STRUCTURAL TRANSFORMATIONS IN ELECTRON BEAM WELDED JOINTS OF CONTINUOUS SAW BLADES. Dumitru OCHIAN, Ion-Dragoş UŢU, Ion MITELEA

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1 STRUCTURAL TRANSFORMATIONS IN ELECTRON BEAM WELDED JOINTS OF CONTINUOUS SAW BLADES Dumitru OCHIAN, Ion-Drgoş UŢU, Ion MITELEA Politehnic University of Timisor, Pt Victoriei, No. 1, Timisor, Romni, Abstrct The ctive prt of the endless sw bldes is mde of rpid steel, HS , nd the nd the holder from X32CrMoV4-1 lloyed steel. The joining of the both prts is relized by electron bem welding which provides the obtining of nrrow sems, smll het ffected zones (HAZ), good reproducibility of the results nd uniform microstructure in welded joint zones. In cse of the smll series production or by the reshuffling of sw bldes broken in exploittion, one propose the replcement of the electricl welding process by butt fusion from intermedite to direct melting with the TIG welding method [1]. The present pper nlyzes the structurl nd chemicl composition modifictions produced in the welded joints zones obtined by the electron bem welding between two dissimilr steels. The phenomen which guide the continuous evolution of the chemicl composition of the weld re highlighted by energy dispersion X-ry technique (EDX) nd scnning electron microscopy. Keywords: fusion welding, tool steels, microstructure 1. INTRODUCTION Sws for cutting of metllic mterils show both producer nd user, some fetures bout the shpe nd size of the blde, the mode of ction nd the hrdness of the mteril to be processed. The selection ctivity of the steels used for the execution of these tools is bsed on their geometricl shpe nd the nture of stresses occurred during opertion. The most representtive generl chrcteristics of such tool steels re: hrdness, wer resistnce nd toughness. The sw bldes exmined in this pper re bimetl type, mde of two mterils, nmely: n lloyed Cr-Mo-V steel, X32CrMoV4-1 grdes, for their support, chrcterized by good toughness chrcteristics nd stisfctory welding behviour, respective rpid steel, lloyed with pprox. 8% Co nd 10%, HS grdes for cutting crown, which provides high resistnce to wer nd ftigue [1,2]. The two mterils re joined through the process of electron bem welding. To obtin continuous, endless sw blde, nd by the reshuffling of the sw bldes broken in exploittion the TIG welding method is economiclly justified [1]. This pper nlyzes the morphologicl chnges of welded joints obtined by electron bem welding. 2. EXPERIMENTAL RESULTS, DISCUSSIONS Electron bem welding belongs to welding processes which re using opticl energy sources. The emission of n electron bem is produces by heting of W or T filment t high temperture. The kinetic energy

2 of the focused electrons on the mteril surfce is converted mostly into het (energy density exceeds 10 6 W/cm 2 ) so tht the mteril from the joint zone not only melts, but lso instntly evportes t temperture of pprox C. As the electron bem is highly focused, the het inserted in the mteril is much lower thn in rc welding processes. Therefore, the effect of the welding process on the mteril ner the weld is miniml, nd the het ffected zone hs smll expnsion. The distortions occurred re reduced, cooling of the welded zone is done t high speed, which cn led to crcking of steels with high crbon nd lloying elements. According to Figure 1 usully re welded unlloyed nd low-lloy steels with reduced crbon content, stinless steels, nickel nd coblt bsed lloys, etc [3]. The process is used lso by welding of wide vrition of mterils combintions [4]. Ni nd Ni lloys Cu lloys Co lloys Unlloyed nd low lloyed steels with low crbon content High lloyes steels with high crbon Pure Cu Al lloys Titnium High lloyed steels with very low crbon content Gry cst irons Weldble with specil precutions Alloy dependent weldbility Unweldble or difficult-to-weld Fig.1 Mterils nd mteril combintions weldble by electron bem In figure 2, b is shown the profile of such welded joint, one cn see tht the metllic continuity obtined fter the contct between the liquid nd the solid metl is pproprite, being bsent the oxidtion nd crcking phenomen. Moving step by step the molten bth, this reches prcticlly to qusi-sttionry size due to the het blnce provided respective evcuted. b Fig.2 Weld joint microscopic imge of the two dissimilr steels: 50 x, b 100 x

