AUSTRALIAN WELDING: MARCH 2018
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ALUMINOTHERMIC WELDING DEFECTS Aluminothermic welding has been undertaken for a century in the joining of rails, however defects do still occur in aluminothermic welds. These defects are generally due to poor set up and preparation and also due to inadequate attention to detail. The following weld defect types, causes and rectification methods have been compiled to reduce, and hopefully eliminate, aluminothermic weld defects, thereby increasing the integrity of rail welds.
An aluminothermic reaction is based on the reduction of heavy metal oxides by aluminium which, as part of the reaction, gives off heat (hence, exothermic). Once the reaction has started, the aluminothermic reacts with metal oxide to produce Al2O3 thus liberating the metal from the oxide and generating heat sufficient to raise the temperature to approximately 2,500°C so that both metal and Al2O3 take on a liquid form. With the reaction complete, the heavier metal separates from the less dense Al2O3, which floats to the top of the mould as slag. This process is perfect for the
joining of rails because it allows for the welding of heavy sections without the need for an external power source, thanks to the high heat energy produced by the exothermic reaction. Aluminothermic welding is most commonly used for butt welds joining heavy sections. A luting medium is used to seal refractory moulds, which are placed around the gaps between sections. The aluminothermic reaction is started by a high heat source fuse, and once the reaction has taken place, superheated liquid is automatically ejected into the mould by the crucible.
For more information, see the WTIA’s Technical Guidance Note TGN-R-04: Aluminothermic Weld Defects.
Welding Procedures When executing aluminothermic welds, rigorous adherence to the qualified welding procedure is essential in order to avoid weld defects. Care must be taken in: rail end preparation, mould fitting and alignment, luting, preheating, crucible cleaning and preheating for multi-use crucibles, and correct portion selection. Black Holes An isolated gas pore in the riser of a weld is known as a ‘black hole’. Black holes have been a major cause for the rejection of welds, however there are very few recorded cases of failure initiating from a black hole as the isolated hole is generally fairly shallow and rounded. This type of defect is caused by gas passing through the weld during solidification. The source of this gas is steam from the sealing material under the rail foot. Using narrower than normal weld widths increases the risk of black holes. To minimise black holes: • Use a railhead gap > 24mm, adjusting the weld gap if required • Ensure that the luting material is not too wet • Observe all checks and procedures to control porosity • Ensure that preheating is performed correctly