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Coated Abrasives for Surface Finishing - Part 3

Our three-part series on coated abrasives concludes with information on choosing the correct abrasive product for the desired finish. Read Part 1. Read Part 2.

THE IMPORTANCE OF SURFACE ROUGHNESS

The surface roughness of stainless steel is an important factor in determining corrosion resistance. Put simply, the smoother the finish the greater the corrosion resistance, whether in the form of sheet or coil or in welded components.

SURFACE TOPOGRAPHY

Interaction between the abrasive belt and the workpiece is affected by surface topography (micro texture). Even a surface which appears perfectly flat to the naked eye has ‘asperities’, undulations between 0.05 μm and 50μm occurring 0.5μm to 5 mm apart.

A variety of instruments are available to measure surface micro texture. They work on the principle of moving a stylus over a representative length of the surface and recording the peaks and valleys.

In Australia, surface roughness is expressed in Ra. The measurement refers to the average variations of the undulations from the average surface of the sample.

Current density & surface roughnessTYPICAL FINISHES

Three stainless steel surface finishes are typically available from the mill:

  • #1 hot rolled, annealed and pickled (Ra 3 to 6μm)
  • 2B cold rolled (Ra 0.1 to 0.2μm)
  • BA bright annealed (Ra 0.06 to 0.2μm)

From these initial surfaces, a wide variety of finishes can be achieved with coated abrasives, satinising wheels and mops, buffing wheels and polishes. The type of finish generated depends on many variables: grit sequence, lubrication, raw material quality, machine type, abrasive type, pressure applied, through feed speed, abrasive belt speed and so on.

Because of all these factors, nominally identical finishes vary slightly from one producer to another. To ensure that the desired finish is delivered, specifiers should nominate the acceptable Ra (surface roughness) range and any other factors necessary for the application (for instance viewing angle or light conditions for architectural samples).

The common ASTM designations for stainless steel surfaces such as ‘No. 4’ specify a process to achieve a finish and not attributes of the surface itself. The result can fall outside the desired
surface roughness range. The Euronorm finishes of EN 10088, provide a larger number of specifications than ASTM A480 and include some which require particular Ra values.1

Although the measurements involved are microscopic, research indicates there is significantly higher resistance to corrosion in stainless surfaces with a roughness below 0.5μm Ra.

THE ROLE OF COATED ABRASIVES

Technically advanced coated abrasives are designed to optimise production by delivering consistent, measurable surface finishes. However, the operator must select the correct abrasives and the right product sequence.

Abrasive grainPolishing is hard work and even with machine operations, it takes time and care. The absolutely essential element is to remove all the polish lines from the previous stage before moving on to a finer grit. If this isn’t done, the final and finest buffing step will be marred by a streak on the surface. Although it is often impossible to rotate the work, removal of polish lines is readily
monitored by polishing at right angles for each new step.

The first grinding step should be as fine as possible. As #80 is usually the finest practicable size, it may require some time to smooth a large weld bead. An effective grit sequence for producing a mirror finish is 80, 120, 240, 320, 400, 600 and 800 before proceeding to mops.

Steps can be missed but at the cost of longer polishing times and the risk of stray scratches. Old abrasives will give a smoother finish but the results are less predictable and are operator dependent.

Lubricants may be necessary because of the poor conductivity of stainless steel. Lubricants also remove debris, improve the quality of the finish and increase the abrasive life. When buying abrasives it is important to choose a reputable product; an unknown quality could mean stray, coarse grit with its attendant final streaks which will mar the result and be especially obvious on a ‘mirror’ finish.

When silicon carbide abrasives are used a brighter, more highly reflective finish results, albeit at the expense of belt life. Other materials, for example aluminium oxide, zirconia or ceramic
grains, will give a significantly longer belt life but will produce a different overall finish.

A quality coated abrasive belt acts as a series of single point cutting tools. Each grain has the optimum shape and angle to accomplish the cutting action and subsequent chip removal. This is partly achieved through electrostatically orienting the grains in relation to the backing during manufacture. The rest comes from choosing the correct abrasive type for the job. A cutting facet which isn’t sharp enough results in random streaks on the stainless steel surface as the grain fails to cut cleanly and drags a chip along the surface. This effect is more prevalent with aluminium oxide belts.

Roughness vs Abrasive Grit Size*

Grit
Ra(μm)
500# 0.10 - 0.25
320# 0.15 - 0.22
240# 0.30 - 0.67
180# 0.42 - 0.96
120# 0.29 - 0.81
60# 2.01

GETTING IT RIGHT

Supplying the desired surface is as much a part of filling a contract as other aspects of fabrication. There are a large number of variables which impact on the surface finish. The previous two issues of Australian Stainless presented an overview of modern coated abrasives and specific information on their composition and manufacture (issues 22 and 23). Data on the hardness of abrasive materials and a relative comparison of grit size was included.

An understanding of what makes a quality abrasive product and how coated abrasives interact with the workpiece helps ensure that the corrosion resistance and aesthetic requirements of
the client are met.

1. EN 10088-2 : 1995 specifies additional specific requirements to a ‘J’-type finish, in order to achieve adequate corrosion resistance for marine and external architectural applications. Transverse Ra < 0.5μm with clean cut surface finish.

This article featured in Australian Stainless Issue 24 - March 2003. It was written by Charles E. Fenton, Managing Director of Klingspor Abrasive Technologies, Australia and Graham Sussex, ASSDA’s technical specialist.

"Dancing Wall" - Colour & Movement in Stainless

To symbolise the wetlands landscape of the Nundah area in Brisbane’s north, sculptor Daniel Della Bosca sought out materials which best convey the fluidity and reflectivity of water and the reedy texture of waterside vegetation.

His choice was 316 stainless steel, finished with specialised surface treatments, combined with translucent blue glass and earthed in basalt.

“Dancing Wall” was commissioned by Brisbane City Council (BCC) as part of its program of Suburban Centre Improvement Projects (SCIPs) which aim to improve economic vitality, focus on community life and enrich local activity. The Nundah SCIP is one of the larger projects in the scheme with a budget of $2.5m.

The artwork is a sculptural balustrade set on the hilltop at the corner of Buckland Street and Sandgate Road. Its design symbolises the local environment which was once rich in waterholes and is now the focus of BCC and Wildlife Preservation Society rehabilitation initiatives.

According to Della Bosca, the piece is not just about the past: the materials and design provide an inspirational link to the future.

The client has expressed satisfaction with the completed project, with Deputy Mayor Councillor Quinn commenting that it fulfills the Council’s objectives of “good design and creative activity to build a prosperous city.”

FABRICATION

“Dancing Wall” was fabricated by Della Bosca in grade 316 stainless plate, flat bar and rod supplied by ASSDA member Austral Wright Metals. It houses five panels of slumped and toughened ‘azurelite’ glass, made by artist Shar Moorman, internally illuminated by concealed LED lighting.

Most of the structure was fabricated from rolled stock to bring an organic quality to the design. The intricate forming was carried out by local firm BJR Metal Rolling & Pressing who specialise in rolling compound curves.

SURFACE FINISH

Integral to the design are the surface treatments which suggest reed and water textures. ASSDA member Australian Industrial Abrasives helped to investigate the products, appropriate tooling and techniques to achieve the desired effects. The finishing on larger areas was completed with a Dynacushion on a variable speed sander polisher, using abrasive belts in a range from P80 to P150 Zirconia/Alox and finishing with 3M Blue Scotchbrite. The tighter, more intricate areas were finished using a Dynafile and various contact arms and the same range of abrasive belts.

An easily achievable, cost-effective maintenance schedule using an activated surfactant cleaner quarterly and a passivation gel as required has been implemented by Brisbane City Council.

The cleaning agent removes oil, grease and dirt, and also removes surface free iron which may cause discolouration or more serious corrosion.

This step is followed by a passivation gel which chemically generates the chrome oxide passive film on the surface to enhance corrosion resistance for stainless steel installed in high corrosion environments.

ARTISTIC POSSIBILITIES

Della Bosca says the qualities of stainless steel can best be conveyed by allowing the material to interact with light. “As fabricators well know it is easy to ‘muddy’ the surface of stainless, but if care is taken and correct procedures followed, the metal can give opportunities to a surface finisher.

“I work with the stainless to allow it to speak of more than itself. This is much more important to me than trying to force a finish.”

This article featured in Australian Stainless Issue 24 - March 2003.

Council Solves Fishy Problem with Stainless

Coastal areas are popular sites for recreational fun and fantastic fishing. However, the City of Albany in Western Australia had one major problem to deal with - fish waste in the nearby waterways 

Local fishermen were cleaning and filleting fish and disposing of the waste overboard. This waste not only stagnated in the water for days, but also attracted seals and stingrays that can become aggressive when feeding.

Faced with a situation of replacing what was a kitchen sink on rusty legs in the water, the Council turned to local ASSDA member, Austenitic Steel Products, to design and fabricate an innovative new stainless steel fish cleaning station for the Emu Point Boat Ramp.

