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Temper embrittlement and creep embrittlement of 1.25 Cr-0.5 Mo steels containing Sb, Sn, P and B as impurities
Scripta
Vol.
METALLURGICA
8,
pp.
Printed
1225-1230,
in the U n i t e d
1974
Pergamon
Press,Inc.
States
TEMPER DfBRITTL~4ENT AND CREEP ~ B R I T T L m q E N T O F 1.25 Cr-0.5 Mo STEELS CONTAINING Sb, Sn, P and B AS IMPURITIES
R. Vlswanathan Westinghouse Research Laboratories Pittsburgh, Pennsylvania 15235
(Received
July
31,
1974)
Introduction It is now well accepted that impurity elements such as Sb, Sn and P can segregate to prior austenlte boundaries during exposure in the range 600-1000"F and cause an increase in the Fracture Appearance Transition Temperature
(FATE) of low alloy steals (1). More
recently it has been shown that the impurity elements also lead to reduced ductilities in stress rupture tests at elevated temperatures (900-1100°Fi (2).
While the presence of the above
impurity elements is generally inadvertent, small additions of B are often made dellberately with a view to increase the hardenabillty of low alloy steels. rupture ductility has not been investigated.
The effect of adding B on
Results pertaining to its effect on the ~mpact
toughness of steels are meager and conflicting (3,4).
In the present study, the combined
effects of Sb, P and Sn as well as B on the FATT and rupture ductility at IO00°F of normalized and tempered 1.25 Cr-0.5 Mo steels were investigated.
Experimental Chemical c o m p o s i t i o n s o f f o u r s t e e l s in Table I.
VM 1706 r e p r e s e n t s
the high purity
p r o d u c e d by vacuum i n d u c t i o n m e l t i n g a r e l i s t e d laboratory
heat.
VM 1715 r e p r e s e n t s
t h e impure
TABLE I Chemical Cemposltlons of 1.25 Cr-0.5 Mo Steels
Stem
c
~_
c__~r
Mo
SAi
S_
_S
O_
~
P_
Sb
S_~n
_B
VH 1706
0.15
0.58
1.25
0.49
0.45
30
5
25
20
20
4
20
VM 1715
0.17
0.55
1.25
0.50
0.45
22
6
50
NA
270
94
350
VH 1707
0.16
0.58
1.22
0.49
0.45
32
20
23
20
20
5
320
45
VM 1716
0.16
0.56
1.25
0.49
0.45
26
7
33
NA
300
97
325
40
Levels of S, N, O, AI, P, Sb, Sn and B are expressed in ppm. 'not analyzed v.
1225
All others in wt %.
NA denotes
1226
TEMPER
AND
CREEP
EMBRITTLEMENT
Vol,
heat, containing large, deliberate additldns of Sb, Sn and P.
8 , No,
11
VM 1707 and VM 1716 contain B
in addition to one or more of the above impurities. All the heata were Rade aa 2 In. x 2 in. ingots and forged to 0.625 in. bar stock at 2000°F.
Specimen blanks were then normalized at
1700°P for 1 hr, tempered at 1250°F for 1 hr and water quenched.
This heat treatment resulted
in the tensile properties listed in Table II for the various steels.
Charpy impact and stress
TABLE II
and
Hardness
Tensile Properties UTS ksi
0.2X YS ksi
at Room Temperature
Steel
Hardness RB
Elongation Z
Reduction in Area Z
VM 1706
94.5
92.3
VM 1715
94.6
92.7
74.4
26.9
73.9
71.8
25.3
69.8
VM 1707
94.0
92.3
74.4
25.7
70.9
VM 1716
92.5
94.0
85.8
26.3
69.2
rupture data were then obtained using specimens of the type shown in Fig. I. The microstructure of all the laboratory steels consisted of upper bainite, see Fig. 2.
L
V
In
all cases two grain boundary networks could be dis0'°"
tinguished:
a prior austenlte grain boundary network
•
and a f~ner network of ferrite boundaries inside 4.625
-,
(a}
the austenlte grains.
Figure 2a shows both types of
boundarles~ while an enlarged view of ferrite
,~,~/. 010R --~0. IW
boundary network is shown in Fig. 2b.
The ferrite
boundary network consisted of clusters of carbides
I.
2.1~5~
t--o.~
that separated regions of ferrite relatively free of carbides (except for small amounts of a fine
FIG. 1
needlelike precipitate).
Schematic drawing of test specimens; a) stress rupture specimen; b) subsize Charpy specimen.
