Scholarly article on topic 'Effect of Ternary Fluxes on Depth of Penetration in A-TIG Welding of AISI 409 Ferritic Stainless Steel'

Effect of Ternary Fluxes on Depth of Penetration in A-TIG Welding of AISI 409 Ferritic Stainless Steel Academic research paper on "Materials engineering"

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{A-TIG / "AISI 409 stainless steel" / "single component flux" / "ternary flux"}

Abstract of research paper on Materials engineering, author of scientific article — G. Venkatesan, Jimin George, M. Sowmyasri, V. Muthupandi

Abstract TIG welding is a preferred process for producing quality weld joints. However, because of its characteristic shallow depth of penetration, productivity achieved by this process is low. Depth of penetration of conventional TIG welding can be improved by Activated TIG (A-TIG) welding. In A-TIG welding, a thin layer of flux comprising mixtures of oxides, fluorides and chlorides is applied in the area to be welded to obtain two to three fold increase in depth of penetration. In this study, three single component fluxes viz., SiO2, TiO2 and Cr2O3 were employed for welding of AISI 409 ferritic stainless steel. Experimental trials were designed for the use of ternary fluxes. From the obtained results on bead geometry, using Minitab 16 software coupled with Matlab R2010 an optimum combination of these fluxes for maximum penetration was estimated. Confirmation test showed that two fold increase in depth of penetration could be achieved in A-TIG welding by using this flux combination over conventional TIG welding.

Academic research paper on topic "Effect of Ternary Fluxes on Depth of Penetration in A-TIG Welding of AISI 409 Ferritic Stainless Steel"

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Procedia Materials Science 5 (2014) 2402 - 2410

International Conference on Advances in Manufacturing and Materials Engineering,

AMME 2014

Effect of ternary fluxes on depth of penetration in A-TIG welding of

AISI 409 ferritic stainless steel

G. Venkatesan*, Jimin George, M. Sowmyasri and V. Muthupandi

National Institute of Technology, Tiruchirapalli, Tamil Nadu, 620015, India,

Abstract

TIG welding is a preferred process for producing quality weld joints. However, because of its characteristic shallow depth of penetration, productivity achieved by this process is low. Depth of penetration of conventional TIG welding can be improved by Activated TIG (A-TIG) welding. In A-TIG welding, a thin layer of flux comprising mixtures of oxides, fluorides and chlorides is applied in the area to be welded to obtain two to three fold increase in depth of penetration. In this study, three single component fluxes viz., Si02, Ti02 and Cr203 were employed for welding of AISI 409 ferritic stainless steel. Experimental trials were designed for the use of ternary fluxes. From the obtained results on bead geometry, using Minitab 16 software coupled with Matlab R2010 an optimum combination of these fluxes for maximum penetration was estimated. Confirmation test showed that two fold increase in depth of penetration could be achieved in A-TIG welding by using this flux combination over conventional TIG welding.

© 2014PublishedbyElsevierLtd.Thisisanopenaccess article under the CC BY-NC-ND license (http://creativecommons.Org/licenses/by-nc-nd/3.0/).

Selection and peer-review under responsibility of Organizing Committee of AMME 2014

Keywords: : A-TIG; AISI 409 stainless steel; single component flux; ternary flux

1. Introduction

Tungsten inert gas welding which is also known as TIG welding is widely applied in the stainless steel fabrication industry. It has become the most widely accepted choice of welding process when a high level of weld quality or considerable precision in welding operation is required (Shyu et al. 2008). The primary limitation of TIG welding is low productivity because of its low deposition rate and shallow joint penetration. To overcome this limitation a novel variant of TIG welding process known as Activated- TIG welding is being practiced. Initially A-TIG was proposed by researchers at the Paton Welding Institute in Ukraine in 1960s to ensure consistency of penetration in welding of titanium alloys with TIG welding process (Gurevich et al. 1965). The depth and consistency of penetration in TIG welds improved by applying a thin layer of activated flux in the form of paste

2211-8128 © 2014 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).

Selection and peer-review under responsibility of Organizing Committee of AMME 2014 doi: 10.1016/j.mspro.2014.07.485

* Corresponding author. Tel.: +91-9976615998 E-mail address: venkimech.trp@gmail. com

to the work piece surface prior to welding. The usage of activated fluxes eliminates the need for edge preparations in welding plates that calls for two or three passes in conventional TIG welding and thus results in increased productivity due to the reduction in the number of weld passes required to make the joint (M Vasudevan).

