THE PROBLEM WITH THE ZINC COATING DIVERSIFICATION OF THE HOT-DIP GALVANIZED STEEL. Sylwia WĘGRZYNKIEWICZ 1

Transcription

1 THE PROBLEM WITH THE ZINC COATING DIVERSIFICATION OF THE HOT-DIP GALVANIZED STEEL Sylwi WĘGRZYNKIEWICZ 1 1- BELOS-PLP S.A., Bielsko- Bił, gen. J. Kustroni 74, e-mil dress: sylwi.wegrzynkiewicz@elos.com.pl Astrct: In industril prctice on vrious production stges of the fittings for overhed power line, there re mny fctors tht cuse the zinc coting thickness diversifiction, tht in the next step determine its corrosion resistnce. This effect is oserved especilly in the therml cutting process. The chnges in the structure of the cut mteril influence the structure nd properties of zinc cotings. The pper presents the results of investigtion regrding the typicl production process of fittings for overhed power lines (doule-eye links). The reserch ws conducted on the link s flt surfce (fter rolling) nd the side surfce (fter oxy-cteylene lowpipe cutting). The zinc cotings thickness nd structure diversifiction were evluted on the sis of metllogrphic nlysis nd corrosion tests (ccording to EN ISO 9227). It hs een proved tht the existence of HAZ in the oxy-cetylene cutting mteril nd, s consequence, igger hrdness hve n impct on the diffusion pce on the edge of steel/coting, nd on the processes of sustrte melting which ssist zinc coting growth. Keywords: oxy-cetylene lowpipe cutting, het ffected zone, zinc coting, hot-dip zinc glvnizing 1. INTRODUCTION The economic losses cused y corrosion, increse the cost of production nd exploittion of steel constructions to lrge extent. The selection of technologicl opertion nd corrosion protection methods is often determined y the mount of finncil resources to e invested. The selection of cutting method depends on mny fctors of the production process. The economic fctor together with the qulity re of gret importnce, especilly in high-volume production. However, considering minimizing the negtive influence on the qulity metllic cotings it is lso importnt. Oxycetylene lowpipe cutting (OAB) elongs to one of the most populr nd economic methods of elements formtion. This process consists in dividing the mteril y locl urning of its prticles y use of the strem of pure oxygen, nd simultneously heting the metl to suitle temperture [1]. The lowest temperture in which the comustion process my occur is the ignition temperture of steel. For pure iron it equls 1050 o C, however for steel with the ddition of 1,5% of cron equls 1380 o C. The item eing cut long the cutting line hs to e heted intensively throughout the process [1]. Liquid oxides lown successively out of the developing crevice re the comustion products. The steel surfce fter therml cutting is lwys covered y metl lyer, which underwent in uncontrollle wy through ll phse chnges - from ferritic structure to ustenite nd the other wy round. The pplied method determines, mong others, the outer lyer structure nd properties, which re relted to the Het Affected Zone (HAZ) ppernce [2]. The chnges in the structure of treted mteril hve n impct on the composition nd properties of coting pplied y hot dip glvnizing [3-5]. It my cuse difficulties with chieving the required thickness nd dhesiveness of zinc cotings. It determines their resistnce nd the period of their exploittion. In prctice

