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1 8nd International Conference on Physical and Numerical Simulation of Materials Processing, ICPNS’16 Seattle Marriott Waterfront, Seattle, Washington, USA, October 14-17, 2016 Simulation of the influence of preheating on stress distribution during multi-pass repair welding of cast steel Wen Wang, Yajun Yin, Jianxin Zhou*, Min Wang, and Yun Ling State Key Laboratory of Materials Processing and Die & Mould Technology, Huazhong University of Science and Technology (HUST), Wuhan 430074, China ABSTRACT Preheating is an important process for multi-pass repair welding of cast steel to prevent bulking, or even crack defects because of stress concentration. In order to obtain the heating parameters, it is important to calculate the stress distribution under various preheating conditions. In this study, a thermal-mechanical computational procedure based on thermal elastic-plasticity theory was developed to understand the evolution of thermal and constraint stress. In the thermal analysis, both temperature dependent property parameters and latent heat of material due to melting and solidification are taken into account; the thermal boundary is a mixture of convection and radiation boundary conditions; a Gaussian volumetric heat source model is adopted to treat the heat put for laser welding; and the multi-pass addition of the material is treated as quiet element method. For the mechanical analysis, a simplified double linear constitutive model is used to reflect the elastic-plasticity behaviors of the base and the filler; and also mechanical parameters are varied with temperature. Based on the program, firstly the thermal and mechanical characterizes in the heating and heat affected zones are studied, and the influence of preheating temperature and holding time on the final stress distribution during multi-pass repair welding are simulated and analyzed. Besides, the effects of heating locations are also simply explored. The present study could do help for increase the effectiveness and lifetime of damaged cast steel treating with repair welding. Keywords: Multi-pass repair welding; Preheating treatment; Thermo-mechanical model; Numerical simulation 1. INTRODUCTION Because of kinds of reasons, defects, like crack, sand inclusion, pore and so on, often occurs during the productive of cast steel [1, 2]. This will worsen the properties of the products, even destroy it. Application of repair welding have been used to repair the defective cast steel in common [3, 4]. Appropriate method could not only save the cost, but also achieve the standard of the properties. During the repair welding process, because of the concentration of heat input, the rate of heating and cooling is very high, thus the steel is easy to crack because of thermal stress and phase stress. As a result, in practice, preheating, slow cooling, controlling of inter-pass temperature are required to ensure the effectiveness of repair welding [5]. During the process of repair welding, residual stress is one of the most important reasons for the defects of the weld; it would deteriorate the strength, fatigue life of the structure and accelerate the propagation of the crack. Meanwhile, the inner physics of the transport of heat and force process is very complex, traditional methods based on experience and experimental test is inefficiency and high scrap rate, the adoption of numeric simulation is not only efficiency and low scrap rate, but also could gain some important data which could not be got by experiments. So carrying out the simulation of residual stress of repair welding of cast steel, and studying the influence of preheating, slow cooling, controlling of inter-pass temperature on residual stress is very important. As early as 1940s, Rosenthal, et al. were the first to gain the analytical solutions of temperature field based on spot/line heat source model, but because of too many simplify in heat source model, the simulated temperature in welding zone has big difference with experimental results [6]; then, the approval of distributed face [7] and volumetric heat source [8] has greatly improved the precise of the simulated temperature field. along with the development of computer software and hardware and finite element theory, great improvement have been achieved in stress simulation during welding process, lots of reports on general finite element simulation software ABAQUS [9, 10], ANSYS [11, 12] and specific finite element simulation software SYSWELD [13, 14] on laser welding were consisted with experimental