Yuan Shijian

Defects caused in forming process of rotary forged parts and their preventive methods

Abstract In this paper, the characteristics of rotary forging deformation and their mechanic analysis are briefly described; meanwhile the four kinds of defects caused generally in the deformation are introduced: chevron crack in the thin workpiece, peripheral crack in the flange with a rod, instability and overlapping, and not thoroughly forging. Then, it is pointed out that the defects such as crazing and pressed-subsidence are frequently caused in the centre of an upper die, because of the particularities of the rotary forging technology: there is a small contact area throughout between the upper die and the workpiece and the pressure is high, and the upper die suffers from the effect of thermal fatigue during hot rotary forging. Finally, the measures to prevent the defects and the methods for improving the service life of the die are proposed. For thick workpieces and flanges with a rod, the feed per rotation should be properly controlled; for thin workpieces, the feed per rotation should be increased. In cold rotary forging of flanges with a rod, the surface quality of blanks should be considered and the die-workpiece clearance should be controlled so that the defects of cracking, instability, overlapping and so on can be decreased.

The numerical simulation of the integral hydrobulging of ellipsoidal shells

The integral hydro-bulge forming of ellipsoidal shells is described. The elasto-plastic finite element method is used to simulate the integral forming process of an ellipsoidal shell. Simulation results show that the plastic deformation does not occur at the same time on the different zone of the shell and the flexural effect exists at the corner of the shell. The expanding of the plastic deformation, the yield paths of the typical points and the thickness distribution after hydrobulging are also researched in detail.

Research on the integral hydrobulge forming of ellipsoidal shells

Abstract Although spherical vessels have sucessfully been made using hydrobulging technology. There are some problems occurring during the hydrobulging of ellipsoidal shells because in the latter case the structure is not centro-symmetrical as with spherical shells. In the present paper, the integral hydrobulge forming of ellipsoidal shells is described, its theoretical analysis, deformation tendency and experimental results being introduced. It is shown that ellipsoidal shells of which the axial length ratio m is less than 2 can be formed easily and that ellipsoidal shells of which m is greater than 2 can be also formed by adopting some technical measures.

Research on the compensatory welding method for preventing welded joints of polyhedral shells from fracturing during hydrobulging

Abstract This paper first introduces the basic principle and the main advantages of the new die-less hydrobulging technology for manufacturing spherical vessels. Then, it analyzes the phenomena that result in the fracture of polyhedral shells during hydrobulging, coming to the conclusion that the concentration of plastic strain at the inside weld toe and the reduction in the plasticity of the heat-affected zone (HAZ) are the main reasons for fracturing. Based on the analysis, a new technological method - the compensatory welding method (CWM), which consists of two transferring beads and a tempering bead - is proposed to prevent the welded joints from fracturing. Finally, it establishes, by means of a substantial amount of experimental research, that the reasons why the compensatory welding method is able to improve the plasticity of the welded joints and prevent fracturing are: (i) that the transferring beads change the direction of the inside bond line, improving the shape profile of the weld toe; and (ii) that the tempering bead improves the microstructure of the HAZ, reducing its hardness such that its plasticity is increased.

Experimental research and finite-element simulation of plates hydrobulging in pairs

In this paper, the new hydrobulging technology is described for manufacturing giant ellipsoidal tank heads which could only be made through press and welding before. First, the head is divided into skirt and center parts; the skirt part is formed through traditional technology, while the center part is formed through hydrobulging. Then these two parts are welded together and if the shape and dimensions of the center part after hydrobulging are those of the required ellipsoidal head, then a whole ellipsoidal head is produced. The experiment was performed by hydrobulging plates in pairs under different boundary conditions to form the centre part of an ellipsoidal head. The deformations of the experimental work were measured and calculated and were compared with those of a standard ellipsoidal curvature which were nearest to the experimental curvatures. Finally numerical simulation of the hydrobulging plates in pairs under stiff boundary conditions was done using FEM.

Research on the residual stresses of hydrobulged spherical vessels

Abstract This paper describes the residual-stress distributions of wide plate specimens for simulating the hydrobulging process of a 200 m 3 spherical vessel, which latter is the first set for the storage of liquefied petroleum gas (LPG) manufactured by die-less hydrobulging technology. The results show that on the inside surface of the hydrobulged spherical vessels, the transverse residual stress is in the compressive state: the longitudinal residual stress usually has a very small tensile value, but is also sometimes in the compressive state. These distributions of residual stresses are favourable for preventing the spherical vessel from stress-corrosion cracking (SCC) and embrittlement fracture and can improve its fatigue strength. This is a significant advantage of the new die-less hydrobulging technology, in ensuring safety.

Research on residual stresses after hydrobulging of spherical vessels

Abstract This paper describes the residual stress distributions on wide plate specimens intended to simulate the hydrobulging process for the 200 m 3 spherical vessel which is the first set for storage of liquefied petroleum gas (LPG) manufactured using dieless hydrobulging technology. The results show that on the inside surface of the spherical vessels after they are hydrobulged, the transversal residual stress is in a compressive state; the longitudinal residual stress has a very small tensile value and sometimes is in a compressive state. These distributions of residual stresses are favourable to prevent the spherical vessel from stress corrosion cracking (SCC) and embrittlement fracture and can improve its fatigue strength. This is a significant advantage of the new dieless hydrobulging technology in guaranteeing safety.