Welding simulation of steels welded with low transformation temperature (LTT) filler materials

Abstract

Welding is a thermal process that creates residual stresses due to a combination of thermal expansion, plastic flow and phase transformation. By using filler material which has a composition that transforms to phase transformation to Martensite at low temperatures, the residual stress state can become more favorable. The purpose of this thesis is to develop a numerical tool to evaluate temperature evolution, phase transformations and residual stresses developing during multi-pass plate-to-plate welding using LTT filler. The commercially available finite element (FE) software Abaqus v6.13 (Simulia 2013) was used for the numerical simulations. In the current thesis a transient 3D simulation of a moving heat source with a distributed heat flux is performed. First a repeated analysis of a single pass weld is performed to validate Abaqus as a good simulation software for welding simulation. Then a heat transfer analysis of a plate with 3 pass of weld is performed. Two different heat source models are used and compared. The Abaqus user subroutine DFLUX is used to define the welding torch heat input as a volumetric heat flux. The subroutine is developed in the FORTRAN language. The heat source travels along the weld line to simulate the torch movement. In the next phase of the work, Metallurgical analysis is performed to model the phase transformation during and after welding. User subroutine Hetval is used to model phase transformation. Finally, the data from the previous heat transfer analysis and metallurgical analysis are used as an input file for a mechanical analysis.