The circular fish cleaning station is believed to be the first and only one of its design currently available in Australia and measures 1400mm in diameter and 1100mm high.

Produced in 316 stainless, the compact design allows six operators at a time and provides a safe environment with no corners or sharp edges.

All plumbing is internal with access only through a hatch on the face of the cone and fitted security locks. Waste water falls to the centre of the table and flows through a circular screen into a collection hopper before entering a two inch waste pipe concealed in the centre.

Initially, the station requires manual removal of offal, but when funding and municipal sewerage is available, the table can be modified to incorporate an automatic processor to pulverise offal into disposable liquid waste.

Albany City's $13,000 station project has generated interest from other Councils and looks set to appear in local boat launching areas, coastal caravan parks and seaside fishing locations throughout Australia.

This article featured in Australian Stainless magazine - Issue 26, November 2003.

Chemical Surface Treatments

Successfully using stainless steel depends on environment, grade selected, surface finish, the expectations of the customer and the maintenance specified.

Stainless steels provide robust solutions, but in harsh or borderline environments with high expectations for durability, surface finish will have a substantial impact on performance. Surface finishes can be applied mechanically (usually with abrasives) and chemically.

Understanding how chemical and mechanical treatments will affect the characteristics of the surface and will enable the best possible outcome for the client and the structure. Chemical treatment can be used to improve the corrosion performance of the steel, and hence its appearance in service.

 

Stainless steels resist corrosion best if they are clean and smooth. Clean means being free of contaminants on or in the surface that can either react with the steel (like carbon steel or salt) or that create crevices or other initiation points where corrosion can start.

Smooth means having a low surface area at the 'micro' level. Mechanically abrading the surface can roughen the steel's surface and may also embed unwanted particles.

The common feature of chemical treatments is that they all clean the surface of the steel. They may also smooth or roughen the steel surface, or leave it unaffected depending on which process is chosen. But if carried out properly, they all increase the corrosion resistance.

Corrosion resistance improves as you go to the right of this graph. The graph shows the relative importance of the smoothness of the surface and chemical treatment of the surface. They can be used together to get the best corrosion resistance.

Corrosion resistance improves as you go to the right of this graph. The graph shows the relative importance of the smoothness of the surface and chemical treatment of the surface. They can be used together to get the best corrosion resistance. The study reported by G. Coates (Materials Performance - August 1990) looked at the effect of various methods of treating an artificial welding heat tint on grade 316, 2B surface.

Stainless Steel Products
During steel making, sulphur in the steel is controlled to very low levels. But even at these levels sulphide particles are left in the steel, and can become points of corrosion attack. This 'achilles heel' can be improved greatly by chemical surface treatment.

Most bar products will be slightly higher in sulphur when produced, so chemical treatment to remove inclusions in the surface of these products becomes more important.

Generally mill finishes for flat products (sheet, plate and strip) will be smoother as their thickness decreases.

A No 1 finish on a thick plate may have dimples or other imperfections and a surface roughness of 5 to 6 micrometres Ra.

A typical 2B cold rolled finish on 1.7mm thick sheet might have a surface roughness of 0.2 micrometres Ra or better as shown in Mill Forms.

New surfaces will be created during fabrication processes, (eg cutting, bending, welding and polishing). The corrosion performance of the new surfaces will generally be lower than the mill supplied product because the surface is rougher, or sulphide inclusions sitting just under the surface have been exposed or mild steel tooling contamination may have occurred.

Chemical treatments correctly performed can clean the surface and ensure the best possible corrosion performance.

Chemical surface treatments can be grouped into four categories:

  • Pickling - acids that remove impurities (including high temperature scale from welding or heat treatment) and etch the steel surface. 'Pickling' means some of the stainless steel surface is removed.
  • Passivation - oxidising acids or chemicals which remove impurities and enhance the chromium level on the surface.
  • Chelating agents are chemicals that can remove surface contaminants.
  • Electropolishing - electrochemical treatments that remove impurities and have the added beneficial effect of smoothing and brightening the surfaces.

Pickling
Mixtures of hydrofluoric (HF) and nitric acid are the most common and are generally the most effective. Acids are available as a bath, a gel or a paste.

Commercially available mixtures contain up to about 25% nitric acid and 8% hydrofluoric acid. These chemicals etch the stainless steel which can roughen and dull the surface.

Care is required with all these chemicals because of both occupational health and safety and environmental considerations. HF is a Schedule 7 poison which has implications for sale or use in most states. See ASSDA's Technical Bulletin on this subject.

Passivation
Nitric acid is most commonly used for this purpose. Passivation treatments are available as a bath, a gel or a paste. Available formulations contain up to about 50% nitric acid and may also contain other oxidisers such as sodium dichromate. Used correctly, a nitric acid treatment should not affect the appearance of the steel although mirror polished surfaces should be tested first.

Passivation works by dissolving any carbon steel contamination from the surface of the stainless steel, and by dissolving out sulphide inclusions breaking the surface.

Nitric acid may also enrich the proportion of chromium at the surface - some chelants are also claimed to do this.

Pickling and passivation: before treatment of fuel tanks for storing helicopter fuel on ships. Pickling and passivation (L-R): after treatment of fuel tanks for storing helicopter fuel on ships.
Pickling and passivation (L-R): before and after treatment of fuel tanks for storing helicopter fuel on ships. Photos courtesy of Alloy Engineers and MME Surface Finishing.

Chelants
Chelants have chemical 'claws' designed to selectively clean the surface.

The carboxylic acid group COOH is the basis for many chelants which are used in cleaners, water softening and lubricants. The pH and temperature must be correct for the chelant to do its job. Turbulent rinsing of pipes and vessels afterwards is important.

Cleaning by chelating agents tends to be based on proprietary knowledge and systems, and is less standardised than the other methods described.

The successful use of these systems needs to be established on a case by case basis.

Electropolishing
Most commonly phosphoric and sulphuric acids are used in conjunction with a high current density to clean and smooth (by metal removal) the surface of the steel.

The process preferentially attacks peaks and rounds valleys on the surface and raises the proportion of chromium at the surface.

The technique can have substantial effect on the appearance increasing lustre and brightness while only changing the measured roughness by about 30%.

Precautions
For chemical processes that etch the stainless steel, reaction times will increase with increasing grade.

More care is required with 'free machining' grades and these will usually require substantially less aggressive chemicals. The sulphur addition in these steels makes them readily attacked by chemical treatments. Care is also required when treating martensitic or low chromium ferritic stainless steels.

Detailed recommendations for each grade of stainless steel are given below.

The four categories of treatment are detailed in a number of Standards, but the most commonly used are:

  • ASTM A380 Cleaning, Descaling and Passivation of Stainless Steel Parts, Equipment and Systems.
  • ASTM A967 Chemical Passivation Treatments for Stainless Steel Parts.
  • ASTM B912 Passivation of Stainless Steels using Electropolishing.

These very useful documents give detailed recommendations on many aspects of selection, application and evaluation of these treatments. Highly recommended reading.

Dirt and grease will mask the surface from treatments outlined above. Therefore, the steel surfaces must be free of these agents before applying chemical treatments.

Many of the chemical treatments described contain strong acids. Before disposal they will require neutralisation. Check with your local authority concerning the requirements for trade waste, neutralisation and disposal.

Many of the chemicals described above will be classified as hazardous substances under State OHS legislation, with implications for purchasing, transport, storage and handling.

Chemical treatments are useful tools in cost effectively achieving peak performance with stainless steels. With appropriate training, hazards associated with their use can be managed.

This technical article featured in Australian Stainless magazine - Issue 26, November 2003.

Specifying Stainless for Suncorp Stadium

PDT Architects, Brisbane in association with HOK Sport, Venue and Event designed and documented the $280m Suncorp Stadium for the Queensland Government.

"It is the top grade material and that's what we wanted for the stadium...

When you take that the client is the people of Queensland, they expect the best to go into the stadium.

If they're paying $280 million for a stadium they want the thing to last and stainless will give you that longevity."

 

Director, John Brown of PDT Architects described the pros and cons of specifying stainless steel for the redevelopment of Lang Park (Suncorp Stadium).

Ticket Counters
When PDT Architects expressed concerns by Ticketek staff that stainless steel ticket counters would be too glary, ASSDA provided the necessary technical assistance required and prevented a switch to timber material.

JB: The client talked about changing from stainless to timber because Ticketek and others had commented that stainless steel would be too glary.

We found that we could use a brushed finish which wouldn’t throw up too much glare and we also did some sun shade diagrams and all sorts of diagrams to make sure the sun wasn’t hitting the ticket counters.

Full Stainless: Commercial Kitchens
JB: It was always going to be stainless steel, mainly because of the health reasons ... but also for cleaniness and ease of cleaning.

All the commercial kitchens at the stadium have them. All the finishing kitchens, all the food outlets, beverage and of course the main kitchen. All the food is prepared in that kitchen and taken up through the service lifts into the finishing kitchens, warming kitchens and then out into the public.

