Weldability-Ferritic Stainless Steels
-
Upload
alislamdeeni -
Category
Documents
-
view
245 -
download
3
Transcript of Weldability-Ferritic Stainless Steels
7/21/2019 Weldability-Ferritic Stainless Steels
http://slidepdf.com/reader/full/weldability-ferritic-stainless-steels 1/8
WELDING RESEARCH
S UP P L E M E NT T O T HE W E L DI NG J OURNA L , A UGUS T , 1 9 8 1
S p o n s o r e d b y th e A m e r i c a n W e l d i n g S o c i e t y a n d t h e W e l d i n g R e s e a r c h C o u n c i l I I B I ,
Weldabil i ty of Ferr i t ic Stainless Steels
Investigation shows that it is possible to provide (1) adequate resistance
to hot cracking during welding and (2) usable mech anical properties
and resistance to intergranular cracking in the as-welded condition
by mean s of stabilization
BY D. H. KAH AN D D. W. D ICK INS ON
ABSTRACT. The annealed fer r i t ic s ta in
less steels containing 16 to 18% Cr
o f f e r accep tab le ox ida t ion and cor ro
s ion res is tance in many environments
ranging f rom rura l a tmospheres to
aggress ive, hot ac id so lut ions at lower
mate r ia l cos t t han t he common aus te
n i t ic s ta in less s teels . They are, howev
e r , sub jec t t o embr i t t lement , suscep t i
b le t o ho t c rack ing dur ing we ld ing ,
and their mechanical proper t ies
( t oughness and duc t i l i t y ) and cor ro
sion resistance are adversely af fected
b y we ld i n g .
The ferr it ic stainless steels are gener
a l l y cons idered t o have poor we ldab i l
i t y when compared to t he aus ten i t i c
s ta in less s teels . Co nse que nt ly , fe w sys
temat ic s tud ies have been conduc ted
on the i r we ldab i l i t y o ther t han t o
de te rm ine t he e f f ec t o f we ld ing on
the i r mechan ica l p roper t ies and cor ro
sion resistance. In the f irst phase of
th is program the ef fects of e lements
such as C, N, Cr, Si, M n , M o , N i, P, S, Ti ,
N b , and Ta on the hot crack ing suscep
t ib i l i t y of Type 430 and Type 444L were
s tud ied .
A laboratory subscale Vare
s t ra in t we ldab i l i t y t es te r mod i f ied t o
a l low tes t ing w i t h a t r ave l ing w e ld ing
arc was used. The au gm ente d st ra ins
ach ieved on de fo rmat ion o f t he
w e l d
ment were con t ro l led a t va lues f rom
0.8 to 3.2%. The values of total crack
length (TCL) , number of cracks, and
max imum and m in imum c rack leng ths
were measured. The resul ts of th is
subscale Varest ra int test ing ind icate
tha t t he e lements exh ib i t ing t he
greatest in f luence on the hot crack ing
suscep t ib i l i t y were t he non-metallics
S, C, N, and P. Ot he r e lem ents pro m ot
ing hot crack ing inc lude Ti , Nb, and
Mn.
Based upon these results, a second
series of Type 430 and Type 444L
a l loys , wh ich sho u ld exh ib i t improv ed
res is tance to hot crack ing, we re pre
pared. This ser ies conta ined low levels
of S, C, N, and P and w ere s tabi l ized
with vary ing levels of T i , Nb, and Ta
added e i t he r s ing ly o r in comb ina t ion
to form al loys wi th a wi de range of
tota l s tab i l ize r / (C + N) rat ios. H ot
c rack ing suscep t ib i l i t y , we ldment
toughness , c ross -we ld t ens i le p roper
t ies,
and res is tance to in tergranular
co r ros ion were de te rm ined fo r each o f
these alloys.
The resul ts of the hot crack ing inves
t igat ion conf i rm the ear l ier resul ts that
low su l f u r and phosphorus con ta in ing
unstabi l ized Types 430 and 444L exhib
i t acceptable res is tance to hot crack ing
for (C + N) contents less than 0.03%.
Acceptable levels of Tota l Stabi l izer /
(C + N) rat io were de term ine d for
both the Type 430 and Type 444L base
compos i t ions s tab i l i zed w i t h T i , Nb ,
Ta, T i + Nb , and Ti + Ta. Stab i l izat ion
of
th e
Type 444L
w i th the Ta +
Nbwas
found unacceptable at a l l levels .
Gas tungsten arc weld ing tests on
0.225 in. (5.7 mm) t hic k m ater ia l of
Typ e 430 and T ype 444L does no t resu I t
in any s igni f icant reduct ion of cross
we ld y ie ld o r t ens i le s t reng ths com
pared to that of the base meta l . The
add i t ion o f ca rb ide s tab i l i z ing e le
ments e i ther had no ef fect on the
tensi le proper t ies or caused s l ight
increases in the s t rength. Cross-weld
Paper based on presentation made at the
61st AWS Annual Meeting held in Los
Angeles, California, during April
13-18,
1980.
D. H. KAH is Senior Research Metallurgist
and D . W. D ICKINSON is Supervi
sor—Welding Research and Development,
Research
Center,
Republic Steel Corpora
tion, Independence, Ohio.
tens i le fa i lures of both a l loys occur red
pre fe ren t ia l l y in t he we ld meta l . The
Type 444L a l loys exhib i ted less tough
ness than the Type 430 alloys and
stabi l izat ion of Type 444L wi th Ti
reduced toughness more t han d id t he
o ther s tab i l i z ing add i t ions .
Research ind icates that adequate
resistance of Types 430 and 444L to
both hot crack ing and in tergranular
co r ros ion is ob ta in ed wh en the
(C + N) con ten t is co nt ro l led at a
value less tha n 0.04%, in co nj un ct i on
wi th a T i add i t ion ac cord ing t o t he
re lat ion Ti
>
12.5 (C + N) w it h a max
imum Ti content of 0 .65%. The same
two a l loys can a lso be ef fect ive ly s tabi
l ized w i th Ta i f the (C + N) cont en t is
less than 0.025% and the Ta/(C + N)
rat io is greater than 25 wi t h a ma xi
mum Ta content of 0 .9%. Type 444L
a l loys s tab i l i zed w i t h a com b in a t io n o f
Ti + Nb or T i + Ta a lso exh ib i te d ade
quate res is tance to fus ion zone hot
c rack ing and in te rg ranu la r co r ros ion .
I n t r o d u c t i o n
The annealed ferr it ic stainless steels
conta in ing 16 to 18% Cr of fer accept
ab le ox ida t ion and cor ros ion res is
t ance in many env i ronments rang ing
f rom rura l a tmospheres to aggress ive
hot ac id so lut ions. Their pr imary
advantages inc lude lower mater ia l cost
t han t he more common ly used aus ten
it ic stainless steels and a greater
resistance to stress corrosion cracking.
A l t hough these p roper t ies make the
al loys commerc ia l ly at t ract ive, they
st i l l exhib i t severa l s ign i f icant draw
backs that l im it the ir use. These
inc lude reduced fo rmab i l i t y , suscep t i
b i l i t y t o em br i t t lem ent , suscep t ib i l i t y
t o ho t c rack ing dur ing we ld ing , and
the adverse ef fect of weld ing on their
W E L D I N G R E SE A RC H S U P P L E M E N T I
135-s
7/21/2019 Weldability-Ferritic Stainless Steels
http://slidepdf.com/reader/full/weldability-ferritic-stainless-steels 2/8
mechanical proper t ies ( toughness and
duct i l i t y ) and res is tance to in tergranu
lar cor ros ion. However , the advent of
t he a rgon-oxygen decarbur iza t ion p ro
cess (AOD) for ref ining stainless steels
has resul ted in the abi l i t y to produce
al loys w i th lo w inters t i t ia l conte nts
and conse quen t ly s ign i f ican t im prov e
ments in the above proper t ies.
The subject of so l id i f icat ion hot
cracking in austenit ic stainless steels
dur in g we l d in g has been the sub jec t o f
numerous invest igat ions over the years
(Ref. 1, 2). Ho we ve r, because of the ir
genera l l y in fe r io r we ldab i l i t y , t he sus
cept ib i l i t y of fer r i t ic s ta in less s teels to
sol id i f icat ion hot crack ing has not
been examined in de ta i l . A num ber o f
theor ies have been proposed to
expla in the ex is tence of the hot crack
ing phenomenon. I n t he genera l case ,
hot crack ing or microf issur ing can
occur in tergranular ly in e i ther the
fus ion zone o r t he hea t -a f f ec ted zone
o f a we ld m e n t . In either case, the
propensi ty for hot crack ing has been
related to the presence of a tensile
st ress across a l iquated boundary. The
most p laus ib le theory for hot crack ing,
that of the general ized gra in boundary
l iqua t ion mechan ism, p roposes t ha t
microsegregat ion of a l loy ing and re
s idua l e lements t o t he g ra in bound
ar ies or in terde ndr i t ic region s occurs
dur ing so l id i f i ca t ion . Th is segrega t ion
produc es a solute r ich region that
exh ib i t s a lower m e l t ing po in t t han
that of the matr ix. These regions,
wh ich exh ib i t a l iqu id f i lm wh i le t he
matr ix is
so l id ,
serve as nucleat ion
points for microf issures tha t form as a
result of the stresses produced by
shr inkage, phase t ransformat ions, or
external tens i le rest ra int .
