The Heat-Affected Zone Toughness of Low-Carbon Microalloyed Steels
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The Heat-Affected Zone Toughness of
Low-Carbon Microalloyed Steels
The influence of N and P on notch toughness
appears to be significant
BY C. THAULOW, A. J. PAAUW AND K. GUTTORMSEN
ABSTRACT. Low-fracture toughness has hardness for structural steels is largely 1970's for offshore applications, was
been correlated with brittle zones in the governed by the carbon content, the used as a reference steel. To understand
HAZ of low-carbon microalloyed steels. reduction of this element was desirable, the brittle behavior, it is important to
This investigation has focused on t w o but its reduction lowered the strength locate which part of the HAZ reveals a
aspects of that problem: level. Developments in steel production low-toughness level, and also to evaluate
1) The extent and the properties of resulted in the introduction of thermo- if brittle areas can reveal brittle fracture
the brittle zones as a function of heat mechanical rolling operations, which cre- behavior in large specimens and can
input, refinement by subsequent weld ated the possibility of a large grain refine- initiate a final fracture behavior. In such a
layers and postweld heat treatment. ment of the microstructure that could context, it is of interest to look at the
2) The possibility of surface crack initi- compensate for strength losses. The roll- situation near the plate surface, since
ation in the cap layer. ing practice is primarily based on the from a theoretical point of view, fractur-
The first aspect has been examined inhibition of austenite grain growth by ing is unlikely to occur from small surface
using weld thermal simulation testing, means of microalloying element additions cracks.
including CCT diagrams, notch toughness of Nb, V and Ti. Consequently, new
measurements and microstructural exam- classes of low-carbon microalloyed steels Background for the Tests
inations. The second has been investi- were developed and were introduced
gated using shallow surface notch CTOD into modern offshore structures. Single Bead Weldment
testing of bead-in-groove weldments. Because of the critical nature of off- Figure 1A illustrates several zones in
with detailed fracture surface examina- shore activities, highlighted by tragic acci- the HAZ of a single bead weldment. Each
tions. A good qualitative relationship was dents, a thorough evaluation of the zone can be characterized by a certain
established between the weld thermal toughness behavior of weldments is microstructural composition, which also
simulation testing, the microstructural required. During such studies, low-frac- can clearly be seen on a macroetched
analysis and the CTOD tests. Two low- ture toughness levels were registered in specimen (Fig. 1B); however, each zone
toughness zones were identified. Of spe- weldments of a low-carbon micro- has no sharp transition, but does have a
cial interest are the crack arrest proper- alloyed steel in both as-welded (AW) and transition region. Descending from the
ties, the influence of elements such as N, postweld heat-treated (PWHT) condi- weld interface into the base metal results
P and Nb, and the significance of local tions. Toughness was evaluated by in a decreasing peak temperature (Tp),
brittle zones with respect to the risk of means of crack tip opening displacement implying that the different zones can be
failure of welded structures. (CTOD) testing. To evaluate the brittle indicated with peak temperatures as fol-
phenomena in the HAZ in more detail, lows: coarse-grained zone, 1590°C
Introduction the Norwegian Petroleum Directorate ini- (2894°F) < T p < 1100°C; fine-grained
tiated a research project including nine zone, 1100°C ( 2 0 1 2 ° F ) < T p < 850°C;
The offshore activities in the North Sea low-carbon microalloyed steels and one partially transformed zone, 850°C
have to a large extent relied on welded C-Mn steel. The C-Mn steel, used in the (1562°F) < T p < 700°C; and tempered
structures of medium-strength steel. In zone, 7 0 0 ° C ( 1 2 9 2 ° F ) < T p .
the past, research indicated that the hard-
The temperatures are only given as
ness requirements in the heat-affected
KEY WORDS indications, since the actual temperatures
zone (HAZ) required welding with low
Low-Carbon Steel depend on chemical composition and
heat inputs. At the same time, it was
Microalloyed Steel welding parameters. The coarse-grained
determined that high heat inputs should
zone is more or less characterized by
be avoided because they contributed to Weldment
quenched microstructures of bainite/
reduced toughness levels in the HAZ. As HAZ
martensite. The increase in peak temper-
Weld Simulation Test
ature also resulted in an increase in aus-
Notch Toughness
tenite grain growth, followed by a subse-
C. THAULOW and A. j. PAAUW are with the Fracture Toughness
quent coarsening of the microstructure.
Foundation for Scientific and Industrial Crack Initiation
Consequently, there is a clear microstruc-
Research, Norwegian Institute of Technology, Fracture Surface
Trondheim, Norway. K. GUTTORMSEN is with tural gradient in each zone; however, the
Influence of P, N
the Norwegian Petroleum Directorate, Sta- microstructure is the same. The fine-
vanger, Norway. grained zone is characterized by a fine
266-s I SEPTEMBER 1987
ferrite grain structure, which resulted
from the normalizing heat treatment of
the base metal. In the partially trans-
formed region, the peak temperature
was too low to produce a complete Peak
transformation. The pearlite has been temperature
transformed, but the transformation at Coarse grained zone Tp
the ferrite grain boundaries was only
partial. Finally, the tempered region is Fine g r a i n e d z o n e
characterized by a thermal treatment of 1100°<
the base material, but no change in the Partially t r a n s f o r m e d z o n e
- 850°C
microstructure was observed by optical
Tempered zone
microscopy. For each point in the HAZ, —700°C
or line parallel the weld interface, the
thermal history is indicated schematically
in Fig. 1A. The temperature cycle is char-
acterized by the peak temperature (TD)
and the cooling time from 800° to 500°C Tp = 1350 C
(Ats/s). The last parameter indicates Temp.
how rapidly the material cools down
and is dependent on the welding para-
meters. Increasing heat input increases
the cooling time, and consequently,
changes the microstructure. Another time
important aspect is that increasing the
cooling time (for instance heat input)
increases the extent of the zones (broad-
er zones).
