| dc.description.abstract | Type 316L(N) stainless steel (SS) is the currently favored structural material
for several high temperature components in the primary side of the liquid metal
cooled fast breeder reactor. The choice of this material is primarily based on its
resistance to sensitization and adequate high temperature mechanical properties. In
liquid metal cooled fast breeder reactors, the components are often subjected to
temperature gradient induced thermal stresses, which are cyclic in nature as a result
of start-ups, shutdowns and transients. Furthermore, steady state loading at elevated
temperatures in combination with cyclic loading leads to creep-fatigue interaction.
Therefore, resistance to low cycle fatigue and creep-fatigue interaction is important
factors in the design of liquid metal cooled fast breeder reactor components. In view
of this, it is necessary to understand the cyclic deformation and fracture behaviour of
this alloy under various loading conditions. Such understanding must not only
include low cycle fatigue behaviour at the maximum operating temperature but also
at various strain amplitudes and temperatures encountered during transients. In
addition, prior cold work might be introduced in the alloy unintentionally during
erection or fabrication of plates and sheets into vessels, tanks, pipings etc.
Therefore, to ensure reliable performance of stainless steels in service, studies
related to effect of prior cold work on strain controlled low cycle fatigue behaviour
assume significance.
In this thesis, the following aspects of strain controlled low cycle fatigue and
creep-fatigue interaction behaviour of type 316L(N) SS have been critically examined
: (i) The effects of temperature and strain amplitude on low cycle fatigue behaviour of
solution annealed alloy, (ii) Time dependent low cycle fatigue behaviour under
various strain rate and temperature conditions, (iii) The influence of prior cold work
on elevated temperature cyclic behaviour and (iv) Creep-fatigue interaction
behaviour of the alloy employing hold times at peak cyclic strains.
The effects of total strain amplitude (+ 0.25% to + 1.0%), and temperature
(298K to 923K) on fatigue behaviour of the solution annealed alloy were studied in
detail. The cyclic stress response of the alloy was characterized by cyclic hardening
to peak stress followed by softening, which ended in a stable stress response until
failure. The regime of stress saturation was longer at lower strain amplitudes of
testing. The peak stress amplitude decreased initially from 298 K to 573K. The peak
stress increased rapidly with increasing temperature between 573 and 873K. Beyond
873 K, the peak stress decreased again. The alloy exhibited a peak in life at
intermediate temperatures around 573 K. Life decreased drastically with increase in
temperature beyond 573 K. The temperature dependence of stress response and
fatigue life have been explained on the basis of several interacting phenomena
which include strain induced transformation of austenite to martensite, substructural
recovery, dynamic strain ageing (DSA) and oxidation.
The fatigue life at elevated temperatures is significantly influenced by testing
parameters such as strain rate and waveform due to combined effects of various
time dependent deformation and damaging processes. The effects of strain rate
(3x1 O'5 s'1 to 3x1 O'2 s’1) on low cycle fatigue behaviour of 316L(N) SS have been
examined at 773, 823 and 873 K. Cyclic stress amplitude at half-life has been found
to increase with decreasing strain rate at 773 and 823 K indicating the negative
strain rate sensitivity of cyclic stresses. The negative strain rate sensitivity of half-life
cyclic stress amplitude at 873 K occurred only over a limited strain rate range. Peak
tensile stress amplitude developed as a function of strain rate has been partitioned to
friction stress and back stress components to establish the mechanism of cyclic
hardening. The rapid hardening and negative strain rate sensitivity of cyclic stress
was attributed to DSA. Transmission electron microscopy studies revealed that there
is an increase in the dislocation density and enhanced slip planarity in the DSA
regime. At all the temperatures, fatigue life decreased with decrease in strain rate.
The factors responsible for the reduction in life have been identified and the life
reduction was explained on the basis of operating deformation and damage
mechanisms. The degradation in fatigue resistance is attributed to the detrimental
effects associated with DSA and oxidation. Quantitative measurement of secondary
cracks indicate that both transgranular and intergranular cracking are accelerated
predominantly under conditions conducive to DSA.
Creep-Fatigue interaction tests have been carried out at 873 and 923 K to
evaluate the influence of duration of hold time (1 to 90 min) and position of hold (at
peak tensile strain, peak compression strain and peak tension plus compression
strain). The alloy under hold time conditions exhibited lower cyclic stresses
compared to those obtained in solution annealed state. The decrease in cyclic stress
response with hold time is attributed to enhanced recovery of the substructure and
increase in grain boundary damage accumulated during stress relaxation period. It
has been observed that tensile hold was more damaging than the compression hold
and fatigue life decreased with increase in duration of hold time in tension. Creepfatigue
interaction was noticed to be more prominent only at lower strain amplitudes
of testing. The effects of oxidation and creep in causing reduction in life under holdtime
conditions are discussed. Quantitative assessment of cracking behaviour has
been made under hold time conditions on longitudinal sections of the low cycle
fatigue tested samples.
The role of 20% prior cold work at 873 K on low cycle fatigue properties of
solutionised 316L(N) SS, at a strain rate of 3x1 O'3 s'1 , has been studied in detail.
Cyclic stress response of the alloy was characterized by a short period of hardening
followed by gradual softening, which ended in a stable stress response at lower
strain amplitudes of testing. Interrupted tests were carried out at different stages of
cyclic stress response and substructural changes were characterized using
transmission electron microscopy to determine the underlying mechanisms causing
such response. The fatigue life of material in prior cold worked condition was lower
at higher strain amplitudes of testing, whereas at lower strain amplitudes (< ± 0.4%),
prior cold worked material exhibited higher life compared to solution annealed alloy.
The lower life of prior cold worked alloy at higher strain amplitude has been
attributed to lower ductility and higher cyclic response stresses. Higher fatigue
endurance of PCW material at lower strain amplitudes of testing is attributed to | en_US |
