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Notes about stainless steel, its use and fabrication.

Click to visit Stainless Selection or Tank Design Notes or Stainless Tanks or Hollow Spheres

Stainless steel is a preferred material whenever hygienic processes or in-service corrosion is anticipated. Its usage varies a lot from place to place, depending on a whole range of reasons, including proximity to the sea and the national economy base. Nowhere in New Zealand is more than 100 miles from the sea and dairy processing is the backbone of our economy. From building components, through domestic and commercial kitchens, to our huge "silver city" dairy factories, we use a lot of stainless.

What is Stainless Steel?

The following paragraph is reproduced from the SSINA Website, which can be visited from the Related Sites page.
"Stainless steel is essentially a low carbon steel which contains chromium at 10% or more by weight. It is this addition of chromium that gives the steel its unique stainless, corrosion resisting properties.
The chromium content of the steel allows the formation of a ... corrosion-resisting chromium oxide film on the steel surface. If damaged mechanically or chemically, this film is self-healing, providing that oxygen, even in very small amounts, is present. The corrosion resistance and other useful properties of the steel are enhanced by increased chromium content and the addition of other elements such as molybdenum, nickel and nitrogen. There are more than 60 grades of stainless steel....(it) is 100% recyclable"

For full cost effectiveness however, there are one or two other aspects to consider.

It costs several times more than carbon or mild steel, so appropriate design is proportionally more critical.

It is also worthwhile to select an appropriate grade. Highly corrosive conditions require specialised knowledge, but for general hygiene quality fabrication, the following guidelines apply-
304 (UNS S30400) Probably the most used grade of all the stainless steels. It is of the group classified as austenitic which are non-magnetic. If the thickness exceeds 3mm and it is to be welded, L grade (UNS S30403) is recommended, which has a carbon content less than 0.03%
316 (UNS S31600) for improved corrosion resistance at temperatures less than 60o Centigrade (120o Fahrenheit) If the thickness exceeds 3mm and it is to be welded, L grade (UNS S31603) is recommended. It is also austenitic.
2205 (UNS S31803) for use in temperatures exceeding 60o Centigrade, where any free halides may be present, to resist stress corrosion cracking. Typically, this includes hot water. This 'duplex' grade is magnetic and approximately 15% stronger than the 'austenitic' grades (304 and 316) which can be a cost effective option when tension governs such as for pressure vessels, not subject to vacuum. It is significantly harder to form however. It work hardens rapidly, requiring any forming processes to be as near as possible to one pass. This is particularly applicable when forming compounded curve shapes such as knuckling and dome forming. Consequently, and depending on the ratio of the ends area to wall area of the tank the additional labour involved can be 10 to 15% more. For more information about duplex, visit http://www.stainless-steel-world.net/duplex/index.asp
 
Series200 With the recent increases in global nickel prices, the 200 series grade of stainless that essentially substitutes manganese for nickel, may be an option to be considered. There are however some significant limitations to its application, not the least of which can be corrosion resistance. For more in-depth information, either download CreviceCorrosion here and/or PittingCorrosion here or visit http://www.worldstainless.org from where both files have been copied.

 

The Relative Costs of Stainless Steel

As noted in the Design Notes page, the materials in stainless steel tanks make up between 30 and 60% of the ex-works price. 316 is roughly 70% and 2205 roughly 100% more expensive than 304. To develop an in-depth understanding of the various relative cost factors, we have developed a unique tank fabrication costing program to assist design engineers, fabrication shops and tank users alike. It has the remarkable ability to estimate market values of practically any thin-walled, shop fabricated, stainless steel tank, without reference to any external historical data. A fully functional, demo version is available. Click here

 

Further Information

For more extensive information, click Stainless Selection which is reproduced adapted from the BSSA website, from "Design Guidelines for the Selection and Use of Stainless Steel" AISI Designers' Handbook Series, published by the Nickel Development Institute (see also the Related Sites page) and includes notes about the maintenance of stainless steel

Another invaluable resource is the Outokumpu website from where you can search Technical Documents by keyword. http://www.outokumpu.com

There are many grades of stainless available, some having been developed for quite specific applications, and there are various international non-profit organisations, dedicated to the promotion of stainless. Most have excellent, comprehensive information available, there is a directory of links to some of the major ones on the Related Sites page.

A very comprehensive database of material properties, including stainless steels, can also be found at the MatWeb site. It is so comprehensive that you will need to use the search facility to find any individual alloy. Another good site is the Hendrix Group where there is a useful relative costs table.

At EngineerOnLine, we include a competitive, specialised stainless steel tank design, drawing and/or specification service that will ensure the best chance for a cost effective, appropriate production. We can also peruse an existing specification for comment if you wish.

The information on this web page is the property of EngineerOnLine Ltd. It may be used free of charge  or reproduced with an appropriate acknowledgement and previous permission. However no responsibility for anything said or implied is accepted by us.

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