DOI:10.1515/adms‐2016‐0014
S.P. Sridhar
1
, S. Arun Kumar
2
, P. Sathiya
1
*
1
Department of Production Engineering, National Institute of Technology. Tiruchirappalli-
620015, Tamilnadu, India
2
School of mechanical engineering, SASTRA University, Thanjavur-613401, Tamilnadu, India
* psathiya@nitt.edu
A STUDY ON THE EFFECT OF DIFFERENT ACTIVATING FLUX ON
A-TIG WELDING PROCESS OF INCOLOY 800H
ABSTRACT
This study investigates the effect of different activating flux such as V
2
O
5
, TiO
2
, MoO
3
, Cr
2
O
3
, and Al
2
O
3
on A-
TIG welding process of
Incoloy 800H. The influence of the flux on the depth of penetration and on mechanical
and metallurgical characteristics of the weld were studied and compared with autogeneous TIG welds which
were welded with the same process parameters and conditions. The use of TiO
2
flux gave full depth of
penetration and the use of V
2
O
5
, Cr
2
O
3
flux gave increased penetration as compared to autogeneous TIG welds
while the use of Al
2
O
3
and MoO
3
led to the detoriation of the effect.
Key words: A-TIG, Incoloy 800H, Activating Flux, Carbide, Corrosion
INTRODUCTION
Incoloy 800H (UNS N08810), which comes under the group of austenitic nickel-iron
chromium steel, exhibits resistance to the carburization, corrosion and oxidation at high
temperature. The combination of appreciable strength and corrosion resistance in a series of
elevated temperature makes this alloy useful in many applications such as cracker tubes in
petrochemical furnace, headers and pigtails, fourth generation nuclear power plant
components, equipment and furnace components etc. [1]. GTAW process is preferred for
producing high quality weld joint where control of weld bead and shape is possible ensuring
the metallurgical stability of the welded joints. On the other hand the major disadvantages of
TIG welding process are the low productivity due to the number of passes required when
welding thick sections and the requirement of highly skilled welders. To overcome these
disadvantages, A-TIG welding process, a variant of TIG welding process, was developed by
Paton Electric Welding Institute (1960). In this process a thin layer of fluxes (oxide, halide,
fluoride and chloride) was applied on the region where the weld was to be made. The
activating flux improved the depth of penetration up to 300% as compared to autogeneous
TIG welding [2]. The reversed Marangoni convection and the arc constriction effect were
considered as the two major mechanisms for the increased depth of penetration in A-TIG
welding process [3]. But research is still going on to find the exact mechanism which causes
S.P. Sridhar, S. Arun Kumar, P. Sathiya: A study on the effect of different activating flux on … 27
the depth of penetration. A-TIG welding process was carried out in Nimonic 263 alloy with
three fluxes and the results suggested that as the thickness of the flux coating increased, the
depth of penetration decreased. The fluxes did not decompose fully and this affected the
viscosity of the molten metal flow and hence resulted in lower depth of penetration [4]. A-
TIG welding was done on stainless steel (0Cr18Ni9) with two different activating fluxes SiO
2
and TiO
2
. The increased depth of penetration using SiO
2
flux was due to arc constriction
effect and reversed Marangoni convection whereas the increase in depth of penetration by
using TiO
2
flux was only due to reversed Marangoni convection [5]. Sakthivel et al.
compared the creep strength of TIG & A-TIG welded austenitic 316LN steel and concluded
that the A-TIG weldments had lower creep rupture life than the TIG weldments because the
TIG welded joints had lower weld strength reduction factor as compared to the A-TIG joints
[6]. Devendranath Ramkumar et al. performed A-TIG welding on 904L super austenitic
stainless steel with the use of 85% SiO
2
+ 15% TiO
2
and reported that the weld penetration
increased three times as compared to the autogeneous TIG weld. A-TIG weldments failed in
the weld zone in tensile testing due to the lower hardness in the weld region as compared to
the base material and HAZ [7]. Monoj Kumar et al. investigated the tensile properties of A-
TIG in Inconel 718 joints welded using SiO
2
& TiO
2
flux. The TiO
2
A-TIG weldments had
higher tensile strength as compared to SiO
2
weldments and also the A-TIG joints had higher
strength than that of the parent material [8]. Shyu et al. reported that the increased δ ferrite
content in the weld zone of A-TIG welded austenitic stainless steel improved the mechanical
properties such as ductility, hardness and strength than the TIG welded joints [9]. The
combination of 50% SiO
2
+ 50% MoO
3
and SiO
2
flux reduced the hot cracking susceptibility
on Inconel 718 weldments and the higher depth to width ratio was obtained using 50% SiO
2
+
50% MoO
3
and 50% SiO
2
+ 50% NiO flux combination [10]. Sairam et al. performed
dissimilar welding of Incoloy 800H and austenitic stainless steel (321) by using two different
filler material, Inconel 617 and Inconel 82 [11]. In this work an attempt is made to find the
effect of different activating flux on A-TIG welding process of Incoloy 800H. The depth of
penetration, mechanical and metallurgical characteristics of the weldments were analysed.