Multipass Weldment
The situation in a multipass weldment
can be constructed from the condition
shown in Fig. 2. Weld 1 in this figure can
be assumed to be the single bead weld in
Fig. 1. The subsequent weldment, Weld
2, will change the HAZ of Weld 1. How-
ever, it is different from the HAZ of a
single bead weldment. The original
microstructure is not the ferrite/pearlite
microstructure of the base metal, but the
bainite/martensite of the coarse-grained
zone. The thermal history in such a HAZ
can be characterized by two tempera-
ture cycles, as indicated in Fig. 2. Subse-
quent weld beads will influence the
microstructures by a tempering heat Fig. 1— The heat-affected zone (HAZ) of a single bead weldment. A-A schematical presentation
treatment mechanism. of the different zones in the HAZ of a single bead weldment; B-a macrosection of a bead in
groove weldment, Steel 2
Investigated Steels
steels, while one steel (Steel 8) is a C-Mn carbon microalloyed steels is low (0.08-
The chemical compositions are pre- steel of the quality used previously for 0.12 wt-%) compared to the C-Mn steel
sented in Table 1. Nine steels are classi- offshore structures. (0.18 wt-%). However, all the steels have
fied as modern low-carbon microalloyed The carbon content of the nine low- a similar Mn content, in the range of
Table 1—Chemical Composition of the Investigated Steels
Chemical Composition (wt-%) ppm
C Si Mn P S Cu Ni Cr Mo V Ti Nb Al N O H
Steel 1 0.09 0.51 1.56 0.021 0.005 0.13 0.14 0.02 0.01 0.01 0.002 0.041 0.036 0.010 40
L= 4W
i._Vi_-
W= B
CTOD TEST SPECIMEN BxB , Surface notched
a/W = 0.05 - 0.1
Cycli Cycle 2
Temp,
(Tp]L,r 900
time
Fig. 2 — The temperature cycle in a multipass weldment Fig. 3 —A — The surface notched CTOD specimen; B — the position of
the electro-discharged notch in the bead-in-groove weldment
1.3-1.6 wt-%. All steels were normalized after con- perature (Tp) and a cooling time from
All steels are microalloyed with Nb, but trolled rolling, resulting in fine-grained 800° to 500°C (1472° to 932°F) (At 8/ s).
in different quantities, ranging from 0.017 ferrite and pearlite in a clear banding By positioning a dilatometer on the spec-
to 0.048 wt-%. Other microalloying ele- distribution. The plate thicknesses ranged imen, the phase transformation was reg-
ments include Mo, V, Ti and Al. The levels from 40 to 60 mm (1.5 to 2.4 in.). istered by recording the change in vol-
are low for M o , V and Ti, with an The low-carbon microalloyed steels ume. After the simulation tests, the spec-
exception for Steel 2, which has a Ti have similar mechanical properties: upper imens were machined to Charpy V-notch
content of 0.011 wt-%. The Al content yield stress, 340-380 MPa (49.3-55.1 ksi); specimens to determine notch tough-
varies between 0.017 and 0.036 wt-%, tensile strength, 480-520 MPa (69.6-75.4 ness. Furthermore, the microstructure
but that is a low level for steels. All ksi); elongation, 30-35%; percentage was investigated by preparing the speci-
microalloying elements are affected by reduction of area, 70-80%; and Charpy mens with grinding, polishing and etching
the N content, which varies from 80 to V-notch absorbed energy at - 4 0 = C , in 2% Nital.
140 ppm. 250-300 ) (184-221 ft-lb). The C-Mn steel The surface fracture toughness speci-
Small amounts of Cu and Ni are exhibits a similar level for mechanical mens were extracted from single-pass
present in the steels at a level
Steel 1 — Steel 6 — •> — .
Steel 2
Steel 3 Steel 9 . u —
Steel 4
200
0-— 300
J 1 \
a
Tp = 1350°C
CVN J
is \
S
100
^
fi"°
27
fi CVN.22oc
200
Li >
11 l v
l\ 1 \
\
\
0
-6 0 -40 -20 0 20 40 60 80 o c 100
. \\l x
Fraction
100
%
*—8^^
100
,U ' -
brittle 50
8 ^ 0
\ \ \ ••• -
fracture 8 \ _ . _ - - '
0
0
-6 0 -40 -20 0 20 40 60 80 0 1 30 c 10 20 s 30
°C
At
*" 8/5
Fig. 4 - The transition curve of the Charpy V-notch energy and the Fig. 5- The influence of the cooling time (At8/5) on the notch toughness
fraction of brittle fracture of the simulated weld thermal microstructure at -22°C of the simulated weld thermal specimens, comparing the
of Steel 1 (T = 1350°G and At8/5 = 12 s) curves of the investigated steels
Weld Thermal Simulation Results on a few steels to investigate: UB — upper bainite which is differen-
1) The influence of a postweld heat tiated between:
The main tests were divided into t w o treatment. The weld thermal specimens SP —side plate structure often
parts: were heat treated for 1 h at 600°C growing from former austenite
1) The influence of the cooling time (1112°F) before testing. grain boundaries with plates
(Ats/s) for a peak temperature of 1350°C 2) The influence of a double weld separated by austenite/mar-
(2462°F). As indicated in Fig. 1, the peak thermal simulation cycle. By performing a tensite/carbide aggregates;
temperature of 1350°C is in the middle second weld thermal cycle, it was intend- AF —acicular intergranular ferrite
of the peak temperature range of the ed to analyze the changes occurring in plate/needle separated by
coarse-grained zone, and this peak tem- multipass welding. high-angle boundaries;
perature is used as a first characterization M —martensite.
