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Materials and Design 25 (2004) 19–23 0261-3069/04/$ - see front matter 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2003.07.004 Experimental investigation of the effect of hydrogen in argon as a shielding gas on TIG welding of austenitic stainless steel Ahmet Durgutlu* Gazi University, Technical Education Faculty, Ankara, Turkey Received 6 February 2003; accepted 30 July 2003 Abstract In this study, the effect of hydrogen in argon as shielding gas was investigated for tungsten inert gas welding of 316L austenitic stainless steel. The microstructure, penetration and mechanical properties were examined. Pure argon, 1.5%H –Ar and 5%H –Ar 2 2 were used as shielding gas. The highest tensile strength was obtained from the sample which was welded under shielding gas of 1.5%H –Ar. After bending test, cracks, tearing and surface defection were not observed on the samples that were welded under 2 all three shielding media. Mean grain size in the weld metal increased with increasing hydrogen content. Additionally, weld metal penetration depth and its width increased with increasing hydrogen content. 2003 Elsevier Ltd. All rights reserved. Keywords: TIG Welding; Shielding gas; Mechanical properties; Microstructure 1. Introduction In the fabrication of equipment made from stainless steels such as pipe, automotive exhaust gas system, chemical industrial equipment, etc., arc welding using shielding gas is often used. Tungsten inert gas (TIG) welding is one of the commonly used welding methods w1x. TIG welding which uses a non-consumable tungsten electrode and an inert gas for arc shielding, is an extremely important arc welding process w2,3x. In this process, an electric arc is formed between a tungsten electrode and the base metal. The arc region is protected by an inert gas or mixture of gases w4x. Although this type of welding is usually done with a single electrode, it may be sometimes done with several electrodes w5x. It is commonly used for welding difficult-to-weld metals such as stainless steel, aluminium, magnesium, copper and many other nonferrous metals w6,7x. In TIG welding, shielding gas plays an important role. Composition of a shielding mixture in arc welding depends mostly on the kind of material to be welded. The selection of the shielding gas should, by all means, take into account chemical–metallurgical processes *Tel.: q90-312-212-3993; fax: q90-312-212-00-59. E-mail address: [email protected] (A. Durgutlu). between the gases and the molten pool that occur during welding w8x. Density of the shielding gas has an important influ- ence on the efficiency of shielding the arc and the weld pool against the ambient atmosphere. The values indi- cating relative density of the shielding gas with regard to air are of primary importance. Argon and carbon dioxide are gases having by far the highest density and therefore, form an efficient gas shielding around the arc. However, the densities of hydrogen and helium are 10– 20 times less than that of argon and thus, are prone to turbulent flow at the exit from the blowpipe nozzle due to thermal buoyancy w9x. Hydrogen is added to argon when welding stainless steels, nickel–copper and nickel based alloys. The addi- tion of hydrogen to argon permits increased welding speeds. Hydrogen is not recommended for use on other metals because it produces hydrogen cracks in the welds w7x. The addition of hydrogen also significantly increases the volume of molten material in the weld pool due to the higher thermal conductivity of argon–hydrogen mixtures at temperatures at which molecules of hydrogen dissociate w10x. The influence of hydrogen additions to an argon shielding gas on the heat input and weld bead morphol- ogy was investigated using the TIG welding process.

Experimental investigation of the effect of hydrogen in argon as a shielding gas on TIG welding of austenitic stainless steel

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Page 1: Experimental investigation of the effect of hydrogen in argon as a shielding gas on TIG welding of austenitic stainless steel

Materials and Design 25(2004) 19–23

0261-3069/04/$ - see front matter� 2003 Elsevier Ltd. All rights reserved.doi:10.1016/j.matdes.2003.07.004

Experimental investigation of the effect of hydrogen in argon as ashielding gas on TIG welding of austenitic stainless steel

Ahmet Durgutlu*

Gazi University, Technical Education Faculty, Ankara, Turkey

Received 6 February 2003; accepted 30 July 2003

Abstract

In this study, the effect of hydrogen in argon as shielding gas was investigated for tungsten inert gas welding of 316L austeniticstainless steel. The microstructure, penetration and mechanical properties were examined. Pure argon, 1.5%H –Ar and 5%H –Ar2 2

were used as shielding gas. The highest tensile strength was obtained from the sample which was welded under shielding gas of1.5%H –Ar. After bending test, cracks, tearing and surface defection were not observed on the samples that were welded under2

all three shielding media. Mean grain size in the weld metal increased with increasing hydrogen content. Additionally, weld metalpenetration depth and its width increased with increasing hydrogen content.� 2003 Elsevier Ltd. All rights reserved.