Ferrit ic stainless steels are also sus
cep t ib le t o embr i t t lement ( loss o f
toughness a nd /o r duct i l i t y ) as a resul t
of we ld in g (Ref. 3, 4). If the mate r ials
are not fu l ly fer r i t ic at h igh tempera
tu res, a smal l vo lu me f ra ct ion of aus
ten i t e may be f o rmed; t h is w i l l t r ans
form to m ar tens i te dur ing co ol in g as a
resul t o f the weld ing thermal cyc le.
The format ion of very large fer r i te
g ra ins and /o r t he f o rmat ion o f g ra in
boundary mar tens i te in the fus ion
zone o r hea t -a f f ec ted zone (HAZ)
leads to decreased duct i l i t y and tough
ness of as-w elde d mater ia ls because of
the hard and br i t t le nature of the
mar tens i te . For mater ia ls that do not
form any mar tens i te dur ing the w e l d
i ng ope ra t ion , g ra in g ro wth in t he
fus ion and heat -af fected zones wi l l be
even more exaggerated and toughness
wil l again be decreased. I t has been
suggested that addit ives such as B, A l ,
V, or Zr might fo rm pre c ip i ta tes th at
w ou ld inh ib i t g ra in g ro wth in t he HAZ
and that T iN par t ic les might help to
cont ro l the gra in s ize of the fus ion
zone (Ref. 5).
The ferr it ic stainless steels are also
suscep t ib le t o a h igh - tempera tu re
embr i t t lement where t he s tee l may
lose duct i l i t y or toughness on water
quench ing o r a i r coo l ing f rom temper
atures in excess of 2000°F (1093°C)
(Ref . 6 , 7) . The embr i t t lement is
acco mp an ied by a severe g ra in g row th
associated wi th the d issolut ion of car
b ides,
e tc . , wh ich w ere ac t ing to in h ib
i t the mot ion of the gra in boundar ies.
Embr i t t lement resu l t s f r om one o f two
mechanisms—either a c luster ing or
segregat ing of carbon atoms in the
fer r i te mat r ix that are prevented f rom
pre c ip i ta t in g as carb ides by the rapid
c o o l i n g ,
or by the mar tens i te mecha
nism where regions re lat ive ly h igh in
carbon t ransform f i rs t to austeni te at
h igh t empera tu res and then t rans fo rm
to b r i t t le mar tens i t e on rap id coo l ing .
Th is t ype o f embr i t t lement can be
e l im in a ted by a pos tw e ld annea l ing
t rea tment .
The ferr it ic stainless steels also
suf
fer f rom notch sensi t iv i ty . The notch
toughness of these steels is af fected by
tem pera ture in a man ner s imi lar to
that of carbon and alloy steels, i .e. ,
t hey exh ib i t a duc t i le - t o - b r i t t le t r ans i
t ion temperature. For typ ica l fer r i t ic
s ta in less s teels th is t rans i t ion tempera
ture l ies at or above room temperature
and is a f un c t io n o f chem ica l co mp os i
t i o n , gra in s ize, heat t reatment , sect ion
size ,
and no tch con f igu ra t ion . I n these
a l loys an improvement in t he t rans i
t ion tem pera ture is most readi ly
achie ved by wa y of a decrease in the
inters t i t ia l co nte nt (C + N) an d, sec
ondly, by a decrease in grain size as
f iner gra ined mater ia ls exhib i t inher
ent ly bet ter toughness than coarse
gra ined mate r ia ls o f t he same compo
s i t ion (Ref. 8-10) . As a con sequ ence of
the sensit iv ity of these alloys to
no tches and impac t load ing , ce r ta in
precaut ions may be taken dur ing
w e l d i n g .
A preheat t reatment is of ten
used to help in reducing shr inkage
st resses and may a id in the prevent ion
o f any spon taneous c rack ing dur ing
t h e we ld i n g o p e r a t i o n .
Suscep t ib i l i t y t o in te rg ranu la r co r ro
s ion in the as-welded condi t ion a lso
plagues some of the ferr it ic stainless
steels. Several invest igators believe
that the format ion of austeni te at h igh
tempera tu res and the subsequen t p re
c ip i ta t ion of readi ly d issolved i ron car
b ides at gra in boundar ies leads to the
co r r o s i o n . Others have p roposed tha t
impover ishment o f t he g ra in boundary
aus ten i t e in ch r om ium c on ten t is
responsib le.
T h e m o s t c o m m o n l y a c c e p t e d t h e o
ry for in tergranular cor ros ion of the
ferr it ic stainless steels is that when
they are heated to h igh temperatures,
as in we ld in g , and subseque n t ly
c o o l e d ,
t he C and N in s o l id - so lu t ion
prec ip i ta tes a long gra in boundar ies at
in te rmed ia te t empera tu res , leav ing be
h i n d a c h r o m iu m - d e p le t e d z o n e t h a t
is suscept ible to corrosion (Ref. 11).
The normal h igh in ters t i t ia l levels
observed in commerc ia l s teels and
their rapid d i f fus ion rate in fer r i te
make i t impossib le to prevent prec ip i
t a t ion even by wa te r quench ing ,
except in very thin sect ions. I t has
been shown by Demo tha t p rec ip i t a
t ion o f
Cr
23
C
6
a n d
Cr
2
N
occurs f rom
about 925 to 1750°F (496 to 954°C) and
that sensi t izat ion or loss in cor ros ion
res is tance occurs by hold ing wi th in or
s low coo l ing t h rough the t empera tu re
range of 925 to
1300°F
(496 to
704°C)
(Ref. 7). At tem pe ratu res abo ve 1300°F
(704°C)
t he ch rom ium d i f f us ion ra te is
suf f ic ient ly rapid to h e a l or redis t r i
bu te ch rom ium to t he dep le ted a reas
as p rec ip i t a t ion occurs . There fo re , sen
s i t izat ion is not a pro ble m for a l loys
heat t reated in the temperature range
of 1300 to 1700°F (704 to 927°C).
Because of the sensi t izat ion k inet ics,
i t can be seen that a postweld anneal
ing t reatm ent w i l l be ef fect ive in
improv ing the cor ros ion res is tance of
we lded mate r ia ls. An impro vem ent in
res is tance to in tergranular cor ros ion
can a lso be achieved by s tabi l izat ion
wi th s t rong carb ide and /o r n i t r ide
fo rm ing e leme nts (T i , Nb , e t c . ) , wh ich
pre fe ren t ia l l y comb ine w i t h C and N
to p reven t p rec ip i t a t ion o f ch rom ium
carbides or n i t r ides, or by a reduct ion
in the tota l in ters t i t ia l (C + N) c o n
cent rat ion to ext remely low levels
( < 0.015%). The on ly prac t ical ap
proach for producing a l loys f ree of
sens i t i za t ion a f te r we ld ing us ing e i t he r
c o n v e n t i o n a l o r A O D s t e e lm a k in g
techniques is to add stabi l iz ing e le
ments.
O b j e c t i v e
The p r imary ob je c t ive o f t h is inves t i
gat ion was to determine the ef fect of
a l loy compos i t ion on t he ho t c rack ing
sus cep t ib i l i t y o f the 16 to 18% Cr fer r i t
ic s ta in less s teels . A second object ive
was to determine the ef fect of carb ide
s tab i l i za t ion and we ld ing on t he
toughness, duct i l i t y , and res is tance to
intergranular cor ros ion of a l loys exhib
i t ing good res is tance to hot crack ing.
