Complex microstructure of austenite-ferrite welded joint has been investigated, focused on its influence on Focal tensile properties. Tensile properties (yield strength and hardening coefficient) have been evaluated by using finite element method to simulate the strain distributions obtained experimentally. The three-dimensional model of V-joint specimen has been used with seven different materials, simulating two base metals, the weld metal and two sub-regions of two heat-affected zones - fine grain and coarse grain. In this way local tensile properties of the whole austenite-ferrite welded joint have been evaluated

Abukresh, M. O.

Bakić, Ramo

Jovičić, Radomir

Samardzić, I.

Sedmak, Aleksandar

English

machinery

335METALURGIJA 54 (2015) 2, 335-338R. BAKIĆ, M. O. ABUKRESH, A. SEDMAK, I. SAMARDŽIĆ, R. JOVIČIĆEFFECT OF MICROSTRUCTURE ON TENSILE PROPERTIES OF AUSTENITE-FERRITE WELDED JOINTReceived – Primljeno: 2014-04-01Accepted – Prihvaćeno: 2014-09-30Original Scientific Paper – Izvorni znanstveni radISSN 0543-5846METABK 54(2) 335-338 (2015)UDC – UDK 669.15:669.14:620.18:620.17=111R. Bakić, Tutin High School, Serbia, M. O. Abukresh, A. Sedmak. Faculty of Mechanical Engineering, University of Belgrade, Serbia, I. Samardžić, Faculty of Mechanical Engineering, Slavonski Brod, Uni-versity of Osijek, Croatia, R. Jovičić, Innovation centre of the Faculty of Mechnical Engineering, Belgrade, SerbiaComplex microstructure of austenite-ferrite welded joint has been investigated, focused on its influence on local tensile properties. Tensile properties (yield strength and hardening coefficient) have been evaluated by using finite element method to simulate the strain distributions obtained experimentally. The three-dimensional model of V-joint specimen has been used with seven different materials, simulating two base metals, the weld metal and two sub-regions of two heat-affected zones - fine grain and coarse grain. In this way local tensile properties of the whole austenite-ferrite welded joint have been evaluated.Key words: Austenite-ferrite weldment, microstructure, tensile properties, strain chain, strain gagesINTRODUCTIONStructural integrity analysis of welded joints re-quires knowledge of complex stress and strain distribu-tion in a heterogeneous mate rial, consisting of weld metal (WM), heat-affected-zone (HAZ) and base metal (BM) of different microstructure and mechanical pro-perties [1-8]. This is even more complex prob lem when two different base metals are welded, like in the case of ferrite-austenite welded joint [9].Due to very complex microstructure of HAZ and its extremely small size compared to WM and BM, it is not possible to determine the tensile properties of HAZ. This problem is also pronounced if ferrite-austenite welded joint is analysed, because two different HAZ are involved [9]. Therefore, the HAZs tensile properties have to be estimated. In this paper an estimation proce-dure of the HAZs tensile properties will be presented, based on nu me ri cal simulation of the experiment, con-sisting of strain measurement with strain gages and chains.MICROSTRUCTUREThe ferrite – austenite welded joint has been made of austenitic steel X6CrNiMo17.12.2 (EN 10088, marked as X6 in this paper), and high strength low al-loyed steel NIOVAL 47 (marked as M), by using shield-ed manual metal arc welding (SMAW) and INOX 29/9 electrode (marked as ASW - all weld metal).Tensile properties of both steels and electro de all weld metal are listed in Table 1. The chemical composi-tions of both steels and electrode all weld metal are given elsewhere [9].Table 1  Tensile properties of welded joint - reference valuesM AWS X6Yield Strength Rp0,2  / MPa 435 545 324Tensile Strength Rm  / MPa 555 755 595Elongation A5  / % 25 35 37Modulus of Elasticity E  / GPa 207 193 193Micrographs of characteristic locations of HAZ of steel M are shown in Figure 1. The HAZ in weld root consists of homogeneous fine-grains ferrite-pearlite mi-crostructure and non-etched WM surface (a), ferrite-pearlite micro structure in the central part (b), coarse grains bainite microstructure close to weld face (c).Figure 2 shows micrographs showing dend ri tic structure of WM. Coarse grains with in c re ased contri-bution of δ ferrite in structure of the last pass are typical for this type of WM.Micrographs of dissimilar welded joint and