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Effect of Welding Variables on Cracking inCobal t -Based SMA
Hardfacing Deposi ts
Investigation focuses on establishing guidelines for
depositingcrack-free hardfacing weld metal
B Y R. V. S H A R PL E S A N D T. G . G O O C H
n t r o d u c t i o n
Weld deposit ion of hardfacing al loyss co m mo nly em ploy ed
to increase theerv ice l i fe of components subjec t tobras ive
wear. A number of a l loys a re
ommerc ia l ly ava i lab le , based la rge lyn i r o n , n icke
l or cobal t mat r ices , andffering various properties in the
depositRefs. 1, 2). In general, greater life is oba ined for many
appl ica t ions by us ingeposits of higher hardness, this
beingbtained via the presence of hard secnd-phase part icles,
especially ca rbides,n the matrix. However, at high hardnessevels,
the tensi le ducti l i ty of the hard-acing is reduced and crack
ing can oc curs a result of we ldin g co ntrac tion strain.uch
cracking does not necessari ly s ig
nificantly reduce the service wear life ofhe com pon ent, and
indeed is sometimeseen as an advantage in reducing resid
ual stress levels (rel ief checking).Nonetheless, in many
instances, crackng i s undes i rab le , whether to obta in aealing
surface or to avoid fat igue f a i l
ure, for example, and a requirement exs ts for depos i t ion of
c rack-f ree , h igh-
hardness surfacing.Cracking can arise ei ther in the solid
tate because of low tensile ducti l i ty orur ing so l id i f
ica t ion . The la t te r mecha
nism of c racking can normal ly be overome by reducing travel
speed, with aten t ion to a rc ex t inc t ion procedure tovoid c ra
te r c racking , bu t the formerrack type represents a rather more
inrac tab le problem in hardfac ing a l loys .
Essentially, the incidence of cracking cane related to the tensi
le ducti l i ty of the
depos i t , and hence , to it s com pos i t ionnd hardness . Whi
le c racking can bevoided by se lec t ion of an a l te rna t
ive
R. V. SHARPLES and T G. GOOCH are withThe W elding Institute,
Abington Hall, Abingon, Cambridge, U.K.
Paper presented at the 72nd Annual AWSMeeting, held April 14-19,
1991, in Detroit,Mich.
consumable composi t ion , th i s wil l genera l ly invo lve a
redu ct ion in depos i thardness , wh ich m ay be unacc eptab lein
te rms of serv ice proper t ies . Wheresuch a mate rial change is
inap plica ble ,the most common preventat ive measure
is to apply preheat (Refs. 1, 2) , on thebasis that the cooling
rate after weldingcan be reduced wi th a concom itant reduc t ion
in the d i ffe ren t ia l co nt ra c t ionstrain between the clad
ding and the substrate. Howeve r, f ew quan t i t a t i ve da t
ahave been, published on the effects ofweld ing condi t ions on the
c racking (orserv ice) behavior of weld-depos i tedha rdf ac ing
(Ref. 3); thus , unless pr iorpractical experience exists ,
substantialp roce du re deve lopmen t i s commo n lynecessary to
achieve crack-free hardfacing.
The present program was ini t iated toexamine the e ffec ts of
vary ing weld ingcondit ions on the cracking sensit ivi ty
ofweld-dep os i ted hardfac ing . A par t icu larobjec t ive was to
explore the feas ib i l i tyof producing nomograms afford ing
general guidance on welding procedures toavoid cracking with
different hardfacingal loys so tha t proce dura l d eve lopm enttr
ials can be minimized. Shielded metalarc (SMA) welding was chosen
to makedepos i t s on s tee l us ing consumablesmeet ing AWS A5.1 3
ECoCr-B specif icat i o n , an exc e l len t hardfac ing a l loy
but
K E Y W O R D S
ECoCr-BHardfac ing A l loysWelding Var iab lesCrackingTensile
Ducti l i tyNomogramsS M AWCo-Based Consumable
Wear ResistanceAbrasive Wear
one known for i ts tendency to crack onc o o l i n g . This
combina t ion of processand material is wi de ly used in hardfacing
applications, especially on site.
