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Collaboration Brings Results

The new facility in Merrifield Estate on Melbourne's outer northern fringe will allow Dulux to step into the next generation of paint manufacturing technology and innovation. Dulux's aim was to design and construct a state-of-the-art facility, so using quality products was essential to achieve this. Stainless steel in 300 series grades was an integral part of the project design, including storage vessels and over 10,000m of food grade, quality tube complying with AS1528.

CONCEPT STAGE

In 2015, Dulux announced its decision to construct the new state-of-the-art, highly automated paint factory at an investment of $165m.

The Dulux plant, the first of its kind, is approximately 20,000m², built on 17 hectares of land and is due for completion towards the end of 2017. For a project of this large scale, it was crucial that a strong collaborative relationship was set up between Dulux and its chosen suppliers. In relation to the tubing, supply flexibility, reliability and quality was a key requirement and ASSDA Sponsor Prochem Pipeline Products stepped up to the mark and was the successful tenderer. At the outset the total amount of tube was unclear and would only become apparent as the building process unfolded.

Working together with the client, Prochem executed a strategy to provide more than 10,000 metres of stainless steel tube over a nine-month period to complete the project.

Whilst the original engineering design specified pharmaceutical standard ASME BPE, Prochem worked with the client and design team to have this changed to AS1528 due to its suitability for the application and also due to availability of this stainless steel standard compliant product in the Australian market.

THE DESIGN

The design included a number of large and small storage vessels which were fabricated locally and supplied by ASSDA Members and Accredited Fabricators A&G Engineering and J Furphy & Sons. 

A major part of the design involved the supply and fabrication of the AS1528 tubing and included stainless steel fitting and flanges. The fabrication work was contracted to ASSDA member John Beever Australia who worked closely with Prochem to ensure a continuous supply of materials was made available as required. 

The project consumed over 10,000 metres of AS1528 tubular products in a size range from ½” to 8” in outside diameter with the bulk being 2”, 3” and 4”. The fittings used in the project included BSM fittings, tri-clover ferrules and clamps and also a significant amount of ANSI flanges which were all supplied by Prochem and fabricated by John Beever. The overall project quantities were significant and as such a large degree of co-ordination and co-operation was required with numerous manufacturers to meet the project schedule. 

An innovative technique was designed to match the tri-clover ferrules and clamps to the tube and machinery to enable a correct fit-out at site. 

With good co-operation and education, the project also allowed the flexibility to use both stainless steel grades 304 and 316 if availability became an issue. 

With an innovative and co-operative approach involving many ASSDA members, this project was completed on time and within budget, and will support the Dulux paints business for decades to come.

 

This article is featured in Australian Stainless Magazine Issue 60 (Summer 2017/18).

Disc Dryers Get a Makeover

Wave Discs Revolutionises Traditional Drying Process

In the search to find a more efficient way of manufacturing industrial disc dryers for the meat-rendering industry, Melbourne-based engineering company Pinches Group has developed a new technology called Wave Disc.

THE CHALLENGE

Traditional rotary discs comprise of a series of stainless steel discs, welded back-to-back and attached to a centre pipe. Together, the pipe and discs make up the shaft, which operates under steam pressure and rotates within a stainless steel shell.

Employed throughout the world in a wide range of industries, disc dryers are commonly used to dry materials such as coal, sand, sludge, paper pulp and animal waste.

There are significant inefficiencies in the manufacturing of disc dryers:

  • Discs are cut from industry standard rectangular plate, with the centres and corners of the donor material often wasted;
  • Flat discs rely on pressure-welded pins for structural support. A typical two-metre diameter disc requires ninety-six support pins pressure welded with full penetration on both sides of the disc. Pressure welds are not only labour intensive, but due to the proximity of discs with the drum, they are difficult to repair once the dryer is in operation.

ASSDA Member Pinches Group began investigating designs that could overcome the need for support pins and reduce raw materials waste. Rather than cutting the discs, Pinches discovered that using specific rectangular sections of material discs could be formed through a series of folds along one edge of the plate. This enable all supplied materials to be used in the formulation of the disc.

BENEFITS

Because folds occur consistently and consecutively around the disc, the folds reinforce the structure of the discs and eliminate the need for added support.

Removing the pins not only improves manufacturing time, it eliminates the chance of weld failure around the support pins, which can cause equipment faults in traditional disc dryers.

Additionally, the natural waves of the discs provide increased agitation to drying material, which can improve evaporation rates.

One design flaw with flat discs is a tendency for product to stick and build up between discs. With Wave Disc, the waves in the disc massage the product and keep it moving within the drum, preventing build up and ensuring that all raw materials are exposed to heat.

Wave Disc also provides greater surface area. Any indirect drying processes (where the heat source doesn't make direct contact with the product) is reliant on the surface area to transfer heat.

