Part 4 Eliminating Oxygen from the Purge Gas and the use of Monitoring Equipment
Even using specialised weld purging equipment does not guarantee defect free welds. Control of the oxygen content of the purge gas is crucial to success.
In this final part of the series the significance of maintaining a low level of oxygen in the purge gas is considered. Several factors will determine what oxygen content can be tolerated in order to prevent oxidation, the most crucial being the material being welded. Sensitive alloys such as titanium may require oxygen to be limited to 50 ppm1 whilst some stainless steels will tolerate 150 ppm without noticeable surface discolouration.
The first, and crucial, step and an aspect often overlooked is the need to provide effective sealing around the weld zone. Poor sealing invariably allows air to enter the weld zone and thus defeat the objective of providing a low oxygen environment.
Attention needs to be given then to sealing any contiguous surfaces. These include services providing gas and other accessories, contact with the material being welded when using pipe purging and trailing shields and, in the case of enclosures, operator access points.
Engineered solutions exploit established technology to achieve reliable solutions to sealing.
|Fig 1 Examples of effective sealing arrangements for, (left to right), inflatable pipe welding system, silicone skirt on Trailing Shield® and glove ports on Flexible Enclosure®.|
Monitoring the oxygen content
Weld discolouration caused by the presence of oxygen in the purge gas may well be acceptable under some circumstances, but the mechanical and metallurgical properties of the joint can be compromised and this can lead to weld failures in service.
Rather than risk weld contamination, measurement and control of purge gas oxygen content during the welding cycle is the recommended approach.
Oxygen measuring devices have been available for many years, but standard instruments are based on primitive technology are totally unsuitable for weld purge gas monitoring. This is especially so in a wide range of industry sectors where the demand for lower oxygen levels has been increasing over the past few years.
A decade ago an oxygen content below 0.1% (1000 ppm) in purge gas was considered low enough for all but the most demanding applications. Not anymore. Quality control during semiconductor, food, beverage, petrochemical and aerospace manufacture has increased awareness of the need to reduce particulate contamination such as might arise from poor weld underbead regulation. Research has also shown that even very low residual oxygen levels can result directly in significant reduction in corrosion resistance in some materials.
The net effect of all this evidence has been to reduce acceptable oxygen levels in some circumstances to well below 0.001% (10 ppm). Accompanying this trend has come a demand for measuring instruments to be more robust and reliable.
Monitoring Equipment Design
Early equipment was based on the ‘wet cell’ technology developed to meet routine needs to measure oxygen contents of 10% or more in air. Many such devices are still marketed for welding applications but the sensing limits are not much below 100 ppm and even that is unreliable.
As long ago as 1990 it was recognised that major changes in instrumentation would be required to meet the increasing demand for sensitivity, reliability and robustness. Taking advantage of technological and engineering developments a range of instruments has emerged using zirconia 3 as a sensing device.
Early products satisfied the immediate requirements for welding, but subsequent changes in demand led to the availability of instruments now capable of meeting the most stringent conditions.
Despite the clear evidence that better equipment was needed, some welding suppliers still offer unsuitable products on the international market. Most of these are based on multi-gas analysers developed for the security and chemical industries. They do not have the oxygen sensitivity required for weld purge gas analysis.
The State of Current Technology
For routine welding of the majority of stainless steels there is a requirement for the purge gas to contain less than 0.01% oxygen. More sensitive materials such as titanium alloys and some special stainless steels, particularly when used by the semiconductor industry, may require oxygen levels well below 0.01% 2.
A basic monitor from Huntingdon Fusion Techniques has been developed to meet the role of providing a rapid and accurate reading of oxygen in the purge gas in a compact and robust instrument rugged enough to be suitable for on-site application. It can be used as a continuously reading instrument with free flow of the purge exhaust gas across the sensor or as a sampling instrument with the hand vacuum pump extracting samples from the purged volume, as and when desired.
For more demanding measurement specialised models have been introduced. These have a measuring range that goes down below 10 parts per million.
The Argweld ‘PurgEye’ range is representative of advanced monitors now available. This series of instruments was the result of several years’ development and fully exploits both scientific and engineering knowledge.
Designed specifically for the welding industry the PurgEye range satisfies the requirements for sensitivity and accuracy whilst at the same time offering reliability and ruggedness. Basic, fully portable, hand held equipment suitable for routine site work is the cornerstone of the product range and more sophisticated instruments offer programmable facilities coupled with data recording for quality control purposes.
Fig 2 General purpose hand held monitors can incorporate push button auto calibration and are usually supplied with sensor indicator
and tripod mount.
A second generation of monitors has been designed to meet the requirements for welding of high specification stainless steels, duplex steels, titanium and other reactive alloys. They have a measurement range from 1000 to 10 ppm.
The recording and auditing capabilities of these advanced weld purge monitors are considered vital to the tube and pipe sector where traceability is now becoming a critical issue. Quality management printout is available. Integrated automatic alarm provides signalling and switching in the event of oxygen levels in the purge gas rising above an operator-set level.
Fig 3 Monitor controller with recording and auditing capability for reading accurately down to 10ppm.
Further development has witnessed the introduction of integral gas sampling pumps for applications where there is may be insufficient flow of exhaust purge gas. These also featured advanced electrical shielding to avoid HF/RF interference.
Fig 4 Rugged monitor for on-site use, with internal electro-mechanical pump to draw purge gas samples as and when necessary together with ‘traffic light’ external alarm accessory.
Fig 5 Computerised monitor with colour touch screen in use with a welding chamber at a leading aerospace manufacturing facility.
Situations can arise when welding joints where physical access for monitoring is impractical such as in long tube and pipe fabrications or where a temperature cycle may be outside the range of monitoring instruments. This has led to the development of a remote sensing head that can be fitted onto the purging system and will measure oxygen level in the purge gas and transmit the information electronically to the monitor up to 1 km away.
Fig 6 Remote sensing head inside pipe connected to weld purge system. Monitor can be up to 1,000 meters away.
Many of the advanced purge monitors now incorporate a networking facility to integrate with a range of accessories to control welding processes from the monitor controller. For example, automatic machines such as orbital welders can be switched on and off automatically by the monitor to meet operator pre-set oxygen levels.
- Oxygen content may be measured as a percentage or as parts per million (ppm). The relationship is linear and as an illustration, 0.01% = 100 ppm.
- Inert Gas Shielding and Purging for Titanium Welding, McMaster J A. Canadian Welding Association Journal 2008.
- Zirconia–based sensors are robust solid-state devices unaffected by position and with no real limitation on shelf life or storage temperature. They are thus well suited for site applications.
- White papers. No 57 Weld Purge Monitor Technology, No 63 Remote Weld Purge Monitoring. Huntingdon Fusion Techniques Ltd, Burry Port UK.
By Dr. Michael J. Fletcher M.Sc. Metallurgy
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