Bending, welding, galvanising -
processes applied to steel sheet
The route from a steel sheet to a finished product usually runs through four groups of operations: cutting, cold forming, joining and corrosion protection. Each of them places different demands on the material, and some of these demands contradict each other. Easily formable steel has lower strength. Steel with a low carbon equivalent is easier to weld, but its composition is often constrained by other requirements. A silicon content that benefits mechanical properties can double the coating thickness in hot-dip galvanising and change its appearance.
Selecting material for a process is therefore a matter of deciding which of these requirements are hard limits and which can be worked around by changing the order of operations.
Bending and cold forming
In bending, the outer layer of the material is stretched and the inner layer compressed. The smaller the bend radius in relation to thickness, the greater the strain in the outer layer and the higher the risk of cracking. Manufacturers state minimum radii as multiples of thickness; for sheet with a zinc-magnesium coating, for example, the recommendation is at least twice the material thickness.
In workshop practice three further phenomena matter.
Bending direction relative to the rolling direction. Rolled material has a structure of elongated grains, which makes it less able to deform in one of the directions. Placing the bend line across the rolling direction reduces the risk of cracking on the outer radius.
Springback. Once the pressure is released, the material partly returns to its original shape. The higher the yield strength, the greater the springback at the same radius and thickness. Switching to a higher-strength grade without correcting the tooling results in a bend angle deviation.
Strain ageing. This is the least obvious phenomenon and the one that most often comes as a surprise. Carbon and nitrogen atoms in solid solution in iron migrate over time to dislocations and pin them, which restores the upper yield point. The result is Lüders bands, visible stretcher strain marks on the surface of the pressing. Nitrogen plays a larger role here than carbon.
Material is protected against this by skin passing or tension levelling. A strain of at least 0.5% removes the upper yield point, and an elongation in the range of 1.25-2.0% gives about six months of resistance to Lüders bands. This period shortens considerably when the material is processed at temperatures above 38 degrees Celsius, at which hardness can rise by about 10 points on the Rockwell B scale within two weeks. Hence the recommendation to use material intended for pressing within about six months of delivery. The phenomenon is eliminated completely by ultra-low carbon steels and limited by additions of aluminium, titanium, niobium or boron, which bind carbon and nitrogen.
Arc welding and resistance welding
The weldability of low-carbon steels is assessed by the carbon equivalent, which describes the combined effect of carbon and alloying elements on hardenability. EN 10025-2 specifies its maximum values for individual grades, and the actual value for a batch is given in the inspection certificate 3.1 to EN 10204.
The equivalent alone is not enough to plan the welding procedure, however. EN 1011-2 makes the preheating temperature dependent on four parameters together: carbon equivalent, combined joint thickness, diffusible hydrogen content of the weld metal and heat input. Switching to a low-hydrogen filler material can lower the required preheating temperature or eliminate it completely for the same parent material.
Thin sheet imposes additional requirements. Too much heat input leads to burn-through and distortion, and welding distortion in large-area components can be harder to control than the quality of the weld itself. Hence the advantage of methods with controlled heat input and of resistance welding in series production.
Welding galvanised components
Welding material that is already zinc-coated is a separate issue. Every weld made on a galvanised surface destroys the coating at the weld and damages it in the adjacent zone. The recommendation of industry institutes is unambiguous: the coating should be removed over a distance of about 25 to 100 mm on both sides of the planned weld, most effectively by grinding, and after welding the protection should be restored with a zinc-rich coating.
The restored coating, however, lacks the metallurgical bond with the substrate that hot-dip galvanising provides, so its durability is lower. This is the main argument for welding to come before galvanising, not the other way round.
Safety requirements come on top of this. Zinc vaporises at arc temperature and oxidises in air to zinc oxide, forming fume. Exposure to this fume causes metal fume fever, a condition with flu-like symptoms that usually appear a few hours after exposure and subside within one or two days. The exposure limits adopted in the United States are 5 mg/m3 as an eight-hour time-weighted average and 10 mg/m3 as a short-term value. Practical recommendations include local exhaust ventilation at the arc, general ventilation, keeping the head out of the fume plume and, where technical measures are insufficient, respiratory protection.
Hot-dip galvanising
The process consists of immersing the finished component in a bath of molten zinc at a temperature of around 443 degrees Celsius. The bath contains more than 98% pure zinc, with additions not exceeding 2% that improve fluidity and the appearance of the coating.
Requirements for coating thickness are set by EN ISO 1461 according to the thickness of the steel component.
| Steel thickness | Min. local thickness | Min. mean thickness |
|---|---|---|
| over 6 mm | 70 µm (505 g/m2) | 85 µm (610 g/m2) |
| over 3 up to 6 mm | 55 µm (395 g/m2) | 70 µm (505 g/m2) |
| from 1.5 to under 3 mm | 45 µm (325 g/m2) | 55 µm (395 g/m2) |
| under 1.5 mm | 35 µm (250 g/m2) | 45 µm (325 g/m2) |
The bath itself does not change the mechanical properties of the steel: tensile strength, yield strength, bending properties and impact toughness remain unchanged. It can, however, cause distortion of asymmetric and thin-walled components, which has to be taken into account at the design stage.
