IGCAR : Annual Report - Indira Gandhi Centre for Atomic Research

IGC
Annual Report 2007
(~15 s -1 ) than at 25 Hz, the
deformation leads to work
hardening and reduces the
tendency for recovery process.
This situation makes the
material less ductile and leads
to fracturing of the material.
The harder material at the
surface results in surface cracks
and material removal. In view
of the above, the wear is high
at 75 Hz. At 125 Hz, the higher
deformation speed (~25 s -1 )
results in higher temperature at
the tip of the pin. The
calculated rise in the
temperature is above
1223 K. This increase in
temperature induces the
oxidation of the surface. The
oxide layer thus formed on the
sliding surfaces forms a glazing
layer. Moreover, the oxide layer
is stronger than that of the base
material. Because of the
presence of glazed layer the
wear rate is low at 125Hz. Fig.
3(a-b) reveals the presence of
oxide particles in the
microstructure of the wear scar.
The Topography of the scar was
examined using 3D surface
profilometer and it is presented
in Fig. 3(c). Fig. 3(c) shows that
the scar was covered with a
layer of material of about 2 mm
thickness. This layer could be
oxide layer. Tests were also
performed at a load of 100 N
at 125 Hz and at 823 K to
confirm the influence of heat
generated during fretting on the
formation of oxide layers.
The above observations
reveal that the wear rate is
higher at 125 Hz. In the steam
generator of PFBR, the level of
oxygen is 2 ppm hence, it is
essential to carry out insotdium
tests at 125 Hz.
III.C.8. Effect of Flowing Sodium on
Corrosion and Tensile Properties of AISI Type 316LN
Stainless Steel at 823 K
Austenitic stainless steels (SS)
of different grades are used as
structural materials in primary
circuit, IHX and piping in
secondary loop of fast breeder
reactors owing to their good
compatibility with sodium in
addition to their desirable
properties of adequate high
temperature mechanical
properties and resistance to
neutron irradiation. Sodium is
used as coolant in liquid metal
fast breeder reactors due to its
multifaceted properties of high
thermal conductivity, low
vapour pressure, high boiling
point, large heat capacity and
low cost.
Long term exposure of
austenitic SS to high
temperature sodium leads to
mass transfer and corrosion.
This corrosion is predominantly
governed by the impurities
present in the sodium,
especially carbon and oxygen.
Presence of oxygen in sodium
significantly influences the
corrosion processes because
leaching is usually preceded by
the formation of ternary
compounds of the steel
constituents with oxygen and
sodium. The most commonly
encountered corrosion product
in sodium containing less than
10 ppm oxygen is NaCrO 2 . The
transfer of elements from
structural materials by liquid
sodium is reported to influence
Fig.1 Microstructure showing
316modified layer
R&D FOR FBRs 55