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A new twist on stainless design

With a striking and innovative design evoking visions of the Singapore based Helix Bridge, stainless steel has formed an integral part in creating one of Sydney’s most exciting new structures.

Commissioned by Landcom as part of the Lachlan Line Precinct development project in North West Sydney’s Macquarie Park, the yet to be officially named cyclist bridge provides visual flair, safe accessibility, and excitement to the area. In an area with typically heavy traffic congestion, the project’s promotion of reduced car dependency creates a significant positive impact to the surrounding environment.

The design is the first of its kind in Australia, utilising a double helix arrangement with a varying diameter along its curving 178m length. While the design elements certainly lend themselves well to aesthetics, structural requirements dictated much of the overall shape. A steel truss arrangement is used, required for the long sections spanning the multiple roads below its footprint. The diameter of the spiral increases at the bridge supports and tapers along its span.  At its narrowest, it is 5.5m in diameter, and 7.8m at its widest.

Approximately 170t of 2205 grade duplex stainless steel was used, along with around 220t of structural mild steel. Due to the large quantity of material required, multiple distributors supplied material mostly on an indent basis, with some delivered ex-stock. ASSDA Members Midway Metals, Stirlings Performance Steels and Vulcan Stainless all supplied material, with the majority of plate (up to 80mm in thickness) produced by ASSDA Member Outokumpu, managed through their Melbourne office. 

A minimum 100-year service life with minimal maintenance (becoming increasingly common in the design of bridges) was a key criterion, particularly important for the hard to access structural components. From the beginning, 2205 duplex grade presented as an ideal material, thanks to its hybridised microstructural properties granting it superior mechanical properties to many forms of mild and stainless steel. 

Put simply, being stronger allows for thinner sections to be used or, alternatively, more scope for efficient design such as larger spanning or increased resistance to bending moments. Mild steel was retained  for the helical outer structure, and painted blue, which was a central design requirement. 

Arup proposed duplex stainless steel for the deck structure and wearing surface within the helix due its increased strength to weight  properties whilst maintaining high durability performance.   

Outokumpu played an instrumental role in advocating the use of the material properties of duplex grade at the early design and concept stages. Backed up by global materials experts and with a wealth of expertise in supplying stainless materials for bridges all over the world, Outokumpu aimed to provide a technical solution through the use of duplex stainless, rather than simply tender for the supply of material. “The use of duplex stainless steels in bridges around the world is becoming more and more the material of choice, so it was great to see Arup in Australia embrace it in its design”, said Con Logos, Vice President APAC at Outokumpu. “A special thanks to George Miech from my team for his tireless effort in the early stages, working closely with both Arup and RMS to have duplex stainless specified”. 

Outokumpu also assisted the fabricator and the end client, Transport for NSW, with expertise, advice, and preliminary procedures in welding the material, particularly vital with the thicker sections which require great care to ensure optimal material properties are realised in the weld and adjacent areas.

The bridge was fabricated in Sydney in four modules, which were trial assembled prior to being delivered to site, where the four segments were positioned and secured over the course of four weekends. Specially designed lifting assemblies were necessary to ensure the segments were not overstressed. 

As bridge design increasingly demands greater durability, aesthetic and creative licence and structural performance, stainless steel, of the duplex family in particular, presents a wonderful option now and into the future.

Photo credit: Landcom

This article is featured in Australian Stainless Magazine issue 71, 2021.

 

Comparisons of hot and cold formed stainless steel

When comparing hot and cold formed stainless steel, the first question you would ask yourself is: are there any chemical differences between the two? ASSDA has previously published articles on the various surface finishes including the few hot and multiple cold finished processes, however this article concentrates on the differences. 

Since the 1970s, most stainless steel is produced by melting in an Electric Arc Furnace (EAF) and then the molten stainless steel is transferred to an Argon Oxygen Decarburisation (AOD) vessel or, less commonly, a Vacuum Oxygen Decarburisation (VOD) vessel. These processes control impurities such as carbon, sulphur, nitrogen, hydrogen and other impurities which could form oxide precipitates. For critical applications such as aerospace or precipitation hardening alloys, further refining is possible, but this is a smaller market. Critically, in AOD, using the injection of inert argon as the stirring agent into the melt allows control of nitrogen additions, e.g., for duplex or high austenitic stainless steels.  

Chemistry

The basic chemistry of a specific grade of stainless steel is the same regardless of how it is subsequently shaped, i.e., as hot or cold rolled, hot forged, cold drawn/shaped or simply cast –  with the proviso that cast stainless steels typically have more chromium and (often) more silicon than their wrought counterparts. In addition, even if cast products have been stress relieved, their microstructure is dendritic so that there can be significant composition differences (localised differences in corrosion susceptibility) between the early solidifying dendrites and the final bulk solidification.

