For those who love to indulge in a life of sun, surf and sand the idea of placing a barbeque in such rust-promising environments is not often an option as their longevity is compromised.

But Gold Coast company Southern Stainless has bridged the gap between a nautical lifestyle and the great Aussie tradition by manufacturing barbeques in stainless steel.

ASSDA member Southern Stainless typically specialises in the manufacture of stainless steel products including wine storage and fermentation tanks and marine fitouts, but supplying stainless steel barbeques, boat mounting systems and accessories to the general public, wholesalers, retailers and building industry has proven to be one of their fastest growth areas.

The barbeques are made from 316  grade stainless steel, with a  2205 duplex grade stainless steel plate, which is more corrosion resistant and easier to clean.

The barbeques are fully welded to enhance strength and use an electropolished finish to aid in corrosion resistance and provide a durable surface.

Southern Stainless Managing Director Phillip Brown says the barbeques can be designed to suit customer requirements and are 100% Australian owned and made.

“Chinese manufacturers retail theirs slighty under our price,” Phillip says.  “A certain percentage of the market will always go for the cheaper alternative, but when consumers are looking for quality, they tend to stick with locally-made products.”

This article featured in Australian Stainless magazine - Issue 38, Summer 2006.

Specifying for industrial-size cooking kettles requires close attention to heating processes, product carried and operating temperature.

ASSDA Accredited Inox Fabrications Australia design and manufacture steam jacketed cooking kettles from 250L to 2000L capacity.

The kettles’ heat-up time is effective through design of the lower hemispherical shell which is constructed from SAF2205 providing increased longevity for use with steam and is designed in accordance
with the Pressure Vessel Code AS1210.

All food contact surfaces are grade 316 and are smooth and crevice free to avoid corrosion. Non-product contact surfaces are grade 304.

A grade 316 horizontal scraped surface agitator ensures even distribution of heat throughout the product and reduces ’burn-on’.

The horizontal agitator also provides good mixing, particularly with products containing particulates. The scraper blades are made from food grade engineered plastic which is capable of withstanding high temperatures.

When selecting correct material usage, careful consideration must be given to the environment in which the equipment is to be used: the type of product and if it contains corrosive elements, the operating temperature and the heating process to ensure success of the final product. Stainless steel meets these requirements.

This article featured in Australian Stainless magazine - Issue 45, Summer 2009.

Safe, Hygienic and of Little Concern to People with Nickel Sensitivity

Posted 31st July 1993

Over the past few years stainless steel cookware has undergone scrutiny to determine its safety as a product for use by people with nickel contact dermatitis and it has been shown to have no ill effect.

Early in 1993 Mr G. Norman Flint visited Australia and described current thinking in Europe concerning the use of stainless steel cookware by nickel sensitised people. Mr Flint is a consultant to the Nickel Development Institute based in the United Kingdom and is responsible for presenting nickel's case in regulatory developments of the European Community.

Mr Flint held meetings attended by manufacturers and health workers and presented conclusions of recent research indicating that pick-up of nickel from stainless steel utensils in domestic cooking operations was of no significance in relation to the naturally occurring level of nickel in the uncooked food. This effectively deems the cookware safe for use by sensitised people.

About 10-15% of women and 1 -2% of men are sensitised to nickel and, on close and prolonged skin contact with nickel, some nickel alloys and solutions of nickel salts, suffer from nickel contact dermatitis. Sensitisation requires a significant initial exposure to nickel such as can occur following the insertion of nickel plated earrings after ear piercing when corrosion of the nickel plating may expose the damaged skin to soluble nickel compounds during healing (which explains the higher rate for women).

The incidence of sensitisation has led to a ban on the use of nickel plated "keepers" in Sweden and a proposal for regulations in the European Community to ban nickel containing studs and earrings from the piercing procedure. The proposal also seeks to restrict the release of nickel from alloys used in articles in close and prolonged contact with the skin such as necklaces, bracelets, watch straps etc. to ~0.5J.lg Ni/cm2/week and in coated products to the same amount for a period of three years.