With the addition of the folds, Wave Disc provides a surface area at least thirteen per cent larger that conventional discs within the same footprint. This means that existing disc dryers can replace their current shaft with a Wave Disc shaft and increase the capacity of the dryer.

AWARD WINNING DESIGN

In 2012, the Wave Disc concept received top level funding from the Australian Federal Government's Commercialisation Australia program.

In 2014, the first Wave Disc unit was installed at animal by-product processor, Australian Tallow, in Brooklyn, Victoria. Since then, further units have been exported to by-product processors in New Zealand.

As Australia's leading supplier of processing equipment to the animal rendering industry, Pinches Group developed Wave Disc to overcome specific inefficiencies within the meat processing industry.

Now that this innovative new technology has been tested and proven in the rendering industry, Pinches has plans to expand their focus to heating and cooling processing technologies in the food and agricultural sector.

Pinches Group own and operate a testing facility in Melbourne which incorporates an operational Wave Disc pilot unit used by prospective clients and educational institutions for running trials on a range of raw materials.

Among other metrics, the pilot units can provide clients with evaporation rates and in-an-out moisture levels for specific products.

PATENT

The Wave Disc design is protected by Provisional Patent Number 593495.

This article in Australian Stainless Magazine Issue 59 (Winter 2017).

Stainless Steel Transforms Meat Processing Plant

Over 17 tonnes of stainless steel has been used for the upgrade of a premier meat processing plant to support the growing local and global demands of Australian red meat supply.

The Australian Lamb Company (ALC) currently exports lamb to more than 60 countries worldwide, and recently secured a 10-year contract to process lamb for Coles supermarkets in eastern Australia.

ALC’s multi-million dollar investment to support demand and increase production capacity included the expansion and upgrade of its meat processing operation in Colac, Victoria.

ASSDA Member and Accredited Fabricator Stainless Steel Associated Fabricators (SSAF) Australia was engaged to design, manufacture and install 65 box conveyors spanning 400m, three access walkovers and 30 production tables for the plant’s re-engineered automated boning room.

The conveyor system was designed by SSAF Australia with input from the ALC’s production team to achieve optimum process flow. The main criterion for the mechanical design was excellent product transfer, mechanical reliability and optimal hygiene through easy cleaning of the conveyor’s belt and frame.

The box conveyors are a semi-modular design using the latest SEW-EURODRIVE MOVIGEAR® SERVO motors and gearboxes. Compared with conventional motors and gearboxes, SSAF Australia’s Managing Director Chris Stacey said these systems are significantly more efficient in reducing power usage and allowing a wider speed range without loss in torque.

Grade 304 stainless steel with a 2B finish was specified and used for the upgrade, supplied by ASSDA Sponsors Atlas SteelsMidway Metals and Vulcan Stainless.

Grade 304 stainless steel is a standard requirement in the food industry where acid and salt are not present in the production process. With rigorous standards in food safety and hygiene to adhere to, the boning room must be washed down daily and to this end, the conveyors incorporate CIP (clean in place) systems.

Stacey said that grade 304 2B stainless steel with a PVC protective coating is the material of choice for their food grade equipment. ‘By taking care during manufacture and polishing welds to 320G, 2B is superior to a No 4 or bead blasted finish. The smoother grain structure is much better than No 4 in inhibiting the growth of microorganisms and is easier to clean. Our equipment is regularly swabbed for surface cleanliness and this is critical to our customers’ Quality Assurance (QA) requirements.’

With the full scope of works completed within a 6-month timeframe in early September 2016, the increased capacity of ALC’s Colac operation has delivered significant benefits for the Australian lamb industry and a boost in the Victorian economy.

This article is featured in Australian Stainless Issue 57 (Spring 2016).

Images courtesy of SSAF Australia.

Revision of AS 1528: Fluid Transfer in Stainless Steel Tube and Fittings

Connections are vital

Any visit to a dairy, beverage or food processing plant will drive home the critical importance of the connections between the tanks, mixers, driers, pumps, etc. The image above (courtesy of TFG Group) showing an image of a brewery is a typical example. These tubes and/or pipes carry the process materials, the heating or cooling or wash water, gases, and also dispose of the wastes.

 

Getting the right standard

Except for high pressure or very aggressive environments, most tube is rolled into shape and welded longitudinally. For mechanical or structural service such as columns or handrails, the weld must penetrate and be sound although to perform its mechanical function, it may not need to provide a seal. This is reflected in the basic test requirements of standards such as ASTM A554 ‘Welded Stainless Steel Mechanical Tubing’ and is a reason why it is cheaper and is sometimes used, in error, for fluid transport. Despite these restricted requirements, the external finish is often critical for aesthetic reasons as seen on the handrails in the figure on the right.