Hot formed

Producing hot formed stainless steel is deceptively simple as shown in the graphic. Currently, for about 95% of production, the molten metal is decanted from the AOD into a cooled continuous caster and emerges horizontally as a slab. The microstructure in the slab is columnar from the outsides (because of the cooling by the caster walls) with a relatively uniform equaxial microstructure in the centre. It is also covered with heavy black oxide scale with a chromium deficient layer underneath – as with weld tints. Typically, the slab is then surface ground to remove solidification features and the scale which would both otherwise be incorporated into the surface during subsequent rolling. The slab is then charged into a reheating furnace and hot rolled to homogenise the microstructure and provide either plate or coil as the product form. The grains will now be more oriented along the rolling direction. The microstructure is then further refined (and internal stresses reduced) by annealing above 1000oC.

Unless the steel is to be used where appearance and maximum corrosion resistance is not critical, e.g., in a furnace, then after hot rolling it would be shot blasted to break up the scale and then pickled. The pickling causes the dimpled appearance of a Hot Rolled Annealed and Pickled (HRAP) plate surface because the pickling acid attacks the base metal at the defects in the black oxide scale. It also gives a typical surface roughness of 5 or 6 µm Ra. There is a potentially cosmetic corrosion issue if the (typically) fast pickle does not completely remove any shallow intergranular oxide penetrations, however this is unusual. For long product such as angles or channels, another visual distinction is that the edges meet at 90o compared to the radius of curvature determined by the thickness and ductility of a cold rolled product.

Cold formed

Cold rolled flat product is quite different because it usually starts at room temperature with a dimpled surface and finishes with thinner material – and watching the increase in speed of the sheet as it gets thinner is quite startling. There is a significantly more elongated microstructure along the rolling direction, and this enhances the anisotropy in transverse to longitudinal strength compared to the relatively slight effect for hot rolled material. Long product is not cold rolled but more correctly it is cold  shaped or formed.

The increase in strength with cold work can be substantial, especially in thin materials as the cold work increase can enhance the strength of the full depth. As an example, the table from ASTM A666 data shows the substantial change in mechanical properties for 304 from annealed to half hard, i.e. half the absolute maximum possible strength – which would have negligible ductility.

Effect of cold work on strength and ductility of 304

One effect of the increase in strength with cold work is that the limit of proportionality will increase with cold work, i.e., the linear deflection occurs up to a higher stress. The reduction in break elongation is simply reflecting the proof stress closing on the tensile stress from 40% to 73% as shown below.

Variation of thickness tolerances for cold and hot rolled materials

There are also differences in the tolerance between cold and hot formed material as shown by comparisons in A480 (flat product – sheet vs. plate) and A484 (sections). However, it is not as simple as “hot formed is less precise than cold formed” as seen below. 

Surface finish

Often it is important to consider appearance and corrosion resistance to the “bright and shiny” benchmark. Hot rolled material is always going to be dimpled, even when it is electropolished and exhibiting a brilliant lustre. It will be marginally more difficult to clean than a smoother cold rolled surface but abrading the surface to give a roughness of less than 0.5 µm Ra is counterproductive. You lose the passive stainless steel and an abraded surface potentially has sulphide inclusions exposed compared to the original pickled, sulphide free HRAP surface.

The inherent roughness of a cold rolled sheet decreases steadily as the plate is rolled thinner as shown in graph. It shows the decrease of Ra from the crushing of the peaks left from the hot rolling as the sheet gets thinner. The graph is for cold rolled annealed and pickled (2B) material and is useful when someone asks for a 2B finish for a thickness not in the band. However, thickness of pickled materials is not relevant to the corrosion resistance whereas the Ra is critical to an abraded surface both for reasons of cleanability and possible crevices from torn surface flaps plus, if not passivated, exposing sulphide inclusions.

 

This article is featured in Australian Stainless Magazine issue 71, 2021.

 

Award-winning stainless food plant

World-class processing systems demand high quality products, innovative features, and long-term yield increase, all of which have been delivered through superior workmanship, engineering, and the use of stainless steel.

ASSDA Member, INOX Australia, was engaged by a Melbourne based stock and soup manufacturer to design the integral process, fabricate, install, and commission a beef, chicken, seafood, and vegetable stock processing system. The processing system is entirely fabricated in stainless steel as it has been essential to the project design and fabrication, being the material of choice in demanding hygienic environments that involve high heat. 

Exceeding their client’s expectations, INOX supplied a processing system that provided multiple innovative features, winning the 2019 Food & Beverage Industry ‘Innovative Technology of the Year’ Award, and being nominated for ASSDA’s 2019 Fabricator Project of the Year Award. This extraordinary processing system features a single user operational interface, safe and ergonomic handling of the product during the process from start to finish and hygienic design of the process including zero wastage at end of production. It also improved the yield of raw materials by pressure processing instead of traditional atmospheric process.

