NUMERICAL SIMULATION OF THERMAL PROCESS IN WELDING

Transcription

1 NUMERICAL IMULATION OF THERMAL PROCE IN WELDING Milan Žmindák, Richard Melicher, Pavol Novák University of Žilina, Faculty of Mechanical Engineering, Department of Applied Mechanics Abstract. The welding is a thermal process with convection between flow fluid and welding body, between welding body and environment. Net type of boundary conditions is radiation and thermo-mechanical contact on the outer surface of gas pipe in the near of weld. The thermal response of a welded joint is a three-dimensional problem that requires a considerable amount of computational time. The computational time required to simulate multi-pass welding increases in portion to the number of weld passes. Therefore it is necessary to develop costeffective procedures to reduce computational time while preserving he accuracy of the solution. The main goal of the contribution is using the simulation model to the evaluation of the metallurgical structures and optimization of the technological parameters of the arc welding process. The purpose of this paper is to develop an appropriate numerical model for analysis of thermal processes during welding. The commercial Finite Element Method (FEM) code ADINA, ADINA-T module was used for thermal analysis. Key words: Thermal process in welding, numerical model, finite element method. Introduction The welding process is very comple time dependent physical phenomenon with material nonlinearity. The nonlinear effects are caused nonlinear dependency of thermal conductivity coefficient, specific heat coefficient, density of material on temperature. Another nonlinearity are caused with specialized boundary conditions. From the thermal cycle point of view, the arc welding performs the strong concentration of the heat source onto the small area of the welding materials [1]. The intense heat of the arc melts both parts to be welded at the point of the metal electrode, which supplies filler metal for the weld. The heat source moves in the desired direction by the certain velocity. After the melting the weld rapidly cools. The change of temperature depends on distance from welding centre. The boundary conditions and the thickness of the base material essentially determine the cooling velocity. This velocity was significantly increased due to the strong convection of the heat into the flowing water inside the test plates. The structural changes of the material depend on the material temperature properties. Therefore the welding parts contain finally a few structural phases, which of the physical and mechanical properties are different. The technological welding procedures depend mostly on desired properties of the weld in the service conditions [2]. Usually the weld does perform the region with a highest hardness, and heat affected zones (HAZ) represent the material with a pure properties. HAZ is crucial to the strength of the welded joint since the cracking and fracture occur inside the HAZ region. The thermal stresses due to epansion and contraction are the other effect of the temperature history during the welding process [3, 4]. The thermal stress analysis is out of the scope of this contribution. The changes of the microstructure and macrostructure together with a residual stress of the welded materials characterize the quality and reliability of the welded joint [5]. Theory background of heat transfer In the thermal analysis, the transient temperature field T of the welded plate is a function of time t and the spatial coordinates (, y, z), and is determined by the non-linear heat transfer equation for body on Fig. 1. Fig. 1. Body subjected to heat transfer 221

2 B k k k q T T T where T temperature, K; q B rate of heat generated per unit volume, W/m 2 ; k heat conductivity, W/kg K. At surfaces of body the following conditions must be satisfied k n T 1 T n 2 T e, (1), (2) q. (3) The equation (2) epress that the temperature can be prescribed at specific points and surfaces of the body, denoted by 1. The equation (3) is the heat flow input can be prescribed at specific points and surfaces. In equation (3) 2 are convection boundary conditions where q h Te T, (4) where h convection coefficient (possibility temperature depend), W/m 2 K; T e the environmental (eternal) temperature; T the body surface temperature. For nonlinear heat transfer the k and h are temperature dependent. For transient problems the heat stored within the material is given by C q c T, (5) p where c p the material heat capacity and the superposed dot denotes differentiation respect to time, J/kg K. Then nonlinear equation has form T T T B T k k k q cp. (6) t Fem model Consider the welded steel joint shown in Fig. 2. The geometric model include: the bottom lower plate is 60 mm long with thickness 8 mm, top plate is long 30 mm with 6 mm thickness. The filled weld 88 mm is generated using five volumes. The gap between plates is 2 mm. Fig. 2. Model geometry and FE mesh 222

