Abrasive blasting is the method generally used to prepare metals for coating, by propelling an abrasive medium forcibly against the metal surface. In shot blasting, the medium used is smooth steel shot.
The better the method used, the better the preparedness of the metal surface and therefore the effectiveness of the applied coating. There are three commonly used systems of standards denoting the condition of the metal surface; SSPC (managed by the USA Coating Society), NACE (managed by the British National Association of Corrosion Engineers), and Swedish Standards.
Since 1967, the Swedish system of classification (SIS 055900) has gained international recognition as a comprehensive set of standards for surface preparation, and has been adopted in its entirety in many countries.
There are four basic levels of surface cleanliness described by the Swedish system, ranging between SA1 (essentially the removal of loose material such as dirt, rust and scale) and SA3, the highest level.
For SA3, described as ‘White Metal’, visible mill scale, dirt, grease, rust and foreign particles are completely removed and the cleaned surface has a uniform metallic colour.
This highest standard of surface preparation is crucial to the success of any subsequent application of a corrosion protection system. SA3 ensures that the surface is not only clean, but also profiled so as to maximise the adhesion of coatings.
We have the most up-to-date welding equipment and our Jamestown welders are qualified for a wide range of Fillet Welds, Partial Penetration and Full Penetration Butt Welds in a large range of positions and a wide range of thicknesses.
Jamestown carries out Metal Active Gas (MAG) Welding, Submerged Arc Welding (SAW), Flux Cored Arc Welding (FCAW) and Drawn Arc Shear Stud Welding.
MAG welding is a versatile technique suitable for both thin sheet and thick section components. An arc is struck between the end of a wire electrode and the workpiece, melting both of them to form a weld pool. The wire serves as both heat source (via the arc at the wire tip) and filler metal for the joint. The wire is fed through a copper contact tube (contact tip) which conducts welding current into the wire. The weld pool is protected from the surrounding atmosphere by a shielding gas fed through a nozzle surrounding the wire. (Ref: TWI).
MAG is widely used in most industry sectors and accounts for more than 50% of all weld metal deposited. Compared to MMA, MAG has the advantage in terms of flexibility, deposition rates and suitability for mechanisation. However, it should be noted that while MAG is ideal for ‘squirting’ metal, a high degree of manipulative skill is demanded of the welder. (Ref: TWI)
Similar to MAG welding, SAW involves formation of an arc between a continuously-fed bare wire electrode and the workpiece. The process uses a flux to generate protective gases and slag, and to add alloying elements to the weld pool. A shielding gas is not required. Prior to welding, a thin layer of flux powder is placed on the workpiece surface. The arc moves along the joint line and as it does so, excess flux is recycled via a hopper. Remaining fused slag layers can be easily removed after welding. As the arc is completely covered by the flux layer, heat loss is extremely low. This produces a thermal efficiency as high as 60% (compared with 25% for manual metal arc). There is no visible arc light, welding is spatter-free and there is no need for fume extraction. (Ref: TWI)
SAW is ideally suited for longitudinal and circumferential butt and fillet welds. However, because of high fluidity of the weld pool, molten slag and loose flux layer, welding is generally carried out on butt joints in the flat position and fillet joints in both the flat and horizontal-vertical positions. Depending on material thickness, either single-pass, two-pass or multi-pass weld procedures can be carried out. There is virtually no restriction on the material thickness, provided a suitable joint preparation is adopted. (Ref: TWI)
Flux-cored arc welding (FCAW or FCA) is a semi-automatic or automatic arc welding process. FCAW requires a continuously-fed consumable tubular electrode containing a flux and a constant-voltage or, less commonly, a constant-current welding power supply. An externally supplied shielding gas is sometimes used, but often the flux itself is relied upon to generate the necessary protection from the atmosphere, producing both gaseous protection and liquid slag protecting the weld.
FCAW may be an “all-position” process with the right filler metals (the consumable electrode). No shielding gas needed with some wires making it suitable for outdoor welding and/or windy conditions. A high-deposition rate process (speed at which the filler metal is applied) in the PA and PB positions. The process is widely used in construction because of its high welding speed and portability.
Drawn Arc Shear Stud Welding is a process for welding Shear Studs to steel sections or plate. The stud is loaded into the stud gun chuck, and a ferrule (a disposable ceramic shield that contains the molten pool of metal) is placed over the end. The gun is placed against the work position. When the trigger is pressed, the dc power supply sends a signal that energizes the gun’s internal lift mechanism lifting the stud, drawing the pilot arc. The pilot arc establishes a path for the weld current which initiates after the pilot arc. Upon completion of the proper arcing time, which is proportional to the square area of the surface being melted, the lift mechanism is de-energized. This causes the stud to plunge into the molten pool of metal. A plunge dampener is often used on larger studs to decelerate the stud’s movement onto the molten pool, minimizing splash. As the stud and the base metal join, the metal begins to solidify and the weld is created. The gun is lifted, and the ferrule is discarded. The Drawn Arc technique uses flux embedded in the stud to cleanse the atmosphere during the weld. During arcing, the flux is vaporized and combines with the contaminating elements in the atmosphere to keep the weld zone clean.
Jamestown Steel operates to International Standard 3834-2: Comprehensive Quality Requirements, which defines quality requirements for steel fusion welding, and also covers issues such inspection, testing, and the calibration of measuring equipment, as well as EN1090-2, Execution Class 4, a Factory Production Control Quality system for the production of Structural Steel Components.
Trial erection, also sometimes called temporary erection, is a well-established method for ensuring the successful completion of a construction project, by first duplicating the process at the same or another site.
It can be argued that modern production methods, using automated fabrication procedures, render trial erection unnecessary in most cases. However, for structures requiring complex geometry, such as skewed bridges, a trial erection may be an important opportunity to make corrections at an early stage.
In locations such as infrastructure bridges in busy locations, and other circumstances when delays could be damaging and expensive, trial erection can be a vital method for ensuring that time is not wasted. In settings where access for remedial work would be difficult and expensive, an offsite trial erection should also be considered.
What Limits the Use of Trial Erection?
In the case of a very large structure, offsite trial erection may be precluded for some fabricators because of space constraints. Jamestown Steel’s large site, which is capable of handling very large temporary erections, is exceptional in this respect. Trial erection may also lengthen the period before the final stages of the project can commence, which may be problematic in some instances; however this, and the additional labour costs involved, may be more than offset by the saving on remedial work.
Clause 9-6-4 of British Standards EN 1090-2 gives examples of situations in which a trial erection should be considered.
Flattening, or levelling, in steel manufacturing is the process of removing unevenness and fluctuations in sheet metal, thus increasing dimensional precision.
Various processes applied to steel plate can cause shape distortion and internal stresses. Cutting, rolling, and applied heat can all make flattening desirable or essential.
Steel flattening was at one time a difficult and time-consuming process performed by hand, using hammers and heat. Modern technology has automated the process, reducing the time taken and improving accuracy. Powerful flattening machinery can achieve flatness with the great precision required by modern construction standards.