The planned surge in new electricity power generation plant and refits across the world over the next two decades will provide outstanding opportunities for the fabrication sector. Recent innovative developments in welding equipment will support the drive towards the production of consistently better quality joints, many of which are in the safety critical class.
Over 300 nuclear reactors have been proposed of which 136 will be in China, 24 in the USA and 23 in Russia1. India’s massively delayed nuclear power programme will see a resurrection after Électricité de France (EDF), the world’s biggest electricity company, agreed build six nuclear plants in the country. The Indian Jaitapur project is expected to become the world’s biggest nuclear contract and one of the world’s largest nuclear sites. The 10,000 MW project will have six reactors of 1650 MW each.
Fossil fuel powered generators are still expected to play a major part in the ever-increasing global demand for electricity. An estimated 1000 GW of new coal fired power stations will be built in the next 20 years 2. Half of these will be in China, but with significant programmes in South Africa and India. Most of the global coal-fired installations are old and maintenance programmes will need to be implemented coupled with the introduction of CCS retrofitting (carbon capture and storage).
All the programmes involve extensive fabrication of steel pipes and tubes, the welding of which presents particular challenges.
Stainless steels are used extensively in the construction of nuclear power plant, primarily for their corrosion resistance. Core and secondary parts of most reactor types in service today such as PWR/VVER, BWR, CANDU, AGR and fast breeder reactors are built from stainless steel, as are reprocessing plants and research reactors. The nuclear decommissioning and waste storage industry is also a prime user of high quality stainless for different types of transport or storage canisters and boxes for low- to high level waste.
The high pressures and temperatures used in steam generation circuits necessitate the use of creep resistant steels such as those based on chromium/molybdenum/vanadium alloys. These materials provide improved oxidation and corrosion resistance together with high strength and are widely used in both fossil fuel and nuclear power plants.
The demand for quality in all these safety critical joints is reflected in the stringent regulations laid down in welding procedures. Nevertheless some welding practices can result in significant reduction both in corrosion resistance and mechanical strength.
Welding of high pressure steam pipe
Some engineering alloys are prone to cracking during welding. Industry sectors having to overcome this problem are principally in the power generation sector. The materials include low and medium alloy steels that have been specially developed for their high strength. Metallurgists have learned that heating the joint prior to and after welding (pre-heating and post-heating) can reduce the sensitivity to cracking quite significantly. These processes involve temperatures in the region of 200°C (392°F)although this may be much higher for certain materials 3, 4, 5, 6.
An example of a commonly used alloy benefitting from this treatment is SA 213 T91 or SA 335 P91. This is a ferritic alloy steel that meets the condition of creep resistance required in high temperature steam generating plant. The material, often simply referred to as P91, has been in successful use for the last two decades in power plant service.
|P91||0.08 - 0.12||0.30 - 0.60||0.020 / 0.010||0.20 - 0.50||8.00 - 9.50||0.85 - 1.05|
|(V% 0.18 - 0.25)||(N% 0.03 - 0.07)||(Ni% 0.40 max)||(Al% 0.02 max||(Nb% 0.06 - 0.10)||(Ti% 0.01 max)|
Pipe welding is one process that is widely used during manufacture. This affects the microstructure. Preheating, maintaining inter-pass temperatures, and post-weld heat treatment procedures are very critical for P91 and similar alloys. Failure to follow the procedures can result in catastrophic failures in service.
Other high temperature creep resistant ferrous alloys requiring this type of heat treatment are;
|ASTM A389 grade C24||A356 grade 9||DIN 21CrMoV 5-11||15CrMoV 5-10||GS-17CrMoV 511||EN G17CrMoV5-10||GE B50A224|
The preferred welding procedures in this type of fabrication are GTAW and GMAW and these offer protection of the exposed upper fusion zone.
The joint around the underbead however needs to be protected simultaneously by purging away the air that could be in contact with this underbead (known as the root weld) - thus protecting the exposed metal by using an inert gas envelope.
Meeting the requirements of inert gas purging when temperatures exceeding 200ºC are involved necessitates the use of purge systems capable of withstanding these temperatures throughout the heating and welding cycles. Typical thermal cycles can exceed 2 hours and it may be necessary to maintain the purge system in place throughout.
Specially engineered purge products have been designed over the past five years that are capable of withstanding the temperatures involved whilst at the same time maintaining excellent gas sealing characteristics. They are also rugged enough to survive multiple-use applications.
Few manufacturers are able to supply weld purging systems that can be used at the high temperatures prevailing during pre- and post-heating. The range of equipment designed by Huntingdon Fusion Techniques HFT® meets the requirement for thermal stability and operational reliability.
Fig 1. HotPurge® systems cover the diameter range from 6" to 88" (152 to 2,235 mm).
These systems are capable of withstanding temeratues up to 300oC / 572oF for 24 hours.
- The inflatable seals are manufactured from flexible, thermally resistant engineering materials.
- Anti burst.
- Guaranteed seal and purge.
- Volume reducing sleeve to keep purge time down and to keep gas costs low.
Welding of Stainless Steels
One area of production receiving little attention is during pipe and tube fabrication where welding is widely used.
Unless strict welding schedules are adhered to, however, not only will discolouration (heat tint) take place but corrosion resistance can be significantly reduced.
The Mechanism of Corrosion
Stainless steels owe their resistance to corrosion to the formation of a very thin (10-5 mm), transparent surface layer of chromium oxide. This provides a passive film that acts as a barrier to penetration by an invasive environment. When heated to a high temperature in the presence of oxygen this film increases in thickness until it becomes visible – the colour becomes darker with increasing film thickness.
