inconel 625 welding metallurgy
TRANSCRIPT
-
8/11/2019 Inconel 625 Welding Metallurgy
1/8
The Welding and Solidif ication Metallurgy
of Alloy 625
Chemical composition and solidification microstructure are
correlated to hot cracking susceptibility
BY M . J. CIESLAK
ABSTRACT. The weld metal microstruc
tu re deve lopment and
solidification
crack
ing behavior o f Al loy 625 gas tungsten arc
(GTA) we lds as a funct ion o f comp osi t ion
has been determined. A th ree-factor , tw o-
level,
factoria l ly-designed set of a l loys in
volv ing the elements C , Si and Nb was ex
amined.
Differen tia l therm al analysis (DTA)
of these al loys indicated that N b, and to a
lesser exten t C a nd
S i,
increased the melt
ing /so l id i fica t ion tem pera ture range. The
DTA revealed that terminal sol id if ication
const i tuents were f o rm ed in the Nb-bear-
ing al loys, the presence of which was
conf i rm ed by opt ica l and e lectron micros
cop y techn iques and ident i fied as 7 / M C -
(NbC) carb ide , 7 /Laves and 7/M
6
C car
bide eutectic-type consti tuents. Addit ion
of carbon to the Nb-bearing al loys was
observed to promote the fo rmat ion o f
th e
7 /MQNbC)
carbide co nsti tuent an d Si
was observed to promote increased fo r
mat ion o f the 7 /Laves const i tuent . Re
gression analysis of Varestraint hot-crack
testing data revea led that addit ions of C o r
Si to A lloy 625 increase d the s uscep tibility
o f the a l loy to ho t cracking . N iob ium-free
a l loys wer e obse rved to have a very low
tendency toward so l id i f ica t ion hot crack
ing,but even amo ng these al loys, C and Si
addit ions were detrimental. I t was con
cluded that the increased sol id if ication
tempera ture range and fo rmat ion o f Nb-
rich eutectic consti tuents were primari ly
responsible for the increased susceptib i li ty
of N b-bearing al loys to
solidification
crack-
Introduction
Al loy 625 (58 min imum Ni-20-2 3 Cr, 8 -
10
M o , 3 .15 -4 .15 Nb+T a -5 m ax imum Fe -
0 .5 maximum M n, 0 .5 m aximum Si, 0 .10
maximum C wt-%) has been a commonly
used nickel-based al loy for over two de-
M.J. CIESLAK iswithSandia NationalLaborato
ries,
Albuquerque,
N.Mex.
Paper presented at the 68th Annual AWS
Meeting, held March 22-27,
1987,
in Chicago,
III.
cades. Although orig inal ly developed as a
turbine al loy (Ref. 1), i ts com bina tion of
good oxidation and corrosion resistance
and moderate mechanical strength have
made it a successful alloy in many other
applications. Among these are cladding
and surfacing for marine environments
(Refs.
2, 3) and for wear resistance as
hardfacing for tool and die steels (Ref. 4).
Al loy 625 is not without i ts problems,
though. Recent studies (Refs. 5, 6), have
indicated that th is al loy can be susceptib le
to hot cracking. Patterson and Milewski
(Ref. 5) no ted that ho t crac ked surfaces in
arc we lds made between A l loy 625 and
304L stainless steel w er e e nric hed in
S,
Nb ,
P and C, and that
eutectic-like
structures
were present in the microstructures of
these welds . Cieslak,
etal.
(Ref. 7), found
that dissimilar metal
CO2
laser beam w elds
between Inconel Al loy 625 and 304 stain
less steel made at slow travel speed (10
in./m in) con tained a Nb-ric h Laves phase.
A l though in compar ison to many o ther
nickel alloys (Refs. 8, 9), Alloy 625 has a
good reputation for resistance to hot
cracking, i t appears fro m the l i terature no t
to be to ta l ly immune f rom the prob lem.
