Characterization and Mechanism of 304 Stainless Steel Vibration Welding

Transcription

1 Materials Transactins, Vl. 48, N. 9 (2007) pp t 2323 Special Issue n Slidificatin Science and Prcessing fr dvanced Materials #2007 The Japan Institute f Metals Characterizatin and Mechanism f 304 Stainless Steel Vibratin Welding Che-Wei Ku*, Chi-Ming Lin*, Gen-Huey Lai*, Yu-Che Chen*, Yung-Tse Chang* and Weite Wu Natinal Chung Hsing University, Institute f Materials Science and Engineering, 250 Ku Kuang Rad, Taichung 402, Taiwan, R. O. China Gas tungsten arc welding (GTW) was perfrmed n ISI 304 stainless steel; steady-state vibratin was prduced by a mass-eccentric mtr. The vibratin weld shws a very small -ferrite structure, unifrm cmpsitin distributin, less residual stress and less -ferrite cntent relative t the weld withut vibratin. The results illustrate that the vibratin reduces the micr supercling and imprves the nucleatin f - ferrite t frm a grain refined structure. Vibratin-induced stacking faults are identified as the majr cause f the line bradening f X-ray diffractin prfile. Crrelating the literature and the result in the study, the mechanism f vibratry stress relief can be represented as the breakdwn f dislcatin int a pair f partial dislcatins. This mechanism can cmprehensively explain all the phenmena that take place during vibratry stress relief. [di: /matertrans.m200706] (Received February 9, 2007; ccepted pril 16, 2007; Published ugust 25, 2007) Keywrds: vibratin welding, -ferrite, X-ray diffractin, residual stress, grain refining 1. Intrductin The micrstructure f steel is largely mdified and psitively influenced by heat treatment, resulting in recrystallizatin, hmgenizatin f segregatin and eliminatin f harmful precipitates. Hwever, in welding, such pssibilities are rather limited, since welded cmpnents can, in rare instances, be affected by heat and be slutin annealed r quenched and tempered. Therefre, the structure f welded metal is cnsiderably less favurable and less hmgeneus than that f the base metal. few years ag, vibratin was used fr imprving the quality f metal which slidified frm the melt, especially t imprve the micrstructures, mechanical prperties and relieve the residual stress. 1 5) Over the past years, the use f vibratin fr stress relief r t enhance the prperties f slidifying metal has becme wide-spread thrughut industry, despite the lack f scientific data regarding the phenmenn. Imprtant infrmatin abut the thery f VSR (vibratry stress relief) shuld be mentined. R. Dawsn and D. G. Mffat indicate that vibratins are essential fr stress relief because they induce micr strain in high-stress regins. 6) That is, the strain can be reduced by lattice slip. Then, a simple thery f bulk plastic flw due t the cmbinatin f the residual stress with the applied stress is ften cited; 7) hereafter, this is referred t as the standard mdel. The standard mdel pstulates that the stress is relieved when the cmbinatin f the residual stress and the vibrating stress exceeds the yield strength f the material; the assumptin is that the subsequent plastic flw is such that when the vibratinal amplitude is remved, the previusly stressed area can nw return t a lw level f residual stress. This is because the plastic defrmatin prcesses in metals are indicative f a prcess f dislcatin mvement. It has been pstulated that the dislcatin segments, released frm weakly pinning pint defects by cyclic lading, migrate twards a mre stable cnfiguratin under the influence f *Graduate Student, Natinal Chung Hsing University internal stresses and thermally excited lattice vibratins. C.. Walker,. J. Waddell and D. J. Jhnstn fund that dislcatin multiplicatin ccurred as the vibratry cycle increased and the multiplicatin stpped at the end f the VSR. 8) This illustrated that the strain reduced as the dislcatin increased. These phenmena imply that the strain energy was released by dislcatin prliferatin during vibratin. ISI 304 stainless steel was used in this research. In austenitic steel welding, the transfrmatin prcess is mre cmplicated, and invlves the transfrmatin f int. ustenitic steel is cmpsed f irn, chrmium and nickel. This cmpsitin is clse t the three-phase sectr (L þ þ ). In 304 stainless steel, the existence f nly ne - austenite is nrmal; hwever, the weld always cntains residual -ferrite f apprximately 5 10%. The -ferrite results frm micrsegregatin during slidificatin, fllwed by slid-state transfrmatin cntrlled by the diffusin during subsequent cling. 