A p p r o a c h
In essence, the risk of cracking in thehard fac ing is govern ed
by the tens i leduct i l i ty of the depos i t and by the applied
shrinkage strain (Refs. 2, 3) . Theforme r is depe nde nt on the
mater ia lcomposit ion and microstructure, and thela t te r on
composi t ion and weld ing c o ndit ions, especial ly, from
practical exper ience, on the preheat level (Ref. 3). Testswere
therefore car r ied out vary ing depos i t d i lu t ion and prehea
t tempera ture ,changes i n d i l u t i on a nd c ompos i t i
onbeing achieved by a l te r ing the weld ingcurrent with single-
and double-layer deposit ion.
Test welds were deposited circumferential onto a steel bar of 1
00- mm (4-in.)d i a me te r and 30 0 -mm (1 2-in.) len gth .This
geometry was selected as constituting a semi-infinite heat sink of
fairly highrestraint . Prel iminary tests showed thatd i ffe rent
ia t ion could be made be tweenthe incidence of cracking in the
ECoCr-B deposits produ ced w ith varyin g w e l ding current .
Ex pe r i men t a l P rocedu r e
Materials
The hardfacing material used was inthe form of SMA elec t rodes
of 4-mm(0.1 6-in.) diameter, obtaine d to the AW SA5.13 ECoCrB spec
i f ica t ion . The substrate bar ma terial was 0.4 % ca rbon
steel080 A42 (Ref. 4) (Table 1), as representa t ive of components
for which suchhardfac ing might be em ployed in practice.
Deposition and Welding Conditions
Depos i t s were made manual ly bywelding in the f lat posit ion
onto the bar
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Ta b l e 1Mater ia l Analys is
M a t e r i a l Fe C S
ECoCr-B< a > 2 . 38 1 .70 - ( c >080A42< b > Ba l
0 . 41 0 . 021
P
(c)
0 . 0 1 2
E lemen t (w t - )
Si M n N i
1.02 -< c > 2 . 4 70 . 1 9 0 . 8 2 0 .2 2
Cr
31 . 50 . 2 3
M o
0 . 2 00 . 0 7
Cu
(c)
0 . 2 4
Co
Bal0 .0 2
W
8 . 7 0(c)
(a) Manufacturer's analysis.(b) TWI Ref. No. S/85/205.(c) Not
determined.
Table 2 Summary of Effects of WeldingConditions on Incidence of
Cracking in theDeposited Hardfacing
P r e h e a t C u r r e n t
Q
20
40
70
100
150
200
250
100250300200
(a) Cracked
(A)
100120150100150120150100120150100120150100150120150100100160100
(b) Not cracked.(c) No t d ete rmrned.
NominalHeatInput
(k ) /mm
11.21.611.61.21.611.21.611.21.611.61.21.6111.81
1st
C ,
N CN CCN CNCN CNC,N CN CNCN CNCNCN CN CN CC(c)
NC_ ( c )
C r a c k i n g
L a y e r 2 n d t a y e r
NC< b>
C
C, CCC
ccccc, ccccccC N C
cNCNC (c)
C(c)
NC
rotat ing at a surface {i.e., t ravel) speedof 130 mm/min (5 in.
/min). Single-layerdepos i t s cons is ted of three beads wi tha p
p r o x i m a t e l y 2 5 % ove r l a p be tweene a c h , and for
dou ble- la yer sa mples , afurther two beads were dep osited on
theoriginal three beads.
