[溶接学会論文集 第 20 巻 第４号 p. 000-000（2002)]

Study on Solidification Cracking Under High-speed Narrow Gap Welding with Tandem Torches*

by Takaaki HARADA**, Kazuki IKUSHIMA***, Masakazu SHIBAHARA***, Fumiaki KIMURA**** and Takuya MORIMOTO****

In this research, the influence of various factors on solidification cracking under high-speed gap welding was investigated by nonlinear thermal-elastic-plastic analysis using the finite element method (FEM). To investigate the formation of cracks, the plastic strain increment in the brittleness temperature range (BTR) was introduced. In addition, to perform thermal-elastic-plastic analysis of a practical three-dimensional model, Idealized Explicit FEM was employed. In the simulations, the penetration shape was compared between the experimental measurement and the analyses to evaluate the analysis accuracy. As a result, it was found that the penetration shapes agree with each other. The heat input, welding speed, and torch distance were changed and the distribution of the BTR plastic strain increment was also compared to show the influence of the welding conditions. From the investigations, it was found that the BTR plastic strain increment increases with heat input, welding speed and decreases with torch distance.

Key Words: Narrow gap welding, Solidification cracking, Hot cracking, Idealized Explicit FEM, MAG welding

1. Introduction

In recent years, undersea pipelines is widely utilized for transporting oil and gas as an economical infrastructure for middle or long distances. For the joining of undersea pipes, butt welds are carried out on the ship. 2-torch fully automatic welding1), shown in Fig. 1, is used to improve the efficiency. In addition, corrosion-resistant materials are employed since undersea pipelines are installed in highly corrosive environments. Due to the change of the welding method and materials, solidification cracking, shown in Fig. 2, may occur due to the use of inappropriate materials and welding conditions. Solidification cracking is a severe defect and its detection is carried out using X-ray or ultrasonic inspection after joining. Repair welds are carried out if cracking is detected. This repair work leads to the increase of labor cost and work time. So, it is important to predict the occurrence of solidification cracking by investigating the

welding conditions in advance. Since solidification cracking is a severe defect, many studies

have been conducted. The influence of metallurgical factors on the occurrence of cracking was investigated by Matsuda et al.2) in the 1980s. From the viewpoint of mechanics, Ueda et al. investigated the deformation and cracking on one-sided automatic welding using finite element analysis (FEA) in the 1970s3). However, existing studies have mainly focused on metallurgical aspects and few mechanical investigations have been conducted. To clarify the occurrence of solidification cracking, it is necessary to consider mechanical factors, including welding conditions and constraints.

In this study, the occurrence of solidification cracking on butt welds of steel pipes is investigated by using a thermal-elastic- plastic FEA. The influence of various factors on solidification cracking is investigated. To analyze solidification cracking, highly detailed nonlinear computation is required and the analysis scale is limited for thermal-elastic-plastic analyses using commercial finite element method (FEM) software. So, in this research, Idealized Explicit FEM (IEFEM)4) is introduced to achieve efficient computation on practical three-dimensional problems.

Fig. 2 Photo of solidification cracking under high-speed

narrow gap welding with tandem torches.

Fig. 1 Tandem torches used in pipe welding.

*Received: 2014.11.27 ** Student Member, Graduate School of Engineering, Osaka

Prefecture University *** Member, Graduate, school of Engineering Osaka Prefecture

University **** Nippon Steels & Sumikin Engineering Co., Ltd.

［溶接学会論文集　第 33 巻　第 2 号　p. 190s-194s （2015）］

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2. Analysis Method

2.1 Thermal-elastic-plastic analysis by IEFEM

IEFEM was developed on the basis of Dynamic Explicit FEM, and convergence to the static equilibrium state is considered for each load step. The virtual inertial term and damping term are added in each load step and the static equilibrium state is obtained by advancing the virtual time step until the virtual inertial term and damping term are negligibly small. Here, fast computation with low memory consumption is expected by using a node-concentrated mass and damping matrix, since the solution of large-scale simultaneous equations is not necessary.

In ordinary Dynamic Explicit FEM, the Courant condition limits the time increment for small values and numerous time steps are necessary to obtain the static equilibrium state. In contrast, in IEFEM, the modified mass matrix is determined based on Courant condition, and the number of virtual time steps to obtain a static equilibrium state is dramatically reduced4).