M a t e r i a l s
Commercial Materials
In the in i t ia l par t o f the hot crack ing
inves t iga t ion , a se ries o f com merc ia l l y
available stainless steel mater ials was
inves t igated. These mater ia ls inc lud ed
a Type 304 austeni t ic s ta in less s teel , a
Typ e 430 ferr it ic stainless ste el, and an
e lec t ron beam me l ted
E-Brite
(26-1)
fer r i t ic s ta in less s teel . The co mp os i
t ions of these three alloys are pre
sente d in Table 1. These alloys served
136-s I A U G U S T 1 9 81
7/21/2019 Weldability-Ferritic Stainless Steels
http://slidepdf.com/reader/full/weldability-ferritic-stainless-steels 3/8
Tab le 1—Compositions o f C o m m e r c i a l
Stain less Steel Invest igated, Wt-%
c
M n
Si
P
S
Cr
Ni
M o
Cu
Al
N
O
Type
430
0.062
0.35
0.30
0.031
0.016
16.6
0.44
0.12
0.14
0.026
0.036
—
E-Brite
26-1
< 0.002
< 0.01
0.26
0.02
0.02
26.4
0.13
1.0
< 0.01
< 0 . 0 1
0.011
< 0.002
Type
304
0.060
1.76
0.50
0.029
0.012
18.3
9.1
0.35
0.28
0.029
0.038
—
a s a b a se l i n e f o r a n a l ys i s o f t h e l a b o
r a t o r y - p r e p a r e d m a t e r i a l s .
Labora to ry Materials—Phase I
T h e b a s i c e x p e r i m e n t a l a p p r o a c h i n
P h a s e I w a s t o e m p l o y a c l a s s i c a l a l l o y
d e s i g n , i. e. , t o m a k e m u l t i - l e v e l , s i n g l e
e l e m e n t v a r i a t io n s t o s t a n d a r d c o m p o
s i t i o n s o f T yp e s 4 3 0 a n d 4 4 4 L . T h e
r a n g e o f e l e m e n t a l v a r i a t i o n s i n v e s t i
g a t e d i s p r e s e n t e d i n T a b l e 2 . T h e C u
a n d N i c o n t e n t s w e r e h e l d c o n s t a n t
f o r t h e s e m a t e r i a l s .
A l l o f t h e a l l o y s w e r e m e l t e d b y
v a c u u m i n d u c t i o n as 5 0 l b (2 2 . 7 k g )
h e a t s ;
t h e s e w e r e s u b s e q u e n t l y s p l i t
i n t o t h r e e 1 6 l b ( 7 . 2 5 kg ) i n g o t s
u t i
l i z i n g i n g o t a d d i t i o n s to p r o d u c e t h e
d e s i r e d e l e m e n t a l v a r i a t i o n s . T h e 3 i n .
<) X 2.5 in . (j) x 8 in . (76 m m tj) X 64
m m ( j) X 2 0 3 m m ) i n g o t s w e r e p r e ss
fo rg ed to s labs 1 .25 X 3 i n . x L
(32 X 76 m m x L) us ing t h e t e m
p e r a t u r e r a n g e o f 2 0 5 0 ^ 1 4 0 0 ° F
( 1 1 2 0 ^
7 6 0 °C ) , a i r c o o l e d , a n d c o n d i
t i o n e d b y g r i n d i n g . S l ab s h a v i n g t h e
d i m e n s i o n s o f 1 .1 2 5 x 3 x 6 i n .
( 2 9 X 7 6 x 1 5 2 m m ) w e r e s o a ke d a t
2300°F ( 1 2 6 0 °C ) , r e d u ce d t o 0 . 2 2 5 x 3
i n .
x L (5.7 x 76 m m x L) in th re e
p a s se s at t e m p e r a t u r e s o f 2 1 5 0 , 1 9 6 0 ,
a n d
1770°F
( 1 1 7 5 , 1 0 7 0 , a n d 1 9 6 5 °C ) ,
a n d f l a t t e n e d .
Tab le
2-Ranges
o f E lementa l Var ia t i ons
Examined fo r t he Study o f We ld Hot
Cracking in Ferr i t ic Stain less Steels—Phase
I, W t - %
C
Cr
Si
Al
M n
M o
N b
Ti
P
S
Ta
Type
430
al loys
0.004-0.099
15.4-20.0
0.25-1.45
0.02-0.70
0.10-0.70
0.10-2.2
0-1.6
0-0.74
0.008-0.052
0.006-0.061
—
Type
444
L
al loys
0.01 -0.05
0.28-0.88
1.6-2.5
0.23-1.03
0.09-0.44
0.005-0.066
0.45-0.92
T h e p l a t e s w e r e g i v e n a s i m u l a t e d
b o x a n n e a l i n a ir w i t h a 4y2 h o u r ( h )
s o a k a t t h e p e a k t e m p e r a t u r e o f 1500°F
( 8 1 5 ° C ) . T h e p l a t e s w e r e s e c t i o n e d t o
g i v e l o n g i t u d i n a l s p e c i m e n s , w h i c h
w e r e g r o u n d t o 0 . 2 0 0 x 1 x 6 i n .
( 5 X 2 5 X 1 5 2 m m ) f o r t h e h o t c r a c k
ing t es t s .
Labora to ry Mater ia l s -Phase I I
A s t h e l a b o r a t o r y t e s t i n g p r o c e e d e d ,
i t b e c a m e o b v i o u s t h a t a s e c o n d s e r ie s
o f h e a t s w o u l d b e n e e d e d . T h i s s e r i e s
c o n t a i n e d l o w l e v e l s o f S, C , N , a n d P
a n d w e r e s t a b i l i z e d w i t h v a r y i n g l e v e l s
o f T i , N b , a n d T a a d d e d e i t h e r s i n g l y o r
i n c o m b i n a t i o n . T h e ra n g e o f e l e m e n
t a l v a r i a t i o n s i n v e s t i g a t e d a re p r e
se n t e d i n T a b l e 3 .
Tab le
3—Ranges
o f E lementa l Var ia t i ons
Examined fo r t he Study o f We ld Hot
Cracking in Ferr i t ic Stain less
Steels—Phase
II ,
W t - %
c
N
Ti
Ta
Type 430
al loys
0.001 -0.032
0.014-0.021
0-0.67
0-1.41
T yp e 444L
al loys
0.007-0.035
0.010-0.019
0-0.67
0-1.06
Ti + Nb: Ti
N b
Ti + Ta: Ti
Ta
Ta + Nb: Ta
N b
—
-
-
0.01-0.22
0.15-0.45
0.12-0.35
0.34-0.68
0.33-0.66
0.16-0.57
A l l o f t h e a l l o ys i n Ph a se I I w e r e
v a c u u m i n d u c t i o n m e l t e d as 5 0 l b ( 22 .7
kg ) h e a t s a n d ca s t as a s i n g l e i n g o t 4 %
i n .
<) X 4 i n .
<>
X 1 0 i n . ( 1 2 1 m m
(j) x 102 mm cj) X 254 m m ). Th ey w e re
f o r g e d ,
c o n d i t i o n e d , a n d h o t r o l l e d t o
pla te s 0.5 X 5 in. X L (13 X 127
m m X L ) a t t h e s a m e t e m p e r a t u r e s a s
in Phase I. T h e p l a t e s w e r e t h e n c o l d
ro l l e d t o e i t h er 0 .225 x 5 x L
(5.7 X 127 X L) or 0.050 X 5 in . X L
( 1 .3 X 1 2 7 m m X L ) a n d a n n e a l e d a t
t e m p e r a t u r e s r a n g i n g f r o m 1 5 0 0 t o
2000°F (815 to 1090°C) f o r e i t h e r 1 5 o r
6 0 m i n u t e s ( m i n ) i n s a l t , a i r c o o l e d t o
1 4 0 0 °F ( 7 6 0 °C ) , a n d w a t e r q u e n c h e d
d e p e n d i n g o n t h e a l l o y c o m p o s i t i o n .
H o t C r a c k i n g
Procedures
The Varestra in t and subsca le Vares
t ra in t tests have been the most w ide ly
used tests fo r de termin ing the hot
cracking suscept ib i l i ty in the fusion
and hea t -a f fec ted zones o f we lded
mater ia ls. The major d i f fe rences be
tween these tests are :
1.
The subscale test uses a sta tion ary
arc spot we ld on th in mater ia ls wh i le
the Va restra in t test uses a t rave l ing arc
weld on heavy mater ia ls.
2.
The m eth od o f st ra in ing the
mater ia ls.
In th is invest iga t ion the subsca le
Varestra in t un i t was redesigned to use
a t rave l ing arc we ld and w i l l be
re ferred to as a mod i f ie d subsca le Va
restra in t test . The opera t iona l se
quence fo r th is test consists o f p roduc
ing a t rave l ing bead on p la te w e ld on
the sample mater ia l using the GTAW
process. The we ld i ng proce dures used
are lis ted in Tab le 4. At an app ropr ia te
t ime in the cycle the specimen is
de fo rmed by an a i r -ac tua ted d ie b lock
to prod uce a g iven st ra in . Th is de fo r
ma t ion p roduces an augmen ted s t ra in
a t the top surface o f the specimen tha t
can be ca lcu la ted as sh ow n in equa
t ion (1) :
t / 2
r i .
e
=
X
(D
w h e r e e = augm en ted s t ra in ( in . / i n . )
o r ( m m / m m ) , t = sp e c i m e n t h ickn e ss
( in . ) o r (m m ), and R = rad ius o f curva
tu re o f d ie b lock ( in . ) o r (mm).