Mr Flint explained that some food stuffs are considerably higher in nickel than others and that avoiding these foods can greatly reduce the intake of nickel for sensitised people. In the light of some reports that dermatologists were concerned that food cooked in stainless steel vessels could exacerbate dermatitis in some cases of nickel sensitisation, research has been undertaken at Bonn University and the BNF/ Fulmer Research Laboratory over the past two years. The research used natural foodstuffs (particularly those high in oxalic acid and salt and likely to create an environment where nickel corrosion from the steel might occur during preparation) and concluded that pick-up of nickel from stainless steel utensils in domestic cooking operations was of no significance in relation to the naturally occurring level of nickel in the uncooked food.

Mr Flint concluded that although in some cases nickel contact dermatitis may be exacerbated by a high nickel diet, many dermatologists in Europe believe that the nickel consumed must exceed about five times that of the average daily diet before any significant effect is observed. Cooking in stainless steel cook-ware will not produce a significant increase in the amount of nickel consumed over that which exists in the food already.

• The average daily intake of nickel is around 200 micrograms (from 2kg of food) per day in the United Kingdom.
• Cocoa, cashew nuts, soya beans and oat flakes have nickel contents well above average (cashew nuts have 50 times the average).
• Nickel Development Institute sponsored tests revealed:
  a) nickel pick-up, even in the most extreme case (rhubarb in a new utensil) is not significant.
  b) pick-up diminishes rapidly below detectable levels with repeated use of any utensil.

This article feature in Australian Stainless magazine - Issue 1, July 1993.

Bruce Harding is not the first person to be frustrated by rusty cast iron hotplates and grills on his barbecue – the difference is that he did something about it.

Drawing on more than 25 years experience in the stainless steel industry, Mr Harding and his team at Equipment Tech Pty Ltd have developed a range of stainless steel hotplates, grills and baking dishes (sold under the name of Topnotch) that can be retro-fitted to almost any barbecue.

The company uses austenitic T304 and ferritic stainless steels, which are mostly supplied by ASSDA members Sandvik and Atlas Specialty Metals.

Mr Harding said these stainless steel grades played an important role in addressing the design challenges, including the ability to expand and contract under heat.

“The grades we have selected really work together with our design to prevent food sticking to the hotplates and grills,” Mr Harding said.

Laser cutting and a specially developed electropolishing procedure are used to fabricate the products.

Equipment Tech has produced over 100,000 Topnotch stainless steel cooking surfaces since launching the product commercially 5 years ago.

This article featured in Australian Stainless Issue 41.

ASSDA member and Accredited Fabricator Stainless Metal Craft has recently completed work on the design and fabrication of stainless steel equipment for Inghams Enterprises’ processing plant at Edinburgh Park, north of Adelaide, and hatchery at Monarto, south of Adelaide.

The projects incorporate numerous custom-designed installations, including a series of 300mm wide channelled drainage at the processing plant in runs of 60m that will withstand the weight of forklifts, and slot drains with integrated sumps (Figure 1) at the hatchery that will tolerate extremely harsh cleaning compounds.

Stainless Metal Craft General Manager Graeme Bunt said the slot drains, in particular, were a specialised project requiring innovative tool work, because of the size and depth required.

He said the slot drains were fabricated at their facility at Emu Plains, NSW, then shipped to Adelaide in 8m lengths where they were site welded to form continuous drains of 44m.

Mr Bunt said grades 304 and 316 were both used at the facilities, depending on the application.

“For most of the general usage areas, such as laundry chutes, bump rails, hand rails, hands-free wash stations, walk-through showers and chequer plate platforms and stairs, we were able to use 304,” Mr Bunt said.

“But in more specialised areas or where harsh chemicals may be used, such as slot drains, some clean points, freezer coving, boot washers and fire hose reel cabinets, we had to use 316 to ensure the material would withstand the harsh environment.”

Mr Bunt said freezer coving (a hygiene requirement to prevent food from being caught where the wall meets the floor) is usually made from epoxy-coated concrete, but there was concern this would wear down with ongoing snap freezing of the chickens. Instead, about 180m of 3mm grade 316 were used.

Strict regulations within the food industry also determined the need for grade 316 stainless steel fire hose reel cabinets.

Mr Bunt said most of the stainless steel was supplied by ASSDA members Atlas Specialty Metals and Midway Metals. The processing plant was handed over to Inghams at the end of September and the hatchery will also be handed over this year.