Verification of leak tightness is the reason why tubing standards for carriage of fluids, e.g. AS 1528.1 or ASTM A269 or ASTM A270, all include either hydrostatic or 100% eddy current testing. Section 8.4 of the ASSDA Reference Manual summarises the test requirements of the plethora of tubing (and piping) standards commonly used in Australia. However, the food and sanitary industries also require surfaces that are readily cleanable. Hence, in addition to a lack of leaks, there are also requirements on the profile of the weld bead in the tubing, potential crevices in fittings and the surface finish of product contact areas. 

System design and installation

Quite apart from the manufactured components, the system design must include adequate slope for self draining (including across welded joins), simple cleaning procedures, velocities above ~0.5m/sec for low solids streams, at least double that for high solids content and avoidance of design features permitting stagnant zones or dead legs. Excess velocity (at least below about 40m/second) is not a concern for stainless steel, although it may increase noise and pumping costs. These are matters for another place.

Material selection

There are quite complete sets of corrosion resistance data for single corrosives (and some mixtures) at a variety of temperatures and concentrations but they are usually for continuous exposure.  For some acidic, hot and salty fluids or slurries such as sauces, high alloy stainless steels or even nickel-based alloys may be required and such components are rarely “off-the-shelf”. However, for apparently aggressive fluids processed in batches, the intermediate cleaning will arrest the initiation of attack and restore the passive layer so that standard 316(L) material is usually adequate especially with the highly polished finish often used to enable cleanability. One operational issue is that cleaning chemicals can be quite aggressive and the procedures must ensure that residues from cleaning do not remain and are not able to be concentrated and cause corrosion or hygienic issues.

Food tube and fittings – AS 1528

The weld bead is a potential source of crevices and for food tube, its effect must be removed without causing additional surface defects. AS 1528.1 requires the weld bead to smoothly blend without harmful markings. It also sets a nominal surface roughness (0.3 μm Ra) for the rest of the interior by requiring the use of fixed (1.6mm) thickness 2B material. ASTM A270 ‘Seamless and Welded Austenitic and Ferritic/Austenitic Stainless Steel Sanitary Tubing’ assumes a sophisticated specifier as it lists a mill finish as well as multiple alternative mechanical or other finishing techniques. Acceptance of minor surface imperfections is by agreement. The specifier may require a surface roughness (Ra) limit – which, of course, would override a grit size specification.

The manufacturing tests (eddy current or hydrotesting) ensure that food tube will hold pressure. For the essential quality assurance purposes, AS1528.1 requires line marking of tube. Finally, food grade tube requires a complementary set of fittings that will fit together. The AS 1528 suite achieves this with screwed couplings (Part 2), butt welding fittings (Part 3) and clamp liners with gaskets (Part 4). Aesthetics may be important and is in the hands of the specifier as the exterior of AS1528.1 tube may be as-produced or “buff polished as agreed”, i.e. polished with grit of a specified size.

The AS 1528 suite started life in 1960 as AS N32, was split into four parts in the mid 1970s and completely revised by an ASSDA driven working group to its present form in 2001. It has been widely accepted especially since the 2006 publication by ASSDA of what is now the Food Code of Practice for the fabrication and installation of stainless steel process plant and equipment in the food and beverage industries.  The New Zealand dairy industry has effectively adopted the AS 1528 requirements for dairy tube and fittings. Multiple overseas suppliers provide tube to the AS 1528 specification.

Food and beverage manufacture is obviously worldwide and this has resulted in national, regional and international standards which are different and locally focused. The sizes of the ISO alternatives (ISO 2037, 2851 – 3) are quite different. The European standard (EN 10357- which supersedes BS4825.1 and DIN 11850) covers similar tube but does not cover the range of sizes commonly used in Australia. The British Standard products (BS 4825) are similar in sizes to the AS 1528, but with a restricted range. The American 3A products also cover a restricted range. 

“As a result, ASSDA is spearheading an industry effort to revise the 15-year-old suite of AS 1528 standards”.

What is in need of review?

There are a number of typographical errors and inconsistencies between the parts, there are only some pressure ratings and the listing of fittings requires some revisions. The tolerance on the tube wall thickness has been narrow and one sided since inception and while the standard allows modification by agreement, the current wall thickness requirement will be reviewed.  Other issues for discussion will be the addition of larger sizes and assessment of differences for internal finishes between parts of the suite. And finally, it is intended that AS 1528 will be converted to a joint Australian and New Zealand standard to formalise New Zealand’s use.

If users of the AS1528 suite of standards have any suggestions for changes or improvements to the standards, ASSDA would welcome your emailed comments to This email address is being protected from spambots. You need JavaScript enabled to view it..

Acknowledgements

This article has drawn heavily on documents produced by the ASSDA/NZSSDA working group dealing with the proposed revision of AS 1528 and in particular Peter Moore from Atlas Steels, Kim Burton from Prochem Pipeline Products and Russell Thorburn from Steel and Tube in New Zealand.

This article is featured in Australian Stainless Issue 56 (Winter 2016).

