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AN ELECTRON MICROSCOPE STUDY OF
INCLUSIONS IN SUBMERGED-ARC STEEL WELDS
A. Bhatti
To cite this version:
A. Bhatti. AN ELECTRON MICROSCOPE STUDY OF INCLUSIONS IN SUBMERGED- ARC STEEL WELDS. Journal de Physique Colloques, 1984, 45 (C2), pp.C2-639-C2-642.
�10.1051/jphyscol:19842149�. �jpa-00223820�
JOURNAL DE PHYSIQUE
Colloque C2, supplement au n°2, Tome 45, fevrier 1984 page C2-639
AN ELECTRON MICROSCOPE STUDY OF INCLUSIONS IN SUBMERGED-ARC STEEL WELDS
A.R. Bhatti
Department of Metallurgy, Sheffield University, Sheffield SI SJD, U.K.
Résumé - On a examiné par microscopie électronique et par microanalyse, le type et la composition des inclusions trouvées dans des perles de soudure en acier déposées par arc submergé pour différentes combinaisons de flux et de fil. On a montré qu'on obtient de grandes proportions de ferrite aciculaire quand la com- position des inclusions s'approche de AI2O3 stoechiométrique. On a montré en plus qu'on obtient un grand pourcentage de ferrite aciculaire au dessus d'un certain niveau de Ti, quelle que soit alors la teneur de l'inclusion en AI2O3.
Abstract - The type and composition of inclusions found in bead-on-plate submerged- arc welds have been examined by electron microscopy and microanalysis for various flux-wire combinations. It is shown that large proportions of acicular ferrite are obtained when the inclusions Chemistry is close to stoichiometric Al^O,. It is further shown that above a certain threshold level of Ti, a large percentage of acicular ferrite is obtained irrespective of the inclusions Al-O, content.
1. INTRODUCTION
It is becoming widely recognised that inclusions play an important role in the nucleation of tough acicular ferrite constituent in the weld metal microstructure
(1-4). However, it is not clear what type of inclusions are required in order to nucleate acicular ferrite. Thus it was considered worthwhile to study the composition and other features of inclusions in the weld metal. The present work examines bead-on-plate submerged arc welds, made on mild steel and a pair of calciur.
free and calcium treated HSLA steel plates. The fluxes cover basicity indices from O.60 to 4.19. The wires vary from a plain C-Mn to a C-Mn alloyed with Mo to a C-Mn bearing Ti and B additions.
2. EXPERIMENTAL DETAILS
A number of bead-on-plate submerged arc welds were made using 16 mm thick plates, Table la. The fluxes with a basicity range from 0.60 to 4.19 were selected.
The analyses of the wires used are given in Table 2. All the welds were deposited at 32V and 600A at 460 mm/min travel speed, giving a heat input of about 2.52 kJ/mm.
An optical microscope examination of the transverse weld sections, including quantification of various phases, was conducted. The type and composition of inclusion were examined by Philips 400 T microscope (fitted with EDAX and STEM facilities), using carbon extraction replicas.
3. RESULTS AND DISCUSSION
3.1. Weld Metal Composition and Microstructure.
The results of the chemical analyses of the weld metals are presented in Table (lb). The typical microstructural features identified in the welds were:
a) Acicular ferrite, b) Grain boundary ferrite, c) Widmanstatten ferrite side plates and d) Polygonal ferrite. The percentage of acicular ferrite (AF) for the welds studied is listed in Table (3).
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19842149
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3.2. I n c l u s i o n S t u d i e s .
TEM showed t h a t m a j o r i t y of t h e i n c l u s i o n s a r e g l o b u l a r , t h e mean s i z e ?.0.5~m F i g 1 . I n welds B and V, g l o b u l a r i n c l u s i o n s were o f t e n supplemented by
g e o m e t r i c a l l y shaped i n c l u s i o n s which when analysed, were found t o be h i g h l y r i c h i n A 1 ( c r y s t a l s t r u c t u r e , Y-A1203). Secondary e l e c t r o n imaging r e v e a l e d duplex n a t u r e o f some o f t h e i n c l u s i o n s . The c o r e had A l , Mn, S i w i t h small amounts of Ti while t h e o u t e r c o a t i n g was r i c h i n Cu and S. This i s c o n s i s t e n t with t h e m e l t i n g p o i n t c o n s i d e r a t i o n s of t h e i n c l u s i o n c o n s t i t u e n t s . I t seems t h a t t h e c o r e forms a t comparatively high temperatures which t h e n a c t s a s n u c l e i f o r t h e formation of s u l p h i d e s .
