The effect of shielding-gas compositions on the microstructure and mechanical properties of stainless steel weldments

The effect of shielding-gas compositions on the microstructure and mechanical properties of stainless steel weldments

MATERIALS CHEM;STP\TPt’dD ELSEVIER Materials Chemisrry and Physics 55 (1998) 145-151 The effect of shielding-gas compositions on the microstructu...

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MATERIALS CHEM;STP\TPt’dD ELSEVIER

Materials

Chemisrry

and Physics 55 (1998)

145-151

The effect of shielding-gas compositions on the microstructure mechanical properties of stainless steel weldments MT. ’ Depnrtrnent b Department

of Mechanical of Mechanical

Received

Manufacturing Marerids

4 September

and

Liao a, W.J. Chen b

Engineering, Engineering,

1997; received

National Huwri Institute qf Technology, National Huwei Institute qf Technology,

in revised form

11 March

Huwei, Yunlin, Taiwan Huwei, Yunlin, Tainban

1998; accepted 30 March

1998

Abstract This study aims to examine how the microstructure and mechanical properties of AISI 304 stainless steel welds are influenced by the shielding gas. The spatter rates increase as the COP content of the Ar+CO, shielding-gas mixtures increases from 2 to 20%. The notch toughness of all the weld metals is affected by the delta-ferrite and oxygen potential. At room temperature, the notch toughness property is strongly dependent on oxygen potential. At - 196”C, both delta-ferrite and oxide inclusions are detrimental to the notch toughness properties, but the delta-ferrite plays a much more important role at this temperature. Both vermicular and lathy ferrite are observed, and the ferrite 0 1998 Elsevier Science S.A. All rights reserved. content decreases as the CO? level of the Ar + CO2 mixtures increases from 2 to 20%. Kqwor~x:

Mechanical

properties;

Microstrucrure;

Shielding

gases; Welding

Ar-He-COz,

I. Introduction

Recently, gasmetal arc welding (GMAW) processes, with either solid- or metal-cored welding wires, have gained the most popularity among the different types of welding, becausehigh-quality and economical welds can be obtained [ I]. The primary function of the shielding gas is to protect the molten tnetal from atmosphericnitrogen and oxygen as the weld pool isbeing formed. Besides,the gasalsopromotes a stablearc and uniform metal transfer. The quality, efficiency andoverall operating acceptanceof the welding operation arestrongly dependenton the shielding gas,sinceit dominatesthe mode of metaltransfer. The shielding gas not only affects the properties of the weld but also determinesthe shapeand penetrationpattern aswell. During welding, the shielding gas also interacts with the welding wire to produce the strength, toughnessand corrosion resistance of particular weld deposits. The shielding gas also affects the residual contents of hydrogen, nitrogen and oxygen dissolved in the weld metal [ 21. Because of the wide applications of austenitic stainless steels under

both mild

and severe corrosive

conditions

at

cryogenic or elevatedtemperatures,the microstructure-property relations of their welds have been paid much attention [ 3-51.

Nowadays

rationalization

of welding

processes

increasingly demandsGMAW for stainlesssteelapplications. 0254-0584/98/$ - see front matter 0 PIlSO254-0584(98)00134-5

1998 Elsevier

Ar-He,

Ar-N,,

Ar-CO,

and Ar-CO,-N,

mixtures are mainly usedasthe shielding gasat present [ 681. The selectionof the shieldinggasfor stainlesssteelwelds hasbeenreported by Lyttle [ 2,7]. Kobayashi and Sugiyama [ 81 examined 13 different types of shielding gases.They have already studied the influences of theseshielding gases on the weldability, the appearanceof beadand the generation of blowholes using bead-on-plateweld specimensprepared by AWS ER308L and ER 308LSi wires. Neverthelessthe related effects on the mechanical properties have not been reported asyet. Thus, this study aimsto examine how different shielding gasesinfluence the mechanicalproperties. 2. Experimental

Premixed gasesare usedas shielding gasesin this study. The compositions of the shielding gases are 90%Ar+ lO%CO, (Ml ) ,8O%Ar + 20%C02 (M2) / 98%Ar + 2%C02 (M3), 98%Ar+2%02 (M4) and 93%Ar+2%02-k5%CO, (M5). Electra-wires usedin the study are 1.2mm in diameter and conformed to the AWS specifications.A 12 mrn( r) X 50 mm(w) X 200 mm(Z) base metal plate of AISI type 304 stainlesssteelwas usedfor the flat position welding. In order to evaluate the properties of the weld deposits, mild steel plates (19 mm(t) X 125 mm(\v) X300 mm(l)) with a V-

Science S.A. All rights reserved.

146

M.T. Liao, W.J. Chen /Materials

Chemist0 and Physics 55 (19981145I51

ing la!iei-S

,--

Tensile

test specimen

0.

O,~

-Op(

8 oz+o.