By using these procedures, IEFEM has almost the same analysis accuracy as Static Implicit FEM and can achieve solutions for large-scale problems, which is difficult with existing methods. In addition, more efficient computation is achieved by introducing a graphics processing unit (GPU) for parallelization of IEFEM4). In this research, to analyze the practical model, IEFEM using a GPU is employed. The computer employed in this analysis has the following specification; CPU is an Intel Core i7 3.33 GHz processor, memory is 24 Giga Byte, GPU is an NVIDIA Geforce GTX 980 processer.

2.2 Evaluation of solidification cracking by BTR plastic

strain increment

Hot cracking is generally classified into the following three types: solidification cracking, which occurs on weld metal in the solidification brittleness temperature range (BTR)5); grain-boundary liquation cracking, which occurs in the heat-affected zone (HAZ) in the grain-boundary liquation temperature range; ductility-drop cracking, which occurs in the

weld metal and HAZ in the ductility-drop temperature range (DTR)7).

The mechanism of solidification cracking in weld metal is as follows: the liquid phase, which has a lower solidification temperature, remains at the boundary between columnar crystals and a crack opens when tensile stress acts on the weld metal on cooling. This solidification cracking occurs in the weld metal and the selection of weld metal is studied to prevent cracking.

With respect to solidification cracking, Senda et al.8) studied the temperature at which the crack occurs and they proposed a hot ductility curve, shown in Fig. 3. From this curve, the BTR, critical strain min and the critical strain rate for temperature drop (CST), which is the combination of BTR and min , were proposed and used as evaluation parameters8). In the current research, the plastic strain increment in BTR during cooling is investigated as a factor indicating the solidification cracking, as shown in Fig. 4, since solidification cracking occurs due to the tensile force and a decrease in the bonding force at the grain boundary. As shown in Fig. 4, BTR plastic strain increment

pBTR is determined by the difference between the plastic

strain at the liquidus temperature and the solidus temperature during cooling. In this research, thermal-elastic-plastic FEA is employed to determine the BTR plastic strain increment.

3. Experiment and Analysis of 2-Torch High-Speed

Circumference MAG Welding

3.1 Analysis model and welding condition

In this chapter, a heat conduction analysis is carried out for the 2-torch heat source model and the results are compared with the experimental measurement to demonstrate the validity of the analysis.

Two-torch high-speed circumferential metal active gas (MAG) welding is carried out in this experiment. The shape, dimensions and FE mesh divisions of the analysis model are shown in Fig. 5. The outer and inner diameters of the model are 609.6 mm and 571.4 mm, respectively. Considering the symmetry of the phenomena, half of the pipe from the welding line in the axial

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Fig. 4 Schematic illustration of plastic strain increment in BTR during cooling.

Fig. 3 Schematic illustration of ductility of material in high-temperature range.

Fig. 5 Analysis model of pipe welding.

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direction is modeled. The model is divided into 360 and 48 elements in the circumferential and thickness directions, respectively. The numbers of elements and nodes are 271,440 and 302,040, respectively. A backing plate is also considered on the inner surface of the pipe. The dimensions of the backing plate are 47 mm in width and 9 mm in thickness. For the welding conditions, the heat inputs are 362.5 J/mm and 337.5 J/mm for the 1st and 2nd torches, respectively. The heat input is determined by the following equation;

vIE=Q (1)

where, Q is heat input (J/mm), E is welding voltage (V), I is welding current (A), v is welding speed and is heat

efficiency. Regarding heat source, a moving volumetric heat source is employed in the heat conduction analysis, and elements in the rectangle are uniformly heated by volumetric heat source. The cross sectional areas of both the 1st weld layer and 2nd weld layer are assumed constant. The pipe material is X65. The temperature-dependent material constants of X65 are shown in Fig. 6. In Fig. 6, the constants are as follows; E :Young’s modulus, Y :Yield stress, :Poisson’s ratio, : Thermal expansion coef., c : Specific heat, :Density, : Heat conductivity coef.

In this analysis, except for yield stress, the same temperature dependent material properties as SM490 are used for x65. Regarding yield stress, it is assumed that the shape of stress –

strain curve are same between SM490 and x65 and only the magnitude of stress is modified.