of the coarse-grained zone. By changing
Influence of the Cooling Time for a Peak
the cooling time (At 8/5 ) of the tempera- What was observed was largely mar-
Temperature of 1350° C
ture cycle, the influence of the heat input tensite and upper bainite consisting of
on the coarse-grained zone can be ana- The cooling time (Ats/s) w a s varied side plate ferrite and acicular ferrite, in
lyzed. between 4 s and 24 s, and a summary of which the side plate ferrite structure was
2) The influence of the peak tempera- the Charpy V-notch toughness is present- the dominating component. At the lon-
ture (Tp) for a cooling time of 12 s. The ed in Fig. 5. Steels 5 and 7 were not gest cooling time, the structures of all
HAZ consists of different microstructural investigated since their compositions are steels (excluding Steel 10), including the
zones, and a gradient exists in each zone. similar to the other steels. The highest C-Mn steel, was upper bainite, as shown
To investigate this situation, the peak toughness is observed for Steel 10, fol- schematically in Fig. 6A. For the low-
temperature of the weld thermal simula- lowed by Steel 4. The other steels carbon microalloyed steels, the same
tion cycle was systematically reduced revealed a similar behavior, with low microstructure was observed at the cool-
from 1350° to 700°C (2462° to 1292°F) toughness at cooling times greater than ing time of 12 s, while the C-Mn steel
for a series of specimens. 10 s. At lower cooling times, the tough- displayed a large amount of martensite.
From the weld thermal specimens, the ness increases significantly. It should be However, at the shortest cooling time,
notch toughness was determined, and noted that the C-Mn steel (Steel 8) fol- deviations were observed. For instance,
the microstructures analyzed. The transi- lowed this tendency, too. Steel 2 had a reduced amount of mar-
tion curve of a simulated microstructure Three cooling times were chosen, tensite compared to the other steels. As
for Steel 1, T p = 1350°C and At 8 / 5 = 12 Af.8/5 = 4 / 5 s, 12 s and 24 s, and the expected, the microstructure of the C-
s, is shown in Fig. 4. The lower shelf of microstructures investigated were as fol- M n steel was martensite.
the curve is positioned at ss —20°C lows: With an increase in the cooling time, a
(—4°F), which implies that this tempera- considerable coarsening of the micro-
ture was probably most sensitive to a P — pearlite; structure was observed. This condition is
change in microstructure with a coincid- F — polygonal, equiaxed ferrite often related to the increase in austenite grain
ing change in transition temperature. located at former austenite grain size with the increase in cooling time —
A secondary project was carried out boundaries; Fig. 6B. Also, Steel 10 showed an increase
WELDING RESEARCH SUPPLEMENT 1269-s
Steel 2 Steel 4 \7 Steel 8 <
Steel 3 Steel 6 • Steel 9 O
B
•tig. y ^ w "
.2 100
o %
2 i,
0) 60
•**
c Tp = 1350 C
(D
CQ 0
30
At 8 / 5
Steel 10 •
150
in
c
a 100
a.
O)
a>
^.
'c
aj
u>
<
50
Tp - 1350 C
10 20 30
At 8 / 5
B
Fig. 6 —A — The influence of the cooling time (Ats/s) on the fraction of Fig. 7 — A comparison between the simulated weld thermal microstruc-
bainite in the microstructure; B — the prior austenite grain size ture of Steel 6 and Steel 10 (Tp = 1350°C, M8/5 = 24-25 s)
in austenite grain size as t h e c o o l i n g t i m e l o w e r bainite m i c r o s t r u c t u r e instead o f Steel 10 p r e s e n t e d t h e l o w e s t level. C o n -
increased. T h e m i c r o s t r u c t u r e o f Steel 10 u p p e r bainite. cerning the C - M n steel, t h e same t o u g h -
d i d not consist of u p p e r bainite —Fig. 7. ness level w a s o b s e r v e d in the partially
A n e l e c t r o n microscopic evaluation is t r a n s f o r m e d z o n e as in the base m e t a l .
Influence of the Peak Temperature (T p ) for
n e e d e d f o r an identification of the m i c r o - A r o u n d a peak t e m p e r a t u r e of 8 5 0 ° C
a Cooling Time of 12 s
structure, but the t r a n s f o r m a t i o n dia- (1562°F), t h e partially t r a n s f o r m e d r e g i o n
grams indicated that l o w e r bainite c o u l d By changing the peak t e m p e r a t u r e of b e c a m e a fine-grained z o n e , character-
b e f o u n d here. the w e l d t h e r m a l simulation cycle, the ized b y a f i n e ferrite grain structure w i t h a
C o m p a r i n g the microstructural obser- different zones in the H A Z o f a w e l d m e n t grain size finer than the n o r m a l i z e d base
vations w i t h t h e C h a r p y V - n o t c h t o u g h - w e r e analyzed. Besides an investigation metal. T h e C h a r p y V - n o t c h toughness w a s
ness, s o m e interesting trends were of the m i c r o s t r u c t u r e and its toughness, it raised t o a high level f o r all the m i c r o a l -
observed: was possible t o f i n d , t o s o m e extent, the l o y e d steels, but l o w e r than the base metal
1) A high C h a r p y V - n o t c h toughness transition peak t e m p e r a t u r e s f o r the ( 2 5 0 - 3 0 0 1 / 1 8 4 - 2 2 1 ft-lb). H o w e v e r , it
level w a s f o u n d f o r a martensite m i c r o - zones and the toughness gradient in the w a s o b s e r v e d that the toughness of the
structure. zones. A s u m m a r y of the C h a r p y V - n o t c h fine-grained z o n e of the C - M n steel w a s
2) W i t h an increasing a m o u n t of u p p e r toughness curves f o r the eight investi- higher t h a n t h e base m e t a l .
