Heat Soaked Glass
If you have worked in the glass industry long enough you will have experienced a mysterious breakage of an in situ glass panel, whether in a shop front, shower screen or glass balustrade. It’s a scary and potentially dangerous experience for a customer and an inconvenient costly event for a business, not forgetting the confidence zapping affect it can have in the eyes of your customer and implications for your business reputation.
Toughened safety glass as its names states is tough but not unbreakable, so what would cause a glass panel to just explode with no physical impact?
The culprit is Nickel Sulphide (NiS) Inclusions.
What are they?
NiS inclusions are small contaminants, “tiny stones”, that are rarely found in batches of float glass during the manufacturing process, they are undetectable to the naked eye and currently there is no test to determine the presence of such an imperfection in the glass.
So how does a NiS inclusion cause the glass panel to explode?
Whilst the glass in its annealed state is relatively safe from the risk of spontaneous explosion, the risk occurs during the toughening process, which transforms the state of the NiS inclusion once the glass has been toughened in a furnace. The trouble begins with a phase transformation in the nickel sulphide inclusion from high to low temperature forms. During the toughening process the rapid cooling of the outer surface of the glass and slow internal cooling results in the NiS inclusion being trapped in its high temperature phase and this can cause spontaneously breakage even years later.
What’s the Solution? – Heat Soaking
How Heat Soaking Works
Heat soaking is a secondary process, completed after the glass has already been toughened and is undertaken to prevent any later problems.
The process is known as “Heat Soaking to EN14179” as this represents the accepted standard. The heat soaking process involves a specialized oven that heats the processed glass to 290˚C and holds it at that temperature for two hours. The test is detructive as 95% of inclusions present in the glass will explode and destroy the glass. As this is a destructive test it also has the potential to damage adjacent glass in the oven.
Heat Soaking adds time and cost to processed toughened glass due to failure rates during testing and rework costs.
The process eliminates up to 95% of spontaneous breakage problems without interfering with the positive properties that toughened glass brings to many projects.
What products are required to be heat soaked?
International Building Codes require different glass products to have different processing attributes. We suggest that glass used a physical barrier should be heat soaked to the European Standard: EN14189.
Testing for Exakt Glass Balustrade Systems are all using heat soaked glass to EN14189
Our Glass Plant holds third party accreditation to EN14189 from CSI International.
Have you any questions or comments about the use of heat soaked glass? If so, please contact our experienced team to discuss.
Stainless Steel Corrosion and Tea Staining
Stainless steel provides structural integrity and aesthetic appeal when utilised in construction, though its propensity to become tarnished and brown in colour over time – especially in coastal locations – has been identified as a significant issue when using the product in construction.
Much research into the brown discolouration of stainless steel – known as tea staining – has been undertaken by research and industry bodies right across the globe.
Although initially just a cosmetic issue, having no impact whatsoever on the structural integrity of the material, tea staining indicates corrosion, and occurs most predominantly in areas within a five-kilometer radius of the ocean.
Research, however, has indicated that tea staining can occur on stainless steel structures more than twenty kilometers from the ocean should factors including wind, sheltering, high temperatures and pollution also be at play.
Tea staining occurs when atmospheric conditions are too harsh for the grade of stainless steel used, with the effects becoming more apparent the closer the material is to salty water.
The factors listed below each play a large role in encouraging the presence of tea staining, and should be considered when attempting to avoid the damage.
- Corrosive substances – one of the biggest causes of tea staining is the presence of sea salt on the material, as it has the ability to remain moist at very low levels of relative humidity. This means the steel remains wet and proceeds to corrode. Corrosion is aggravated further by the presence of industrial pollutants that may exist in the environment.
- Atmospheric conditions – High humidity and temperatures, like those commonly experienced in coastal areas, create ideal conditions for tea staining. A thin layer of moisture that develops due to high humidity allows salt to diffuse, leaving behind a corrosive compound.
- Design – if drainage systems in the construction are not well designed, e.g. the surface is almost horizontal, or the steel captures pollutants, tea staining will likely become apparent. Severe corrosion beyond tea staining occurs in structures with corners and crevices, including intermittent welds, as these tend to readily trap water and other contaminants.