The system works by depositing 1000kg of raw materials into a stainless steel basket which is lifted by an electric hoist into the automatically opened pressure vessel. The touchscreen operational interface is used to set and supply the water volume. The process is fully automatic to the set parameters and then alarms when the process is completed. Following the cooking procedure, the CIP (Clean In Place) water is circulated through the heating jacket of the vessel which serves two purposes. The first is to cool the cooking vessel to a temperature that allows the product to be discharged, and the second, to use the heat recovered from the cooking vessel to heat up the cleaning water. This reusable use of heating allows for no external heating of the cleaning water.

The liquid stock produced is then pumped through a specially designed filtration system, to a holding tank which is then ready for the product to be received by the external filling line. The stainless steel basket is then removed from the vessel and the waste product is dumped into a hopper underneath the basket, ready to be removed to a disposable waste bin. The basket is then cleaned within the vessel during the CIP process and does not exit the food processing room at any point, ensuring the equipment is cleaned and cannot be contaminated externally. 

An impressive 5000kg of 304 and 316L grade stainless steel was used to complete the project. All stainless steel material was supplied by ASSDA Members, Midway Metals, Vulcan Stainless and Tubesales Stainless. This included stainless steel tube, pipe, plate and sheet with 3mm to 12mm thickness. Several components (with thicknesses up to 75mm) required a large amount of specialised machining. The material was mechanically polished where required to achieve a better than 0.8 µm Ra surface finish. This superior material of choice meets the project’s sanitary requirements, offers structural integrity and excellent corrosion resistance in high temperature applications. 

 

This article is featured in Australian Stainless Magazine issue 71, 2021.

 

Stainless steel – a mainstay in water assets

A mainstay in the processing of sewage water in water recycling applications, stainless steel once again plays a critical part in working towards a sustainable future.

Western Water Treatment Plant (WWTP), south of Werribee in Melbourne’s Western District, is a critical asset responsible for processing around half of the city’s total sewage. The process requires a massive footprint, where a series of large lagoons use anaerobic (without oxygen) and aerobic (with oxygen) bacteria to sequentially break down and clean the feed water of solids and gases. The resultant recycled water is used for multiple non-drinking purposes, including irrigation and firefighting. 

WWTP is also largely energy self-sufficient. This energy is captured during the sewage treatment process by combusting biogas, which is captured under the covers. Biogas is used to meet much of the Plant’s electricity needs – preventing hundreds of thousands of tonnes of carbon dioxide entering the atmosphere each year. 

The site carries significant environmental importance to both the immediate and wider areas. Biogas production from the process is used to meet all the Plant’s energy needs through on-site power generation. The Plant is an enormous site – almost the same size as Phillip Island and a world-renowned wetland, declared a Ramsar site in 1983. It is home to many species of flora and fauna and a haven for birdwatchers – in summer it can host more than 100,000 birds in a month.

With current capacity concerns in the existing plant network, stretching back 120 years, and a forecasted increase in processing demands due to a growing city, the need for an additional nutrient removal plant (to complement the existing two) was established. More efficient in design and with more advanced monitoring and controls to provide class A recycled water, the new plant provides an additional 140ML of treated water per day. The plant was designed by Jacobs and built by UGL and CBP Contractors, with commissioning in 2019.

Grade 316L stainless steel was selected as a default material for the pipework, in 2B finish. Offering strong corrosion resistance even against contaminated effluent water, its ease of fabrication and ability to be installed without the need for coating and subsequent maintenance was also important.

ASSDA Member and Accredited Fabricator Roladuct Spiral Tubing was selected to manufacture and supply approximately 50t of spiral welded large diameter pipe, ranging from 300 to 1422mm in diameter. Feed material was supplied  by ASSDA Member, Midway Metals to Roladuct’s  Melbourne manufacturing site. 

Roladuct was nominated in 2019 for the ASSDA Fabricator Project of the Year Award, with judges impressed by the quantity of stainless steel used, as well as the significant environmental benefits realised by the plant upgrade.

As water assets continue to be upgraded and optimised for a growing country, stainless steel will continue to play a vital role in providing material performance and plant reliability for years to come.

 

 Photo credit: Roladuct Spiral Tubing

This article is featured in Australian Stainless Magazine issue 71, 2021.

 

Posted 25th September 2009

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Australian Stainless is a leading industry magazine devoted to showcasing the unique diversity and durability of stainless steel. Since its inception in 1993, Australian Stainless has assisted and encouraged specifiers and end users of stainless steel, with a focus on local fabrication.

For 17 years, Australian Stainless has been enjoyed by more than 8,000 readers across the country. In October 2009, Australian Stainless Online was launched to provide a readily available live news feed to a wider audience around the world. Australian Stainless is published by the Australian Stainless Steel Development Association (ASSDA), a non-profit industry group that aims to increase the consumption of stainless steel in Australia, through publication, education, accreditation and more.

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