Stainless Wire Rope for Visibility
PDT Architects has recommended replacing the stadium’s flat bar rails with stainless steel wire rope to improve visibility.

JB: We've been able to convince the State Government code people that we can take a lot of the galvanising rails out and put in stainless steel wire rope which would give a better view.

We’ve put in a report to the Government whereby we can cut out a lot of these flat bars and put in stainless steel wire rope through them which will just open the whole place up.

Stadium Stainless Statistics
ASSDA member Fagersta supplied 60 tonnes of stainless steel coil of various widths and thicknesses to ASSDA member Tom Stoddart. A further 10 tonnes of stainless steel was supplied to Eziform for gutters and box gutters.

Tom Stoddart produced works at a cost of $11 million and approximately 50,000 man hours including full kitchen and bar fitouts, refrigeration units, cash register stations balustrading and some food service equipment.

Fagersta was also the main supplier of stainless steel for Colonial Stadium in Melbourne and Telstra Stadium in Sydney.

The Post Game Wrap-Up
JB: Firstly, it satisfied us as far as an architectural feature went, it satisfied us as far as life cycle went and most certainly it then satisfied us as far as safety issues went.

This article featured in Australian Stainless magazine - Issue 26, November 2003.

Stainless Bathing Box of Shattered Dreams

Tamarama Beach or as local Sydneysiders call it 'Glamarama Beach', is well known as the place for beautiful people to be seen and for the rest of us to dream.

 

Artist, Graham Chalcroft set out to distort self-image in the name of fun by capitalising on the beach's former fairground history to design and fabricate a sculpture - 'Bathing Box: the impossible change room of shattered dreams'.

The stainless steel sculpture was a crowd favourite at the 'Sculpture by the Sea' exhibition held in November 2003 from Bondi Beach to Tamarama Beach in Sydney NSW.

The sculpture's design aesthetic was inspired by archival images of traditional late 19th century bathing boxes and how they have evolved.

Each sculpture profile is based on the curves of the human form - one male and one female. On approaching the convex/concave doors, the mirror distorts the body with startling effects.

ASSDA member, Stainless Sections, kindly donated six sheets of 304 with a No. 8 mirror finish for profiles fitted inside the doorframes.

For accuracy and efficiency, Chalcroft use T & M Engineering Group in Smithfield to computer design and laser cut the sheets to be clamped on the structural wood faces and strips.

"The stainless steel performed extremely well. It handled well in fabrication and the thick protective coating saved the steel from potential scratching," said Graham Chalcroft.

"The mirrored steel coped with the heat, wind, salt spray and suffered no scratching from the many thousands of hands touching the artwork," he said.

'Sculpture by the Sea' is in its seventh year and is the largest free outdoors sculpture exhibition in the worlds with over 200,000 visitors.

Photos by Graham Chalcroft.

This article featured in Australian Stainless magazine - Issue 27, February 2004.

Restoring Stainless

Brewed for Efficiency

When Carlton & United Breweries (CUB) announced plans last year to expand production capacity at the Yatala site in Queensland, the company considered both purchase of new stainless tankage as well as transfer of tanks from its Sydney brewery and re-use of second hand tanks in its inventory.

 Six such vessels had been stored by CUB on leased land in the City of Redcliffe, unsealed and exposed to the weather.

Constructed in 321 stainless, the 3 x 12 metre tanks were out of service for the last ten years and placed in the yard for future use.

The six beer storage tanks are part of a series of 16 tanks built in 1966, that have proven the test of time and reinforced the material's durability and returned back to service 'as good as new'.

ASSDA member, D & R Stainless was approached to upgrade the six 1,000 hectalitre (hl) vessels to the new fittings with design verification and tested to meet AS 1220 Standard.

The Brisbane-based fabricator modified, checked and tested the beer storage tanks so as to meet CUB's exacting standards for beer production which would include popular CUB brands such as VB, Fosters Lager, Carlton Midstrength and Crown Lager, allowing them to be used as part of the ongoing expansion.

Work on the six tanks involved cleaning down and refurbishing with new stainless steel tubes and fittings supplied by Matrix Process Solutions.

The six tanks were taken to D & R Stainless and pumped with 4,000 litres of water constantly through the system and back through the CIP spray balls so that all of the tank area was wet to see if there was any carbon steel pick up.

All tanks were checked thoroughly for any rust spots and repolished where required. New nozzles were also installed and internally polished.

D & R Stainless Manager, Ray Powell was initially concerned that the carbon steel supporting rings around the vessel would bleed through onto the stainless as there were no compensating plates.

"We only had one tank [the first tank] that had a problem and that was laminating in the heat effected zones. And that was only on one tank on the bottom of the seams, so we came back in and dye checked, found the problems, ground back and repaired," Mr Powell said.

"The quality of the tank is in good condition. The beauty of stainless is that if it's looked after it will last forever and proof's in the pudding, they're coming up nearly 40 years old - they've certainly proved the test of time," he said.

The expansion of CUB's Yatala facility follows an announcement in April last year of the closure of the Kent Brewery in NSW by February 2005.

This operational review will see the relocation of over 50 beer fermentation and storage tanks from Sydney, a brand new twin stream brewhouse and new filtration equipment to the Queensland facility, which is CUB's most efficient brewery. Total project cost is around $170 million.

As a result of the review, the Yatala facility will double its capacity from 230 million litres of beer a year to over 500 million litres.

Despite being out of service for more than a decade, the excellent condition of the tanks ensures that stainless steel has significantly contributed to product quality and cost efficiency for Carlton & United Breweries long into the future.

This article featured in Australian Stainless magazine - Issue 27, February 2004.

Threaded Fittings to ISO 4144 Standard

For many years there has not been a Standard to cover the low pressure stainless steel cast pipe fittings commonly used in Australia and other countries around the world.

These are commonly termed “150lb” or “BSP” pipe fittings. In most cases the fittings that have been supplied were a mismatch of various Standards.

The fittings were dimensionally in accordance with a number of American Standards, whilst British Standard threads were used. This led to the fittings having threads that in some cases were non-compliant - basically there was insufficient length to accommodate the thread.

ASSDA, through its Technical Committee, identified this problem in the early 90s and through the publication of ASSDA’s Technical Bulletin No 1, highlighted the problems to the Australian market. ASSDA also looked for a mechanism to have these problems rectified.

ISO 4144 "Pipework - Stainless Steel Fittings Threaded in Accordance with ISO 7-1"
After investigating the alternatives it was decided that International Standard ISO 4144 could be the conduit to rectifying the problems. ISO 4144 in its 1979 form covered most of the committee’s concerns, but it did not allow for cast fittings - only wrought stainless steel.

After correspondence with the Australian and International Standard bodies, it was ascertained that ISO 4144 was due for revision, which presented a golden opportunity to have the standard rewritten to cover all of the Technical Committee’s concerns.

ASSDA was invited to represent Australia on the committee established to review the Standard and actively took part in the full process of its revision. Not all of the Committee’s recommendations were accepted. Finally, in early 2003 the new Standard was published.

What has been achieved?
The major improvements that have been adopted in the new Standard are:

a.    The use of castings as well as wrought materials.
b.    All cast fittings are to be properly heat-treated by solution annealing.
c.    The reduction in dimension, a more economical fitting.
d.    The thread standards allowed have been clearly defined.
e.    An introduction of pressure-temperature ratings for application of the fittings.
f.    The inclusion of eight new types of fittings into the Standard.
g.    The inclusion of DN 100 (4”) fittings.

Now that ISO 4144:2003 allows for the use of castings, Australia finally has a Standard that covers the products that have been in common use for many years.

The requirement that all castings are to be fully heat-treated will alleviate some of the corrosion problems that have been encountered in the past.

The dimensions of the fittings have been revised dramatically, thus giving a lighter and more economical fitting.

The wall thickness is the major dimension that has been reduced and it can be reduced by a further 20% if the fitting is made from wrought material.

ISO 7-1 sealing pipe threads are to be used on all fittings. The external and internal threads are to be tapered, but the internal threads may be parallel. The only exception to this is the threads on the Unions and their mating nut, which are allowed to have a variety of parallel threads.

Pressure temperature ratings for application of fittings have been specified (refer to Table 1).

Table 1 - Pressure-Temperature Rating

ome fitting types supplied into Australia were not covered in the old ISO 4144 Standard.

Eight new types have been included in the new Standard: these are 90° Reducing Female Elbows, Reducing Female Tees, 45° Equal Female Elbows, 90° Male x Female Elbows, Crosses, Reducing Nipples, Male x Female Unions and Male x Male Unions.

With the inclusion of the DN 100 (4”) fittings, the Standard now has a comprehensive range of products.

Some Disappointments with the New Standard
In the new Standard, apart from some minor editorial errors, there are two points of concern to the ASSDA Technical Committee.