Stainless Delivers State-of-the-Art Production Facility

Stainless steel has helped deliver improved environmental performance and increased efficiency for a major food production company.

In 2014, Australian agribusiness GrainCorp announced a $125 million investment in a consolidation strategy to integrate its GrainCorp Foods’ manufacturing operations, including the relocation of its Brisbane plant to the existing West Footscray facility in Victoria. This move effectively terminates the use of its coal-fired equipment, giving GrainCorp Foods the opportunity to invest in efficient and environmentally sustainable technology and significantly reduce its carbon footprint.

As a result, GrainCorp Foods’ West Footscray operation commenced its expansion and upgrade in 2015 to deliver a state-of-the-art food processing plant.

GrainCorp awarded the design, engineering and installation to SPX Flow Technology Australia, and SPXFlow awarded ASSDA Member and Accredited Fabricator TFG Group the contract to install and fabricate specialised components for the facility’s new margarine production line.

TFG Group’s Foodline Projects division installed and assembled the stainless steel equipment under the direction of SPX Flow Technology Australia, including numerous pumps, valves, heat exchangers, vessels and specialised processing equipment.

The TFG Foodline Projects team also mechanically installed over 12km of stainless steel pipe, AS 1528 304L and 316L tube ranging from 25mm to 100mm in diameter, and over 6000 fittings supplied by ASSDA Sponsor Prochem Pipeline Products.

Hygiene and cleanability of equipment used in food production is paramount, and the correct specification and fabrication quality of stainless steel ensured this criterion was met.

The TFG Foodline Projects team consisted of 35 specialised welders, fitters and installation specialists to ensure the project’s tight lead-time of 24 weeks to completion was met with zero safety incidents. Orbital welding was applied to ensure speed, accuracy and prevention of bacterial contamination in the products.

As part of the factory upgrade, TFG Group’s Austline Fabrications division assisted with the fabrication and installation of the specialised scalloped stainless steel tank access platforms, break tanks, stainless steel chemical bunds and support racks. Jacketed pipework was fabricated to ensure the internal temperature of the process pipework was controlled to prevent the viscous product from solidifying.

These specialised items were all fabricated at TFG Group’s purpose-built factories in both Western Australia and Victoria, and transported to the West Footscray facility for installation by the Foodline Projects division.

GrainCorp’s investment in its West Footscray plant has delivered a fully integrated facility and a more efficient focal point for the sourcing, refining, and distribution of GrainCorp’s locally-produced edible oils and food ingredients.

This article is featured in Australian Stainless Issue 56 (Winter 2016).

Images courtesy of TFG Group.

Cutting a Carbon Footprint

Coca-Cola Amatil is reducing the carbon footprint of its 600ml PET bottles by 22% with the help of stainless steel.

Innovation in process technology and the successful application of stainless steel has led to efficiency gains and sustainable outcomes for one of the world's most recognised brands in the food and beverage industry.

In 2011, Coca-Cola Amatil (CCA) announced a $450 million investment in PET bottle self-manufacture, or ‘blowfill’ technology at its production facilities across Australia, New Zealand, Indonesia, Papua New Guinea and Fiji.

Blow-fill technology is a manufacturing technique that allows companies to produce their own PET (polyethylene terephthalate) bottles within their own facility. It allows manufacturers to form, fill and seal bottles in one continuous process at the one location without human intervention. Blow-fill has enabled CCA to make its PET bottles using significantly less PET resin, resulting in production of the lightest-weight bottles in the global Coca-Cola system.

Previously, CCA would buy blow-moulded bottles from a third party supplier, transporting them to its own facility to sterilise and fill with product. CCA’s integration of these three steps into one operation has automated its production lines, creating economies of scale and
optimising efficiencies of operation.

CCA’s Kewdale facility in Perth is one packaging line that recently completed its installation of blow-fill equipment, procured from Krones AG, a German-based process manufacturer.

CCA engaged ASSDA member and Accredited Fabricator TFG Pty Ltd for the installation and fabrication of the stainless steel interconnecting pipework for the facility’s new blow-fill equipment.

Sydney-based ME Engineering detailed the scope of works, and coordinated the process engineering and installation of the new equipment.

Over 6km of 304L and 316L AS1528 standard grade stainless steel tube was supplied by ASSDA sponsor Prochem Pipeline Products, ranging in size from 25mm-200mm diameter.

The TFG team purge TIG welded all stainless steel components on site and internally passivated the stainless steel using citric acid.

ME Engineering’s Project Manager Andrew Meagher said grade 316L was specified for CCA’s Kewdale facility because of the high chloride content of the water supply in Perth.

With spring water one of CCA’s main products, sanitation is key to avoiding microbiologically-influenced corrosion.

Tom Moultrie, General Manager of TFG, said that whilst there are other materials that can be specified for equipment using compressed air, stainless steel provides aesthetic appeal, trusted hygiene and longer life span.