The r e s u l t s o f t y p i c a l i n c l u s i o n composition a n a l y s e s i n t h e weld m e t a l s a r e given i n Table ( 4 ) . A s t h e average i n c l u s i o n composition f o r a l l t h e welds e s s e n t i a l l y l i e s w i t h i n t h e MnO-A1 0 -Si02-Ti02 system, i t can be r e p r e s e n t e d i n a t e r n a r y diagram a s shown i n Fig. (1)! Ti02 because of i t s r e l a t i v e l y small amounts has been i n c l u d e d w i t h S i 0 2 . I t i s worth n o t l n g t h a t t h e r a t i o of MnO/Si02+Ti0
i s almost c o n s t a n t . 2
Data p r e s e n t e d i n Table ( 4 ) and Fig. ( 2 ) , show t h a t t h e high p r o p o r t i o n of a c i c u l a r f e r r i t e a r e found f o r t h e welds which have h i g h A1203 c o n t e n t i n t h e i n c l u s i o n s . I t would t h e r e f o r e seem t h a t t h e g r e a t e s t amount of a c i c u l a r f e r r i t e
w l l l be o b t a i n e d when t h e ~ n c l u s i o n s approach p u r e alumina i n composition. Indeed
t h e c a l c u l a t i o n s show t h a t i n t h e weld B, which c o n t a i n s t h e l a r g e s t amount of a c i c u l a r f e r r i t e i n t h e welds s t u d i e d , t h e l e v e l of A 1 i s approximately e q u a l t o t h a t r e q u i r e d t o combine w i t h a l l t h e oxygen i n t h e weld t o form A 1 03. This r e s u l t i s borne o u t by r e c e n t work of Terashima and H a r t (3) where i t can l?e seen t h a t optimum toughness i n weld metal i s o b t a i n e d w i t h approximately s t o i c h i o m e t r i c A 1 and oxygen c o n t e n t s f o r t h e formation of A 1 0 . There i s an a p p a r e n t anomaly i n t h e c a s e of welds L and V which d e s p i t e t h e i r i d e n z i c a l l e v e l s of a c i c u l a r f e r r i t e show a 2 s u b s t a n t i a l d i f f e r e n c e i n t h e A1203 c o n t e n t of t h e i r i n c l u s i o n s . An examination of t h e t a b l e ( 4 ) , shows t h a t t h e inclusions i n weld L c o n t a i n c o n s i d e r a b l y more T i 0
(".12wt%) t h a n t h o s e i n t h e weld V (Ti02 = 1.90 w t % ) . I t i s i n f e r r e d t h a t above a 2 c e r t a i n t h r e s h o l d l e v e l of Ti02 i n t h e weld m e t a l i n c l u s i o n s , high p r o p o r t i o n s of AF can b e o b t a i n e d i r r e s p e c t i v e o f t h e l e v e l o f A1203 i n t h e i n c l u s i o n s .
D i f f r a c t i o n s t u d i e s showed t h a t t h e i n c l u s i o n s w i t h low A1203 c o n t e n t a r e amorphous while t h o s e r i c h i n A 1 O3 tend t o be c r y s t a l l i n e . The c r y s t a l l i n i t y of i n c l u s i o n s can have a p a r t i c u l a r ? y p o t e n t e f f e c t ( v l a c r y s t a l l o g r a p h i c matching between t h e f e r r i t e and i n c l u s i o n s ) i f some e p i t a x i a l n u c l e a t i o n mechanism of AF formation i s o p e r a t i v e ( 5 ) .
CONCLUSIONS
Large p r o p o r t i o n s of a c i c u l a r f e r r i t e a r e o b t a i n e d when t h e i n c l u s i o n comp- o s i t i o n i s c l o s e t o s t o i c h i o m e t r i c A 1 03. A t a c r i t i c a l amount of T i i n t h e i n c l u s i o n s , a t o l e r a n c e t o t h i s s t o i c i i o m e t r i c composition i s p o s s i b l e . ACKNOWLEDGEMENTS
Thanks a r e due t o Oerlikan I n t e r n a t i o n a l f o r supplying some of t h e weld d e p o s i t s and D r . J . A . Whiteman f o r u s e f u l d i s c u s s i o n s .
REFERENCES
1. Abson D . J . , e t a 1 - Welding I n s t i t u t e Research Report 67/1978/M.
2. B h a t t i A.R., e t a 1 - 64th Annual AWS Convention - P h i l a d e l p h i a A p r i l 1983, paper 26D.
3. Terashima H. and H a r t P.H.M. - 6 4 t h Annual AWS Convention - P h i l a d e l p h i a A p r i l 1983, paper 26C.
4. Ricks R . A . , e t a 1 - J o u r n a l of M a t e r i a l s Science, 1982, c, 732-740.
5. Koukabi A.H., e t a 1 - Metal C o n s t r u c t i o n 1979 1 1 ( 1 2 ) , 639-644.
TABLE l(a): Base Plate Composition, wt% TABLE l(b): Weld Metal Com~osition
Plate Code *P1 (Ca-Free *~2 ($'?&ted)o.060 P3
(Mild Steel)
*~ccording to Ref. (2)
C 0.052 0.160 TABLE 2: Wire Composition, wt%
S 0.005 0.006 0.023 - Wire *S2
P 0.018 0.024 0.015 C 0.10
Cr -
Mo -
Mn 1.37 1.45 1.38 - B -
Mn 1.06
Cu -
Ti -
Si 0.224 0.142 0.30 Si 0.22
Ni 0.003 0.003 0.03 Ni -
S 0.012
Cr 0.32 0.32 <0.02 P 0.013
Mo 0.012 0.007 (0.02
Cu 0.34 0.32 0.03
V 0.069 0.070 <0.02
Ti - - -
Nb 0.036 0.032 0.03
A1 0.054 (sol) 0.025 (sol) 0.025
N 30 60 77
O2 - - 14
IJOURNAL DE PHYSIQUE
,
Fig.1. A typical C-replica micrograph 5.2K
TABLE 4: Inclusion Compositions, wt%
1 I 1 I 1