Prior austenlte boundaries
were also delineated by continuous networks of carbides.
Average ferrite grain size and austenite
grain size were estimated to be about 5 and 45 microns respectively for all the laboratory steels.
No appreciable differences in structure could be
discerned between steels. Results Creep Embrit tlement Stress rupture tests were conducted at 1000°F with stresses in the range of 30-50 ksi.
* Initial nominal stress.
The tests were carried out under constant load.
Vol.
8, No.
ii
TEMPER
AND
Photomicrograph of overall structure.
l~g.
EMBRITTLEMENT
1227
Figure 3 illustrates observed variation of Z RA* as a function of log tr and log ~.
e)
CREEP
225X
The plot for each steel is characterized
by an initial region of constant ductility, at short times, followed by a region of decreasing ductillty with further increase in t crease in ~).
(or deE Data for both 1706 (control
heat) and 1715 (Sb + Sn + P) are described by the same plot, indicating that even a large
Extraction replica. F e r r i t e g r a i n s and stain boundary carbide networks.
increase in the concentration of impurities did not have any discernible influence on ductility.
b)
Lowest values of ductility are
encountered for VM 1707 (Sn + B).
Fag. 1000X
Due to
scatter in results, the exact shape of the plot
Extraction replica. Triple point between two f e r r i t e g r a i n s and an island of cluster "carbides. c)
Meg. 3000x
for 1716 (Sn + P + Sb + B) can not be defined; nevertheless, rupture ductilities for this steel appear to be intermediate between those for 1706 and for 1707.
The lower ductillties
of VM 1707 and 1716 compared to VM 1706 and 1715 can be interpreted as either being due to B alone or due to the combined presence of
FIG. 2 Details of mlcrostructure in VMI706.
Sn + B in the steels.
Temper Embrlttlement Fracture Appearance Transition T~,peranEes (FATT) were determined for each steel before and after an embrlttllng treatment.
The embrlttllng treatment consisted of a stepwlse
coollng of 950°F/4 hr, 915°F/3 days, 860"F/4 days, 750°F/7 days, followed by furnace coollng to room temperature.
For each FATT determination at least 12 Charpy specimens were utilized.
The error in FATT determination was estimated to be ±5°F. T-hle IIl contains results of the Charpy impact tests.
In the nonembrlttled condition,
FATT is somewhat hlgher for VM 1707 and appreciably higher for VM 1715 and 1716 compared to the control composition.
Presumably some embrlttlm,,ent had already occurred in VM 1707, 1715 and
1716, even in the nonembrlttlad condition, during cooling from auetenlte.
In view of this, the
FATT in the embrlttled condition may provide a better index of the total amount of embrlttlement, than AFATT. VM 1716.
Values of FATT in the embrlttled state are appreciably higher for VM 1715 and
The upper shelf energies are also much lower for VM 1715 and 1716.
These results
indicate that from a temper embrlttlement point of view the impurities P, Sb and Sn are more
* Z RA, t r and ~ d e n o t e p ~ c e n t a g e r e d u c t i o n o f a r e a a t r u p t u r e , minimum c r e e p r a t e , r e s p e c t i v e l y .
t i m e t o r u p t u r e and
1228
TEMPER
AND
CREEP
EMBRITTLEMENT
Vol.
8, No.
Ii
: ::',,'," 557592-B I
f
f
90
8O
70
o VMIT06
50
VM1715 6 VM1707 • VM1716 •
t0 I
311
I
I0
100 tr. hours
1000
I
i
I
I
I 1
I 10 - 1
I 10 - 2
I 10 - 3
8O
46
2O 10
FIG. 3 Variation of Z IRA at rupture with (e) log tr and (b) log e for 1.25 Cr-0.5 Mo steel.
TABLE III Results of Impact Tests on 1.25 Cr-0.5 Mo Steels.
Steel
Nonembrlttled Upper Shelf FATT ~ °F Energy, ft-lb
Embrittled Upper Shelf FATT~ "F Energy, ft-lb
AFATTp °F
VM 1706(Control)
i0
78 -+ 2
10
75 -+ 5
0
VM 1707(Sn+B)
20
66 -+ 6
15
67 -+ 2
0
VM 1715(Sb+Sn+P)
50
67 -+ 5
90
54 -+ 2
40
VM 1716(Sb+Sn+P+B)
50
66 + 5
85
58 -+ 2
35
Vol.