The augmen ted s t ra ins u t i l i zed we re
0.8, 1.6, and 3.2%. The amount of
c rack ing obse rved in the we ldmen t a t
the to p surfa ce is take n as a mea sure o f
the ho t c rack ing suscep t ib i l i t y o f the
ma te r ia l .
A f t e r w e l d i n g a n d d e f o r m a
t ion the we lds we re p ick led fo r one
min u te us ing a so lu t io n o f 10%
H N 0
3
-
2% HF at 140°F (60°C), r insed in water,
and d r ied . The c lean we ldm en t was
then exam ined a t a mag n i f i ca t ion o f
X40 to de te rm ine the ex ten t o f ho t
c rack ing in bo th the fus ion and hea t -
a f fected zone s. The to ta l crack lengt h ,
ma x im um crack leng th , ave rage c rack
length , and number o f cracks were
de te rm ined fo r each samp le u t i l i z ing a
b i n o cu l a r m i c r o sco p e a n d a m i c r o m e
ter re t ic le .
Results
Commercial Materials
In o rder to estab l ish a ho t crac king
base l ine , a ho t cracking study was
conducted on the th ree commerc ia l
materia ls. The results of th is testing are
presented graph ica l ly in F ig . 1 .
From the data i t can be seen that the
overa l l resistance to ho t cracking o f
the h igh pur i ty, fe rr i t ic E-Brite 26-1 is
super io r to tha t o f Type 304. Commer
cia l Type 430, however, exh ib i ted as
much c rack ing a t an augmen ted s t ra in
of 0.8% as Type 304 did at an aug
m en ted s train of 3.2%. The average
total crack length (TCL) at 0.8%
aug
men ted s t ra in o f the commerc ia l Type
430 was 0.061 in. (1.5 mm ) co mp are d
to an average TCL fo r n ine labora tory
Type 430 al loys of 0.038 in. (0.97 mm)
wi th a standard dev ia t io n o f 0 .028 in .
(0.71 mm).
The genera l cr i te r ion se lected fo r
acceptab le resistance to ho t cracking ,
W E L D I N G R E S EA R C H SU P P LE M E N T 1137-s
7/21/2019 Weldability-Ferritic Stainless Steels
http://slidepdf.com/reader/full/weldability-ferritic-stainless-steels 4/8
Tab le
4—Welding
Procedures
Subscale Varestraint Test
T o r c h :
Type
Gas cup
Elec t rode : Type
D i a m e t e r
Extens ion f rom
co l lec t
T ip ang le
T i p - t o - w o r k
d is tance
Sh ie ld i ng :
Travel speed:
Vo l t age :
Cur ren t :
A i r accumula to r p ressure :
Bo l t t o rque :
Mechanical properties
Process:
Jo in t des ign :
F i l ler metal :
Sh ie ld i ng :
W eld ing co nd i t i ons f o r passes 1
Cur ren t
Vo l t age
Travel speed
Heat i npu t
Intergranular corrosion
Process:
Joint design:
Sh ie ld i ng :
W eld ing co nd i t i o ns f o r pass 1 :
Cur ren t
Vo l t age
Travel speed
Heat i npu t
and 2:
Wa ter -c oo le d gas t ungs ten a rc
N o.
10
2%
t h o r i a t e d t u n g s t e n
0.125 in.
1.40 in.
-
90 deg
0.055 in.
-
Argon gas @ 40 chf
10.5 ipm
14 V
300 A
40 psi
200 in.-lb
Gas tungsten a rc we ld ing (DCSP)
Square bu t t
N o n e
Arg on gas 25 cfh
300 A
12 V
8 ipm
27.0
k j / i n .
Gas tungs ten a rc we ld in g
Bead on plate
Argon gas (cover and back ing)
115 A
11 V
15 ipm
5.1 k j / i n .
w h i c h w a s u s e d t h r o u g h o u t t h e
r e m a i n d e r o f t h is i n v e s t i g a t i o n , w a s a
t o t a l c r a ck l e n g t h o f 0 . 0 2 0 i n . ( 0 . 5 1
m m ) a t 0 . 8% a u g m e n t e d s t r a i n . T h i s
r e p r e s e n t s a n i m p r o v e m e n t o f a t l e a s t
5 0 % i n h o t c r a c k i n g r e s i s t a n c e o v e r
t h a t o f c o m m e r c i a l T y p e 4 3 0. T h e c h o
s e n a u g m e n t e d s t r a i n o f 0 . 8 % i s g r e a t e r
t h a n t h a t w h i c h w o u l d n o r m a l l y b e
e n c o u n t e r e d i n a w e l d i n g o p e r a t i o n .
P r i o r t o e x a m i n i n g t h e e f f e c t o f s p e
c i f i c e l e m e n t s o n h o t c r a c k i n g , i .e ., t h e
l a b o r a t o r y s t u d y , s o m e g e n e r a l o b s e r
v a t i o n s o n t h e c o m m e r c i a l m a t e r i a l
w i l l b e n o t e d . F i g u r e 2 i l l u s t r a t e s a
t y p i c a l h o t c r a c k in c o m m e r c i a l T y p e
4 3 0 w h e n s u b j e c t e d t o t h e m o d i f i e d
s u b s c a l e V a r e s t r a i n t t e s t . T h i s s c a n n i n g
e l e c t r o n m i c r o g r a p h c l e a r l y s h o w s t h e
b l u n t e d d e n d r i t e s i n t h e c r a c k , w h i c h
h a v e s t o p p e d g r o w i n g b e c a u s e o f a
l a c k o f l i q u i d f e e d m e t a l . T h i s
i n d i
c a t e s t h a t t h e d e n d r i t e i n t e r f a c e s w e r e
s t il l l i q u i d a t t h e t i m e o f d e f o r m a t i o n ,
w h i c h is c o n s i s t e n t w i t h t h e g e n e r a l
i z e d l i q u a t i o n t h e o r y f o r h o t c r a c k
i n g .
V i s u al e x a m i n a t i o n o f w e l d e d s p e c i
m e n s i n d i c a t e d t h a t t h e l a r g e s t t o t a l
c r a c k l e n g t h s w e r e a s s o c i a t e d w i t h
w i d e r w e l d m e n t s e x h i b i t i n g a t e a r d r o p
r a t h e r t h a n a n e l l i p t i c a l l y s h a p e d p o o l .
P o o l s h a p e is a f u n c t i o n o f a l l o y c o m
p o s i t i o n a n d t h e w e l d i n g p a r a m e t e r s ,
i n p a r t i c u l a r t h e h e a t i n p u t . T e a r d r o p -
s h a p e d p o o l s a r e g e n e r a l l y m o r e s u s
c e p t i b l e t o h o t c r a c k i n g .
Labora to ry Material—Phase I
P l o t s o f t o t a l c r a ck l e n g t h vs . p e r
c e n t a l l o y i n g a d d i t i o n w e r e
c o n
s t r u c t e d f o r e a c h s e t o f a l l o y i n g
a d d i
t i o n s a s i l l u s t r a t e d i n F i g . 3 f o r t h e S
c o n t e n t i n T y p e 4 3 0 a l l o y .
I n g e n e r a l , d u e t o t h e a m o u n t o f
s c a t t er o b s e r v e d , t h e h o t c r a c k i n g
t e n
d e n c i e s w e r e r e p r e s e n t e d b y s t r a i g h t
l i n e s . W h i l e t h e t o t a l r e a s o n f o r t h e
s c a t t e r h as n o t b e e n e x p l a i n e d , a t l e a s t
a p o r t i o n o f i t is d u e t o t h e s u b j e c t i v e
n a t u r e o f t h e c r a c k m e a s u r e m e n t a n d
t h e le v e l o f c r a c k i n g e n c o u n t e r e d . A t
l o w l e v e l s o f c r a c k i n g o f a b o u t 0 . 0 1 0
i n .
(0 .25 m m ) , a A TC L o f 0 .010 in . (0 .25
m m ) r e p r e s e n t s a 1 0 0 % v a r i a t i o n w h i l e
t h e s a m e A T C L a t a c r a c k i n g l e v e l o f
0 . 1 0 0 i n . ( 2 .5 m m ) r e p r e s e n t s o n l y a
1 0 %
v a r i a t i o n .
W h e n t h e d a t a a r e t r e a t e d a s s t r a i g h t
l i n e s , t h e s l o p e o f t h e l i n e r e p r e s e n t s
t h e r a t e o f c h a n g e o f t h e h o t c r a c k i n g
t e n d e n c y d u e t o t h e a l l o y a d d i t i o n .
