The Southern African Institute of Welding (SAIW) conducted its first ISO 3834 certification process in September this year, in accordance with the International Institute of Welding and the European Federation for Welding certification system, awarding local manufacturer Stainless Fabricators with the first ISO 3834 certification in the country. The SAIW reports that end-users such as Sasol, Eskom, and Mittal Steel, have supported the implementation of the scheme since inception.

Stainless Fabricators MD Peter Viljoen says previous dealings with end-users such as Sasol entailed rigorous auditing, since the ISO 9000 system did not incorporate welding fabrication-specific issues. 'The emphasis of the ISO 3834 system is on quality management systems specifically in the welding systems, and covers all facets of welding.'

Viljoen adds that in the past, end-users were expected to cover the expenses incurred during duplication work, which resulted from a lack of a standard specification. He says the ISO 9000 system lacked the depth to deal with sophisticated projects, and often manufacturers were unable to deliver to end-user specification.

'With the implementation of the ISO 3834, there is a standard and norm for welding, and end-users are realising that there is really no need for duplication work, and the cost that implies. 'Welding is considered a special process because the final result may not be capable of being verified by routine testing.'

Viljoen says the quality of the weld has to be manufactured into the product, and not inspected; this means that welding normally requires continuous control and that specified procedures be followed. The ISO 3834 standard concerning quality requirements in welding has been specifically prepared to identify the controls and procedures required to produce welds of a quality level acceptable to the end- user of the product.

'It should be noted that it is not a quality system standard replacing ISO 9001: 2000, but it can form a useful tool when ISO 9001 is applied by manufacturers. However, ISO 3834 can be used independ- ently of ISO 9001: 2000.'

The main advantages for a fabricator choosing to implement the International Welding Fabricator Certification Scheme, through the ISO 3834 standard, includes the welding process-specific quality tool, which can stand alone, or be used with ISO 9001: 2000.

The welding process quality management tool is audited by welding industry experts so that true value is added for the fabricator's benefit. This avoids the system being relegated to a 'paper exercise'. Large end-users of fabricated equipment have already realised the potential value in this standard and are considering requesting fabricator compliance for future work.

The anticipated revised Occu-pational Health and Safety Act may include the certification of Pressure Vessel Fabricators and this ISO standard could be adopted to also align our industry with Europe's. Export opportunities are becoming more attractive and the alignment of our fabrication industry to Europe will create more business.

SAIW executive director Jim Guild says the larger end-users want to have confidence in the technical capabilities of their supplier organisations, and that the ISO standard will be invaluable in accomplishing this. He adds that, further, the potential changes to regulations in the Factory Act, which is likely to happen this year, makes it advisable for pressure vessel and boiler manufacturers to have their systems certified by an independent third party.

'Customers and end-users will want to be completely assured that the quality standards laid down by the Act have been adhered to,' says Guild.

He says that welding is a unique process in that the final result cannot be verified by testing only, but that the quality of the finished product is incorporated in the entire process thorough a continuously monitored control system, which follows specific procedures.

'These procedures are laid down in the ISO 3834 quality standard, and the basic reason for having a company certification scheme is to make sure that the manufacturers and fabricators are constantly and consistently working to the standards set by ISO 3834,' says Guild.

Although the need for such a standard was identified many years ago, Viljoen says it was not until recently, when the European Federation for Welding developed the standard, owing to competitiveness and unification with Europe, that the local industry responded to the need. Since January this year, the system has been available to members of the International Institute of Welding, says Viljoen. With the accreditation of Stainless Fabricators, the SAIW also received its international accreditation from the International Institute of Welding.

Guild says that with the the acquisition of an international accreditation, the large end-users will not be the only benefactors, but that the system will be a great help to exporters as the quality of product will now be recognised internationally.

This is a description of welding of rail joints using Thermit welding. In this process, the highly exothermic reaction between aluminium and iron oxides results in the production of molten steel which is poured into a mould around the gap to be welded. The superheated molten metal causes the rails to melt at the edges of the gap to be welded, and it is also the filler metal, so that the material from the rails coalesces with and joins the added molten steel as it solidifies to form a weld. Thermit is the trade name for one of the granular mixtures of aluminium metal and powdered ferric oxide. Ignition of the Thermit is usually carried out by lighting a magnesium ribbon or sparkler.

Procedures for Thermit welding:

