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The effect of pre-strain on the high temperature ductility of a stainless steel
JOURNAL
OF NUCLEAR
MATERIALS
37 (1070)
THE EB’FECT OF PRE-STRAIN
118-120.
@
NORTH-HOLLAND
ON THE HIGH A STAINLESS
Received
PUBLISHING
TEMPERATURE
CO., AMSTERDAM
DUCTILITY
OF
STEEL
11 May 1970
It is known I-3) that low-temperature straining affects the response of material to subsequent high-temperature creep conditions. Such pre-strain is valuable in incurring shortterm strength increases but can also reduce ductility and time to rupture. The nett benefit can be assessed only for the particular material under consideration. The present note reports some results on the cladding alloy, 2OCr/25Ni/Nb stainless steel. Tensile specimens of gauge dimensions 25.4 x 6.4 s 0.7 mm were prepared from cold-rolled strip and annealed in argon at 1050 “C for 3 h to give a recrystallised grain size of about 30 ,um. Before testing, the specimens were aged at 750 “C for 65 h to produce a stable carbide dist’ribution. The tests were performed in air on a Hounsfield tensometer at a strain
01
0
, 0.1
I
02
I
I
I
0.3
0.4
0.5
PRE-STRAIN
Fig. 1.
rate of 2.8 x lo-s/see ; the load was continuously monitored on a chart recorder. The lowtemperature pre-strain was applied at 20 “C prior to straining to fracture at 750 “C. After straining in each temperature regime, crosssectional area measurements were mads on a travelling microscope sensitive to lo-” mm ; strains were defined as true reduction of area. Longitudinal sections of the fractured specimens were electrolytically etched ; in a 10% oxalic acid solution after mechanical polishing. The effect of pre-straining on the high temperature ductility is shown in fig. 1. Within the scatter of the results, increasing pre-strain causes a continuoue decrease in ductility. The shape of the curve is similar to that reported by Williams and Lindley 4) for an Al/Zn alloy.
The effect of pe-strain 760 “C.
AT
I 0.6
20°C
on the ductility
at
The effect of pre-straining on the steady-state stress reached at 750 “C is shown in fig. 2. An almost linear relationship exists with the amount of prior low temperature strain. None of the specimens showed any indication of recrystallisation at 750 “C and for all levels of pre-strain failure was intergranular. For the specimen strained continuously at high temperature, cavities near the fracture surface were considerably enlarged. Increasing pre-strain however, reduced the tendency for the development of large cavities and the fracture characteristics took on the appearance of wedge-type cracking (figs. 3 and 4). Closer examination though showed that a cavitational process was still operative (fig. 5) and that the
118
HIGH
TEMPERATURE
119
DUCTILITY
so
20
Fig.
2.
I
0
0.1
The
I
I
I
O-2 0.3 O-4 PRE-STRAIN AT 20°C
effect
of
pre-strain
stress obtained
1
O-5
on the
0.6
maximum
at 750 “C. Fig.
4.
Specimen
pm-strained
fractured
Fig.
3.
Specimen fractured
pre-strained at 750 “C.
0.141 x 200
at
20
at, 5’50 “C.
0.293
at
20
x 200
“C, Fig.
5.
Same
specimen
as in fig.
4.
x 1000
“C,
120
J.
S.
.__I
WADDINGTON
ANlJ
-_
-
H.
-__.
fl;.
.l!iVANY
__^
large aspect ratio was due to the link-up of a
grain boundary.
large number
applicable to this material since grain-boundary
the growth
of smaller cavities
rather than
of a wedge crack.
The nature of the cavity
cavities
nuclei introduced
at low temperature is not certain. They may be grain-boundary ledges 1) which would fracture
brittling
on subsequent
cannot
cracked
high-temperature
precipitate
straining,
particles *) lying
or
in the
can
temperature
The latter is probably
be
produced
straining
solely
by
more room-
(fig. 6).
It is clear from these results that the emeffect of a low-temperature be attributed
singly
pre-strain
to either an in-
creased cavity nucleation rate or to an increased growth rate. Thus, it has been shown that the number of cavity nuclei can be readily increased by me-strain and a rough estimate from the metallography suggests an increase by a factor of about 2-3 at the higher pre-strains. Assuming a eonstant growth rate, the ductility would decrease by the same factor if the failure criterion is one of cavity impingement. However, since the observed ductilities show a decrease by a factor of about 7-8, it is necessary to postulate that the cavity growth rate is also increased by pre-straining. This can be reasonably attributed to the higher stress levels obtained in pre-strained material. This note is published by permission Central electricity Generating Board.
of the
References 1) P. W. Davies, J. D. Richards and B. Wilshire, J. Inst.
6.
Specimen
strained
continuously
x 1000
at
20
“C.
90 (1961/62)
431
0. D. Sherby, a. Goldberg and J. E. Darn, Trans.
3)
R. M. Sergeant,
4)
5. A. Williams
Am. Fig.
Metals
2)
Sot. Metals
(1969)
957
46 (1954) J. Inst.