A review of the l i terature reveals no
published study that correlates the sol id i
f ication microstructure with weldabil i ty in
Alloy 625 as a function of chemical com
posit ion. In addit ion, no published report
describes the sequences of sol id if ication
events leading to the development of the
K E Y W O R D S
Microstructure Deve lopment
Solid if ication Crack
GTA Al loy 625 Welds
Differentional Analysis
Thermal Analysis
Eutectic Constituent
7 /Laves Const i tuents
7 / M C Ca rb ide
7/M6 Carbide
Nb-Containing Alloys
observe d microstructure in A l loy 625, The
purpose o f th is work,
then,
i s two fo ld .
First, the welding metal lurgy of Al loy 625
is described in some de tai l . That is, the ev
o lu t ion o f we ld meta l microstructure upon
cooling from the l iquidus is explained.
Second, a correlation is establ ished be
tween al loy chemistry and both sol id if ica
t ion microstructure and weldabil i ty (hot-
cracking s usceptib i l ity). These results may
provide for intel l igent al loy optimization
schemes for Al loy 625 and similar materi
a ls from a weldabil i ty perspective.
Exper imental Procedure
The al loy design decision for th is ex per
iment was driven by the desire to estab
l ish the fundam ental s ol id if ication and ho t-
cracking mechanisms in th is al loy system.
The e f fect o f t ramp e lements (S,P, B, etc .)
on the sol id if ication and weldabil i ty be
havior of n ickel-based al loys has been w ell
establ ished. It was concluded that study
ing these e lements wou ld no t add much
new insigh t . They were e l iminated f rom
this study by being held at constant low
levels. Also, i t had been observed earl ier
(Ref. 10) that the solidificatio n m icros truc
ture in Alloy 625 and similar alloys could
conta in minor const i tuents composed o f
Laves phase and MC carbide. Both of
these phases we re N b-rich a nd Laves had
been shown (Ref .
11)
to be stabi l ized by
Si alloying additions. Based upon these
tw o factors, C, S i and Nb we re chosen as
the composit ion variables for th is experi
ment .
A three-factor, two-level factoria l series
of a l loys was designed around these ele
ments. The aim low level for C, 0.005 wt-
%, was effectively the l imit of industria l
processing capabil i ties. The high level for
C was set at what would l ikely be ex
pected in a high C commercial heat of Al
loy 625, 0.035 wt-%. The low level for Si
was set as none intentionally added. The
high level a im was 0.35 wt-%. The low
level for Nb was set as none intentionally
added. The high level a im was 3.5 wt-%.
The levels for Nb would clearly indicate
the d i f fe rence between Nb-bear ing and
WELDING RESEARCH SUPPLEMENT
149-s
-
8/11/2019 Inconel 625 Welding Metallurgy
2/8
Table 1 -
Element
C
M n
P
5
Si
Cr
M o
Ti
N b
Fe
Ni
-Al loy Composit ions
1
0.006
0.02
0.005
0.002
0.03
22.10
9.54
0.06
0.01
2.56
bal
2
0.031
0.02
0.005
0.002
0.03
21.95
9.61
0.06
0.01
2.55
bal
( w t - % )
3
0.006
0.02
0.005
0.002
0.35
21.63
9.60
0.06
0.02
2.18
bal
A l l oy Number
4
0.036
0.03
0.005
0.003
0.39
21.57
9.63
0.06
0.02
2.59
bal
5
0.009
0.03
0.006
0.003
0.03
21.81
9.81
0.06
3.61
2.30
bal
6
0.038
0.03
0.006
0.003
0.03
21.83
9.81
0.06
3.60
2.31
bal
7
0.008
0.03
0.006
0.004
0.38
21.65
9.68
0.06
3.57
2.26
bal
8
0.035
0.03
0.006
0.003
0.46
21.68
9.67
0.06
3.53
2.29
bal
Nb-free alloys. The alloys were double
vacuum melted at the Sandia Nat ional
Laborator ies melt ing and solidif icat ion fa
cility. Init ial melting was done in a vacuum
induct ion furnace from virgin raw mater i
als.