bainite, t h e n o t c h toughness decreased, g a t e d steels is p r e s e n t e d in Fig. 8. Starting Increasing t h e p e a k t e m p e r a t u r e t o
a n d especially f o r the C - M n steel, w h e r e at t h e l o w e r peak t e m p e r a t u r e s , all higher values ( > 1 0 0 0 ° C / 1 8 3 2 ° F ) re-
a rather small a m o u n t w a s e n o u g h t o m i c r o a l l o y e d steels s h o w e d a m i n i m u m sulted in a transition f r o m fine-grained t o
r e d u c e the n o t c h toughness considera- toughness in the partially t r a n s f o r m e d coarse-grained z o n e . T h e transition range
bly. r e g i o n . T h e microstructures revealed the for Steels 1 , 4 a n d 10 a n d the C - M n steel
3) A c o m p l e t e u p p e r bainite m i c r o - t r a n s f o r m a t i o n of pearlite t o martensite, w a s 1 0 0 0 ° - 1 1 0 0 ° C . A s o m e w h a t higher
structure has a l o w n o t c h toughness f o r w h i l e t r a n s f o r m a t i o n p r o d u c t s w e r e also transition w a s f o u n d f o r Steel 2, w h i l e t h e
almost all o f the specimens, including the o b s e r v e d at the ferrite grain boundaries. highest transition (1150°-1250°C/
C - M n steel. H o w e v e r , Steel 4 s h o w e d a Increasing the peak t e m p e r a t u r e in- 2 1 0 2 ° - 2 2 8 2 ° F ) w a s o b s e r v e d f o r Steels
rather high n o t c h toughness w i t h a c o m - creased t h e t r a n s f o r m a t i o n r e g i o n t o t h e 6, 9 a n d 3.
plete u p p e r bainite m i c r o s t r u c t u r e . extent that w h o l e ferrite grains w e r e For Steels 1 , 6, 9 and 3 a n d the C - M n
4) Superior toughness has been t r a n s f o r m e d in the fine-grained z o n e . steel, t h e transition f r o m high toughness
o b s e r v e d f o r Steel 10, w h i c h s h o w s a Steel 4 s h o w e d the highest values, w h i l e in t h e fine-grained z o n e t o l o w toughness
2 7 0 - s | SEPTEMBER 1 9 8 7
in the coarse-grained zone coincides with
the transition temperature of the micro- Steel 1 — Steel 6
structure. The microstructure is upper
bainite for the microalloyed steels, while Steel 2 Steel 8
the C-Mn steel has a considerable Steel 3 Steel 9
amount of martensite. Steel 10 exhibited
Steel 4 Steel 10
a special behavior, with the toughness
increasing in the transition region to a
very high level. Both Steels 2 and 4 show
upper bainite in the coarse-grained zone;
however, in a large part of the coarse-
grained zone (up to 1350°C/2462°F), a
high toughness level was found.
Influence of PWHT on the Weld
Thermal Specimens
The investigations were limited to
Steels 1, 2, 4 and 8. The peak tempera- Fig. 8 — The influence
ture was varied between 700° and of the peak
1350°C, and after the weld thermal sim- temperature (Tp) on
ulation experiments, the specimens were Ihe Charpy V-notch
heat treated for 1 h at 600°C. toughness at —22°C
of the simulated weld
The most dramatic influence of the
1400 thermal specimens,
heat treatment was observed in the par- comparing the curves
tially transformed region. The Charpy of the investigated
V-notch toughness level was raised to the steels
same toughness as the fine-grained
zone —Fig. 9. The fine-grained zone in
largest increase, although the scatter was Fracture Toughness Test Results
turn is also improved by the heat treat-
considerable for Steel 4. In the partially
ment. However, the low toughness part
transformed zone, transformation prod-
of the coarse-grained zone revealed at Surface Fracture Toughness
ucts were observed at the prior austenite
least the same low toughness level after
grain boundaries, but since the toughness All weldments were made with the
the heat treatment.
was low initially, the same low level was same type of electrode, using the same
Although the Charpy V-notch tough- observed here. welding procedure and a heat input of 4
ness seems not to be altered, the fracture
surfaces of Steels 1, 4 and 8 revealed a
large degree of intercrystalline fracture Steel 1 Steel 2
appearance after the heat treatment, 300
300
' P W H T * 1 hour)800 0 C
compared with a transcrystalline mode of "... : 8 I 0 J
PWHT : 1 h o u r / 6 0 0 C
fracture before heat treatment. Howev-
er, Steel 2 has a transcrystalline fracture • \ 200
200
both before and after a heat treatment. • (
' As simulated J,
CVN
CVN -22t
-22°C
' ' / ' - - ' " " - - " • \
Influence of a Second Weld
Thermal Simulation Cycle 100 100
The initial microstructure of the speci-
mens was the coarse-grained zone
\ OaV At
8/5
= 12 • VJ '
\;
/ \ o
(Tp = 1350°C and Atg/s = 12 s). The 0
900 1000 1100 1200 1300 1409
600 700 800 900 1000 1100 1200 1300„140C eo 0 700 800
specimens were simulated with a second °c
cycle, with varying peak temperatures.
