| The
first and most important step toward successful use
of stainless steel or nickel alloy is selection of the
proper type for the application. Stainless steels and
nickel alloys include a large number of standard types,
but these types differ greatly from one another in composition,
corrosion resistance, physical properties, and mechanical
properties, and selection of the optimum type for a
specific application is the key to satisfactory performance
at a minimum total cost. Below is a suggested checklist
of properties to be considered in the selection of the
proper type for a specific application. This includes
not only the obvious properties but also some less frequently
required, but occasionally overlooked, properties.
Property Checklist for Type Selection
Corrosion resistance
Oxidation and sulfidation resistance
Strength and ductility at service and ambient temperatures
Suitability for intended fabrication techniques
Suitability for intended cleaning procedures
Property stability in service
Toughness
Abrasion and erosion resistance
Galling and seizing resistance
Reflectivity
Magnetic properties
Thermal conductivity
Thermal expansion
Electrical resistivity
Sharpness (retention of cutting edge)
Rigidity
Dimensional stability
Corrosion resistance is frequently the
most important characteristic of a stainless or heat
resistant steel but is often also the most difficult
to assess for a specific application. General corrosion
resistance to natural conditions and to pure chemical
solutions is comparatively easy to determine. However,
general corrosion is often much less serious than localized
forms such as stress corrosion cracking, crevice corrosion
in tight spaces or under deposits, pitting attack, intergranular
attack in "sensitized" material such as in
weld heat affected zones, etc. Such localized corrosion
can cause unexpected and sometimes catastrophic failures
while most of a structure is unaffected and must, therefore,
be considered carefully in design and in steel selection.
Corrosive attack can also be dramatically increased
by seemingly minor impurities in the medium, which may
be difficult to anticipate but can have major effects
even in parts-per-million concentrations. At elevated
temperatures, an attack on the metal can be significantly
accelerated by seemingly minor changes in atmosphere
which affect scaling rate, sulfidation, or carburization.
Despite these complications, a suitable
steel can be selected for most applications on the basis
of experience, perhaps assisted by suggestions from
the steel producer. However, it must be recognized that
laboratory corrosion data can be misleading in predicting
service performance of a particular type. Even service
data have limitations because similar corrosive media
may differ substantially due to slight variations in
some of the above corrosion factors. For difficult applications,
extensive study of comparative data may be necessary,
sometimes followed by pilot or service testing.
Mechanical properties at service temperature
are an obvious consideration, but sometimes overlooked
is the necessity for satisfactory properties at other
temperatures which are likely to be experienced. Thus,
a product for arctic service must have suitable properties
at subzero temperatures even though the steady-state
operating temperature may be much higher, and post-service
room-temperature properties can be important for a structure
which may be intermittently shut down after operating
at an elevated temperature.
Selection must consider not only performance
requirements but also fabrication and cleaning requirements.
Frequently, a particular type is chosen for a fabrication
characteristic such as formability or weldability over
other types, which would perform adequately, but cost
more to fabricate. Even a required or preferred cleaning
procedure may dictate steel selection. Sometimes, it
is overlooked that a welded fabrication, which is to
be cleaned in a medium which attacks sensitized stainless
steel, such as nitric-hydrofluoric acid, should be produced
from stabilized or low-carbon types even though sensitization
may be unimportant under service conditions. Other properties
listed in the checklist are of vital importance for
some specialized applications but are of little concern
for many other applications. Surface finish is important
for many applications, and stainless steels are sometimes
used because of the variety of attractive finishes available.
Selection among these finishes may be made on the basis
of characteristics such as appearance, slideability,
or cleanability. Effect of finish on the cleanability
is not as simple as sometimes thought, and tests of
available finishes may be advisable. Selection of finish
may in turn influence selection of type because of differences
in availability of the various finishes with grade or
differences in finish durability. A more corrosion resistant
type, for example, will maintain a bright finish in
a corrosive environment, which would dull a lower alloy
steel. |
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