A STUDY ON THE PREDICTION OF

WELDING DISTORTIONS OF HULL IN SHIPBUILDING (I)

S.B. Shin*, J.G. Youn*

ABSTRACT

In order to evaluate principal factors controlling welding distortions in particular angular distortion, both dimensional analysis and Finite Element Analysis (FEA) have been performed. The principal factors were found to be the heat intensity, rigidity and restraint, which were verified by experimental results. Angular distortion both at the fillet and at the butt weldment of hull structure can be predicted by an equation expressed with function of heat intensity, bending rigidity and internal restraint.

KEYWORDS

Angular Distortion, Heat Intensity, Bending Rigidity, Restraint, Dimensional Analysis and Finite Element Analysis (FEA)

INTRODUCTION

Welding process has been widely used in fabricating steel structures such as ships and pressure vessels. However, some inherent problems of the welding still need to be solved (Ref.1-3). A major problem is welding distortion. This is attributed to incompatible strains over the welded joint, which is developed by uneven temperature distribution during/after welding. Welding distortions adversely affect the service behavior of welded structure including static and dynamic stability and buckling characteristics. In order to reduce and prevent distortions, lots of researches have been performed but welding distortion for actual structures has not yet been controlled. This may be associated to the fact that the previous studies have been limited generally in a laboratory scale with simple welding variables. Welding distortions at actual structures like ships depend not only on welding process and conditions, but also on internal and external restraint acting on the weldment. It is, therefore, necessary to establish proper predictive and control methods of welding distortion for actual structures on the basis of these factors, combined with material itself.

This study has been tried to identify the principal factors controlling welding distortion of the hull structure, in particular angular distortion and transverse shrinkage both at the fillet and butt weldment by the dimensional analysis and Finite Element Analysis (FEA). Comprehensive experiments were also carried to verify the results of the dimensional analysis and FEA. Based on these results, a predictive equation of welding distortion applicable to actual hull structures has been proposed. In this study, results and discussion on the angular distortion are described Material Research Dept., Hyundai Heavy Industries Co. Ltd., Jeon Ha Dong, Dong Gu, Ulsan, South Korea

ANALYSIS AND EXPERIMENT PROEDURE

Dimensional analysis was performed to identify the principal factors controlling welding distortion as given in below. If effects of material properties including yield strength and thermal expansion coefficient were not taken into consideration because of their temperature dependence, residual angular distortion of fillet and butt weldment could be expressed as shown in Eqn (1).

Where, Q is heat intensity and D, and W are bending rigidity and width of weldment respectively. Here, equation (1) is rearranged as following equation by substituting the dimensional formula.

The equation corresponding to the dimensional homogeneity is defines as follow:

F: a+b=0

L: a+b+c=1

Solving Eqn (3), the following exponents are obtained

a=1 b = -1 c=1

(3)

(4)

Substituting these values into Eqn (1), the dimensionless form of angular distortion is defined as following equation

As shown in Eqn (5), the angular distortion could be defined as a function of heat intensity (Q) and bending (D). It is, therefore, deduced that the principal factors controlling welding distortion are the heat intensity and rigidity.

From the results of the dimensional analysis, the variables for FEA and experiment were selected to establish a predictive equation for welding distortion for actual weldment as shown in Table 1. Fig. 1 shows the schematic configuration of the models for FEA and experiment. Welding parameters used in this study were typical FCAW (Flux Cored Arc Welding) and SAW (Submerged Arc Welding) conditions for actual fillet and butt weldment. In order to evaluate the effect of internal restraint on the angular distortion, width of weldment is varied from 700mm to 4000mm. External restraint condition at the weldment was simulated by attaching temporary fixture to the lateral side of the weldment as shown in Fig. 2. The amount of external restraint is defined as a function of bending or in-plane rigidity and restraint length (Ls). (Ref. 4, 5) Table 1 Variables used For FEA and experiment

Fig. 2 Schematic diagram of external restraint of fillet and butt weldment

Finite Element Analysis

The transient temperature distribution was calculated based on the assumed quasi-stationary condition. (Ref. 6) The volume heat source of welding arc was specified by a Gaussian distribution. Heat loss at all surfaces of the solution domain was governed by natural convection. Thermal properties of the material used depend on the temperature and an effective conductivity may be assumed at the temperature above the melting point. To consider the effect of latent heat at the phase change, the specific heat in the solidification range was modified to be very high. (Ref. 7)

Mesh design used for thermo-mechanical stress analysis consisted of 8-nodes plane element with general plane strain condition and spring element as shown in Fig. 3. Spring elements attached to bottom surface of fillet and butt weldment are to prevent reverse angular distortion during the welding and cooling stage. (Ref. 8) While thermo-mechanical strains along the longitudinal direction of welding line were assumed to be uniform, free expansion and bending in the transverse direction of welding line were allowed. When the temperature of elements reached the liquidus temperature, the plastic strains accumulated by that time were assumed to be relieved. In this region above the melting temperature, thermal strain increment was set to zero. Mechanical properties of weldment were postulated to behave as an isotropic, elasto-plastic and strain-hardening continuum. Yielding of material was assumed to be governed by von-Mises criteria.

RESULTS AND DISCUSSION

Verification of Principal Factors

Fig. 4 shows the variation of angular distortion at the fillet and butt weldment as a function of the heat intensity (Q) and bending rigidity (D). Angular distortion at the fillet and butt weldment is strongly dependent on the function of heat intensity and bending rigidity. Good agreements between calculated results and measured results for angular distortion are found. This verifies that heat intensity (Q) and bending rigidity are the principal factors controlling the angular distortion, which was already proposed from the dimensional analysis described in the previous section.

Fig. 4 Variation of the angular distortion at the fillet and butt weldment with the ratio of Q and Db

Internal Restraint

Welding distortion of the actual welded structure is different from that of small size welding specimen in terms of amount and deformed shape. Welding distortion of actual weldment could be over-estimated by a predictive method established by the small size weldment. Fig. 5