micro-structures of HAZ fusion region of steel X6 are shown in Figure 3. Structure at fusion line is shown in Figure 3(a), whereas the auste nitic microstructure in HAZ in the middle of the sample in Figure 3(b), together with the dark etched WM surface.Both HAZs comprise fine and coarse grains regions, having significantly different micro structure. In this way austenite-ferrite welded joint has at least seven zones of different properties. Finally, microstructures of both base metals are shown in Figure 4, the base metal M in Figure 4(a), and the base metal X6 in Figure 4(b).EXPERIMENTAL PROCEDUREFlat tensile specimens were machined from two base metals and the whole welded joint to determine the 336  METALURGIJA 54 (2015) 2, 335-338R. BAKIĆ et al.: EFFECT OF MICROSTRUCTURE ON TENSILE PROPERTIES OF AUSTENITE-FERRITE WELDED JOINTstress-strain curves. The weld metal itself was tested by using circular cross-section specimen. The results indi-cate different behaviour of two base metals - continuous curve for steel X6 and the curve with typical yielding behaviour for steel M, as shown in Figure 5(a) and (b), respectively, and explained in more details in [9]. Also, complex behavior of the whole welded joint should be noticed, Figure 5(c), caused by lower value of Yield Strength and higher value of Tensile Strength of the base metal X6, com pared to the base metal M. The modulus of elas ti city, yield strength and harden-ing coefficient are given in Table 2 for both base metals and the weld metal. The hardening coefficient were deter mined as the ratio of the difference bet ween tensile and yield strength and different ratios of total strain: 1/2  (a) Magnification 200 x (b) Magnification 200 x (c) Magnification 200 xFigure 1 Micrograph of HAZ - base metal MFigure 2 Micrograph of WM Magnification 200 x(a) Magnification 200 x(b) Magnification 200 xFigure 3 Micrograph of HAZ – base metal X6Figure 4 Micrograph of base metals, M and X6(a) Magnification 200 x(b) Magnification 200 x337METALURGIJA 54 (2015) 2, 335-338R. BAKIĆ et al.: EFFECT OF MICROSTRUCTURE ON TENSILE PROPERTIES OF AUSTENITE-FERRITE WELDED JOINTA total of 12 strain gages and 4 strain chains were used for each tensile panel, positioned at each side of the specimen, Figure 6. Re sults for average value of measurements at four sides of a specimen are shown in Table 3. Table 3 Experimental results - average strain valuesmat. 2 2 2 2 2,7 7,6 6,3 3ave. 11,38 12,18 12,67 11,05 7,75 4,88 4,12 2,22mat. 3 3,5 5,4 4,1 1 1 1 1ave. 1,66 1,46 2,67 3,94 5,04 5,98 6,27 7,04Materials are denoted from 1 to 7, as follows:  – 1 and 2 are BM, M and X6, respectively, – 3 is WM,  – 4 and 5 are CGHAZ and FGHAZ of M,  – 6 and 7 are CGHAZ and FGHAZ of X6.NUMERICAL SIMULATIONThe finite element method has been used to simulate the strain distributions obtained experimentally. The three-dimensional model of V-joint (45) specimen is shown in Figure 7, with seven different materials indi-cated by numbers 1-7. The three-dimensional iso par a-met ric finite ele ments with 20 nodes were used to create a mesh. The maximum remote stress was 425 MPa. The specimen was modeled with seven different ma-terials, as defined in Table 4 and explained in more de-tails in [9]. Initial values were obtained according to micro-hardness measurement [9].(a) (c) (b) Figure 5  Stress-strain curves for base metals and welded joint as the wholefor steel X6, 2/3 for steel M and 4/7 for weld metal, as explained in [9].In order to evaluate strain distribution in the welded joint, two tensile panels have been instrumented with strain gages, as shown in Figure 6. Table 2 Tensile properties - measured Material Rp0.2 / MPa Rm / MPa A5 / % E / GPa H‘ / MPaSteel X6 335 670 48 193 1400Steel M 455 616 33 207 730AWM 545 685 35 193 700Figure 6 Experimental setupFigure 7 The Finite Element Model with 7 different materials338  METALURGIJA 54 (2015) 2, 335-338R. BAKIĆ et al.: EFFECT OF MICROSTRUCTURE ON TENSILE PROPERTIES OF AUSTENITE-FERRITE WELDED JOINTTensile properties of both CG HAZ and FG HAZs, needed for the calculation (Yield Strength and harden-ing coefficient), were varied until nu merical strain distribu tions matched closely enough the experimental ones. Five iterations, i.e. different sets of ten sile proper-ties, were used to match the expe ri men tal strain distri-bution, as shown in more