results well, these findings were 2 helpful for guiding for production practice. Dean Deng, et al. investigated the effects of solid-state phase transformation on residual stress in low carbon and medium carbon steels welds by TIG arc welding process, results indicated that the residual stresses were significantly affected by low temperature phase transformation [9].Fu. GM, et al. estimate the influences of the effects of preheat and inter-pass temperatures on residual stresses in welded structures representatives of ships and offshore platforms [5]. However, because of process particularity of repair welding: high degree of constraints of the base on the welds, complexity of local heat treatment, related reports is rarely and needs to go further. Thus the present research on the influence of preheating on stress distribution during multi-pass repair welding of cast steel is necessary. In the present research, a three dimensional transient heat-mechanical combined model are proposed for the repair welding of ZG45 steel. Based on the process characteristics, Gaussian volumetric heat source model are adopted as the heat input, convection and radiation heat transfer boundary conditions are considered; Mises yield criterion is used to judge the elasticity or plasticity state of the material, double linear constitutive model are adopted. What is more, based on the simulation results, the characteristics of distribution and magnitude of residual stress, as well as influence of different preheating parameters on evolution of residual stress during multi-pass repair welding process are analyzed. 2. MATHEMATIC MODEL The present numeric simulation is based on finite element method. The thermal analysis and the mechanical analysis were uncoupled and conducted in sequence in order to reduce calculation time, as well as guarantee calculation precise. As shown in Fig. 1, heat transfer, mass transfer, temperature dependant material parameters, evolution of mechanical properties, heat source models are considered in the present physical model. While flows of the weld pool, phase transformation are ignored. Heat transfer Heat input Material properties Deformation in solid Fig. 1 Main physical model for simulation. 2.1. Temperature Field The conduction of heat in the workpiece follows the law of Fourier heat conduction, as described in Eq. (1): T T T kx ky t x x y y T & kz Q z z cp (1) , c p and k is the density, specific heat and thermal conductivity of the material; T is the temperature field in the workpiece; Q& is the density where of the generated heat. Convection and radiation heat transfer boundary conditions are adopted to describe the heat interaction between the workpiece and the environment, as shown in Eq. (2): kn T q h T T0 T 4 T04 0 n (2) where h is the convection coefficient, gained by experiment; is the Boltzmann constant; is the emissivity of the material, for ZG 45, 0.25 could be adopted. According to the process characteristics, the present research used the Gaussian volumetric model to describe the heat input of arc, as shown in Figure 2. The density of the heat source of different locations in the workpiece in Cartesian coordinate system are shown in Eq. (3): 3 Linear isotropic reinforce model are adopted to describe the plastic behavior of the material. The influence of the phase transformation is ignored, the total strain could be calculated as the sum of elasticity strain, plasticity stain and thermal strain, as shown in Eq. (7): ij ije ijp ij (7) ij could where thermal strain be calculated as Eq. (8): Fig. 2 Gaussian heat source model q ( x, y, z ) q (0, 0, 0) exp[ where heat which 3Cs ( x 2 y 2 )] H log( ) z (3) 2.3. Addition of material could be calculated as Cs 3 R : q (0, 0, 0) is the density of the heat 2 source at the original location, as calculated with Eq. (4): q (0, 0, 0) Quiet element method was used to deal with the addition of the material. That is: the whole element was included in the calculation model before the start of the simulation, but unfilled elements are multiplied with a very small factor, 1.0e-6. The factor of the elements are dynamically dismissed when the material were filled on the workpiece. (4) 3Cs Q H (1 (8) T Cs is a parameter related to the shape of the source, = T T0 1 1 1 0 0 0 3. SIMULATION PARAMETERS AND FINITE CALCULATION MODEL 1 ) e3 where H is the depth of the heat source, Q is the power density. The actual parameters of heat source model could be adjusted as the actual profile of the welds. The heat enthalpy method is adopted to conduct the latent heat of fusion and