The last years improvements in steel making technologies have resulted in
steel grades with higher tensile strength, hardness, weldability etc.
compared to conventional construction steel. This category steel gives
lighter and smaller sized structures and cost reductions.
Since the weld metal is still only a cast alloy whose strength mainly
depends on its chemical composition, the differences in mechanical
properties between the base plate and the consumable are increasing. The
question is how to choose the undermatching level between the base plate
and weld metal without compromising the global strength of the joint.
In this master's thesis, the effects of different weld geometries on the
mechanical properties of undermatched welds in high strength quenched and
tempered steel, are studied. Different undermatching levels have been
accomplished by combining two different steels: Weldox 960 and Weldox 1100
together with two consumables: Filarc PZ6145 and Filarc PZ6149.
Two methods have been used to gather information: 30 test specimens have
been manufactured and tested at SSAB's plant in Oxelösund and previous
studies in this field have been analyzed in a literature review.
Three different parameters were chosen to be studied in the laboratory
tests, only one parameter was changed at a time:
? Width- to thickness relation
? Undermatching
? Relative thickness
The hypothesis was that if the weld metal is softer than the adjacent
steel, it will yield before the base metal does. When the weld starts to
deform, this deformation is constrained by the non-yielded surrounding
metal. Tension is developed in both the width- and thickness directions, in
addition to tension in the longitudinal direction due to the applied load.
When the weld experiences tension in two or three material directions, the
mean stress or hydrostatic stress in the weld is increased. As the
hydrostatic stress is increased, by constraint, the magnitudes of the
deviatoric stresses, which govern yielding, are reduced. This results in a
stronger weld where increases in the applied load can be achieved.
When the width- to thickness relation was studied, five different specimen
widths, from 6 mm to 96 mm, were created out of the same plate. All
specimens were taken from the same coupon which means that the welding
parameters were identical for all specimens. In this case the effect of the
width was obvious: the ultimate strength of the joint increased from 1070
MPa (6 mm) to 1192 MPa (96 mm). The ultimate strength of the weld metal
was, in this case, 1015 MPa.
When it comes to undermatching, all test specimens in Weldox 960 steel,
welded with the stronger electrode, fractured in the base metal. This in
spite of that the ratio between the weld metal's and base metal's yield
strengths ...