The use of stainess steel has been successful in the output of this project, with CCA’s State Projects Engineer Simon Wall stating that ‘as a beverage manufacturer, food safety aspects of our processes and equipment are critical to ensuring the integrity and quality of our products – an area that stainless steel ensures.’

Kewdale’s new blow-fill line commenced operation in June 2012. It features 14 blowing stations, 108 filling nozzles and 18 capping stations, and has the capacity to produce 26,000 bottles per hour.

Mr Wall said the investment in PET bottle self-manufacture will continue to deliver savings in raw materials - bottles are made using less PET resin and less water is used in the bottling process - and meet future consumer growth and demand.

CCA’s ongoing commitment to innovation and sustainability has maximised production capabilities whilst minimising the use of resources.

By the end of 2012, 10 blow-fill lines will have been deployed across CCA’s production facilities in Australia, bringing self-sufficiency to over 70%. Once all 26 production lines are implemented, CCA estimates a saving of 7000 tonnes of PET resin per year, a 15% reduction in bottle weight and 50,000 truck movements eliminated per year. Overall, this is reducing the carbon footprint of every 600ml bottle by an average of 22%.

Images courtesy of TFG Pty Ltd.

This article is featured in Australian Stainless magazine, issue 52.

Stainless Steel and Nickel - 100 Years of Working Together

This is an abridged version of a story that first appeared under the same title in Stainless Steel Focus No. 07/2012.

The Nickel Institute's director of promotion, Peter Cutler, and consultant Gary Coates, reveal some of the reasons for the continuing popularity of nickel in stainless steels.

Stainless steel is everywhere in our world and contributes to all aspects of our lives. We find stainless steel in our homes, in our buildings and offices, in the vehicles we travel in and in every imaginable industrial sector. Yet the first patents for stainless steel were issued only 100 years ago.

How did this metal become so desirable over the past century that more than 32 million tonnes was produced in 2011? And how does nickel, a vital alloying element in most stainless steel alloys, contribute to the high demand for stainless steel?

THE 'CREATION' OF STAINLESS STEEL
By definition, a ‘stainless’ steel has a minimum level of about 10.5% chromium, so the discovery of chromium in 1799 by Nicolas Louis Vauquelin in France was the first key event in the creation of stainless steel. In 1821 another Frenchman, Pierre Berthier, published research that showed a correlation between increasing chromium content and increasing corrosion resistance, but the high carbon content of his alloys prevented them from showing a true ‘stainless’ behaviour.

Still in France, in 1904 Leon Guillet first published his metallographic work on alloys that today would be classified as ferritic and martensitic stainless steel. In 1906 Guillet published his work on the nickel-containing austenitic stainless steel family, but his studies did not include corrosion resistance. Albert Portevin then continued to build on Guillet’s work.

In 1911, a German scientist named Philip Monnartz reported that as the chromium content neared 12% in a steel with a relatively low carbon content, the alloy exhibited ‘stainless’ properties. Further developments then rapidly occurred in many other countries. In the United States, Elwood Haynes started working with martensitic alloys while Becket and Dantsizen were developing a ferritic stainless steel as lead-in wires for electric light bulbs. In 1912, Great Britain’s Harry Brearley worked on a 13% chromium martensitic alloy, initially for high temperature service in exhaust valves for aeroplane engines.

Meanwhile in Germany, Eduard Maurer and Benno Strauss were testing nickel-containingalloys and, in 1912, two patents were awarded. One of these grades, containing about 20% chromium and 7% nickel, was called V2A, and was found to have exceptional corrosion resistance in nitric acid. That grade had a relatively high carbon content compared to today’s stainless steel, and would be
similar to a Type 302 (EN 1.4317) stainless steel. 100 years later, the most commonly used alloy for nitric acid is 304L (EN 1.4307) with approximately 18.5% chromium and 8.5% nickel, quite similar to the V2A composition other than having a much lower carbon content.

Brearley’s martensitic stainless steel alloy would not rust when wet. He worked with Sheffield cutlery manufacturers to forge it into knife blades and then harden it, replacing the carbon steel blades they were then making. Stainless steel knives rapidly became a common household item. However, for forks and spoons, where high hardness was not so important, the 18-8 (302) composition became the most commonly used alloy.

300 SERIES
We normally think of the austenitic or 300 series family of stainless steels as the ‘nickel stainless steels’, but many other families contain nickel. One of the prime reasons for using nickel in the 300 series alloys is that nickel is an austenite former, but other reasons include:

  • Nickel adds corrosion resistance, especially in certain aqueous environments, and in certain high temperature environments.
  • Nickel can retard the formation of embrittling intermetallic phases at elevated temperatures, a major downfall of the non-austenitic families.
  • The austenitic structure can mean high toughness at cryogenic temperatures.
  • The advantages of the 300 series extend to welding and forming operations.

A fuller discussion of these topics can be found in 'The Nickel Advantage - Nickel in Stainless Steels', available on the Nickel Institute website.

