THE EFFECT OF HEAT INPUT & TRAVEL SPEED ON THE WELDING RESIDUAL STRESS BY FINITE ELEMENT METHOD

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1 International Journal of Mechanical and Production Engineering Research and Development (IJMPERD) ISSN Vol. 2 Issue 4 Dec TJPRC Pvt. Ltd., THE EFFECT OF HEAT INPUT & TRAVEL SPEED ON THE WELDING RESIDUAL STRESS BY FINITE ELEMENT METHOD 1 P.D.SUDERSANAN & 2 U.N.KEMPAIAH 1 Assistant Professor & Head, Dept. of Mechanical Engineering, Dr.T.Thimmaiah Institute of Technology, K.G.F, India. 2 Dr.U.N.Kempaiah, Professor & Head, Dept. of Mechanical Engineering, Govt. Engineering College, Ramanagar, India ABSTRACT The welding is one of the most common joining processes in the industry today. Despite the development in welding technology the undesirable part of this joining process, i.e. residual stress is still a major concern. These residual stresses can be detrimental to the life of the structures and the factors affecting these are studied by many investigators. This paper studies the residual stress induced in the dual phase steel with changes in heat input and travel speed during Butt welding. Multi pass butt welding is carried out on DP Steel specimens using MAG welding. The residual stress in the weld metal, heat affected zone and base metal are measured using X-ray diffraction technique. A sequentially coupled thermo mechanical analysis is carried out using ANSYS to validate the result. The analysis shows good agreement with the experimental result. Further analysis using the validated FEM model is carried out to investigate the effect of heat input on welding residual stress. The result shows a significant increase in residual stress with the increase in heat input. It is also found to decrease with the increase in travel speed. KEY WORDS: Dual Phase Steel, Residual Stress, X-Ray Diffraction, Thermo Mechanical Analysis INTRODUCTION It is well known that the any form of welding induces residual stress in the weld metal, heat affected zone and base metal. This is due to the non uniform heating of the weld metal and base metal during welding and shrinkage thereafter. These stresses may be detrimental to the life of the structure. Residual stresses induced may be either tensile or compressive. Tensile residual stresses are more damaging to the structure since it leads to fatigue failure. The growing demand for newer materials with improved mechanical properties has led to the development of dual phase steels with varying percentages of martensite. Low carbon steels can be transformed to dual phase steels having superior strength by Intercritical heat treatment. The heat treatment involves heating the steel to intercritical temperature range to obtain ferrite and austenite followed by quenching to get ferrite-martensite dual phase structure. These steels have high strength, ductility, toughness, high initial strain hardening and exhibit continuous yielding. In dual phase steel martensite is dispersed in a soft ductile ferrite matrix. In this study multi pass welding is carried out on different samples of dual phase steels with varying percentage of martensite. The longitudinal and transverse residual stress are measured using X-ray diffraction test in the weld bead, heat effected zone and the base metal region. A sequentially coupled thermo mechanical analysis using ANSYS is carried out to verify the result. LITERATURE SURVEY The dual phase steels with a high strength to weight ratio have created great interest in the automotive sector [1]- [3]. Dual phase steels possess high tensile strength, high work hardening rate at the starting of plastic deformation and also