STAINLESS STEEL STATISTICS

Further Processing Plant
•    55 hands-free wash stations
•    80 clean points
•    180 metres of freezer coving
•    12 boot washers
•    18 fire hose reel cabinets
•    300mm wide channelled drainage in runs of 60 metres – strong enough to withstand weight of forklifts

Hatchery
•    495 metres of slot drains
•    10 hands-free wash stations
•    800 metres of Schedule 40 pipe bump rails
•    45 clean points
•    7.7 square metre egg shell hopper

This article featured in Australian Stainless Issue 41

Over a five-month period, a team of Austline fabricators and specialised welders installed 25 vessels, five kilometres of stainless steel pipework, heat exchangers and over 3,000 fittings and valves into the
new brewhouse.

Austline Manager, Tom Moultrie, said the construction process was quite unique. “The whole project was carried out with the old brewery, bar and restaurant sandwiched between the new brewery and fermentation cellar,” he said.

Little Creatures Project Manager, Roger Bailey, said stainless steel was specified for its corrosion resistance and low maintenance requirements.

Stainless also provides micro-biological stability, ensuring a very high level of sanitation and minimal possibility for contamination, he said.

“Using stainless steel is an innovative way to achieve reusable water in the brewery,” Mr Bailey said.

Grades 304 and 316 were both specified in different applications, depending on the product being carried.

Not one hour of trading time was lost during installation. “Any cross over where we worked on the areas still being used were done at night,” Mr Moultrie said.

This article featured in Australian Stainless magazine - Issue 45 Summer 2009.

Great attention to detail is what immediately comes to mind when you visit the home and kitchen of Kenmore Hills property developer, Matt Sproule.

Sproule's residence in Brisbane's westside features a modern stainless steel kitchen designed by Brian Grant Kitchens and Bathrooms that would be the envy of most interior designers and home chefs.

According to Sproule, friend and business associate, Royce Amos of Royce Amos Interior Design was the driving force for the inclusion of stainless steel benches in the design.

Fabricated by ASSDA Accredited Fabricator Bell Stainless, the large prestige kitchen has four food preparation areas all designed for entertaining a large group of people.

The main area of the kitchen design features a five metre long stainless steel benchtop with a highly, scratch resistant textured finish and seamless welded Murray triple bowl sinks (another full size sink is under the drainage tray).

Other kitchen areas include a stainless steel barbeque with hood and in-built drain for oils and grease plus textured stainless side benching, stovetop gas burners, hood and textured stainless bench and a four metre long marble island bench for food preparation.

Bell Stainless' craftmanship and attention to detail is highlighted in the enduring finish. Sproule said that he was delighted with the way Bell Stainless handled the job. After one year in service and with careful treatment by its owners, the stainless steel benches show no sign of scratches or dents.

Bell Stainless is a family-owned business operating since 1956, mainly manufacturing for the domestic market, an area not often tackled by many metal fabricators.

Bell Stainless is the only Australian company and ASSDA Accredited Fabricator to have won an international award for kitchen design and manufacture from the South African Stainless Steel Development Association.

This article featured in Australian Stainless magazine - Issue 35, Autumn 2006.

Food retail shops and bars are constantly vying for the attention of consumers. Presentation is everything and stainless steel appeal can often be an essential ingredient to business success.

LENARD'S POULTRY SHOP

One such Australian business success is Lenard's Poultry. Lenard's was voted the Best Food Retailer in Australia by the National Retail Association at the 2005 Rewards for Excellence.

With more than 194 stores throughout Australia and more planned to open in the next financial year, Lenard's went head-to-head with some heavy-weight competitors including supermarkets, takeaway food and fresh food retailers to secure the prestigious title.

According to ASSDA Accredited Fabricator, Koolall Manufacturing, creating consumer appeal requires working closely with business franchisees and shopowners to ensure a quality presentation and functional design.

Established in 1979, current owner Igor Theodoridis purchased the company in 1986. Situated between Brisbane and the Gold Coast, Koolall Manufacturing specialises in the design and manufacture of custom stainless steel commercial kitchens and refrigeration cabinets.

Mr Theodoridis says often franchisee owners must “run to a deadline that's set by shopping centres” to ensure that all shops open simultaneously and on schedule.

With more than 17 years experience working with franchisees for Lenard's and the Coffee Club, Koolall's personal approach to handling fitouts for commercial kitchens and hotels is an enviable achievement.

