SS400 Base Steel
SS400 is a general structural steel defined in JIS G3101, the Japanese standard for rolled steels for general structure. The designation follows the JIS convention in which the number indicates minimum tensile strength rather than yield strength, so SS400 carries a specified tensile range of 400 to 510 MPa. For thickness up to 16 mm the minimum yield strength is 245 MPa. The grade is fully killed or rimmed according to the order, is readily weldable with common electrodes, and is the usual economic choice for frames, supports and lattice work where a higher strength grade such as S355 or Q355 is not needed. H beam sections rolled from SS400 are supplied in a range of depths and masses, with the flange and web proportions selected so that the section resists bending about the strong axis efficiently.
Coating Requirements for Galvanized H Beams
Continuous hot dip galvanizing after fabrication is specified either to ASTM A123/A123M or to EN ISO 1461, both of which set minimum zinc coating values according to the thickness of the steel being coated. Because an H beam has a web and flanges of different thickness, the controlling value is normally taken from the thinnest element, although in practice both the web and the flange of a heavy beam fall in the same band.
| Steel thickness band | ASTM A123/A123M minimum average coating mass | Equivalent nominal coating thickness | EN ISO 1461 requirement |
|---|---|---|---|
| 6.4 mm and above | 610 g/m2 | about 85 micrometres | 85 micrometres average, 70 micrometres local minimum |
| 1.6 mm up to 6.4 mm | 505 g/m2 | about 70 micrometres | 70 micrometres average, 55 micrometres local minimum (3 to 6 mm band) |
| Below 1.6 mm | 375 g/m2 | about 52 micrometres | 45 micrometres average, 35 micrometres local minimum |
A galvanized H beam with an 8 mm web and 13 mm flanges therefore falls into the heaviest band and must reach a minimum average coating mass of 610 g/m2, which is equivalent to about 85 micrometres of zinc, matching the average thickness required by EN ISO 1461 for steel 6 mm and thicker. Quoting a coating in micrometres only, without stating whether the figure is an average or an individual minimum reading, is a common source of dispute on delivery, so the order should state the standard, the thickness band and the sampling rule.
Why Galvanizing Works on Structural Sections
The zinc layer protects steel in two ways. It acts as a physical barrier that keeps moisture and chlorides away from the surface, and it provides sacrificial protection at cut edges, drill holes and minor coating damage, where the surrounding zinc corrodes preferentially and protects the exposed steel. This combination is why hot dip galvanizing performs well in atmospheric corrosivity categories C3 and C4 to ISO 9223, and why it is preferred over paint in humid, coastal and industrial environments where access for repainting is difficult. Coating life is governed by the local deposition rate of the environment and by the coating mass applied, so specifying the correct thickness band is the single most effective way to extend time to first maintenance.
Fabrication Rules Before and After Dipping
All cutting, drilling, bending and welding should be completed before galvanizing, because zinc cannot be applied to welded areas afterwards without removing the coating locally. The designer must provide vent and drain holes so that molten zinc can enter and leave every enclosed volume and no air pocket is trapped; the openings also allow the section to drain during cooling. Welds should be continuous and free of slag and porosity, as rough weld surfaces and weld spatter produce local thin spots. Distortion is controlled by keeping dip time to the minimum needed and by avoiding very slender unsupported assemblies. Any damage or uncoated area identified after inspection is repaired with zinc rich coating applied in accordance with ASTM A780/A780M, which sets out the surface preparation, coating composition and thickness for the repair. One metallurgical point worth noting on the base steel is that silicon content influences the appearance and thickness of the coating: steel with silicon in the range associated with the Sandelin effect produces a thicker, darker and sometimes less adherent coating, so the silicon level is normally agreed with the galvanizer when SS400 sections are ordered for dipping.
Applications
Galvanized SS400 H beams are used for greenhouse and shade structures, walkway and platform framing, transmission and lighting supports, crane rails and gantry frames, water treatment equipment supports, agricultural sheds, bus shelters and other outdoor frames where a maintenance free service life of several decades is required. The section is also used in warm humid interiors such as laundries and food processing buildings where condensation would attack a painted finish.
Frequently Asked Questions
Q: Does the 400 in SS400 mean yield strength?
A: No. It indicates a minimum tensile strength of 400 N/mm2. The minimum yield strength is 245 MPa for thickness up to 16 mm.
Q: How thick is the zinc coating on a galvanized H beam?
A: For steel 6 mm and thicker, both ASTM A123/A123M and EN ISO 1461 call for an average coating of about 85 micrometres, equivalent to a coating mass of 610 g/m2, with a local minimum of 70 micrometres.
Q: Can galvanized beams be welded on site?
A: Welding after galvanizing destroys the coating locally and produces zinc fume, so joints should be shop welded and the weld zone repaired with zinc rich coating. Where site welding is unavoidable, the affected area is stripped, welded, cleaned and repaired.
Q: Why are vent and drain holes needed?
A: Molten zinc must be able to flow into and out of every enclosed space during dipping. Without venting, trapped air prevents coverage and trapped zinc creates a hazard when the part is withdrawn.
Q: What causes a very thick, dark coating on some beams?
A: Silicon and phosphorus content in the steel, in the range associated with the Sandelin effect, accelerates the zinc iron reaction and produces a thicker, darker coating. The silicon level should therefore be agreed with the galvanizer when the steel is ordered.
Q: Can holes be drilled after galvanizing?
A: They can, but the cut faces lose their coating and must be repaired with zinc rich coating to restore corrosion protection at the edge.