|
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)
Another invaluable resource is the Outokumpu website from where you
can search Technical Documents by keyword. http://www.outokumpu.com/pages/Search____9170.aspx
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.
|