3.2 Experimental and analysis results

Fig. 7 shows the maximum temperature distribution, which is obtained by the heat conduction analysis, on the cross section at 90° from the start point of welding. In the figure, the gray region indicates the molten region, whose temperature higher than 1450°C. From Fig. 7, it is found that the forming of the molten region is well represented. In this analysis, thermal fluid phenomena in molten pool is not considered. However, as shown in Figs. 7 and 8, molten region agrees very well between analysis and experiment. In addition, as shown in Fig. 6, Young’s modulus E and yield stress Y are set to very small value in higher temperature than melt point of 1450℃ to model the melting of the material.

In this analysis, the element birth technique is employed. In the element birth technique, elements corresponding to the weld metal are initially disabled and sequentially enabled with progression of the torch. That means that elements are enabled and heated for each torch of tandem welding. By using this technique, it is possible to represent the realistic overlay of each welding torch and to evaluate the influence of the heat input of each torch.

4. Evaluation of Solidification Cracking Using BTR Plastic

Strain Increment

The previous chapter shows the validity of the heat conduction analysis, which considers the tandem heat source and the element birth technique. In this section, thermal-elastic-plastic analysis using IEFEM is applied to the same steel pipe model used in the experiment. The obtained BTR plastic strain increment is evaluated along the center line of the weld bead to investigate solidification cracking. In addition, since the occurrence of solidification cracking is reported at the 1st welding pass, as shown in Fig. 2, various conditions are studied in detail at the 1st welding pass.

Fig. 8 Photo of penetration shape of weld metal of 2-torch MAG welding.

Fig. 7 Maximum temperature distribution obtained by FEA.

Fig. 6 Temperature dependent material constants.

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192s 研究論文　　SHIBAHARA et al.: Study on Solidification Cracking Under High-speed Narrow Gap Welding with ……

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4.1 Analysis conditions

In this chapter, the same steel pipe model as the previous chapter is used. For the welding method, a left half-circle is welded in the counterclockwise direction. After that, the remaining right half-circle is welded clockwise as shown in Fig. 5. The same welding conditions as in the previous chapter are used as the standard conditions, and the influence of heat input, welding speed and distance between torches on the BTR plastic strain increment is investigated. The pipe material is X65 and the temperature-dependent material constants are shown in Fig. 6.

4.2 Influence of heat input

Figure 9 shows the distribution of the BTR plastic strain increment on the cross section at 90° from the start point of welding. In the figure, (a) and (b) show the distribution of BTR plastic strain increment for the cases in which the total heat input is set to 700 J/mm and 875 J/mm, respectively. The total heat input is the sum of the heat input of the 1st and 2nd torches. The welding speed and the torch distance were fixed at 20 mm/s and 65 mm, respectively. Figure 10 shows the distribution of the BTR plastic strain increment along line A-B on the cross section at 90° from the start point of welding. From the figures 9 and 10, it can be seen that the larger BTR plastic strain increment occurs in the case of heat input of 875 J/mm than in the case of 700 J/mm on the weld metal of the 2nd torch. It is also found that the position of the maximum BTR plastic strain increment is not on the outer

or inner surface; instead, it is inside the bead. Next, Fig. 11 shows the temperature history of plastic strain in

the y direction at point C for each heat input. In the figure, the horizontal axis and the vertical axis indicates temperature and the plastic strain, respectively. The circles and triangles show the cases where the heat inputs are 700 J/mm and 875 J/mm, respectively. Point C is located at 4.7 mm from the inner surface on line A-B. From the figure, it can be seen that the BTR plastic strain increment increases with the heat input.

4.3 Influence of welding speed

To discuss the influence of welding speed, the welding speed was changed to the following four cases: v=20 mm/s, 22 mm/s, 35 mm/s and 40 mm/s. The torch distance and heat input were

Fig. 10 Influence of heat input on pBTR .

Fig. 11 History of plastic strain in y direction for temperature

at point C.

(a) Q=700 J/mm (b) Q=875 J/mm Fig. 9 Distribution of BTR plastic strain increment along the y direction.

Fig. 12 Influence of welding speed on pBTR .

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0.00650.00530.00410.00290.00170.0006-0.0006-0.0017-0.0029-0.0041-0.0053-0.0065

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fixed at 65mm and 700 J/mm, respectively. Figure 12 shows the comparison of the distribution of the BTR

plastic strain increment along the line A-B for the welding speed. From the figure, it can be seen that BTR strain increment is larger at the higher welding speed on the weld metal of the 2nd torch.

