Evaluation of WC-9Co-4Cr laser surface alloyed coatings on stainless steel

Transcription

1 Evlution of WC-9Co-4Cr lser surfce lloyed cotings on stinless steel B A Odele 1, P A Olumi 1, S L Pityn 2 nd A P I Popool 1 1 Deprtment of Chemicl nd Metllurgicl Engineering, Tshwne University of Technology, P.M.B. X680, Pretori, South Afric, Center for Scientific nd Industril Reserch- Ntionl Lser Centre, P.O.Box 395, BLD 46F, Pretori, South Afric, E-mil: ioduno@tut.c.z Astrct. In order to exmine the microstructure nd hrdness ehviour of WC cermets, cotings hve een otined y lser surfce lloying technique. WC-9Co-4Cr prticulte ws injected onto the surfce of AISI 304L ustenitic stinless steel (ASS) under different lser processing vriles. The morphologies nd microstructures of the composite cotings were investigted using opticl microscopy nd high resolution field emission scnning electron microscopy (FESEM) equipped with energy dispersive spectrometer (EDS), while the phse chnged were oserved using x-ry diffrction (XRD). The surfce hrdness ws determined using the Vickers microhrdness tester. The decomposition of WC-9Co-4Cr into W 2 C, C nd W is s result of low het of formtion of WC nd low ffinity of tungsten for Cron. Free Co nd C in the meltpool formed intermetllic phses such s Co 6 W 6 C nd M 23 C 6 (M=Fe, Cr, W). A considerle increse in hrdness vlue of the mtrix 246 H v0.1 compred to the coting 1331 H v0.1 ws chieved when lloying ws crried out t 2.0 kw lser power nd scn speed of 0.6 m/min. 1. Introduction Tungsten cride (WC) elongs to the group of dvnced cermic mterils with gret industril importnce nd well known s hrdfcing mteril with Co or Ni lloys s inders. Aprt from high hrdness, WC hs unique set of properties; high melting point, wer resistnce, good therml shock resistnce, therml conductivity nd good resistnce to oxidtion [1-3]. However, WC cermets melt nd dissolve in melt pool formed y lser irrdition of the surfce lyer of the sustrte due to low free formtion enthlpy of 38.5 kj/mol. According to Nerz et l, [4] depending on time nd temperture WC decrurize to form W 2 C, nd lter free tungsten nd cron. The cron cn rect with tmospheric oxygen to form CO/CO 2, which could e trpped in the melt pool during rpid solidifiction. As result of this, pores re formed in the composite coting [2] nd this could limit the wide-scle industril recognition of this composite. Lser lloying gives perfect dhesion to the interfce of the ulk steel nd the coting with homogeneous microstructure due to strong mrngoni convection cused y surfce tension grdients nd high cooling rtes [5, 6]. With optimum lser processing prmeters, relile coting tht is free of crcks nd pores cn een produced on the mtrix. In this pper, WC-9Co-4Cr composite cotings were prepred on 304L ASS the lser processing prmeters.

2 SECTION C LASERS, OPTICS AND SPECTROSCOPY 2. Experimentl methods The specimen investigted ws 304L stinless steel with dimension of 64x40x4 mm. The reinforced powder ws pure gglomerted nd sintered WC-9Co-4Cr cermet s shown in figure 1. Figure 1. SEM imge nd EDS of WC-9Co-4Cr prticle showing the () Co-Cr inder nd () WC. The verge prticle size ws pproximtely 26 µm. The surfce melting opertion ws conducted using 4.4 kw Nd:YAG lser. The specimen surfce ws shielded y rgon gs flowing t 2 L/min during the lser scnning to prevent oxidtion. The lser em ws focused to 3 mm dimeter size while the lser power used ws 2.0 kw nd the scnning speed ws vried from 0.6 to 1.2 m/min. After lser injection, the cotings were sectioned, mounted nd polished for metllogrphic exmintion s well s hrdness testing. XRD of the coted surfce ws performed on PW1710 Philips diffrctometer, using monochromtic Cu K he phses were identified using X pert High Score Plus softwre. The microstructures nd the distriution of hrd phses were exmined using field emission scnning electron microscope (FESEM) equipped with energy dispersive spectrometer (EDS). Microhrdness profiles of the cross section from the coted zone to the mtrix were mesured using n EMCO TEST Durscn microhrdness tester t lod of 100 g. 3. Results nd discussion Figure 2 shows the SEM microgrphs of the specimen reinforced with WC-9Co-4Cr t lser power of 2.0 kw nd vrying scn speeds of 0.6 to 1.2 m/min. No pores nd crcks were present from the SEM imges. It could e seen tht undissolved WC cermets re present t the top surfce of the cotings nd this vry for ll the cotings with specimen 1 lloyed t lser power of 2.0 kw nd scn speed of 0.6 m/min hving most of such undissolved crides. This could e ttriuted to the volume of crides injected nd the coted surfce exposure to the tmosphere. At low scn speed, more crides re injected into the melt pool nd due to high cooling rte, the crides t the top solidifies efore eing dissolved. High wettility

