Page 129 - SSAB Welding Handbook Edition 2

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

Another type of fracture is the inter-crystalline frac-
ture, defined as a crack that propagates through the
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= Dislocation movement grain boundaries, which is illustrated in fig. 22.7.
= Grain boundaries The grain refinement effect is, however, not as
pronounced for this kind of fracture. The main frac-
ture mechanism for sound steel structures in the Har-
dox and Strenx grades is a trans-crystalline fracture.
However, if discontinuities, such as hydrogen cracks, or
stress corrosion cracking are present, cracks can form
Fig. 22.5: Schematic illustration of dislocation movement in a
structure, which is illustrated by the dashed red line. The gray as inter-crystalline or as a combination of trans- and
dots show the orientation of the iron atoms in each grain. inter-crystalline cracks.

If the stress level in the parent metal is further As previously mentioned, the welding performance
increased, at or above its yield strength, the stress causes grain growth in the HAZ. However, alloy
levels finally reach such a high level that a separation elements in the parent metal, including niobium,
between the atoms in an individual grain can take titanium, vanadium, and aluminum, can react with
place. A crack can then form, which can develop and carbon and nitrogen in the steel and form precipitates
propagate through the grains into a trans-crystalline that can lower the grain growth.
fracture, according to fig. 22.6.
The grain growth supports reduced mechanical
The ability to withstand crack formation also means properties in the HAZ compared to the unaffected
increased toughness in the structure. The mechanism parent metal. However, the Strenx and the
behind this is similar to the strength mechanism. Hardox grades are characterized as high strength
Below a certain temperature, which depends of the steels that still can attain favorable mechanical
material, the dislocation pile-up will form a crack properties throughout the complete HAZ, even if
instead of transferring the plastic deformation to the the minimum toughness value in the HAZ is below
next grain. This will lead to trans-crystalline crack that of the unaffected parent metal.
propagation. The fine grained structure that has a
lower number of dislocations at the grain boundaries
than a larger grained structure, leads to a higher
resistance to this kind of crack formation.

= Fracture location = Fracture location
= Grain boundaries = Direction of fracture

Fig. 22.6: A trans-crystalline fracture. Fig. 22.7: A inter-crystalline fracture.

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