F i g u r e 4 a n d T a b l e 5 s u m m a r i z e t h e
e f f e c t o f
0 . 1 %
c h a n g e s i n a l l o y e l e m e n t
a d d i t i o n o n t h e T C L o b s e r v e d a t 0 . 8%
a u g m e n t e d s t r a i n .
It
is r e a d i l y o b
s e r v e d f r o m t h e s e d a t a o n n o m i n a l
1 8 %
C r a l l o y s t h a t S , C , P, a n d M n a r e
t h e p r i m a r y c o n t r i b u t o r s t o h o t c r a c k
i n g w h e r e S is a b o u t t h r e e t i m e s a s
d e t r i m e n t a l a s C a n d a b o u t 4 8 t i m e s as
d e t r i m e n t a l a s M n . W h i l e t h e e f f e c t o f
N w a s n o t s t u d i e d d i r e c t l y , a m u l t i p l e
l i n e a r r e g r e s s i o n a n a l y s i s o f t h e d a t a
i n d i c a t e d t h a t it s e f f e c t w o u l d b e
s i m
i l a r t o t h a t o f C . T h e e l e m e n t s C r , A l ,
a n d S i e x h i b i t e d l i t t l e , i f a n y , e f f e c t o n
t h e h o t c r a c k i n g s u s c e p t i b i l i t y o f t h e
1 6 t o 1 8 % C r f e r r i t i c s t a i n l e ss s t e e l s .
M o l y b d e n u m a p p e a r s t o b e s l i g h t l y
b e n e f i c i a l i n r e d u c i n g h o t c r a c k i n g a s
i ts c o n t e n t i n c r e a s e s f r o m 0 t o 2 .5 % .
T h e e f f e c t o f T i a n d N b o n h o t
c r a c k i n g is n o t as c l e a r b e c a u s e o f t h e
p r e c i p i t a t i o n o f c a r b o - n i t r i d e s , w h i c h
p r e v e n t s t h e s t u d y i n g o f T i a n d N b
d i r e c t l y . It w a s f o u n d t h a t a p l o t o f T C L
vs . N b / ( C + N ) c o u l d b e f i t b y a
s t r a i g h t l i n e a t t h e 0 . 0 3 % C l e ve l w h i l e
f o r t h e 0 . 0 6 % C l e ve l t h e r e i s a n a p p a r
e n t m a x i m u m . T h e s l o p e s o f t h e s e
l i n e s a r e q u i t e s m a l l a n d t e n d t o i n d i
c a t e t h a t c a r b o n i s t h e p r i m a r y d e t r i
m e n t a l e l e m e n t . A d d i t i o n a l l y , m e t a l l o
g r a p h i c e x a m i n a t i o n o f a n u m b e r o f
w e l d c r o s s s e c t i o n s f r o m t h e P h a s e I
m a t e r i a l s r e v e a l e d t h e f o l l o w i n g :
1.
T h e s o l i d i f i e d w e l d m e t a l w a s
ep i tax ia l ly r e l a t e d t o t h e p a r t i a l l y
m e l t e d g r a i n s a t t h e e d g e o f t h e h e a t -
a f f e c t e d z o n e .
2. M o s t o f t h e w e l d m e n t s s o l i d i f ie d
w i t h e i t h e r a c e l l u l a r o r c e l l u l a r - d e n
d r i t i c s t r u c t u r e . T h e s e s t r u c t u r e s a r e
m o r e s u s c e p t i b l e t o h o t c r a c k i n g t h a n
is a f i n e e q u i a x e d d e n d r i t i c s t r u c t u r e .
3 . A l l o f t h e a l l o y s e x h i b i t e d s u b
s t a n t i a l g r a i n g r o w t h i n t h e h e a t -
a f f e c t e d z o n e .
4 .
U n s t a b i l i z e d o r u n d e r - s t a b i l i z e d
a l lo y s e x h i b i t e d u n t e m p e r e d m a r t e n
s i t e a t g r a i n b o u n d a r i e s i n b o t h t h e
f u s i o n a n d h e a t - a f f e c t e d z o n e s . T h e
a m o u n t o f m a r t e n s i t e is a f u n c t i o n o f
t h e i n t e r s t i t i a l c o n t e n t a n d n o m a r t e n
s i te w a s p r e s e n t a t v e r y l o w i n t e r s t i t i a l
c o n t e n t s .
L a b o r a t o r y Material—Phase II
B a s e d u p o n t h e r e s u l t s o f t h e h o t
c r a c k i n g i n v e s t i g a t i o n i n P ha s e
I,
a
s e c o n d s e ri e s o f T y p e 4 3 0 a n d T y p e
4 4 4 L a l l o y s t h a t s h o u l d e x h i b i t i m
p r o v e d r e s i s t a n c e t o h o t c r a c k i n g w a s
p r e p a r e d .
T h i s s e r ie s c o n t a i n e d l o w
l e ve l s o f S , C , N , a n d P , w h e r e t h e
( C + N ) l e v e ls w e r e c o n t r o l l e d a n d
t h e s e m a t e r i a l s w e r e s t a b i l i z e d w i t h
v a r y i n g l e v e l s o f T i , N b , a n d T a .
T h e r e s u l t s o f t h e h o t c r a c k i n g
s t u d i e s c o n d u c t e d o n t h e s e a l l o y s a r e
s u m m a r i z e d i n F ig s . 5 t h r o u g h 8 . I n
t h e s e f ig u r e s , t h e a l l o y v a r i a t i o n s w e r e
g r o u p e d i n t o t h o s e h a v i n g ( C + N )
l e v e l s o f a p p r o x i m a t e l y 0 . 0 3 % , a p p r o x i
m a t e l y 0 . 0 6 % , a n d i n o n e ca se as h i g h
a s a p p r o x i m a t e l y 0 . 0 9 % . T h e T C L a t
0 .8 % a u g m e n t e d s t r a in as a f u n c t i o n o f
t h e s t a b i l i z e r / ( C + N ) r a t i o w a s t h e n
p l o t t e d f o r e a c h a l l o y a n d g r o u p . T h e
138-s I
AUGU ST 1981
7/21/2019 Weldability-Ferritic Stainless Steels
http://slidepdf.com/reader/full/weldability-ferritic-stainless-steels 5/8
10.0
E
E
I- 7.5
O
z
UJ
* 5 . 0
o
<
tr
o
-J 2 . 5
<
15
3.2 % STRAIN
COMMERCIAL MATERIAL
1.6
STRAIN
-
0 . 8 % ST R A I N
E-BRITE
(26-1)
TYPE
4 3 0
TYPE
3 0 4
E-BRITE
(26-1)
- . 3 5 :?
x
- . 3 0 H
. 25
TYPE
4 3 0
TYPE
3 0 4
E-BRITE
(26-1)
TYPE
4 3 0
TYPE
3 0 4
UJ
. 2 0 -^
O
.15 Q£
O
Kio _i
- . 0 5 O
Fig. l—The hot cracking susceptibility of comm ercial stainless steels; EB 26-1,
Type 304, and Type 430.
-A -
- y^i
Fig.
2—A
scanning electron microscope
view ot a typical hot crack in Type 430.
7.0
E
x
O
5.0
U
<
O
3 0
•
•
.26
- . 2 2
.18
.14
.10
.06
O
UJ
<
rr.
o
0 .02 .04 .0 6 .08
SULFUR IN T Y P E 4 3 0 , %
Fig. 3—The variation of total crack length for Type 430 at 0.
augmen ted strain as a function of S content.
8
E
E
UJ*
H
3
- I
O
CO
o
<
10.0
7.5
5.0
r
P H A S E I
TY P E 4 3 0
PI
J_L
S C P Mn Cr Al SI Mo N
T i
Nb
TYPE 444L
_L_t
. 45
-.40
S Mo Si N
T i
Nb
- . 3 5
.30
- . 2 5
.20
.15
.10
- . 0 5
0
UJ
I -
_ l
o
co
O
_ l
CJ
I -
<
g. 4—Summary
of the change in total crack length for Types 430 and
a function of a 0.1 increase in solute content.
444L
Tab le
5—Effect
o f A l l oy ing Add i t i ons on Hot Crack ing o f Fer r i t i c Sta in less Stee l We lds
Element
C
Cr
Si
M n
Al
M o
P
S
Type 430
0.120
0.00046
0.00035
0.0079
0.00043
-0 .0021
0.0128
0.38
A TCL per 0 .1% so lu te
Type
4 4 4 L
l a l
N D
N D
-0 .0052
N D
N D
-0 .00025
N D
0.42
ND-Not determined.
cu r v e s p r e s e n t e d i n F i g s . 5 - 8 a r e a
r e g r e s s i o n f i t o f t h e d a t a . A c t u a l d a t a
p o i n t s h a v e b e e n o m i t t e d f o r c l a r i t y .
