Slow cooling
Regardless of whether or not preheating of
the cut parts is employed, a slow cooling
rate will reduce the risk of cut edge crack-
ing. Slow cooling can be achieved if the parts
are stacked together while still warm from
the cutting process, and are covered with an
insulating blanket. Allow the parts to cool
slowly down to room temperature.
Post-heating
Heating of the parts immediately after
cutting is another method that can be used.
This will prolong the time at temperature
to allow the hydrogen to escape from
the plate and, to some extent, reduce the
residual stresses at the cut edge. The soaking
temperature should be the same as that given
in Table 2, and the soaking time should be at
least 5 minutes per mm of plate thickness.
Burner lances, electric heating mats or heat
treatment in a furnace can be used for post-
heating.
Diagram 2
Submerged cutting.
Reducing the risk of softening
The resistance of the steel to softening de-
pends on its chemistry, microstructure and the
way in which it has been processed.
The smaller the part that is thermally cut,
the greater the risk of the whole component
being softened. If the temperature of the steel
will be reduced, according to diagram 2.
Cutting method
When small parts are cut, the heat supplied
by the cutting torch and by preheating will be
accumulated in the workpiece. The smaller
the size of the cut part, the greater the risk of
softening. When oxy-fuel is used for cutting
30 mm or thicker plate, the rule of thumb is
that there is risk of loss of hardness of the
entire component if the distance between two
cuts is less than 200 mm.
The best way of eliminating the risk of
softening is to use cold cutting methods, such
as abrasive water jet cutting. If thermal cut-
ting must be performed, laser or plasma cut-
ting is preferable to oxy-fuel cutting.
This is because oxy-fuel cutting supplies
more heat and thus raises the temperature of
the workpiece.
Submerged cutting
An effective way of limiting and reducing
the extent of the soft zone is to water-cool
the plate and the cut surfaces during the cut-
ting operation. This can be done either by
submerging the plate in water or by spraying
water into the cut during cutting. Submerged
cutting can be done both in plasma cutting
and in oxy-fuel cutting.
Some advantages offered by
submerged cutting are listed below.
whole component
cutting
650
600
550
500
450
400
350
300
Surface hardness vs. tempering temperature
Tempering temperature, [�C ]
100 150 200 250 300 350 400 450
HARDOX 600
HARDOX 500
HARDOX 400
Hardness HBW
650
600
550
500
450
400
350
300