Welds were made a t prehea ts ranging fro m 20 to 30 0C (68 to
572 F) fo rcurrents between 100 and 160 A at voltages between 21
and 24 V, as summarized in Table 2. W eld ing was p erformedusing
DC, electrode posit ive condit ions.In most cases, the same nominal
conditions were used for both layers. The preheat was applied by
placing the test barin a furnace at the required temperature,and i
t was m ain tain ed as an interpasstempera ture for depos i t ion
of ad jacentbeads . On comple t i on o f we l d ing o feach layer,
the tes t p iece was a l lowedto cool in air to room tem perature.
Sometests under specif ic condit ions were repeated to clarify
behavior as necessary.
Thermocouples were harpooned in tothe 3rd and 5th bead weld
pools duringdepos i t ion , and the cool ing cyc le downto about 1
00C (21 2F) wa s r ec o rded .Co olin g rates from 80 0 to 500C
(1472to 932F) we re de t e rmined . Th i s c o o ling parameter was
taken as corresponding to a temperature range close to thata t wh
ich c r a c k ing comm enc e s d u r ingcooling (Ref. 2), and
because data exist(Refs. 5, 6) to pred ict the effect of cha nging
weld ing condi t ions , jo in t hea t s ink ,etc. , on the deposit
cooling cycle.
Examination
Cracks were detected both aurally asthey occurred during ini t
ial cooling andby dye penetrant testing carried out afterdepos i t
ion of the 3rd bead and aga inafter the f inal run. Sections were
takenf rom depos i t s , moun te d , g round andpolished to a 1 pm
finish. They were examined under an opt ica l microscope to
assess de pos it mic ros truc ture and as afurther check on the
incidence of cracking.
To de termine the ex tent of d i lu t ion ,the amount of iron
present in the depositwas measured by using energy dispersive x-ray
analysis in conjunction with ascanning electron microscope.
Analyseswere taken in each of beads 3 and 5.
Hardness measurements on t ransverse sections were made using a
Vickers pyra mida l d iam ond indentor unde ra load of 5 kg.
Results
Material Microstructure andCracking Behavior
Figures 1 and 2 show represen tat ivedeposits and cracking. Both
the first andsecond layers showed pr imary so l id i f icat ion to
the metall ic a-phase, with subsequent formation of interdendri t
ic carbides, as in Fig. 3. The carbide contentin the second layer
was, however, verymuch higher than in the first beads.
The cracking observed was v i r tua l lyall t ransverse to the
weld bead, roughlyperpendicular to the substrate, and hadformed
apparent ly randomly a long thedepo s i t (F igs . 1 , 4 ) . The
cra cking occurred wi th negl ig ib le p las t ic s t ra in inthe
matrix, and developed along the carb ide phase . No so l id i f ica
t ion crackingwas observed.
In addi t ion to c racking in the depos i ted bead , fus ion l
ine and hea t -a ffec ted zone (HAZ) , hydrogen cracking
Fig. 1 View of test weld: 1 kj/mm and 10CPC preheat.
Fig. 2 Transverse section through test weld: 1.2 kj/mm and
150Cpreheat.
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ig. 3 Microstructure of test weld in Fig. 1, 320X. A Third run;
B fifth run.
a s o b s e r v e d . F i g u re 5 s h o w s c r a c k i n gn i
t i a t e d at t h e d e p o s i t t o e i n a s a m p l er o d u c
e d a t 1 k j / m m (25 k j / i n . ) a nd 40C
104F) p r e h e a t . T h e H A Z c r a c k s w e r ef t h e t o
e a n d u n d e r b e a d t y p e l y i n g
o u g h l y p a r a l l e l t o t h e w e l d i n g d i r e co n
, t h e f o r m e r s o m e t i m e s b e i n g a s s o
i a t e d w i t h c r a c k i n g i n t h e c l a d d i n g m ae
r i a l . C r a c k i n g a p pe a r e d p r e d o m i n a n t l yn
t e r g r a n u l a r , a n d w a s l a r g e l y c o n f i n e do
t h e t r a n s f o r m e d H A Z . T h e H A Z m ir o s t r u c t
u r e p r o d u c e d b y t h e f i rs t l a y e ra r i e d f r o m
f u l l y m a r t e n s i t i c i n t h e d eo s i t s w i t h th e
m o s t r a p i d c o o l i n g t o
m i x e d h i g h e r t e m p e r a t u r e t r a n s f o r m ao
n p r o d u c t s at t h e l o n g e r c o o l i n g t i m e s .h e
H A Z c r a c k s w e r e o b s e r v e d o n l y i n
h e m a r t e n s i t i c m i c r o s t r u c t u r e s .