8, No. 11
pot~m~ ~ i t t l o r 8
8 t u ~ . U a~e ~ ~s d ~ f ~ t
~
B. c o n L ~ r y to t h e ~ t u a t i o n oboez~ed i n crsep ~ t £ ~ .
¢o ~ * £ y
i f tim n e c b m u m of sraXa ~
~ t t ~
£ b f l t t y to t ~
by m
~rittlemeut
1000"F and t h e s u s e ~ t -
of n o t u a l t z e d Bud tempered 1.25 Ct-O.$ No m ~ s
uwte ~ t i -
i n s t r e s s r u p t u r e s t u d i e s a t 1000"F, presence o f B caused an apgrec~able r ~ u c t ~ o n o f
t he r u p t u r e d u c t i l i t y , ~foet
~
from t h a t 4ue to P, Sb end Sn.
The e f f e c t s of Sb, P, Sn aed B on the r u p t u r e d u c ~ l £ t y - a t Kated.
1229
TEMFERANDCREEPHQ~ITTI/BaI~
w h i l e l a r s e i n c r e ~ s t s JJa c o n c ~ r a t i o n
on r u p t u r e d u c t i l i t y ,
r e l a t i v e to the c o n t r o l h e a t .
on t h e other hand, l a r s e r chauKe8 i n touKImess £ o 1 1 ~ due to Sb, P and Sn than due to B.
o£ fib, P and h In t ~ e r
had no d i s c e r n i b l e
embrittlemsnt studies,
an emb~ttr.ll~K t ~ l : m m a t were ~ r v e d
Furthez- s t u d i e s a r e needed to clar4.Yy i f t h e mechmniem of
Kratn boundary e m b r i t t l e s e n t produced by B t8 d i f f e r e n t from t h a t due to P, Sb and Sn. a~fereuces l.
C. J . McNshon, 3 r . • Temper l a b r i t t l a a e n t
in Steels•
ASTHSTP 407, 127 ( I ~ ( ~ ) .
2.
B. E . ~ u ,
3.
a. e. C L . ~ , a . L. Cryde~n~ and M. S ~ h y ~ , ci~ u~a~emt ~ (1~9).
4.
E. 3. I r v t u e and P. B. PtckerinS, 3 I S l • 201, 518 (1963).
H. R. T i p l e r and G. D. Branch, 3 I S I , 209, 745 (1971).
Stuel S t r ~ - ~ _
~
,
Vol.
METALLURGICA
8,
pp.
Printed
1225-1230,
in the U n i t e d
1974
Pergamon
Press,Inc.
States
TEMPER DfBRITTL~4ENT AND CREEP ~ B R I T T L m q E N T O F 1.25 Cr-0.5 Mo STEELS CONTAINING Sb, Sn, P and B AS IMPURITIES
R. Vlswanathan Westinghouse Research Laboratories Pittsburgh, Pennsylvania 15235
(Received
July
31,
1974)
Introduction It is now well accepted that impurity elements such as Sb, Sn and P can segregate to prior austenlte boundaries during exposure in the range 600-1000"F and cause an increase in the Fracture Appearance Transition Temperature
(FATE) of low alloy steals (1). More
recently it has been shown that the impurity elements also lead to reduced ductilities in stress rupture tests at elevated temperatures (900-1100°Fi (2).
While the presence of the above
impurity elements is generally inadvertent, small additions of B are often made dellberately with a view to increase the hardenabillty of low alloy steels. rupture ductility has not been investigated.
The effect of adding B on
Results pertaining to its effect on the ~mpact
toughness of steels are meager and conflicting (3,4).
In the present study, the combined
effects of Sb, P and Sn as well as B on the FATT and rupture ductility at IO00°F of normalized and tempered 1.25 Cr-0.5 Mo steels were investigated.
Experimental Chemical c o m p o s i t i o n s o f f o u r s t e e l s in Table I.
VM 1706 r e p r e s e n t s
the high purity
p r o d u c e d by vacuum i n d u c t i o n m e l t i n g a r e l i s t e d laboratory
heat.
VM 1715 r e p r e s e n t s
t h e impure
TABLE I Chemical Cemposltlons of 1.25 Cr-0.5 Mo Steels
Stem
c
~_
c__~r
Mo
SAi
S_
_S
O_
~
P_
Sb
S_~n
_B
VH 1706
0.15
0.58
1.25
0.49
0.45
30
5
25
20
20
4
20
VM 1715
0.17
0.55
1.25
0.50
0.45
22
6
50
NA
270
94
350
VH 1707
0.16
0.58
1.22
0.49
0.45
32
20
23
20
20
5
320
45
VM 1716
0.16
0.56
1.25
0.49
0.45
26
7
33
NA
300
97
325
40
Levels of S, N, O, AI, P, Sb, Sn and B are expressed in ppm. 'not analyzed v.