These experiments were limited to Steels
1, 4 and 8.
Steel 4 Steel 8
In the coarse-grained zone, transition
from 1000° to 1100°C (1832° to
/ PWHT : 1 hoUr/BOO C
2012°F), the notch toughness was low, as J J
"a/« = 12
*
the microstructure was similar to the
original microstructure of upper bainite —
Fig. 10. In the fine-grained zone, the CVN
200
i 'i
/ \
,' As simulated \ v
A
i 200 .
PWHT : 1 hour/600 0 C
-22°C a
CVN
coarse-grained initial microstructure was -22t
transformed, resulting in some grain
100
refinement of the austenite, although the 100
austenite grain size was still t o o large to Aa (Imulaiecl - ~". \ \'
result in a fine ferrite grain structure. Only
at the prior austenite grain boundaries '^v&tefc.
was the fine ferrite grain structure
600 ,
700 800 B00 1000 1100 1200 1300 140-0 600 700 600 900 1000 1100 1 2 0 0 1 3 0 0 . 1 4 00
Tp —•- Tp
observed, while upper bainite was found
within the grains. A certain improvement
Fig. 9- The influence of a postweld heat treatment (PWHT) on the Charpy V-notch toughness at
in the Charpy V-notch toughness was
—22 "C for simulated weld thermal specimens with varying peak temperatures and a cooling time
observed. Steels 4 and 8 showed the of At8/5= 12 s
WELDING RESEARCH SUPPLEMENT 1271-s
Steel 4 Steel 8
(Tp) = 13SO°C / (M ) = 12 a ( T p ) i = 1350°C / ( A t a / 5 ) 1 = 12 s
J j
1
"Wl' *' "V5V ,2 •
200
' 200
CVN o CVN
-22°C
-2 2 C
/ \ )
100 100
/. -\ (y
IX V" ~'\. \ . . • • •y^'^' s <
"^._ w __
~"~^-o-' " " °
600 700 800 900 1000 1100 t?00 1300 1400 600 700 BOO 900 IOOO 1100 1200 1300 1400 800 700 800 900 1000 1100 1200 1300 1400
°C
°c °C
(Tp),
Fig. 10— The influence of the peak temperature of the second simulated weld thermal cycle (Tp)j on the Charpy V-notch toughness at —22°C. The
peak temperature of the first cycle was 1350°C while the cooling time from 800' to 500°C for both cycles was 12 s
MJ/m (101.6 kj/in.). All steels, except for Small-Scale CTOD Testing of the partially transformed region. The
Steel 2, reveal a pop-in or a final brittle results are presented in Figs. 12 and 13,
Small-scale CTOD specimens (10 X 10
fracture —Fig. 11. A pop-in through the respectively.
mm/0.4 X 0.4 in., a / W = 0.3) were
coarse-grained zone followed by ductile
machined from the weld thermal speci-
crack growth through the fine-grained
mens. The test program was limited to Discussion
zone, resulting in a final brittle fracture or
t w o simulated conditions:
further ductile crack growth, were The simulated weld thermal tests and
observed for the group of steels with low 1) T p = 1350°C and At 8 / 5 = 12 s
the CTOD tests identified t w o embrittled
CTOD values-Table 2. A final brittle 2) T p = 720°C and At 6 / 4 = 14 s
areas in the heat-affected zone:
fracture, with or without ductile crack The first condition represented the 1) A coarse-grained zone, consisting
growth, is shown for the group of steels coarse-grained HAZ, while the second of a coarse microstructure of martensite
with high CTOD values. simulation represented the investigation or bainite.
Table 2—Schematic Presentation of the Type of Fracture in the CTOD Specimens
r;
?
Number of specimens for e a c h f r a c t u r e type w i t h l o w e s t CTOD in p a r e n t h e s i s
Zone
Coarse grained Zone
PartiaHy Transformed
Flne grained Zone
Tempered Zone
Steel Steel Steel Steel Steel Steel Steel Steel Steel Steel
Weld
1 2 3 4 5 6 7 8 9 10
\
s
6 8 2
(0.47mm) (0.26mm] (1.1mm)
"j [^,^^^4*4^^® 2 1 9 9
(0.7mm) (0.25mm) (0.18mmX0.29mm)
\ I ! I •» 1 3 3 4 1 1
1 1 I I * "
(3.3mm) (1.8mm) (1.9mm) (1.6mm) (2.3mm) (1.76mm)
in - I 1 I » 6 5 8
(2.5mm) (1.9mm) (1.85mm)
6
* * *
(5.0mm)
= EDM notch a„~, Ductile fracture • • B l ittle frac ture
•—>- Final brittle fra cture 0 Unk aded
272-s I SEPTEMBER 1987
T p = 720°C / A t - 1 2 s
8 / 5
1.5
6.0 mm 5
Q.
o
>
5.0
1.0 • — A LU
• * Q
CTOD 0„ \
-10 C I
O
• ce
• \ <
4.0
0.5 / /—* UJ
V)
"A UJ
CTOD o T
cc
-10 C
UJ
3.0
0-
0
Stee 1 2 3 4 5 6 7 8 9 10 o
—I
ui
2.0 Fig. 13 — The small-scale CTOD results from the simulated weld thermal >
UJ
specimens with Tp = 720°C and Atb/4=14 s. BXB specimens: Q
a/'W = 0.3 I
O
CC
1.0 <
UJ
tr>
ATp
CVN o
z
-22 C UJ
Steel 1 2 3 4 5 6 7 8 \ / V ' Q.