Firstly, the new wall thicknesses that are stated as minimum could lead to a product being supplied that may not meet the expectations of the customer.

Even though the Standard allows for thin wall product, such thin walled fittings could be subject to distortion during the threading process or during installation. Care must be taken that this does not occur.

The second concern within the Standard is the length of the minimum external thread that has been adopted. The title of ISO 7-1 is “pipe threads where pressure tight joints are made on the threads”.

The minimum length specified can accommodate a thread that seals if it is manufactured to close tolerance. Care is required in manufacture to achieve this outcome.

Although it was recommended to the International Committee that it accept external thread lengths that could accommodate a thread at both ends of the tolerance range, the Committee did not adopt these recommendations.

Table 2 highlights the external thread lengths that were adopted compared to the external thread lengths that were recommended by Australia.

Table 2 - External Thread Length Comparisons

Conclusion
ASSDA believes minimum thread length is a concern to all suppliers and users of these fittings and care should be taken in their selection.

If mating fittings do not seal on the threads and interfere with the washout they may leak.

It is recommended that fittings should only be sourced from reputable and experienced manufacturers and supplied to ISO 4144:2003.

Overall, the Standard is a considerable improvement on what was available, and with care in the selection, the end user will be in a more certain and much safer environment than in the past.

Credits
This article was written by Kim Burton, Group Supply Manager of Prochem Pipeline Products Pty Ltd and an ASSDA Technical Committee member. ASSDA also acknowledges the contribution of Technical Committee member Peter Moore, Technical Services Manager of Atlas Specialty Metals, in the development of this article.

Download Technical Bulletin (April 1997 - pdf 920k)

This technical article featured in Australian Stainless magazine - Issue 27, February 2004.

Cleaning and Protecting Stainless Sculptures

"Windhover" is a dramatic stainless steel sculpture created by the late Lenton Parr, located on the eastern foreshore of Port Phillip Bay in Sandringham, Melbourne.

Unveiled in December 2001, the sculpture's vertical lines and arcs are evocative of the yachts often seen sailing out on the Bay.

However, two and a half years of zero maintenance and exposure to salt spray from the bay have taken their toll, turning the surface of the stainless steel a blotchy brown.

Called tea staining, it's caused by deposition of salt on the surface which is then trapped in the crevices of the brushed finish.

Regular reactivation by rain has perpetuated a corrosion cycle leading to quite rapid and severe surface staining.

The problem was how to clean the sculpture and then to ensure that it would remain protected from tea staining in the future.

Conventional weld pickling products containing hydrofluoric acid are very aggressive and risk damage to or discolouration of the surface.

Strong acids may also create an environmental and safety hazard when used in such a public place.

Many cleaning formulations are available based on phosphoric, sulphamic, oxalic or nitric acids. They have various degrees of handling and disposal restrictions.

The formulations may also contain mid abrasives and wetting agents/detergents to aid the cleaning process.

In July 2004, ASSDA Member, Revolution Advanced Metals and Materials, used a cleaning paste based on a moderate concentration of phosphoric acid which is relatively safe to handle.

Inadvertent skin contact by this product does not cause the burning and possible ulceration associated with strong concentrations of nitric and hydrofluoric acid preparations.

The cleaning product was brushed on and left to react for 3-4 hours. The brown tea staining gradually disappeared.

In some particularly bad sections a second application was necessary to completely remove all traces of the staining, but it left a completely blemish-free surface.

In this case, residue from the cleaning product was simply washed away with water. In other cases, however, check with local authorities for correct disposal procedures.

One of the problems when washing stainless steel with water is the streaking caused by uneven drying.

This was very noticeable on the sculpture.

Also, because it is unlikely that ongoing regular cleaning will occur, it is also important to limit the access of chlorides to the surface. Otherwise the staining problem will recur.

To overcome both these problems, a water-based protective product with oils and non-ionic surfactants but no phosphates was sprayed on and wiped over.

After polishing with a dry cloth all streaking vanished. It left an invisible film that stopped further streaking and fingermarks.

Best of all, it brought up the lustre of the brushed finish, and left Windhover looking as good as the day it was made.

Regular re-application should maintain the finish and help prevent tea staining in future.

Correct design, fabrication and on-going maintenance will all assist in keeping stainless steel sculptures and other structures erected adjacent to the coast in good condition.

Words and images courtesy of Jim Picot, Revolution Advanced Metals & Materials.

This article featured in Australian Stainless magazine - Issue 29, September 2004.

Crevices and Corrosion

A crevice is a narrow gap between a piece of metal and another piece of metal or tightly adhering material like plastic or a film of bacterial growth.

Many metals and alloys are susceptible to crevice corrosion, but in stainless steel, crevices are the first and most common place for corrosive attack to begin. With a little understanding, crevice corrosion can either be avoided or minimised.

Crevices can be:
• The space under a washer or bolt head.
• The gap between plates bolted together.
• The gap between components intermittently welded.
• The space under a sticky label.
• The space between a gasket and the metal in a flange (especially if the gasket is absorbent).
• Any other tight gap.

Crevices can be designed into the structure, they can be created during fabrication or can occur during service.

Prevention measures should therefore also aim at design, fabrication and service.

Why crevices can corrode
To work at its best, stainless needs free access to oxygen. Crevices are wide enough to permit entry of moisture, but narrow enough to prevent free circulation.

The result is that the oxygen in the moisture is used up. In addition, if chlorides are present they will concentrate in the stagnant conditions and, by a combination of reactions, the moisture can become acidic.

These are all conditions that can lead to the breakdown of the passive film on the stainless. Attack can then progress rapidly.

Crevices can create conditions much more aggressive than on adjacent surfaces. Having crevices builds in weak spots where attack can begin and begin in much less severe conditions than anticipated for the remainder of the structure.

Table one shows laboratory measurements of critical temperatures needed to cause pitting on an open surface (CPT) and crevice (CCT) attack of a metal plate beneath a PTFE washer in a 10% ferric chloride solution.

The CCT is at least 20˚C lower than the temperature to cause pitting corrosion in this aggressive liquid. (Ferric chloride solution is an aggressive corrodent and is used because it is similar to the liquid in a pit when it is actively corroding.)

Factors influencing crevices:

Crevice Shape
The geometry of the crevice will influence its susceptibility to attack and the speed of progress. The narrower and deeper (relative to its width) a crevice is the worse attack will be.

Metal to flexible plastic crevices tend to be narrower than rigid metal to metal gaps so metal to plastic joints provide more aggressive crevices.

Environment
The more aggressive the liquid outside the crevice, the more likely it is that the crevice will be attacked.

This is why crevice attack can be a problem in a salty swimming pool but not in a fresh water tank.

In the atmosphere, crevices beside the sea give more problems than in rural environments. If the liquid outside the crevice is very oxidising, eg with bleach, hydrogen peroxide or ozone, then crevice attack will tend to be more severe.

Temperature
Once the CCT is exceeded, then as with pitting corrosion, higher temperatures mean corrosion is more rapid. The rule of thumb is that a 10˚C rise in temperature will double the corrosion rate.

This means that when comparing Far North Queensland to Tasmania, not only are crevices more likely to start corroding but also that once they do, they will corrode faster because the temperature is consistently higher.

Alloy resistance
Using a more corrosion resistant alloy gives less crevice attack. For example, in seawater at ambient temperature, crevices will form on 304 if there is a 0.9mm gap, on 316 if there is a 0.4mm gap and on 904L (similar corrosion resistance to 2205) if there is a 0.15mm gap.

Minimising the risk of crevice corrosion

Good design, fabrication and operating practices will anticipate and hence minimise crevice corrosion.

Design
Design to minimise the occurrence of crevices.  If a crevice is a necessary part of a component’s design – can it be made wider?

Full penetration butt welds are best for joints.  Seal lap joints and avoid gaps between pipes and fittings.

Minimise use of bolted connections and other fasteners. Where crevices can’t be avoided use a steel grade resistant to crevice corrosion in the operating environment. It is also possible to seal the crevices to keep out corrosive liquids, but care must be taken that the seal is permanent.

Be careful that the sealant “wets” the surface. If it doesn’t it may form its own crevice. Sealants that dry and shrink can form their own crevices.

Gaskets between flanges will probably form a slight crevice, but if the gasket does not absorb the liquid and is compressed between the surfaces (and not bulging around the flange), then the crevice is usually shallow enough so that crevice corrosion is not a problem.

Fabrication
Ensure full root penetration of welded joints with smooth weld bead. Avoid under cut and cracks in welding. Use of sticky labels or markers of various kinds (such as crayons) should be avoided, as should smears of grease or oil.

“Smooth and clean” at all times. ASSDA Accredited Fabricators are assessed on their knowledge of crevice corrosion.

Operation
Sediment and scale can both result in crevices.

If the problem can’t be designed out, routine maintenance will minimise risk. Crevice corrosion under bacteria film can occur. Maintaining circulation reduces the risk that debris will collect and form crevices in dead legs or low flow areas.

