FATIGUE FAILURE OF CENTRIFUGAL BOMB AXIS

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1 Copyright 2005 y ABCM 18th Interntionl Congress of Mechnicl Engineering Novemer 6-11, 2005, Ouro Preto, MG FATIGUE FAILURE OF CENTRIFUGAL BOMB AXIS Jôneo Lopes do Nscimento Ntionl Institute of Technology INT - Brzil joneolop@int.gov.r José Luiz Fernndes Federl Center of Technology Celso Suckow CEFET-RJ - Brzil jlfernndes@cefet-rj.r Cássio Bros Ntionl Institute of Technology INT - Brzil cssio@int.gov.r Iêd Mri Vieir Cminh Ntionl Institute of Technology INT - Brzil iedmri@int.gov.r Astrct. Among the fctors tht cn cuse the premture filure in metllic components re the use of indequte mterils, the presence of defects tht ppered during the production nd errors of project, ssemly or mintennce. Filure nlysis llows the identifiction of cuses, contriuting to improve the opertion nd performnce of similr equipments, concerning oth functionlity nd sfety requirementsnd lso to void the repetition of the sme filure type. In this work it is mde study of ftigue nlysis filure in centrifugl om xis fricted in duplex stinless steel nd used in petroleum industry. This study comprises frctogrfic oservtion in scnning electron microscope, microstructurl nlysis in n opticl microscope nd hrdness tests. The results thus otined llow determining the cuse of the filure s ftigue of the mteril. Keywords: Filure nlysis, ftigue of mterils, duplex stinless steel 1. Introduction Petrochemicl industry is one of the most dynmic segments of the world economy nd its growth in the lst yers hs led to the development of new mterils which cn fit to requirements which re getting more demnding. Duplex stinless steels (DSSs), which were firstly proposed s new mteril in 1927, were further developed in the following decdes until the 1990s nd hve een mentioned s possile option to the use of more trditionl stinless steels. A NACE/ISO stndrd descries the requirements for such ppliction (NACE MR0175 / ISO , 2003). DSSs hve two phse microstructure, composed y expressive frctions of ustenite nd ferrite nd hve some dvntges, minly higher strength, higher resistnce to intergrnulr corrosion nd lower cost, together with comprle resistnce to uniform corrosion, when compred with ustenitic stinless steels. On the other hnd, DSSs hve some disdvntges like difficult thermomechnicl processing nd low resistnce to pit corrosion (Reick, Pohl nd Pdilh, 1992). In order to rech some requirements of toughness, strength nd corrosion resistnce, the presence of some phses, like σ phse, for instnce, must e minimized (Lee, Cho nd Choi, 1999; Chen, nd Yng, 2001; Chen, Wing nd Yng, 2002). Another deleterious phse, formed during thermomechnicl processing of DSSs, is Cr 2 N, the min precipitte found in the het ffected zone (HAZ) of welded joints. This phse presents competitive ehvior with the ustenite phse, so tht the conditions which fvor the formtion of ustenite, such s high nitrogen content nd low cooling rte, re exctly the ones which minimize the formtion of Cr 2 N precipittes (Liou, Hsieh nd Tsi, 2002). On the other hnd, σ phse is formed s consequence of the decomposition of ferrite, which lso increses the volume frction of ustenite. This rection is fvored y ging t higher tempertures ( ºC) nd longer periods (30 min 8 h). However, the kinetics of the rection of formtion of σ phse is slower thn the one of Cr 2 N precipittion (Lee, Cho nd Choi, 1999; Chen, Wing nd Yng, 2002). The sic mechnicl properties of ny mteril re otined through stress-strin test. A certin led is pplied to stndrd specimen until the rupture of this one, nd these loding conditions re known s lmost sttic ones. However, frequently stresses vry etween certin vlues of tension nd compression nd this kind of loding is clled cyclic loding. Ftigue filure is consequence of cyclic loding. When crck nucletes nd propgtes in the mteril, this one will undergo rupture t stress level much lower thn the one necessry to cuse frcture under sttic loding. The min fctors which cn cuse ftigue filure re the rnge of vrition of pplied stresses, the numer of cycles, stress risers,

