Non-magnetic Stainless Steel??? -Reply

[PAT DOYLE]

Since the subject of facts came up yesterday, I
would like to address some comments made on
stainless steel.  

I don't intend on making this a dissertation on
stainless steel, but I thought that some further
clarification was required.

Stainless steel is a ferrous (iron based) alloy. 
The American Society for Metals (ASM) defines
stainless steel as an alloy containing at least
10% chromium (Cr) (with or without other
elements).  Few of the alloys contain more than
30% Cr or less than 50% iron (Fe).  Cr is the
element that makes the stainless steel more
resistant to rusting and staining than plain
carbon or low-alloy steels.  Other elements are
added to give specific properties.

There are four metallurgic categories for
stainless steel:  1) martensitic; 2) ferritic; 3)
austenitic; 4) precipitation-hardened.  The most
common types found in general use are martensitic
and austenitic.  I will not address the less
common types.

Austenitic stainless is non-magnetic.
Martensitic stainless is magnetic.  

Nickel (Ni) is found in austenitic stainless and
is makes it non-magnetic.  It has to do with the
alloy structure and where the various elements of
the alloy end up in that structure.

Martensitic is stronger and harder than
austenitic, primarily because it is heat treated. 
The heat treating causes the alloy's structure to
form such that it is not magnetic.
 
Alloy 304 is an austenitic stainless and has the
widest range of applications and is available in a
variety of forms for exterior architectural
applications.  If you encounter stainless steel,
it is most likely 304.  It possesses excellent
resistance to corrosion from exposure to weather. 
It is non-magnetic.

304L is a low carbon version having slightly
higher corrosion resistance.  This is a good
choice if the piece is to be welded.  Welding
reduces stainless steels' corrosion resistance.
Carbon will "steal" Cr to form Chromium Carbide at
welding temperatures (remember that Cr is what
makes stainless corrosion resistant). 

There are many other alloys (each with there own
number and specification) with specific material
properties.  ASM publishes volumes on the subject.
 The Society of Automotive Engineers publish
specifications for the alloys.

The talk about using a magnet to find "better
stainless" does have some merit, but "better" is
relative to what you want to use it for.  If a
metal is magnetic, it is steel and might be
stainless.  If it is not magnetic, it might be
stainless or one of the many other non-ferrous
metals (aluminum, titanium, magnesium, etc.).  For
corrosion resistance and strength, stainless steel
is better choice than most other alloys of steel. 
There are other metals that are more corrosion
resistant and lighter (titanium and aluminum are
two), but they tend to be more expensive, weaker,
or more difficult to work with, especially if you
intend on welding parts.

I've gone on long enough.  This is a vast subject.

If anyone wants to pursue the study of the
subject, I recommend a good Materials
Engineering/Science Handbook as a start.  If you
are building or designing something to last a long
time and you want to make a good materials
selection, there are very few encyclopedias that
provide enough information to make a well informed
decision.

KA2GSL


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