3 The solid model contains of 3D conduction and surface boundary convection elements [6]. This model has elements. It was used 4 nodes element for D conduction and 6136 surface boundary convection elements. The material of the plates is low carbon steel (0.18% carbon) TN 12022, welding material is steel TN EN 499 E 38 3 B 41 H10 (PERFECT). The thermal conductivity, specific heat and density at room temperature are defined to be k = 50.2 W/m oc, c = 460 J/kg oc and = 7850 kg/m 3, respectively. The dependences of specific heat and thermal conductivity on temperature is given in Fig. 3. The temperature is fied at 20 o C along the surfaces. On bottom surface is prescribed forced convection.the calculation of a heat transfer coefficient h c to be used as a boundary condition within FEM models was based on a non-dimensional approach. An empirical calculation by Dittus and Boetler [6]: Nu 0.012Re Pr (7) is formed basis for calculation of the heat transfer coefficient for fully development turbulent flow within smooth surfaces, where Nu Nusselt number (Nu = hd/k, N); Re Reynolds number; Pr Prandtl number. From (7) we calculated heat transfer coefficient for the force convection of the surrounding air h c = 1500 W/m 2 K. The weld pool is assumed to be 5 mm long. In this heat transfer analysis, the surrouding air is at a temperature T = 20 o C. The initial temperature for the weld pool is 1500 o C, and for the remaining volume is 20 o C. For a transient analysis, the effect of heat capacity is included in the governing equation system; thus the time derivative T, term appears and is integrated by the -method with 0 1 and for specific values we have [8] = 0 Euler forward (eplicit); = 2 trapezoidal rule (implicit); = 1 Euler backward (implicit). We note that the choice of time step size t is important; if t is too large then the system may become unstable for eplicit time stepping, or in implicit integration the equilibrium iteration process may not converge for nonlinear problems. On the other hand, too small a time step may result in etra effort unnecessarily being made to reach a given accuracy. (a) (b) Fig. 3. Dependence of material properties on temperature: a specific heat; b conductivity Therefore it is useful to provide some guidelines as to the choice of time step size t. We would like to use as large a time step as the accuracy/stability/convergence criteria allow. Thus the guidelines are phrased as upper limits on the time step size t, i.e. where size mesh size. 2 (size) c p t, (8) 4 k 223

4 Results Fig. 4 and Fig. 5 shows the temperature distribution of the welded plates at t = 3 s and t = 20 s. As can be seen from the figures, the temperature around the torch reaches 1440 o C suggesting melted material in the fusion zone (FZ). High temperatures are present at immediate vicinities of the FZ, which defines the heat affect zone. The temperatures are calculated in three points which has coordinates B1 [0.0; 1.3; 0.0], B2 [0.0; -4.0; 0.0], B3 [0.0; 0.0; 7.0] where origin of coordinate systems is at corner of filled weld. Fig. 4. Temperature distribution at t = 3 s Fig. 5. Temperature results at t = 20 s It can be seen from Fig. 6. that the calculated temperatures of points B1, B2, B3 are increased almost immediately after the weld arc passes their position. The maimum temperature of point B3 is higher than that of B2 as ecepted because B3 is located closer to the weld-pool boundary than B2. ince the change of temperature with time from 800 to 500 o C is fairly similar for points in the vicinity of the heat source, the cooling time t 8/5 for B3 is approimately 10 s. The cooling curve we transformed to the ARA diagram (steel TN ). The outgoing guess of the final structure tends to presence of phases: Ferit, Perlit and Bainit with a hardness cca 190 HV. 224

5 Fig. 6. Temperature results (T-s) Conclusions The work has shown that it is possible to reliability simulate the process of in service welding. Three-dimensional finite element models of plane fillet welds have been developed. These models have been used to predict weld zone geometries and t 8/5 weld cooling times. The final microstructure structure tends to presence of phases: Ferit, Perlit and Bainit with a hardness cca 190 HV. The predictted values t 8/5 have shown that it is possible calculate the thermal fields and weld cooling times to a useful accuracy. Acknowledgement. This work has been supported by grant project KEGA (No. KEGA-K ) Literature 1. BANG, I.-W., ON, I.-P., OH K.H., KIM, I.-P., AND KIM, W.-.: Numerical simulation of sleeve repair welding of in-service gas pipelines. Welding Journal, Vol. 2001, s NGUYEN, N.T.: Thermal analysis of Welds, Wit press, ELEZTÖ, P., VOŠTINÁR, V., MRÁZ,.: Numerical and eperimental analysis of the deformational and stress state of welded joints. Acta Mechanica lovaca. Vol. 10, (in lovak) 4. YAGHI, A.H. HYDE, T.H. BECKER, A.A. WILLIAM, J.A. UN, W.: Residual stress simulation in Welded sections of P91 pipes. Journal of Materials Processing Technology 167 (2005) CHAPETTI, M.D., OTEGUI, J.L., MANFREDI, C. AND MARTIN, C.F.: Full scale eperimental analysis of stress states in sleeve repairs of gas pipelines. Journal of Materials Processing Technology, Vol.78, 2001, pp ADINA: Theory and Modelling Guide, ADINA RD, Inc. (2005). 7. ABAPATHY, P.N., WAHAB, M.A., PAINTER, M. J.: Numerical models of in-service welding of gas pipelines. Journal of Materials processing technology 118 (2001), BATHE, K.J.: Finite Element Procedures. Prentice Hall,