Page 128 - SSAB Welding Handbook Edition 2

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22.0 Hardening mechanisms in the heat-affected zone and the unaffected parent metal Welding handbook

22.2 Grain refinement
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The size of the individual grains in the parent
metal affects the mechanical properties in the
steel. A decreased grain size leads to higher yield
strength, tensile strength, and increased toughness
in the parent metal. The Hardox and Strenx steels
are developed to attain very small grains in the
Fig. 22.3: A dislocation in a cubic structure. The dislocation unaffected parent metal.
represents an extra plane of atoms marked with a .
The strength of the parent metal is enhanced by
One example of their presence is illustrated in the smaller grain sizes because it counteracts dislocation
following example, which shows an atomic structure movement. This change in strength is caused by
within a grain. In order to show a dislocation dislocations that move in favorable directions within a
schematically, this example illustrates a cubic atomic grain. When dislocations reach a grain boundary, their
structure. Each dot represents the center part of the movement is usually obstructed since it is very likely
atom, see fig. 22.3. that the favourable directions for further movement
in the next grain are not the same. As a result, there is
When external stresses are applied to the structure, a pile-up of dislocations at the grain boundary. If the
which are below its yield strength, the dislocations will applied stress is sufficiently high, the dislocation
not move. If the stresses are increased to the parent movement can then be transferred to the next grain
metal’s yield strength or above, the dislocations start but, very likely, in another direction, see fig. 22.5.
to move and a permanent plastic deformation of the
steel structure is encountered, see fig. 22.4. Additionally, larger grains have a higher amount of
dislocations than smaller grains. Fewer dislocations
If the stresses on the structure are high enough to at a grain boundary require higher external stress
cause distortions above what is feasible for the parent levels to cause dislocation movement to the next
metal, a crack is initiated that can propagate to a grain. Consequently, decreased grain size increases
fracture. If this kind of fracture appears, it is of a the strength in the parent metal.
ductile type and is oriented through the grains.
A more pronounced plastic deformation over a
larger section of the steel structure is encountered
when dislocation movements are transferred from
grain to grain.

: Dislocation
: External stresses
Fig. 22.4: Dislocation movement in a atomic structure.

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