Biomedical Engineering Reference
In-Depth Information
6.2 Common metallic biomaterials
6.2.1 Austenitic stainless steels
The first stainless steel utilized for fabrication of implants was developed
in 1926 by Krupp (essen-Germany). It contained 18% chromium and 8%
nickel and became popular as 18-8 (type 302 in modern classification). The
key function of chromium is to allow the formation of a protective Cr-rich
passive film on the surface. Later some molybdenum was added to improve
the pitting corrosion resistance. It was called 18-8Mo and became later type
316 stainless steel. With the introduction of vacuum technology in 1950,
it was possible to reduce the carbon content from 0.08 to about 0.03 wt%
maximum, yielding an increase in the corrosion resistance. The main rationale
for addition of nickel, in the range 9-15 wt%, is to ensure an austenitic state
at room temperature for the high chromium content used. Compositions of
this and other alloys are listed in Table 6.2.
nowadays, about 1% of the total production of stainless steel is used for
biomedical applications. surgical and dental instruments are manufactured
from commercial-grade stainless steel. However, for implant-grade stainless
steels, special production routes such as vacuum melting (VM), vacuum
arc remelting (VAR) or electroslag refining (ESR) are used to increase the
resistance to pitting and crevice corrosion, as well as to decrease quantity and
size of the non-metallic inclusions. austenitic stainless steels constitute the
largest stainless steel family in terms of number of alloys and use. They are
popular because are relatively inexpensive, easy to machine using common
techniques and their mechanical properties can be controlled over a wide
range for optimal strength and ductility.
Vacuum melted 316L stainless steel remains the most widely used stainless
steel for implant devices. They find applications mostly as bone screws, bone
plates, intramedullary nails and rods and other temporary fixation devices.
The microstructure consists of austenite with a single face-centered cubic
(fcc) structure called the g phase. The ferrite, a phase with a body-centered
cubic (bcc) structure, should not be present, not only from the standpoint
of its corrosion resistance but also because of its ferromagnetic behaviour.
sulphide inclusions predispose the steel to pitting-type corrosion at the
metal-inclusion interface and, therefore, their presence must be also avoided.
Passivation treatments applied at the last moment of the fabrication of the
components are used to remove inclusions emerging at the surface. The
recommended grain size is asTM no. 6 (< 100 mm) or finer. Another feature
of the microstructure is the texture as the result of a preferred orientation
of deformed grains. Figure 6.2 shows a typical micrograph of the 316 LVM
steel in the cold worked and aged condition. Within the grains, thermal twins
that developed during ageing are observed.
The austenitic stainless steels work-harden very rapidly and therefore