details in [9]. The results of this procedure are given in Table 4 for the first and last iteration.Table 4 Results for the 1st and 5th iteration 1st Iteration 5th IterationRp0,2 / MPa H’ / MPa Rp0,2 / MPa H’ / MPa1 - X6 355 1 400 355 1 4002 - M 455 730 455 7303 - WM 545 700 650 4004 - CGHAZ-f 550 300 550 3005 - FGHAZ-f 400 1 000 450 1 2006 - CGHAZ-a 400 400 630 5007 - FGHAZ-a 300 1 000 400 1 600RESULTS AND DISCUSSIONBased on the comparison of numerical and experi-mental results one can see that minimum seven different materials (WM, two BMs, two CGHAZs and two FG-HAZs) should be taken into account when austenite-ferrite welded joint is modeled. Since it was not possi-ble to extract tensile specimens from small regions like CGHAZs and FG HAZs, the iteration procedure has been applied, providing reliable results for all 7 differ-ent regions. One should notice that not only these two basic tensile properties differ, but the whole tensile be-havior as well. It is also to be noted that the austenite BM has the largest strengthening modulus and the smallest yield strength, whereas the WM has the small-est strengthening modulus and the largest yield strength. Finally, one should keep in mind that HAZs in both base metals, being also heterogeneous, should be modeled by taking into account the fine grain and coarse grain regions, at least. Having in mind complexity of the problem analysed in this paper, one should also keep in mind new possibilities for more precise measurement of strains, e.g. Digital Image Correlation (DIC) method, as described in [10].CONCLUSIONBased on the results presented in this paper, the fol-lowing conclusions may be drawn:•  Different microstructures in austenite-ferrite welded joint significantly affect its stress-strain behaviour, due to complex material behavior, involving differ-ent yield strength and strengthening levels.•  The iterative method for evaluation of elastic-plastic tensile properties of different re gions in a welded joint has been used, based on matching of numerical and ex peri mental results. •  The iterative method proved that HAZ has to be modeled with at least two different regions, coarse-grain and fine-grain.AcknowledgementsThe authors acknowledge financial support by the Ministry for education, science and technology devel-opment, Republic of Serbia, projects 174004 and 35002.REFERENCES [1]  R. Jovičić, A. Sedmak, S. Sedmak, Lj. Milović, K. Jovičić, Leakage of an Austenitic Steel CO2 Storage Tank, Structu-ral Integrity and  Life, 12 (2012) 2,105-108[2]  R. Jovičić, S. Petronić, S. Sedmak, U. Tatić, K. Jovičić, Integrity Assessment of Tanks with Micro cracks in Welded Joints Structural Integrity and Life, 13 (2013) 2, 131-136[3]  B. Petrovski, M. Kocak, S. Sedmak, Fracture Behavior of undermatched weld joint with short surface crack, GKSS 91/E/93, 1993[4]  D. T. Read, Fracture Toughness of Weldments: Elastic-Plastic fracture Analysis, ASME Con fe ren ce, MPC-22, 1984, 75-90[5]  N. Gubeljak, J. Predan, I. Rak, D. Kozak: Integrity asses-sment of HSLA steel welded joint with mis-matched strength, Structural Integrity and Life, 9. (2009) 3, 157-164[6]  V. Grabulov, I. Blačić, A. Radović, S. Sedmak Toughness and ductility of high strength steels welded joints, Structu-ral Integrity and Life, 8 (2008) 3, 181-190[7]  M. Manjgo, M. Behmen, F. Islamović, Z. Burzić, Beha-viour of Cracks in Microalloyed Steel Welded Joint, Struc-tural Integrity and Life, 10 (2010) 3, 235-238[8]  Lj. Milović, T. Vuherer, Z. Radaković, M. Janković, M. Zrilić, D. Daničić, Determination of Fatigue Crack Growth Parameters in Welded Joint of HSLA Steel, Structural Inte-grity and Life 11 (2011) 3, 183–187[9]  R. Jovičić, A. Sedmak, K. Čolić, M. Milošević, N. Mitrović, Evaluation of the local tensile properties of au-stenite-ferrite welded joint, Chemicke Listy, 105 (2011), 754-757[10]  J. Lozanović, Application of stereometric strain measure-ment at macro and micro level, Structural Integrity and Life, 7 (2007) 3, 201-208Note:  For English language translation responsible is prof. Martina Šuto, University of Osijek

Effect of microstructure on tensile properties of austenite-ferrite welded joint

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Effect of microstructure on tensile properties of austenite-ferrite welded joint

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