Electrodes weighing approximately
15 0Ib (68.2 kg) were poured in vacuum.
These electrodes were then vacuum arc
remelted to 6- in. (152-mm) diameter in
gots in preparat ion for hot working. Table
1 lists the compositions of the eight alloys
studied.
Hot working of the ingots began with
extrusion at 117 5C (2147F) do w n to a
3-in.
(76-mm) diameter bar. These bars
we re then f la t tened at
1175C
to app rox
imately 0.6-in. plates. From these plates,
specimens were taken for dif ferent ial ther
mal analysis (DTA). The plates were fur
ther reduced by hot rol l ing (1175C) to a
thickness of ap proxim ately 0.18 in. (4.6
mm).Further reduct ion was done at room
temperature to a thickness of approxi-
Alloy
S ( Mb
O N
HEATING
O N C O O L I N G
mately0.12 in. (3 mm). These sheets w ere
given a final anneal at 1010C (1850F)
and water quenched. A cold straightening
pass ( < 1 % co ld work) was then made to
prepare the sheets for machining into
Varestraint test specimens.
The autoge nous gas tungsten arc (GTA)
Varestraint test was used to quant ify the
suscep tibility of these alloys tofusionzone
hot cracking. The GTA welding parame
ters used were 100 A, (direct current,
electrode negat ive at a t ravel speed of 8
i n . /m in .
The machine voltage was ra 12 V
and argon was used as the shielding gas.
These condit ions produced welds that
were approx imate ly 0.20 in . wide at the
top surface. The test specimens measured
ra 6.5 X ra 1 X ra 0.12 in . (165 X 25 X
3 mm). All tests we re p erf orm ed at a strain
level of ra 2.5% to simulate high restraint
we lding c ondit ions. R eplicate test ing (4 to
5 tests per alloy) was employed to de
velop acceptable stat ist ics. The order of
test ing was randomized to eliminate sys
tematic error. Maximum crack length
(MCL) was the quant itat ive measure of
hot-cracking susceptibility used in this
study.
Differential thermal analysis (DTA) test-
A l l o y 6 ( N b . C )
O N H E A T I N G O N C O O L I N G
, 0 0 0 1 * 0 0
1 0 0
TEMPERATURE C)
Fig.
1-DTA
thermogram
(20 C/min)
for Alloy
5.
TEMPERATURE (C)
Fig.
2-DTA thermogram 20
C/min)
for Alloy
6.
ing was done on a Netsch thermal ana
lyzer STA 429. Samples were machined
f rom blocks taken f rom the hot -worked
plates that had been subsequent ly an
nealed in vacuum at1200C (2192F) for
4 h and water quenched. The samples
weighed ra 0.8 g. All tests were con
ducted in a hel ium environm ent w i th pure
W used as the reference mater ial. To
cal
ibrate the system, pure Ni was found to
melt wi th in 2C (3.6F) of the established
literature value. The experiments involve d
heat ing and cooling the Nb-bearing alloys
(Alloys 5-8) through the melt ing/solidif i
cat ion temperature range as fast as was
possible(20 C/m in 36F/min) wi th the
available equ ipme nt. The purpose of these
experiments was to ident ify any terminal
solidif icat ion react ions that were occur
ring in these alloys.
Varestraint test and DTA specimens
were examined metallographically. After
polishing through 0.05
-
8/11/2019 Inconel 625 Welding Metallurgy
3/8
Results
The DTA thermograms obta ined f rom
Alloys 5-8 are shown in Figs. 1-4. The al
loys containing C at the high level (6, 8)
show an event prior to matrix melt ing,
which is l ikely the dissolution of Nb car
bide. The high-tem peratu re annealing (4 h
at 1200C) was insuff icient to dissolve
these stable carbides. Figure 5 shows the
Nb carbides present in the microstructure
of Al loy 6 prior to DTA testing. The
solid
i f ication behavior of Al loys 5-8 can be
seen by examination of the individual
thermograms. A l loys 5 and 6 show two
exothermic events, while Al loys 7 and 8
show three exothermic events.