R e m e m b e r i n g t h a t t h e c r i t e r i o n
a d a p t e d a s a b a s e l i n e f o r a c c e p t a b l e
h o t c r a c k i n g r e s i s t a n c e f o r t h e c o m
m e r c i a l m a t e r i a l w a s 0 . 0 2 0 i n . ( 0 . 5 1
m m ) o f t o t a l c r a c k l e n g t h a t 0 . 8 %
a u g m e n t e d s t r a i n , t h e r e s u l t s o f t h e s e
t r i a l s c a n n o w b e a n a l y z e d .
T h e e f f e c t o f N b s t a b i l i z a t i o n o n h o t
c r a c k i n g s u s c e p t i b i l i t y is s h o w n in F i g .
5 . I t ca n be seen tha t t he le ve l o f
c r a c k i n g i n c r e a s e s w i t h t h e ( C 4 - N )
c o n t e n t i n b o t h t h e T y p e 4 30 a n d T y p e
444L a l lo y s . T h e t o t a l c r a c k l e n g t h a l s o
i n c r e a se s w i t h i n c r e a s e i n N b , i. e .,
i n c r e a s e i n t h e N b / ( C + N ) r a t i o . F u r
t h e r m o r e , t h e t o t a l l e v e l o f c r a c k i n g is
l e ss i n t h e T yp e 4 4 4 L a l l o ys t h a n i n t h e
T y p e 4 3 0 a l l o y s . I n t h e T y p e 4 3 0 a l l o y s ,
t h e c r i t e r i o n o f 0 . 0 2 0 i n . ( 0 . 5 1 m m ) T C L
i s m e t o n l y f o r t h e l o w e s t ( C + N )
l e v el s w i t h N b / ( C + N ) r a ti o s b e l o w
a b o u t 2 . I n t h e T y p e 4 4 4 L a l l o ys t h e
c r i t e r i o n i s o b t a i n e d a t t h e l o w
( C + N ) l e ve l u p t o a N b / ( C + N )
ra t i o i n excess o f 16 .
T h e e f f e c t o f T a s t a b i l i z a t i o n o n t h e
h o t c r a c k i n g s u s c e p t i b i l i t y i s s h o w n i n
Fig . 6 . T h e t o t a l c r a ck i n g is l e ss t h a n
t h a t o b s e r v e d i n t h e N b s t a b i l i z e d
m a t e r i a l .
F o r t h e l o w ( C + N ) l e ve l i n
t h e T y p e 4 3 0 a l l o y , T a c o n t e n t s c o r r e
s p o n d i n g t o a T a / ( C + N ) r a t i o u p t o
a b o u t 3 6 ( 1 . 0 8 % T a ) p r o v i d e a c c e p t -
W E L D I N G R E S E A R C H S U P P L E M E N T I 1 3 9 - s
7/21/2019 Weldability-Ferritic Stainless Steels
http://slidepdf.com/reader/full/weldability-ferritic-stainless-steels 6/8
3 . 0
2.0
1.0
0
(C + N) « 0.06
1 \ l
— (C+N)W 0.03
1 \J
TYPE 444L.
C
o
.10 <
tr
.0 8 o
.0 6 _i
<
J .04 H
O
02 I -
2 4
Fig
of
4 8 12 16 2 0
N b / ( C + N )
. 5—The effect of Nb content on the hot cracking susceptibility
Types
430 and
444L
at 0.8 augmented strain.
8 16 2 4
T a / ( C + N )
Fig. 6—The effect of
Ta
content on the hot cracking susceptibility of
Types 430 and
444L
at 0.8 augmented strain.
3.0
12 16 2 0
T i / ( C + N )
Fig.
7—The
effect of Ti content on the hot cracking susceptibility of
Types 430 and
444L
at
0.8
augmented strain.
E
E
5 3
z
UJ
* 2
o
<
ce
o
<
o
^^^^
( C +N ) .03
(Nb + Ti )
TYPE 444L
M T I
+
To)
-
-
4 8 12 16 2 0
T i + T a / C + N OR Nb + T i / C + N
.14 ~
I
I
12 I-
O
z
.io UJ
.06 *
o
<
.0 6 (E
O
.0 4 -J
i
.02 O
24
Fig. 8—The
effect of dual stabilization on the hot cracking suscep-
tibilty ot Type
4441
at 0.8 augmented strain.
ab le resistance to ho t cracking . In the
Type 444L a l loys, acceptab le resistance
to ho t crack ing is ob ta in ed a t lo w
(C + N) levels for a l l Ta levels investi
ga ted .
Ac cep tab le resistance to ho t
crac king is ob ta i ne d a t h ighe r (C + N),
i .e., 0.06%, up to a Ta /( C + N) ratio of
about 24 (1.44% Ta).
The e f fect o f T i stab i l iza t ion on the
ho t c rack ing susce p t ib i l i t y is show n in
Fig. 7. For the lo w (C + N) Ty pe 430
a l loy acceptab le resistance to ho t
c rack ing is ob ta in ed up to a T i /
(C + N) ra t io o f abo ut 12 . fo r the low
(C + N) Typ e
444L
a l loy ac cepta b le
resistance to ho t cracking is ob ta ined
up to a T i / (C 4- N) ratio of about 24.
Dua l s tab i l i za t ion o f Type 444L w i th
T i + Nb a nd T i + Ta was a lso exam
i n e d . The results are presented in Fig.
8 . Ac cep tab le resistance to ho t crack
ing is observ ed wi th b o th o f these dua l
stab i l izers up to a sta b i l iz er / (C + N)
ra t io o f about 20 .
In summary, i t appears tha t accept
ab le resistance to ho t cracking can be
obt a in ed a t low (C + N) leve ls w i t h T i ,
Ta,
o r dua l s tab i l i za t ion , bu t no t w i th
Nb a lone.
M e c h a n i c a l P r o p e r t i e s
Procedures
Each p la te f rom the Phase I I mater i
a ls was sec t ioned long i tu d ina l l y a long
the p la te cen te r l i ne , su r face g round in
the v ic in i t y o f the we ld jo in t , c leaned ,
and we lde d w i th a two -pass gas tun g
sten arc process as ind ica te d in Fig. 9.
The we ld ing p rocedu res a re de ta i led
in Table 4.
The p la tes were rad iographed a f te r
we ld ing to ensu re tha t they we re
de fec t - f ree . The we lds we re sec t ioned
as sh ow n in Fig. 9 to p rov ide f lat
tens i le , ha l f -s i ze Cha rpy V -no tc h im
pac t , and m e ta l log raph ic samp les. The
no tch on the we ld me ta l Cha rpy sam
p les was loca ted a t the we ld center-
l i ne ,
wh i le the no tch on the hea t -
a f fected zone samples was loca ted a t
the 50% we ld me ta l , 50% hea t -a f fec ted
zo n e l o ca t i o n .
The impact and tensi le samples
were tes ted acco rd ing to s tanda rd
ASTM pract ices.
Results of Tensile Tests
The resu l ts o f the tensi le test ing
ind ic a te tha t , in gene ra l , the ten si le
and yie ld st rengths o f the base meta l
vary on ly s l igh t ly as the T i o r Ta stab i l
iza t ion increases in Type 430, a l though
some random va r ia t ions we re no ted
w i th Ta add i t i ons . The tens i le
strengths were o f the order o f 65 to 70
ksi (448 to 483 MPa) and the yie ld
strengths 40 to 45 ksi (276 to 310 MPa).
The c ross-we ld samp les exh ib i ted ap
p rox ima te ly the same tens i le s t reng th ,
bu t the y ie ld st rength averaged about
5 ksi (34 MPa) higher. Fai lure in the
cross-we ld samples was a lways in the
we ld me ta l w i th T i s tab i l i za t ion and in
e i ther the base meta l o r heat-a f fected
zone w i th Ta s tab i l i za t ion .
The e longa t ions exh ib i ted by the
cross-we ld samp les we re reduced to
abou t 20 to 30% in 2 in . (50.8 mm ) f ro m
140 -s I A U G U S T 1 9 81
7/21/2019 Weldability-Ferritic Stainless Steels
http://slidepdf.com/reader/full/weldability-ferritic-stainless-steels 7/8
METALLOGRAPHY
SAMPLE
CHARF
(2-1/2 )
-
6 3 . 5
mm
(2-1/2 )
5.72 mm (.225 )
Fig. 9—Schematic illustration of sample location and sectioning for mecha nical
property determination.
va l u e s o f a b o u t 3 2 t o 4 0 % i n t h e b a s e
m e t a l s a m p l e s . It i s u n c l e a r a s t o
w h e t h e r t h i s d e c r e a s e i n e l o n g a t i o n i s
a r e s u l t o f w e l d i n g , s t a b i l i z a t i o n , o r
a n i s o t r o p y o f t h e m a t e r i a l s .
