What is perforated stainless steel used for?
Architectural/Civil Engineering – cladding, street furniture, structural sections, reinforcement bar, lighting columns, lintels, masonry supports Transport – exhaust systems, car trim/grilles, road tankers, ship containers, ships chemical tankers, refuse vehicles Chemical/Pharmaceutical – pressure vessels, process piping. Filtration Oil and Gas – platform accommodation, cable trays, subsea pipelines. Food and Drink – Catering equipment, brewing, distilling, food processing. Water – Water and sewage treatment, water tubing, hot water tanks.
At Bion we have 55 years of acquired knowledge and experience within the perforating market, with a large proportion of this being within the stainless steel market. Our state of the art levelling equipment means a superior flat product (where required) along with knowledge of any specialist material and how this performs (see below).
Stainless steel is usually divided into 5 types:
Ferritic – These steels are based on Chromium with small amounts of Carbon usually less than 0.10%. These steels have a similar microstructure to carbon and low alloy steels. They are usually limited in use to relatively thin sections due to lack of toughness in welds. However, where welding is not required they offer a wide range of applications. They cannot be hardened by heat treatment. High Chromium steels with additions of Molybdenum can be used in quite aggressive conditions such as sea water. Ferritic steels are also chosen for their resistance to stress corrosion cracking. They are not as formable as austenitic stainless steels. They are magnetic.
Austenitic – These steels are the most common. Their microstructure is derived from the addition of Nickel, Manganese and Nitrogen. It is the same structure as occurs in ordinary steels at much higher temperatures. This structure gives these steels their characteristic combination of weldability and formability. Corrosion resistance can be enhanced by adding Chromium, Molybdenum and Nitrogen. They cannot be hardened by heat treatment but have the useful property of being able to be work hardened to high strength levels whilst retaining a useful level of ductility and toughness. Standard austenitic steels are vulnerable to stress corrosion cracking. Higher nickel austenitic steels have increased resistance to stress corrosion cracking. They are nominally non-magnetic but usually exhibit some magnetic response depending on the composition and the work hardening of the steel.
Martensitic – These steels are similar to ferritic steels in being based on Chromium but have higher Carbon levels up as high as 1%. This allows them to be hardened and tempered much like carbon and low-alloy steels. They are used where high strength and moderate corrosion resistance is required. They are more common in long products than in sheet and plate form. They have generally low weldability and formability. They are magnetic.
Duplex – These steels have a microstructure which is approximately 50% ferritic and 50% austenitic. This gives them a higher strength than either ferritic or austenitic steels. They are resistant to stress corrosion cracking. So called “lean duplex” steels are formulated to have comparable corrosion resistance to standard austenitic steels but with enhanced strength and resistance to stress corrosion cracking. “Superduplex” steels have enhanced strength and resistance to all forms of corrosion compared to standard austenitic steels. They are weldable but need care in selection of welding consumables and heat input. They have moderate formability. They are magnetic but not so much as the ferritic, martensitic and PH grades due to the 50% austenitic phase.
Precipitation hardening (PH) – These steels can develop very high strength by adding elements such as Copper, Niobium and Aluminium to the steel. With a suitable “aging” heat treatment, very fine particles form in the matrix of the steel which imparts strength. These steels can be machined to quite intricate shapes requiring good tolerances before the final aging treatment as there is minimal distortion from the final treatment. This is in contrast to conventional hardening and tempering in martensitic steels where distortion is more of a problem. Corrosion resistance is comparable to standard austenitic steels like 1.4301 (304).
How do I choose which stainless steel to use for my perforated application?
Most decisions about which steel to use are based on a combination of the following factors:
What is the corrosive environment? – Atmospheric, water, concentration of particular chemicals, chloride content, presence of acid.
What is the temperature of operation? – High temperatures usually accelerate corrosion rates and therefore indicate a higher grade. Low temperatures will require a tough austenitic steel.