Improvement in depth of penetration in A-TIG can mainly be attributed to two types of mechanisms: one based on the weld arc behaviour and the other on Marangoni convection effect. In the theory of arc constriction proposed by Howse and Lucas (Zhang et al. 2011) the activated flux which gets ionized during welding will constrict the arc by capturing electrons in the outer region of the arc and restrict the current flow to the central region. This increases the current density in the plasma and resulting in a narrow arc and a deeper weld pool. Heiple and Roper proposed that the presence of activated fluxes causes reversal of normal outward pulling surface tension gradient to inward pulling surface tension gradient thereby producing a strong inward fluid flow. This mode of convection current can transfer heat towards bottom and produce deep and narrow weld pool (Tseng and Hsu 2011; Qing-ming et al. 2007). Studies conducted by Yushchenko et al. (2005) on the effect of 32 different oxide fluxes on A-TIG welding of stainless steel have shown that Ti02, Si02 and Cr203 are the best performing fluxes when considering depth of penetration.

Based on this suggestion, in the present investigation, the effect of three single component activating fluxes viz., Si02, Ti02 and Cr203 and ternary activating fluxes that are mixtures of Si02, Ti02 and Cr203 on weld morphology of AISI 409 stainless steel is studied and the results are presented and discussed.

2. Experimental procedures

2.1. Material

The material used in this study was AISI 409 ferritic stainless steel and its chemical composition is listed in Table 1. Plates cut to have dimensions of 450mm x 55mm x 8mm were used for autogenous TIG welding trials. Commercially available Si02, Ti02 and Cr203 in powder form were used as flux constituents.

Table 1. Chemical composition of the AISI 409 steel plate

ELEMENTS WEIGHT %

C 0.025

Ni 0.12

Cr 12.9

Si 0.919

Mn 0.854

P 0.0189

s 0.016

Nb 0.0097

V 0.0487

Fe Balance

2.2. Weldingprocedure

To study the effect of single and multicomponent fluxes on depth of penetration, three single component fluxes viz., Ti02, Si02 and Cr203 and 16 different combinations of these fluxes were employed. These flux combinations were made based on the trial plans suggested by Minitab 16 software. Welding trials were carried out as autogenous TIG melt runs in flat position with a torch angle of zero degree using a HOBART CYBERWAVE 300S power source. The size of the coupons used for getting the weld bead was 70 mm X 55 mm X 8 mm. Just prior to welding trials specimens were roughly polished with abrasive paper to remove surface impurities and then cleaned with acetone. Plates were rigidly clamped to avoid distortion during welding.

Before welding, the flux powders (Ti02, Si02 and Cr203) were weighed to required proportions and mixed with acetone to produce a paint-like (slurry) consistency. A uniform thin layer of activated flux was applied manually with a paint brush on the surface, where the melt run was to be made. The autogenous melt runs were made with an automatic control system in which the torch was moved at a constant speed. The welding parameters employed in this investigation are given in Table 2.

Table 2. Welding parameters usedfor autogenous meltruns.

Welding speed 80 mm/min

Polarity DCEN

Welding current 160 A

Shielding gas 99.9% pure Argon

Shielding gas How rate 15 1pm

Electrode 4 mm diameter

2% thoriated tungsten

3. Results and discussions

3.1.Bead appearance

Usage of fluxes produced an inferior weld bead appearance compared to conventional TIG welding and left a surface slag residue, that was to be removed after welding. With increase in Si02 content in the mixture, weld bead appearance was improved. But with increase in Ti02 content in the flux, the bead appearance became rather poor. With the addition of Cr203 in the flux, surface finish obtained was between the result of Si02 and Ti02.

3.2. Bead profile

After the completion of welding, specimens were cut across the weldment to observe the weld profile. Specimens were polished using different grades of emery papers with increase in fineness in sequence and using alumina and diamond pastes in succession and then etched with a mixture of HN03 and HF.

The macro structures were recorded using a stereo zoom microscope fitted with image capturing facility. Depth of penetration and bead width were measured using Image J software. Macrostructures of weldments obtained with different combinations of fluxes in their standard order are shown in Fig. 2 Depth of penetration, bead width and aspect ratio of the weld bead obtained are tabulated in Table 3.

3.3. Heat affected zone

The heat affected zones (HAZ) close to the face of the weld beads are more or less of same width in all the cases as the width of the plate used in all the trials are same. Since the heat flow in the thickness direction is restricted by the small thickness of the plate in all the cases, HAZ width is increased considerably at the root of the weld. Asymmetric bead structure is observed in the melt runs produced using fluxes devoid of Si02.