- Roughness of surface – surfaces featuring grooves or folds are more likely to see tea staining because of their propensity to trap salts and chlorides. As the surface dries the salt becomes more concentrated, aggravating the corrosion and increasing the rate at which tea staining will occur. Surface roughness should be measured at no more than 0.5µm Ra, as cut and abraded surfaces smoother than this are far less likely to corrode.
- Surface characteristics – stainless steel should be clean and free of pollutants in order to avoid corrosion, which can be achieved by conducting acid pickling, acid passivation and electro polishing for satisfactory amounts of time. If stainless steel has been welded, heat input will have destroyed the materials passive layer. Details on how this can be restored are explained later.
- Appropriate grade – the hundreds of stainless steel grades available owe their corrosion resistance to the chromium oxide film on the surface. A material’s corrosion resistance is measured using the Pitting Resistance Equivalent (PRE), formulated by testing the grade’s chromium, molybdenum and nitrogen levels. Increasing a grade’s PRE will ensure it is more corrosion resistant.
- Maintenance – while stainless steel is a relatively low maintenance material, it is not maintenance free, and should be given a regular wash. Oils and waxes can be applied to reduce the accumulation of corrosives, though these need to be renewed often, as they tend to attract debris.
In order to best preempt and subsequently avoid tea staining, several steps can be taken during fabrication.
- Design and fabrication – surfaces should be able to drain freely, with runoff not channeled, and the surface exposed to rain. Abraded surfaces should have their grain running vertically, and be no more coarse that 0.5µm Ra, finished with nitric acid passivation to achieve the best resistance to corrosion.
- Grade selection – stainless steel is graded by the limit of salts it can tolerate before corrosion. A minimum grade 316 should be chosen for areas within five-kilometres of the surf, as lower grades, i.e. 304 and 430, will likely see tea staining and higher rates of corrosion in a coastal environment, grade 2205 Duplex offers the most cost effective protection against surface corrosion designed into the product from manufacture.
- Treatment of welds and hot work – pickling following welding removes the welding oxide and chromium depleted layer from the surface, restoring the passive layer, meaning the steel retains its corrosion resistance.
- Installation and inspection – a completed structure must be inspected for damage and contaminants, with any suspected pollutants treated with repeated cycles of a misting and drying test using tap water.
- Avoid hydrochloric acid – though it is often used to clean cement and mortar, hydrochloric acid will stain a surface and begin serious corrosion if used around stainless steel.
The following design recommendations should be taken into consideration when constructing with stainless steel.
- Planning – forward planning must take place to specify the expectations of the structure, by way of a maintenance program. If maintenance is kept up and corrosion resisted, structures are likely to stay structurally sound for many years.
- Environment – while areas within five kilometres of the surf are most susceptible to tea staining, there is no real rule, as wind and weather conditions can see structures twenty kilometres from the surf experience tea staining.
- A smooth and clean surface finish – when it comes to reducing corrosion levels, the smoother the surface the better. It is recommended that a surface roughness of less than 0.5µm Ra is used. Methods including passivation, electropolishing, and pickling can be utilised to remove contamination from surfaces, though these processes have been known to decrease a surface’s aesthetic appeal. Mirror polish of components offers the best protection.
- Get the right grade – in marine and tropical environments, stainless steel with a corrosion resistance grade of 316 should be used as a minimum. A higher corrosion resistant steel could be used if the structure requires a higher aesthetic appearance using grades such as Duplex 2205.
- Weld treatment – generally in construction a weld should comply with AS/NZS 1554.6 Level 2, Class B, as explained in the relevant international standards. However, this doesn’t ensure minor defects won’t occur. A protruding weld should be ground flat and polished to a 320 grit to provide true corrosion resistance. It must be remembered that passivation takes place in moist air within 24 hours, though a chemical passivation treatment using a nitric acid solution can be used to reduce the time for passivation to occur, create a corrosion avoidance film, deplete iron contamination and rid the surface of exposed manganese sulphide. If the environmental circumstances are especially harsh, the process of chemical passivation requires completion following abrasion. Alternatively, a grade II blasting finish can be undertaken, which sees the surface’s heat tint and chromium depleted layer removed, while ensuring the surface roughness is never higher that 0.55µm Ra. It should also be notes that this will not control tea staining. Provided there is no surface crevices or defects, a grade II pickled finish will protect against tea stain corrosion without polishing down the weld. Pickling should take place as the final process in the weld, to completely remove any contaminants and replace the passive layer, which restores a corrosion resistant finish.
