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

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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

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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