Logo


purge welding to minimise heat tint

Posted 19th May 2010

PIpes thumbnail

Stainless steel is frequently specified for food production, pharmaceutical, chemical and industrial applications due to its corrosion resistance and cleanability. It is vital in these sorts of applications to avoid or remove the oxide heat tint or scale that forms when weld metal is melted, because this heat tint is non-protective and provides a place for bugs to settle or for corrosion to start in certain conditions. Purge welding is particularly useful in these circumstances if no post weld cleaning is possible, e.g. inside tubes.

Figure-1---thickest-oxide

What is heat tint?

Figure 1 (right) shows the typical heat tint formed on the welded side if stainless steel is welded without excluding oxygen. The thickest, darkest oxide is in the centre (where the metal was hottest for longest) and a similar double rainbow will form on the opposite, root side of the stainless steel.

However, if access is good, such as in a tank or large vessel, then the back of the weld can be protected by gas flowing through a backing bar or even by manually or automatically blanketing the weld root with an inert gas from a lance. Unfortunately, this is not practicable in small diameter tubes. Further, post weld cleaning of the surface may not be permitted in pharmaceutical or food industry tubing with highly polished surfaces.

How is heat tint minimised?

Purge welding is a method used to ensure that, with no post weld treatment, the root of TIG welds in tube or pipe has no more than a pale straw heat tint. This level of colouration is specified in AS/NZS 1554.6 and AWS D18.1/D18.1M:2009 (level 3) as the maximum permitted for tube to be used in the as-welded condition both for corrosion resistance and hygienic applications (see Figure 2 below).

Figure-2-AWS-image-2

The heat tint control is achieved by maintaining oxygen levels <50ppm (0.005%) while the metal is hotter than ~250oC. It is assumed that weld preparation, heat input and weld technique are controlled to provide a full penetration weld with a smooth, cleanable profile suitable for Clean In Place (CIP) procedures.

Figure 3 purge welding diagramMechanical orbital TIG welding equipment should give the same result if the manufacturer’s instructions are followed. Modern orbital welders are relatively narrow and can weld close to an elbow, i.e. near the edge of the welding head, as shown by the offset distance in Figure 3 (right), which represents a side view of an elbow being welded. The orbital welder clamps around the pipe and, after purging, rotates automatically while TIG welding the join.

If a consumable is used it must be at least as corrosion resistant as the tube or pipe material. Otherwise, the narrow weld could corrode rapidly if the tube was used in a corrosive environment. Purge gas must be dry and is normally argon, although low oxygen nitrogen can be used (even for duplex tubing). However, if there is excessive leakage into the arc, then phase balances can be disturbed and cause either cracking, poor toughness or lower corrosion resistance.

For long lengths of tube or pipe it is common to use removable dams to contain the purge gas. There are two main types of dams illustrated in Figure 4 below:

  • water soluble paper and adhesive tape inserted on either side of the weld area before assembly and flushed away afterwards; or
  • rubber lipped dumbell shaped assemblies with one end of the assembly attached to a purge feedline and cable for removal after the weld has cooled. The other dam disc contains a vent to avoid pressurising the purged area. Inflatable bladders can also be used instead of the rubber seals.

Custom-made tapered foam discs with a rubber backing and a covering hat may also be used if externally welding a flange to a pipe.

Figure-4---removable-dams

Purge welding tips and tricks

Purge welding is a skill and it is important that the welder is qualified for the weld. It is also essential to assess if he/she is competent to weld on the day. The weld preparation must include verification that the longitudinal weld profile in the tube will permit a gas tight seal for purging.

When the dams are inserted into each section of the tube or pipe, the feed tube and extraction wire must not be tangled. The dam spacing must be large enough so they are not overheated but, typically, a couple of hundred millimetres is adequate. The weld area must be cleaned with a new wipe and volatile solvent, and then allowed to dry before checking the area is clean. The weld area must not be touched.

Figure-5---gas-meterAlign the matching faces and start the pre-purge. The flow should be turbulent enough to remove air from the surface of the pipe, i.e. ensure the stagnant, boundary layer is very thin. Venting must be sufficient to prevent pressurisation or reverse, swirling flow which will mix purge gas with the existing air and reduce the effectiveness of the pre-purge. Either monitor the exit purge gas with a meter (as shown in Figure 5, right) until the oxygen level is acceptable or purge until at least 10 times the dammed volume has flowed. If a significant root gap is required then it can be taped over during this purge. However, care is needed to avoid contaminating the clean weld preparation with the tape adhesive. After pre-purging, reduce the gas flow to avoid blowing out the weld and commence welding.

Plan the welding to minimise positional welding with its less controlled weld profile and heat input. If the ends are not well restrained by a jig, tack them (but ensure the tack is also gas shielded). Thicker wall materials may require a trailing shield to ensure air does not contact the external metal while it is hot enough to oxidise. This is not an issue if external mechanical cleaning is acceptable.

Summing up

Stainless steel’s unique characteristics make it the ideal material in many highly-sensitive applications, but it is vital that it is handled appropriately so it performs as required. Purge welding to avoid heat tint is one example where getting it right from the outset ensures corrosion resistance, cleanability and, ultimately, longevity.

