Technical Info Sheet #3
During hot-dip galvanizing, composite structures can deform. By heating to 450°C, the stresses present in the object can lead to a play of forces that causes deformation. What are the causes of distortion and how do we prevent it?
CAUSES OF DISTORTION
Because steel heats up to 450°C when immersed in the zinc bath, it loses strength in the process. Upon cooling, the steel regains its original strength. However, during the heating process, which is also accompanied by expansion of the steel, there is a chance that the tension present in the object exceeds the reduced strength of the steel. As a result, the object will deform until the forces present cancel each other out. This phenomenon is clarified in the figure below.’
Thus, stresses are present in all composite steel structures. In general, they do not give rise to deformation. Self-strains present in the structure, for example, in the form of rolling and/or welding stresses, often keep each other in balance and therefore do not give rise to deformation. Sometimes the force is so great that welds or profiles crack. Factors that play a role in this interplay of forces in deformations include:
PREVENTION OF DISTORTION
Good welding sequence
It is good practice to take into account the possibility of deformation as early as when assembling the structure. A correct welding sequence minimizes the occurrence of stresses during assembly. Stresses resulting from welding joints are usually the cause of deformation of steel structures during galvanizing. It is therefore important to keep the stresses in a steel structure as low as possible. Even at the design stage, it is sometimes better to choose an object consisting of several separate parts and connect them with bolted connections. Where possible, choose profiles with a symmetrical cross section. Weld a fence crosswise from inside to outside.
Sheet steel
With sheet steel structures, take into account possible expansion of the structure during dipping in the zinc bath. Therefore, do not choose sheets that are too large and keep the sheet flat. Larger plate surfaces should be stiffened to prevent or limit the formation of dents and waves as much as possible. Again, symmetry is very important. For example, do not set a raised edge on one side while keeping the other side flat. Due to the settlement, the sheet cannot shrink evenly after expanding during dipping when cooling. The still flat side will then show folds after cooling the object after galvanizing.
Immersion in zinc bath
In order to prevent deformation due to expansion differences as much as possible, it is important for the galvanizing plant to ensure that immersion takes place as quickly as possible. Unfortunately, however, the immersion speed depends on what is possible. This is because, from a design point of view, all advice must be followed with regard to openings, countersink holes and hanging options. If there are not enough correctly positioned openings, there is a risk that the object will float at high immersion speeds. This leads to stagnation in production and a cost item due to the fact that if an object floats, the entire process has to be repeated. This is because floating causes the necessary pretreatment fluids to burn, preventing the zinc layer from forming. For this reason, the object will have to be dipped at a lower speed and thus the risk of deformation increases.
LARGE STEEL STRUCTURES
Due to large dimensions of composite, flat or bulky structures and the dimensions of the available zinc baths, it is not always possible to galvanize a construction part in one operation. In such cases, it is possible to galvanize the structure in several steps (double dip or tilt dip). In these cases, there is an increased risk of deformation due to uneven heating. Already during the design process it is better to opt for division or some other method of limiting the size of the object.
The double-dip galvanizing of long, slender rolled sections often used as columns and beams will not easily lead to such thermal deformation. Only in the case when another profile is welded on along its entire length, and possibly not symmetrically, can it lead to deformation. Try to avoid such welded-on sections as much as possible. Consult with your galvanizer in these cases.
Careful coordination
Client and/or design office, drafting office or construction shop should coordinate the method of construction of a complex workpiece with the galvanizing plant to minimize the possibility of deformation damage.
EN ISO 1461
Coatings applied by hot-dip galvanizing to iron and steel objects – Specifications and test methods.
EN ISO 14713 part 1
Zinc coatings – Guidelines and recommendations for the protection of iron and steel in structures against corrosion – Part 1: General design principles and corrosion resistance.
EN ISO 14713 part 2
Zinc coatings – Guidelines and recommendations for the protection of iron and steel in structures against corrosion – Part 2: Hot dip galvanizing
