Structure Improvement of Aluminothermic Welding Joints by Using Modifiers

Aluminothermic rail welding was from the beginning a great success because its low price even in 1895 in Germany. This method is now, widely used all over the world for the railways construction, maintenance and modernization. Instructions give you guidelines for preparing papers for conferences or journals. After 1989, the welding needs of the potentials beneficiaries (Romanian Railways, Urban Transportation Companies) keep raise because of the railways maintenance and modernization necessity. The main materials that determine the Thermit (T) composition result from manufacturing scraps all over the country. This can help the environment by consuming these scraps. The Romanian need for alumino-thermic welding is now by 11300 per year, and in a favourable economical environment, this amount can reach 30000 units. This paper tries to show the effect of two types of modifiers introduced in the T composition on the structure and properties of an alumino-thermic welding. Keywords—aluminothermic rail welding, modifier, Thermit. I. THE ALUMINOTHERMIC WELDING PRINCIPLE HE alumino-thermic welding is base on the extremely exothermic reaction between iron oxides (FeO, Fe2O3, Fe3O4) and aluminium powders, by the following formulas [1]-[2]-[3]: ) 3088 / 3009 ( 9 4 8 3 0 3 2 4 3 C KJ Fe O Al Al O Fe + + → + (1) ) 2500 / 783 ( 3 2 3 0 3 2 C KJ Fe O Al Al FeO + + → + (2) ) 2960 / 850 ( 2 2 0 3 2 3 2 C KJ Fe O Al Al O Fe + + → + (3) The Thermit powder (charge) reacted in the crucible and after that, the welding process started (see Fig. 1). Thermit powder Welding process welding joint Fig. 1 The welding process [4] The molten steel temperature as a result of aforementioned reaction is about 2500oC. Mihai Brânzei. is with the Faculty of Materials Science and Engineering, from University POLITEHNICA of Bucharest, 313 Splaiul Independentei, Sector 6, 060042Bucharest, Romania (phone: 0040-21-4029782; Fax: 0040-21-3169562; e-mail: [email protected]). Tudor Adrian Coman was with S.C.Quark EXE S.R.L., 121 Str. Răsadniței, Sector 6, Bucharest, Romania (e-mail: [email protected]). The amount of added alloy, inclusive modifier will decrease molten temperature below 2100C. The melting temperature must be in concordance with pre-heating rail ends (about 950C) in order to avoid under-cooling in contact with cold rail end. It is also very important to insure the cleanness of the mould and melt, because of direct influence on weld joint inclusions rate quality and weldment mechanical properties. II. EXPERIMENTAL CONDITIONS This paper exhibit the effect of two types of modifiers introduced in the T composition on the structure and properties of an aluminothermic welding. The aluminothermic welding structure improvement was ensuring by the presence of modifier in the melting. The chemical composition of TRQ Thermit – steel, before and after modifier with FeSi34V25Ti12 (M1) and FeSi45Zr35 (M2), is indicate in Table I. TABLE I CHEMICAL COMPOSITION OF TRQ THERMIT – STEEL Modifier (wt.%) Unmodified Modified with FeSi34V25Ti12 Modified with FeSi45Zr35 C 0.50 0.47 0.46 S 0.018 0.019 0.018 P 0.026 0.026 0.022 Si 0.68 1.62 1.85 Mn 1.37 0.48 0.62 Cr 0.18 0.07 0.10 Ni 0.95 0.10 0.10 Al 0.12 0.055 0.23 Ti <0.005 0.092 0.006 V 0.24 Zr 0.09 There were performed different types of aluminothermic welding, depending of the T chemical composition (see Table II). It was also measured the HBW (SR EN ISO 6506-1/2002) after welding of all types of combinations between the moulds (FF – from France; FRQ – from S.C.Quark S.R.L.) and the Thermit (TF – from France; TRQ – from S.C.QUARK EXE S.R.L.) [5]. TABLE II HARDNESS FOR DIFFERENT TYPES OF MOULDS AND THERMIT Weld No. Weld type HBW 2.5 (N/mm) HBW 2.5 Mean value HBW 2.5 Rail end 2 TF-FF 223, 216, 203, 193, 221 211.2 220-250 6 TRQ-FRQ 249, 182, 206, 256, 200, 222 226.6 3 TF-FRQ 182, 182, 231, 197,207 198.4 Mihai Brânzei, Tudor Coman Structure Improvement of Aluminothermic Welding Joints by Using Modifiers T World Academy of Science, Engineering and Technology International Journal of Chemical, Molecular, Nuclear, Materials and Metallurgical Engineering Vol:6, No:8, 2012 745 International Scholarly and Scientific Research & Innovation 6(8) 2012 scholar.waset.org/1999.2/8439 In te rn at io na l S ci en ce I nd ex , M at er ia ls a nd M et al lu rg ic al E ng in ee ri ng V ol :6 , N o: 8, 2 01 2 w as et .o rg /P ub lic at io n/ 84 39 7 TRQ-FF 228, 265, 232, 228, 219 234.4 III. RESULTS AND DISCUSSION A. Qualitative analysis of welding joints structure In order to characterize the Thermit-steel as a distinctive entity and the alumino-thermic welding of the railways metallographic analysis were perform, these being the second stage of the thermit-steel and welding