Biomedical Engineering Reference
In-Depth Information
Alloying elements that increase the solubility of nitrogen are
molybdenum, manganese, chromium, vanadium, niobium, and
titanium. However, vanadium, niobium and titanium must not be
used in large quantities due to their responsibility for the formation
of nitrides. Elements that reduce the solubility of nitrogen are
carbon, silicon, and nickel.
Austenitic steels with higher nitrogen content were produced
for the irst time in the forties of the 20th century. The main driving
force for their creation was the need to replace the nickel with
cheaper nitrogen. The resulting 18−8 steels with a nitrogen content
of 0.3%, while retaining the 6% nickel, retained austenite structure
and were characterized by both increased strength and yield point.
The increase in these parameters was, however, accompanied with
decreased ductility. As demonstrated by subsequent studies, the
increase in strength without reducing ductility comes with the
addition of at least 0.4% nitrogen.
The development of technology has enabled the introduction of
more than 1% of nitrogen to the steel, which resulted in increase in
strength by 134%, with decrease of elongation by only 8%.
Steels with high nitrogen content are examples of positive effects
of nitrogen on the resistance to cracking. However, the addition of
nitrogen to alloy does not guarantee resistance to cracking in the
environment containing chloride ions. According to the graph
shown in Fig. 6.2, it can be concluded that the addition of nitrogen
to 18Cr-10Ni type steel results in reduced resistance to corrosion
cracking, expressed in shortening the time to rupture. It is also the
result of boiling in aqueous solution of MgCl
2
. Nitrogen was added to
stabilize the austenite, but in this case the presence of nitrogen does
not eliminate the harmful effects of carbon.
Modern steels with carbon completely replaced by nitrogen and
with very high concentration of the alloying elements show a much
higher resistance to cracking. In Fig. 6.3, a measure of resistance
to cracking is cracking initiation temperature — the higher the
temperature, the less prone to cracking. The addition of nitrogen,
nickel and molybdenum results in a high temperature to initiate
cracks.
