Page 28 - SSAB Welding Handbook Edition 2

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4.0 Hydrogen cracks Welding handbook

Tension
Tension
©SSAB
Compression
Compression

Fig. 4.9: Residual stresses in and around a butt joint, viewed longitudinal and transverse to the direction of the joint.

The weld metal is first heated to a molten state by As the level of restraint for a joint is raised, the more
the welding process in section B-B in fig. 4.8. The it will promote increased levels of residual stresses
parent metals adjacent to the weld pass in this section in the welded structure. However, the residual stress
expand as the temperature increases. The strength of level is always maximized to the local yield strength
the weld metal at this temperature is negligible and it in the joint.
is not as strong as the surrounding parent metal. This
causes the weld metal to be plastically deformed by Even though residual stresses occur during welding,
the parent metals. Due to the high temperatures, the the risk for hydrogen cracks is minimized, in all
stresses in the weld metal and its surroundings are structures produced with the Hardox and Strenx
very low at this stage. grades, if the recommendations from SSAB are
followed.
During the cooling cycle of the welding process,
when the weld metal transforms from a liquefied The exact measure of the levels and distributions of
state to a solid state, it has a natural tendency to residual stresses for a joint are seldom analyzed in
shrink. However, the surrounding parent metals in detail. They are complex and depend on the
the joint counteract this movement. The strength of appearance of the joint and how the welding
the weld metal increases gradually as its temperature performance is carried out. A more detailed
decreases. As a result, there are interactions between evaluation of the residual stresses for a particular
the weld metal and the parent metals, causing very joint can be preformed with, for instance, a
high residual tensile stresses in the weld metal and the computerized FEM (fenite element method) study.
weld metal creates compressive residual stresses in
the parent metals – on both sides of the weld metal –
which is illustrated in fig. 4.9.

Residual stresses are also formed transverse to the
direction of the joint and in the through thickness
direction of the joint. A criterion for the residual
stresses in the joint is that their levels are maximized
to the local yield strength in the joint. The outcome
of the residual stresses transverse to the joint is
illustrated in fig. 4.9.

28 This handbook contains general suggestions and information without any expressed or implied warranty of any kind. SSAB hereby expressly disclaims
all liability of any kind, including any damages, in connection with the use of the information and for their suitability for individual applications.
It is the responsibility of the user of this brochure to adapt the recommendations contained therein to the requirements of individual applications.