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QUICK FACT GUIDES FOR MACHINISTS
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GUIDES |
SIMPLIFYING THE SELECTION OF STAINLESS STEEL Everyone
is familiar with stainless steel knives and other tableware, auto trim, and
razor blades, but few understand that the versatile family of stainless steels
is an alloy system capable of providing a wide variety of corrosion resistance,
strength, and workability. Many recognize a finished stainless steel product,
but just what are stainless steels? How are they unique compared with other
materials? Typically,
stainless steels are alloys of iron to which a minimum of about 12% chromium has
been added to impart the same type of corrosion resistance conferred by pure
chromium in chrome plate. A 12% chromium stainless steel will not corrode or
“rust” when exposed to the weather. To obtain a greater corrosion
resistance, more chromium is added to the alloy, so that there are stainless
steels with chromium contents of 15% - 17% - 20% and even 27%. In
addition, all stainless steels are uniform in composition. There need be no
concern about chipping, scratching, or other surface damage, since any freshly
exposed surface is as resistant to corrosion as the original surface. This
remarkable attribute is the result of an invisible, self-forming and
self-healing, passive film (non-reactive chromium oxide), which always forms in
the presence of oxygen. Along
with iron and chromium, all stainless steels contain some carbon. It is
difficult to get much less than about 0.03% and sometimes carbon is deliberately
added up to 1.00% or more. The more carbon there is, the more chromium must be
used, because carbon can take from the alloy about seventeen times its own
weight of chromium to form carbides. This chromium in the form of carbides is of
little use for resisting corrosion. The carbon, of course, is added for the same
purpose as in ordinary steels, to make the alloy stronger. Other alloying elements are added for improved corrosion resistance, fabricabililty and variations in strength. These elements include appreciable amounts of nickel, molybdenum, copper, titanium, silicon, aluminum, sulfur and many others which cause pronounced metallurgical changes. The most important of these is nickel. If enough nickel is added, the entire nature of the alloy changes. |
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Copyright © 2002 QUICK FACT GUIDES FOR MACHINISTS
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