Deformations and stresses in welded pipes : numerical and experimental investigation

In this dissertation, deformations and stresses in welded pipes have been studied both numerically and experimentally. The aim of this work has been to investigate and verify finite element models for simulation of the fabrications of two types of pipe joints. The first joint considered is a butt-welding of thin-walled pipes where residual stresses and deformations were obtained numerically and experimentally verified. The second type of joint which has been investigated twice, is a pipeflange joint, i.e. a flange is attached to one end of a pipe by multi-pass welding. The aim of this study was to predict the distortion of the flange after completed welding. Results obtained from simulations have been compared and verified with corresponding experimental quantities. In the latter part of the pipe-flange joint study, a large amount of work has been devoted to experimental verifications of results obtained during the welding process. Furthermore, an application of additional simulations of single-pass butt-welded pipes has been performed by turning the residual fields of stresses and deformations into a finite element model for buckling analysis, investigating which of the quantities, i.e. residual stresses or residual deformations, have most influence on the reduction of the axial load carrying capacity for welded pipes.Godkänd; 1996; 20070428 (ysko

Deformations and stresses in welded pipes : numerical and experimental investigation

In this dissertation, deformations and stresses in welded pipes have been studied both numerically and experimentally. The aim of this work has been to investigate and verify finite element models for simulation of the fabrications of two types of pipe joints. The first joint considered is a butt-welding of thin-walled pipes where residual stresses and deformations were obtained numerically and experimentally verified. The second type of joint which has been investigated twice, is a pipeflange joint, i.e. a flange is attached to one end of a pipe by multi-pass welding. The aim of this study was to predict the distortion of the flange after completed welding. Results obtained from simulations have been compared and verified with corresponding experimental quantities. In the latter part of the pipe-flange joint study, a large amount of work has been devoted to experimental verifications of results obtained during the welding process. Furthermore, an application of additional simulations of single-pass butt-welded pipes has been performed by turning the residual fields of stresses and deformations into a finite element model for buckling analysis, investigating which of the quantities, i.e. residual stresses or residual deformations, have most influence on the reduction of the axial load carrying capacity for welded pipes.Godkänd; 1996; 20070428 (ysko

Green body behaviour of high velocity pressed metal powder

High velocity compaction (HVC) is a production technique with capacity to significantly improve the mechanical properties of powder metallurgy (PM) parts. Several investigations indicate that high-density components can by obtained using HVC. Other characteristics are low ejection force and uniform density. Investigated here are green body data such as density, tensile strength, radial springback, ejection force and surface flatness. Comparisons are performed with conventional compaction using the same pressing conditions. Cylindrical samples of a pre-alloyed water atomized iron powder are used in this experimental investigation. The different behaviour of HVC-pressed green bodies compared to conventional pressed green bodies are analysed and discussed. The HVC process in this study resulted in a better compressibility curve and lower ejection force compared to conventional quasi static pressing. Vertical scanning interferometry (VSI) measurements show that the HVC process gives flatter sample surfaces.Validerad; 2007; 20070216 (cira)</p

The effect of mechanical behavior on bendability of ultrahigh-strength steel

Abstract Bendability is an important property of ultrahigh-strength steels since the typical applications of such materials include structures manufactured by air-bending. Conventional methods to evaluate bendability, such as the bending test according to the standard VDA-238 or the conventional tensile test do not provide sufficient information to evaluate bendability of ultrahigh-strength steels due to the average nature of the material response in these tests. In this study, the mechanical properties were determined using thin tensile specimens cut from the surface of the sheet and the evaluation of bendability was carried out using frictionless bending tests. The results of the experiments and FE-modelling presented in this paper reveal that the mechanical properties of the sheet surface have a significant impact on bendability. Novel ultrahigh-strength steel with better work-hardening capacity at the surface caused by a layer of relatively soft ferrite and lower bainite has good bendability, especially when the bend line is aligned transverse to the rolling direction. Microstructural investigations reveal that in a conventional steel with a relatively hard surface microstructure, the deformation localizes into shear bands that eventually lead to fracture, but similar shear banding was not present in the novel steel surface. This can be attributed to the better work-hardening capacity which delays the onset of shear localization and fracture

Superior bendability of direct-quenched 960 MPa strip steels

Abstract The present paper shows the effect of microstructure on the press brake and frictionless 3-point bending of 6 mm thick ultrahigh-strength steel strips with a yield strength of 960 MPa. With a traditional press brake machine the minimum bending radii of the studied steels varied from 1.3 times the thickness to 3.0 times the thickness for the bend axis perpendicular to the rolling direction and in the range 2.0–3.5 times the thickness for the bend axis parallel to the rolling direction. The frictionless 3-point bending-equipment incorporating rotatable die-rollers has been applied to characterize the material work hardening behavior in a way relevant to the bending process, i.e. by using measured punch force vs. position data to derive the bending moment and the evolution of the flow stress and the strip curvature during the bending process. The main aim of the present paper is to establish an understanding of how bendability can be significantly improved and made more isotropic by modifying the subsurface microstructure to include a relatively soft polygonal ferrite and granular bainite layer and why the subsurface microstructure plays such a dominant role