The results of Varestraint test experi
ments are shown in Fig. 6. As can be
readily seen, the hot-cracking susceptib i l
i ty under these test condit ions is greater
forthe Nb-b earing al loys (5-8) than for the
Nb-free al loys (1-4).
The microstructures observed in the
GTA weld metals could be easi ly d iscrim
inated on the basis of Nb content. Al loys
1-4 had microstructures that were essen
tially single phase and will not be further
discussed in th is paper. Al loys 5 -8 had mi
crostructures that contained minor inter
dendrit ic consti tuents in addit ion to the
dendrit ic m atrix. As a general observ ation,
Alloy 5 had the smallest minor consti tuent
population and Alloy 8 had the greatest
amount o f minor const i tuent . No a t tempt
was made to further quantify th is obser
vation in the weld metal. I t had been
shown earlier (Ref. 16) that the DTA sam
ples had bet we en 0.3% (Alloy 5) and 1.3%
(Alloy 7) by volume of minor consti tuent.
The SEM exam ination of Varestraint test
samples from Alloys 5-8 revealed that
these interdendrit ic consti tuents were as
socia ted wi th the fo rmat ion o f fusion
zone hot cracks. As a represe ntative ex
ample of these ob servation s, Fig. 7 (A, B)
shows the microstructure associa ted wi th
a fusion zone hot crack in Al loy 7.
Identif ication of these phases was ac
compl ished by perfo rming se lected-area
electron diffraction on th in fo i ls and ex
t ract ion rep l icas made f rom the Vare
straint test samples. The primary micro-
consti tuents ob ser ve d wer e a Laves phase
(hexagonal, a ra 0.476 nm , c ra 0.713 nm)
and MC carbide (cubic, a ra 0.441 nm).
The observations made in examination of
the th in fo i ls we re cons istent with t he SEM
examination of the Varestraint test sam
ples relative to minor consti tuent popula
t ion. That is, Al loy 5 had a much lower
vo lume f ract ionofm inor const i tuent than
did Alloys 6- 8. In addit ion t o the Laves and
MC carb ide phases observed, A l loy 7
(high Nb, Si) was found to contain a
coarsely lamellar M
6
C carb ide (d iamond
cubic, a=a 1.12 nm) constituent on its ex
t ract ion rep l ica . Tw o morpho log ies o f MC
wer e observed, a dendr i t ic o r Ch inese-
scr ip t morp ho logy and a smaller b locky
O N HE AT I NG
Al l o y 7 No.SO
A l l o y a Nb .C.S i )
O N C O O L I N G
T E M P E R A T U R E ( C )
Fig. 3
DTA
thermogram 20
C/min)
for Alloy
7.
morpho logy .
It was qualitatively observed (Ref. 16)
tha t the predominant microconst i tuent in
Alloys 5 and 7 was Laves, whereas for Al
loys 6 and 8, it was M C carbide. In the case
of Al loy 8, large quantit ies of both MC
carbide and Laves we re o bse rved . Figures
8A and 8B are representative TEM micro
graphs from Varestraint specimens show
ing M C carbide (A lloy 6) and Laves phase
(Alloy
7),
respectively. Table 2 summarizes
the phases identif ied during the TEM
anal
yses.
Two types o f e lementa l segregat ion
were observed in the GTA weld meta ls.
The f irst was the discontinuous composi
t ion changes associated with the micro-
const i tuents observed. The second was
the periodic pattern of dendrit ic segrega
tion associated with the sol id if ication pro
cess.
Using AEM techniques, as described
above, the composi t ions o f the var ious
phases in the weld metals of Al loys 5-8
we re determ ined. These are l is ted in Ta
ble 3. In the cases of the carbide phases,
a composi t ion w as ca lcu la ted based upon
ideal stoichiometry (MC or M
6
C). As can
TEMPERATURE C)
Fig. 4DTA thermogram 20 C/min) for Alloy
be seen, al l of the microconsti tuents are
enriched in Nb and depleted in Ni re lative
to the nominal composit ions.