ffect of Welding Conditions
A s u m m a r y o f t h e e f f e c t s o f c u r r e n tn d p r
e h e a t o n d e p o s i t c r a c k i n g i s g i v e nn Ta b l e
2 . F o r b o t h l a y e r s , c r a c k i n g i nh e h a r d f a
c i n g w a s r e d u c e d b y i n c r e a sn g c u r r e n t o r
p r e h e a t t e m p e r a t u r e , a n dr a c k i n g o c c u r
r e d m a i n l y i n t h e s e c o n da ye r, r equ i r i n g a
subs t an t i a l i nc r ea s e i nr e h e a t l e v e l f o r i ts
a v o i d a n c e . A s i l l u s
r a t e d i n F i g . 6 , c o o l i n g t i m e s f r o m 8 0 0
o 500C (At a _ 5) i n c r e a s e d a t h i g h e r p r e
h e a t t e m p e r a t u r e s , e s p e c i a l l y w i t h p
r eh e a t t o a b o v e 2 0 0 C ( 3 9 2 F ) . N o p a r t i cu l a
r d i f f i c u l t i e s w e r e e x p e r i e n c e d w i t hs l
a g r e m o v a l , a n d t h e re w e r e n o i n d i c at i o n s
t h a t r e s i d u a l s l a g c o n t r i b u t e d t o
c r a c k i n g i n t h e s e c o n d l a y e r.T h e H A Z c r
a c k i n g o c c u r r e d o n l y i n
d e p o s i t s p r o d u c e d w i t h p r e h e a t l e v e l
su p t o 1 0 0 C . R e s u l t s o f h a r d n e s s m e as u r e m
e n t s o n t y p i c a l c r a c k e d a n d u nc r a c k e d H A
Z s a r e g i v e n i n Ta b l e 3 . T h eh i g h e s t h a r d n e
s s w a s f o u n d w i t h 2 0 Cp r e h e a t , b u t , e s p e c
i a l l y i n t h e o t h e rw e l d s , i t m u s t b e p r e s u
m e d t h a t s o m et e m p e r i n g a n d s o f t e n i n g f r
o m t h e as -w e l d e d h a r d n e s s h a d o c c u r r e d d u
r i n gs u b s e q u e n t p r e h e a t i n g a n d d e p o s i t
i o no f t he s ec ond l aye r.
Dilution
D i l u t i o n a s a s s es s e d b y t h e i r o n c o nt e n
t o f t h e d e p o s i t i n c r e a s e d w i t h i nc r e a s i
n g c u r r e n t a n d , t o a le s s e r e x t e n t ,p r e h e a
t F i g . 7 . T h e d e p o s i t h a r d n e s sm e a s u r e m e
n t s a r e p l o t t e d a g a i n s t d i l ut i o n i n F ig . 8
. Ha rdn es s f e l l a t h ig he r d il u t i o n l e v e l s . N
o e v i d e n c e o f d e p o s i tc r a c k i n g a r o u n d h a
r d ne s s i n d e n t a t i o n sw a s s e e n .