- Regular maintenance – regular maintenance is absolutely necessary in order to ensure tea staining does not occur. Contaminant deposits that inflict corrosion, such as that of salt, can be rain washed clean, however, it is imperative the surface has good drainage. Soap or a mild detergent combined with warm water will produce the best finish on stainless steel surfaces, and it is important that harsh cleaners that contain chlorides or bleach and never used to clean stainless steel surfaces.
Should tea staining occur on a surface, the seven points first mentioned can be addressed, in order to correctly identify where the problem has occurred leading to surface corrosion.
By utilizing 2205 duplex stainless steel for investment castings and CNC machining of glass hardware we can all but eliminate the risk of tea staining corrosion if usage guidelines are adhered to.
Photo (left) is an example of extreme surface tea staining due to NO maintenance .
- Choose the correct grade of stainless steel to suit your environment
- Don’t ever use Hydrochloric Acid on or around stainless steel
- Mirror polished finishes offer superior corrosion protection over satin and brushed finishes
- All welds and hot work on the metal must be treated to reinstate the protective chromium coating
- Duplex stainless steels offer superior corrosion resistance over 304, 316 and 316L
- Maintain the metal by washing with warm soapy water and rinsing to remove surface contaminants .
How do you educate your customers on cleaning stainless steel products?
What is 2205 Duplex Stainless Steel and Why Do We Use it?
To begin, what is stainless steel?
A: As the name suggests stainless steel is a stain/corrosion resistant metal and it’s corrosion resistance is due mainly to its chemical composition and namely the greater than 10% of chromium present in the composite of metals called an alloy.
There are five main types of stainless steel alloys,
Ferritic, Austenitic, Martensitic, Duplex and Precipitation Hardening.
Duplex stainless steel is approximately 50% ferritic and 50% austenitic offering the very best characteristics of strength and corrosion resistance.
Of the duplex grades, 2205 duplex stainless steel is the most widely used of its kind, found in industries where petrochemicals, oil, gas and chemicals are used due to the superior properties its microstructure contains. Only in recent years has 2205 become more readily available for construction hardware.
Able to withstand temperatures between -50 and 300 degrees Celsius, 2205 is composed of equal parts austenite and ferrite, leading to stronger, highly corrosion resistant steel that is also resistant to stress corrosion cracking and erosion.
Grade 2205 contains large amounts of chromium, molybdenum and nitrogen, and lower amounts of nickel, which further benefits the alloy by lowering cost and weight. 2205 is far superior to marine grade 316, in not only marine and tropical environments but in all common residential and industrial environments.
This is due to its low carbon content, which provides the steel with a high resistance to corrosion in the grain, as well as protection from pitting and crevicing. Its duplex properties make it less susceptible to stress cracking, though this also means it is unable to handle temperatures above 300 degrees Celsius for event short periods of time but not a consideration when used in construction applications of glass barriers.
|Tensile strength||620 MPa min||515Mpa min|
|Yield strength||450 MPa min||290 MPa min|
|Elongation||25% min||50% min|
|Brinell hardness||293 HB max||217HB max|
|Rockwell hardness||31 HR C max||95HRB max|
|Elastic Modulus||450 MPa min||200GPa|
|Mean coefficient of thermal expansion 0-100˚C||13.7µm/m/˚C||15.9µm/m/˚C|
- ASTM/ASME: A240 UNS S32205/S31803
- EURONORM: 1.4462 X2CrNiMoN 22.5.3
- AFNOR: Z3 CrNi 22.05 AZ
- DIN:W.Nr 1.4462
When it comes to fabricating grade 2205 the processes become more difficult, as greater capacity equipment is necessary than is required for the cold forming of most other austentic steels. Cutting speeds for machining and milling are usually at least 20% slower than grades 304 and 316
Care should be taken when stretch forming, spinning and deep drawing grade 2205, though welding can be undertaken, providing the following conditions are followed:
- Heat input must be kept low before, during and after
- The material must cool to below 150 degrees Celsius between passes
- Filler grade 2209 must be used
Why use 2205 duplex stainless steel for glass balustrade and glass pool fence hardware?