3 The molten bth shows in this cse symmetry with medin plne which psses through the welding xis (welding direction). Since the mixing degree (dilution) of the two mterils tht prticipte by the formtion of the molten bth is 100%, the solidifiction microstructure is close to tht of the Cr-Mo-V high speed steel in the mteril zone djcent to the tool body (fig. 3, 4, 5). If fter hrdening t C followed by tempering t C of the rpid steel one obtins microstructure consisting of crbides embedded in tempered mrtensitic mtrix (Fig. 3) nd in the steel holder of the tool, the microstructure resulted fter the secondry hrdening het tretment t C followed by tempering t C is composed of tempered troostite (fig.4), the weld presents microstructure consisting of fine prticles of crbides rrnged in mrtensito-troostitic mtrix bse (fig. 5). Fig. 3 SEM imge of HS steel Fig. 4 SEM imge of X32CrMoV4-1 steel b

4 c d e Fig. 5 SEM imge of the weld:, b-microstructure of the trnsition zone tooth weld, c, d- microstructure of the weld, e-microstructure of the trnsition zone weld steel tool holder In the first moments when the welding bth is formed some importnt movements tke plce cused by the ction of the therml nd dynmic het source. The overheted liquid is the plce of complex peripherl movement which begin from the melting front to the bck ozone of the bth, with n immedite genertion of trnsfer of mtter. The mintining time of the toms in the liquid phse is vrible. Some of them re blocked by the dvncing of the solidifiction front nd the others rech to this front lter, depending on the pth it is imposed. The rest of the toms leve the environment by voltiliztion or through vrious chemicl rections. As result, the chemicl composition of the molten zone evolves both continuously nd discontinuously. Metllogrphic investigtions together the with EDX nlysis (Fig. 6, 7 nd Tb. 1) come to confirm these locl chnges of chemicl composition nd microstructure.

5 f c h d g b e -CoK1 b-mol1 c-crk1 d-vk1 e-nik1 f-fek1 g-mnk1 h-sik1 Fig. 6 Liner distribution of the chemicl elements on the weld surfce: -coblt, b-molybdenum, c-chromium, d-vndium, e-nickel, f-iron, g-mngnese, h-silicon

6 b Fig. 7 SEM imge of the welded joint () nd EDX spectrum of the weld (b) Tb. 1 Compositionl nlysis of the weld Element Weight% Atomic% Si K V K Cr K Mn K Fe K Co K Ni K Mo L Totl Since the welding is performed without filler mteril, the prticiption of the two metls to form the molten zone is complete. In this wy it is justified the chemicl composition of the weld, which differs little from both of the rpid steel nd of the Cr-Mo-V steel (Tble 1). Beside dilution, the continuous evolution of the chemicl composition of the weld is cused by voltiliztion phenomen of some chemicl elements (W, Mn, Co, Mo, etc..), by the gses emissions of both steels due to melting, by the vrition of the het regime, by the fluctution of the solidifiction rte, constitutionl subcooling, dendritic segregtion, etc.. From the presented dt it cn be seen tht the chromium concentrtion in the weld is similr to tht of the bse metls, this element fvours the incresing of hrdening penetrtion nd prticipte to the crbides formtion. Coblt is present only in the chemicl composition of the rpid steel nd if it is ssumed tht the two bse metls prticipte in pproximtely equl proportions to the weld formtion it cn be justified its concentrtion of pprox. 4%. It plys n importnt role in incresing of the tempering stbility nd the hot hrdness. Molybdenum is present in both bse metls nd consequently is found in the weld t high enough concentrtion so tht hs positive effect on the incresing of the toughness nd the wer resistnce (by the formtion of specil crbides) nd of the tempering stbility.

7 Also vndium is s n lloying element in both steel so tht it ppers in weld. It cuses n improvement of the wer resistnce. 3. CONCLUSIONS Compred to the conventionl welding processes s well s lser welding, the electron bem welding process input in the mteril the lowest specific het nd provides the nrrowest melting zone with the lowest oxidtion nd the lowest strins. The heterogeneous welded joints between the rpid steel HS nd the lloyed X32CrMoV4-1 steel show suitble geometry without metllic continuity defects, with chemicl composition nd microstructure close to the two bse metls REFERENCES [1] MITELEA, I., OCHIAN, D., BURCA, M., UTU, I.D., TIG welding opportunities of bimetllic endless sw bldes. Metl 2011, 20 th Interntionl Conference on Metllurgy nd Mterils, Brno, s , ISBN [2] UTU, I.D., MITELEA, I., OCHIAN, D., BURCA, M., Contributions on MIG/MAG welding of bimetllic endless sw bldes. NANOCON 2012, Brno. [3] FERRO, P., ZAMBON, A., BONOLLO, F., Investigtion of electron bem welding in wrought Inconel 706- experimentl nd numericl nlysis. Mterils Science nd Engineering A 392, 2005, s [4] SUN, Z., KARPPI, R., The ppliction of electron bem welding for the joining of dissimilr metls: n overview. Journl of Mterils Processing Technology, Vol. 59, Iss. 3, 1996, s