9) The - transfrmatin nrmally ccurs quickly during welding, prducing dendrite grwth and segregatin. It is difficult t discriminate between the and cmpsitins f the weld. This prblem was studied by Lichtenegger and löch; 10) it was fund that the Cr and Ni cntents in the residual -ferrite were higher in a metal ally weld than thse in the riginal specimen. Therefre, bserving the residual -ferrite is useful fr understanding the difference between the slidifying prcesses in vibratin welding and nn-vibratin welding. 2. Experimental Prcedures 2.1 Welding prcess s-received ISI 304 austenitic stainless steel plate was used. The plate was 70 mm 25 mm 3 mm. Specimens were welded using the gas tungsten arc welding (GTW) prcess withut a filler metal. The welding current was 60 ; the vltage, 9 V; and the traveling speed, 96 mm/min. tungsten rd f 2.4 mm diameter, prtective argn gas (15 IPM) and an acetne-cleaned pre-weld were used. efre determining the vibratin frequency fr welding, the welding

2 2320 C.-W. Ku et al Relative amplitude, / V Frequency, f/ Hz Fig. 1 Harmnic curve f the welding system. Fig. 3 Weld bead f welding withut vibratin. () Tp view () Crss sectin. Fig. 2 Synchrnized vibratin welding system. table was cntinuusly vibrated fr 20 min until the system reached the steady-state. Then, the data fr the harmnic curve f the vibratin welding system was btained by using a transducer with a vibratin cntrller and a data (frequency and amplitude) recrder; this curve is shwn in Fig. 1. Figure 2 shws the synchrnized vibratin welding system. The vibratin welding was perfrmed at a sub-resnant frequency. 2) The sub-resnant frequency was defined as the frequency which had ne-third the resnant amplitude r which was 10 Hz less than the resnant frequency. Subresnant frequency was thught t input mre energy t the bject than the resnant frequency. The sub-resnant frequency was 39.4 Hz (with ne-third the resnant amplitude). The resnant frequency f the welding system was 43.3 Hz. The specimens were welded under tw cnditins: subjected t vibratins f 0 Hz and 39.4 Hz. 2.2 Micrscpy, chemical cmpsitin, and -ferrite cntent measurement The micrstructures f the welded specimens were investigated using ptical micrscpy. Cnventinal etching cmpunds react particularly with the secndary structures. If these cmpunds are emplyed, the primary structure is nly hardly, r nt at all, visible. Revealing the primary structure requires the use f the s-called primary etchants that react with the segregatins f the primary slidified structure. The L 1 clur etchant (Lichtenegger and löch etch) was used, which makes bth primary and secndary Fig. 4 Weld bead f vibratin welding. () Tp view () Crss sectin. structures visible. 10) Etching was perfrmed using 0.5 g f K 2 S 2 O 5 and 20 g f NH 4 FHF in 100 ml H 2 O at C until the sample became tinted. Clur etching was set up in such a way as t illustrate the situatin during the slidificatin f the weld metal when abut three-furths f the weld-metal vlume has slidified and ne-furth is still present as liquid residual melt. In Fig. 5, the blue area f the structure represents the already slidified cellular structure, which cnsists f primary -ferrite. bright structural cnstituent is als visible in the primary precipitated cellular -ferrite. This represents the prtin f the residual -ferrite that remains after - transfrmatin frm the primary precipitated -ferrite at ambient temperature. The residual melt exists between all the blue structures, which appears as a brwnish-tinted structural cnstituent. The chemical cmpsitins f residual -ferrite (white), primary precipitate - ferrite (blue) and residual melt (range) in Figs. 5 and 6 were identified by field emissin scanning electrn micrscpe (FESEM, JEOL JSM 7000F) equipped with energy disper-

