TPT November 2021

AR T I C L E

Huntingdon Fusion Techniques

1. Effective sealing Now recognised as the optimum solution is the use of inflatable purge systems. 2. Supply and exhaust connections In addition to the need for effective sealing of the dams inside the pipe, the connectors and tubing used to provide inert gas and remove displaced gas must also be fit for purpose. This not only applies to any tubing used but also to the fittings and since these need to be easily demountable and re-connected, often on site, attention to the quality is crucial. The most reliable and efficient tube and tube connectors are those widely used in the pneumatics industry and therefore comfortably capable of meeting the weld purge demands for repeated use, rapid connect and disconnect and sustained pressures without leaking. Oxygen measurement Once depletion of chromium due to combination with oxygen during welding of chromium containing steels was recognised, it was realised that the consequent reduction in corrosion resistance could be serious. The need for instruments to measure residual oxygen in the purge gas led quickly to the development of equipment specifically designed for welding applications. At the time, standard available sensors based on electrochemical measurement – made for general purpose applications – had a slow response and offered a detection range quite inadequate to record reliably below 200ppm. These are clearly unsuitable for weld applications where a 50ppm maximum oxygen level is required. Weld purge oxygen monitors using rapid response, accurate solid-state detectors such as the Argweld ® PurgEye ® instruments, introduced from 1996 satisfied the need for measurement as low as 50ppm oxygen. Later developments led to the current PurgEye series of 7 robust instruments capable of meeting site applications, reading oxygen levels down to 10ppm and with an operating range up to 1km. Many of these products now incorporate software for downloading of data for quality control purposes, fault finding diagnosis, alarm features to warn of high oxygen levels, full colour touch screen control and IP65 rating. Dew point measurement Control of oxygen content in purge gas is only one issue facing welders: the presence of moisture on the joint surface can give rise to porosity in the weld metal. The point at which condensation of water vapour takes place to form moisture is referred to as the ‘dew point’. This is the temperature at which the air becomes saturated with water, in other words when the relative humidity reaches 100 per cent.

The recently introduced dew point monitor developed specifically for the welding industry and for use in conjunction with weld purge monitors

It is therefore important that the dew point is monitored so that welders are made aware when moisture is likely to be present. Just for further clarification, when the ambient temperature is below the freezing point of water, the dew point is called the ‘frost point’. Shielding gas supplies are generally controlled to very low moisture content (typically -60°C dewpoint or lower) and if the shielding gas at the joint is identical to that of the supply then the dew point is not likely to be reached. However, between the supply and the purge zone there exists the possibilities of leaks caused, for example, by inefficient connectors, delivery tubing and air passing through the joint gap. Such sources of ambient air can increase the dew point to a level that can give rise to moisture. Under arc conditions, where temperatures can approach 20,000°C, water is broken down into hydrogen and oxygen in atomic form which, with nitrogen when present in weld metal, are known to rise to cracking. [2-4] Methods for measuring dew point are well-established and equipment is available. However, recent development work by Huntingdon Fusion Techniques in Carmarthenshire, has resulted in the production of instruments specifically designed for welding applications and suitable for direct integration with the company’s oxygen monitoring range of equipment. This development allows welders to measure and control purge gas oxygen and dew point with a single instrument. References [1] Welding can reduce corrosion resistance in stainless steels. Gascoigne G. Stainless Steel World Conference & Exhibition, Maastricht 2015 [2] Moisture/hydrogen in shielding gas of gas metal arc welds. The Welding Institute. Technical Knowledge Series [3] Hydrogen production by water splitting using a thermal plasma. Boudesocquea N et al. World Hydrogen Energy Conference. June 2006 – Lyon France. [4] Weld Discontinuities – Part 1 Porosity. ESAB Knowledge Center

Huntingdon Fusion Techniques – UK hft@huntingdonfusion.com www.huntingdonfusion.com

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NOVEMBER 2021