D i s c u s s i o n
Effect o f Welding Conditions
C r a c k i n g o c c u r s i n a d e p o s i t as a re
su l t o f t he s t r a i n s s e t up n o t on ly by u ne q u a
l c o o l i n g r a te s w i t h i n t h e d e p o s i t ,b u t a l
s o b y a n y e x p a n s i o n m i s m a t c h b et w e e n d e p
o s i t a n d s u b s t r a t e . In t h e c a s eo f h a r d f a c
i n g , t h e p r o b l e m i s e x a c e rb a t e d b y t h e h i
g h m a t e r i a l s t r e n g t h o v e ra r a n g e o f t e m p
e r a t u r e s t h a t r es i s ts a cc o m m o d a t i o n o f s
h r i n k a g e s tr a i n . I t f o ll o w s t h a t a n y d e g r
e e o f d i l u t i o n o f aE C o C r - B o r s i m i l a r a l l
o y b y a s t e e l s u b -
Table 3Representative Results of HAZHardness Measurements
Nominal MaximumHeat HAZ
Preheat Cu rrent Input HA Z Hardness(C) (A) (kj/mm) Cracking
HV5
20 100 1100 100 1150 100 1300 160 1.8
C 473C 349N C 289NC 317
(a) Vickers hardness wi th 5 kg indent ing load.(b) C ra ck ed
.(c) Not cracked.
ig. 4 Typical deposit cracking, 100X. Fig. 5 - HAZ cracking from
deposit toe, 50X.
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25
20 -
S 15
1 h
cncoo 5 =
1
D = 1.6kJ/mm
D aa
^n Q a ^ . ^B .
9
1
^- - _ ' ,,
i
^ ^
i
/ // /
a S /' //
y Y
^
i
100 200P r e h e a t t e m p e r a t u r e , C
300
40
30
20
10
n
i iP r e h e a t = 20Co = 100*Ca = 250T
// D
//
y
^ - ^ s
i i
i
/y
First -l a y e r ,
yy
Second jjLayer , '
i
100 120 140C u r r e n t , A
160
Fig. 6 Effect of preheat temperature on deposit cooling times,
800-500C, for 1 Fig. 7 Effect of current on dilution, measu red as
ironand 1.6 kj/mm, with bounding lines. content of the deposit.
0
Fig. 8 -
20Iron c o n t e n t , %
Effect of deposit dilution, measu red as iron content, on
hardness.
25 -
20 -
n
cu
eC
oo
15
10 -
5 -
1
- o = F i r s t
1
l aye rQ = Second lay erSol id symb
D
,
**~ __ n __
V. #
i
ol s = c rac k ing
*1\
.4 *0 0: \ o
\\
0
o
8 o
o
o
-
-
10 20 30% iron in deposi t
40
Fig. 9 Effect of cooling time from 800 - 500 C ondeposit
cracking.
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ra te w i l l be par t ic u lar ly imp or tan t invoiding
cracking because dilut ion botheduces composi t iona l (and to a la
rgextent expans ion) mismatch and a lsoakes a softe r and more a
ccom mo dat
ng depos i t by decreas ing the carb ideontent . In pr inc ip le
, the overa l l s i tuaon is made more complex when a secnd layer
of deposit is added becauseomposi t iona l d i ffe rences be tween
the
ew layer and its substrate be com emuch less, but so does the
degree of acom mod at ion requi red . In addi t ion , misatch
strains generated in the first layeri l l , to some extent , be t
ransmi t ted to
he second layer, but the concept of to lrance to shrinkage
strains being deter
mined by dilut ion should remain val id.I t was further argued
above that the
ffective applied strain and hence crackng risk is influenced by
preheat levelnd subsequent cool ing ra te . Cer ta in lyhis is
indicated by practical experiencei th the proviso that a genera l
prehe at
applied to the substrate so that severemperature gradients do
not exist.On this basis , the present data were
lotted as in Fig. 9, considering dilu t ionnd cool ing condi t
ions . I t can be seenhat a bounding curve can be identif ied,i v
id ing c r a c k e d a n d unc rac ke d d eos i t s . There would
seem to be a d isnc t ion be tween the f i r s t and second
ayer results, as discussed further be low ,ut it is considered
that a diagram of thisype could well form the basis for aomogram sy
s t em t o de f i ne we ld ingondit ions giving crack-free deposits
.
