The paper presents research of weldability of microalloyed high strength steels TStE 420 and S960QL after weld thermal cycle simulation. Beside mechanical properties hardness and impact strength, the microstructures of characteristic weld thermal cycled structures for both steels are given. The results will contribure to determination of weakest points in Heat Affected Zone of both steels

I. Samardžić

M. Dunđer

T. Vuherer

English

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Weldability of microalloyed high strength steels TStE 420 and S960QL

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335METALURGIJA 53 (2014) 3, 335-338M. DUNĐER, T. VUHERER, I. SAMARDŽIĆWELDABILITY OF MICROALLOYED HIGH STRENGTH STEELS TStE 420 and S960QLReceived – Prispjelo: 2013-08-29Accepted – Prihvaćeno: 2014-01-10Original Scientifi c Paper – Izvorni znanstveni radISSN 0543-5846METABK 53(3) 335-338 (2014)UDC – UDK 621.791.052:669.15:620.17=111M. Dunđer, EWE, Department for Polytechnics, Faculty of Philosophy, University of Rijeka, CroatiaT. Vuherer, University of Maribor, Faculty of Mechanical Engineering, SloveniaI. Samardžić, Josip Juraj Strossmayer University of Osijek, Mechanical Engineering Faculty in Slavonski Brod, CroatiaThe paper presents research of weldability of microalloyed high strength steels TStE 420 and S960QL after weld thermal cycle simulation. Beside mechanical properties hardness and impact strength, the microstructures of char-acteristic weld thermal cycled structures for both steels are given. The results will contribure to determination of weakest points in Heat Aff ected Zone of both steels.Key words: weldability, high strength steels, mechanical properties, TStE 420, S960QLINTRODUCTIONMore intensive utilization of improved and high-strength steels occurred more decades ago, fi rstly in in-dustrialized countries around the world, and somewhat later also in our country. High-strength steels greatly re-duce required mass and increase capacity and tough-ness.These steels are used mostly for:- construction of road and rail vehicles - construction of civil engineering machinery so  called heavy loads (high speed rotors)- construction of airplanes- rocket and space technology - construction of ships - construction of cranes (harbor and mobile), etc.Depending on the fl ow stress, three groups of these steels are distinguished (there are different classifi ca-tions of authors, however, the most commonly cited is: -  fi ne grained steels with yield strength of up to 500 MPa, called steels of increased strength, which is reached after rolling and normalizing-  fi ne grained steels with yield strength from 500 to 1 000 MPa, called high strength steels, which is achieved after improving-  fi ne grained steels with yield strength above 1 000 MPa, called ultra high strength steels, achieved by thermomechanical rolling.In this work authors will present results of weldability investigations after weld thermal cycle simulation on representant of fi ne grained steel with yield strength up to 500 MPa (TStE 420 steel) and of fi ne grained steel with yield strength from 500 to 1 000 MPa (S960QL steel).EXPERIMENT PLANThe experiment plan was to assess the hardness and impact toughness after welding of microalloyed steel of increased strength TStE 420, and high strength steel S960QL. Chemical composition and mechanical proper-ties of both tested steels were obtained in laboratory meas-urements by the author, as presented in the Tables 1-4. Table 1 Composition of the TStE420 steel [1,2]C 0,18Si 0,3Mn 1,47P 0,017S 0,005Ni 0,22N 0,016Al 0,023V 0,13Cu 0,02Table 2 Mechanical properties of steel TStE420 [1,2]Yield Strength Rp0,2 / MPa 422Tensile Strength Rm / MPa 577Elongationa A5 / %  30Contraction Z / % 61,9Bending α =180 ° Longitudinal +Transfersal +Toughness, KV / J longitudinallyat 20 °C 261at -20 °C 245at -40 °C 182Specimens were cut out in the direction of the base material rolling. Each panel was checked for direction of fi bers during rolling. Thickness of base material was 15 mm. Calculated values  of preheating temperatures for the steel TStE 420 are shown in the Figure 1, and in the Figure 2 the same values refer to the steel S960QL.336  METALURGIJA 53 (2014) 3, 335-338M. DUNĐER et al.: WELDABILITY OF MICROALLOYED HIGH STRENGTH STEELS TSTE 420 AND S960QLDetermination of hydrogen content in the weld was performed by the glycerin method according to HRN C.H3.018/83.Table 3 Composition of S960QL steel [2,3]C 0,17Si 0,47Mn 1,42P 0,008S 0,003Cr 0,59Mo 0,56Ni 0,79Nb 0,02V 0,05Cu 0,03Ti 0,01Table 4 Mechanical properties of S960QL steel [2,3]Yield Strength Rp0,2 / MPa 1 020Tensile Strength Rm / MPa 1 080Elongationa A5 / %   16Contraction Z / % 61,9Toughness, KV  / J longitudinallyat 0 °C 158at -20 °C 76at -40 °C 58    Figure  1  Calculated values  of preheating temperatures for the steel TStE 420 [4]Figure 2  Calculated values  of preheating temperatures for the steel S960QL [2]Figure 3  Dependence of the mean value of hardness of TStE 420 steel on the temperature of single pass simulation (peak temperatures are 20, 600,700, 780, 960, 1 100 and 1 350 °C)EXPERIMENT RESULTSWeld thermal cycle simulation was performed on Smitweld TCS 1405 simulator. After single pass weld thermal cycle simulation on different weld cycle peak temperatures, hardness of specimens are shown at Fig-ures 3, 4. Measurement of impact toughness was performed on simulated