Glass hardware requires a certain level of corrosion resistance to be present in the stainless steel used, more importantly one strong enough to withstand the extreme testing requirements of structural glass barriers to meet international building codes.
While 304 and 316/316L grades of stainless steel were previously used for glass hardware, the strength characteristics of 304/316 stainless steel fall short when achieving the high requirement of test loads. Grade 2205 Duplex more easily passes load testing, and has become more commonly used grade to achieve high load capacity in structural balustrade and railings allowing the use of larger glass panel sizes than previously permitted using lesser grades.
The corrosion resistance properties are much better suited to coastal areas that if properly maintained can all but eliminate the risk of surface corrosion known as tea staining.
- 2205 has twice the yield Strength of 304, 316 and 316L Stainless Steel
- Demonstrates good fatigue stress
- Superior resistance to corrosion cracking, crevice, pitting, erosion and general corrosion in severe environments when compared to 316, 316L and 304
- Enables light weight yet incredibly strong components allowing more cost effective products
- Stronger metal allows for thin wall investment castings offering more scope for new product designs
How can 2205 Duplex Stainless Steel be used in your business to give you the competitive edge?
3D modelling & fine element analysis of glass hardware
Finite Element Analysis modelling has long been used for metal components in construction. By calculating the forces that will act on a component, engineers can ensure that a given structural element is up to the challenges it will face in the real world. However, FEA can also be used when it comes to glass hardware components.
What Is FEA?
Finite Element Analysis, or FEA, is a type of 3D modelling that looks at how something will perform in the real world. It predicts how the forces that act upon a product will affect it. These include everything from heat, wind, water, and other environmental elements, to vibration.
FEA allows industrial designers and engineers to create products and systems that can withstand the real world pressures they’ll be subjected to. By doing this in the computer modelling stage of design, we are able to create a better product that is safer, stronger, and less expensive. Integrating this into both design and engineering processes also allows us to create glass hardware components that look great while still easily handling their structural purpose.
How does FEA help?
By using Finite Element Analysis, we are able to simulate products with 3D modeling and then apply the equivalent of test loads to the products to see how they actually perform before we commit to expensive manufacturing tooling. This simulation allows us to see first hand how our new designs stack up. We are then able to use “what if” scenarios using different designs and materials.
Using FEA modeling in conjunction with 3D printing we can fast track new products by many months and save many thousands of dollars in development cost by finding potential product issues before we go to manufactured samples.
It is possible to produce low weight yet incredibly strong glass hardware components. The lower weight also means a lower cost without sacrificing the quality of the piece or the overall structure by removing material that serves no structural purpose. Through 3D modelling of glass hardware, we can find the right combination of design and material to produce the results we want and our customers need.
One key use of these glass components is in frameless pool fencing and frameless balustrading. By using FEA to create the spigots for these situations, Exakt Glass has been able to engineer an innovative design. The result is a set of hardware components that offers a top-level design and strength at a cost effective pricing.
Glass spigots are amongst the most expensive component in a glass barrier system due to 3 main components of the manufacturing process:
1. Weight – the finished weight of the product has the single most significant cost as the material for investment castings and machining is charged out per kg. Reducing this weight by creating hollow recesses in the post in non-structurally critical zones we can save significant cost.
2. Material Selection – The type of metal used for the manufacturing bears greatly on cost and performance. By using higher strength materials such as 2205 Duplex stainless steel we can use less material but still retain required strength to meet test load parameters.
3. Surface Finishing – Polishing of outer surfaces is the second greatest cost after the metal substrate.
Predicting the future
While 3D modelling and Finite Element Analysis allows us to design a better product, it can also allow us to show how our glass balustrade or glass pool fencing will hold up in use. There’s no need to build the actual balustrade or fencing to figure out whether it will be strong enough to handle the external forces of daily use. This greatly reduces our development cost and fast tracks design to final product testing process and therefore our final selling price making our products amongst the most cost effective engineered solutions in the marketplace.
By making use of the technological abilities of FEA, we are able to offer a far superior product to what is currently on the market. Rather than producing a low quality lightweight solution or a durable but expensive option, we have used our design and engineering expertise to create a spigot for frameless glass installations that is the best of both worlds.
To find out more about what Exakt Glass products can do, how they can be used for your frameless glass balustrade or frameless glass pool fencing project, and how we’ve developed them to meet your needs, get in touch with us today.