681
Metals
96 (1968)
and T. C. Lindley,
197
2. Metallk.
60
OF NUCLEAR
MATERIALS
37 (1070)
THE EB’FECT OF PRE-STRAIN
118-120.
@
NORTH-HOLLAND
ON THE HIGH A STAINLESS
Received
PUBLISHING
TEMPERATURE
CO., AMSTERDAM
DUCTILITY
OF
STEEL
11 May 1970
It is known I-3) that low-temperature straining affects the response of material to subsequent high-temperature creep conditions. Such pre-strain is valuable in incurring shortterm strength increases but can also reduce ductility and time to rupture. The nett benefit can be assessed only for the particular material under consideration. The present note reports some results on the cladding alloy, 2OCr/25Ni/Nb stainless steel. Tensile specimens of gauge dimensions 25.4 x 6.4 s 0.7 mm were prepared from cold-rolled strip and annealed in argon at 1050 “C for 3 h to give a recrystallised grain size of about 30 ,um. Before testing, the specimens were aged at 750 “C for 65 h to produce a stable carbide dist’ribution. The tests were performed in air on a Hounsfield tensometer at a strain
01
0
, 0.1
I
02
I
I
I
0.3
0.4
0.5
PRE-STRAIN
Fig. 1.
rate of 2.8 x lo-s/see ; the load was continuously monitored on a chart recorder. The lowtemperature pre-strain was applied at 20 “C prior to straining to fracture at 750 “C. After straining in each temperature regime, crosssectional area measurements were mads on a travelling microscope sensitive to lo-” mm ; strains were defined as true reduction of area. Longitudinal sections of the fractured specimens were electrolytically etched ; in a 10% oxalic acid solution after mechanical polishing. The effect of pre-straining on the high temperature ductility is shown in fig. 1. Within the scatter of the results, increasing pre-strain causes a continuoue decrease in ductility. The shape of the curve is similar to that reported by Williams and Lindley 4) for an Al/Zn alloy.
The effect of pe-strain 760 “C.
AT
I 0.6
20°C
on the ductility
at
The effect of pre-straining on the steady-state stress reached at 750 “C is shown in fig. 2. An almost linear relationship exists with the amount of prior low temperature strain. None of the specimens showed any indication of recrystallisation at 750 “C and for all levels of pre-strain failure was intergranular. For the specimen strained continuously at high temperature, cavities near the fracture surface were considerably enlarged. Increasing pre-strain however, reduced the tendency for the development of large cavities and the fracture characteristics took on the appearance of wedge-type cracking (figs. 3 and 4). Closer examination though showed that a cavitational process was still operative (fig. 5) and that the
118
HIGH
TEMPERATURE
119
DUCTILITY
so
20
Fig.
2.
I
0
0.1
The
I
I
I
O-2 0.3 O-4 PRE-STRAIN AT 20°C
effect
of
pre-strain
stress obtained
1
O-5
on the
0.6
maximum
at 750 “C. Fig.
4.
Specimen
pm-strained
fractured
Fig.
3.
Specimen fractured
pre-strained at 750 “C.
0.141 x 200
at
20
at, 5’50 “C.
0.293
at
20
x 200
“C, Fig.
5.
Same
specimen
as in fig.
4.
x 1000
“C,
120
J.
S.
.__I
WADDINGTON
ANlJ
-_
-
H.
-__.
fl;.
.l!iVANY
__^
large aspect ratio was due to the link-up of a
grain boundary.
large number
applicable to this material since grain-boundary
the growth
of smaller cavities
rather than
of a wedge crack.
The nature of the cavity
cavities
nuclei introduced
at low temperature is not certain. They may be grain-boundary ledges 1) which would fracture
brittling
on subsequent
cannot
cracked
high-temperature
precipitate
straining,
particles *) lying
or
in the
can
temperature
The latter is probably
be
produced
straining
solely
by
more room-
(fig. 6).
It is clear from these results that the emeffect of a low-temperature be attributed
singly
pre-strain
to either an in-
creased cavity nucleation rate or to an increased growth rate. Thus, it has been shown that the number of cavity nuclei can be readily increased by me-strain and a rough estimate from the metallography suggests an increase by a factor of about 2-3 at the higher pre-strains. Assuming a eonstant growth rate, the ductility would decrease by the same factor if the failure criterion is one of cavity impingement. However, since the observed ductilities show a decrease by a factor of about 7-8, it is necessary to postulate that the cavity growth rate is also increased by pre-straining. This can be reasonably attributed to the higher stress levels obtained in pre-strained material. This note is published by permission Central electricity Generating Board.
of the
References 1) P. W. Davies, J. D. Richards and B. Wilshire, J. Inst.
6.
Specimen
strained
continuously
x 1000
at
20
“C.
90 (1961/62)
431
0. D. Sherby, a. Goldberg and J. E. Darn, Trans.
3)
R. M. Sergeant,
4)
5. A. Williams
Am. Fig.
Metals
2)
Sot. Metals
(1969)
957
46 (1954) J. Inst.
681
Metals
96 (1968)
and T. C. Lindley,
197
2. Metallk.
60