Biomedical Engineering Reference
In-Depth Information
With 316L, the main rationale for the alloying additions
involves the metal's surface and bulk microstructure. The
key function of chromium is to permit the development of
corrosion-resistant steel by forming a strongly adherent
surface oxide (Cr
2
O
3
). However, the downside to adding
Cr is that it tends to stabilize the ferritic (body-centered
cubic (BCC)) phase of iron and steel, which is weaker
than the austenitic (face-centered cubic (FCC)) phase.
Moreover, molybdenum and silicon are also ferrite stabi-
lizers. So to counter this tendency to form weaker ferrite,
nickel is added to stabilize the stronger austenitic phase.
The main reason for the low carbon content in 316L is
to improve corrosion resistance. If the carbon content of
the steel significantly exceeds 0.03%, there is increased
danger of formation of carbides such as Cr
23
C
6
. Such
carbides have the bad habit of tending to precipitate at
grain boundaries when the carbon concentration and
thermal history are favorable to the kinetics of carbide
growth. The negative effect of carbide precipitation is
that it depletes the adjacent grain boundary regions of
chromium, which in turn has the effect of diminishing
formation of the protective, chromium-based oxide
Cr
2
O
3
. Steels in which such grain-boundary carbides
have formed are called ''sensitized'' and are prone to fail
through corrosion-assisted fractures that originate at the
sensitized (weakened) grain boundaries.
m
is approximately 0.5. From this equation it follows that
higher yield stresses may be achieved by a metal with
a smaller grain diameter
d,
all other things being equal. A
key determinant of grain size is manufacturing history,
including details on solidification conditions, cold-working,
annealing cycles, and recrystallization.
Another notable microstructural feature of 316L as
used in typical implants is plastic deformation within
grains (
Fig. 3.2.9-4B
). The metal is often used in a 30%
cold-worked state because cold-worked metal has
a markedly increased yield, ultimate tensile, and fatigue
strength relative to the annealed state (
Table 3.2.9-2
).
The trade-off is decreased ductility, but ordinarily this is
not a major concern in implant products.
In specific orthopedic devices such as bone screws
made of 316L, texture may also be a notable feature in
the microstructure. Texture means a preferred orienta-
tion of deformed grains. Stainless steel bone screws show
elongated grains in metallographic sections taken parallel
to the long axis of the screws (
Fig. 3.2.9-5
). Texture
arises as a result of the cold drawing or similar cold-
working operations inherent in the manufacture of bar
rod stock from which screws are usually machined. In
metallographic sections taken perpendicular to the
screw's long axis, the grains appear more equiaxed. A
summary of representative mechanical properties of
316L stainless is provided in
Table 3.2.9-2
, but this
should only be taken as a general guide, given that final
production steps specific to a given implant may often
affect properties of the final device.
Microstructure and mechanical properties
Under ASTM specifications, the desirable form of 316L
is single-phase austenite (FCC); there should be no free
ferritic (BCC) or carbide phases in the microstructure.
Also, the steel should be free of inclusions or impurity
phases such as sulfide stringers, which can arise primarily
from unclean steel-making practices and predispose the
steel to pitting-type corrosion at the metal-inclusion
interfaces.
The recommended grain size for 316L is ASTM #6 or
finer. The ASTM grain size number
n
is defined by the
formula:
Cobalt-based alloys
Composition
Cobalt-based alloys include Haynes-Stellite 21 and 25
(ASTM F75 and F90, respectively), forged Co-Cr-Mo
alloy (ASTM F799), and multiphase (MP) alloy MP35N
(ASTM F562). The F75 and F799 alloys are virtually
identical in composition (
Table 3.2.9-3
), each being about
58-70% Co and 26-30% Cr. The key difference is their
processing history, as discussed later. The other two alloys,
F90 and F562, have slightly less Co and Cr, but more Ni in
the case of F562, and more tungsten in the case of F90.
N ¼
2
n
1
(3.2.9.1)
where
N
is the number of grains counted in 1 square inch
at 100-times magnification (0.0645 mm
2
actual area).
As an example, when
n ¼
6, the grain size is about
100 microns or less. Furthermore, the grain size should
be relatively uniform throughout (
Fig. 3.2.9-4A
). The
emphasis on a fine grain size is explained by a Hall-Petch-
type relationship (
Hall, 1951; Petch, 1953
) between
mechanical yield stress and grain diameter:
Microstructures and properties
ASTM F75
The main attribute of this alloy is corrosion resistance in
chloride environments, which is related to its bulk
composition and surface oxide (nominally Cr
2
O
3
). This
alloy has a long history in both the aerospace and bio-
medical implant industries.
When F75 is cast into shape by investment casting
(''lost wax'' process), the alloy is melted at 1350-1450
C
and then poured or pressurized into ceramic molds of the
t
y
¼ t
i
þ kd
m
(3.2.9.2)
Here
t
y
and
t
i
are the yield and friction stress, respectively;
d
is the grain diameter;
k
is a constant associated with
propagation of deformation across grain boundaries; and