THE COFFEE CLUB

Koolall Manufacturing offers uncomplicated, full service fitouts that effectively streamlines the shopfitting process leaving clients like The Coffee Club free to focus on the business - not the fitout.

Since the opening of the first Coffee Club franchise in 1989, Koolall Manufacturing's team work hard to ensure that the Coffee Club brand and culture is consistently achieved.

The company designs, manufactures and installs fridges, cake displays and kitchen preparation benching and equipment for more than 105 Coffee Club outlets in Australia and New Zealand.

GRAND CENTRAL HOTEL

A major refit of one of Brisbane's great railway bars, Fihelly's Arms Hotel, has seen the city venue transform into the Grand Central Hotel.

Koolall Manufacturing designed, manufactured and installed the stainless steel refrigeration units and benching for the main public bar, commercial kitchen and Platform - a bar for trainspotters and travellers looking for a tipple.

And with a hotel that offers about 20 beers on tap, Koolall's design assistance was critical in positioning beer fonts, glass racks and benches for staff to operate as efficiently as possible.

This article featured in Australian Stainless magazine - Issue 34 - Summer 2005

An Australian firm has won the kitchen design category of the prestigious Stainless Steel Awards conducted by the South African Stainless Steel Development Association.

Competing against entrants from all over the world, Queensland fabricator Bell Stainless won with its Crerar design, which was judged to be the most attractive and functional domestic kitchen installed during the past two years. The competition criteria were that the kitchen had to be designed using stainless steel appliances, kitchen accessories as well as stainless steel wash-up equipment, such as sinks and preparation bowls.

The objective of the Awards program is to encourage creativity, competitiveness and productivity in the stainless steel industry. Awards are made in fourteen categories covering all aspects of stainless steel design and manufacturing. Bell Stainless also achieved finalist status in the welding category.

Other 2002 finalists and winners came from South Africa, Finland, India, France and Canada.

Bell Stainless manufactures and installs both domestic and commercial kitchens, specialising in custom design and manufacture for clients' specific needs, including benchtops, rangehoods, custom designed furniture, water features and balustrading.

Bell Stainless has spent years refining designs and manufacturing techniques. Its winning design utilises stainless steel in several applications.

The rangehood and bench surfaces are made from 5WL Rimex textured stainless steel with curved mirror finished bullnose edging, providing both scratch resistance and visual interest. Other features have been constructed from stainless steel tube, round and flat bar in different finishes as well as satin finish sheet.

This article featured in Australian Stainless magazine - Issue 23, December 2002.

That deserves a drink!

A joint venture between two ASSDA Members has seen 21km of stainless steel pipe work installed as part of the greenfields Bluetongue Brewery recently completed at Warnervale on the Central Coast of NSW.

The $120 million Pacific Beverages brewery, which officially opened in November 2010, contains more than 2000 tonnes of stainless steel, including more than 120 tonnes of tube sourced in Australia through ASSDA Sponsor Atlas Steels.

The brewery construction was overseen by German brewery manufacturer Ziemann, who contracted ASSDA Accredited Fabricators TFG Pty Ltd and TripleNine Stainless Pty Ltd as the sole installation partners for the stainless steel components.

TFG/TripleNine assembled and installed the pumps, heat exchangers, valves, brewing vessels and fermentation tanks, as well as fabricating and installing all the pipework.

TFG Manager Tom Moultrie said they used grade 304 and 316 tube ranging from 25mm to 100mm in diameter. To ensure accuracy, speed, efficiency and quality, specialist sanitary welders orbital welded the tube on site. The construction phase lasted 8 months and, at the height of the project, TFG/TripleNine had 60 fabricators on site.

Mr Moultrie said the scope and size of the project were the motivating factors behind the first-time joint venture.

“The joint venture made sense because both companies could continue to service our other clients during the construction phase, as well as meeting the tight deadline,” he said.

Ziemann project manager Sven Mauchnik said TFG/TripleNine were chosen due to their brewery experience and their ability to match the tight time schedule.

“We were able to build the complete brewery within 8 months and make the first brew on the original planned date,” Mr Mauchnik said. “The quality TFG/TripleNine delivered was beyond our expectation, which is obvious for everybody who visits the brewery.”

TFG/TripleNine installed 48 silos, including fermentation and storage vessels around 18m high. Two cranes were used to install four vessels a day.