EXPERIMENTAL PROCEDURE
Incoloy 800H of dimension 100x75x4 mm was chosen as the base material. The chemical
composition of Incoloy 800H is shown in Table 1. Five different fluxes (TiO
2
, Cr
2
O
3
, MoO
3
,
V
2
O
5
, and Al
2
O
3
) were mixed with acetone in the ratio of 1:6.5 separately to form a paste. A
thin layer of flux coating was applied on the region where the weld was to be made. Prior to
the application of flux, the base material was cleaned with emery sheets and acetone to keep
the surface away from contaminants. A-TIG welding and autogeneous TIG welding were
performed using Lincoln TIG welding machine and the process parameters used to complete
the weld are shown in Table 2. After welding the weldments were cut in the cross section of
the weldments using a wire cut EDM machine to analyse the effect of different flux on the
mechanical and metallurgical characteristics of the weldments. The schematic representation
of coupons obtained from the A-TIG weldments for mechanical and metallurgical
characterization is shown in fig. 1. The cross sectioned weldments were polished with
different grade of emery sheets followed by alumina and diamond polishing to get mirror
finish. The etchant with the combination of 15 ml HCL + 10 ml HNO
3
+ 10 ml CH
3
COOH
was used to reveal the macro and microstructure of the welded joints. The depth and width of
28 ADVANCES IN MATERIALS SCIENCE, Vol. 16, No. 3 (49), September 2016
the weld bead were measured using IMAGE-J software. The SEM with EDS analysis was
made using Carl Zeiss σ version machine in high vacuum mode. The hardness was measured
using Vickers micro hardness tester, loaded with 500 g for a dwell time of 15 s transverse
cross-section of the weldments at different locations.
Table 1. Chemical elements of Incoloy 800H
C Mn S Si Cu Cr Fe Al Ti Ni
0.065 0.688 <0.010 0.094 0.091 20.79 46.60 0.277 0.280 30.65
Table 2. Welding parameters
Parameters Values Units
Welding Current 100 Amps
Arc Length 2 mm
Shielding Gas Argon 99.999% pure
shielding gas flow rate 15 lpm
Electrode Diameter 2.4
Welding Speed 50 mm/min
Power Polarity DCEN
Electrode type 2%-wt ThO
2
thoriated tungsten.
Fig. 1. Schematic representation of coupons obtained from the A-TIG weldments for metallurgical and mechanical
characterization studies
S.P. Sridhar, S. Arun Kumar, P. Sathiya: A study on the effect of different activating flux on … 29
RESULT AND DISCUSSION
Macrostructure of the weld region
a)Autogenous TIG weld b) A-TIG weld with V
2
O
5
d) A-TIG weld with Cr
2
O
3
e) A-TIG weld with TiO
2
f) A-TIG weld with MoO
3
g) A-TIG weld with Al
2
O
3
Fig. 2. Macrostructure of the weldments
Fig. 2 shows the macrostructure of the autogeneous TIG and A-TIG weldments. From the
figure 2 (a) it can be seen that the autogeneous TIG gave a weld depth of 2.494 mm.
Compared to the autogeneous weld zone depth, the use of TiO
2
flux (fig. 2 (e)) gave a full
30 ADVANCES IN MATERIALS SCIENCE, Vol. 16, No. 3 (49), September 2016
depth penetration and V
2
O
5
, Cr
2
O
3
flux (fig. 2 (b & d)) gave a deeper penetration. The use of
Al
2
O
3
flux (fig. 2 (g)) gave a lesser depth of penetration. Most likely the increased depth in
penetration was due to the reversed Marangoni convection which relates the penetration depth
to the fluid flow direction. The molten fluid flow direction is determined by the temperature
co-efficient of surface tension. In TIG welding the convection movements were centrifugal
and surface tension gradient in negative led to shallow penetration. The addition of activating
flux improved the oxygen content which induced an inversion of the convection currents
changing the sign of the surface tension gradient from negative to positive and the resulting
convection current movements changed to centripetal. Hence the depth of penetration
increased [12]. Also the depth of penetration depended upon the interaction between base
material composition and the flux, the type of flux used and the process parameters involved
during the welding [9]. The use of MoO
3
flux led to hot cracking issue in Incoloy 800H
whereas this defect was not reported in materials such as duplex stainless steel, Inconel 718
and 316L austenitic stainless steel [13-16]. Hot cracking is a welding defect that occurs with
temperature higher than 1200⁰ C and it is due to the presence of impurities with lower melting
than the base metal. The cooling is greater on the weld region and it is from there that grain
grows forwards the centre of the weld which is the last to become solid. Since almost all
alloys, solidify over a range of temperature, the first metal to solidify will have higher melting
point and the last will have the lowest melting point. As a consequence of this, the lowest
melting point composition is pushed ahead of the solidifying dendrite until it gets trapped
between the adjacent dendrites, along the grain boundaries. If the difference in melting point
between the lowest melting point constraint and rest of the bulk material is sufficiently great,
the liquid films by the grain boundaries will be separated as cools and contracts leading to hot
cracking. Some theories suggest that hot cracking is dependent on the metallurgical and
mechanical factors that are supposed to be independent of each other [17-18]. Further studies
have also shown that cracking could also be initiated at the early stage of solidification [19].
Microstructure of the base material
Fig. 3. Microstructure of the base material