TIG WELDING OPPORTUNITIES OF BIMETALLIC ENDLESS SAW BLADES Ion MITELEA a, Dumitru-Daniel OCHIAN a, Mircea BURCA a, Ion-Dragos UTU a a POLITEHNICA UNIVERSITY OF TIMISOARA, Pta Victoriei,no. 1, Timisoara,Romania, ion.mitelea@mec.upt.ro, danochian@yahoo.com, mircea.burca@mec.upt.ro, utu.dragos@mec.upt.ro Abstract The paper contains the results of the experimental researches regarding the process parameters by TIG welding of the bimetallic saw blades. The active part of these bands is made of high alloyed tools steel and the holder from Cr-Mo-V low alloyed steel. The joining of the both parts is realized by electron beam welding. By mechanical tests and metallographic examinations one appreciates the quality of the welded joint. The established technological solution is the economical efficiency especially by unique or small series production and also in case of reconditioning of saw blades broken during exploitation. Keywords: welding, bimetallic blades, examinations 1. INTRODUCTION In the case of large series and mass production, joining of endless band-saw blades is made using the electrical welding process by butt fusion from intermediate to direct melting which ensures high productivity and quality of processing. However for a small series manufacturing or in case of broken saw blades rewelding during exploitation, the acquisition of such equipment by companies is expensive and not economically justified because the return of investment is on long term. This paper presents an alternative to the saw blade welding using the TIG welding process, accessible to most companies producing welded steel structures. The performed experimental researches led to the optimization of the TIG welding technology for two DUOFLEX M42 saw blades sizes produced by the German company EBERLE, having the dimensions of b x s = 34 x 1.1 mm, respectively b x s = 41 x 1.3 mm. 2. EXPERIMENTAL PROCEDURE The bimetal saw bands (Fig.1) are used for cutting of high carbon content steels, of stainless steels and other hard materials chips, where the developed temperature during their processing does not exceed 500-600 C. Their active part (the teeth) is made of high speed steel and the holder is of Cr-Mo-V alloyed steel (Table 1). The joining of the two parts is achieved by electron beam welding. The experimental researches followed the establishing of the process parameters using a filler material with a solid state transformation point which was not compatible with the base material of the cutting tool holder. According to Figure 1 the ITH welding was made in the gap area between two neighbouring teeth. After welding a local tempering heat treatment was applied. The quality appreciation of the welded joint was made by macro- and micrographic investigations, sclerometric examinations and liquid penetrant testing.
Bi-Metal WELD Cr-Mo-V alloyed steel High speed steel Fig.1. Bimetal saw band Table 1. The chemical composition of the blade material Active part High speed steel HS2 10 1-8 Elements Chemical composition, wg. % Prescriptions Actual composition C 1,00 1,10 1,06 Si 0,15 0,40 0,23 Mn 0,20 0,45 0,37 Pmax. 0,025 0,016 Smax. 0,015 0,011 Cr 3,50 4,25 3,94 W 1,25 2,00 1,81 Mo 9,00 10,00 9,58 V 1,00 1,50 1,29 Co 7,75 8,75 8,12 Tool holder Alloyed steel X32CrMoV4-1 C 0,29 0,33 0,32 Si 0,20 0,35 0,28 Mn 0,90 1,10 0,96 Pmax. 0,020 0,012 Smax. 0,010 0,08 Cr 3,80-4,00 3,90 Ni 0,60 0,80 0,62 Mo 1,00 1,20 1,08 V 0,30 0,40 0,32 3. EVALUATION AND INTERPRETATION OF THE RESULTS 3.1 Establishing of the process parameters TIG welding is made with or without filler material in form of wire, in AC or DC, the welding supply having the external characteristic with abruptly descending. The method can be applied in one of the variants: semimechanized, mechanized or manual. The experiments were performed using the welding equipment having the MAGIC WAVE 300 invertor. In order to avoid the danger of breakdown, respectively of band overheating, the welding was carried out in a device having the holder and the clamping dies made of copper (Fig. 2). The weld joint was in form of I with the opening b = 0. In order to form a penetrated root, the holder is provided with a channel having 1.5 mm width and 0.5 mm penetration. Welding is the point I with the opening b = 0. To achieve high quality welds along the whole band length, the beginning and ending of the welding is done on technological plates.