1225
All others in wt %.
NA denotes
1226
TEMPER
AND
CREEP
EMBRITTLEMENT
Vol,
heat, containing large, deliberate additldns of Sb, Sn and P.
8 , No,
11
VM 1707 and VM 1716 contain B
in addition to one or more of the above impurities. All the heata were Rade aa 2 In. x 2 in. ingots and forged to 0.625 in. bar stock at 2000°F.
Specimen blanks were then normalized at
1700°P for 1 hr, tempered at 1250°F for 1 hr and water quenched.
This heat treatment resulted
in the tensile properties listed in Table II for the various steels.
Charpy impact and stress
TABLE II
and
Hardness
Tensile Properties UTS ksi
0.2X YS ksi
at Room Temperature
Steel
Hardness RB
Elongation Z
Reduction in Area Z
VM 1706
94.5
92.3
VM 1715
94.6
92.7
74.4
26.9
73.9
71.8
25.3
69.8
VM 1707
94.0
92.3
74.4
25.7
70.9
VM 1716
92.5
94.0
85.8
26.3
69.2
rupture data were then obtained using specimens of the type shown in Fig. I. The microstructure of all the laboratory steels consisted of upper bainite, see Fig. 2.
L
V
In
all cases two grain boundary networks could be dis0'°"
tinguished:
a prior austenlte grain boundary network
•
and a f~ner network of ferrite boundaries inside 4.625
-,
(a}
the austenlte grains.
Figure 2a shows both types of
boundarles~ while an enlarged view of ferrite
,~,~/. 010R --~0. IW
boundary network is shown in Fig. 2b.
The ferrite
boundary network consisted of clusters of carbides
I.
2.1~5~
t--o.~
that separated regions of ferrite relatively free of carbides (except for small amounts of a fine
FIG. 1
needlelike precipitate).
Schematic drawing of test specimens; a) stress rupture specimen; b) subsize Charpy specimen.
Prior austenlte boundaries
were also delineated by continuous networks of carbides.
Average ferrite grain size and austenite
grain size were estimated to be about 5 and 45 microns respectively for all the laboratory steels.
No appreciable differences in structure could be
discerned between steels. Results Creep Embrit tlement Stress rupture tests were conducted at 1000°F with stresses in the range of 30-50 ksi.
* Initial nominal stress.
The tests were carried out under constant load.
Vol.
8, No.
ii
TEMPER
AND
Photomicrograph of overall structure.
l~g.
EMBRITTLEMENT
1227
Figure 3 illustrates observed variation of Z RA* as a function of log tr and log ~.
e)
CREEP
225X
The plot for each steel is characterized
by an initial region of constant ductility, at short times, followed by a region of decreasing ductillty with further increase in t crease in ~).
(or deE Data for both 1706 (control
heat) and 1715 (Sb + Sn + P) are described by the same plot, indicating that even a large
Extraction replica. F e r r i t e g r a i n s and stain boundary carbide networks.
increase in the concentration of impurities did not have any discernible influence on ductility.
b)
Lowest values of ductility are
encountered for VM 1707 (Sn + B).
Fag. 1000X
Due to
scatter in results, the exact shape of the plot
Extraction replica. Triple point between two f e r r i t e g r a i n s and an island of cluster "carbides. c)
Meg. 3000x
for 1716 (Sn + P + Sb + B) can not be defined; nevertheless, rupture ductilities for this steel appear to be intermediate between those for 1706 and for 1707.
The lower ductillties
of VM 1707 and 1716 compared to VM 1706 and 1715 can be interpreted as either being due to B alone or due to the combined presence of
FIG. 2 Details of mlcrostructure in VMI706.
Sn + B in the steels.
Temper Embrlttlement Fracture Appearance Transition T~,peranEes (FATT) were determined for each steel before and after an embrlttllng treatment.
The embrlttllng treatment consisted of a stepwlse
coollng of 950°F/4 hr, 915°F/3 days, 860"F/4 days, 750°F/7 days, followed by furnace coollng to room temperature.
For each FATT determination at least 12 Charpy specimens were utilized.
The error in FATT determination was estimated to be ±5°F. T-hle IIl contains results of the Charpy impact tests.