The standard also sets design requirements. The component must have a sufficient number of holes of adequate size to allow safe venting and zinc drainage. Unvented closed spaces must not be galvanised, as this creates a risk of explosion. Industry guidelines recommend holes of at least 12 mm diameter, placed diagonally opposite, as close to the component wall as possible, with a minimum of two holes at each closed end. Details of design solutions are given in EN ISO 14713-2.
Steel composition and coating appearance
The thickness and appearance of the coating depend not only on the process but above all on the composition of the steel, especially its silicon and phosphorus content. This phenomenon, known as the Sandelin effect, is a frequent source of misunderstanding between the buyer and the galvaniser.
| Silicon content | Effect |
|---|---|
| below 0.03-0.04%, aluminium-killed steel | thinner coatings, around 50-65 µm, bright and shiny, dulling over time |
| about 0.04-0.06%, peak reactivity range | thick, matt grey coatings with poorer aesthetics |
| about 0.15-0.25%, silicon-killed steel | thick coatings, around 120-300 µm, matt grey but predictable |
Phosphorus acts in a similar way: a content above 0.02% can produce an excessively thick, brittle coating prone to flaking. Industry guidelines indicate as safe ranges a silicon content below 0.04% or within 0.15-0.22%, avoiding the intermediate range.
The practical conclusion for the buyer is important: if the product is to be hot-dip galvanised and its appearance matters, the chemical composition of the steel has to be agreed before the material is purchased, not before it is dipped in the bath. A matt grey, thick coating on steel with an unfortunate silicon content is not a workmanship defect but a consequence of the material.
Painting and duplex systems
Surface preparation before painting is described in ISO 8501-1, which defines cleanliness grades: Sa 1 removes loosely adhering contamination, Sa 2 most of the mill scale and rust, Sa 2.5 leaves only slight staining in the form of spots or stripes, and Sa 3 gives a uniform metallic appearance. Hand and power tool cleaning are designated St 2 and St 3.
A duplex system, meaning a paint coating applied over zinc, requires separate preparation of the galvanised surface. Recognised methods include a zinc phosphate solution with added copper salts, etch primers for weathered coatings, sweep blasting at a pressure not exceeding about 2.7 bar, and natural weathering of the coating for at least six months followed by mechanical cleaning. In all cases the surface must first be cleaned of oil, grease and dirt, and the paint must be intended for use on galvanised steel.
Order of operations
All of the above relationships lead to one guiding rule: all mechanical processing and welding should precede galvanising.
Cutting, bending, drilling and welding a galvanised component destroys the coating exactly where it is most needed, at the welds and edges. Restoring it with paint gives weaker protection than a metallurgical coating. Galvanising carried out last covers the entire surface of the finished component, including edges, holes and welds.
The exception is structures made of continuously coated sheet, where the coating is applied to the strip before forming. There the order is reversed by definition, and protection of cut edges relies on the cathodic action of zinc, which is effective mainly in thin material.
When planning production, it is worth settling this sequence together with material selection, as the requirements of the individual processes may point to different grades.
Available steel grades, coating types and formats of coils, slit strip and sheets are presented in the HEN-STOL steel sheet offer, with separate pages for galvanized steel sheet, cold-rolled steel sheet and hot-rolled steel sheet.
See how we can help
HEN-STOL helps select material that takes forming, welding and corrosion protection requirements into account together. Send us the specification and a description of the process, and we will check availability and propose the most favourable option.
Frequently asked questions
What is the minimum bend radius for steel sheet?
It is given as a multiple of the material thickness and depends on the grade and on the bending direction relative to the rolling direction. For sheet with a zinc-magnesium coating, the manufacturer recommends a radius of at least twice the thickness. Data for a specific grade can be found in the material data sheet.
Why do marks appear on a pressed part?
These are Lüders bands, an effect of strain ageing. Over time carbon and nitrogen migrate to dislocations and pin them, restoring the upper yield point. This is prevented by skin passing or tension levelling and by using the material within a reasonable time after delivery.
Can an already galvanised component be welded?
Technically yes, but the coating must be removed over a distance of 25 to 100 mm on both sides of the weld and restored after welding with a zinc-rich coating. The restored protection is less durable than a metallurgical coating, which is why galvanising after welding is preferred.
What zinc coating thickness is required?
Under EN ISO 1461 it depends on the component thickness: for steel over 6 mm the mean thickness is at least 85 µm, for 3-6 mm 70 µm, for 1.5-3 mm 55 µm, and below 1.5 mm 45 µm.
Why is the coating sometimes grey and matt after galvanising?
Most often because of the silicon content of the steel. In the peak reactivity range the coating grows excessively and loses its sheen. This is a consequence of the chemical composition of the material, not a defect in the coating work.
In what order should processing and galvanising be carried out?
First cutting, bending, drilling and welding, and galvanising last. This requires vent and drainage holes to be designed into the final geometry of the component.
Sources
- EN ISO 1461, Hot dip galvanized coatings on fabricated iron and steel articles
- EN 1011-2, Welding - Recommendations for welding of metallic materials, arc welding of ferritic steels
- ISO 8501-1, Preparation of steel substrates before application of paints
- GalvInfo Center, GalvInfoNote 2.8.1, Strain Ageing in Galvanized Low Carbon Steel Sheet
- American Galvanizers Association, Silicon Content of Steel and Can You Weld Galvanized Steel
- Galvanizers Association, guidance on vent and drainage holes and on painting galvanised steel
- American Welding Society, Safety and Health Fact Sheet No. 25, Metal Fume Fever