What strength is required? – Higher strength can be obtained from the austenitic, duplex, martensitic and PH steels. Other processes such as welding and forming often influence which of these is most suitable. For example, high strength austenitic steels produced by work hardening would not be suitable where welding was necessary as the process would soften the steel.
What welding will be carried out? – Austenitic steels are generally more weldable than the other types. Ferritic steels are weldable in thin sections. Duplex steels require more care than austenitic steels but are now regarded as fully weldable. Martensitic and PH grades are less weldable.
What degree of forming is required to make the component? – Austenitic steels are the most formable of all the types being able to undergo a high degree of deep drawing or stretch forming. Generally, ferritic steels are not as formable but can still be capable of producing quite intricate shapes. Duplex, martensitic and PH grades are not particularly formable.
What product form is required? – Not all grades are available in all product forms and sizes, for example sheet, bar, tube. In general, the austenitic steels are available in all product forms over a wide range of dimensions. Ferritics are more likely to be in sheet form than bar. For martensitic steels, the reverse is true.
What are the customer’s expectations of the performance of the material? – This is an important consideration often missed in the selection process. Particularly, what are the aesthetic requirements as compared to the structural requirements? Design life is sometimes specified but is very difficult to guarantee.
There may also be special requirements such as non-magnetic properties to take into account.
It must also be borne in mind that steel type alone is not the only factor in material selection. Surface finish is at least as important in many applications, particularly where there is a strong aesthetic component. See https://www.bssa.org.uk/cms/File/surfacefinishbssaVer2.pdf
Availability. There may be a perfectly correct technical choice of material which cannot be implemented because it is not available in the time required.
Cost. Sometimes the correct technical option is not finally chosen on cost grounds alone. However, it is important to assess cost on the correct basis. Many stainless steel applications are shown to be advantageous on a life cycle cost basis rather than initial cost.
The final choice will almost certainly be in the hands of a specialist but their task can be helped by gathering as much information about the above factors. Missing information is sometimes the difference between a successful and unsuccessful application
Although stainless steel is much more resistant to corrosion than ordinary carbon or alloy steels, in some circumstances it can corrode. It is ‘stain-less’ not ‘stain-impossible’. In normal atmospheric or water based environments, stainless steel will not corrode as demonstrated by domestic sink units, cutlery, saucepans and work-surfaces. In more aggressive conditions, the basic types of stainless steel may corrode and a more highly alloyed stainless steel can be used.
Getting the best from your perforated Stainless Steel
Although the vast majority of stainless steel applications work perfectly as intended by the designer and fabricator, there are a significant number of instances where someone, often the end-user, is disappointed by the performance of the material. The causes of these disappointments tend to fall into only a few basic categories. In nearly all cases, a little basic knowledge would have prevented or significantly improved the situation.
On the assumption that prevention is better than cure, this short article addresses these issues.
The causes of disappointment can arise at any point in the long supply chain that often applies to a stainless steel project. This helps to explain why problems occur. Getting the appropriate knowledge to all parts of the supply chain is difficult and it only takes ignorance in one small part to create a problem later on.
The main issues are:
- Importance of surface finish in determining corrosion resistance
- Lack of knowledge in this area is a major cause of problems. Most specifiers and designers understand the importance of selecting a grade of stainless steel, for example 1.4301 (304) or 1.4401 (316). But surface finish is at least as important.
- Briefly, a bright polished surface gives maximum corrosion resistance.
- A directional polish equivalent to the EN 10088-2 2K (Ra = 0.5 micron max), usually produced using silicon carbide (SiC) abrasives, will give adequate corrosion resistance in many severe environments notably heavy urban and coastal ones.
- A common surface finish achieved with 240 grit alumina abrasives has been implicated in the corrosion of stainless steel in urban and coastal environments. In some cases, surface roughness Ra values have been measured at well above 1 micron which is known to be inadequate in these environments.
- The lack of any specified surface finish on architectural drawings can be the source of the final problem.
- If, at any stage of the supply chain, there is any doubt about the appropriate surface finish, specialist advice should be sought. Bion are on hand to provide support and advise you throughout your project.