I n b a s e m e t a l s a m p l e s o f T y p e 444L
n o s i g n i f i c a n t i n c r e a s e i n y i e l d o r t e n
s i le s t r e n g t h s w a s o b s e r v e d f o r a l l o y s
s t a b i l i ze d w i t h T i , T i + N b , o r T i + T a .
T h e t e n s i l e s t r e n g t h r a n g e s f r o m 7 0 t o
7 7 ks i ( 4 8 3 t o 5 3 1 M Pa ) a n d t h e y i e l d
s t r e n g t h f r o m a b o u t 4 6 t o 5 3 ks i ( 3 1 7 t o
3 6 5 M P a ) . T h e t e n s i l e s t r e n g t h i n
c r e a se d f r o m a b o u t 7 0 t o 8 8 ks i ( 4 8 3 t o
6 0 7 M P a ) f o r a l l o y s s t a b i l i z e d w i t h T a
o r T a + N b , w h i l e t h e y i e l d s t r e n g t h
i n c r e a s e d f r o m a b o u t 4 6 t o 6 3 ks i ( 3 1 7
t o 4 2 7 M P a ) f o r a l l o y s s t a b i l i z e d w i t h
Ta.
In a m a n n e r s i m i l a r t o t h e T y p e 4 3 0
a l l o y s , t h e c r o s s - w e l d s a m p l e s o f T y p e
4 4 4 L g e n e r a l l y h a d a s l i g h t l y h i g h e r
y i e l d s t r e n g t h a n d e s s e n t i a l l y u n
c h a n g e d te n s i l e s t r e n g t h w h e n c o m
p a r e d t o t h e b a s e m e t a l s a m p l e s . W i t h
t h e e x c e p t i o n o f a l l o y s s t a b i l i z e d w i t h
T i + T a , a l l o f t h e c r o s s - w e l d s a m p l e s
f a i l e d in t h e w e l d m e t a l , i n d i c a t i n g
t h a t s t a b i l i z e d T y p e 4 4 4 L m a y b e p r o n e
t o w e l d m e t a l f a i l u r e s . T h e e l o n g a t i o n s
o f c r o s s - w e l d s a m p l e s w e r e al s o l o w e r
t h a n t h o s e o f t h e b a s e m e t a l .
Resul ts of Impact Test ing
I n o r d e r t o c h a r a c t e r i z e t h e i m p a c t
p r o p e r t i e s o f t h e s e a l l o y s , t h e i m p a c t
e n e r g y a t 1 4 0 °F (60°C) a n d t h e 1 5 f t - l b
( 20 .3 J) t r a n s i t i o n t e m p e r a t u r e w e r e
o b t a i n e d u s i n g h a l f - s i z e C h a r p y V -
n o t c h s a m p l e s . R e s u l ts f o r t h e T y p e
4 3 0 a l l o y s e x h i b i t m u c h s c a t t e r a n d n o
c l e a r t r e n d s o f t h e e f f e c t o f i n c r e a s i n g
s t a b i l i z a t i o n w i t h e i t h e r T i o r T a . T h e
w e l d m e t a l s a m p l e s , h o w e v e r , c o n s i s
t e n t l y e x h i b i t a l o w e r i m p a c t e n e r g y a t
140°F (60°Q) a n d a h i g h e r 1 5 f t - l b
( 20 .3 J) t r a n s i t i o n t e m p e r a t u r e t h a n
e i t h e r t h e b a s e m e t a l o r h e a t - a f f e c t e d
z o n e s a m p l e s .
T h e i m p a c t e n e r g i e s a t 1 4 0 °F (66°C)
f o r s t a b i l i z e d T y p e 4 4 4 L a l l o y s w e r e a l l
b e l o w t h o s e e x h i b i t e d b y t h e T y p e 4 3 0
a l l o y s . T h e h i g h e s t i m p a c t e n e r g i e s
w e r e e x h i b i t e d b y a l l o y s s t a b i l i z e d
w i t h e i t h e r T a o r T a + N b . L i k e w i s e ,
t h e c o r r e s p o n d i n g 15 f t - l b ( 2 0. 3 J)
t r a n
s i t i o n t e m p e r a t u r e s w e r e e q u a l t o o r
h i g h e r t h a n t h o s e o b s e r v e d f o r t h e
T y p e 4 3 0 a l l o y s .
T h e r e s u l t s i n d i c a t e t h a t t h e T y p e
4 4 4 L a l l o y s a r e n o t a s t o u g h a s t h e
T y p e 4 3 0 a l l o y s a n d t h a t s t a b i l i z a t i o n
w i t h T i s i n g l y o r i n c o m b i n a t i o n w i t h
N b o r T a is d e t r i m e n t a l t o t o u g h n e s s
w h e n c o m p a r e d t o o t h e r s t a b i l i z a t io n
e l e m e n t s . T h i s e ff e c t o f T i o n t o u g h
n e ss o f f e r r i t i c s t a i n l e s s s t e e ls c o n f i r m s
o b s e r v a t i o n s b y B o n d a n d o t h e r s ( R e f .
1 0 ) .
H o w e v e r , i n t h i s i n v e s t i g a t i o n n o
c l e a r t r e n d s w e r e n o t e d s h o w i n g t h a t
t o u g h n e s s d e c r e a s e s a s t h e T i c o n t e n t
i n c r e a se s .
I n t e r g r a n u l a r C o r r o s i o n
Procedures
T h e a ll o y s w e r e e x a m i n e d t o d e t e r
m i n e if t h e y w e r e s u s c e p t i b l e t o i n t e r
g r a n u l a r a t t a c k ( I G A ) o r s e n s i t i z a t i o n
a s a r e s u l t o f w e l d i n g . S h e e t m a t e r i a l s
w e l d e d a s i n T a b l e 4 w e r e s u b j e c t e d t o
P r a c t i c e Z ( c o p p e r - c o p p e r s u l f a t e - 1 6 %
s u l f u r i c a c i d o r M o d i f i e d S t ra u s s t e s t )
o f t h e p r o p o s e d A S T M s t a n d a r d f o r
d e t e c t i n g s u s c e p t i b i l i t y t o i n t e r g r a n u
l a r a t t a ck i n f e r r i t i c s t a i n l e ss s t e e l s . T h e
St rauss t es t is t h e leas t seve re o f t h e
r e c o m m e n d e d t es ts f r o m t h e s t a n d
p o i n t o f w e i g h t l o s s, b u t p r o d u c e s t h e
g r e a t e s t i n t e r g r a n u l a r p e n e t r a t i o n .
T h e S tr a us s t e s t is r e c o m m e n d e d
s p e c i f i c a l l y f o r us e w i t h t h e f e r r i t i c
s t a i n l e s s s t e e l s h a v i n g C r c o n t e n t s o f
1 6 t o 2 0 % ; i t d e t e c t s s u s c e p t i b i l i t y t o
I G A a s s o c i a t e d w i t h o n l y t h e p r e c i p i
t a t i o n o f c h r o m i u m c a r b i d e s a n d / o r
n i t r id e s a n d t h e a c c o m p a n y i n g c h r o
m i u m - d e p l e t e d z o n e s in b o t h s t a b i
l i z e d a n d n o n - s t a b i l i z e d m a t e r i a l s . T h e
p r i m a r y e v a l u a t i o n c r i t e r i o n is a 1 8 0
d e g , 2 -T b e n d tes t ; t o pas s o r t o f a i l i s
b a s e d o n w h e t h e r o r n o t i n t e r g r a n u l a r
f i s s u re s o c c u r i n t h e b e n t r e g i o n .
T h e a x is o f t h e b e n d w a s l o c a t e d a t
t h e in t e r f a c e o f t h e h e a t - a f f e c t e d z o n e
a n d b a s e m a t e r i a l . V i s u a l e x a m i n a t i o n
o f t h e w e l d m e n t f o r g r a i n d r o p p i n g
a n d c o r r o s i o n r a t e d a t a w a s a l s o
u t i
l i z e d .
2.0
.02
0 6
.080 4
C + N , %
Fig. 10—The effect of Ta on the intergranular corrosion resistance of
Types 430 and
4441.
s«
a.
IT
LU
N
_J
m
\
co
1.6
1.2
.8
Ta»27.5(C +
X)
Af Nb + Ta-20( C+N)
X^-'**̂ — —
^
Ti 12.5
Ti + Nb 0
I
9(C4
1
(C + N)
» T l+ l
N)
a =
0 .01 .02 .03 .04 .05 .06 .07 .08
C + N , %
Fig. 11—The effect of stabilization on the intergranular corrosion
resistance of Types 430 and
4441.