solidification. The heat enthalpy is the sum of sensible heat capacity and latent heat, could be described as Eq. (5): H C (T Tm ); T Tm H C (T Tm ) H ; T Tm H 0 H ; T Tm The material of the work piece and fill material are both ZG 45. The dimensions of the base are 300 mm, 300 mm and 30 mm in longitudinal, width and height directions respectively. All three passes are weld using TIG welding method, the welding current, voltage, and travel speed are 160 A, 30 V and 2.5 mm/s. The length of each pass is 100 mm. Chemical composition of ZG 45 is listed in Table I, and simulation physical parameters are listed in Table II. Table I Chemical composition of ZG 45. (5) Element C Si Mn S P Ni Cr Cu Content (%) 0.50 0.6 0.9 0.04 0.04 0.30 0.35 0.3 2.2. Stress Field Table II Physical parameters used in the simulation. Mises yield criterion is used to judge the elasticity or plasticity state of the material, as shown in Eq. (6): Physical parameters =0.5 11 22 0.5 22 33 2 2 2 0.5 33 11 6 12 6 23 6 2 where 2 is the equal stress. 2 2 31 Symbol Value Density (kgm ) 7850 Specific Heat (Jkg-1K-1) Cp 480 Thermal conduct (Wm-1K-1) k 50.2 Solidus temperature (K) Ts 1708 Liquidus temperature (K) Tl 1738 -3 (6) 4 Latent heat (Jkg-1) Tm 1.02×105 Thermal emissivity 0.25 Young modulus (GPa) E 210 Poisson ratio 0.31 Yield strength 0.2%(MPa) s 310 Thermal expansion rate (10-6/K) 17.0 The calculation mesh model is shown in Figure 3. Element type is tetrahedron element, Sum of the nodes and elements are 38165 and 198240. To consider the real repair welding constrain conditions, x-, x+, y-, y+, z- are assumed as strong spring boundary conditions; z+ are free boundary. (a) 20.0 s Fig. 3. Finite element calculation model. 4. RESULTS AND DISCUSSION 4.1. The temperature field As shown in Fig. 4, during the 1st weld pass, after the very first unsteady period, isotherm of the temperature become nearly unchanged, and move along the welding direction with welding speed. Evolution rule of temperature field during the 2nd and 3rd weld pass is the same with the 1st weld pass, and the peak value is almost the same (Fig. 5). But the area of the high temperature zone is enlarged because of the preheating effect of the previous pass. (b) 40.0 s Fig. 4 Temperature field of the 1st weld pass. (a) 60.0 s 5 (a) 20.0 s (b) 100.0 s Fig. 5 Temperature field of the 2nd and 3rd weld pass. 4.2. The stress field. Fig. 6 shows the front and top views of the longitudinal stress field during 1st weld pass. Along the welding direction, the interaction weld zone of beam and work piece shows compressive stress due to the expansion of heating. But the weld zone which has efficiently cooled presents tensile stress. This is because that, shrinkage of the weld pool cooled would be blocked with the effect of the cooled nearly zone. The peak value of the tensile stress is about 150 MPa. Along the thickness direction, below the tensile stress zone, there exists a large compress stress zone with the peak value of 200 Mpa. Along the transverses direction, away of the weld mental and heat affected zone presents compress stress with the peak value of 200 Mpa. But the size is smaller than the compress zone in the thickness direction. The distribution of longitudinal stress field after the 2nd and 3rd weld passes is similar to the 1st one, but the area of both tensile and compress zone is enlarged (Figure 7). This indicated that, under the influence of hear input of the next weld pass, the stress of the previous pass has been effetely reset. (b) 40.0 s Fig. 6 Longitudinal stress field of the 1st weld pass. (a) 80.0 s 6 (b) 120.0 s Fig. 7 Longitudinal stress field of the 2nd and 3rd weld pass. Fig. 8 illustrates the distribution of residual longitudinal stress field after full cooled. Along the thickness direction, the surface zone of the arc interaction zone presents tensile stress with peak value of about 250Mpa, below is compress zone, which is highest in the start and end location the weld passes. The magnitude of the start location is 300 Mpa, and that of the end location is 150 Mpa. Along the transverses direction, the tensile stress becomes gradually bigger away of the weld center; outside of the tensile stress zone is compress zone with small value of about 100 Mpa; and the rest of the workpiece presents nearly no stress, which indicate that there exists no plastic deformation. cooled presents tensile stress with peak value of about 150 MPa. Different from longitudinal stress, the front direction of the compress stress zone presents tensile stress with value of about 200 Mpa. This indicates