2 44 P.D.Sudersanan & U.N.Kempaiah possess good ductility. These favourable properties are due to dual phase structure of dual phase steel. The soft ferrite phase provides the required ductility where as the hard phase martensite imparts the required strength [5]-[7]. Dual phase steels also possess continuous yielding behaviour, uniform plastic deformation and good formability [8]. The hardness of dual phase steel is considerably improved due to the presence of harder martensite phase compared to the normal steel with ferrite pearlite microstructure [9],[10]. The dual phase steels with martensite content up to 60% are found suitable for structural welding applications [11]. A detailed study of the temperature dependent material properties reveals that the yield stress is the most influencing mechanical property on residual stress simulation using FEM. Young s modulus and thermal expansion coefficient were found to have little influence on residual stress [12]. Hardenability and presence of hydrogen coupled with residual stress in the weld initiates cracks in the welding of high strength steel. The residual stress is analysed using FEM and the instantaneous stress levels are found to be exceeding the yield strength of the material [13]. Residual stresses induced in butt welding of dissimilar pipes are simulated using ANSYS by S.Nadimi and et al. The peak longitudinal residual stress is observed in the metal with highest strength [14]. Comparison of residual stresses induced in butt welded joint is studied using full 3D and Shell/3D modelling using both ABAQUS and ANSYS in order to reduce the computing time [16]. No significant difference in computed residual stress is observed by the study. The effect of thermal conductivity on welding residual stresses is studied by two dimensional FEM analysis by E. Armentani and et al [17]. The study concluded that both the form of residual stresses are affected to some extend by changes in thermal conductivity. No systematic study is reported in the literature relating the effect of heat input and travel speed on the residual stresses induced during multi pass welding of the dual phase steel. METHODOLOGY The composition of HSLA steel samples were tested using Optical Emission Spectrometer and Carbon & Sulphur analyser. The composition obtained after the analysis of the base metal is tabulated in table 1. Table 1: Chemical Composition of Specimen Element Weight % C 0.13 Mn 1.18 S P Si 0.30 Cr Mo B Ni The specimens sizes of 100mm x 100mm x 14mm are first austenized by heating using an electrical Muffle furnace. They are heated to 920 o C which is above the austenizing temperature for low carbon steels. The specimens are held at 920 o C for 30 minutes and then quenched in 9% iced brine solution at -7 o C. The critical annealing temperatures of the base metal are estimated using Andrew s equation [18], [19]. The lower and upper critical temperatures are approximated as 720 o C and 816 o C respectively. The specimens are then heated to different intercritical temperatures ranging from 730 o C to 810 o C before finally quenched in Servo quench 707 oil at 25 o C to obtain dual phase microstructure. The fig.1 shows the joint design for butt welding and the sequence of welding used in the study. The specimens are butt welded using MAG welding using ER filler material after edge preparation. Four passes are used for the

3 The Effect of Heat Input & Travel Speed on the Welding Residual Stress by Finite Element Method 45 welding of 14mm thick dual phase steel plates. The specimens are cooled to room temperatures before next pass. The voltage, current and travel speed are recorded during each pass. The residual stresses induced in the welded plates are measured using X-ray diffraction test in the weld bead, heat effected zone and base metal region. Fig. 1: The Welding Joint Design and the Sequence of Welding Used in the Present Study A model is generated in ANSYS using the graphical user interface. The root gap and the V-groove are provided to accommodate the filler metal and to complete the welding in four passes. The elements in the root gap and groove are separately created. The filler material is assumed to have the same properties of the base metal. The temperature dependent material properties are defined [20]. Simplified properties constituted by a piece-wise linear function with temperature for the yield stress as suggested by X.K.Zhu and et al [12] are used for weld simulation and analysis. The temperature dependent material properties used in the study are shown in figures 2&3.The convection heat transfer coefficient and the ambient temperature are taken as 10W/m 2 K and 298K respectively. 3 Young's Modulus x 100 GPa Poissons Ratio Temperature o C Fig. 2: The Temperature Dependent Thermo Mechanical Properties. 1.2 Thermal conductivity (W/mk) x 125 Speciific heat (J/kgK) x Temperature o C Fig. 3: The Temperature Dependent Thermo Physical Properties.

4 46 P.D.Sudersanan & U.N.Kempaiah The model is then meshed using solid70 elements before conducting the transient thermal analysis. The elements for the filler metal are separately meshed to identify the element numbers. For thermal analysis an APDL macro is used to apply the simulation of moving heat source. By knowing the element numbers a Do loop is used in the macro to apply heat flux on the elements. Element birth and death capability of ANSYS is used to deal with the filler material deposition as the welding progresses. All the elements including the filler material are generated before the start of the simulation. All the elements in the filler metal region are initially deactivated. They are activated using separate commands before coming in to contact with the heat source. The heat source is assumed to have double ellipsoidal nature as suggested by Goldak and et al [15]. The heat flux distribution is shown in fig4.the heat flux distribution at the front portion and rear portion of the double ellipsoidal arc, q f and q r are given by the governing equations (1) and (2). q f = q r = η π π η π π (1) (2) Where, and are the heat flux distribution factors in the front and rear portion of the arc. Q = VI + =2 η = efficiency of arc (0.85 is taken for this analysis) Q = Heat Input (Watts) V = Voltage (Volts) I = Current (Ampere) Fig. 4: The Heat Source Model Used in the Present Study The heat source parameters used to model the heat source in the present study is given in the table2. Table 2: Heat Source Parameters Used for Welding Parameter a f a r b c f f f r Size 4 mm 12 mm 8 mm 3.5 mm