2 the prolem of zinc cotings qulity is significnt, especilly tking into considertion the corrosion protection of network equipment elements used in electricl overhed power lines. EN ISO stndrd indictes the prolem of chieving, in the zone of therml cutting, the required thickness nd dhesiveness of the coting to the surfce. Prcticlly, we chieve cotings of the thickness not exceeding 100 μm in thermlly cut surfces, while on the remining element prt, the thickness of zinc coting is from 2 to 6 times higher [3,4,6]. Diversifiction of zinc coting thickness is considered s filure nd it lowers the product qulity. Due to the fct tht fittings for overhed power lines re used in corrosive environment of vrying ggressiveness, it is importnt to try to reduce descried difficulties sing upon the ction modifying the forming process. Using the products which re invlid with the stndrds (e.g. EN 61284), or which qulity does not meet the requirements my led to disstrous consequences. The im of reserch ws the indiction of the cuses of difference in thickness of the zinc coting on the surfce shped y OAB cutting. 2. TESTED MATERIAL The study ws conducted on links commonly used to connect fitting elements on insultor strings. The products re prt of so clled doule tension insultor strings, instlled on the superstructure of electric power lines. Doule tension insultor string is presented in Figure 1. The reserch ws focused on doule eye link type SLINK (Fig. 1 ) mde of S355JR steel. c Fig. 1 Fittings for overhed power line: - doule tension insultor string (ŁO2 110kV) 1 - hinge U-olt type, 2 - doule eye link, 3 - yoke tringulr, 4 - doule eye link twisted, 5 - olt, 6 - rcing horns, 7 - insultor, 8 - doule eye link type SLINK, 9 - suspension clmp, L1, L2 - dimensions depends of the length of the insultor; - the flt surfce of link type SLINK, c - the side surfce of link type SLINK

3 Chemicl composition of mteril used in experiment ws s follows: 0,18 %C; 0,23% Si; 1,5 %Mn; 0,012 %P; 0,008 %S; 0,030 %Cu. Cron nd sulphur were determined using LECO CS-125 nlyzer. Other elements were nlyzed on the ICP-OES spectrometer. Links were cut from steel sheet with thickness of 20mm y oxy-cetylene lowpipe (OAB) - CNC 500 MESSER cutter (temp o C, v=400 mm/min). Then the prepred mterils were sujected to n rsive lsting - steel shot GL40. The holes (d = 21 mm [+0,5]) were drilled nd chmfered in tested mterils. In the next stge links were treted chemiclly: pickling (hydrochloric cid 12 %, Fe contents 30 g/dm 3, pickling time t = 600 s), rinsing in cold wter, fluxing (TIBFLUX60 - ph 4,9, Fe contents: 0,17 g/dm 3, fluxing time t = 180 s). Hot-dip Zn glvnizing process ws mde in industril conditions in temperture: 457 o C nd time t=150 s in Zn th enriched in: nickel, ismuth nd luminum. The th chemicl composition ws s follows: 99,859 %Zn, 0,0481 %Ni, 0,0417 %Bi, 0,0002 %Al, 0,037 %Fe, 0,0058 %P, 0,0014 %Sn, 0,0067 %Cu, 0,0006 %Cd. 3. METHOD OF INVESTIGATION AND RESULTS ANALYSIS 3.1. Roughness nd surfce topogrphy Perthometer Concept (MAHR), with 3D equipment nd softwre, ws used to mesure surfce roughness nd topogrphy. The surfce roughness ws descried ccording to EN ISO 4287 nd EN ISO stndrds [8,9]. The surfce roughness mesurement ws mde on the flt surfce (fter rolling) nd on the side surfce (fter OAB cutting). It ws mde efore nd fter shot-lsting. The results of surfce topogrphy re shown in Figure 2. The verge vlues of presented prmeters to descrie R, Rp nd Rv my e nlyzed in Tle 1, where R - it is n rithmetic men of profile ordintes, Rp - height of highest pek of the profile, Rv - depth of the lowest vlley of the profile. c d Fig. 2 Surfce topogrphy of links:, - the flt surfce (fter rolling), c,d - the side surfce (fter cutting);, c - efore shot-lsting,, d - fter shot-lsting