Hydrostatic Testing of Stainless Steels

Guidelines to Ensure Long Service Life

Design engineers frequently specify stainless steel in industrial piping systems and tanks for its excellent corrosion resistance. While stainless steel’s unique characteristics make it a standout leader in the durability stakes of alloys, it is not completely immune to corrosion.

Premature failures of the stainless steel can occur due to Microbiologically Influenced Corrosion (MIC). This corrosion phenomenon usually occurs when raw water used for hydrostatic pressure tests is not fully removed from the pipework and there is an extended period before commissioning of the equipment. The result is localised pitting corrosion attack from microbacterial deposits that, in severe cases, can cause failure within a few weeks. MIC is easily prevented using proper hydrostatic testing techniques.

MIC

MIC failures occur by pitting corrosion, often at welds, where colonies of bacteria may form. A number of different bacterial species are known to cause the problem, but the detailed mechanism is not known.

Iron utilising bacteria appear to be the dominating microbial species involved with MIC occurring in stainless steel. Anaerobic sulphate-reducing bacteria pose a greater risk of instigating or accelerating corrosion often under a layer of aerobic slime or microbial deposits. However others, such as manganese utilising bacteria (generally from underground waters), have also been discovered.

MIC is extremely aggressive and difficult to eliminate once established, so it is surprising and disappointing that there is limited knowledge of MIC within the engineering community. Fortunately, MIC is easily avoided by using good practices during the initial hydrostatic testing. Education and promotion of proven hydrostatic testing practices which prevent MIC are vital to minimising its potential impact on the stainless steel industry.

                    

Hydrostatic testing practices to eliminate MIC

In order to eliminate MIC, it is recommended that the following practices are used.

1. Fabrication practices

Crevices should be eliminated or at least minimised during the fabrication process, as they are the preferred sites for attachment and growth of microbial colonies. They also provide traps for chemicals which could concentrate and cause pits.

The likelihood of MIC will also be reduced by:

  • Using full penetration welds; and
  • Purge welding to prevent the formation of heat tint; or
  • Removing heat tint by grinding or pickling.

Arc strikes and weld splatter should also be ground off and pickled.

2. Use clean water

The cleanest water available should be used in a hydrostatic test, such as demineralised, steam condensate or treated potable water. Untreated or raw water from dams or bores should be avoided when conducting a hydrostatic test but, where this is not possible, the water should be sterilised (e.g. by chlorination) before use. If sterilisation is not practical, the requirements for short residence time and subsequent drying of the system are extremely important. The cleaner the water, the less ‘food’ there is for MIC bacteria to live off and multiply.

It is important to ensure that there is no trace of sediment in the stainless steel system during testing to avoid silting, as the water is normally not circulated during a hydrostatic test. This may require the test water to be filtered to ensure it is free of all undissolved solids. Sediments can provide the conditions for crevice attack.

3. Draining and drying

Thoroughly draining and drying the stainless steel system immediately following a hydrostatic test (preferably within 24 hours, certainly within 5 days) will almost certainly prevent the occurrence of MIC.

Horizontal pipelines should be installed in a sloping direction to make them self-draining.

Drying can be achieved by pigging (cleaning with foam or rubber scrapers), followed by blowing dry air through the system. Beware of blowing higher temperature moist air through cold pipework unless the air is dried before being introduced to the system. If warm air is used, it should not be from a gas burner as condensation may occur.

Draining and drying of systems following a hydrostatic test should only be disregarded when the system is placed into service immediately following the test. Partial draining is potentially very serious as subsequent slow evaporation of even clean residual water can produce very concentrated and aggressive solutions.

4. Chloride content and temperature

During hydrostatic testing of stainless steel equipment, the chloride content of the test water must be within the range to which the stainless steel grade is resistant. Figure 1 shows the maximum temperatures and chloride contents to which stainless steels are resistant in water with residual chlorine of about 1 ppm.

The limits shown in Figure 1 may be exceeded provided the contact time of the water is brief, i.e. 24-48 hours.

If the chloride content of the test water is uncertain, the water should be analysed.

5. Standards

NACE and API standards for a number of products and installations provide guidelines for hydrostatic testing, including limits for water quality and contact times. These standards should be consulted for specific details for the fabrication in hand.

Conclusion

The benefits of stainless steel’s corrosion resistance are well proven in many industrial applications involving piping systems, but failures can occur during hydrostatic testing if care is not taken. Attention to a few simple details will prevent surprises a few months down the track, allowing the long service life available from stainless steel to be fully realised.

This article featured in Australian Stainless Issue 47 - Spring 2010.

Water Farming

Stainless Technology Essential

Guaranteeing water supply in Australia is thirsty work. Western Australia’s new Southern Seawater Desalination Plant, currently under construction north of Bunbury, will help quench Perth residents and businesses with up to 100 billion litres of water a year. In such a highly-corrosive salt water environment stainless steel is a natural fit.