Further Reading
The Nickel Institute’s free publication #11021 “High Performance Stainless Steels” contains much of the information used in this article.

This publication and a mathematical model useful for assessing crevice corrosion risk can be downloaded from the Nickel Institute website - www.nickelinstitute.org

If more detailed corrosion mechanism information is required, then “Corrosion of Stainless Steels” by A. John Sedriks is a good intermediate point.


Reinforcing pad, staggered welds - adequate strength.


Reinforced pad, seal weld - best corrosion resistance.


Staggered fillets - severe crevice


Continuous fillets both sides - crevice sealed


Figure 1: Typical crack defects around a weld (WTIA)

Credits
The Australian Stainless Steel Development Association (ASSDA) would like to acknowledge the contribution of the following Technical Committee members for their contribution to the production of this article.
•  Richard Matheson - Executive Director, ASSDA
•  Graham Sussex - Technical Specialist, ASSDA
•  Peter Moore - Technical Services Manager, Atlas Specialty Metals

This article featured in Australian Stainless magazine - Issue 29, January 2004.

Versatile Style with Stainless Decorative Tiles

A revolution is about to take place in the tiling industry with the introduction of stainless steel decorative tiles with a versatile design that will add style to any kitchen, bathroom, restaurant, bar and even as a feature wall.

There are many benefits to using stainless steel tiles. The tiles cannot burn or crack, graffiti can be easily removed, will not rust and is a stylish upgrade to the standard one piece splashbacks.

Available from Futura Tiles (Border Sheetmetal), the tiles are available in a wide variety of sizes, patterns, finishes and grades. Size ranges include: 65 x 290mm freeze, 290 x 290mm tile, 390 x 390mm tile.

Runaway Bay Designer Builder, Kerri Phillips, was so happy with the product that he installed the stainless tiles in his own kitchen and found cutting the tiles no problem at all.

"They provide you with special blades. It's a delicate operation, as long as you're careful it's fine," says Phillips.

ASSDA major sponsor, Fagersta Steels, supplies grade 304 stainless and patterned sheet from ASSDA member, Rimex Metals to Futura Tiles (Border Sheetmetal) to produce the tiles.

The tiles are also available in various Rimex patterns and finishes including the standard brush finish, 5WL and 6WL surface finishes.

Manufactured from grade 304 stainless as standard, the tiles are also available in grade 316 stainless for applications with a close proximity to marine environments.

Installing stainless steel tiles
Stainless steel tiles are basically the same as laying most floor and wall tiles. Cut tiles using similar ceramic grinding methods by using blades with 1mm steel cutting wheels.

Grouting
When grouting, apply grout to tiles leaving the protective film covering on the face of the tile. Use only fine, sanitised water resistant grouts and do not apply sanded or abrasive grouts. Remove excess grout with a clean sponge leaving a smooth grout line.

Removing film covering
Peel the protective film covering from the tiles, then with a clean sponge, remove any grout left behind from the edges of the covering. Leave approximately 30 minutes and dust off with a clean lint free cloth.

Protecting
After all dust film from the grouting has been removed apply a small amount of polishing oil to a clean rag and wipe with the grain of the tile to remove any excess oil. This will resist finger printing and water spotting and will enhance and protect the stainless steel surface.

Images courtesy of Karl Johnson, Futura Tiles (Border Sheetmetal).

This article featured in Australian Stainless magazine - Issue 29, September 2004.

Stainless Steel Enhances Hayman Island Views

North Queensland's Hayman Island Resort welcomes thousands of guests every year to the Great Barrier Reef island destination. Also attracted by the beauty of the resort, cockatoos have eaten away at the timber balcony railings and balustrades

 

To combat the work of the troublesome cockatoos, the resort management called for stainless steel to replace the timber railings and balustrades on the fifteen year old building.

ASSDA major sponsor, Atlas Specialty Metals, supplied approximately 1,000 linear metres of grade 316 stainless steel including 76 x 42mm oval tube and 38mm diameter round tube in high polish to Mackay-based fabricator, Jeff Eales Sheetmetal for the project.

Stainless steel was used extensively for the balcony top rail and posts on all three levels of the pool wing accommodation block. Because many of the balconies are at the edge of the pool, oval profile tube was specified to prevent glasses or bottles being placed on the rail and then being bumped into the pool.

Also, to ensure guests receive uninterrupted ocean views, stainless steel wire rope was installed on each of the balustrades. This helped to eliminate the restricted views given by the previous timber material.

ASSDA member, Arcus Australia Pty Ltd supplied stainless steel wire for the balcony balustrades and ASSDA member Bridco supplied the wire fittings, turnbuckles and swages for the resort balustrading redevelopment.

Guests have commented on how the use of stainless steel complements the surroundings, improves the views and suits the building style.

Hayman Island Resort management are also impressed with the new stainless steel railing as it stops the bird problem, requires low maintenance and is easy to clean. All these qualities make the tropical ocean views much better.

This article featured in Australian Stainless magazine - Issue 30, January 2005.

Solving the puzzle with stainless mainline fittings

Choosing mainline fittings for irrigation applications can often seem like building a giant puzzle with elbows, tees, crosses and coupler sets - various fittings required to connect irrigation pipework together.

Pierce AustraliaHowever, Geoff Mellows from Yarrawonga Irrigation in Victoria may have solved the puzzle by using stainless steel mainline fittings - something that plastic fittings cannot yet match.

Poly, pvc and avs fittings are common materials in irrigation applications but because they are produced out of a mould, the combinations of size and outlet configuration are restricted.

Mellows said that by using stainless mainline fittings by ASSDA member, Pierce Australia, he can now “manipulate the angle, the shape, the variation and combination of outlets.“

The difference is simple. PVC and poly are bolted on, or welded and glued - making it difficult to change fittings”.

Stainless steel mainline fittings are the only rubber-ring jointed fittings available on the marketplace manufactured to the customers specific needs and can be fitted on any other combination because they are fabricated.

Stainless steel mainline fittings also provide flexibility of design in the angle of the fittings.

This also applies to the combination of outlets on those fittings and any other additional connections to that fitting.

With versatile stainless steel fittings, Mellows advice to customers is simple - “lay the pipe first and worry about the fittings later!”

This article featured in Australia Stainless Issue 28, May 2004.

Photos courtesy of Pierce Australia.

Stainless technology to remember war heroes

The Australian War Memorial in Hyde Park Corner, London stands as a symbol of freedom and an enduring spirit of strength.

To commemorate the men and women who fought and died for Australia alongside Britons in the two World Wars, Australian architectural firm Tonkin Zulaikha Greer and artist Janet Laurence designed the Memorial to reflect the sweep of the Australian landscape.

The result was a highly durable structure featuring a long, curved wave wall constructed out of West Australian green granite and supported by grade 316L stainless steel.

ASSDA Major Sponsor, Atlas Specialty Metals, and ASSDA members, M & S Stainless Supplies and Dalsteel Stainless supplied approximately 9000 kilos of stainless steel for the structure including 8mm plate, pipe, angles and 3mm sheet.

Grade 316L was specified for its corrosion resistance, particularly as the Memorial comprises a water feature that periodically cascades water across the wall to highlight the names of the hometowns of our soldiers.

Stainless steel was used primarily in the construction of stainless steel cradles which were fabricated in Australia, shipped, positioned and lifted into place to support the granite.

All welds were pickled and passivated to provide protection from the bromine and chlorine’s likely to be deposited on the frames from the water forms built into the Memorial.

Australian-based firm Design and Survey Neon (DSN) played a leading role in the design and manufacture of the supporting structure by using 3D modelling techniques.

The 3D modelling allowed the manufacture of components and assembly of the job to become a seamless process.

DSN modelled the granite wave wall and supporting cradles. The templates for the granite blocks and their fixings were then lifted from the model to enable the fixings to be pre-drilled prior to assembly.

The use of laser cutting and CNC technologies allowed DSN to fabricate to near machining tolerances. Laser etching of assembly notches were added for simple fabrication and installation.

Coordinates for supporting cradles from the model were used to determine correct on-site positioning via electronic theodolites.

The granite blocks were positioned with a 6mm gap vertically and horizontally to a tolerance of plus or minus 1mm. Precise accuracy was required to avoid accumulation of errors because of its wave like design.

Most of the components for the Memorial were imported from Australia. Water features and water effects were created by Waterforms International and all the stone work was assembled by Australians.

This article was featured in Australian Stainless Issue 28, May 2004.

Photos courtesy of Department of Veteran Affairs & Design and Survey Neon (DSN).

Main image: The Australian War Memorial 'Dedication Day Wreaths' placed in front of the curved granite wave wall. Photo courtesy of the Department of Veterans Affairs, copyright Commonwealth of Australia. Reproduced by permission.

Other images: 316 stainless steel cradles were lifted into place to support the granite blocks that form the wave wall.