3 sive spectrmeter (EDS). The -ferrite cntent was measured by a magnetic scpe (Fischer Feritscpe MP30). Characterizatin and Mechanism f 304 Stainless Steel Vibratin Welding X-ray diffractin stress measurement The measurements were perfrmed with a MC SCI- ENCE MXP3 (40 kv, 30 m, CuK radiatin, K ¼ 1:54056 nm. The -Fe reflectin under study was {331}, which prduces a ragg peak at 2 ¼ 148:4 fr the afrementined wavelength. Within each crss-sectin, tw generatrices were sampled, lcated at 0 and 45. The applicatin f X-ray-diffractin (XRD) stress measurements t practical engineering prblems began in the early 1950s. s in all diffractin methds, the lattice spacing is calculated frm the diffractin angle, 2, and the knwn X- ray wavelength using ragg s law. The precisin necessary fr strain measurement in engineering materials can be achieved using the diffractin peaks prduced in the high back-reflectin regin in which 2 >120. The macrstrain is determined frm shifts, typically <1, f the mean diffractin-peak psitin. Residual stress can be expressed as fllws. Details f this equatin are given elsewhere 11) (Yung s mdulus: 193, 100 MPa; Pissn s rati: 0.29). ¼ E 1 þ 1 ðhklþd 0 : Residual stress : Pissn s rati E: Yung s mdulus d 0 : Stress-free sin 2 3. Experimental Results and Discussin Fig. 5 Tp f the weld bead (welding withut vibratin). The mrphlgies f weld bead withut and with vibratin are shwn in Figs. 3 and 4, respectively. The tp view f vibratinless welding shws a flatness surface, different frm the wavy surface f vibratin welding. s t the shape f crss sectin, the utline f vibratin welding displayed a salient at the bttm f welding pl (Fig. 4) which implies that vibratin ffered an t frce burst the bttm f weld pl. The perturbatin causes liquid cnvectin f vibratin welding different frm withut vibratin welding, and the phenmenn will affect the slidificatin parameter and further mre n the micrstructure and ferrite percent. Figure 5 shws the slidificatin structure f 304 stainless steel weld metal, which has slidified t frm primary - ferrite. In this area, the -ferrite dendrite was a hetergeneus nucleatin that frmed frm the liquid melt. Dendrite grwth and the resulting dendrite structure are related t the undesirable effects in interdendritic liquid, with its inhmgeneus cncentratins. The primary precipitate, -ferrite, als emerges as the secnd dendrite arm frm the majr arm because f the huge cnstitutinal supercling. This phenmenn has been described elsewhere. 12) The micrstructure f the weld bead fr vibratin welding is shwn in Fig. 4. The dendrite arm f -ferrite is much shrter, nly 4 5 mm in length, than that in the case f the vibratinless welding in which it is a big, thick structure, apprximately mm in length. The difference in dendrite size shws that the cnstitutinal supercling f liquid metal significantly decreased under vibratin. On the ther hand, the Fig. 6 Tp f the weld bead (welding with vibratin). amunt f nuclei increased. Fr welding withut vibratin, the amunt f nuclei was apprximately 1.04 per 100 mm 2 ; this value increased t 3.65 per 100 mm 2 fr vibratin welding. This increase in the amunt f nuclei limited the grwth f the -ferrite dendrite arm which cnsequently frmed a very fine structure. The increase in the rate f nucleatin resulted frm the vibratin-induced cnvectin in the weld pl, which caused partially melted grains t detach frm the slid liquid mixture surrunding the weld pl. Like dendrite fragments, these partially melted grains, if they survive in the weld pl, can act as nuclei fr the frmatin f new grains in the weld metal. 4) nther imprtant distinctin between general (withut vibratin) and vibratin welding is the cmpsitin unifrmity. In this case, clur etched ptical micrgraphs can clearly display the divergence f chemical cncentratin. Clur etching is a cntrlled crrsin prcess based n an electrlytic actin between the surface areas at different ptentials. In the etching f ISI 304 stainless steel, a ptential difference exists between the and phases and cncentratin gradients in the phase. Figure 6 shws a mre unifrm cmpsitin distributin than that in Figure 5. The blue (primary precipitate -ferrite) area embracing bright tiny part (residual -ferrite) is smaller in Fig. 6 than in Fig. 5. The chemical cmpsitins f the residual, primary precip-