3 of WC nd strong mrngoni convection lso ids the uniform distriution of dissolved crides in the melt pool nd segregtion of the undissolved crides t the coting surfce s cn e seen in figure 2. Melting, dissolution nd resolidifiction of WC took plce to form WC, W 2 C, Co 6 W 6 C nd M 23 C 6 (M=Fe, Cr, W) s indicted in the XRD nlysis in figure 3. The EDS point nlysis in figure 4 lso confirms the presence of W, C, Co, Cr elements in the mtrix. c d Figure 2. SEM microgrph of 304L stinless steel lloyed with WC-Co-Cr t lser power of 2.0 kw nd scnning speed of () 0.6, () 0.8 (c) 1.0 nd (d) 1.2 m/min Figure 3. XRD nlysis of specimen 1t lser power of 2.0 kw nd scn speed of 0.6 m/min.

4 SECTION C LASERS, OPTICS AND SPECTROSCOPY Microhrdness mesurement ws performed on the cross section of the specimens from the coted lyer to the mtrix s shown in figure 5. Specimen 1 t lser power of 2.0 kw nd scn speed of 0.6 m/min gve the highest hrdness of 1331 Hv 0.1. The coting thicknesses for the specimens were found to e pproximtely 1750, 1260, 1140 nd 1055 µm respectively s shown in figure 5. The vrition in hrdness nd thickness is dependent on the speed of the lser em. At lower scn speed, the lser em irrdites the specimen longer, thus lrge nd deep meltpool forms. Figure 4. SEM imge nd EDS point nlysis t lser power of 2.0 kw nd scn speed of 0.6 m/min Figure 5. () Microhrdness profiles of the lloyed lyers nd () Depth of coted zone s function of lser speed.

5 4. Conclusion It hs een shown tht 304L ASS mtrix composite cn e prepred y lser surfce lloying without pores nd crcks. Lser surfce lloying with WC cermet powder result in n increse in hrdness of 304L ASS. At lser power of 2.0 kw nd scn speed of 0.6 m/min, hrdness vlue ws out 5 times tht of the mtrix. This is ttriuted to the formtion of hrd phses such s WC, W 2 C nd Co 6 W 6 C. Acknowledgements The uthors grtefully cknowledge the finncil support from the Africn Lser Centre (ALC) (LHEAB01 Tsk 300), Tshwne University of Technology nd lser time from the Ntionl Lser Centre- CSIR. References [1] Wu P, Zhou C Z nd Tng X N 1995 Surf. Cot. Technol [2] Wu P, Du H M, Chen X L, Li Z Q, Bi H L nd Jing E Y 2004 Wer [3] Zhou S, Zeng X, Hu Q nd Hung Y 2008 Appl. Surf. Sci [4] Nerz T C, Nerz J E, Kushner B A nd Riggs W L 1993 Surf. Eng [5] Xio-Hu Y nd Xi C 2002 Chin. Phys Lett [6] Gugliotti M, Bptist M S, Politi M J, Silverstein T P nd Slter C D 2004 J. Chem. Edu [7] Cheng F T, Lo K H nd Mn H C 2003 Surf. Cot. Technol