200 SERIES
The 200 series stainless steels are also austenitic in structure. The standardised 200 series grades, which have chromium contents close to the level of a 304L alloy (about 18%), have an intermediate level of nickel. The ‘non-standardised’ 200 series not only have lower contents of nickel, but also lower contents of chromium, with the net effect of significantly reduced corrosion resistance, although still an improvement over the 11-13% chromium ferritic stainless steels.

DUPLEX
The duplex (austenitic-ferritic) family of alloys also need some nickel as well as nitrogen to ensure proper austenite formation. Most ‘matching’ duplex filler metals are actually over-alloyed with nickel to ensure that the welds have the required properties.

PRECIPITATION HARDENABLE
The precipitation hardenable (PH) stainless steel family contain nickel, which increases their corrosion resistance, ductility and weldability compared with hardenable non-nickel-containing stainless steel alloys. One of the other major advantages of the PH grades is that, unlike the martensitic grades, they do not need a quenching operation, which considerably reduces risk of distortion. Some of the martensitic grades also contain a small nickel addition. In the higher chromium types, the nickel is needed for the martensitic transition. In all nickel containing martensitic grades, nickel improves their corrosion resistance, ductility and weldability.

Some of the lower alloyed ferritic grades such as UNS S41003 (EN 1.4001) and S40975 contain a small intentional nickel alloying addition that allows for grain size control, which aids especially in welded constructions. A few of the higher alloyed ferritic grades also have a small nickel addition to increase toughness and ductility, which is beneficial during both hot rolling and in their end use.

Clearly, it is important for each specific application to select the appropriate alloy or alloys to give the desired properties.

GROWTH IN DEMAND FOR STAINLESS STEEL
According to the ISSF, 300 series stainless steel still dominates the worldwide production figures, as shown in Figure 1.

The properties of the various 300 series grades - created by the addition of nickel - are clearly valued by users, both in industry and the general public. Upwards of two thirds of all stainless steel produced in 2011 fell within the 300 series and close to three quarters of all stainless steel produced contains nickel.

The growth of worldwide production of stainless steel over the past 100 years has been steady, if not spectacular. This has meant that the demand for new nickel has steadily increased along with the demand for stainless steel, as shown in Figure 2. Recycled stainless steel is also a very important component in the alloy supply chain.

EFFICIENCY AND 'GREEN' CREDENTIALS
Resource efficiency is a recurring theme as the global economy faces economic challenges. Stainless steel not only contributes towards efficiency in many applications, it also shows continuous improvement in the resource efficiency related to stainless steel itself.

There are three important factors:

  1. Stainless steel’s long service life, which might average 15 to 20 years, although much longer in prestigious buildings.
  2. The extent of recycling: The percentage recovered and recycled at end-of-life - around 90% - is amongst the highest of all materials. Moreover, this recycling can be repeated many times without loss of quality. While the recycled content may appear to be relatively low, this is simply a result of stainless steel’s long service life (15 to 20 years) coupled with much lower global production 15 to 20 years ago.
  3. Continual production improvements for stainless steel and its raw materials. For example, whilst the ores being processed today are of lower grade than before, the extraction and recovery processes are more efficient.

THE FUTURE
The history of stainless steel would be incomplete without celebrating the extent to which it has enabled innovation not just in the area of improved performance, but also in the more intangible, aesthetic aspects. From chemical plants to medical equipment to iconic stainless steel-clad buildings, stainless steel has made - and will continue to make - a major contribution to almost every aspect of our lives.

With durability, recyclability, versatility and aesthetic appeal at the core of its appeal, stainless steel - with nickel as one of its trusted alloys - is well placed to continue to innovate and expand its applications.

STAINLESS STEEL IN USE

FOOD AND BEVERAGE INDUSTRY
The popularity of stainless steels in kitchens did not go unnoticed in the food and beverage industry.

If we take milk, we know of an early stainless steel bulk milk tank truck from 1927 in the USA. A paper entitled ‘The Corrosion of Metals by Milk’ from the January 1932 Journal of Dairy Science by Fink and Rohrman states: ‘It has long been known that milk in contact with iron and copper will not only acquire a metallic taste, but corrode these metals readily’. At that time, tin-coated metals were commonly used. It went on to say that ‘High chromium nickel (18-8) iron alloys … are very resistant to corrosion by milk and are satisfactory for dairy equipment …’. The modern milk processing industry is filled with stainless steel equipment, mostly of Type 304 (EN 1.4301) or 304L.

The report also went on to state that some materials that are otherwise suitable for processing of milk ‘…do not stand up well to the action of cleaning compounds that are commonly used in dairies’, but that the 18-8 alloy was suitable for those cleaning compounds. Today, the typical cleaning acids and hypochlorite sanitising compounds that are used not only in the dairy industry but also in most food and beverage plants worldwide, require that same 18-8 alloy as a minimum. A correctly chosen stainless steel alloy will not change the taste or appearance
of the food product. However, it is the ability to withstand repeated use of the sanitising chemicals over the lifetime of the equipment that has led to the widespread use of stainless steel in all sectors of the food and beverage industry. Producers are then able to guarantee the
safety of their food products.

