Biomedical Engineering Reference
In-Depth Information
1.2.2 StainlessSteels
Stainless steels are the most widely used family of alloys for medical appli-
cations. h ey contain 17-21% chromium which imparts good corrosion
resistance due to the adherent chromium oxide i lm that forms and heals
in the presence of oxygen. ASTM standards F138, F139, F1314, F1586, and
F2229 dei ne the chemical, mechanical, and microstructural requirements
for various types of stainless steels that are used for medical applications.
Austenitic stainless steels (American Iron and Steel Institute 300 series)
such as 304L or 316L are used in mainly in temporary implant applications
such as bone screws, bone plates, and intramedullary nails. Despite their
passive oxide layer, they nevertheless corrode, releasing chromium and
nickel into the body. Only minute quantities of these metals can be toler-
ated in the blood. Some stainless steels contain high amounts of nitrogen
(per ASTM F2229) so that the nickel content can be reduced to less than
0.05 wt.%, minimizing the risks associated with nickel allergy reactions.
h ese steels fall in the AISI 200 series, and are used for bone screws, plates,
and fracture i xation. Martensitic stainless steels from the AISI 400 series
are exceedingly strong and hard. However, they are ferromagnetic and are
generally used outside the body, for example, for surgical instruments.
h e austenitic stainless steels (AISI 300 series) are single phase and are
ot en strengthened through cold or hot working. Austenitic stainless steel
alloys are also readily strengthened by severe plastic deformation. For exam-
ple, Idell
et al.
used SPD to increase the strength of 316L stainless steel from
515 MPa to 1647 MPa [212]. Similarly, Chen
et al.
obtained a yield strength
of 1460 MPa in a duplex 32304 stainless steel at er just 4 ECAP passes [213].
High nitrogen variants of stainless steel (ASTM F1586 and F2229) may also
be cold worked to have ultimate tensile strengths above 1400 MPa.
While stainless steels are highly responsive to SPD, the adoption of SPD-
processed stainless steels for commercial applications has been slow, in part
because SPD introduces complex changes in microstructure that need to
be more thoroughly understood. SPD alters the phase compositions from
those normally expected in stainless steel. h is occurs through mecha-
nisms including stress-induced and strain-induced martensite formation.
For example strain-induced martensite was nucleated during ECAP of
301 stainless [214] and 304 stainless steels [215]. SPD introduces micro-
structural features such as nano-twins, micro-twins, micro-shear bands,
very high dislocation densities, and dif use subboundary structures. SPD
also alters the formation and distribution of carbides [216]. h ese micro-
structural ef ects in combination can signii cantly alter the annealing and
recrystallization behaviors of stainless steels [217, 218].