Cleaning of exterior stainless steel

The visual performance of outdoor stainless steel depends on five interrelated factors:

• Surface finish - smooth and clean and free of crevices.
• Grade selection - appropriate for environment.
• Good design - rain washing and uniform draining.
• Maintenance program - regular cleaning.
• End user expectations.

This technical article provides suggestions on a maintenance program for cleaning of exterior stainless steel, together with some recommendations for remedial action if stains occur beyond regular maintenance or where such maintenance has not been performed.

Maintenance: routine removal of grime

Stainless steel holds its appearance best if it is washed regularly. When washing use soap or detergent or 1% ammonia solution in warm, low chloride water with cloths or soft brushes to avoid scratching the surface.

Smears will be reduced if the surface is dried afterwards. This treatment applies to bare stainless steel but care should be taken with coloured surfaces.

Coloured and very smooth finished (eg BA or No. 8) surfaces subjected to excessive brushing or rubbing may lose gloss or even become scratched. Bleaches are not recommended.

Simply wiping with a damp cloth is not adequate as it smears corrosive deposits without removing them.

Table 1 from the ASSDA ‘Tea Staining’ Technical Bulletin provides a guide to the recommended frequency for cleaning exterior stainless steel. This Bulletin is available for viewing or download from ASSDA’s website.

Grease, oily films and other organic contamination

Oils and grease may be removed by alkaline formulations or hot water and detergents or, if necessary, by hydrocarbon solvents such as alcohol, acetone or thinners or eucalyptus oil. In all cases the surface should be rinsed with clean water and preferably dried.

For directionally grit polished finishes, wiping along the polish direction with very hot clean water and a soft, absorbent cloth is a good final step to reduce smears.

Heat from a hair dryer or glue gun may soften adhesive remnants from labels or protective films for removal.

After exposure to UV degradation from sunlight, adhesives may require similar treatment to grease stains or even abrasion, with the probability of a bright or scratched spot.

Adherent Scales and Mortar

Adherent scales and mortar may be removed chemically but NOT using chemicals containing chlorides.

NEVER use brick cleaning liquids that contain hydrochloric acid. Hot 25% acetic acid (vinegar) or warm 10% phosphoric acid are effective in removing hard water scales and dried mortar splashes.

Following the acid wash, the surface should be neutralised with dilute ammonia or sodium bicarbonate solution, rinsed and dried.

Remedial Work

The brown surface stains that can occur on stainless steel during atmospheric exposure are simply cosmetic rust stains.

This brown ‘tea staining’ on stainless steels will not progress to potential structural damage as could occur with a carbon steel structure.

The procedures outlined below may enable you to remove the tea staining. However, if the progression of damage is beyond these recommendations it is advisable to employ an experienced contractor.

Cleaning Rust Stained Flat Surfaces

Early action after the onset of tea staining is desirable, before the appearance of the underlying surface is changed.

If the surface is pitted, then it is probable that it will require mechanical repolishing. After mechanically cleaning off tea staining, it is preferable to passivate the surface by using a nitric acid gel or, if the item is portable, by immersion in a nitric acid bath. For marine exposures, passivation is very strongly recommended.

In contrast to other acids, nitric acid is a strong, oxidising acid cleaner and has the added advantage that it is a passivating agent.

The Nickel Institute has suggested that rust may be removed by the use of a 10% phosphoric or oxalic acid followed by a 1% ammonia solution neutralisation and then a water rinse.

Alternatively a mild acid based cleaner such as sulphamic acid (used in some saucepan cleaners) can be used with some care to avoid local changes in appearance. NEVER EVER use hydrochloric or sulphuric acids.

There are also proprietary chemical cleaning treatments often based on citric acid or other chelating compounds. Although these agents passivate in the sense of removing free iron and other foreign matter, they do not augment the surface oxide film.

Use of liquid acids on site is generally unsatisfactory as contact time is short and the acid may run off and damage adjacent components.

Unlike the hydrofluoric acid pickling process used after welding, a nitric acid passivation process does not normally change the surface appearance of stainless steel, although it may cloud a mirror polished surface. Careful trials on inconspicuous areas are recommended prior to full scale cleaning.

Electropolishing is also used by some contractors to smooth rough edges and both clean and passivate the surface. It can be carried out on site or, more usually, in purpose-built tanks.
Afterwards – prevention of recurrence

If tea staining has occurred, one or more of the five factors outlined in the introduction have not been considered carefully enough when the structure was designed and/or built.
To improve the structure, the following steps may be taken to prevent recurrence:
• Increase the frequency of maintenance.
• Improve the surface finish - mechanical polishing and chemical treatment on-site.
• Alter the design of the structure - redesign and replace the affected part of the structure.
• Improve grade selection - replace the structure with a more suitable grade of stainless steel.


ABOVE: A successful stainless steel installation in an outdoor application.

If consideration of the aforementioned steps indicates an uneconomic result, the stainless steel can be painted.

Paint systems using lacquers and polyurethane top coats are available and have been used successfully, but care and understanding is required.

Painting the stainless steel is a step that should only be used as a last option as it is irreversible.

This technical article was published in Australian Stainless Issue 28, May 2004. It is an extract of a Technical FAQ on ‘Exterior Cleaning of Stainless Steel’.

For technical support and advice contact ASSDA on 07 3220 0722 or email This email address is being protected from spambots. You need JavaScript enabled to view it.

Stainless delivers savings for Sydney Water

Cleaner beaches and major water savings will be the chief benefits of the largest Sydney Water construction project ever undertaken on the New South Wales south coast.

ASSDA Member, Roladuct Spiral Tubing, supplied approximately 60 tonnes of grade 316 and 316L stainless steel tubing and associated fittings for the Wollongong Sewage Treatment Plant.The $197 million Illawarra Wastewater Strategy will see an overhaul of the 40-year-old Wollongong sewage treatment plant (STP) including the construction of a major new water recycling plant, high level (tertiary) treatment processes and ultraviolet disinfection systems.

ASSDA member, Roladuct Spiral Tubing, supplied approximately 60 tonnes of grade 316 and 316L stainless spiral tubing and associated fittings in 2mm to 5mm thicknesses for the project.

These materials were provided to Total Process Services for use throughout the Wollongong STP for the majority of the above-ground process lines.

An additional supply of 35 tonnes of grade 316 stainless tube and pipe fittings were provided by ASSDA Major Sponsor, Atlas Specialty Metals.

Sydney Water’s head contractor for the project is the Walter-Veolia Joint Venture. Walter Construction Group (Walter) is responsible for managing the delivery of the project and undertaking civil infrastructure construction at the treatment plants. Veolia Water Systems Australia (Veolia) is responsible for the process, mechanical and electrical design, supply, installation, commissioning and operational advice.

An additional supply of 35 tonnes of grade 316 stainless tube and pipe fittings were provided by ASSDA Major Sponsor, Atlas Specialty Metals The project’s most dramatic transformations are taking place at the Wollongong sewage treatment plant, where a 21 million litre bioreactor forms the centre-piece of the upgraded plant. Designed to remove organic impurities and nutrients from wastewater, the base of the bioreactor tank was poured over a continuous 15-hour period.

The design approach redirects wastewater from treatment plants at Bellambi and Port Kembla to the Wollongong facility. The Bellambi and Port Kembla plants will be converted to specialised storm-flow treatment facilities which will be used only in extreme wet weather.

The new Wollongong plant will also operate a reuse facility - supplying high quality treated wastewater to BlueScope Steel and cutting demand for fresh water from the local Avon Dam by about 20 percent. The upgraded sewage treatment plant is due for completion in mid 2005. The Illawarra Wastewater Strategy is part of WaterPlan 21, Sydney Water’s long-term strategy for sustainable water and wastewater management.

ASSDA provides technical advice and access to resources on the water and wastewater industries - for details phone 07 3220 0722.

ASSDA Major Sponsor, The Nickel Institute, can provide essential information on waste water. This information is available for download from www.stainlesswater.org.

Standards Australia distributes the Water Services Specification (WS-SPEC:2000) incorporating guidelines for stainless steel. Visit www.standards.com.au for purchase.

This article featured in Australian Stainless Issue 28, May 2004.

Grade 2205 for High Corrosion Resistance and High Strength

Combining many of the beneficial properties of both ferritic and austenitic steels, 2205 is the most widely used duplex stainless steel grade. Its high chromium and molybdenum content gives the stainless steel excellent corrosion resistance. The microstructure provides resistance to stress corrosion cracking and ensures high strength.

The grade is generally not suitable for use at temperatures above 300oC or below -50oC because of reduced toughness outside this range.

You are most likely to encounter 2205 stainless steel being used in industrial environments such as petrochemical, chemical, oil, gas and paper plants.

Alternative Grades
2205 has been available for several years - in general this complies with UNS grade designation S31803. More recently, product has become available complying with the higher corrosion resistant composition UNS S32205, as in table 1. Both these alternatives are known as 2205.