Keywords: TIG Welding; Shielding gas; Mechanical properties; Microstructure

1. Introduction

In the fabrication of equipment made from stainlesssteels such as pipe, automotive exhaust gas system,chemical industrial equipment, etc., arc welding usingshielding gas is often used. Tungsten inert gas(TIG)welding is one of the commonly used welding methodsw1x.TIG welding which uses a non-consumable tungsten

electrode and an inert gas for arc shielding, is anextremely important arc welding processw2,3x. In thisprocess, an electric arc is formed between a tungstenelectrode and the base metal. The arc region is protectedby an inert gas or mixture of gasesw4x. Although thistype of welding is usually done with a single electrode,it may be sometimes done with several electrodesw5x.It is commonly used for welding difficult-to-weld metalssuch as stainless steel, aluminium, magnesium, copperand many other nonferrous metalsw6,7x.In TIG welding, shielding gas plays an important

role. Composition of a shielding mixture in arc weldingdepends mostly on the kind of material to be welded.The selection of the shielding gas should, by all means,take into account chemical–metallurgical processes

*Tel.: q90-312-212-3993; fax:q90-312-212-00-59.E-mail address: [email protected](A. Durgutlu).

between the gases and the molten pool that occur duringwelding w8x.Density of the shielding gas has an important influ-

ence on the efficiency of shielding the arc and the weldpool against the ambient atmosphere. The values indi-cating relative density of the shielding gas with regardto air are of primary importance. Argon and carbondioxide are gases having by far the highest density andtherefore, form an efficient gas shielding around the arc.However, the densities of hydrogen and helium are 10–20 times less than that of argon and thus, are prone toturbulent flow at the exit from the blowpipe nozzle dueto thermal buoyancyw9x.Hydrogen is added to argon when welding stainless

steels, nickel–copper and nickel based alloys. The addi-tion of hydrogen to argon permits increased weldingspeeds. Hydrogen is not recommended for use on othermetals because it produces hydrogen cracks in the weldsw7x. The addition of hydrogen also significantly increasesthe volume of molten material in the weld pool due tothe higher thermal conductivity of argon–hydrogenmixtures at temperatures at which molecules of hydrogendissociatew10x.The influence of hydrogen additions to an argon

shielding gas on the heat input and weld bead morphol-ogy was investigated using the TIG welding process.

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20 A. Durgutlu / Materials and Design 25 (2004) 19–23

Table 1Chemical composition of base metal(wt.%)

C Si Mn Cr Ni Mo Fe

0.008 0.45 1.75 18.5 12 2.7 Balance

Fig. 1. Preparation of welded samples for the tests.

Fig. 2. Hardness values and its region.

Fig. 3. Macro image of bending specimens.

Table 2Tensile testing results

Shielding gas Ultimate tensile Yield strength Elongationstrength(Nymm )2 (Nymm )2 (%)

Pure argon 612 384 321.5%H –Ar2 695 415 425%H –Ar2 647 402 36

Variations in weld bead size and its shape with hydrogenadditions were related to changes in the ability of thearc to generate heat and not to generate perturbations inthe weld pool caused by Marangoni fluid floww11x.As stainless steel is a condidate material for the

structural material in fusion reactors, joining technologywith irradiated structural has been under investigationusing TIG weldingw12x.It has already been mentioned earlier that hydrogen

addition to a shielding gas in arc welding affects thestatic characteristic of the arc and increases arc powerwhich results in a larger quantity of the material melted.But the majority of the researchers on shielded-gaswelding recommends the use of hydrogen in argonranging 0.5–5%w8x.In the present work, type 316L stainless steel speci-

mens were bonded by TIG welding method, and themechanical properties of their weldments wereevaluated.

2. Experimental procedure

In this study, 316L austenitic stainless steel specimens4 mm in thickness were welded to each other by TIGwelding method using a welding wire made from thesame material. Chemical composition of 316L is givenin Table 1. Three different gases were used as theshielding media, pure argon, 1.5%H –Ar and 5%H –2 2

Ar.Mechanical properties, penetration and microstructur-

al properties of the samples which were bonded underdifferent shielding media were examined. Firstly, stain-less steel specimens in 200=80=4 mm dimensions3

were cut and then the edges to be bonded were cham-fered at 308. Surfaces of machined samples were cleanedand then placed adjacent to each other by 2-mm distancebetween them on a copper plate. CEMONT T 301 typewelding machine was used. Welding was carried outwith a 2.4-mm tungsten electrode. The arc length was 3mm. Gas shielding for welding was kept in the flowingrate of 10 lymin. Current intensity and welding speedwere chosen as 115 A and 100 mmymin, respectively.During the welding, welding gun was controlled auto-matically and the welding wire was fed manually intothe welding area. To check the penetration, only thesurfaces of the workpiece were melted without usingany filler material under the same welding parameters.After welding, samples were cooled in the air. Threesamples were prepared for each gas media which werepure argon, 1.5%H –Ar and 5%H –Ar. As it is seen2 2

from Fig. 1, the welded samples were cut into piecesfor tensile test, bending test and microstructuralexaminations.Tensile testing and bending testing samples were

prepared according to the EN 895 and pr EN 910,respectively. Standard metallographic grinding wasapplied and then grinded samples were polished byusing 3 mm diamond paste. Polished samples wereetched in a solution of 4 g CuSO , 20 ml HCl and 204

ml distilled water. Microphotographs were taken with aPRIOR optical microscope. Hardness measurementswere performed on the samples which were used for

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21A. Durgutlu / Materials and Design 25 (2004) 19–23

Fig. 4. Macrosections of the specimens welded under different shield-ing mixtures.

