Abstract:
Fusion welding processes cause residual stress due to the uneven heat
distribution produced by the moving welding torch. These residual stresses are
characterised by a large tensile component in the welding direction. Due to the
self-equilibrated nature of the residual stress, compressive ones are present in
the far field next to the weld seam, which can cause different kind of distortion
such as bending or buckling. Welding residual stress can be responsible of
premature failure of the components, such as stress crack corrosion, buckling,
and reduction of fatigue life. Localised rolling is a stress engineering technique
that can be used to reduce the residual stress and distortion caused by welding.
It induces plastic strain in the rolling direction, counteracting the plastic strain
produced during welding.
In this thesis three techniques were investigated, pre-weld rolling, post-weld
rolling, and in situ rolling. These techniques have been seldom studied in the
past, particularly pre-weld rolling; consequently the mechanisms are poorly
understood. Finite element models allow stress and strain development during
both welding and rolling processes to be better understood, providing an
improved understanding of the mechanisms involved and aiding process
development.
A literature survey was done to find the state of the art of the computational
welding mechanics simulations, stress management, and the residual stress
measurement techniques, as well as the knowledge gaps such as, the thermal
losses through the backing-bar in the thermal simulation, the frictional
interaction in the rolling process, and the material properties of the steel used in
the models. In the literature not many models that investigate the management
of welding residual stress were found.
After this, the general considerations and assumptions for the welding thermal
mechanical models presented in this thesis were discussed. The effect of
different backing-bar conditions, as well as different material properties where
investigated. Both influenced the residual stress profile to varying degrees. In
particular, temperature dependent heat loss to the backing-bar was necessary
to capture the improved heat loss near the weld. The distortion predicted by the
model was investigated to determine whether it was due to bending or buckling
phenomena. Lastly, the temperature distribution and residual stress predictions
were validated against thermocouple and neutron diffraction measurements
conducted by Coules et al. [1–3].
Pre-weld rolling was the first of the three rolling methods considered, in which
rolling is applied to the plates before performing GMA butt-welds. The principle
behind this technique consisted in inducing tensile residual stress in the weld
region before welding; therefore, it is similar to mechanically tensioning the
weld, which can significantly reduce the residual stress and distortion. However,
there was no significant change in the tensile residual stresses. On the other
hand, it was possible to achieve a small reduction in the distortion, when the
plates were rolled on the opposite surface to the weld; rolling in this way
induced distortion in the opposite direction to the distortion induced by welding,
reducing the magnitude of the latter. These results were compared with
experiments conducted by Coules et al. [1,4]. A subsequent investigation
combined pre-weld rolling with post-weld heating. With this additional process
the residual stress and distortion were significantly reduced, and flatter residual
stress profile was achieved.
The post-weld rolling and in situ rolling techniques were discussed afterwards.
In the post-weld rolling models, rolling was applied after the weldment was
cooled to room temperature. In in situ rolling the roller was applied on top of the
weld bead at some distance behind the torch, while it was still hot. The principle
behind these techniques consisted in applying positive plastic strain to the weld
bead region by a roller, counteracting the negative plastic strains produced in
the welding process. Two roller profiles were investigated, namely, grooved,
and double flat rollers. The post-weld rolling on top of the weld bead models,
which used the grooved roller, showed good agreement against experimental
results, producing a large reduction of the residual stress and distortion. Some
discrepancies were present when the weld toes were rolled with the dual flat
roller. The former roller was more efficient for reducing residual stress and
distortion. The influence of different friction coefficients (between the roller and
weldment, and between the backing-bar and the weldment), were investigated.
It showed significant dependency on the residual stress distribution when high
rolling loads were used. The frictional interaction constrained the contact area
inducing more compressive stress in the core of the weld bead; therefore it
produced more tensile residual stress in the surface of the weldment.
Additionally, the influence of rolling parameters on the through-thickness
residual stress variation was investigated. Low loads only influence the residual
stress near the surface, while high loads affected the material through the entire
thickness.
When the dual flat roller was used to roll next to the weld bead, significant
compressive residual stress was induce in the weld bead; however, the residual
stress reduction was very sensitive to the contact of the roller to the weld toes;
therefore, when rolling a weld bead that varies in shape along the weld, the
residual stress reduction is not uniform and varies along the length. On the
other hand, the in situ rolling did not produced significant residual stress or
distortion reduction in all the cases analysed. The rolling occurred when the
material was still hot and the residual stress was subsequently formed as the
material cooled to room temperature. Numerical modelling was a very useful
tool for understanding the development of stress and plastic strain during the
welding and rolling processes.