Biomedical Engineering Reference
In-Depth Information
form a thin, invisible layer of chrome-containing oxide, called the
passive ilm.
Stainless steel is usually divided into ive types: ferritic, austenitic,
martensitic, precipitation-hardened, and duplex. Only austenitic
and martensitic stainless steels are used in medical applications.
Austenitic, or 300 series, stainless steels make up over 70% of
total stainless steel production. They contain a maximum of 0.15%
carbon, a minimum of 16% chromium and suficient nickel and/or
manganese to retain an austenitic structure at all temperatures from
the cryogenic region to the melting point of the alloy. Superaustenitic
stainless steels, such as alloy AL-6XN and 254SMO, exhibit great
resistance to chloride pitting and crevice corrosion due to high
molybdenum content (>6%) and nitrogen additions, and the higher
nickel content ensures better resistance to stress-corrosion cracking
versus the 300 series. The higher alloy content of superaustenitic
steels makes them more expensive. They are nominally non-
magnetic but usually exhibit some magnetic response depending on
the composition and the work hardening of the steel.
On the other hand, martensitic steels are similar to ferritic steels
in being based on chromium but have higher carbon levels up as
high as 1%. This allows them to be hardened and tempered much
like carbon and low-alloy steels. They are used where high strength
and moderate corrosion resistance is required. They have generally
low weldability and formability and are magnetic.
These types of steel are well-suited for making surgical
instruments: they are easy to clean and sterilize, strong and corrosion-
resistant. The nickel/chrome/molybdenum alloys are also used for
orthopedic implants as aids in bone repair, as a structural part of
artiicial heart valves, and other implants. Immune system reaction
to nickel is a potential complication. Most surgical equipment is
made out of martensite steel. It is much harder than austenitic steel,
and it is easier to keep it sharp.
Implants are made out of austenitic steel, often 316L and
316LVM, because it is less brittle (see Tables 3.6 and 3.7). 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 pit and crevice corrosion in chloride
environments. Grade 316L, the low carbon version of 316 and is
immune from sensitization (grain boundary carbide precipitation).