Such a no m ogr am s hou ld b e ap p l iab le to hardfac ing sys
tems genera l ly,nd not only to the ECoCr-B consumbles ut i l ized
in the present work. Howver, the practical usage of data in theorm
of Fig. 9 wil l require con siderat ionf a number of aspects of
behavior. Inhe f irst instance, the control of di lut ion par t icu
lar ly impo r tan t in hardfa c ing
s it markedly affects the wear and corosion resistance of the
deposit (Refs. 3, 7). Low levels of iron dilution may be
cc ep table (as is the case w ith ECoCr-), but in general , the
greater the per
entage of iron in a hardfacing layer, theoorer i ts proper t ies
wi l l be . The t enency for increased dilution to decreasehe l ike
l ihood of c racking may lead tohe use of we ld i ng cond i t ions
caus ingigher d i lu t ions than are compat ib lei th the serv ice
condi t ions of the comonen t , wh en m a x i m u m p e r fo rm a
nc e
will not be given by the hardfacing deosit . Hence , wh en e mp
loy ing Fig. 9, aec ision must be taken as to the perm isible di
lut ion level . If , for example, deosit hardness can be taken as a
usefuluide to the resistance of a surface unergoing low- load abras
ive wear (Ref .), data as in Fig. 8 may be applicable,ut will need
to be generated for partic-
300 200 100P r e h e a t t e m p e r a t u r e , C
10 20 30 40% iron in deposit
Fig. 10 Proposed nomogram for derivation of SMA welding
conditions giving a specified
iron content in the deposit, with no brittle hardfacing
cracking.
ular hardfacing/substrate combina t ions .It will be necessary
also to define the
c oo l i n g r a te a nd d i l u t i on ex pe r i en cedin any
par t icu lar hardfac ing opera t ion .Some guidance on the lat ter
is given inFig. 7 , bu t c lear ly more informat ion i sneeded. In
regard to coo ling rate, a number of nomograms exist (Refs. 5, 6)
forp r ed i c t i on o f A t8 . 5 . For infini te heatsinks, and
using the bounding l ines fromFigs. 6 and 9, i t is possible to
develo p anomogram cons t ruc t ion as in F ig . 10,
whereby the cool ing ra te necessary toavoid c rack ing a t the
requi red d i lu t ionis directly related to we ldin g co ndit ions
.
At present, the nomogram must be regarded as on ly a tentat ive
pro posal , andsubs tantially mo re testing is essential todef ine
i t s prac t ica l v iab i l i ty. In dee d , anumber of
reservations must be expressed. First , Fig. 9 was obtained for
aspec i f ic mater ia l /weld ing process combina t ion and for
only one consumableb a t c h ; whereas, other hardfacing al
loysmust be expec ted to show a d i ffe rentcracking response with
changes in d i l ut i o n , depending on the composi t ion inv o l
v e d . Further, the use of dilution as aninput presupposes tha t
process andwe l d ing c ond i t i ons i n f l uence c r ack ingo n
ly by de t e rmin ing t he de p th o f subs t ra te penet ra t ion
. For prac t ica l purposes, this may be true but appropriatestudy
of such welding variables is requi red . Moreover, it is assumed
that thecracking test piece geometry used is relevant to service.
Even if this is the case,Fig. 10 is limited to substrate
geometrieswh ich ac t as a sem i- inf in i te hea t s inkand a l
low no accommodat ion of res idual stresses by distortion. In this
last respect , the nom ogram wi ll err on the c o nservative
side.