Composition
Grade 2205 has a micro structure of roughly equal amounts of ferrite and austenite, hence the 'duplex' description. The duplex structure of 2205 has the following properties:

  • High strength.
  • Lower thermal expansion coeffecient than austenitic steels but greater than carbon steels.
  • High resistance to corrosion, particularly stress corrosion cracking, corrosion fatigue and erosion.

The high content of chromium and molybdenum and the addition of nitrogen gives the steel further beneficial characteristics:

  • High general corrosion resistance.
  • High pitting and crevice corrosion resistance.
  • Good sulphide stress corrosion cracking resistance.

The addition of nitrogen gives a further increase in pitting and crevice corrosion resistance.

Table 1: Composition of 2205 and Alternative Grades (Single Values are Maximum)

Grade Common Name C% Mn% Si% P% S% Cr% Ni% Mo% N%
S31803 2205 0.030 2.00 1.00 0.030 0.020 21.0-23.0 4.5-6.5 2.5-3.5 0.08-0.20
S32205 2205 0.030 2.00 1.00 0.030 0.020 22.0-23.0 4.5-6.5 3.0-3.5 0.14-0.20

 

Corrosion Resistance
The grade has excellent corrosion resistance and is superior to grade 316, performing well in most environments where standard austenitic grades may fail. 2205's low carbon content gives the grade a high resistance to intergranular corrosion and has better resistance to uniform, pitting and crevice corrosion due to its high chromium and molybdenum content.

As 2205 is a duplex stainless steel, the grade is also less sensitive to stress corrosion cracking in warm chloride environments, unlike austenitic stainless steels. The grade also has good resistance to stress corrosion cracking when exposed to hydrogen sulphide in chloride solutions.

High mechanical strength combined with excellent corrosion resistance gives 2205 high corrosion fatigue resistance.

Heat Resistance
Although 2205 has good high temperature oxidation resistance, this grade, like other duplex stainless steels, suffers from embrittlement if held for even short times at temperatures above 300oC. If embrittled this can only be rectified by a full solution annealing treatment. 2205 is annealed at 1020-1100oC followed by rapid cooling. This treatment applies for both solution annealing and stress relieving.

Mechanical Properties
Mechanical properties for grade 2205 stainless steels are given in table 2.

Table 2: Mechanical Properties of 2205 (Annealed Condition)

 

Table 3: Physical Properties of Grade 2205 (Typical Values in Annealed Condition)

Tensile strength 620MPa min   Density 7,805kg/m3
Yield strength 450MPa mi   Elastic modulus 200GPa
Elongation 25% min  

Mean coefficient of thermal expansion

Brinell hardness 293 HB max   0-100oC 13.7µm/m/oC
Rockwell hardness 31 HR C max   0-315oC  

 

Physical Properties
Typical physical properties for grade 2205 stainless steels are given in table 3. There are surprisingly large variations in the values from different manufacturers for notionally identical materials.

2205 has a microstructure containing approximately 50% ferrite in the annealed condition, quenched from about 10500C. Higher annealing temperatures often result in an increase of ferrite content.

Fabricability
Due to the high yield strength of 2205, greater forces are required for the cold forming of this duplex steel, and will require larger capacity equipment than would be required for austenitic steels.

Processes such as stretch forming, deep-drawing and spinning are more difficult to perform.

Welding of 2205 is good by all standard methods, however, with the following restrictions:

  • Do not pre-heat or post-heat the material, heat input must be kept low.
  • Allow the material to cool between passes, preferably to below 150oC.
  • Use correct filler grade 2209. Autogenous welding should be avoided.

Forms Available
Grade 2205 is available in hot rolled plate and strip, cold rolled sheet, plate and coil, forgings/bar, tube and pipe and in threaded fittings and flanges.

Applications
Grade 2205 is typically used in the construction of heat exchangers, pressure vessels, tanks, tubes and pipes for the following industry areas:

  • Chemical processing, transport and storage.
  • Oil and gas exploration and processing equipment.
  • Marine and other high chloride environments.
  • Pulp and paper digesters, liquor tanks and paper machines.

Specifications
For many products grade 2205 is covered by the same specifications that include the common austenitics - ASTM A240M for flat rolled and ASTM A276 for bar. Duplex grades of tube have their own specification - ASTM A789M and A790M covers duplex grades of pipe.

Credits
ASSDA would like to thank Peter Moore of Atlas Specialty Metals and Graham Sussex of ASSDA in the development of this article.

Main image: 2205 hot water tank for beef abbatoir. Photo courtesy of ASSDA Accredited Fabricator, G&B Stainless Pty Ltd.

This article featured in Australian Stainless magazine - Issue 30, January 2005.

Specifying Stainless Steel Pressure Piping for High Rise Buildings

Brisbane's tallest residential tower, The Aurora will stand 69 levels and will set an important precedent in the use of stainless steel pressure piping in high rise buildings when the Bovis Lend Lease project is completed in January 2006

 

Situated on the corner of Queen, Eagle and Wharf Streets in the Brisbane CBD, The Aurora utilises stainless steel pressure piping instead of conventional copper piping to ensure adequate water pressure for each of the 478 two and three bedroom apartments in the $250 million development.

ASSDA member, Blucher Australia, supplied approximately 250m x 108mm OD x 2mm Mapress tube and fittings, 90 degree and 45 degree bends, sockets, flange adaptors and tees in grade 316 stainless steel.

The Aurora project is different to conventional installations due to a single metered water supply being provided to a common pump set for both potable and fire fighting services.

The potable supply is then directly pumped to a reservoir at the top of the building, thus eliminating large costs of having to set aside floors for transfer tanks, pumps etc.

The fire service is branched off the potable supply immediately after the pump set and separated with a non-return valve allowing potable supply to continue the 70 storey rise to the top floor gravity feed tank. The potable water supply is to be an approved system and also able to withstand both the head pressure created by the vertical rise and pressure of emergency back up pumps in the event of a fire, which in this case is 2490 kPa.

The Mapress 316 Stainless Steel Press-Fit System was recommended by Mark Tapley of Plumbing Contractor Tapworth and Booth and specified by Hydramellenia, the subsidiary of Brisbane Hydraulic consultants Steve Paul & Partners. The system is able to withstand a high working pressure of up to 2600 kPa or 26 Bar. The system can be pressure tested up to 4000 kPa or 40 Bar.

Blucher Australia is presently proceeding with Standards Australia to obtain MP 52 certification for potable water supply and once obtained, the Mapress System will be the only stainless steel system, complete with tubes and fittings to achieve this certification.

The Mapress Stainless Steel Press-Fit System carries European pressure certification suitable for this particular application and the pressure rating for the system. No other stainless steel 'system' holds an MP 52 certification so the major change was acceptance by Brisbane City Council.

The Mapress Stainless Steel Press-Fit System was chosen for a combination of reasons including longevity, ease of installation and the system's ability to handle high pressure.

As a part of Blucher Australia's guarantee and OH&S requirements, Blucher Australia Technical Manager, Ian Johnson trained Project Manager Mark Tapley, Site Foreman Steve Woods and four other employees involved in The Aurora project in the use of the specialised Hydraulic Press-Fit tool and installation procedures.

Blucher Australia hold stock of all the stainless steel components required to do the installation. Delivery, in conjunction with the good organisation skills of Steve Wood of Tapworth and Booth, was straight forward.

Hydramellenia, the subsidiary of Steve Paul and Partners, is convinced of the benefits and is currently specifying the Mapress system for other projects. Blucher Australia has already supplied to the smaller Metropole Apartments Project, also in Brisbane, and has received inquiries from other consultants who have heard of this project.

This article featured in Australian Stainless magazine - Issue 30, January 2005.

Strata: A Majestic Streetscape in Stainless Steel

When South Australian developer, John Culshaw of Pentroth Pty Ltd proposed the design for the Majestic Roof Garden Hotel in Adelaide, the original proposal was for plain vertical bars for the car park grille of the Bent Street facade

Instead of surrendering to drab, pre-fab vertical bar grille designs, the developer actively sought a stylish alternative to the car park status quo. He commissioned artists Sue Rodwell and Trevor Rodwell to create ‘Strata’, a stainless steel grille design that aesthetically showcases the hotel building and provides a ‘majestic streetscape’ experience for Bent Street pedestrians.

The artists’ intention was to excise the barrier that would be created by the vertical security bars, which were uninviting at street level. Taking this as their starting point, they researched the replacement of verticals with horizontals to create a calm but dynamic form.

Strata lines of sandstone (used on the Frome Street facade of the hotel), contour lines, isobars from weather charts and the winged roof line of the hotel all helped to drive the inspiration for the unique design. From these lines, the artists devised a series of twenty-two panels, in sets of one, two and three. Each set is a unique design so that the complexity of the concept was acknowledged.

The grille runs the length of the hotel on Bent Street, integrates with the building and the electrically operated entrance doors to the car park and storage areas of the hotel. The nature of these horizontal waving lines creates a dynamic effect because it seems that they swing in and out because of an optical illusion when viewed from an oblique angle. By day the sky and daylight are reflected from the satin surface of the steel and at night the street and hotel lighting is reflected.