metallographic characterization of the train and tram railways. The microstructure of the railway active zone (RAZ), shows that the ferrite proportion is below wt.1%, so the steel can be considered an eutectoid one (see Fig. 2) [6]. Fig. 2 Microstructure of RAZ; the rail end part The microstructure aspects, both in the weld zone (WZ), and Heat Affected Zone (HAZ), in mushroom zone (MZ) and heart zone (HZ), before and after modification with FeSi34V25Ti12 and FeSi45Zr35 , are present in Fig. 3 and Fig. 4. The effect of the modification is visible in both cases and in all parts of the rail. The Widmanstattenn structure is less evident, and the perlite finishing degree is very advanced, ensuring a good tenacity. The etching reagent was nital 2%. The weld has a bigger and different grain size and distribution in mushroom zone, in compared with the heart one. The oxide and silicate inclusions are place mainly in ferrite grains and at the interface with perlite grains boundaries. WZ HAZ a) Unmodified WZ HAZ b) Modified with FeSi34V25Ti12 WZ TAZ c) Modified with FeSi45Zr35 Fig. 3 The microstructure aspects of Thermit – steel in MZ The inclusion rating is better in mushroom zone, in compared with the heart one, so we estimate a promising static band press, fatigue testing rig and rolling road test. WZ HAZ a) Unmodified WZ HAZ b) Modified with FeSi34V25Ti12 WZ HAZ c) Modified with FeSi45Zr35 Fig. 4 The microstructure aspects of Thermit – steel in HZ B. Quantitative analysis of welding joints structure Inclusions rating correlation (see Fig. 5) perform with a image analyses system (Buehler Enterprise System) highlight that the inclusion state is better after modification than before. This fact will ensure an increase fatigue and wear resistance. Inclusions rating in MZ are much more uniform after using M1 modifier. Inclusions percent area measured in MZ (mean value decrease from 0.590, to 0.356) is better after using M2 modifier. World Academy of Science, Engineering and Technology International Journal of Chemical, Molecular, Nuclear, Materials and Metallurgical Engineering Vol:6, No:8, 2012 746 International Scholarly and Scientific Research & Innovation 6(8) 2012 scholar.waset.org/1999.2/8439 In te rn at io na l S ci en ce I nd ex , M at er ia ls a nd M et al lu rg ic al E ng in ee ri ng V ol :6 , N o: 8, 2 01 2 w as et .o rg /P ub lic at io n/ 84 39 In compare, in HZ inclusions rating registered the same variation, but percent area increase to 0.801after using M1 modifier. a) MZ b) HZ Fig. 5 Inclusions rating correlation a) MZ World Academy of Science, Engineering and Technology International Journal of Chemical, Molecular, Nuclear, Materials and Metallurgical Engineering Vol:6, No:8, 2012 747 International Scholarly and Scientific Research & Innovation 6(8) 2012 scholar.waset.org/1999.2/8439 In te rn at io na l S ci en ce I nd ex , M at er ia ls a nd M et al lu rg ic al E ng in ee ri ng V ol :6 , N o: 8, 2 01 2 w as et .o rg /P ub lic at io n/ 84 39 b) HZ Fig. 6 ASTM E112-10 Average Grain Size correlation The ASTM E112-10 Average Grain Size (see Fig. 6) lay out the correlation between unmodified and modified states of the Thermit – steel. The grain size is smaller and uniform in all the railway zones (mainly in RAZ, HZ) and there are no micro pores. The mean value for ASTM grain size number (G) is comparable in WZ after using the modifiers: 5.1 for M1 in compare with 4.8 for M2, in WZ, and 5.3 for M1 in compare with 5.1 for M2, in HZ. C. Mechanical Tests The welds joints were subject of static bending test, sagging and tensile strength tests (SR EN 13674-1, SR EN 100021/95), in conformity with quality control system for Thermitsteel (see Table III). TABLE III MECHANICAL TESTINGS RESULTS Weld No. Weld type Static band (daNx10) Sagging δ (mm) Tensile strength σr (daN/mm) 2 TF-FF 76.800 10 80.000 6 TRQ-FRQ 65.700 8 68.437 3 TF-FRQ 58.600 7 61.046 7 TRQ-FF 78.000 17 81.250 IV. CONCLUSIONS There were two types of modifiers used to experiment the grain refinement by using alloyed Thermit in aluminothermic railways welding joints, FeSi34V25Ti12 (M1) and FeSi45Zr35 (M2). A reduction of Mn content was induced by both modifiers and this fact influences the grain size of the Thermit steel. The inclusions rating are better after modification than before. The inclusions are smaller and more uniform distributed in the structure. This fact will ensure a good fatigue resistance and a higher wear resistance. Using modifiers in Thermit kit can be considered a solution to increase the performance in aluminothermic railways welding joints. Presented results are base for the near future investigations in order to correlate and optimize the chemical and granular composition of the Thermit kit with grain size refinement and inclusion rating decreasing.