Fig. 5. Optical photomicrographs of sample welded under pure argon.

microstructural examinations. For hardness measure-ment, an INSTRON WOLPERT device was used byapplying 5-kg load. Tensile and bending tests werecarried out on a INSTRON MFL SYSTEM testingmachine.

3. Results and discussions

Results of bonded samples, which were welded underdifferent shielding gases, are given in Table 2. It shouldbe noted that fracture for all the tensile tests occurredin the welded region. As it is seen from Table 2, thehighest tensile strength(695 Nymm ) was obtained2

from the sample welded under shielding gas of1.5%H –Ar. Other two samples welded under 5%H –2 2

Ar and pure argon have tensile strength of 647 and 612Nymm , respectively. For pure argon shielding, tensile2

strength values were found to be slightly lower than thevalues supplied by the manufacturer of the filler mate-rial. However, it is seen that reducing H content in the2

shielding gas definitely increases the mechanical prop-erties of welded sample.From Table 2 it can be observed that increasing

hydrogen content in the shielding gas reduces themechanical properties. This can be attributed to theincreasing stresses in the weld metal with increasinghydrogen. As it is seen from Fig. 2, the hardness ofweld metal is lower than that of the HAZ and the basemetal. This finding is in agreement with Tsuchiya et al.w12x, who reported that the hardness of weld metal andHAZ was lower than that of base metal. Higher hardnessof the sample welded using 5%H –Ar than that of the2

sample welded using 1.5%H –Ar led to the lower tensile2

strength of this sample.Three point bending test was carried out until 1808

and test results showed that; all samples have a bendingstrength of 85 Nymm . After bending the samples until2

1808, no cracks were observed visually in the bendedarea(Fig. 3).Fig. 4 shows the three macrosections of the fusion

zone obtained in TIG welding in different shieldingmedia. It can be clearly seen from Fig. 4 that meltingefficiency of TIG welding increases with increasing

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22 A. Durgutlu / Materials and Design 25 (2004) 19–23

Fig. 6. Optical photomicrographs of sample welded under 1.5%H –Ar.2

hydrogen content in argon. Tusek and Subanw8x alsoˇreported that melting efficiency of TIG welding increas-es with the increasing current intensity and hydrogencontent in argon.Photomicrographs of the transition zone of the sam-

ples which were welded under pure argon are given inFig. 5a and b. The transition zone between the base andweld metal can be clearly seen from Fig. 5. Grains inthe weld metal oriented parallel to the heat flow. Likemany other researchers, Durgutluw13x also reportedsimilar result about grain orientation. In Fig. 5c, micro-structure of the weld metal is given. Weld metal grainswere formed as dendrites.Photomicrographs of the transition zone of the sam-

ples welded under shielding gas of 1.5%H –Ar are2

given in Fig. 6a and b. It is well known that hydrogenaddition into shielding gas increases the heat inputw8x.Increasing heat input to the weld metal causes longersolidification time for the melt metal. Since longersolidification time, some grain orientations occurred indifferent directions due to the heat flow in weld metal(Fig. 6a and b). In Fig. 6c, microstructure of weld metalis given. Since more heat input, mean grain size islarger than that of sample welded under pure argonatmosphere.Photomicrographs of the transition zone of the sam-

ples welded under shielding gas of 5%H –Ar are given2

in Fig. 7a and b. 5%H –Ar is more effective than2

1.5%H –Ar in terms of weld metal grain characteristics.2

It can be seen from Fig. 7c that grain growth in weldmetal of the sample welded under shielding gas of5%H –Ar, is more distinguishable than that of THE2

other two samples.

4. Conclusions

On the basis of the mechanical properties and micro-structural experimental studies accomplished and theresults obtained on the effect of hydrogen in the shield-ing gas on the TIG welding processes of 316L austeniticstainless steel the following conclusions may be drawn:

1. For tensile strength, the best result is obtained from1.5%H –Ar as gas shielding.2

2. Cracks, tearing and surface defection were not withnaked eye observed after bending the samples weldedunder all three shielding media.

3. For all shielding media, hardness of weld metal islower than that of the HAZ and base metal.

4. Penetration profile examinations for all three differentshielding gases show that penetration depth and weldbead width increases with increasing hydrogencontent.

5. Mean grain size in the weld metal increases withincreasing hydrogen content. More hydrogen content

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23A. Durgutlu / Materials and Design 25 (2004) 19–23

Fig. 7. Optical photomicrographs of sample welded under 5%H –Ar.2

caused deviations in the grain orientation of weldmetal.

Acknowledgments

We would like to thank BOS A.S. for their valuablesupport.

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