4 2322 C.-W. Ku et al. Table 1 Chemical cmpsitins by EDS. r : residual -ferrite, p : primary precipitate -ferrite, R m : residual melt. Weight% Withut vibratin With vibratin r p R m r p R m Cr Fe Ni Table 2 Results f -ferrite cntent measurement. Values are given in %. Frequency verage 0 Hz Hz Intensity, I/ cps 4500 γ (111) γ (200) 3000 γ (220) γ (311) γ (222) Withut vibratin 2500 () Synchrnized vibratin () Vibrate after welding 0 (C) ngle, 2 θ / Fig. 7 XRD pattern fr 304 stainless steel. () Withut vibratin. () With vibratin. (C) Vibrate after welding. itate -ferrite and residual melt are listed in Table 1. The same cmpsitin tendency is bserved fr bth the cases f welding, namely, withut vibratin and with vibratin. The Cr cntent is the highest fr the residual -ferrite, fllwed by the primary precipitate -ferrite; the residual melt has the lwest Cr cntent. This result implies that the difference in divergence f chemical cncentratin was limited t a small zne. The difference culd be attributed t the small cnstitutinal supercling and fine grain structure such that the lcal chemical cncentratins were mre finely and smthly distributed. This can avid the serius segregatins caused by the slidifying prcess in the weld. The -ferrite cntent was measured using a ferritescpe, and the results are listed in Table 2. The average -ferrite cntent fr the nn-vibratin and vibratin welding is 4.48% and 2.19%, respectively. s shwn in Table 1, -ferrite has a higher Cr cntent than -austenite, and less -ferrite implies less segregatin. These -ferrite cntents indicate that vibratins decrease the temperature gradient f melt pl and enable the liquid metal t slidify t frm a unifrm structure with less segregatin. The residual stresses in the nn-vibratin and vibratin welding are 262 MPa and 206 MPa, respectively. In the vibratin welding, the residual stress is smaller by 56 MPa; this implies that apprximately ne-fifth f the residual stress can be reduced by vibratin. Vibratin applied during welding needs less time fr residual stress relief. In this study, a residual stress test was perfrmed n samples cut frm the center f the weld bead; it melted and slidified under vibratin applied nly fr s. The vibratin and the welding stpped simultaneusly. s cmpared t vibratin welding, vibratin after welding takes mre time and energy because the yield pint at rm temperature is greater than that at a higher temperature. Figure 7 shws the XRD prfiles; and shw the weld frmed withut and with vibratin, respectively. In rder t discriminate the effect f vibratin but nt the nucleatin prcess, a test was perfrmed in which the specimen was vibrated fr 20 min; the results are shwn in Figure 7(C). The intensity shwn in Fig. 7() is the highest intensity, fllwed by that in (C); and () has the lwest. Their FWHM (full width at half maximum) were determined by curve fitting using a Gaussian functin. The mst bvius distinctins between the three cnditins are the peak intensity and the Table 3 FWHM f the XRD planes. () Withut vibratin. () With vibratin. (C) Vibrate after welding. Plane index FWHM, C (111) (200) (220) (311) (222) width, especially when the vibratin was applied. The diffractin line bradening, 13) is caused by three majr factrs: nnunifrm strain, small particles and stacking fault. Nnunifrm strain was induced by manufacturing int lcal area and causes the lattice cnstant t deviate frm a unifrm value. This leads the diffractin line t distribute in a range f 2 values, resulting in line bradening (Table 3). In the study, three samples have the same stress distributin because they were fabricated using the same welding prcess. Hence, nnunifrm strain cannt be the cause f line bradening. Further, the grain size must be smaller than 0.1 mm in rder t cause line bradening. Thus, the stacking fault can be identified as the majr cause f diffractin line bradening. The significant implicatins f the VSR mentined abve are as fllws: (1) Plastic defrmatin is necessary fr stress relief and is crrelated with the amplitude f vibratin but nt the frequency. 6) (2) Dislcatin multiplicatin takes place during stress relieving; when the stress relief ends, the dislcatin multiplicatin stps, t. 8) n increase in the dislcatin density during plastic manufacturing is very usual. The distinctin between VSR and plastic manufacturing is that the vibratin can induce nly a very small frce. ccrding the standard mdel, 7) the frce needed fr plastic defrmatin is cntributed by the residual stress and the vibrating stress, and their sum must exceed the yield strength. Fr this reasn, the vibratin has n effect n the area with a lw residual stress and the effect f vibratin can be ignred. T integrate the phenmenn f VSR frm that in the pervius study and this study, dislcatin mvement, dislcatin multiplicatin and increase f stacking fault