4 Tle 1 The verge vlues of the sic prmeters of roughness (cc. Fig. 2) Roughness prmeters The flt surfce of link (crude- fter rolling) The side surfce of link (fter OAB cutting) Before shot-lsting After shot-lsting Before shot-lsting After shot-lsting R [µm] 1,42 2,44 5,31 5,72 Rv [µm] 10,38 14,66 16,20 11,12 Rp [µm] 9,85 9,84 14,59 9, Hrdness mesurement The hrdness mesurement ws crried out using Vicker s method ccording to EN ISO 6507 [10]. The exmintion ws divided in two stges. In the first stge the hrdness (HV10) of flt nd side surfce ws mesured. The mesurement ws conducted perpendiculrly to cutting pln. The verge vlues from dozen plces of the mesurement were s follows: on the flt surfce HV10, on the side surfce HV10. In the second stge the hrdness mesurement (HV0,5) ws crried out strting from the cutting edge towrd the smple core. The step of the mesurement ws estlished for 200 µm. The microhrdness mesurement results re presented in Figure 3. Fig. 3 Results of hrdness mesurement (HV0,5) on the flt nd side surfces of link in direction from cutting edge to the smple core 3.3 Metllogrphic nd X-ry nlysis Metllogrphic exmintions for smples fter cutting nd hot-dip zinc glvnizing were crried out. Metllogrphic specimens were prepred in clssic wy. The surfce ws etched with 4% HNO3. To microscopic oservtion the microscope AxioImger M1m Crl Zeiss ws used with mgnifiction: 50, 100, 200 nd 1000x. The results of selected oservtion re presented in Figure 4. In order to fully identify the prticulr phses occurring in the surfce lyer fter OAB cutting, n exmintion y scnning microscope PHILIPS XL30 with X-ry nlyzer ws conducted. The cross section of the steel structure oserved fter OAB cutting efore shot-lsting nd hot dip zinc glvnizing is shown in Figure 5, together with typicl EDS grph of the oxides lyer generted y therml impct (Fig. 5 ).

5 c HAZ HAZ Fig. 4 The S355JR steel structure: - the flt surfce (rolled), - the side surfce (fter OAB cutting), c - the side surfce (fter OAB cutting) nd shot-lsting B Fig. 5 The nlysis of selected micro re in the surfce lyer fter OAB cutting: - the plce of exmintion, - EDS grph in micro re B Metllogrphic exmintion were mde for link fter hot dip glvnizing nd lso confirms the diversifiction of zinc coting thickness nd its structure. The mesurement of Zn coting thickness ws mde for ll smples fter glvnizing. The verge vlues from dozen plces of the mesurement on flt nd side surfces re 158 μm ( Fig. 6 ) nd 59 μm (Fig. 6 ). Fig. 6 The zinc coting structure: - on the flt surfce of link, - on the side link surfces of link

6 X-ry nlysis ws executed in order to determine chemicl composition of zinc cotings. Reserch of chemicl composition ws crried out in selected cross-sections of smples with zinc cotings tht were stted s representtive smples. The mesure step ws 10 μm. Intermetllic phses distriution ws determined t the coting cross section. The cross section of the zinc coting structure on the flt surfce of link is shown in Figure 7, together Zn distriution t the cross section of the coting (Fig. 7 ). The cross section of the zinc coting structure on the side surfce of link is shown in Figure 7 c together Zn distriution t the cross section of the coting (Fig. 7 d).. c d Fig. 6 The microstructure nd Zn distriution t the cross section of the coting creted on link:, - flt surfce; c, d - side surfce; 3.4 Corrosion test in neutrl slt fog (spry) The NSS test ws conducted ccording to the requirements of EN ISO 9227 stndrd [11]. The following process prmeters were pplied: 5 % NCl; ph 6,7-6,9; temperture 35 o C; slt fog fll 1,6 ml/h. The results of NSS test fter 840 h in slt chmer were s follows: time to white corrosion ppernce (time to red corrosion ppernce): flt surfce - 24 h (840 h); side surfce - 24 h (408 h). 4. RESULTS DISCUSSION Although the OAB cutting elongs to the chepest methods of steel elements forming, in some cses, i.e. when the hot-dip zinc glvnizing is pplied, this method cn cuse serious, difficult to void prolems - diversifiction of zinc coting growth dependent on the se steel surfce condition. The structure of the mteril in the initil stte ws little it nisotropic, minly ferritic with smll mount of perlitic. Figure 4 presents drk perlite re nd light ferrite re. In the oserved HAZ zone the needle shpe structure ppered - lower inite, mrtensite (Fig. 4, c) creted s result of undesirle quenching tretment cused y cutting. The HAZ re oundry is running prllel to cut edge. Its width in the nlyzed re mounts to out 800 µm. After OAB cutting it is necessry to use mechnicl tretment -

