IGCAR : Annual Report - Indira Gandhi Centre for Atomic Research

IGC
Annual Report 2007
III.C.7. High Temperature Fretting Wear Studies on
Stainless Steel Type AISI 316LN
In Fast Breeder Reactor,
stainless steel tubes supported
on stainless steel rollers are
being used in the primary heat
exchanger for transferring heat
from radioactive sodium to
sodium in the secondary circuit.
Because of the flow induced
vibration as a result of the flow
of sodium the tubes and roller
joints undergo fretting wear,
which is a serious concern
because of the presence of
radioactive sodium in the heat
exchanger. The role of
environment especially that of
temperature and the frequency
of fretting wear of stainless steel
is not well understood in the
literature. Hence an in-depth
study was takenup to
characterise the fretting wear
behaviour of stainless steel and
stainless steel pairs. The fretting
wear studies of stainless steel
type AISI 316LN against
Stainless steel type AISI 316LN
23.2 o
were conducted under various
operating conditions such as
load (10 N and 50N),
frequency (25 Hz, 75 Hz and
125 Hz) and temperature (623,
783 and 823 K). Temperature
was the main variable input
parameter while coefficient of
friction (µ) and wear were the
output parameters. It was
observed that µ did not depend
appreciably on load, frequency
or temperature while wear rate
showed high dependency on
these parameters. Influence of
temperature on wear depended
on the frequency selected. For
higher frequency of oscillation
(125 Hz and 75 Hz), wear rate
reduced with increase in
temperature from 623 to
823 K. For 25 Hz, however, it
increased with temperature
possibly because of fretting
fatigue. The wear rate was
observed to be maximum at
75Hz. On other hand, wear
30.83 N/mm 2
24.7 o
13.56 N/mm 2
Maximum Shear Plane
Fig.1 Microstructure of the sample tested at 823 K, 125 Hz and 70 N load
shows the evidence of crack because of shear deformation
rate decreased as load
increased.
Further,
microstructural examinations
were conducted on the worn
samples to analyze the wear
behaviour and to understand
the wear mechanisms. The
microstructural examinations at
SEM level revealed that the
failure (i.e., the cracking and
thus removal of material) on
stainless steel 316LN is due to
fatigue at lower frequencies.
The interesting new observation
is that adiabatic shear
deformation (Fig. 1) and
crushing are responsible for the
failure at higher loads and
higher frequencies.
A typical microstructure is
given in Fig. 2(a) and the crack
propagates perpendicular to
the shear bands and the
material fractures. Fig. 2(b)
shows the fractured grain in
which the fracture is
perpendicular to the shear
bands. These shear bands are
observed to be perpendicular
to the motion of the pin. The
above observations revealed
that at low 623 K and at 25 Hz
the material failure is due to
fatigue. The microstructure of
the sample tested at 25 Hz and
350 Hz under a load of 100N
R&D FOR FBRs 53