IGCAR : Annual Report - Indira Gandhi Centre for Atomic Research

IGC
Annual Report 2007
III.C.9. Effect of Metallurgical Variables on the
Stress Corrosion Cracking Behaviour of
AISI Type 316LN Stainless Steel
High-nitrogen stainless steels
(SS) are an important class of
engineering materials with their
better property combinations
such as strength, toughness,
creep resistance, nonferromagnetic
behaviour,
corrosion resistance and stress
corrosion cracking (SCC)
resistance. Sensitisation is a
common problem encountered
with normal grades of
austenitic SS. The problem of
sensitisation can be overcome
by using low carbon austenitic
SS. To overcome the loss of
strength due to carbon
reduction, nitrogen is added to
these SS. For PFBR, nitrogen
content is limited to 0.08 wt. %
in view of improved weldability,
code data availability and to
minimise scatter in mechanical
properties.
The response of nitrogen
additions to SCC is varied.
Addition of nitrogen to
austenitic SS could influence its
SCC resistance by influencing
two opposing and competing
factors viz. decrease in stacking
fault energy, which causes an
increase in slip planarity and
thus deteriorates SCC
resistance, and improved
passive film stability which
improves SCC resistance. In the
present study, AISI type 316LN
SS from two different sources,
henceforth referred to as SS-1
and SS-2, were tested for SCC
crack growth behaviour in
boiling aqueous solution of 5M
NaCl + 0.15M Na 2 SO 4 + 2.5
ml/l HCl (b.p = 381.5 K; pH =
1.3) at various values of K I
(stress intensity factor). SS-2
had a slightly higher Cr, Ni and
Mo contents than SS-1, while
the nitrogen contents were
similar (0.086wt. %). SS-1 was
tested for its SCC crack growth
behaviour in the mill-annealed
condition while SS-2 was tested
in the mill-annealed, 15% cold
rolled and thermally aged (973
K/800 hours) conditions. The
present results were compared
with those of AISI types 316
and 304N SS carried out in our
laboratory earlier.
The comparison between the
SCC behaviour of SS-1 and SS-
2 is shown in Fig.1. SS-2
possessed higher K ISCC
(threshold stress intensity factor
for stress corrosion cracking)
and J ISCC (integral threshold
stress intensity factor for stress
corrosion cracking) and lower
plateau crack growth rate
(PCGR) than SS-1, despite the
contents of major alloying
elements and carbon and
nitrogen being nearly the same.
Initially, the lower average
grain size ( ≅ 55 µm) of SS-2,
vis-à-vis that of SS-1 ( ≅ 70 µm)
was thought to be the cause for
this significant difference in the
crack growth rate. However, the
difference in the grain sizes was
very small to cause the large
differences observed. Based on
orientation microscopic studies,
it was found that the effective
grain boundary energy, which is
a function of energies of
different low angle grain
boundaries and grain size, was
very low for SS-2. The presence
of these special grain
boundaries resulted from the
processing history of the
material. This difference in the
nature of the grain boundary
characteristics were correlated
to a higher resistance to SCC in
SS-2.
Fig.2 shows that (i) on cold
working, the values of K ISCC and
PCGR of mill-annealed SS-2
reduced, (ii) high temperature
aging caused a decrease in the
value of K ISCC and an increase
R&D FOR FBRs 57