Grade
316 is the standard molybdenum-bearing grade, second
in importance to 304 amongst the austenitic stainless steels.
The molybdenum gives 316 better overall corrosion resistant
properties than Grade 304, particularly higher resistance to
pitting and crevice corrosion in chloride environments. It has
excellent forming and welding characteristics. It is readily
brake or roll formed into a variety of parts for applications
in the industrial, architectural, and transportation fields.
Grade 316 also has outstanding welding characteristics. Post-weld
annealing is not required when welding thin sections.
Grade 316L, the low carbon version of 316 and is immune from
sensitization (grain boundary carbide precipitation). Thus it
is extensively used in heavy gauge welded components (over about
6mm). Grade 316H, with its higher carbon content has application
at elevated temperatures, as does stabilized grade 316Ti.
The austenitic structure also gives these grades excellent toughness,
even down to cryogenic temperatures. |
| Martensitic
Stainless Steels / Surgical grade stainless steels
High carbon and lower chromium content are the distinguishing
features of martensitic stainless steel when compared with
ferritic stainless.
Martensitic stainless steels include 410, 416, 420 and 440.
Hardened martensitic steels cannot be successfully cold formed.
They are magnetic, have moderate corrosion resistance and
poor weldability.
For surgical grade application: 420, 440 are used.
Grade 420 stainless steel is a higher carbon version of 410;
like most non-stainless steels it can be hardened by heat
treatment. It contains a minimum of 12 per cent chromium,
just sufficient to give corrosion resistance properties. It
has good ductility in the annealed condition but is capable
of being hardened up to Rockwell Hardness 50HRC, the highest
hardness of the 12 per cent chromium grades. Its best corrosion
resistance is achieved when the metal is hardened and surface
ground or polished.
Related grades to 420 are high carbon high hardness martensitic
stainless steels such as the 440 series and also variations
to 420 containing molybdenum (for increased corrosion resistance),
sulphur (for increased machinability) or vanadium (for higher
hardness). A slightly higher carbon version of 420 is the
non-standard grade 420C.
Grade 440C is capable of attaining, after heat treatment,
the highest strength, hardness and wear resistance of all
the stainless alloys. Its very high carbon content is responsible
for these characteristics, which make 440C particularly suited
to such applications as ball bearings and valve parts.
Grades 440A and 440B are identical except for slightly lower
carbon contents (0.60 - 0.75% and 0.75 - 0.95% respectively);
these have lower attainable hardnesses but slightly higher
corrosion resistances. Although all three versions of this
grade are standard grades, in practice 440C is more available
than the A or B variants.
A free-machining variant 440F (UNS S44020) also exists, with
the same high carbon content as 440C. Again this grade is
not readily available in Australia.
Martensitic stainless steels are optimised for high hardness,
and other properties are to some degree compromised. Fabrication
must be by methods that allow for poor weldability and usually
also allow for a final harden and temper heat treatment. Corrosion
resistance is lower than the common austenitic grades, and
their useful operating temperature range is limited by their
loss of ductility at sub-zero temperatures and loss of strength
by over-tempering at elevated temperatures.
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