Shinji KOBAYASHI*
Esao YAMADA**
Tetsu GO**
Shigetaka OKANO***
Masahito MOCHIZUKI***
Kenyu KIMURA****
Akiyoshi ANDO****
*
**
***
Graduate School, Osaka University
****
Elliott Ebara Turbomachinery Corporation
Among many problems with welding is welding distortion. Since welding distortion significantly affects the performance and reliability of products, it has become increasingly required in recent years to appropriately foresee, control, and reduce distortion. One of the approaches to this issue is numerical analysis. As significant improvements have been made to welding dis- tortion analysis technology, its application has been increasing every year. So far, however, numerical analysis has only been applied to a limited number of large scale and/or complicated structures. Under such circumstances, with a compressor impel-ler having complicated structure as a target, distortion analysis and experiment was performed. The analysis and experiment results have revealed that it is possible to evaluate welding distortion occurring to an impeller with high accuracy by using numerical analysis with the help of the construction of a heat source model and taking into account phase transformation.
Keywords: Compressor, Impeller, Welding distortion, Numerical simulation, Distortion measurement, Phase transformation, Martensitic stainless steel, Displacement of discharge width, Displacement of cover height
In fabricating machines and structures, welding plays an important part as a basic manufacturing technology. However, welding can be accompanied by defects such as poor welds and cracks, unevenness in strength and toughness ascribable to a heat-affected zone, mismatch in the shape of, for example, a bead toe, and residual stress and distortion, which may result in the decreased strength of the structure and other problems. These phenomena must be appropriately predicted evaluated, and controlled to build sound welded joints/structures.
Fig. 1 General view of the compressor impeller
Fig. 2 Welding sequence
Figure 3 shows a finite element model of the compressor impeller. As the boundary condition in heat transfer analysis, heat transmission and the thermal radiation that obeyed the Stefan-Boltzmann law was considered, and as the boundary condition in thermo-elastic-plastic analysis, the model is restricted only the rigid-body movement and rotation. Figure 4 shows examples of material properties used for the numerical analysis. In order to take into consideration the phase transformation behavior associated with the thermal cycle, yield stress and the temperature dependence of the linear expansion coefficient (thermal strain curve) as shown in the figure were used in numerical analysis. A heat-source model was modeled by reference to the temperature history and angular distortion at the welds of T-shaped fillet welded joints observed with the same welding heat input as for the compressor impeller. In this analysis, heat source length was 6 mm, and thermal efficiency was 0.6.
Fig. 3 Finite element model of the compressor impeller
Fig. 4 Material properties used in numerical analysis in consideration of phase transformation
Fig. 5 Measurement targets and their positions
Fig. 6 Simulated and measured displacement H
Fig. 7 Simulated and measured displacement C
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