Common material qualities

The main advantage to use steel mullions and transoms for curtain walls, if compared to aluminum, is the higher strength of the material, which allows building façades with slim cross-sections and high spans. If architects choose steel for technical or design-specific reasons, the mullions and transoms are usually designed in common structural steel grades. In the case of prestige objects, clients often decide to upgrade steel to the nobler stainless steel, or, in case of projects near the sea, even to duplex steel. Duplex stainless steels are also used when a stainless steel finish is desired, but the planned strength values cannot be achieved by the common austenitic stainless steels (e.g. 304/304L or 316/316L) with the same profile geometry.

Following are some common grades used for facades with sharp-edged steel profiles.

Mild steel

  • S235
  • S275
  • S355

Stainless steel

  • 1.4301 (AISI 304)
  • 1.4307 (AISI 304L)
  • 1.4401 (AISI 316)
  • 1.4404 (AISI 316L)
  • 1.4571 (AISI 316Ti)

Duplex steel

  • 1.4162 (LDX 2101)
  • 1.4362 (AISI 2304)
  • 1.4462 (AISI 2205)
  • 1.4410 (AISI 2507)

Surfaces:

The sharp corners and filigree weld seams of the laser-welded façade profiles are decisive for the clear light-shadow separation and, together with the shape of the profile, form the aesthetics of a steel façade. However, this is not the only feature for a valuable aesthetic appearance. The surface finish of the material also contributes significantly to the quality of the curtain wall and hence to the overall building.

Carbon Steel Factory Finish Sa3

Montanstahl manufactures laser-welded profiles for architectural projects from raw materials with aimproved surface finish. The sheet metal strips that make up the profiles are laser cut from hot rolled sheets, annealed and finally pickled or sandblasted.  The sharp-edged, laser-welded steel profiles from Montanstahl are sand blasted after the welding and straightening operations, which leads to a clean and homogeneous surface appearance. This standard surface finish of the material is defined by the surface class Sa 3 according to the standard EN (ISO) 8501-1. The blasted surface Sa 3 has a uniform gray-white metallic finish. It is slightly roughened by the blasting grains. This leads to a good adhesion for further processing by coating, painting, hot-dip galvanizing and so on. Oil, dirt, corrosion elements, grease, visible scale, rust or oxides, paint or other foreign bodies are removed from the surface. The surface is already suitable for the application of the corrosion protection.
Please note that the raw material used is generally suitable for hot-dip galvanizing in accordance with EN 10025-2, but this must be specified explicitly when ordering.

Stainless Steel Factory Finish 1D

Similar to profiles made of carbon steel, Montanstahl manufactures façade profiles exclusively from laser-cut sheet metal strips that come from hot-rolled, annealed and pickled stainless steel sheets. The surface is blasted after the welding and straightening process and finally passivated. The stainless steel blasting sand confers the profiles a slight surface roughness. The surface is free of scale, tarnish or oxidation from the welding process. This ensures a homogeneous appearance over the entire length of the bar. According to the EN 10088-2 standard, this surface is defined as 1D finish. This factory finish can already be used for mullion-transom constructions. This neatly blasted surface gives the facade a matte elegant appearance.

Surface Finishing

The surfaces of stainless steel or duplex steel are usually upgraded afterwards. High-quality refinements are very often found in stainless steel sheets or panels, e.g. it can be used for cladding entire building facades or for decorative interiors. Panels made of stainless steel sheets can be refined relatively easily, e.g. by electrochemical coloring, bluing, electro polishing, mechanical grinding or by rolling patterns and structures on to the surface. On stainless steel bars, which is the case with mullions and transoms, the finishing is usually limited to mechanical grinding and polishing. The risk of damage to the surface during installation on the construction site is very high and a subsequent surface repair would be associated with considerable effort or even impossible.

The following are the most common types of surface finishing, which are standardized according to EN 10088-2.

* Scroll the table horizontally < >

Execution Description Grain Typical Ra-values
1G Ground 120 2.5-2.0 µ
1J Brushed 180 1.2-1.0 µ
1J Dull-polished 240 0.6 µ
1K Satin-polished 320 0.5 µ max.
1P Bright-polished 600 0.1 µ
1P Mirror-polished 800 0.05 µ
Laser-Welded-ss-polished

For stainless steel profiles, we recommend defining the desired surface not only on the basis of roughness values, but also, if possible, by means of a representative sample, as the surfaces can have a different appearance depending on the polishing company, even if the defined roughness values are the same.