2 corrosion, temperture, overloding, residul stresses nd comined stresses. According to Fernndes nd Cstro (2002) ftigue is kind of locl, progressive nd ccumultive mechnicl filure, or in other words, prtil loss of functionlity of some component, s result of nucletion nd progressive propgtion of crck due to cyclic loding. Ftigue filure is locl prolem which depends on detils of geometric chrcteristics of the piece, on the mteril nd on the loding t the most stressed point of the piece. According to Suresh (1991) the phenomenology of ftigue crcks cn e nlyzed s prolem of locl plstic deformtion nd it cn e explined y the ppernce of sher nds. When there is disloction movement certin grins, with min slip direction which coincides with the mximum shering direction, help the disloctions to e grouped in cells, forming persistent slip nds, which pper minly on the surfce of the piece. Filure nlysis uses severl kinds of techniques for the investigtion of the cuses of the defects which led to the collpse of n equipment or structure. Generlly these cuses re relted to: the use of indequte mterils, the presence of defects which ppered during friction nd errors of project, ssemly or use. The knowledge out these cuses nd the correction of these nomlies llow improving the performnce of similr equipments nd void the reppernce of the sme kind of filure, concerning oth functionlity nd sfety requirements (Azevedo, 2004). Usully filure nlysis ttempts to correlte the visul spect of frcture surfce to its possile cuse nd with this purpose it cn use microscopy techniques, minly scnning electron microscopy (SEM) (Wouters nd Froyen, 1996). In this work it is crried out frcture nlysis of centrifugl om xis used in petrochemicl industry, y using techniques such s SEM frctogrphy, microstructurl oservtion of duplex stinless steel in n opticl microscope nd hrdness tests. These techniques complement ech other nd thus, in n efficient wy they llow the identifiction of the cuse of the component filure. 2. Methodology The chemicl composition of duplex stinless steel, otined through X-ry fluorescence nlysis, is presented in tle 1. Tle 1. Chemicl composition of duplex stinless steel. C Si Mn P S Cr Ni N Mo V Cu W Co Fe Bl. The centrifugl om xis ws prtilly cut for otining smples for microstructurl nlysis in opticl microscope, SEM frctogrphic nlysis nd hrdness tests. Smples for microstructurl nlysis were sujected to stndrd metllogrphic preprtion: grinding (100 to 600 meshes), dimond pste polishing (6 to 1 µm) nd etching with 30 ml nitric cid, 10 ml cloridric cid nd 60 ml distilled wter etchnt. Smples for frctogrphic nlysis were oserved nd photogrphed in equipment operting t 20 kv. Rockwell C Hrdness test (lod: N) comprised 5 mesurements t different points of the smple, nd the verge vlue ws clculted nd considered s representtive of the smple hrdness. 3. Results nd Discussion Figure 1 shows the frctured centrifugl om xis s received, while figure 1, otined in stereo-microscope, presents the mcroscopic overview of the frcture surfce. Figure 2 shows the microstructure of the duplex stinless steel, with ustenite delineted in ferritic mtrix, which is in ccordnce to the NACE/ISO stndrd (NACE MR0175 / ISO , 2003). Through opticl microscopy it ws not found σ phse. Figure 3, otined in scnning electron microscope (SEM), presents detiled spects of the frcture surfce, in which stritions re clerly visile. These stritions indicte filure y ftigue of the mteril (Metls Hndook, 1986). Figure 4 shows detiled spects of other regions of the sme frcture surfce. Tle 2 shows results of Rockwell C hrdness test.

3 Copyright 2005 y ABCM 18th Interntionl Congress of Mechnicl Engineering Novemer 6-11, 2005, Ouro Preto, MG Figure 1. () Frctured centrifugl om xis () Frcture surfce: mcroscopic overview. 20µm 20µm Figure 2. Duplex stinless steel microstructure: () trnsverse section; () longitudinl section. Figure 3. SEM imges: () Frcture surfce presenting ftigue stritions. () Detiled spect of the sme region.