ARCHITECTURE
Another area of quick acceptance was in architecture. The first recorded use for that purpose was in 1929 in London at the Savoy Hotel where a sidewalk canopy and a sign were erected with the 18-8 alloy. These were soon followed by two iconic skyscrapers in New York that used stainless steel as a dominant element on their exteriors: the Chrysler Building in 1930 and the Empire State Building in 1931.

Since then, many prestigious buildings around the world have used stainless steel, including the Petronas Tower in Kuala Lumpur, the Trump Tower in Chicago, and the Jin Mao Tower in Shanghai. Related to architecture is sculpture, and Isamu Noguchi convinced the Associated Press in 1940 to approve stainless steel instead of bronze for his sculpture above the entrance to its building in New York. Since then, artists around the world have been using stainless steel, mostly either 304L or 316L (EN 1.4404), in their works. The St Louis Arch in the USA, Frank Gehry’s Peis (Fish) in Barcelona, Spain, and more recently Genghis Khan in Mongolia are examples of what can be done with stainless steel.

TRANSPORTATION
During the Great Depression in the USA, Edward Budd realised the untapped potential for stainless steels. Although their use in aeroplanes was his first application, his legacy remains the building of more than 10,000 passenger railcars, some of them still in use today.

Around the world, stainless steel is used extensively for passenger rail cars for subways, commuter trains and long distance trains, ensuring safety plus long life and low maintenance costs. In addition, stainless steels are used to transport cargoes such as food products, petroleum products and corrosive chemicals by rail, road, water and even air, both domestically and internationally.

ENERGY
In the broad field of energy, stainless steels have been used to extract oil and gas containing hazardous substances as well as for use in the refining stages. For power plants, stainless steel is used extensively at both low and high temperatures, whether the fuel is coal, oil, gas, uranium or waste products. Hydroelectric stations use stainless steel for dam gates as well as turbines. Many of the established sustainable
energy technologies such as solar and geothermal are using stainless steel, as well as the present biofuels industry with corn or sugar cane as feed stock.

WATER
Fresh water is an essential commodity for mankind, and stainless steel is used extensively in treatment plants for potable water as well as for wastewater. Cost effectively producing fresh water from seawater or brackish water by desalination also requires the use of stainless steel. In some countries, underground stainless steel pipe is used to deliver potable water to homes to prevent leakage, or in other special cases to protect either the environment outside the pipe or the water inside the pipe. Stainless steel plumbing is also common in certain countries and offers a long lasting, low maintenance option.

SURGERY
The first recorded example of an austenitic stainless steel surgical implant is from 1926. Medical instruments are also known from that time period. The ability to easily and repeatedly sterilise components that come in contact with the human body or are used in hospitals and clinics contributed to the early acceptance of stainless steel. Today, there are well-established international specifications for materials used in this industry. For example, stainless steel alloys for implants must meet stringent metallurgical cleanliness requirements and be completely non-magnetic so that the patient can safely undergo diagnosis by Magnetic Resonance Imaging.

FUTURE USES OF STAINLESS STEEL
Strong growth in the use of stainless steel has continued in the past decades despite the rapid and diverse developments in other materials and the more recent economic turmoil. The nickel-containing alloys in the 300 series still account for nearly two thirds of current stainless steel production worldwide, and there is nickel in the 200 series, duplex and precipitation hardening families, as well as in some of the martensitic and ferritic alloys. The reason for this is the great value that is placed on the properties which nickel provides.

Society is rapidly evolving and facing challenges on a global scale. Population is increasing, expectations are growing and resources are limited. Therefore we must use those resources more efficiently. This is particularly apparent for energy where stainless steel, and especially the nickel-containing alloys, already plays a major role in the more difficult to extract fossil fuels. Stainless steel’s corrosion and heat resisting properties are key to more cost-efficient operations. This also applies to the renewable sources that are now being developed, such as wave power and biofuels from new organic sources.

The worldwide need for higher quality, safe food and beverages and water will only increase, especially as food products can come from anywhere in the world. Stainless steel has evolved as the material of choice in this industry, both industrially and domestically, and it is likely to continue to meet the demands of a global population that is predicted to increase to nine billion by 2050.

This growing population, combined with a rapid movement to urbanisation, requires an expanded and more efficient transport infrastructure. The characteristics of stainless steel enable it to deliver lightweight and durable designs, leading to more efficient performance, safety, lower energy requirements and reduced emissions while giving lower life-cycle costs.

Image of Trump Tower (Chicago, USA) pictured above courtesy of C.Houska.

