, Eects o surce inish on tigue in ustenitic stinless steels, ECF 6, (26). 9. A.P. Jivkov, N.P.C. Stevens nd T.J. Mrrow, The roles o microstructure nd mechnics in intergrnulr stress corrosion crcking, First Interntionl Conerence on the Simultion o Electrochemicl Processes (ElectroCor 5), Cdiz, Spin, (25).. A.P. Jivkov, N.P.C. Stevens nd T.J. Mrrow, A three-dimensionl computtionl model or intergrnulr stress corrosion crcking, submitted to Computtionl Mterils Science (26).