At a critical film thickness the film becomes unstable and begins to break down. The fractured zones created offer sites for localised corrosion.
Four principle mechanisms are involved:
- Crevice corrosion
- Pitting corrosion
- Stress corrosion cracking
- Microbiologically induced corrosion (MIC).
Localised corrosion of a metal surface attributable to proximity of another metal such as a weld. It is a locally accelerated type of corrosion and is one of the major corrosion hazards in stainless steels.
Fig 2. Crevice corrosion adjacent to stainless steel pipe weld
This produces attacks in the form of spots or pits and takes place at points where the passive layer might be weakened: it occurs in stainless steels where oxidation has reduced the passivity. Once the attack has started, the material can be completely penetrated within a short time.
Fig 3. Extensive penetration following pitting corrosion in stainless steel pipe.
Stress Corrosion Cracking
Characterised by cracks propagating either through or along grain boundaries. It results from the combined action of tensile stresses in the material and the presence of a corrosive medium. It can be induced in some stainless steels by adverse heat treatments such as those occurring in weld heat affected zones.
Microbiologically induced corrosion
Corrosion promoted or caused by microorganisms, usually referred to by the acronym ’MIC’ and is common in welded sections.
To avoid these forms of corrosion it is essential that heat tints are properly removed before the stainless steel equipment or piping is exposed to aggressive or aqueous environments. The alternative is to prevent heat tinting during the welding process by using an inert environment to protect the surface.
Removal of Heat Tint
Bright annealing or acid pickling can remove light discolouration but heavier deposits may require machining such as grinding and polishing. Removal clearly requires access to the area in question, not only for treatment but also for debris removal. Even when access is available none of these treatments is easy and most can be very expensive.
Fig 4. Even very low oxygen comtent of protective gas can cause discolouration in stainless steel.
Fig 5. In order to eliminate discolouration oxygen content of protective gas must be reduced to 20 ppm (0.002%)
Whilst it is not too difficult to protect the outside surface of a weld by using an inert gas as coverage, preventing oxidation and loss of corrosion resistance on the inside is often overlooked. The technique of inside protection is known as ‘weld purging’ and uses inert gas to flush out air and thus oxygen before, during and after welding while waiting for the joint to cool below its oxidation temperature.
Weld Purging Techniques
Equipment has been developed over the past decade to make purging much easier. Currently available systems are robust and suitable for multi-use applications. They can be supplied to cover the size range between 6” to 88” (152 to 2,235 mm).
The most effective devices are those based on connected inflatable dams and shown in Fig 6.
Fig 6. The inert gas weld purging concept can be illustrated using the QuickPurge® systems.
These are programmed to control gas flow and pressure during inflation and purging and once placed in position require little more input from an operator. The dams are fabricated using advanced engineering polymers and are thus suitable for use with oil and gas pipelines and processing plant where elimination of contamination is essential.
Fig 7. PurgElite® range of fully integrated systems covering the 1" to 24" (25 to 610 mm) thin wall tube and pipe range.
Purge gas oxygen content can be controlled by using special oxygen monitoring instruments called Weld Purge Monitors®.
These instruments not only measure oxygen levels but will inhibit welding if the level is above that set by the operator. Recording and analysing software provides information for quality control purposes.
Fig 8. Weld Purge Monitor® designed specifically for use in a welding environment. The PurgEye® range is capable of measuring residual oxygen content down to 10 ppm. This model with PurgeNet™ aloows connection of accessories to run automatic machines such as orbital welders.
Even very low oxygen concentrations in weld gases can give rise to discolouration, loss of corrosion resistance and reduction in mechanical strength. Controlling oxygen level in purge gas can be effected simply and efficiently using contemporary integrated purge systems.
- World Nuclear Power Reactors & Uranium Requirements, World Nuclear Association, November 2016.
- Analysis of Globally Installed Coal Fired Power Plant, Finkenrath et al, International Energy Agency,2012.
- BS EN ISO 13916:1997: ‘Welding: Guidance on the measurement of preheating temperature, British Standards Institution, 1997.
- BS EN 1011-2: 2001: ‘Welding: Recommendations for welding of metallic materials. Arc welding of ferritic steels’, British Standards Institution, 2001.
- The Welding Institute. Technical-knowledge series.
- Bailey, N. Weldability of Ferritic Steels. The Welding Institute, 1995.
- Huntingdon Fusion Techniques Ltd, UK. www.huntingdonfusion.com.
- Eastwood et al. 1993. Welding stainless steel to meet hygienic requirements. Document 9. European Hygienic Engineering Design Group (EHEDG).
- Microbiologically influenced corrosion of stainless steel. Jörg-Thomas Titz. 2nd symposium on orbital welding in high purity industries, La Baule, France.
- Effects of purge gas purity and Chelan passivation on the corrosion resistance of orbitally welded 316L stainless steel tubing. Pharmaceutical Engineering. Vol 17 Nos 1 & 2 1997.
- Considerations for Orbital Welding of Corrosion Resistant Materials to the ASME Bioprocessing Equipment Standard. Dr B. K. Henon, Arc Machines, Inc. Stainless Steel America conference 2008.
- Heat Tint Guide with references from Norsok, AWS, and Huntingdon Fusion Techniques own research. (https://www.huntingdonfusion.com/index.php/en_gb/technical-support/technical-material/heat-tint-charts).
By Dr. Michael J. Fletcher M.Sc. Metallurgy
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