Biomedical Engineering Reference
In-Depth Information
Tabl e 2. 4
Stainless steel and Cobalt-chrome alloys: typical values of mechanical properties.
Fatigue strength is expressed as the stress below which the material will not fail at specified
conditions for R and number of cycles
Alloy
Condition
a
Fatigue
b
Design.
E
YS
UTS
ASTM
GPa
MPa
MPa
MPa
420
an.
> 720
h
C
t
1,420
>1,720
304
an
193
205
515
250
316L
an
F138
200
330
590
250
316L
cw
F138
190
790
930
390
CF
F138
190
1,210
1,350
820
Co29Cr5Mo
c
C
an
F75
210
480
770
260
Co29Cr5Mo
hf
F799
210
1,050
1,500
750
Co20Cr15W10Ni
cw 44%
F90
230
1,600
1,940
590
Co20Cr35Ni10Mo
cw
C
a
F562
230
1,500
1,800
740
a
Condition:
a
:aged;
f
:forged;
an
:annealed;
ca
:as cast;
cw
:cold worked;
CF
:cold forged;
h
C
t
:hardened
C
tempered
b
At 10
7
cycles with R
D
1. R is the ratio of min.stress to max.stress
gives rise to the formation of carbides with Co, Cr or Mo (M
23
C
6
) and these car-
bides enhance the wear resistance. The intrinsic limitations of this cast alloy were
the millimeter-sized grains and the development of Cr, Mo and C rich domains (with
carbides) and dendrites depleted in Cr and rich in Co. The large grain size is unfa-
vorable for its fatigue characteristics while the domains of different composition are
unfavorable for its corrosion resistance (galvanic differences between the segregated
domains; see next chapter). Figure
2.6
a is a metallographic section through an ortho-
pedic screw: oddly shaped large grains and carbides along the grain boundaries are
the main feature, the latter also shown into more detail in Fig.
2.6
b. The finer grains
at the surface of the material are already produced during solidification. Because of
a higher cooling rate at the surface as compared to the inner part and because of
imperfections at the mould walls, more nuclei are formed producing finer grain size
(screw of unknown manufacturer).
Another metallographic feature is the smaller grains at the surface, a result of
thermal treatment after machining and probably performed to improve the torque
strength.
An alternative alloy is F799. It has about the same composition but is forged.
It lifts important shortcomings of F75: smaller grain size and fine more homo-
geneously distributed carbide precipitates. Superior fatigue and wear resistance
compared to F75 made it for many years the alloy of preference for hip prostheses.
Another respected member of the family is a CoNiCrMo alloy, called originally
MP35N. High corrosion resistance, UTS and fatigue strength and excellent abrasive
wear properties are the strong points and made it a suitable candidate for hip stems
