IGCAR : Annual Report - Indira Gandhi Centre for Atomic Research

IGC
Annual Report 2007
Carbon content(wt%)
0.14
0.12
0.10
0.08
0.06
0.04
type 316SS
type 316LN
0.02
0 5 10 15 20 25 30 35
Distance(µm)
Fig.2 Measured carbon profile for
types and 316LN stainless
steel exposed for 16000 hours
obtained by SIMS analysis
the corrosion and mechanical
properties.
Here we report the results of
the changes in microstructure,
tensile and corrosion properties
of AISI type 316LN SS on
exposure to high temperature
liquid sodium at 823 K for
16000 hours and compare
them with the corresponding
changes in type 316 SS
exposed for the same duration
in high temperature liquid
sodium.
In both the steels, thermal
ageing at 823 K caused
carbide precipitation at the
grain boundaries. Due to
leaching of elements, there was
a surface modified layer of
ferrite up to around 10 µm
(Fig.1) in type 316LN SS while
the modified layer was up to
about 15 µm in type 316 SS.
The average microhardness of
mill-annealed type 316LN SS
was 174 VHN. Thermally aged
type 316LN SS showed an
average hardness of 236 VHN.
An increase of nearly 50 VHN
was observed nearer the
surface in sodium-exposed
material vis-à-vis thermally
aged material. The hardness
values decreased and reached
the matrix value at around
100µm. This increase in
hardness indicated surface
carburization of the material
due to sodium exposure.
The concentration profile of
carbon determined by SIMS
showed that, in type 316LN SS,
the peak concentration of
carbon was attained at 10 µm,
immediately after the end of the
degraded layer, while the bulk
concentration was attained at a
distance of 38 µm. In type 316
SS, carbon concentration had
attained a maximum of 0.135
wt% at a depth of 43 µm and
equaled the bulk concentration
at a distance of 74 µm from the
surface. The carbon content
was less than the bulk
concentration up to a depth
where a degraded layer which
contained ferrite was formed on
the surface. The reduction in
carbon content in the degraded
layer was because the solubility
of carbon in ferrite is lower
resulting in carbon being
rejected from the degraded
layer. Comparative carbon
profiles of type 316 and 316LN
SS, for the period of exposure
of 16000 hours, measured by
SIMS analysis is shown in Fig.2.
Based on the measured carbon
profiles by SIMS, the probable
carbon profile after 40 years
(Fig.3) was predicted by
calculating and using an
effective diffusion coefficient.
The expected carburization
depth was around 260 µm for
type 316LN SS.
Tensile test results showed an
increase of 22% in yield
strength (YS) and a reduction in
total elongation by 33% on
sodium exposure of annealed
material. Thermal exposure of
the annealed material at 823 K
for 16000 hours, without
exposure to sodium, caused an
increase in YS by 8% and a
decrease of 13% in total
elongation. The increased
changes on sodium exposure
vis-a vis thermally aged
condition was attributed to
carburization. Similar changes
in mechanical properties were
observed for type 316 stainless
steel.
Carbon Content (wt%)
0.14
0.13
0.12
0.11
0.10
0.09
0.08
0.07
0.06
0.05
0.04
0.03
type 316SS
type 316LN
0 50 100 150 200 250 300
Distance (µm)
Fig.3 Calculated carbon profile of
types 316 and 316LN stainless
steel after 40 years
56 R&D FOR FBRs