Biomedical Engineering Reference
In-Depth Information
the formation of martensite, even in the most extreme conditions of
cold-working or cryogenic cooling. These steels, even after a strong
deformation, remain paramagnetic.
In a simulated biological environment, 316L steel shows
pitting corrosion. Nickel-free austenitic stainless steel showed no
corrosion of this type in all the electrolytes. Moreover, by preventing
precipitation of M
23
C
6
carbides, nitrogen reduces the risk of
intergranular corrosion [4, 6, 16, 17].
Nickel-free austenitic stainless steels containing nitrogen are
very promising metallic biomaterials [7]. They can be implemented
as implants in the form of: stabilizing bone plates, screws or wires
[9].
The most commonly used nickel-free stainless steels are shown
in Table 6.1 [9, 14, 26, 32]. However, due to the large strengthening
and low thermal conductivity of these steels, precise machining is
obstructed. Therefore, the production of small, precise products,
such as stents, from such steel is very expensive, and the range of
possible dimensions is limited.
Table 6.1
Chemical compositions of commercially available nickel-free
austenitic stainless steels
Chemical composition
Fe-(19-23)Cr-(10-12)Mn-(3-6)Mo-(0.85-1.1)N
Fe-15Cr-(10-15)Mn-4Mo-0.9N
Fe-18Cr-18Mn-2Mo-0.9N
Fe-(15-18)Cr-(10-12)Mn-(3-6)Mo-0.9N
Fe-23Cr-2Mo-1.5N
Efforts were taken to remedy this problem, and solution to this
day is a new method of manufacturing. This method relies on a pre-
cise machining of small products made of nickel-free ferritic stain-
less steels, and then nitriding their surface at 1200
o
C where they be-
come austenitic stainless steel with improved properties of strength
and corrosion resistance [11].
Schematic representation of the process is shown in Fig. 6.4,
and comparison of the properties of ferritic steels and nickel-free
austenitic steels (after nitriding) is presented in Table 6.2.