• The rails are cut square and the gap to be welded is prepared within prescribed limits. (If the rail ends are cut skewed, the gap will be non-uniform and the fusion of the rails will be asymmetric.)
• The cut faces are cleaned with kerosene oil and a wire brush to remove rust, dust, or greasy material, etc. (Otherwise, this material may get fused with the weld material and this may render the weld defective.)
• A 1m-long steel straightedge is used to align the running edge of the rail head. The rail ends are 'peaked' to accommodate contraction during solidification and cooling of the 'Thermit' steel. If 'rising' of the rails is not done, the joint will sag due to differential cooling of the rail head (where more material is available and hence the cooling is slower) and rail foot after cooling. A sagged joint gives bad riding and becomes a maintenance problem. Such a joint will be subject to larger stresses due to the dynamic augment. For lateral and vertical alignment, wedges are used.
• Stands for crucible and torch are fixed on the railhead, at appropriate locations, on opposite sides of the welding gap and position and the height of the torch stand is checked and adjusted by placing the preheating burner or welding torch on it which is then removed and set aside for later use.
• A set of prefabricated moulds of the appropriate rail section is selected and examined for suitability. The rail profile of the mould is checked by placing the mould against the side of the rail to be welded. If required, small adjustments to the mould profile are made by rubbing the mould gently against the sides of the rail. Then the moulds are placed in the mould shoe (i.e., clamp), seating it properly using luting sand. The placement of the mould should be central over the gap as otherwise while pouring the molten metal, one rail end will get more heat than the other and the fusion of the metal at the other rail end may not be complete. The recess, if any, between the mould and the rail profile is sealed with luting sand. A slag bowl is attached to the mould shoe to collect the overflowing slag and molten metal during the pouring.
• The magnesite lines crucible is housed at the correct height and alignment on the swiveling crucible stand. A closing pin is placed at the bottom over the opening. This pin's head is covered by about 5g of asbestos powder, so that it does not melt in contact with the molten metal and 'auto tapping' takes place.
• The crucible is swung away from the rail and the 'portion' (self-igniting mixture which yields the molten metal) is poured into the crucible, heaped in a conical shape.
• Using LPG (commercial use cylinders) and oxygen (or petrol and compressed air, an older technique, but still in use), the preheating burner or welding torch is lit and the flame is tuned. This torch is placed in its stand which is fixed over the gap, and the flame is directed into the mould through the central opening. The flame heats the rail ends and this is done for a specified time for each rail section and the pre-heating gases employed.
• As the preheating is completed, the Thermit reaction is initiated by igniting a sparkler and putting it into the crucible. The reaction is allowed a specified time and the slag is allowed to be separated from the molten metal. Thereafter, the closing pin is tapped from the outside, thus discharging the metal into the top central cavity of the mould. Thereafter, the crucible and torch stands are removed.
• The excess Thermit steel over the head of the rail (head riser) is removed after solidification (but when the metal is still red hot) by either manual chiseling or using hydraulic weld trimmers.
• The remaining refractory material is removed and the steel vent risers attached to the collar of the foot of the weld are snapped off.
• The wedges, etc., are removed, any fastenings that were removed, are re-fixed and the railhead is ground manually or using grinding machines.

Electron beam welding is a welding process which produces coalescence of metals with the heat obtained from a concentrated beam composed primarily of high-velocity electrons impinging upon the surfaces to be joined. Heat is generated in the workpiece as it is bombarded by a dense stream of high-velocity electrons. Virtually all of the kinetic energy-the energy of motion-of the electrons is transformed into heat upon impact.

The electron beam welding process had its inception in the 1950s in the nuclear field. There were many requirements to weld refractory and reactive metals. These metals, because of their affinity for oxygen and nitrogen of the air, are very difficult to weld.

The original work was done in a high vacuum. The process utilized an electron gun similar to that used in an X-ray tube. In an X-ray tube the beam of electrons is focused on a target of either tungsten or molybdenum which gives off X-rays. The target becomes extremely hot and must be water-cooled. In welding, the target is the base metal which absorbs the heat to bring it to the molten stage. In electron beam welding, X-rays may be produced if the electrical potential is sufficiently high.

As developments continued, two basic designs evolved: the low-voltage electron beam system, which uses accelerating voltages in 30 kV to 60 kV range and the high-voltage system with accelerating voltages in the 100 kV range. The higher voltage system emits more X-rays than the lower voltage system.

In both systems, the electron gun and the work piece are housed in a vacuum chamber. There are three basic components in an electron beam-welding machine. These are the electron beam gun, the power supply with controls, and a vacuum work chamber with work-handling equipment. The electron beam gun emits electrons, accelerates the beam of electrons, and focuses it on the work piece.

Recent advances in equipment allow the work chamber to operate at a medium vacuum or pressure. In this system, the vacuum in the work chamber is not as high. It is sometimes called a "soft" vacuum. This vacuum range allowed the same contamination that would be obtained in atmosphere of 99.995% argon. Mechanical pumps can produce vacuums to the medium pressure level.

One of the major advantages of electron beam welding is its tremendous penetration. This occurs when the highly accelerated electron hits the base metal. It will penetrate slightly below the surface and at that point release the bulk of its kinetic energy which turns to heat energy. The addition of the heat brings about a substantial temperature increase at the point of impact. The succession of electrons striking the same place causes melting and then evaporation of the base metal. This creates metal vapors but the electron beam travels through the vapor much easier than solid metal. This causes the beam to penetrate deeper into the base metal. The width of the penetration pattern is extremely narrow. The depth-to-width can exceed a ratio of 20 to 1. As the power density is increased penetration is increased.

The heat input of electron beam welding is controlled by four variables: the number of electrons per second hitting the work piece or beam current, the electron speed at the moment of impact, the accelerating potential, the diameter of the beam at or within the work-piece, the beam spot size, and the speed of travel or the welding speed. The first two variables, beam current and accelerating potential, are used in establishing welding parameters. The third factor, the beam spot size, is related to the focus of the beam, and the fourth factor is also part of the procedure.

Since the electron beam has tremendous penetrating characteristics, with the lower heat input, the heat-affected zone is much smaller than that of any arc welding process. In addition, because of the almost parallel sides of the weld nugget, distortion is greatly minimized. The cooling rate is much higher and for many metals this is advantageous; however, for high-carbon steel this is a disadvantage and cracking may occur.

The weld joint details for electron beam welding must be selected with care. In high vacuum chamber welding special techniques must be used to properly align the electron beam with the joint. Welds are extremely narrow and therefore preparation for welding must be extremely accurate.

Filler metal is not used in electron beam welding; however, when welding mild steel highly deoxidized filler metal is sometimes used. This helps deoxidize the molten metal and produce dense welds.

Almost all metals can be welded with the electron beam welding process. The metals that are most often welded are the super alloys, the refractory metals, the reactive metals, and the stainless steels. Many combinations of dissimilar metals can also be welded.

One of the disadvantages of the electron beam process is its high capital cost. The price of the equipment is very high and it is expensive to operate due to the need for vacuum pumps. In addition, fit up must be precise and locating the parts with respect to the beam must be perfect.