3.4. Influence of fluxes on bead geometry

Aspect ratio of a weld bead i.e., the depth to width ratio is an important factor as it indicates the tolerance for the deviation in weld fit up and edge preparation. Wider the beads better the tolerance. Generally for a given heat input wider the bead shallower the depth of penetration and therefore more number of passes are required to weld thick plates which in turn can affect productivity. Any effect that can improve the depth of penetration without reduction in bead width is desirable as it can improve the productivity without compromising on the tolerance for deviation in edge preparation.

In the present study on welding of 409 stainless steel for the given heat input, conventional TIG produced a

weld metal having 11.5 mm bead width and 3.5 mm depth of penetration.

For weld metals produced with 100% Si02, Ti02 or Cr203 the bead width is nearly the same. However, there is a noticeable increase in depth of penetration with the use of single component fluxes. With the addition of Ti02 to Si02 (standard order 2) width is reduced to a greater extent than with the addition of Cr203 (standard order 3) and corresponding increase in depth of penetration could be noticed.

On addition of Si02 to Cr203 aspect ratio is increased with associated reduction in bead width (standard order 10), but similar effect is not observed with Ti02 addition to Cr203 (standard order 14). The influence of Ti02 on bead width with the addition of Si02 (standard order 7) and Cr203 (standard order 12) is not significant, however depth of penetration increases with Si02 addition and decreases with addition of &2O3. The bead width obtained with a flux mixture having all the three components in equal proportion (standard order 16) is same as that of 100% Si02 and 100%Ti02- Similarly other trials also show that there is an interaction effect among the flux constituents in deciding the bead profile

3.5. Mixture contour plot

Mixture contour plot for depth of penetration generated using Minitab software is shown in Fig. 1. The iso contour lines and the regions confined by them indicate the compositional fields that can produce different range of depth of penetration. These field covered by the various regions identified different colours in the mixture contour plot. In the plot, compositions of flux mixtures used in the experiments are indicated with black dots. The maximum possible depth of penetration could be achieved without even using Cr203 and made rich in either Si02 or Ti02. However, the plot shows that the depth of penetration is minimum for fluxes having near equal percentages of Si02 and Ti02. Similarly almost for entire range of binary compositions of Cr203 and Ti02 the achievable depth of penetration predicted to be poor. From mixture contour plot it can be seen that maximum depth of penetration can be attained by employing fluxes having 75-90% Si02 and the rest Ti02.

Fig. 1. Mixture contour plot showing depth of penetration

Table 3. Bead profile obtained with different composition of fluxes

Standard

Order % Composition of flux Weld Bead Dimensions

SÍ02 TÍ02 Cr203 Depth of Penetration (mm) Bead Width (mm) Aspect Ratio

1 100 0 0 4.36 10.31 0.42

2 75 25 0 6.51 8.62 0.76

3 75 0 25 5.64 9.98 0.57

4 50 50 0 3.61 11.53 0.31

5 50 25 25 5.39 9.22 0.58

6 50 0 50 6.01 9.13 0.66

7 25 75 0 5.78 10.93 0.53

8 25 50 25 4.13 11.19 0.37

9 25 25 50 5.49 9.57 0.57

10 25 0 75 5.78 9.78 0.59

11 0 100 0 4.38 10.26 0.43

12 0 75 25 3.98 10.96 0.36

13 0 50 50 3.89 10.75 0.36

14 0 25 75 4.21 11.23 0.37

15 0 0 100 5.28 10.28 0.51

16 33.333 33.333 33.333 4.79 10.03 0.48

17 66.667 16.667 16.667 6.03 9.13 0.66

18 16.667 66.667 16.667 5.51 10.19 0.54

19 16.667 16.667 66.667 5.52 9.73 0.57

20 0 0 0 3.45 11.37 0.30

3.6 Mathematical modelling and optimisation

The response function representing depth of penetration can be expressed as P = f (Si02, Ti02, Cr203), where P is the response. The mathematical model to establish the relationships between input and output parameters is developed using Minitab 16 software at a confidence level of 95%, based on the experimental data collected. In this model, depth of penetration is expressed as a non-linear function of percentage combination of Si02, Ti02 and Cr203.