4.4 Influence of torch distance

To investigate the influence of torch distance on the BTR plastic strain increment, the following four torch distances are assumed: L=35mm, 45mm, 65mm and 100mm, in the fixed welding speed and heat input at 20 mm/s and 700 J/mm, respectively. Figure 13 shows the comparison of the BTR plastic strain increment along line A-B for the torch distance. From the figure, it can be seen that the BTR plastic strain increment increases with the decrease of torch distance. It appears that the apparent heat input of the 2nd torch becomes large since heat from 1st torch becomes large at the smaller torch distance.

5. Conclusion

In this research, a mechanical investigation was carried out on the narrow-gap 2-torch high-speed circumferential MAG welding of steel pipe. In the investigation, a three-dimensional nonlinear thermal-elastic-plastic FEA, which considers a practical model, was carried out by using IEFEM, and BTR plastic strain increment was studied. Before the thermal-elastic-plastic analysis, the validity of the heat conduction analysis was verified by comparing the penetration shapes between the experimental and analytical results. Based on the history of the temperature distribution obtained by the heat conduction analysis, thermal-elastic-plastic analysis was conducted and the influence of the welding conditions was investigated. The following results were obtained: 1) The penetration shape was compared between the

experimental measurement and the heat conduction analysis. It was found that the penetration shapes obtained by the heat conduction analysis agree well with those of the experiment.

2) To investigate the influence of the welding condition on the BTR plastic strain increment, analyses with changing heat input was conducted. It was found that the BTR plastic strain increment increases with heat input.

3) Welding speed and torch distance were also changed and analyses were conducted. As a result, it was found that the BTR plastic strain increment increases with welding speed. For torch distance, it was clearly seen that the BTR plastic strain increment is larger for a smaller torch distance, since cooling after the pass of the 1st torch becomes small and the region of BTR temperature becomes large after the pass of the 2nd torch.

Reference

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3) Kunihiko SATOH, Yukio UEDA, Toshio YADA, Ryoichi KAMIDHIKA, Toyoo MAEDA and Yuchol KIM: Studies on Deformations and Cracking in One Sided Welding (1st Report), Journal of the Society of the NAVAL Architects of Japan, Vol.136 (1974), pp.441-458.

4) Kazuki IKUSHIMA, Shinsuke ITOH and Masakazu SHIBAHARA: Development of Parallelized Idealized Explicit FEM Using GPU, Quarterly Journal of The Japan Welding Society, Vol.31, No.1 (2013), pp.23-32.

5) Fukuhisa MATSUDA, Hiroji NAKAGAWA and Jong-Bong LEE: Numerical Analysis on Solidification Brittleness Temperature Range in Stainless Steel : Weld Crack Susceptibility of Duplex Stainless Steel (Report V), Quarterly Journal of The Japan Welding Society, Vol.9, No.1 (1991), pp.93-98.

6) Ken-ichi KARIMINE, Kouichi UCHINO and Makoto OKUMURA: Susceptibility and Occurrence Condition of HAZ Liquation Cracking on Rail Steels - Study on Rail Welding with High-C Welding Materials (Report 4) -, Quarterly Journal of The Japan Welding Society, Vol.14, No.3 (1996), pp.578-585.

7) Yoshikuni NAKANO, Kenji SHINOZAKI, Tadao OGAWA and Hideo SAKURAI: Effectsof Cr and S on Ductility –dip Cracking Susceptibility in Reheated Weld Metals of Ni-Cr-Fe Ternary Alloys -Study on Microcracks in Multj−pass Weld Metals of Ni−base Alloys (Part 2), Transactions of the Japan Welding Society, Vol.24, No.2 (1993) pp.101-106.

8) Tomio SENDA, Fukushima MATSUDA, Genta TAKANO, Kiyoshi WATANABE, Tadaaki KOBAYASHI and Takeshi MATSUZAKA: Studies on Solidification Crack Susceptibility for Weld Metals with Trans-Varestraint Test (1), Journal of the Japan Welding Society, Vol.41, No.6 (1972), pp.709-723.

Fig. 13 Influence of torch distance on pBTR .

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194s 研究論文　　SHIBAHARA et al.: Study on Solidification Cracking Under High-speed Narrow Gap Welding with ……