In the nonembrlttled condition,
FATT is somewhat hlgher for VM 1707 and appreciably higher for VM 1715 and 1716 compared to the control composition.
Presumably some embrlttlm,,ent had already occurred in VM 1707, 1715 and
1716, even in the nonembrlttlad condition, during cooling from auetenlte.
In view of this, the
FATT in the embrlttled condition may provide a better index of the total amount of embrlttlement, than AFATT. VM 1716.
Values of FATT in the embrlttled state are appreciably higher for VM 1715 and
The upper shelf energies are also much lower for VM 1715 and 1716.
These results
indicate that from a temper embrlttlement point of view the impurities P, Sb and Sn are more
* Z RA, t r and ~ d e n o t e p ~ c e n t a g e r e d u c t i o n o f a r e a a t r u p t u r e , minimum c r e e p r a t e , r e s p e c t i v e l y .
t i m e t o r u p t u r e and
1228
TEMPER
AND
CREEP
EMBRITTLEMENT
Vol.
8, No.
Ii
: ::',,'," 557592-B I
f
f
90
8O
70
o VMIT06
50
VM1715 6 VM1707 • VM1716 •
t0 I
311
I
I0
100 tr. hours
1000
I
i
I
I
I 1
I 10 - 1
I 10 - 2
I 10 - 3
8O
46
2O 10
FIG. 3 Variation of Z IRA at rupture with (e) log tr and (b) log e for 1.25 Cr-0.5 Mo steel.
TABLE III Results of Impact Tests on 1.25 Cr-0.5 Mo Steels.
Steel
Nonembrlttled Upper Shelf FATT ~ °F Energy, ft-lb
Embrittled Upper Shelf FATT~ "F Energy, ft-lb
AFATTp °F
VM 1706(Control)
i0
78 -+ 2
10
75 -+ 5
0
VM 1707(Sn+B)
20
66 -+ 6
15
67 -+ 2
0
VM 1715(Sb+Sn+P)
50
67 -+ 5
90
54 -+ 2
40
VM 1716(Sb+Sn+P+B)
50
66 + 5
85
58 -+ 2
35
Vol.
8, No. 11
pot~m~ ~ i t t l o r 8
8 t u ~ . U a~e ~ ~s d ~ f ~ t
~
B. c o n L ~ r y to t h e ~ t u a t i o n oboez~ed i n crsep ~ t £ ~ .
¢o ~ * £ y
i f tim n e c b m u m of sraXa ~
~ t t ~
£ b f l t t y to t ~
by m
~rittlemeut
1000"F and t h e s u s e ~ t -
of n o t u a l t z e d Bud tempered 1.25 Ct-O.$ No m ~ s
uwte ~ t i -
i n s t r e s s r u p t u r e s t u d i e s a t 1000"F, presence o f B caused an apgrec~able r ~ u c t ~ o n o f
t he r u p t u r e d u c t i l i t y , ~foet
~
from t h a t 4ue to P, Sb end Sn.
The e f f e c t s of Sb, P, Sn aed B on the r u p t u r e d u c ~ l £ t y - a t Kated.
1229
TEMFERANDCREEPHQ~ITTI/BaI~
w h i l e l a r s e i n c r e ~ s t s JJa c o n c ~ r a t i o n
on r u p t u r e d u c t i l i t y ,
r e l a t i v e to the c o n t r o l h e a t .
on t h e other hand, l a r s e r chauKe8 i n touKImess £ o 1 1 ~ due to Sb, P and Sn than due to B.
o£ fib, P and h In t ~ e r
had no d i s c e r n i b l e
embrittlemsnt studies,
an emb~ttr.ll~K t ~ l : m m a t were ~ r v e d
Furthez- s t u d i e s a r e needed to clar4.Yy i f t h e mechmniem of
Kratn boundary e m b r i t t l e s e n t produced by B t8 d i f f e r e n t from t h a t due to P, Sb and Sn. a~fereuces l.
C. J . McNshon, 3 r . • Temper l a b r i t t l a a e n t
in Steels•
ASTHSTP 407, 127 ( I ~ ( ~ ) .
2.
B. E . ~ u ,
3.
a. e. C L . ~ , a . L. Cryde~n~ and M. S ~ h y ~ , ci~ u~a~emt ~ (1~9).
4.
E. 3. I r v t u e and P. B. PtckerinS, 3 I S l • 201, 518 (1963).
H. R. T i p l e r and G. D. Branch, 3 I S I , 209, 745 (1971).
Stuel S t r ~ - ~ _
~
,