The regression equation thus obtained with a prediction accuracy of 82% is as follows:

P= DEPTH =1.47*x(l )+ 1.48*x(2) + 1.73*x(3) -0.38*x(l)*x(2) + 0.54*x(l)*x(3)-0.41*x(2)*x(3) -0.44*x(l)*x(2)*x(3) + 0.12*x(l)*x(2)*(x(l) - x(2)) + 0.82*x(l)*x(2)*x(3)*x(3)+0.64*x(l)*x(2)*(x (l)-x(2))A2 where x (1) =Si02, x (2) =Ti02and x (3) = Cr203.

From the mathematical model developed, optimized composition of the flux to yield maximum depth of penetration is predicted by using optimization toolbox of Matlab R2010a software. The prediction is based on Genetic algorithm and the optimum composition is estimated to be 87.23%Si02+12.76% Ti02 + 0%Cr203. For the optimized flux composition the estimated depth of penetration is 7.48 mm.

STANDARD ORDER: 1

STANDARD ORDER: 3

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r «f» | ' ' f^ M v ftW '"l? r y, y ^ « . •X » 9 4 « / ( % K t—

STANDARD ORDER: 5

STANDARD ORDER: 2

STANDARD ORDER: 4

STANDARD ORDER: 6

STANDARD ORDER: 8

STANDARD ORDER: 10

STANDARDORDER: 13

■Maarr«! gnats'----Ç'

STANDARDORDER: 15

STANDARD ORDER: 12

Fig. 2 (Standard Order 1 to 20). Macrostructures of weldments produced by different composition of fluxes (Etchant: 1:1 mixture of nitric acid

and hydrofluoric acid)

3.7. Confirmation Test

A-TIG welding is performed under identical conditions of the earlier tests, using the flux having the optimized composition and the macro structure of this weldment is shown in Fig. 3. The bead profile data of this weldment are tabulated in Table 4.

Optimized multicomponent flux (87.23%Si02+12.76%Ti02+ 0% Cr203) is expected to give 7.48 mm penetration whereas, the resulted depth of penetration is only 6.31 mm. However, the difference between the expected and resulted values is 15.6% which is well within the prediction accuracy of 82%, where maximum error allowed is 18%.

Table 4. Weld bead profile obtained with optimum composition of flux % Composition of Flux Weld Bead Dimensions

Bead Width Depth of Aspect

Si02 Ti02 Cr203 (mm> Penetration (mm) Ratio

87.23 12.76 0 8.98 6.31 0.70

Fig. 3. Macrostructuxe of weldment produced using the flux having optimized composition

4. Conclusion

• The A-TIG process is capable of increasing the productivity and quality of conventional TIG welding in a simple and reproducible manner which will have practical industrial use.

. The results of this work show that the use of a flux can greatly increase the depth of penetration in TIG welding; in the present investigation nearly 86% increase in depth of penetration has been observed.

• Study on the effect of multicomponent fluxes on A-TIG welding reveals that Si02 flux is having maximum influence in improving depth of penetration though the effect of the flux having 100% Si02 on depth of penetration, is not very significant.

References

Shyu, S.W., Huang, H.Y., Tseng K.H., and Chou, C.P., 2008. Study of the Performance of Stainless Steel A-TIG Welds. JMEPEG, ASM International, 17:193-201.

Gurevich, S.M., Zamkov, V.N, Kushnirenko, N.A., 1965. Improving the penetration of titanium alloys when they are welded by argon tungsten arc process. AvtomaticheskayaSvarka 9, 1-4.

M. Vasudevan, Penetration Enhancing Activated Flux for TIG Welding of Stainless Steels, TECHNOLOGY-7, 182-183.

Rui-Hua ZHANG., Ji-Luan PAN., and Seiji Katayama., 2011. The mechanism of penetration increase in A-TIG welding. Front. Mater. Sci. 5(2): 109-118

Kuang-Hung Tseng., and Chih-Yu Hsu., 2011. Performance of activated TIG process in austenitic stainless steel welds. Journal of Materials

Processing Technology 211, 503-512. LI Qing-ming., WANG Xin-hong., ZOU Zeng-da., WU Jun., 2007. Effect of activating flux on arc shape and arc voltage in tungsten inert gas welding. Trans. Nonferrous Met. Soc. China 17,486-490.

Yushchenko, K.A., Kovalenko, D.V., Kovalenko, I.V., 2005. Peculiarities of A-TIG welding of stainless steel. 7^ International Conference on trends in Welding Research, May 16-20,Georgia,USA