Steel fabrication is an interesting process. There are two ways in which steel fabrication takes place. First is raw material approach which is also called integrated route and the other one is electric arc furnace(EAF) method of steel fabrication. In first method materials are heated up ,melted down and mixed into steels. In other method recycled steel is put into a furnace and melted down. Once mixed with some other materials, steel is the end product.
The efficiency of fabrication in steelwork dictates the success of any project involving steel-intensive construction. Steel passes through various operations during the course of its fabrication. The sequence of activities needed for fabricating steel is given below
· Surface cleaning
· Cutting and machining
· Punching and drilling
· Straightening, Bending and Rolling
· Fitting and Reaming
· Fastening Methods
· Surface Treatment
Structural sections from the rolling mills may require surface cleaning to remove mill scale prior to fabrication and painting. Blast cleaning is the accepted way of carrying out surface preparation in a well-run fabrication. Flame cleaning is another method of surface cleaning. The surface is cleaned using an oxy-acetylene torch which works on the principle of differential thermal expansion between steel and mill scale. In Dubai places like Horiya does great job in cleaning the steel surface at very great rates.
Cutting to length is always the first process to be carried out, and this is done by any of the following methods.
· Shearing and cropping
· Flame Cutting or Burning
· Arc Plasma Cutting
Shearing and cropping: By shearing and cropping sections can be cut to length by using hydraulic shears. By specialist plate shears long plates can be shaped and cut to length.
Flame cutting and burning: The steel is heated locally by a pressurised mixture of oxygen and a combustible gas such as propane, which passes through a ring of small holes in a cutting nozzle .The desired cuts are obtained quickly by this process. Flame cutting are treated specially in the design of steel .
Arc Plasma Cutting: The cutting energy is produced electrically by heating a gas in an electric arc produced between a tungsten electrode and the workpiece . The cut produced by plasma jet is very clean and its quality can be improved by using a water injection arc plasma torch.
Steel fabrication can be done by a range of machines which can form holes for connections in steelwork. The traditional drilling machine is the radial drill, a manually operated machine, which drills individual holes in steelwork. Punching creates distortion and material strain hardening around the holes, which increase with material thickness.
Straightening, Bending and Rolling
During fabrication the material should be straightened, either rolls or gag presses are used to straighten structural shapes Gag press is generally used for straightening beams, channels, angles, and heavy bars. Long plates, which are cambered out of alignment longitudinally, are frequently straightened by rollers. When heat is applied to a small area of steel, the larger unheated portion of the surrounding material prevents expansion. Upon cooling, the subsequent shrinkage produces a shortening of the member, thus pulling it back into alignment.
The strength of the entire structure depends upon the proper use of fastening methods. There are three methods of fastening namely bolting, riveting and welding. Welding is the most common method of steel fabrication.
The term finish or mill is used on detail drawings to describe any operation that requires steel to be finished to a smooth even surface by milling, sawing or other machines.
Steel is protected against corrosion by applying metal or paint coating.
The corrosion protection afforded by metallic coating largely depends upon the surface preparation, the choice of coating and its thickness. Metal coating to steel surfaces are hot-dip galvanizing, metal spraying, and electroplating. Electroplating is generally used for fittings and other small items. Galvanizing is the most common method of applying a metal coating to steel.
Painting is the principal method of protecting steelwork from corrosion. Paints are usually applied one coat on top of another, each coat having a specific function or use.