5 Characterizatin and Mechanism f 304 Stainless Steel Vibratin Welding 2323 () nther clse-packed plane (111) () High residual stress area Clse-packed plane (111) Fig. 8 Stress relief during vibratin welding. density are cnsidered. The reasnable prcesses that tk place during the VSR can be utlined. First, we cnsider the plastic defrmatin. Its mechanism in an fcc crystal has been illustrated. 14) During VSR, the dislcatin shwn in Figure 8 tends t mve twards the left. tms with a high residual stress are n the meta-stable sites. tms cannt slip t the lw-energy sites unless they btain an extra energy t vercme the barrier. When vibratin is applied, atms have sufficient energy t slip and fit int the dtted circles shwn in Fig. 8; then, the strain is released. The prcess invlves the breakdwn f the ttal dislcatin int tw partial dislcatins and can be expressed accrding t the relatin f urgers vectr. 14) 1 2 ½ 110Š ¼ 1 6 ½ 121Šþ 1 6 ½ 211Š nther mechanism, extended dislcatin, 14) induce the rearrangement f the atms; cnsequently, the strain wuld be released. If dislcatin density stps increasing, the VSR is terminated. fter the dislcatins break dwn, the dislcatin density and the number f stacking faults increase because the tw sides f the dislcatin cnstitute new stacking faults. These phenmena crrespnded with the literature referred and were bserved in the study. It can als be well expressed by the energy state diagram. The atms with high residual stress are meta-stable, and their energy state is at (M), as shwn in Fig. 9. If the vibratin prvides energy Q v, then the meta-stable atms with internal energies higher than (M) can crss ver the saddle pint (H) and release their internal energies Q r t a stable state (L). If the vibratin energy is higher than Q v, then the atms with lw residual stress can crss slip and fit int the lw-energy sites and release the strain energy. This is why VSR has n relatin with vibratin frequency; the higher the amplitude, the mre is the stress relief. 4. Cnclusins In this study, we investigated the effect f vibratin welding n the micrstructure, -ferrite cntent, residual stress and XRD prfile f 304 stainless steel. The mechanism f VSR was discussed. The fllwing results were btained. (1) Vibratin affects the welding pl shape and the cnvectin f welding pl strngly. It will affect the slidifying parameter and metallurgic prperties. (2) Dendrite arm f the -ferrite became very shrt and fine under vibratin welding because the cnstitutinal Fig. 9 supercling was decreased by vibratin. (3) Vibratin imprves the nucleatin f -ferrite, which precipitates frm the liquid phase. The large amunt f -ferrite blcks its grwth, resulting in the frmatin f a fine and unifrm structure with lw -ferrite cntent. (4) The residual stress reduced frm 262 t 206 MPa when vibratin was applied during welding even if the vibratin time was nly 1520 s. (5) XRD line bradening f the vibratin weld revealed that the vibratin welding induces stacking faults. ccrding t pervius studies, the dislcatin mvement fllwed by dislcatin breakdwn is identified as the majr cause f stress relief. cknwledgement This wrk was partially supprted by the Natinal Science Cuncil, Taiwan, R.O.C., under prject NSC E REFERENCES Energy states f atms in the VSR prcess. 1) G. Gnirss: Indian Weld. J. 22 (1990) ) Jr. G. Hebel ugust: Metal Prg. 128 (1985) ) W. Wu, D. Y. Lin and S. H. Chen: J. Mater. Sci. Lett. 18 (1999) ). Wei: cta Mater. 40 (1992) ) W. Wu: Scr. Mater. 42 (2000) ) R. Dawsn and D. G. Mffat: J. Eng. Mater. Technl.-Trans. SME. 102 (1980) ) E. Kltzbucher and H. Kraft: Residual Stresses in Science and Technlgy, (DGM Metallurgy Infrmatin, US, 1987) p ) C.. Walker,. J. Waddell and D. J. Jhnstn: Prc. Instn. Mech. Engrs. 209 (1995) ) P. ilmes,. Gnzalez, C. Llrente and M. Slari: Welding Int. 10 (1997) ) E. Flkhard: Welding Metallurgy f Stainless Steels, (Springer-Verlag, Wien, 1984) pp ) M. E. Hilley, J.. Larsn, C. F. Jatczak and R. E. Richlefs, editrs. Residual Stress Measurement by X-ray Diffractin, SE Infrmatin Reprt J784a (1971) pp ) Sind Ku: Welding Metallurgy, 2nd ed., (Jhn Wiley & Sns, New Yrk, 2003) pp ). D. Cullity: Elements f X-Ray Diffractin, 2nd ed., (ddisn Wesley, Lndn, 1978) pp ) Rbert E. Reed-Hill: Physical Metallurgy Principles, 3rd ed., (PWS., stn, 1994) pp