The artists chose stainless steel for their artwork for several reasons. The material used had to be strong to fulfill the structural requirements of a security grille. The artists also required it to be aesthetically complementary to the contemporary and stylish design of the hotel.

The artists went through an intense design development stage to arrive at a formula for the curving lines that worked visually. Using this formula each panel was then drawn on a computer as vector lines, which were then converted to DXF files for driving the laser cutting equipment.

The panels were fabricated by Donato Steel Fabrications from 4mm thick 304 grade stainless steel sheet supplied by ASSDA major sponsor, Sandvik Australia Pty Ltd. The artists’ designs were laser cut by Molnar Laser Cutting Services and then plate linished both sides to a number four satin finish. The sheets were then welded on the inside into the frames constructed of 38mm square stainless steel tube supplied by ASSDA major sponsor, Atlas Specialty Metals.

The intention with these panels was to create a frame within a frame, the second frame being the walls and pillars of the hotel into which they were bolted. Therefore, the construction of the panels became part of the artwork.

The artists are very pleased with the result of the installation of ‘Strata’. Pentroth Pty Ltd and the Adelaide City Council are pleased that it adds to the ambience of Bent Street and creates a lively streetscape for pedestrians. Opposite the Hotel (and currently under construction) are Bent Street Apartments which will have retail facilities at ground floor level – ‘Strata’ provides a pleasant environment on the street which is also overlooked by the apartments above.

The stainless steel fabrication meets all the requirements of a car park security grille while providing a lively streetscape in the city. The artists believe this is a good example of creating something meaningful out of a necessity. That is, a car park grille that had to be created but it did not need to be ugly and aggressive for pedestrians at street level or for those living in the adjacent apartments.

Images courtesy of Sue Rodwell and Trevor Rodwell.

This article featured in Australian Stainless magazine - Issue 30, January 2005.

Low nickel austenitic stainless steels

The most common grades of stainless steel are 304 and 316, which are particularly popular because their austenitic microstructure results in an excellent combination of corrosion resistance, mechanical and physical properties and ease of fabrication.

The austenitic structure is the result of the addition of approximately 8-10% nickel. Nickel is not alone in being an austenite former; other elements that are used in this way are manganese, nitrogen, carbon and copper.

The Cost of Nickel and Its Addition to Stainless Steel

The cost of the common stainless steels is substantially determined by the cost of ingredients. The cost of the chromium that is the essential "stainless ingredient" is not high, but additions of elements that improve the corrosion resistance (especially molybdenum) or that modify the fabrication properties (especially nickel) add very much to the cost.

Costs for nickel have fluctuated from US$5,000 or US$6,000 in 2001 to US$15,000 per tonne in 2004.

Similarly, molybdenum has dramatically increased from approximately US$8,000 per tonne in 2001 to around US$50,000 per tonne in 2004.

These costs impact directly on the two most common grades: 304 (18%Cr, 8%Ni) and 316 (17%Cr, 10%Ni, 2%Mo). The impact is most keenly felt in grade 316, which has suffered an increase to its cost premium above 304.

Other grades such as the duplex 2205 (22%Cr, 5%Ni, 3%Mo) and all more highly alloyed stainless steels are also affected.

Relative costs of the ingredients are shown in Figure 1, but these do vary widely and sometimes rapidly over time. These costs were correct in late 2004.

Relative Costs for Alloy Ingredients (Late 2004) Alloying Additions - Manganese Replacing Nickel

The point of the alloying elements is that they achieve certain changes to the corrosion resistance or to the microstructure (which in turn influences the mechanical and fabrication properties).

Chromium is used to achieve corrosion resistance, and molybdenum adds to this.

A common evaluation of corrosion resistance of stainless steel grades is the Pitting Resistance Equivalent (PRE), where this is usually evaluated as PRE = %Cr + 3.3 x %Mo + 16 x %N. A neat equation, but unfortunately only a guide.

The PRE gives a guide to ranking of grades, but is not a predictor of resistance to any particular corrosive environment. What is apparent is that pitting corrosion resistance can be increased by molybdenum, but also by chromium or by nitrogen additions. These are much cheaper than molybdenum. Despite its high PRE factor, nitrogen has limited effect on corrosion resistance because of low solubility, ie. <0.2%.

The microstructure of the steel is largely determined by the balance between austenite former elements and ferrite former elements.

On the austenite former side carbon, manganese, nitrogen and copper are all possible alternatives to nickel. All these elements are lower cost than nickel.

As is the case for the PRE, the Ni-equivalence formulas are a guide but do not tell the full story; each element acts in slightly different ways, and it is not possible to fully remove nickel and replace it with, for example, copper or nitrogen.

Manganese acts as an austenite former but is not as effective as nickel, and Cr-Mn steels have higher work hardening rates than do apparently equivalent Cr-Ni steels.

Carbon is a very powerful austenite former, but has only limited solubility in austenite, so is of limited value in a steel intended to be fully austenitic.

Although not recognised by the PRE formula, nickel has positive effects on resistance to some corrosive environments that manganese does not duplicate.

There can also be synergy between the elements. Addition of nitrogen has the double effect of forming austenite and of increasing the pitting corrosion resistance. And manganese is a strong austenite former in its own right, but also has the effect of increasing the solubility of nitrogen.

The Rise of the "200 Series" Steels

Manganese is therefore a viable alternative to nickel, ranging from a minor addition to an almost complete replacement.

The development of high manganese austenitic grades first occurred about fifty years ago, during one of the (several) previous periods of high nickel cost.

At that time some Cr-Mn-Ni grades were sufficiently developed to be allocated AISI grade numbers Ü 201 (17%Cr, 4%Ni, 6%Mn) and 202 (18%Cr, 4%Ni, 8%Mn) are high Mn alternatives to the straight chromium-nickel grades 301 and 302, and are still included in ASTM specifications as standard grades.

Their consumption over the following decades has been low relative to their Cr-Ni equivalents. The reasons for the poor take-up of these lower cost grades have been:

  • Very high work hardening rate (this can be an advantage in some applications).
  • Slightly inferior surface appearance Ü considered unacceptable for certain applications.
  • Additional production costs Ü higher refractory wear in melting in particular.
  • Corrosion resistance is lower in some environments, compared to Cr-Ni grades.

An additional issue is that Cr-Ni and Cr-Mn-Ni austenitic scrap is all non-magnetic, irrespective of nickel content, but scrap merchants evaluate scrap on the basis of assumed nickel content.

This has the potential to destabilise scrap markets. The upshot is that the cost reduction (due to lower ingredient cost) has been generally insufficient to move applications from the traditional Cr-Ni grades.

Take-up has often been because of technical advantages of 200-series in niche applications, not cost-driven.

Some New Contenders

The last decade has seen the rise of some new contenders in the Cr-Mn-Ni austenitic group. The main development work has been in India and the principal application has been kitchen ware Ü cooking utensils in particular.

The very high work hardening rate of the low nickel / high manganese grades has been acceptable to a point in this application, but additions of copper have also been made to reduce this problem.

India has been a fertile development and production venue for these grades because of local economic factors.

Other Asian countries have also become strong markets and more recently also producers. The Chinese market is particularly strong, and there is substantial demand in China for the Cr-Mn-Ni grades, often referred to generically as ?200-seriesî stainless steels.

Other centres of production are Taiwan, Brazil and Japan. Alloy development has resulted in a range of austenitic grades with nickel contents ranging from 1% to 4% and up to over 9% manganese. None of these grades are included in ASTM or other internationally recognised standards as yet.

The growth rate in production of these low-nickel austenitic grades has been very rapid. The most recent data published by the International Stainless Steel Forum (ISSF) shows that in 2003 as much as 1.5 million tonnes (7.5% of the worlds stainless steel) was of this type. In China the proportion has been estimated to be 25% in 2004.

Problems still exist however, and large-scale conversion of Cr-Ni applications to Cr-Mn-Ni-(Cu) grades is not likely.

The principal issue is lack of control ‹ unscrupulous suppliers misrepresenting low-nickel product as grade 304 with some resultant service corrosion failures, and degradation of scrap due to contamination by low-nickel material.

As at the start of 2005 the future is unclear. Although it seems logical that there should be a place for the low nickel austenitic grades, the practical issues may mean that grade selectors will instead choose to either continue to use the higher cost Cr-Ni grades, or to seek lower cost alternatives amongst the ferritic or duplex grades.

Credits

ASSDA would like to thank Mr Peter Moore, Technical Services Manager of Atlas Specialty Metals, for the contribution of this article.

This technical article was featured in Australian Stainless magazine # 32 - Winter 2005 and is an extract from the Grade Selection section of the Australian Stainless 2005 Reference Manual.

To order a copy of this essential technical resource for the stainless steel industry, download an order form from the ASSDA website - www.assda.asn.au or phone the ASSDA office on 07 3220 0722.