7 shot-lsting in order to remove scle, which hs n influence on the surfce roughness. In this cse, lyer of out 30 µm scle (Fig. 5) ws removed. Roughness oserved on the exmined surfce resulted from the steel shot used during the tretment efore glvnizing. The more stress steel surfce fter shot-lsting thn fter OAB cutting is presented in Figure 4 c. It hs een oserved tht the rsive lsting method - shot-lsting which ws crried out in order to clen the surfce efore hot dip glvnizing is insufficient for surfce fter OAB cutting. Zn coting thickness diversifiction on the flt surfce nd side surfce is uncceptle. Shot-lsting my proly eliminte the lck of coting dhesiveness to the sustrte. In this study no prolems with this phenomenon occurred. The zinc coting structure diversifiction. The Zn coting put on the flt surfce (fter rolling) revels typicl structure for this grde of steel - composed of phses: Г, δ, ζ nd η (Zn) (Fig. 6 ). Lyers creted y iron diffusion (δ+ζ) occupy the greter prt of the coting thickness (135μm - 84% of totl thickness). The coting growth ws not distured/hindered y the het ffection results of cutting (there is no HAZ) nd the ppliction of shot-lsting efore glvnizing dditionlly supported proper process course. In cse of Zn coting creted on the side surfce (fter OAB cutting) the oserved structure is quite different: the lloyed re is much thinner (40μm - 57% of totl thickness) - the rest is occupied y pure zinc. The differentition of corrosion resistnce mesured during the NSS test on the rolled surfce nd side surfce is closely relted to the structure of the zinc coting. It ws stted tht so-clled "white corrosion/rust" ppers on ll smples fter 24 h. The time of red rust ppernce on the flt surfce of link is twice s long s the surfce fter cutting. 5. CONCLUSION The reduced iron diffusion rte to Zn coting on the cut side surfce is proly the reson of oth thickness of the entire coting nd its individul sulyers diversifiction due to the reduction of the soluility of the steel sis resulting from the cretion of HAZ zone. The insufficient clening of the steel surfce fter OAB - remining oxide lyers cn lso crete the rrier hindering the diffusion process. The prolem with Zn coting diversifiction my e solved in two wys: using different cutting methods (lser nd wter-jet cutting)- thnks to tht eliminting OAB cutting from the production of difficult fittings or using n dditionl tretment fter OAB cutting for exmple normlizing nneling or grinding. REFERENCES [1] KLIMPEL A., Spwnie, zgrzewnie i cięcie metli. Technologie, Wrszw, WNT, [2] RUDNIK S., Metloznwstwo, Wrszw, PWN, [3] SCHULTZ, W., THIELE, M., Cynkownie ogniowe jednoskowe. Mterił - Technologi - Powstwnie powłoki- Włściwości- Błędy, Bd Sulgu: Eugen G. Leuze Verlg KG, [4] KUKLIK V., KUDLÁČEK J., Žárové zinkování, Asocice českých slovenských zinkoven, Prh, [5] KUKLIK V., Povlky žárového zinku n pálených plochch, in 17 konference zroveho zinkovni, Ostrv, Czech Repulic, [6] EN ISO :2009 Zinc cotings. Guidelines nd recommendtions for the protection ginst corrosion of iron nd steel in structures. Hot dip glvnizing [7] EN 61284:1996 Overhed lines. Requirements nd tests for fittings [8] EN ISO 4287:1998 Geometricl product specifiction (GPS). Surfce texture: Profile method. Terms, definitions nd surfce texture prmeters

8 [9] EN ISO :1998 Geometric product specifictions (GPS). Surfce texture: Profile method. Surfces hving strtified functionl properties. Height chrcteriztion using the liner mteril rtion curve [10] EN ISO :2005 Metllic mterils. Vickers hrdness test. Test method [11] EN ISO 9227:2012 Corrosion tests in rtificil tmospheres. Slt spry tests