Figure 3 presents the relative position of the welding head and of the filler material (rod electrode) by TIG welding. The performed experiments permitted the optimization of the following proces parameters: 1. Filler material: G3Si1 wire Wire diameter: 1 mm Shielding gas Ar 100% Electrode type: non-fusible Electrode diameter: 2.4 mm 2. Technological parameters presented in table 2. Fig.2. Welding device Fig. 3. Relative position of the welding head Figure 4a, b presents the outer and the root aspect of the welded samples before removing the technological plates. One can notice an esthetical seam with an uniformly width on the entire bandwidth, respectively a penetrated root with an increased height of about 0.5 mm. No marginal indentations or defects at the beginning or ending of the welding are observed. The forced cooling effect of the clamping dies is shown by the silver color of the weld surface. Table 2. Technological parameters of the saw blades Technological parameters Blade sizes b x s = 34 x 1.1 mm b x s = 41 x 1.3 mm - welding current, I s [A] 38 42 - welding arc voltage, U a, [V] 11 12 - welding speed, v s, [cm/min] 6 6 - welding arc length, mm 2 2 - gas flow, Q g, [l/min] 8 8 - welding direction to the left (by pushing) to the left (by pushing) inclination of the welding torch 80 (to the components) 80 (to the components) of the wire 15 (to the components) 15 (to the components)
2.2 Heat treatments after welding a - - b Fig.4. The aspect of the welded joint: a- surface; b- root After the band saw welding a high temperature tempering heat treatment must be done, resulting a more favorable structure in the heat affected zone (HAZ) by decreasing the hardness and respectively increasing the material toughness. In lack of it a fragile fracture in the HAZ zone appears under the bending load during exploitation. The researches aimed to establish the optimal technological parameters of heat treatment to avoid the danger of sample braking by its bending on a 300 mm diameter role at an angle of 180 º, practical in exploitation conditions. In order to realize the heat treatment a welding equipment for soldering by overlapping and pressure of the saw blades was used. Practically the heating was realized by Joule-Lenz effect by passing the current through the welded samples fixed in the machine clampling dies. According to figure 5, the equipment contains: - pressure welding installation; - the autotransformer to control the primary circuit voltage respectively for regulating the current through the transformer secondary circuit; - Yokogawa digital device for measuring the temperature of the assembly welded sample-thermocouple. To check the heating and maintaining temperature an assemblage was realized using a Cromel-Alumel thermocouple fixed in the seam axis by inserting the thermocouple hot point in a TIG molten bath, providing an intimate contact between the thermocouple hot point and the base metal, increasing the temperature measurement accuracy. Figure 6 shows a mounting detail of the thermocouple on the welded sample. During the optimization proceess of the two main parameters, the heating temperature and the maintaining time, the following two values resulted: - heating temperature T i = 500 ± 10ºC ; - maintaining time t m = 13-16 s for the saw blade 34 x 1.1mm, respectively t m = 15 18 s for the saw blade 41 x 1.3mm.
Fig. 5. The stand for the tempering heat treatment Fig.6. Thermocouple fixed in the seam axis 3.3 Quality of the welded joints 3.3.1 Bending test The bending test seeks to determine the angle at which the cracking phenomenon is initiated. It was performed on both heat-treated and not heat treated samples. In case of the not heat treated samples, a brittle fracture was observed in the heat affected zone (HAZ) at a bending angle of 15-45º; applying the heat treatmentand bending at an angle of 180º on a 300 mm bar no cracking of the welded joint appeared (Figure 8). Such a test is more severe than the requests from exploitation where the diameter roll of the machine is larger. 3.3.2 Testing with penetrant liquids This inspection reveals the defects like any surface-breaking cracks or cavities; the method is based on liquid penetration by capillary diffusion (soaking) in very fine discontinuities. Subsequently, this liquid is absorbed and by color contrast indicates the presence of the defects. (a) (b) Fig.8. Bending test of the TIG welded and tempered saw blade Fig.9. The aspect of the surfaces after applying the liquid: a- outer zone; b-root zone 3.3.3 Hardness measurements Hardness indentation marks arrangement beside the measurements values carried out on the saw blade 34 x 1.1 mm are shown in Figure 10. As expected, the highest hardness values appear in the base metal and
region from the heat affected zone (HAZ) adjacent to the weld metal. In contrast, the filler material dilution with the base metal lead to smaller hardness values in the weld seam which ensures high resistance to brittle fracture of the welded joint. However, due to the effect of overtempering of the HAZ region adjacent to the base metal HAZ, promoted by the welding heat cycle, a hardness decreasing to values about 460-480 HV was noticed. Fig.10. Arrangement of the hardness indentation marks HV for the 34 mm saw band 3.3.4 Macro and micrographic examinations Metallographic investigations come to confirm the results obtained in previous attempts. Thus, the macrographic image of a parallel section with the longitudinal axis of the weld (Fig. 11a) shows a suitable geometry of the joint, with no structure and chemical composition heterogeneities. For the micrographic examination it has been used cross side samples cut to the longitudinal axis of the weld allowing the analysis of all structural domains presented in the deposited metal and base metal HAZ. In Figures 11b and 11c one can observe the formation of a bainito-martensitic structure in the weld seam and of a martensitic structure in HAZ with a nonuniform carbides distribution.
-a- -b- -c- Fig.11. Structure of TIG welded joint: a macrographic image; b,c welded joint micrographs 4. CONCLUSIONS TIG welding of the endless saw blades is economically efficient for the small series production and for repairing of broken blades during their exploitation. The selected filler material (wire G3Si1) is a soft steel having a solid state transformation point, which leads to the formation of a deposited metal providing an optimal combination between characteristics of mechanical strength, ductility and toughness. Metallographic investigations showed the formation of an esthetic welded seam, with an uniform width over the entire area of the joint, without metallic continuity defects and with a favorable microstructure for longterm exploitation. REFERENCES [1] Mitelea, I., Cr ciunescu, C. Parameter influence on friction welding of dissimilar surface - carburized/ volume hardened alloyed steels. Materials & Design, 2010, s. 2181 2186, ISSN 0261 3069; [2] Sun, Z., Karppi, R. The application of electron beam welding for the joining of dissimilar metals: an overview. Journal of Materials Processing Technology, 1996, Volume 99, Issue 1-3, s. 257-267, ISSN: 0924-0136; [3] Modenesi,P.J., Apolinário E. R., Pereira I.M. TIG welding with single-component fluxes, Journal of Materials Processing Technology, 2000, Volume 99, Issue 1-3, s. 260-265, ISSN: 0924-0136;