specimens at a temperature of 20 °C. Dia-gram of dependence of impact toughness within double simulation on Tmax of the second simulation cycle is pre-sented on the Figures 5 and 6.As seen in the diagram, it is to conclude that maxi-mum values for impact toughness were obtained when Tmax of the second simulation cycle was 780 °C.The above diagram shows that maximum values for impact toughness were obtained when Tmax of the sec-ond simulation cycle was 600 °C. After measuring of hardness, metallographic examination was performed Figure 4  Dependence of the mean value of hardness of S960QL steel on the temperature of single pass simulation (peak temperatures are 20, 600,700, 800,  900, 1 100 and 1 350 °C)337METALURGIJA 53 (2014) 3, 335-338M. DUNĐER et al.: WELDABILITY OF MICROALLOYED HIGH STRENGTH STEELS TSTE 420 AND S960QLon samples of base materials, as well as on simulated specimens, as shown in Figures 7 -10. Examination of microstructure of TStE 420 was performed on normal optical microscope and microstructure of S960QL on scanning microscope (for base metal and for weld cy-cled specimen).  Figure 5  Diagram of dependence of impact toughness on Tmax of the single simulation cycle for the steel TStE 420 for diff erent peak thermal cycle temperaturesFigure 6  Diagram of dependence of impact toughness on Tmax of the single simulation cycle for the steel S960QL for diff erent peak thermal cycle temperaturesFigure 7  Microstructure of base metal steel TStE 420; magnifi cation 200 xFigure 8  Microstructure of base metal steel steel S960QL; magnifi cation 200 xFigure 9  Microstructure of of the steel TStE 420, peak temperature of thermal cycle 1 300 °C, magnifi cation 200 xFigure 10  Microstructure of the steel S960QL, peak temperature of thermal cycle 1 300 °C, magnifi cation 200 x  338  METALURGIJA 53 (2014) 3, 335-338M. DUNĐER et al.: WELDABILITY OF MICROALLOYED HIGH STRENGTH STEELS TSTE 420 AND S960QLCONCLUSIONMicroalloyed steel of increased strength TStE 420 shall be welded at such parameters to reach time of cooling from 800 - 500 °C for 8 - 10 s. Within such pa-rameters, hardness in the HAZ at the fusion line in heat affected zone is reduced (as wel as the risk of cold cracks), while the values of impact toughness are higher than marginal values of transition into fragile condition. [5-8] The author’s values for impact toughness are re-lated to fi ne ferrite grain as a result of the maximum at a temperature little higher than Ac3, being 780 °C in the performed experiment. Reduce of impact toughness oc-curs above that temperature. Moreover, maximum of impact toughness at 780 °C reduces hardness. Further increase of temperature causes decrease of impact toughness (Figure 5), which is correlated to the increase of hardness (diagram of the Figure 3). Welding technol-ogy of the steel S960QL is based on the controlled input of energy during welding process (preheating, tempera-ture between passes, heat input achieved by electric arc, additional heating) and strict compliance with the de-fi ned procedure, all with the aim to avoid occurrence of cold cracks and other failures, and to achieve the re-quired properties of welded joints. Insuffi cient input of heat usually affects the increase in strength and hard-ness of the welded joint, which, along with residual stresses and the presence of hydrogen in the weld, may cause cold cracks, while reducing deformability and in-creasing susceptibility to brittle fractures [8]. The high-est values for impact toughness are obtained for tem-pered martensite with smaller portion of bainite, which occurs due to hardness of 320 - 350 HV (Figures 4 and 6). Simulation of weld thermal cycle provides results that can be used in optimization of parameters for weld-ing of microalloyed high strength steels. These results can be used in real welding process to achieve optimal mechanical properties of welded joints of microalloyed high strength steels.REFERENCES[1] I. Samardžić, A. Čikić, M. Dunđer. Accelerated weldabili-ty investigation of TStE 420 steel by weld thermal cycle simulation. Metalurgija 52(2013)4, 461-464.[2]  European standard for structural steel (EN 10025-6/2004)[3] F.Werner, Numerische Untersuchungen zu Temperaturfel-dern und Eigenspannungen an einer Schweißnahtverbin-dung bei WIG - Nachbehandlung, Bauhaus Universität Weimar 2005.[4] M. Dunđer, T. Vuherer, I. Kladarić, Weldability Investiga-tion of TStE 420 After Weld Thermal Cycle Simulation, Journal Strojarstvo 52 (2010) 2, 97-104.[5] SEW 88 Stahl - Eisen - Werkstoffbläter (SEW)088, Sch-weissgeeignete Feinkornbaustähle, Verlag Stahleisen MBH, Düsseldorf, 1983. Fakultät Bauingenieurwesen, Weimar 2005.[6] V. Grdun, B. Godec, Neugodne mikrostrukturne sestavine v zvarnih spojnih konstrukcijskih jekel, Materijali in teh-nologije 36 (2002) 5.  [7] M. Novosel, S. Kralj, Čelici povišene i visoke čvrstoće, Zavarivanje 34(1991)3-4, Hrvatsko društvo za tehniku za-varivanja, Zagreb.[8] J. Hildebrand, F. Werner, Change of structural condition of welded joints between high-strength fi ne grained and structural steels, Journal of civil engineering and manage-ment 10(2004)2.Note:  Responsible translator: Martina Šuto, prof., University of J. J. Strossmayer of Osijek, Croatia 

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Weldability of microalloyed high strength steels TStE 420 and S960QL

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