W E L D I N G R E S E A R C H S U P P L E M E N T I 1 4 1 - s
7/21/2019 Weldability-Ferritic Stainless Steels
http://slidepdf.com/reader/full/weldability-ferritic-stainless-steels 8/8
Results of Corrosion Testing
The resu l ts o f the in te rgranu lar cor
rosion tests were p lo t ted fo r each
g roup o f ma te r ia l s (examp le shown in
Fig. 10 for the Types 430 and 444L
a l loys stab i l ized wi th Ta). I t can be
no ted tha t a re la t io nsh ip exists be
twe en the in te rs t i t i a l con ten t (C + N )
and the stab i l izer content o f the a l loy.
The measured corrosion ra te o f the
Ta -con ta in ing a l loys was gene ra l l y
h igher, and they appeared to be more
suscep t ib le to g ra in d ropp ing in the
Modi f ied Strauss test .
It
is appa rent tha t a cr i t ic a l stab i
l i ze r / in t e rs t i t i a l ra t io ex is ts fo r w h ic h
acceptab le resistance to in te rgranu lar
corr osio n is ob ta in ed . For the Ta stab i
l ized a l loys, Ta contents
>
27.5
(C + N) con fer ac cepta b le resistance
to in te rgranu lar a t tack. Th is va lue rep
resen ts app rox ima te ly tw ice the Ta
requ i red
stoichiometrically
to t ie up
the C and N as TaC and TaN.
F igure 11 summarizes the resu l ts o f
a l l the in te rgranu lar corrosion tests
co n d u c t e d o n w e l d e d sp e c i m e n s o f
Types 430 and 444L. The c r i t ica l va lues
o f s tab i l i z ing e lemen ts requ i red to
obta in resistance to in te rgranu lar a t
tack fo r these a l loys in the as-we lded
cond i t ion a re as fo l lows:
1. Ta
27.5 C + N)
2. Nb + Ta
>
20.0 (C + N)
3. Ti > 12.5 (C + N)
4. Ti + Nb
>
9.0 (C + N)
The best cho ice o f e lements fo r use
in stab i l iz ing these a l loys is e i ther T i o r
a com b in a t io n o f Ti + Nb .
It shou ld be no ted tha t the T i
c o n
ten t requ i red fo r stab i l iza t ion is
approximate ly 3 .4 t imes the average
factor o f 3 .7 requ i red to t ie up a l l o f
the C and N as TiC and TiN. It is
general ly necessary to use and excess
of T i to ensure stab i l iza t ion and
resistance to
IGA
because the T i may
a l so co m b i n e w i t h t h e
O
a nd S in th e
steel.
C o n c l u s i o n s
1.
Th is inves t iga t io n has resu l ted in
the des ign o f a mo d i f ie d subsca le t rav
e l ing Varestra in t test tha t can be
u t i
l i zed in the s tudy o f the ho t c rack ing
resistance o f meta l l ic mater ia ls. The
ho t c rack ing da ta ob ta ined tend to
agree w i t h the resu lts expe cted f ro m
the gene ra l i zed l i qua t ion theo ry fo r
ho t c rack ing .
2. The ho t c rack ing suscep t ib i l i t y o f
16 to 18% Cr ferri t ic stain less steels is at
leas t pa r t ia l l y dependen t on the chem
ica l cons t i tu t ion . Ho t c rack ing i s p ro
mo te d by the fo l low ing e lemen ts in
the app rox ima te o rde r l i s ted :
S > C > N > N b > T i > P > M n
The C co nte nt is especia l ly cr i t ica l
because o f i ts de t r imen ta l i n f luence
on not on l y ho t crac king , bu t a lso on
m i c r o s t r u c t u r e , d u c t i l i t y , t o u g h n e ss ,
and corrosion resistance o f bo th the
fus ion zone and hea t -a f fec ted zone .
N i t rogen appa ren t l y behaves s im i la r l y ,
and bo th e lemen ts shou ld be l im i ted
to a level less than 0.025%.
Unstabilized
Types 430 and 444L
exh ib i t acceptab le resistance to ho t
crac king fo r (C + N) c onte nts less
than 0 .03% i f the S is ma in ta ine d a t a
low leve l . Acc ep tab le res is tance to ho t
cracking can be ach ieved in Types 430
and 444L stab i l ized w i th T i , Ta ,
Ti + Nb , or Ti + Ta i f th e (C + N )
co nte nt is co ntr o l l ed a t leve ls less than
0.04%.
3. In genera l , we l d i ng caused no
de t r imen ta l e f fec ts on the tens i le
prope rt ies o f Types 430 and 444L. Y ie ld
strengths were increased about 5 ksi
(34 MPa) and e long a t ions w e re de
creased, bu t i t was not c lear whether
th is was a resu l t o f we ld ing o r an iso
t ropy o f the mater ia ls. Some increase
in tensi le st rength was noted in Type
444L stab i l iz ed w i th Ta or Ta + Nb .
Cross-we ld tensi le fa i lu res in Types
430 and
444L
a re genera l ly in the we ld
me ta l ,
part icu la r ly when T i is used fo r
stab i l iza t ion . Type 444L appears to be
prone to we ld meta l fa i lu res. The
impact resu l ts exh ib i ted mu ch sca t te r
but do ind ica te tha t the Type
444L
a l loys are less tough than the Type 430
a l loys; they a lso ind ica te tha t T i stab i l
i za t ion is mo r e de t r im en ta l to tou gh
ness than o the r s tab i l i z ing e lemen ts .
4. Types 430 and 444L w i t h (C + N)
contents less than 0.04% are not sus
cep t ib le to IG A a f t e r w e l d i n g w h e n
stab i l i zed w i th T i acco rd ing to the
f o r m u l a T i
>
12.5 (C + N) or w it h Ta
acc ord in g to the f o rm ula Ta > 27.5
(C + N). Dua l stab i l iza t ion can a lso be
u t i l i zed w i th Type 444L to p reven t
susce p t ib i l i t y to IGA . A l loys success
ful ly pass the Strauss test i f the
(T i + Nb) or (T i + Ta )/ ( C + N) ra
t io > 9 or i f the (N b + Ta) / (C + N)
ratio is B > 20.
In con c lus ion th is inves t iga t ion has
shown tha t fe rr i t ic sta in less stee ls
c o n
ta in ing 16 to 18% Cr can be designe d to
provide adequate resistance to ho t
c r a ck i n g d u r i n g w e l d i n g a n d , by t h e
use o f stab i l iza t ion , re ta in usab le
mechan ica l p ropert ies and resistance
to in te rgranu lar a t tack in the as-
w e l d e d c o n d i t i o n .
Acknowledgments
The au tho rs acknow ledge the sup
po r t and con t r ibu t ions o f the i r c o l
leagues at the Republic Steel Research
Cen ter, in pa rt icu lar, G . D. Ries, J. M .
Haser, and J. B. Lee. They also
ack now ledge L. M . Huse , K. W eed en ,
and M. Dav is who pe r fo rmed the
e xp e r i m e n t a l w o r k .
References
1.
Puzak, P. P. and Rischall, H. 1957.
Further studies on stainless steel hot crack
ing.
Welding lournal
36 (2): 57-s to 61-s.
2.
Hull ,
F. C. 1967. Effect of delta ferrite
on the hot cracking of stainless steel.
Weld
ing lournal
46 (9): 399-s to 409-s.
3. Thielsch, H., 1951. Physical and
we ld
ing metallurgy of chro miu m stainless steels.
Welding lournal
30 (5): 209-s to 250-s.
4. Demo, J. J. 1977.
Structure, constitu
tion, and general characteristics of wrough t
ferritic stainless steels.
ASTM STP 619.
5. The So uthern Cross Steel Com pany
(PTY) Ltd., 1976. Cost saving with stainless
steel. 68.
6. Thielsch, H. 1950. We ld em brittlem ent
in chromium stainless steels.
Welding jour
na l
29 (3): 126-s to 132-s.
7. De mo , J. J. 1977. Mec hanism of high
temperature embrittlement and loss of cor
rosion resistance in AISI Type 446 stainless
steel. Corrosion 27
(12): 531 to 544.
8. Plum tree, A. and C ullb erg , R. 1974. The
influence of interstitial content on the duc
tile-brittle transition temperature of Fe-25
Cr ferritic stainless steels,
journal of Testing
and Evaluation
2 (9): 331 to 336.
9. Wright, R. N. 1971. Mechanical behav
ior and weldability of a high chromium
ferrit ic stainless steel as
a
function of purity.
Welding lournal
50 (10): 434-s to 440-s.
10. Semchyshen, M.; Bond, A. P.; and
Dundas, H. J. 1971. Effects of com position
on ductility and toughness of ferritic stain
less steels.
Toward Improved Ductility and
Toughness
Climax: 239 to 253.
11.
Demo, J. J., and Bond, A. P. 1975.
Intergranular corrosion and embrittlement
of ferritic stainless steels.
Corrosion
31 (1):
21-22.
Authors
See page 154-s in this
issue of the W elding journal