o
-J
Fig. 11—The surface fracture toughness results for the investigated UJ
1540 C
steels. BXB specimens: a/W = 0.05. Fillet symbols: CTOD at maximum Tp >
load Weld thermal simulation test UJ
Q
I
1 O
CC
Stee 2 10 4 8 3 9
6 <
Tp = 1350°C UJ
/ A t 8 / 5 = 12 s 500 ID
C/Mn-steeK UJ
1.0
°c oc
-
mm 400
Or CL
CTOD
10°C 0 ATp 3 0 0
O
0.5
200 A
/ o •
•
o / >
100 UJ
a / Q
/ i
o / \
~/fi
I
fit ° y.r> — 0
3 0.004 0.008 0.012 0 016
O
CC
0
Stee 1 2 3 4 5 6 7 8 9 10 - °oN <
UJ
Fig. 12 — The small-scale CTOD results from the simulated weld thermal Fig. 14 — The peak temperature range (A Tp) of the coarse-grained zone
specimens with Tp= 1350°C and Atg/s = 12 s. BXB specimens: with low Charpy V-notch toughness plotted as a function of the total
a/W = 0.3 nitrogen content
Q.
o
—I
UJ
2) A partially transformed zone, thermal simulation cycle had only a limit- Coarse-Grained Zone
>
where only microstructures with a high- ed improvement in the temperature UJ
Important variables to be taken into Q
carbon content (pearlite, banding) are range of 800° to 1100°C-Fig. 10. Post- -^
account with steels are processing, chem- I
transformed. weld heat treatment had no beneficial
All steels, except for Steel 10, exhibit effect on the low toughness in the
istry and HAZ microstructure. O
CC
low Charpy toughness values in a part of coarse-grained zone —Fig. 9. <
5fee/ Processing
the coarse-grained zone. The part of the All steels had low Charpy toughness in UJ
zone showing low toughness varies with the partially transformed zone (Fig. 8), but v>
All steels are delivered in the normal- UJ
the steel quality (Fig. 8) and the cooling very high toughness values were ized condition, but there is no informa- cc
rate —Fig. 5. A subsequent second weld obtained after P W H T - F i g . 9. tion regarding the detailed processing.
WELDING RESEARCH SUPPLEMENT 1273-simproving the toughness (Ref. 3). Addi-
tions of trace amounts of Ti, in the range
(x Number of specimens in the population for the minimum CTOD value. of 0.004-0.01%, are intended to inhibit
4 1 the formation of free N in the heating
stage of the weld thermal cycle. During
Steel 9
6
cooling, it is suggested that Al may play
-C
an important role in fixing free nitrogen to
5
6.0 l o form AIN (Ref. 3).
a Pop-in/Ductila cr g(
| O) In addition, niobium carbonitrides may
mm • Pop-in/Due III a Cf.gr ./Buttle lr. o form at lower temperatures; hence, it is
co difficult first to estimate the amount of
o Brittle it.
I o free N, and second, the proportion of the
1 e Ductile cr.Qf /Brillle lr.
5.0 6
CD HAZ where nitrides have been dissolved
w ' -t- Ductile cr.flr.
o
3
during heating, leaving free N after the
Q subsequent cooling. The total amount of
\ I\ N should, however, give an indication of
\ \ \ the potential for free N. In Fig. 14, the
\ temperature range of the coarse-grained
4.0
VW # (8>. \
\
\\
zone with low toughness is plotted as a
function of the total content of N. There
CTO[ \\ is a trend towards decreasing the width
\ATp
CVN o \ i i
-22X
la-iot
Weld thermal simulation test Weld thermal simulation test
2
Steel 2 10
Steel 4 10 3 9 6 18
1.0
500 C/Mn-steel
o„
mm Stoel 10 Lower Bainite Ductile CTack growth C
400
CTOD o |
-10 C O
300
/ o \ ATp
0.5 + / \ .even with the problems of notch posi-
(x) Number of specimens in the population for the minimum C T O D value. tioning, it is strange that no low CTOD
values were recorded.
4 2
St eel 5 1073 9 6 18 HAZ Microstructure
a. sz
o *J Most of the steels exhibited the same
_i 6.0 rl behavior of Charpy notch toughness as
UJ
> mm
a Pop-in/DuCitla cr gt. O) related to the cooling time —Fig. 5. A
UJ • Pop-in/Oucnie ct g, /Brittle It.
CJ
sharp reduction in toughness was
o Brittle It ra
ka
observed in the cooling time interval
""a.
a Ductile cr gr /Briltie lr. o from 5 to 12 s, followed by a low notch
6)
I 5.0 ^ _0>
toughness level at 24 s.
o o
CC Steels 10 and 4 deviate from this trend,
Q
< with a higher Charpy notch toughness.
UJ
IS) \ \ \\ Examination by optical microscope identi-
fied a shift in microstructure with a
4.0 \ \ change in cooling time —Fig. 6A. The
Q. \ general trend is for a structure to be
O CTOD \\ dominated by martensite at the shortest
0 CD
-10 C cooling time, Ats/s = 5 s, and then for a
>
|\^l \
\
13
CO
gradual increase in the fraction of upper
bainite with longer cooling times. Steel 10
I
, 3.0 Ks \\ l
1
. \\
7\
/
toughness because autotempering can
take place from a high M s temperature.
a.
o
—i
UJ 1.0
r
//A
/
f>
/
/ /
_c
The hardness gives no indication of the
toughness, because this kind of marten-
site has a high toughness and hardness,
> \A / A
/ Q. while the brittle upper bainite has a lower
UJ
Q
o'/A o
DL toughness —Fig. 5. The upper bainite is
"aa. /
o dominated by a side plate structure with
cc /
the plates separated by austenite/mar-
< "tl)" X
C/Mn-steel / tensite/carbides, giving favorable condi-
UJ 0
tf) () 0.01 0.02 0.03 tions for initiation and propagation of
UJ
oc cleavage fracture. Both Steels 2 and 4
exhibit a high notch toughness in a large
» %P
part of the coarse-grained zone with
upper bainite (Fig. 8), indicating that some
a. Fig. 18— The minimum CTOD values of the groove-welded specimens plotted as a function of additional embrittling effect is necessary
O the phosphorus content and related to the different fracture types in order to obtain a brittle low-carbon
_i
UJ upper bainite structure.