Hardfacing Cracking Behavior
The very high strength and low ducti l i ty of the deposit from
ECoCr-B electrodes, and i ts great sensit ivi ty to d i l ut ion ,
dominate the behavior of a depositalmost to the exclusion of other
factorsand are respons ib le for the somewhatdifferent response of
the f irst and secon d layers. It was diff icu lt to generatecracks
in the first layer, but the c rack/no-crack boundary appears to
approach a
near-ver t ica l l ine . This impl ies tha t d ilu t ion was the
main fac tor inf luencingcrack format ion in the f i r s t l ayer,
p resum ably via its effect on the carbid e cont en t , and tha t
about 20% was the c r i t ical leve l , irrespective of coolin g
rate insofar as this was an inde pen de nt v a r ia b l e . Thus,
from Fig. 8, any f irst layerdeposit with a hardness below say
450HV would be expected to be crack-freea lmost wi thout regard to
i t s condi t ionsof deposit ion, noting that only one c o nsumable
was tested and given that thepresent level of restraint was
sufficient
to represent a wor st case. In prin cip le,depos i t c r ack in
g cou ld b e i n f l uencedalso by the volume expans ion assoc ia
ted wi th t ransform at ion to m ar tens i tein the subs t ra te .
However, s ince c o o ling rate had l i t t le effect on f irst
layercracking (Fig. 9) , such an effect is unlikely to be of
particular significance.
On the other hand, di lut io n of the second layer was restr
icted to low levels.H e r e , t he c r ack /no - c r ac k bounda
rywould appear to be near ly hor izonta l ,that is , cooling rate
was the controll ingfactor with dilut ion having much less effect .
I t is dif f icu lt or im pos sible toachieve such low di lu t ion
in a s inglelayer with the SMA process, but this is
W E L D I N G R E SE A RC H S U P P L E M E N T I 199-s
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QJC L
QJc_
Q _
200
150
100
50
i I
Increasing parent metalhardenabil i ty or weldmeta l hydrogen
leve l '
v /
\ '\ /
//
/(b) / 1
/i i
l
N7
a) /
i
1 1oo
/
/ Reducing parent metalhardenabil i ty or weldmetal hydrogen
level
M
0 0 1 0.5.2 0.3 0.4Carbon conten t , %
Fig. 11 Guide to preheat temperatures using austenitic SMA
electrodes at about 1 to 2 kj/mmto avoid HAZ hydrogen cracking when
welding ferritic steels Ref. 10 . A tovv restraint; B high
restraint. The present results are indicated as: = not cracked, =
cracked.
readi ly obta ined wi th a l te rna t ive deposit ion methods,
for example, by plasmatransferred arc surfacing (Ref. 8), and as
tudy of low-di lu t ion f i r s t l ayers produced by other
processes is necessaryto indicate the general applicabil i ty
(orotherwise) of Figs. 9 and 10.
I t should also be noted that the present inves t iga t ion has
conce nt ra ted onbr i t t le , so l id-s ta te c racking . Sol id
i f icat ion cracking was not found to be a part icular problem,
but may require furtherat tention in ECoCr-B deposits at veryhigh d
i lu t ion leve ls or in a l te rna t ivehardfacing materials .
Heat-Affected Zone Cracking
The loca t ion and morphology of theHAZ cracking observed indica
te th i ss tems f r om h yd ro g e n em br i t t l emen t .Clear
ly, th i s problem must be recognized in any scheme in tended to g
iveguidance on weld ing procedures for production of crack-free
hardfaced compo
nents. Vi r tua l ly no da ta ex is t on w eldmeta l hydrogen
leve ls assoc ia ted wi thcobalt-based consumables, but in termsof
hydrogen solu bil i ty and diffusion rate,a c lose para l le l can
be dra wn wi thaustenitic stainless steel and nickel alloyelectro
des (Ref. 9). The risk of HA Z h ydrogen cracking us ing s ta in
less s tee lconsumables was d iscussed by Gooch(Ref. 10) in terms
of base metal transforma t ion beh av io r a nd consumab le
hydrogen level , and i t is considered probable that the guidelines
proposed wouldbe generally applicable to cobalt-basedSMA electrodes
as presently employed.