This article is featured in Australian Stainless magazine, issue 52.

Stainless = Freshness

As bottled water continues to gain popularity in Australia, maintaining the quality and purity of the water extracted from natural springs is paramount.

This is just one example within the food and beverage sector where hygiene is vitally important and, therefore, stainless steel continues to be the material of choice for processing and storage facilities.

In 2011, Coca-Cola Amatil (CCA) commissioned ‘Project Flint’ to upgrade three spring water storage tanks for their Moorabbin plant in Victoria plus an additional two tanks for their Thebarton plant in South Australia.

GEA Process Engineering Australia engaged Byford Equipment on behalf of CCA to fabricate and install the five storage tanks.

GEA Engineering’s General Manager Operations, Andrew Fillery, said stainless steel was an important specification as the tanks had to cope with the chemical and thermal rigours of cleaning processes.

“Stainless steel was chosen for process and hygienic reasons, and the vessels needed to withstand the process and cleaning conditions where mild caustic and acid CIP solutions were used,” said Fillery.

Strength and durability was key for the 200,000L capacity silos, which measured 4.7m in diameter by 14.5m high for the Moorabbin site and 5.5m in diameter by 10m high for the Thebarton plant.

ASSDA Sponsor Midway Metals supplied 27 tonnes of grade 304 stainless steel coil with a 2B finish in 2mm, 2.5mm, 3mm and 4mm thicknesses. The coil widths were 1219mm and 1500mm.

With a team of five fabricators on the project, the tanks were welded together using a semi-automatic MIG welding process. The welds were then pickled to restore the chromium oxide layer and abstain from rusting.

Byford Equipment’s Project Manager Geoff Smallwood said coordinating the delivery of the tanks was a challenge, given the logistics of travelling through three states by road.

The delivery of the vessels was critical added Fillery, as there were specific installation windows to work within.

The storage tanks were delivered from Byford’s workshop in New South Wales to Moorabbin in March 2011. The two remaining tanks were delivered to Thebarton a month later for installation. It took one day and one crane to install each tank on site.

The connecting pipework was positioned on site, which was grade 304 polished tube in diameters ranging from 38mm to 150mm and purge welded prior to installation.

Images courtesy of Byford Equipment.

This article is featured in Australian Stainless magazine, issue 51.

Brewery to Excel with Local Fabrication

A worrying trend among Australia's major resource companies is the increasing amount of engineering, detailing and fabrication work being sent offshore - a move that has had significant impact on local fabrication. But there are some positive signs in the food and beverage sector that local fabricators are more than capable of meeting design and fabrication expectations.

When ASSDA member and Accredited Fabricator, A&G Engineering, put in a bid to build 10 x 100 hectolitre beer fermenters for Casella Estate - a company best known for their Yellowtail wine label - they had to compete against companies as far away as Europe for the coveted project.

But A&G had a few advantages over the offshore companies: they had worked with Casella before, fabricating 88 x 1.1 million litre wine tanks for the company’s tank farm in Yenda, NSW; they have supplied stainless steel tanks to Australia’s leading breweries, wineries and beverage companies; and they are one of the largest users of stainless steel in Australia.

A&G’s win is an important victory for the Australian industry as a whole and another milestone for A&G Engineering, which was founded in 1963.

The five-month Casella Brewery project, completed in August 2011, saw 25 of A&G’s 200 staff use 65 tonnes of 304 grade stainless steel (including 2-4mm coil and 8mm plate) to build the 10 vessels.

A&G’s Design Manager Heath Woodland said the tanks were designed to AS1210-2010 pressure vessel standards, in order to withstand a pressure rating of 115kPa.

The stainless was welded with A&G’s semi-automated welding process and the internal welds were polished to achieve a 0.6Ra surface finish, to meet beverage industry standards of a food grade finish.

A&G built the vessels at their Griffith and Irymple plants, before transporting them to Yenda. With the beer fermenters now in place, it is hoped the Casella Brewery will be operational by the end of 2011.

Images courtesy of A&G Engineering.

This article is featured in Australian Stainless magazine - Issue 50, Summer 2011/12.

Specifying Quality

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.

Stunning Little Creatures
ASSDA Accredited Member TFG Austline Pty Ltd was recently entrusted to install a new brewhouse and fermentation cellar for Little Creatures Brewery in Fremantle, Western Australia.

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.

Pipes run half marathon

That deserves a drink!

A joint venture between two ASSDA Accredited Fabricators 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.

Bluetongue Brewery 3The $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.

Bluetongue Brewery 2“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.

Grade 431

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.
Grade 431_Table 1

 

 

 

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”.

Grade 431_Table 2

 

 

 

 

 

 

 

 

 

 

 

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.

Grade 431_Figure 1

Physical Properties

Density

7700kg/m3

Elastic Modulus

200GPa

Thermal Expansion (0-100°C)

10.2µm/m/°

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.

Stunning stainless

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

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.