Sea water is pumped from the ocean and its high salinity is extremely corrosive. The desalination plant uses reverse osmosis to purify the sea water, essentially pushing it through a fine membrane at high pressure.

The first pass (first membrane) is the most corrosive environment which is why super duplex stainless steel is essential. Following passes, which have much lower levels of salt and are almost fresh water, require duplex and grade 316 stainless steels.

A collaborative effort of WA stainless steel expertise ensured the best knowledge was applied to the 200-plus tonnes of piping in the plant.

Alltype Engineering was contracted to supply the complete reverse osmosis racks (see image above) with super duplex, duplex and 316 stainless steels required for all the connecting pipe spooling. Project Manager Keith Thomas-Wurth said the the energy recovery devices and the pipe spooling connecting the reverse osmosis racks with the pressure pumps were subcontracted to ASSDA Accredited Fabricator Weldtronics Australia.

International Corrosion Services’ pickling and passivation treatments were central to ensuring the performance of the stainless steel entering the plant. They use Avesta Finishing Chemicals supplied by Bohler Welding Australia (a division of Bohler Uddeholm Australia).

ICS Business Development Manager Stuart Norton said the opportunity to apply the pickling and passivation processes to 200 tonnes of piping came at the right time.

“We’ve just developed the largest nitric and hydrofluoric acid tanks in the southern hemisphere, and they’ve been used to treat the stainless steel to ASTM380-06,” he said.

The near 20m3 tank is a realisation that the industry will move towards longer pieces, particularly in piping, saving on fabrication time and reducing the number of joins – ultimately providing less opportunity for corrosion.

Southern Seawater Joint Venture Mechanical Engineer Juan Jose Perez said that the stainless steel piping in particular is one of the most important elements of the desalination plant’s construction.

“The membrane is the core of the plant and, in turn, the core of the filtration process. The salt water is passing through the stainless steel pipes to get to the membrane and any corrosion, any tiny particle, can damage the membrane which is extremely expensive,” Mr Perez said.

“Suppliers of the membranes run regular checks to detect for corrosion and, if they detect it, it could potentially affect functionality, even warranty of the membrane. So we rely on the stainless steel, particularly inside the pipes, to be of the highest quality. This is why the pickling and passivation process is so important.”

Mr Norton said that ICS heard the industry screaming out for larger tanks for pickling and passivation jobs such as the one undertaken for Southern Seawater and undertook the two-year journey to get the required authorisation.

“Obviously there are some key environmental and waste treatment factors involved in this. Our waste-water process was made easier by constructing an in-house acid neutralisation tank plus a filter press to push heavy metals out of the acid before sending it off to be further treated,” he said.

The trend towards desalination as a water supply method is clear: when Southern Seawater comes online in late 2011 desalinated water will account for 30 per cent (up from 16 per cent) of WA’s total water supply.

This trend means that further use of large-scale pickling and passivation is likely as stainless steel continues to prove to be an essential and trustworthy component of the desalination plant’s construction.



This article featured in Australian Stainless magazine - Issue 47, Spring 2010.

Coloured Facade

Maximum impact two years on

Coloured stainless steel has helped revitalise what has become one of Victoria’s largest and most recognisable shopping precincts – Westfield Doncaster.

In late 2008 Westfield completed a major redevelopment and refurbishment of the Doncaster shopping centre (located 20 minutes east of Melbourne’s CBD), doubling the complex’s size.

Central to the centre’s new look and feel is the building’s ultra contemporary and striking cladded facade that features coloured and patterned stainless steel supplied by Steel Color Australia Pty Ltd.

Steel Color Australia owner Vince Araullo said more than 600 square metres of grade 304 stainless steel were used to construct the eye-catching “Red Wall”.

“The brief from the designers, Westfield Design and Construction, was to deliver a contemporary looking facade that not only provided the Doncaster shopping centre with plenty of colour but would also be hard wearing against Melbourne’s diverse weather conditions,” he said.

“Our coloured stainless steel, which we import from Italy and distribute exclusively in Australia and New Zealand, is manufactured by Europe’s leading specialist in coloured stainless steel and special metal finishes – Steel Color S.p.a.”

The stainless sheeting was fabricated and installed by Melbourne-based Barden-Steeldeck Industries. Manager and part-owner Michael Shacklock said this was the first time his company had worked with coloured stainless steel.

“By attaching the sheets to a sub-frame we were able to make certain that all 300 sheets of coloured stainless steel were accurately positioned to deliver the distinctive looking facade,” Mr Shacklock said.

Mr Araullo said the colour refraction from the Rosso (Italian for red) stainless steel provided a changing colour palette depending on the time of the day and viewing angle.

“The unique movement of colour across the stainless steel clad entrance is a major shift forward from traditionally sterile looking facades that appear on many shopping centres,” he said.

To avoid the potential reflectivity of the facade hindering nearby traffic safety, a Perla pattern was specified. The indentations of the pattern diffuse light and provide an optical flatness, which effectively eliminates reflections.

The pattern also provided improved strength, allowing for a lighter gauge of 1.2mm instead of, typically, 1.5mm or more.

This article featured in Australian Stainless magazine - Issue 47, Spring 2010.