The maximum HAZ hardness in Table3 of over 470 HV is high enough
to en
gender s igni f icant sens i t iv i ty to hydrogen cracking. In
plain carbon steels suchas 080 A4 2, some cont ro l over H AZ m
icros t ruc ture and hardness can beachieved by vary ing we ld ing
condi t ions .However, in SMA hardfac ing , the hea tinput emp
loyed may be restr icted by theneed for pos i t iona l weld ing .
In suchcases, rel iance must be placed on theuse of prehea t ,
espec ia l ly to a l low hy
drogen diffusion away from the depositwh ile the material is at
sufficien tly hightemperature for hydrogen embrit t lementto be
negligible. In this regard, referencecan be made to Fig. 1 1 , d e
r i v ed f r omwelds in t ransformable fe r r i t ic s tee lsm ad e
u s ing au s t en i t i c SMA consumables. The present HAZ cracking
resultsare shown, and i t can be seen that theobserved beh avior is
pred icted we ll bythe diagram.
S umm ary an d Conc lu s ions
Study has been carried o ut on the sensi t ivi ty of weld
deposited hardfacing tocracking stemming from low tensi le ducti l
i ty of the deposit . Cobalt-based al loyto AWS A5.1 3 Grade
ECoCr-B was depos i ted o nto a 0 .4% C s tee l by theshie lded m
eta l a rc process , wi th v arying current and preheat levels .
The fo llowing conclusions were reached.
1) Cracking in the hardfacing was reduced by increasing current
and preheattempera ture for both s ingle- and two-layer
deposits.
2) Sensit ivi ty to deposit cracking wassubstantial ly higher in
the second layerthan the f irst , as a result of lower d i l ution
and higher deposit hardness.
3 ) B ound ing cond i t i o ns fo r depos i tc racking were def
ined in te rms of depos i t d i lu t io n and coo l ing ra te . The
approa ch is proposed as a basis for a no m ogram system to predict
the risk of cracking in d i ffe rent hardfacing/substratecombinat
ions .
4) For the par t icu lar e lec t rodes andsubstrate steel s
tudied, cracking of f irstlayers was avoided by selection of w e l
ding condi t ions g iv ing over 20% d i lu t ion :prevent ion of c
racking in the secondlayer required a we ldin g proced ure suchthat
a co olin g t ime from 800 to 500Cabove 20 s was obtain ed.
5) Heat -affec ted zone hydrogencracking was observed . I t i s
p robabletha t ex is t ing guide l ines for the avoidance of such
cracking us ing aus ten i t icstainless steel electrodes are
applicablealso to cobalt-based consumables.
AcknowledgmentsThe authors thank their colleagues at
The W el d in g Inst i tute for assistance in
the course of the pro gram . Part icular a ckno wled gm ent is
made to D. N. N oblefor in i t ia t ing the pro jec t , to Dr. I. A
.Bucklow for advice, to C. S. Hunt for directing the welding, and
to N.) . Tebbit ,G. H . D ixo n and J. E. Clark for carry ingou t t
he ex pe r i men t a l w o rk . The workwas jointly funded by
research membersof The Welding Inst i tute and the Minerals and
Metals Division of the U.K. Department of Trade and Industry.
References
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coatings and materials se lection. Met C on 12(12): 685-6 90.
2. Mathew, M. D., Mannan, S. L, andGupta, S. K. 1980. Influence
of preheat temperature on stellite deposits. Welding journa l
59(7): 213-s to 216-s.
3. Noble, D. N. 1 985. Abrasive wear resistance of hardfacing we
ld d eposits. Met Con17(9): 605-611.
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Bragard, A. 1975.
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2 0 0 - s I M A Y 1 99 2