The fermentation tanks and brewing vessels were manufactured abroad by Ziemann to assist in meeting the tight time frames.

The Bluetongue Brewery is unique in its design because it has twin-stream brew houses under one roof, which allow for brewing flexibility.

This article featured in Australian Stainless magazine - Issue 48, Autumn 2011.

A versatile, high strength martensitic stainless steel

Martensitic stainless steels are a less well-known branch of the stainless family. Their special features – high strength and hardness – point to their main application area as shafts and fasteners for motors, pumps and valves in the food and process industries.

The name “martensitic” means that these steels can be thermally hardened. They have a ferritic microstructure if cooled very slowly, but a quenching heat treatment converts the structure to very hard martensite, the same as it would for a low alloy steel such as 4140. Neither the familiar austenitic grades (304, 316 etc) nor the duplex grades (2205 etc) can be hardened in this way.

Grade 431 (UNS 43100) is the most common and versatile of these martensitic stainless steels. It combines good strength and toughness with very useful corrosion resistance and in its usual supply condition can be readily machined.

Chemical Composition

The composition of 431 specified in ASTM A276 is given in Table 1 below.

The inclusion of a small amount of nickel in grade 431 is different from most other martensitic grades. This small but important addition makes the steel microstructure austenitic at heat treatment temperatures, even with such a high (for a martensitic grade) chromium content. This high temperature austenite enables formation of hard martensite by quenching.

Corrosion Resistance

The relatively high chromium content gives grade 431 pitting, crevice and general corrosion resistance approaching that of grade 304, which is very useful in a wide range of environments including fresh water and many foods.

Grade 431 has the highest corrosion resistance of any of the martensitic grades. Corrosion resistance is best with a smooth surface finish in the hardened and tempered condition.

Grade 431 is sometimes used for boat shafting and works well in fresh water but is usually not adequate for sea water.

Heat Resistance

Grade 431 has good scaling resistance to about 700°C but, as martensitic steels are hardened by thermal treatment, any exposure at a temperature above their tempering temperature will permanently soften them. 600°C is a common limit.

Mechanical Properties

The application of grade 431 is all about strength and hardness. Table 2 below lists mechanical properties of the grade annealed and in hardened and tempered “Condition T”.

Heat Treatment

A feature of grade 431 is that it can, like other martensitic steels, be hardened and then tempered at various temperatures to generate properties within a wide spectrum, depending on whether the requirement is for highest possible hardness, or best ductility, or some balance between these. Hardening is by air or oil quenching, usually from 950-1000°C.

The tempering diagram in Figure 1 shows properties typically achieved when the hardened steel is tempered at the indicated temperature. A tempering temperature within the range 580 – 680°C is usual. Tempering between 370 and 570°C should be avoided because of resulting low impact toughness.

Tempering should follow quenching as quickly as possible to avoid cracking. Softening is usually by sub-critical annealing, by heating to 620 – 660°C and then air cooling.

Physical Properties

Fabrication

Machining is readily carried out in the annealed condition, and also in the common Condition T. Modern machining equipment enables high speed machining at this hardness of about 30HRC.

Welding of 431 is rarely carried out — its high hardenability means that cracking is likely unless very careful pre-heat and post-weld heat treatments are carried out. If welding must be done this can be with 410 fillers to achieve high strength but austenitic 308L, 309L or 310 fillers give softer and more ductile welds.

Cold bending and forming of hardened 431 is very difficult because of the high strength and relatively low ductility.

Forms Available

Grade 431 is available in a wide range of bar sizes — virtually exclusively round but some hexagonal. Most other martensitic grades are only available in round bar, although the higher carbon 12% chromium “420” series of grades may also be available as hollow bar and as blocks and plates intended for tooling applications.

Alternatives

Another approach to high strength stainless steel bar is a precipitation hardening grade, such as 17-4PH. These grades have similar corrosion resistance and offer some advantages in producing long, straight, higher strength shafts.

Shafts to be used in more corrosive environments are likely to be a duplex or super duplex or nitrogen-strengthened austenitic grade. These, however, have lower achievable strengths than martensitic or precipitation hardening grades.