5 The Effect of Heat Input & Travel Speed on the Welding Residual Stress by Finite Element Method 47 The temperature history is stored for every time step to be used later for the structural analysis. In the structural analysis the element type is changed to solid185 using element change command. The boundary constraints are applied and a transient structural solution is obtained using a separate macro written using APDL. The structural analysis uses the same time steps and loop used for the thermal analysis. RESULTS AND DISCUSSIONS The percentage of ferrite and martensite in the different samples of dual phase steel are measured using point counting method as per ASTM E562 standard. The percentage of martensite in the dual phase steel is influenced by the intercritical annealing temperature. The residual stresses in the samples of different martensite content are measured in the welded region, heat affected zone and base metal by X-ray diffraction test. No significant change in the residual stress pattern is observed with changes in martensite content. A sequentially coupled thermo mechanical analysis using ANSYS is carried out to verify the result obtained. The predicted result using ANSYS analysis shows a deviation up to 18%. The model is further used to study the effect of heat input and travel speed on the residual stresses. The heat input is varied from 2000W to 3000W and the travel speed from 1mm/sec to 4mm/sec. The residual stresses are found to vary significantly with the heat input and travel speed. The temperature contour plot at the end of 10 seconds from the start of the welding is shown in fig.5. Fig. 5: The Temperature in Kelvin at the End of 10sec from the Start of the Welding for a Heat Input of 2000w and Travel Speed 4mm/Sec Longitudinal Residual Stress - MPa W 2500W 3000W Travel Speed - mm/sec Fig. 6: Variation of Maximum Longitudinal Residual Stress with Travel Speed

6 48 P.D.Sudersanan & U.N.Kempaiah The variation of longitudinal residual stress with travel speeds for different heat input is shown in fig6.the longitudinal residual stress has decreased considerably with arc travel speed for all heat input values analysed. The same is found to increase with the heat input for every input of travel speed. Residual stress in MPa Distance in mm FEM EXP Fig. 7: Comparison of Experimental Longitudinal Residual Stress with FEM for a Heat Input of 2000W and Travel Speed 4mm/Sec The longitudinal residual stress obtained from 3D FEM analysis and the experimental result for a heat input of 2000W and travel speed of 4mm/sec is plotted in fig 7. Figures 8-10 shows the ANSYS path plots for Longitudinal and transverse stress variation across and along the weld line. The contour plot for longitudinal residual stress at the end of welding is shown in fig.11. Fig. 8: Longitudinal Residual Stress Distribution in the Transverse Direction at the Centre of the Plate for a Heat Input of 2000w and Travel Speed 4mm/Sec. Fig. 9: Longitudinal Residual Stress Distribution Along the weld Bead for a Heat Input of 2000w and Travel Speed 4mm/Sec.

7 The Effect of Heat Input & Travel Speed on the Welding Residual Stress by Finite Element Method 49. Fig. 10: Transverse Residual Stress Distribution Along the Weld Bead for a Heat Input of 2000w Speed and Travel 4mm/Sec. Fig. 11: Contour Plot of Longitudinal Residual Stress Distribution at the End of Welding for a Heat Input of 2000w and Travel Speed 4mm/Sec CONCLUSIONS The Intercritical temperature has shown a considerable influence on the volume fraction of the dual phase steel and increased nonlinearly with it. X-ray diffraction tests were carried out on various samples to study the effect of percentage of martensite in the dual phase steel on the residual stress introduced after welding. The results show no significant influence of percentage of martensite on the residual stress. The FEM simulation and consequent thermo mechanical analysis carried out has shown the residual stress increases with heat input. It is also observed that the residual stress considerably decreases with travel speed. The transverse residual stress induced in the welding is negligible except at the starting and ending of the weld. REFERENCES 1. S Hayami, T Furukawa, Microalloying75, Proceedings of International Symposium on HSLA alloys, Washington D.C., Pp , M S Rashid, G.M 980X-A Unique strength sheet steel with superior formability, SAE, No , Pp , K Hulka, Dual phase and Trip steels, ASM Metals Park Ohio, Pp.1-4, 2000.

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