Welding before hot-dip galvanising
Aandachtspunten bij lassen
Voor een goede corrosiewering is het noodzakelijk dat alle laswerkzaamheden zoveel mogelijk zijn uigevoerd voordat het materiaal thermisch verzinkt wordt. Het lassen van niet verzinkt staal is eenvoudiger omdat het minder voorbereiding vergt en achteraf geen herstelwerkzaamheden vereist zijn. Om een optimaal verzinkresultaat te behalen dient bij het lassen op de volgende aandachtspunten gelet te worden:
- netheid van de omgeving van de lasnaden;
- samenstelling van de laselektrode of lasdraad.;
- voorkomen van roestwatervlekken;
- minimale lasspanningen.
Neatness of the areas around the welds
The neatness of the areas around the welds is one of the factors that determine the quality and the appearance of the zinc coating on and near the welds. The use of welding sprays should be avoided as much as possible. If they are used nevertheless, they should not contain any silicates or grease and they must be applied as thinly as possible. The chemical pre-treatment at the galvanisation plant does not remove excess traces of welding spray. As a result, the adequate interaction between the steel and the zinc is prevented locally, leading to black, non-galvanised, spots.
Traces of welding spray are virtually undetectable for the galvanisation operative and they should therefore be removed immediately after welding. Welding slag on and near the weld is not removed by the chemical pre-treatment either and it should therefore be removed in advance also.
Composition of the welding electrode or welding wire
It is known that the Silicon (Si) component in steel affects the thickness and the appearance of the zinc coating (see also Galvanisation-Technical Information-Effect of steel composition on zinc coating).
If the chemical composition of the welding electrode or the welding wire is very different from the composition of the steel to be welded, there may not only be clear visual differences after galvanisation, but also differences in the thickness of the zinc coating in the vicinity of the weld.
Some welding electrodes contain almost 1% Si, and this may result in very thick, dull-grey and sometimes poorly bonded zinc coatings on the welds. The term 'swollen welds' is used to refer to this problem. It can be prevented by using welding electrodes or welding wires containing no more than 0.7% Si.
Prevention of rust stains
Welding large steel surfaces to one another should be avoided as much as possible because this may result in large overlaps and enclosed spaces into which the liquid zinc cannot penetrate as it should.
In most cases, the liquid zinc will cover the joints but small joints and pores may still not be closed off adequately, resulting in unsightly rust stains and stripes on and around these locations at a later stage. It should be pointed out that these rust stains do not affect the quality or the endurance of the corrosion-resistant zinc coating.
Minimal welding stresses
During hot-dip galvanisation, materials heat up and cool down, and this may result in welding stresses that lead to the warping of steel structures (see also: Preventing warping). During welding, large amounts of heat are applied locally to the steel in a concentrated way. The rise in temperature, and the subsequent cooling, result in shrinkage stresses. The number of welds in the structural member determines the extent of the negative impact of the applied stress.
Structural measures are required to minimise the stresses resulting from the welding work. Weld numbers and weld size must be limited. Where possible, the welds should be located in the axis of gravity of the structural member. When this is not possible, the welds should be applied symmetrically at equal distances from the axis of gravity of the structural member.
With careful weld sequencing, it is usually possible to distribute welding stresses equally across the surface of the structural member. As a result, the steel structure can be more flexible in the zinc bath and absorb inherent stresses* completely without warping. Welds that provide a structure with stiffness should preferably be applied last.
* Every structure has inherent stresses, for example in the form of rolling stresses, weld stresses and alignment stresses.