Specifications

Grade 431 is usually specified by ASTM A276, with composition as in Table 1. In the Australian market, however, there are usually two deviations from A276:

1. It is most common to find this grade supplied in the hardened and tempered “Condition T” to AS 1444 or BS 970, with specified tensile strength of 850-1000MPa. Yield and elongation are typically in conformance with the limits listed above. ASTM A276 only lists a Condition A version of grade 431 — this is the annealed condition that would normally require hardening heat treatment after machining.

2. The second deviation is that it is usual for cold finished stainless steel bars stocked in Australia to be with the all-minus ISO h9 or h10 diameter tolerances. Hot finished “black” bars with all-plus ISO k tolerances may also be available.

This article was prepared by ASSDA Technical Committee member Peter Moore from Atlas Steels. Further technical advice can be obtained via ASSDA’s technical inquiry line on +617 3220 0722.

This article featured in Australian Stainless magazine - Issue 48, Autumn 2011.

Strength and corrosion resistance vital

As wild fish stocks decline globally, the spotlight is increasingly being shone on humane stun and slaughter methods in the rapidly growing aquaculture industry. Stainless steel components fabricated by Pryde Fabrication (ASSDA Accredited) are an integral part of a Brisbane innovation that is leading the way internationally in a shift towards faster and more humane automated percussive stun methods.

Seafood Innovations International Group Pty Ltd has spent around 10 years developing fish harvest technology which enables fish to swim naturally until the second they are stunned, reducing stress on the fish and improving flesh quality.

They have collaborated extensively during this period with Pryde Fabrication (Cleveland, Queensland) to develop the system, which incorporates a base, ramp and trigger plate made from grade 316 stainless steel.

Up to 400 of the units are being produced each year, of which around 98 per cent are for export.

Pryde Fabrication General Manager Darren Newbegin said Grade 316 stainless steel was chosen for the components primarily due to its corrosion resistance and strength. He said other design and fabrication requirements included:

• No bacterial traps
• Robust enough to withstand the harsh environment and repetitive shock loading
• Light enough to enable easy handling of the modules for cleaning
• Configured to enable easy dismantling for cleaning

“We never considered any grade other than 316 because of the harsh environment – the majority of the units are exported overseas, where they are being used in minus temperatures, fully immersed in sea water,” he said.

There is about 15kg of stainless steel in each machine, which is laser cut, enabling a high level of accuracy for both cutting and fold marks. The rest of the procedure is performed manually, including welding, polishing and glass bead blasting to provide a pleasing surface appearance.

“Stainless steel is the perfect material to laser because it’s so clean to cut,” Mr Newbegin said.

Seafood Innovations’ Business Manager Noel Carruthers said the development of the system had benefited from choosing a fabricator in the company’s local area, as it enabled a close collaboration.

Mr Newbegin agreed with this sentiment, suggesting it was this relationship between the two companies which had contributed to making the product fit for purpose and tailored to cost and operational efficiency.

“This relationship has allowed Pryde Fabrication to be involved in a solution to world fish farming and we are excited about further growth in this Australian initiative,” he said.

Mr Carruthers said the patented system represented an enormous change to the industry, with a single unit processing 15-20 fish per minute automatically, compared with other processes such as electrocution, carbon dioxide gas, and the use of wooden clubs.

The system works by pumping a current of water, which the fish are naturally inclined to swim towards. They then reach a point where their nose hits a trigger, which releases and immediately retracts a small, blunt-nosed piston at high speed, making the fish irreversibly unconscious. The fish are then turned upside down and enter a bleed machine where they are automatically bled.

In addition to improved flesh quality, the automated system means fewer operator injuries and immediate bleeding, resulting in improved appearance of fillets when fish are processed. The ability to slaughter at the point of capture means fish potentially carrying diseases will not contaminate other waters in transit.

Although originally developed for Atlantic salmon, the system has also been refined to cater for different varieties of fish, including tilapia, pangasius, barramundi, yellowtail kingfish and cobia.

A recent installation on a Marine Harvest vessel in Norway (incorporating three sets of a four channel system) is slaughtering 20,000 fish an hour at 98% efficiency.

The equipment has been independently tested by laboratories in Norway and ongoing developments to the system are tested at Huon Aquaculture in Tasmania.

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

purge welding to minimise heat tint

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.

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).

The heat tint control is achieved by maintaining oxygen levels <50ppm (0.005%) while the metal is hotter than ~250^oC. 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.

Mechanical 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.

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.

Align 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.