4 c d Figure 4. (),(), (c), (d) Detiled spects of other regions of the sme frcture surfce Tle 2. Hrdness vlues of the duplex stinless steel smple. Points Hrdness (HRC) Averge (Rockwell C) 22,4 The verge hrdness vlue (22.4 HRC) is lower then the upper limit (25 HRC) prescried y the NACE/ISO stndrd for the ppliction of this component nd thus it cn e considered cceptle (NACE MR0175 / ISO , 2003). According to Reick (1992), duplex stinless steels, with chemicl composition very similr to the one nlyzed in this study, hve tensile strength round 640 to 750 MP, yield strength etween 400 nd 450 MP nd totl elongtion round 25 %, eing these vlues comptile with the kind of ppliction defined for this mteril in the present cse. If one considers the results with different techniques, which eliminted the possiility of the cuses eing relted to indequte mteril (chemicl composition nd microstructure), one relizes tht mislignment hrmed the opertion of the equipment leding to ftigue filure.

5 Copyright 2005 y ABCM 18th Interntionl Congress of Mechnicl Engineering Novemer 6-11, 2005, Ouro Preto, MG 4. Conclusion The present study on filure nlysis of centrifugl om xis mde of duplex stinless steel, sed on opticl microscopy, scnning electron microscopy nd Rockwell C hrdness tests, led to the following results: Microstructure, composed y ustenite in ferritic mtrix, which is in ccordnce to the stndrd, nd no σ phse ws oserved in opticl microscope. The hrdness of the duplex stinless steel smple (22.4 HRC) lso meets the specified vlues (lower thn 25 HRC). There is no evidence of corrosion neither ny other kind of degrdtion in the mteril. SEM imges revel evident presence of ftigue stritions on the frcture surfce. The previous results suggest tht the ftigue filure ws proly cused y mislignment during the ssemly of the equipment. 5. References Azevedo, C.R.F.; Cescon, T., 2004, Filure Anlysis nd metlogrfy, Selected cses ( ), (in Portuguese) IPT (Technology Reserches Institute), São Pulo - Brzil, 1 st edition, 416 p. Chen, T.H.; Yng, J.R., 2001, Effects of solution tretment nd continuous cooling on σ-phse precipittion in 2205 duplex stinless steel, Mterils Science nd Engineering A, v.311, pp Chen, T.H.; Weng, K.L.; Yng, J.R., 2002, The effect of high-temperture exposure on the micro structurl stility nd toughness property in 2205 duplex stinless steel, Mterils Science nd Engineering A, v.338, pp Fernndes, J.L., Cstro, J.T.P., 2002, Ftigue Crck Propgtion in API-5L-X60, (in Portuguese) Technology nd Conference of Equipments VI COTEQ, August, 10p. Lee, K.M.; Cho, H.S.; Choi, D.C., 1999, Effect of isotherml tretment of SAF 2205 duplex stinless steel on migrtion of δ/γ interfce oundry nd growth of ustenite, Journl of Alloys nd Compounds, v.285, pp Liou, H.-Y.; Hsieh, R.-I.; Tsi, W.-T., 2002, Microstructure nd stress corrosion crcking in simulted het-ffected zones of duplex stinless steels, Corrosion Science, v.44, pp Metls Hndook, 1986, Filure Anlysis nd Prevention, ASM (Americn Society for Metls), Ninth Edition, v. 11, Metls Prk, Ohio, 843p. NACE MR 0175 / ISO , 2003, Petroleum nd nturl gs industries Mterils for use in H 2 S-contining environments in oil nd gs production Prt 3: Crcking-resistnt CRAs (corrosion-resistnt lloys) nd other lloys. Reick, W.; Pohl, M; Pdilh, A.F., 1992, The Development in the stinless ferritic-ustenic steel with duplex microstructure, (in Portuguese) Metllurgy nd Mterils, ABM - Brzil, v. 48, n. 409, Septemer, pp Suresh, S., 1991, Ftigue of Mterils, Cmridge University, 1991, 605p. Wouters, R.; Froyen, L., 1996, Scnning electron microscope frctogrphy in filure nlysis of steels ; Mterils Chrcteriztion, 36, pp Responsiility notice The uthors re the only responsile for the printed mteril included in this pper.