>
UJ
Q Partially Transformed Zone
\ Fig. 19-The CTOD
I Recognizing brittleness in the partially
O results of the Weld thermal simulation test
or simulated weld transformed zone is not a new discovery.
< thermal specimens Stee 4 10 3 9 6 18 In a paper published in 1947, the exis-
UJ (Tp = 1350°C and 2 tence of this low-toughness region was
cn Ata/s = 12 s) plotted examined with notched bend specimens
1.0
UJ as a function of the and heat-treated Charpy bars to simulate
Z phosphorus content mm Steel 10 . Lower Bainite - Ductile crack growth the thermal effect of welding (Ref. 6).
UJ
2 Two temperatures were found to be
o
CTOD o
-10 C ] highly deleterious —a coarse-grained
structure formed at 1150°C (2102°F) and
o.s the lowest values of all, observed at
>
UJ
o
O
O
i\ /
»
\ /
/
Upper Bainite
-\
»
\
760°C (1400°F). Postheating at 450°C
(842 °F) and above restored a large
degree of the toughness drop at 760°C.
X Ov & ^ \ C/Mn- steei . Microscopic examinations revealed that
o Martensite
oc 0 upon heating to 760°C, pearlite islands
< transformed to austenite with a nearly
UJ c) 0.01 0.02
V) eutectoid composition. Upon cooling,
UJ
cr these high-carbon islands were partially
transformed to martensite. Postheating
276-s | SEPTEMBER 1987served to temper and partially spheroid-
ize the martensite. ATp
The observations from 1947 corre-
spond very well to the present results.
r i
°c
\r\ i
C V N
-,-a
Low toughness is observed in the partially -22
transformed zone after single cycle ther- Q.
mal simulation (Fig. 8), and also in the
1540"C
o
temperature range which corresponds to - Tp UJ
the partially transformed zone after dou- >
UJ
ble simulation of the coarse-grained Weld t h e r m a l simulation t e s t
Q
zone —Fig. 10. \
6 I
The optical microscopic examination St e e l 4 10 289 1 3 O
CC
and work in progress on similar low-
carbon microalloyed steels (Ref. 7) have 500
C/Mn-Steel / <
identified high-carbon twinned marten- °c UJ
400
* /o tf)
site as the main embrittling factor. After / / UJ
single cycle transformation, the banding cc
ATp 300 / UJ
with high-carbon areas (pearlite) is aus- a
tenized and subsequently transformed to 2
CL
twinned martensite upon cooling. The 200 Fig. 20-Thepeak o
—I
martensite will reduce the toughness temperature range UJ
partly because of the residual stresses in A o / (ATp) of the >
100
the surrounding matrix and partly coarse-grained zone UJ
with low Charpy Q
because of the preferred distribution of / N
the martensite islands along the ferrite 0 V-notch toughness I
grain boundaries within the banding area.
() 0.02 0.04 0.06 plotted as a function o
—-** %Nb of the niobium cc
The work in progress (Ref. 7) has indi- <
content UJ
cated that the brittleness can be avoided
If)
if the pearlite is finely dispersed. UJ
oc
After double simulation of the coarse- ~~~
(x) N u m b e r of s p e c i m e n s in t h e p o p u l a t i o n f o r t h e minimum C T O D v a l u e . \-
grained upper bainite, the existence of
separate phases (islands) along the prior 4 6 3 z
UJ
austenite grain boundaries is even visible Steel 5 10 289 1 7
at low magnification in the optical micro- JZ o
scope—Fig. 23. The embrittling effect of 6.0
5
o
>
twinned martensite in a less ductile matrix Pop-ni/Ouciila cr.gr UJ
" DJ
(upper bainite) is obvious. mm • Pop-in,DuCllla cr gr. /Brntla lr.
O
o
° Mil/ CO I
PWHT has a remarkable effect on the
o
o
toughness of the partially transformed • Ouciila c V3' \
CTOD standard was substituted with an \
electro-discharge machined notch. Both
the shallow cracks and the EDM notch /
A .(?> / OL.
result in higher fracture toughness than if 1.0 / / _c
measured in accordance with the CTOD
o' ' 1
/ ' /
Q.
O
o
_J
standard (Ref. 5). 0. UJ
// .«) / >
Most specimens were characterized ' r t . MW» / n (3 » /- / LU
with a pop-in or a final brittle fracture — C/Mn-steel^iB." / . &
' / a
0 « • * • > " . I
Table 2. Only for Steel 2 was ductile () 0.02 0.04 o
crack growth, resulting in a maximum oc
<
load recording, observed for several •a CV U L
UJ
V)
specimens. For all the other steels, the •^ rolND
UJ
CTOD values (Fig. 11) represent a brittle or
fracture or a pop-in. The large scatter in Fig. 21- The minimum CTOD values of the groove-welded specimens plotted as a function of the
the test results is partly caused by the niobium content and related to the different fracture types
WELDING RESEARCH SUPPLEMENT 1277-s• ; , . ! . .
