NICkEL-fREE STAINLESS STEEL SPIRALS TO SUCCESS IN WATER HEATERS MATERIALS IN DESIGN & PROTOTYPING PRODUCTS


is made by taking long strips of stainless steel and forming them into spirals to create lengths of tube that are sealed shut with one continuous laser weld in closely controlled factory conditions. Talking about Sole’s approach to material


The longer the length of the coil inside a domestic water heater, the greater the heat transfer capacity. However, butt-welded joints between sections of tube inside the heater can be points of failure – as a focus for scale to build up in hard water areas, for instance. Alternatively, the heat affected zone may become corroded in soft water or could become a propagation point for stress corrosion cracking. As a result, designers often prefer to buy tube


in long continuous lengths, which gives maximum flexibility for producing water heaters with longer coils that can heat water more quickly. The ability to secure longer lengths of high


quality tube is a competitive advantage and enables the manufacturer to create domestic heaters that are high performance and long-lasting. Norwegian tube manufacturer, Sole, is one supplier of this type of tubing. Up to 40m long, this


selection, managing director, Thor Reidar Braathen, said: “Every length of spiral tubing we manufacture is pressure tested. If we detect even the slightest leak then we scrap it. That’s why we rely on high quality steel strip – because what may seem like tiny surface imperfections can result in a poor laser weld. Currently, no other manufacturer can produce the same type of tubing in such a long continuous length. That’s critical to the quality and long life of our product, because when you have to join two lengths together you introduce another potential point of failure. “In all the time we have been making spiral


tubing we have never had a failed product returned from the field. Not one. That is a testament to the quality of our processes and the quality of the stainless steel we use.” When Sole started production in 2005 it used


grade 316L from Outokumpu, which provides very good corrosion resistance, formability and weldability – an ideal set of properties. But wanting to work more efficiently without compromising on quality, it switched to Supra 444/4521 in 2012. This is a nickel-free ferritic stainless steel that


contains molybdenum and titanium as alloying elements. It has similar properties to 316L, with good cold formability, weldability and high strength, all of which are ideal for producing long lengths of


tubing. The addition of titanium has the benefit that it can be welded in all axes without becoming susceptible to intergranular corrosion. Once in operation, Supra 444/4521 has good resistance to chloride-induced stress corrosion cracking, giving it a long life for domestic water heaters. The main driver for switching grades was that


the new grade contains no nickel. Nickel is an important alloying element for many grades of stainless steel as it provides enhanced corrosion resistance and thermal properties. The price of Nickel is volatile, and that makes it hard to plan and forecast accurately as the price of raw materials can change significantly between the day a quote is submitted and the day the material is ordered. So, by switching to a nickel- free grade, Sole saw an opportunity to work with stable and predictable costs, and also pass on that certainty to its customers. Since adopting the new grade in 2012, Sole


has been using around 500 tonnes of it every year to produce more than a million metres of spiral-welded tubing. Braathen commented: “There were a number of


reasons for making the switch to Supra 444/4521. First is that it is an extremely cost-effective material and also met our requirements for formability and weldability. Also, we knew that our customers were adopting it for their water tanks, so it made sense to use the same steel grade.”


Outokumpu www.outokumpu.com/en


Validating designs with form, fit and function prototypes


Form, fit and function prototypes validate a design by providing the opportunity to physically examine specific parts and features, test functionality, or test the fit of a part against other components, such as electronics, in the same assembly. The lines and features can also be examined through form models. Being able to see, touch and feel a part or


prototype allows for meticulous checks to take place and any changes to be made. It can also spark ideas on how your design can be optimised. Most of the processes available at Ogle will be


used in some form or another. However, because each process and material will have individual properties to support the testing phase or match as close to individual production and testing requirements, the specific processes and the associated materials chosen will be dependent on each product. For example, the company knows that stereolithography (SLA) 3D printing provides greater accuracy (perfect for testing fit) but possesses a lower material strength, and SLS will give a more robust part, more suited to functional testing. For larger models, CNC machining is usually more appropriate, but also gives the opportunity to test in the actual production material if desired. Lines and form of the model can also be assessed in greater detail by utilising model board with a higher density. At Ogle, both SLA and SLS have been


successfully used in the production of wind tunnel models to verify and validate the aerodynamics


of the materials, processes and lead time. There are plenty of finishes available – from


‘straight off the machine’, to fully finished and painted. Sometimes, if you have fine details, lines and features on an SLA print, it may be better to bead blast the surface because you get less reflection on the parts to further highlight the lines and features. This is especially used for a large part or vehicle (such as a yacht) that has been scaled down for the purposes of a form model. Ogle produced the early development steering


of buildings, parts and components. Exploring the serviceability of a product or


component, through the use of form, fit and function prototypes, helps avoid prolonged assembly and lengthy repair times. Those involved in the production line can use the prototype to look carefully at how the product as a whole is assembled and disassembled as well as examining how it interacts with the components around it. To achieve the best results for its clients,


Ogle asks a few questions: What’s the end use? What are your core objectives from this prototype? What environment will this be used in? Do you have a specific material you’d like used or properties to imitate? Is surface finish important at this stage? Using the answers, the company can prepare a proposal providing details


wheels for the Bloodhound Supersonic Car in SLS to test the form and fit around the driver’s hand. The purpose of the prototype was to enable testing and amends prior to moving to metal 3D printing for the production wheel, thus saving costs. It was also approached to support a project


assessing car parking sensor calibration and test fits. CNC was used to produce front and rear bumpers from a mid-density model board with the SLA process being utilised for adding inserts for the finer, more complex details, to keep costs down. In addition, when producing an aircraft seating


mock-up, the company used production composite materials to check whether the wiring harnesses could be physically routed through and around the parts; and also to make the wiring as efficient and as short as possible, saving weight.


Ogle Models & Prototypes www.oglemodels.com


OCTOBER 2021 DESIGN SOLUTIONS 53


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