constraint conditions of workpiece are strong during repair welding. Along the thickness direction, similar with longitudinal stress below the tensile stress zone, there exists a large compress stress zone with the peak value of 300 Mpa. However, different from longitudinal stress, below the compress stress zone, presents tensile stress with value of about 300 Mpa which further illustrates that the workpiece is strongly constricted during repair welding practice. The distribution of longitudinal stress field after the 2nd and 3rd weld passes is similar to the 1st one (Figure 10). But the tensile zone arising from constraint conditions becomes weaken due to the preheating effect of the previous weld pass. (a) 20.0 s Fig. 8 Residual longitudinal stress field. Fig. 9 shows the front and top views of the transverse stress field during 1st weld pass. Along the welding direction, similar with longitudinal stress, the interaction weld zone of beam and work piece shows compressive stress due to the expansion of heating; and the weld zone which has efficiently (b) 40.0 s Fig. 9 Transverse stress field of the 1st weld pass. 7 Fig. 11 Residual transverse stress field. (a) 80.0 s 4.3. Influence of preheating temperature As shown in Fig. 12, after local preheating with different temperatures, the temperature is uniform in welding and heat affected zones. The maximum temperature difference is no more than 20℃. Fig. 13 shows the simulation results of temperature field with different preheating temperatures. As increasing of preheating temperatures, the temperature gravity of the frontier of the welding zones is therefore smaller. This indicates that temperature distribution is apparently relaxed with preheating process. (b) 120.0 s Fig. 10 Transverse stress field of the 2nd and 3rd weld pass. Fig. 11 illustrates the distribution of residual transverse stress field after full cooled. Along the thickness direction, the surface zone of the arc interaction zone presents tensile stress with peak value of about 150Mpa, below is compress zone, which is highest in the start and end location the weld passes. The magnitude of the start location is 300 Mpa, and that of the end location is 200 Mpa. Along the transverses direction, the weld start and end zones presents weak compress stress, with value of 100 Mpa. Away of the weld center, the tensile stress is firstly become bigger, than get smaller with peak value of about 150 Mpa. (a) no preheating (b) 100℃ 8 (c) 200℃ conditions after 1st welding pass. Comparison results present that, although the peak value of stress is almost the same, the range of compress stress has been enlarged apparently with increasing of preheating temperatures. That is to say that the stress gravity gets smaller with preheating process. Therefore, preheating could be used to decrease the potential of crack defects during repair welding of cast steel. (d) 400℃ Fig. 12 Temperature field after preheating. (a) no preheating (a) no preheating (b) 100℃ (b) 100℃ (c) 200℃ (c) 200℃ (d) 400℃ Fig. 14 Longitudinal stress field at 40.0 s with different preheating temperatures. (d) 400℃ Fig. 13 Temperature field at 20.0 s with different preheating temperatures. Fig. 14 and 15 illustrate top view of longitudinal and transverse stress fields after different preheating (a) no preheating 9 (b) 100℃ (c) 200℃ (c) 200℃ (d) 400℃ Fig. 16 Residual longitudinal stress field with different preheating temperatures. (d) 400℃ Fig. 15 Transverse stress field at 40.0 s with different preheating temperatures. (a) no preheating As illustrated in Fig. 16 and 17, the distribution of longitudinal and transverse stresses make nearly on difference after full cooled with preheating process. The range of longitudinal tensile stress is enlarged in small extent. Hence simulation results indicated that preheating process have small influence on distribution of residual stresses with the present welding assumptions. (b) 100℃ (a) no preheating (c) 200℃ (b) 100℃ (d) 400℃ 10 Fig. 17 Residual transverse stress field with different preheating temperatures. 5. CONCLUSIONS (1) As the increase of welding passes, temperature gravity gets smaller on the influence of previous welding pass; the peak value and evolution rules are almost the same in different welding passes. (2) During repair welding, as the increase of welding passes, both the area of longitudinal tensile and compress zone is enlarged; the transverse tensile zone arising from constraint conditions becomes weaken due to preheating effect of previous weld pass. 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