Weld thermal simulation t e s t
6
Steel 4 10 2 8 9 1 3
1.0
mm
Steel 10 Lower Bainite - Ductile crack growth
CTOD o
'&•,
-10 C t -tlSSE^
O
0.5 """"^-c-
O
\ • X
o "Upper X
Bainite \ v ^
Fig. 22-The CTOD rv o At^-iy-: ^
results of the 0 Wch>ii a'**',X
simulated weld
thermal specimens
—1
C/Mn-steel . 70% Martensite £ -;;; ~>.Ap, * •V •"'. A
0.02 0.04 0.06 :
(Tp = 1350°C and $• * ••• •' S •
Ats/s = 12 s) plotted
as a function of the °oNb
b%}~-' -.-•-••-A*#&.%*
niobium content
:< /mm
Ay-". - "Ct,: • j
'Tp = 1 3 5 0 C /At 8 / 5 = 12 s
1.0
CTOD ou
-10 C
, 0.5
Steel 4 Steel 8
Brittle fracture
i
Steel 1 2 3 4 5 6 7 8 9 10
p
/ Pop-in
J i
P fi'-\ 300
J At 8 / 5 :12 s
^ j Unloaded 200
CVN o„
-22 C
i
I I f 100 .
\ 1
50k fa' 50kN[
1 1mm
1mm
Vg
! - Vg
0 =7J^t>--:
Specimen 4B3 Specimen 8C3 Steel 1 2 3 4 5 6 7 8 9 10
Fig. 25 — Examples of load/clip gauge records from the groove weld Fig. 26 — The small-scale CTOD results and the Charpy V-notch tough-
CTOD specimens of Steel 4 and Steel 8 ness at —22 "C for the simulated weld thermal specimens with a peak
temperature of 1350°C and a cooling time of Ats/s = 12 s
coarse-grained z o n e of a real H A Z c o m - 1) W e l d t h e r m a l simulation testing References
p a r e d w i t h the single t e m p e r a t u r e simula- identified t w o l o w toughness zones — t h e 1. Nippon Steel Corporation Technical
t i o n . For Steel 2, the C h a r p y toughness is coarse-grained z o n e a n d the partially Symposium. Low Carbon Heavy Section Steel
still high at a peak t e m p e r a t u r e o f 1 2 5 0 ° C transformed zone. for Offshore Applications. Stavanger, May 30,
(2282 °F), indicating that t h e high C T O D 2) Shallow surface n o t c h e d C T O D 1983.
values f r o m t h e g r o o v e w e l d testing may testing revealed high C T O D values, b u t 2. Loberg, B., Nordgren, A., Strid, I., and
reflect that the size o f the brittle z o n e is m o s t specimens w e r e characterized w i t h Easterling, K. E. 1984. The role of alloy compo-
o f i m p o r t a n c e ; i.e., the small brittle z o n e a p o p - i n o r final brittle f r a c t u r e initiating sition on the stability of nitrides in Ti-microal-
in t h e coarse-grained H A Z at t h e w e l d loyed steels during weld thermal cycles. Metal-
f r o m the l o w toughness zones.
lurgical Trans. A 15A(1):33-41.
interface m a y b e of limited c o n s e q u e n c e . 3) The influence of elements, such as
3. Watanabe, I., Suzuki M., Tsukada, K.,
H o w e v e r , the significance of local brittle Ti, N, P, N b a n d Ni is discussed, a n d it is Yamazaki, Y., and Tokunaga, T. 1983. Devel-
zones w i t h respect t o the risk of failure of suggested that N a n d P m a y b e of m a j o r opment of NK-HIWEL Steels Suitable for High
w e l d e d structures should b e f u r t h e r eval- importance. Heat Input Welding. Nippon Kokan Technical
uated. 4) T h e r e f e r e n c e C - M n steel w a s at Report, Overseas No. 38.
least as brittle as the l o w - c a r b o n steel, 4. McKeown, D., )udson, P., Apps, R. L.,
a n d it is q u e s t i o n e d w h y n o l o w H A Z Dumphrey, W . I. 1983. The weldability of low
Conclusions C T O D values w e r e r e c o r d e d in the c o m - sulphur steels. Metal Construction (11):667-
673.
p r e h e n s i v e testing of this steel 10 years
Nine m o d e r n l o w - c a r b o n m i c r o a l l o y e d 5. Thaulow, C , Paauw, A. )., Cunleiksrud,
ago.
steels, d e l i v e r e d f o r recent applications in A., Naess, O. |. 1985. Heat affected zone
o f f s h o r e structures, and o n e C - M n steel, toughness of a low-carbon microalloyed steel.
used in the 1970's f o r o f f s h o r e p u r p o s e s , Metal Construction (2):94R-99R.
have b e e n e v a l u a t e d w i t h respect t o t h e 6. Stout, R. D., McCeady, L. ). 1947.
f r a c t u r e toughness level in the heat- Metallurgical factors in the embrittlement of
welded plate. Welding journal'26(11):683-s to
affected zone of weldments. The tough-
A ckno wledgments 692-5.
ness w a s investigated by means o f w e l d
7. Work in progress at SINTEF, Trondheim,
t h e r m a l simulation testing and surface T h e authors a c k n o w l e d g e the s u p p o r t Norway.
fracture toughness testing. T h e results are f r o m the N o r w e g i a n Petroleum D i r e c t o r - 8. Private communication with The Weld-
as f o l l o w s : ate and f r o m M r . O . ). Naess. ing Institute and Det Norske Veritas.
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