The periodic dendrit ic segregation pat
tern representative of the GTA welds
made on the Nb-b earing al loys is shown in
Fig. 9 (Al loy 7). What is observed is the
enrichment of Ni and Fe in dendrite core
(DC) regions and the segregation of Nb,
M o a nd Si to inte rdendr it ic (ID) regions.
There is very l i t tle dendrit ic segregation o f
Cr
observed in th is or any of the other al
loys. Car bon co uld not be analyzed by th is
techn ique because o f i ts low concentra
t ion. The interdendrit ic consti tuents asso
cia ted wi th ho t cracks in A lloys 5-8 wer e
also chemically analyzed with the electro n
microprobe. The results of these analyses
are given in Table 4. All constituents are
enr iched in bo t h Nb and M o re la t ive to the
nominal a l loy composit ions and the
c o n
sti tuents obse rved in the high-Si heats (7
8) are enriched in Si.
Table 2Phases Observed in the TEM
Al loy No.
5 (Nb)
6 (Nb, C)
7 (Nb, Si)
8 (Nb, C, Si)
Thin Foil
Laves
(a)
Small MC (NbC) carbides at
7/Laves interface
Dendri t ic MC (NbC)
(a)
Laves
,a)
Small MC (NbC) carbides at
7/Laves interface
Dendri t ic MC (NbC)
(a)
Laves in vicinity of M C (N bC)
Small MC (NbC) carbides at
7/Laves interface
Extraction Replica
Dendr i t i c MC
(NbC)
(a
>
Blocky MC
Laves
Dendrit ic MC (NbC)
Blocky
MCW
Thin Foi l /
Extract ion
T.F.
Extr.
Extr.
Extr.
T.F.
Extr.
Extr.
Extr.
T.F.
Extr.
Extr.
from AEM Analysis
C
12.1
11.6
11.9
12.4
2.8
12.3
12.2
12.2
Ni
45.6
(0.2)
3.3
(0.1)
0.1
(0.1)
2.1
(0.1)
48.2
(0.2)
2.5
(0.1)
31.6
(0.2)
3.5
(0.1)
46.7
(0.2)
4.5
(0.1)
4.1
(0.1)
Fe
1.4
(0.1)
0.3
(0.1)
0.0
0.1
(0.1)
1.0
(0.1)
0.3
(0.1)
0.5
(0.1)
0.0
0.9
(0.1)
0.4
(0.1)
0.4
(0.1)
C r
15.6
(0.6)
4.1
(0.2)
1.3
(0.1)
4.1
(0.2)
13.9
(0.5)
8.6
(0.4)
14.6
(0.6)
7.4
(0.4)
13.6
(0.5)
5.2
(0.2)
5.1
(0.3)
N b
19.2
(0.3)
73.4
(1.2)
82.6
(1.1)
65.5
(0.8)
18.2
(0.3)
69.7
(1.1)
31.0
(0.5)
68.5
(1.1)
16.8
(0.2)
60.0
(0.8)
65.0
(1.2)
M o
18.2
(0.3)
7.2
(0.3)
4.4
(0.1)
16.3
(0.2)
17.6
(0.2)
6.5
(0.2)
18.7
(0.3)
8.2
(0.3)
19.8
(0.3)
17.7
(0.3)
13.2
(0.4)
Si
< 0 . 1
0.0
0.0
0.0
1.2
(0.1)
0.0
0.9
(0.1)
0.0
2.2
(0.1)
0.0
0.0
(a) Absolute error, wt-%.
(b) Calculated assuming ideal MC stoichiometry.
(c) Calculated assuming idealMeCstoichiometry.
Table 4 -
Alloy
N o
5
6
7
8
-Compositions of Constituents Associated with Hot Cracks
N b
17.45