Page 61 - SSAB Welding Handbook Edition 2

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Welding handbook 11.0 Cutting of Weldox and Hardox

or abrasive water jet cutting in very hard materials. These factors interact with each other, by keep-
In the first phase, small cracks form in the center of ing their levels suitably low the risk for hydrogen
©SSAB
the plate that runs horizontally within the HAZ. cracks is minimized.
They form just behind the cut edge and they typically
appear within a couple of hours after cutting is
finalized. The cracks are not visible to the naked The influence from alloy
eye at this stage. elements in the steel
The influence of alloy elements is the same during
In the second phase, which typically occurs after a thermal cutting as for welding. This means that high-
couple of days, the cracks propagate to the cut edge er carbon equivalent values in the steel correspond to
surface and build longer horizontal cracks, typically increased sensitivity to hydrogen cracks. In general
up to 5-10 cm. terms, the carbon equivalent rises with increased

thickness, hardness and strength of the Hardox and
A third phase may take place, which normally Strenx grades.
occurs after a week or more, where the crack
propagation continues, changes direction and Consequently:
spreads up to the plate surface. Although vertical 4 There are more restrictions regarding cutting of
cracks are unusual, the risk for this emergence rises the Hardox and Strenx grades with increasing
with increasing hardness and thickness of the steel. strengths and hardness values.
Hydrogen cracks due to cutting are illustrated in 4 Larger plate thicknesses have more restrictions
fig. 11.3.
for minimizing the risk of hydrogen cracks during
cutting.
Small cracks behind the cut edge
Phase 1 Further information about the carbon equivalent is
found in chapter 4.

Tensile stresses due to cutting

The cutting operation causes a non-uniform heat
Phase 2
treatment of the cutting edge and its surroundings.
For instance, the peak temperatures in the plate
decrease as the distance from the cutting edge
increases. This non-uniform thermal heat treatment
causes the mechanical properties in the HAZ to
Phase 3
vary. Thermal cutting methods always induce tensile
stresses in and around the area to be cut. Thermal
cutting of Hardox and Strenx will result in a HAZ
that can be divided in two zones, one re-quenched
= HAZ
and one tempered zone.
Fig. 11.3: Formation of hydrogen cracks at, and around, the cut edge.
The outer part of the HAZ has been heated to above
900°C during the cutting procedure, at this
Three conditions must be present in the steel at the temperature the material has been transformed to
same time in order for hydrogen cracks to form. austenite. Directly after the cutting torch has passed,
They are: the heat is rapidly absorbed by the cold surroundings
4 A brittle microstructure which cool the material in this zone rapidly enough
4 A high level of tensile stresses to re-quench the material again. This re-quenched
4 Presence of hydrogen
zone is here called zone 1.
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all liability of any kind, including any damages, in connection with the use of the information and for their suitability for individual applications. 61
It is the responsibility of the user of this brochure to adapt the recommendations contained therein to the requirements of individual applications.