Biomedical Engineering Reference
In-Depth Information
World Health Organization concluded that metallic nickel and
alloys containing nickel are carcinogenic. Nickel content of less
than 0.2% is considered adequate for the human body. However,
the reduction of nickel content below 0.2%, using conventional
manufacturing methods, is associated with a signiicant increase in
the cost of manufacture of the material.
In the following chapter, a concept of nickel-free austenitic
stainless steels will be presented.
6.1 NickelFree Stainless Steels
From the chemical composition point of view, nitrogen is the most
promising substitute for nickel. It is a gas at room temperature,
stabilizes austenitic phase, increases the corrosion resistance,
and prevents the formation of the sigma phase. Nitrogen dissolves
interstitially in austenite structure in relatively large quantities,
causing a large number of solution hardening. Drastic reduction
in resistance to brittle fracture and corrosion resistance occurs
with the presence of sigma phase with low nitrogen content and
nitrides of chromium, such as Cr
2
N with high nitrogen content.
Therefore, the amount of nitrogen should be limited to obtain a
homogeneous microstructure without inclusions (Fig. 6.1). Nitrides
inclusions can lead to a local drop in chromium content, which in
turn leads to local corrosion around the inclusions [14].
Figure 6.1
Schematic representation of strength and corrosion resistance
as a function of nitrogen content in nickel-free austenitic
stainless steel [14].
