Page 91 - SSAB Welding Handbook Edition 2

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Welding handbook 14.0 Avoidance of discontinuities in the joint

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Fig. 14.18: Internal porosity in the weld metal. Fig. 14.19: Surface breaking porosity.
(Reproduced by permission, TWI Ltd.) (Reproduced by permission, TWI Ltd.)

14.7 Porosity primed surface. For the best results, remove the
primer from the joint prior to welding. However,
Description of porosity the low levels of pores that can be caused by the
primer are accepted in most types of regulations.
Pores can occur in the weld metal and they are formed
during solidification of the molten weld metal. Pores 4 Paints other than the shop primer may also
can occur in different parts of the weld metal, which include substances that cause unacceptable levels
means that they can be embedded in the weld metal, of porosity. The recommendation is, therefore, to
shown in fig. 14.18, or surface breaking, see fig. remove these kinds of paints from the joint prior
14.19. They can appear in various geometrical forms to welding.
and concentrations. The principal gases that are 4 A lowered welding speed may counteract pore
found in embedded pores are nitrogen, oxygen and formation. This measure increases the size of the
hydrogen. weld pool allowing more time for the pores to
bubble up to the surface of the weld pool and
Pores are commonly caused by the gases that are disappear from the joint.
introduced during welding. These gases are 4 For shielding gas related welding methods:
introduced into the liquid weld pool since the molten - Regularly check that the gas shielding equipment
weld metal dissolves them, however, when the weld is fit for purpose to avoid the surrounding air
metal solidifies, their solubility decreases. As a result, from getting into the shielding gas.
pores are formed in the weld metal. Some remain - Use the proper amount of shielding gas. Flow
completely trapped in the weld metal while others rates that are too low or too high can cause
escape from the joint through the weld metal that is pores. As a rule of thumb, the shielding gas,
still liquefied. Surface breaking pores appear when measured in l/min, should be approximately
pores become trapped just as they reach the surface the same as the inner diameter of the nozzle,
of the weld metal.
measured in mm.
Causes of porosity and remedies to prevent this - Keep the nozzle of the torch clean from spatter,
discontinuity as it could otherwise disturb the gas flow.
4 Keep the joint clean from contaminants such as - Do not weld in a draft. This can cause the
water, snow and rust. shielding gas to blow away casing an
4 Keep the consumables dry and store them in insufficient protection of the melt.
accordance with their manufacturer. - It is beneficial to weld with a short arc length,
4 Substances in the shop primer used on the Hardox especially during the start sequence of a weld,
and Strenx plates may cause small fractions of to improve the gas shield of the weld metal.
pores when welding is performed directly on the
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. 91
It is the responsibility of the user of this brochure to adapt the recommendations contained therein to the requirements of individual applications.