10 12 14 16 7CO.) & ----COs(

18

20 a)

Fig. 2. The spatter rate as a function of oxygen potential (0,) and percentage coz. (b) Fig. 1. (a) Details of test plate and assembly; (b) machining of specimens for tension test and impact test of all weid metals.

shapedgroove were usedand multipass welding wascarried out. In accordancewith the AWS A5.22-80 specification,the V-grooved surfacewascoatedwith a JIS 304L stainlesssteel before the multipass welding was performed. A schematic diagram of the test plate and assemblyis shownin Fig. 1(a). The automaticwelding systemconsistsof a PanasonicYD356KRI welding machine and a cantilever-beamsupporting frame. A constant-potentialpower supply was usedand the welding voltage was held constant at 30 V for the welds. A constant wire-feeding rate was maintained for all the welds. However, a slight variation in current was observeddue to the small changein the cover gas.Consequently,thewelding current was in the range200 110 A. Cylindrical tensilespecimensandsquareimpactspecimens were machinedfrom the depositedmetal. Cylindrical tensile specimenswere takenparallel to the weld direction at a fixed distancefrom the weld centreas shown in Fig. 1(b) . Square impact specimenswere takenperpendicularto the weld direction. This is also shownin Fig. 1.Tensile testswereperformed at room temperature.Impact testswere performedat temperaturesranging from - 196 to 25°C. The fracture surfacewas investigated by a scanning electron microscope (SEM) to observethe fracture mode. Chemical microanalysiswascarried out using the SEM equippedwith an EDAX detector. Specimensfor optical metallography were prepared by mechanical polishing and chemical etching. Thin foils for transmission electron microscope (TEM) analysis were taken from the longitudinal section andpreparedby ion milling. They were examined in a JEOL JEM-2010 operatedat 200 kV. Magnetic gauge readings were taken to determine the ferrite number of the depositedmetal. The spatterrates were measuredfor each mixture by collecting the spatters within a certain period of time. 3. Results and discussion

It is observedthat spattersare generatedin the flat position welding with all the gasmixtures. The 98%Ar + 2%C02 and

the 98%Ar + 2%0, mixtures create a stable spray arc with few spatters.The spatterrates of these two mixtures are the lowest amongall themixtures, asindicated in Fig. 2. It is also shown in Fig. 2 that the spatterratesincreasewith increasing CO, content of the Ar + CO2 mixtures with CO, from 2 to 20%. The spatter createslarger particles with higher C& content, and theseparticles areredepositedon the basemetal as shown in Fig. 3. In general,the oxygen potential (0,) meansthe oxidizing effect of the shielding gas and/or the significance for the oxygen contentof the weld metal.Oxygen andcarbondioxide areoxidizing gases.They arevery active at high temperature, therefore their direct chemical effect on the filler metal or baseplateis strong.The oxygen potential of the shielding gas is estimatedby using the formula 0, = O2+ ,&O1 (where p is the oxidizing factor) [I$]. Usually, p is taken as 0.5 or 0.7 [ 1,8], In this study, p is assumedto be 0.7. Fig. 2 shows the spatterratesas a function of oxygen potential; the spatter rate increaseswith the oxygen potential. It is apparentthat the oxidizing effect will increasethe spatterrate. The chemical composition of all the depositedmetalsand their ferrite numbers measuredby the magnetic gauge for each weld are shown in Table 1. The ferrite numbersof all the depositedmetals as a function of nickel equivalent and CO, or 0, are shown in Fig. 4(a) and (b) , respectively.The ferrite number decreaseswith increasing CO, content of the Ar + COZmixtures with CO, from 2 to 20%. The increaseof CO2percentageof the mixtures will raisethe C contentin the weld deposits,therefore the ferrite number will decreaseand the Ni equivalent will become larger. Also, the increaseof CO, percentagewill speedup the consumptionof Cr and Si elementsdue to oxidation andmaketheCrequivalent smaller. In the 98%Ar4 2%02 mixture, thereis no CO2gas,therefore the carbon content of the weld depositsis not increasedand the amount of ferrite is the highest. The ferrite numbers of weld deposits using 98%Ar+2%C02 and 93%Ar+ 2%02 + 5%C02 mixtures are almost the same.This may be becausethe decompositionof CO?is retardedby oxygen in the 93%Ar+2%0,+5%C02 mixture. It will reduce the carbon content of the depositedmetal.

M.T.

Liao,

W.J.

Chen

/hfarerials

Chemistq

and Phq‘sics 55 (1998) 145-151

Nickel

147

Equiva!ent

10

5

/,//,/

0

2

4 -Op(

6

6 02to.

10 12 14 18 16 7CO*) B ----CO.(X)

/,,,

20

:

Fig. 4. The ferrite contents of all the deposited metals as a function of (a) nickel equivalent; (b) oxygen potential and percentage CO,.

The structure observations on a microscopic scale are illustrated in Fig. 5 (optical micrographs) and Figs. 6 and 7 (TEM images). Fig. 5 shows the microstructure of all the deposited metals. The austenitic and ferrite phases were found in all deposited metals. Both vermicular and lathy ferrites are observed in Fig. 5. The vermicular (or dendtitic) ferrite morphology results from peritectic solidification. Ferrite in plates

during cooling [ 41. Fig. 5 also shows that the ferrite volume fraction decreases by increasing the amount of CO2 in the Ar + CO2 mixtures. The results are in agreement with the ferrite number measured by the magnetic gauge. It is also shown that the ferrite volume fraction increases with increasing ferrite number. Fig. 6 shows a dislocation substructure in an austenitic phase and ferrite phase. The dislocations are formed in the course of a rapid weld-pool solidification due to an accommodation process of the mismatch between adjacent deltaferrite dendrites in the austenitic weld [ 91. Fig. 7(a) shows that there are a large number of lath substructures in the austenite matrix of the weld metal. These substructures are twins that are identified by selected-area electron diffraction techniques (Fig. 7(b)). Fig. 7(b) shows that the [ 1011 pole of the substructure is parallel to the [ i4i] pole of the matrix. This indicates that the substructures are twins. In general, only a little deformation twinning is found in fee metals except under cryogenic temperature conditions and extremely high strain stress [ lo], The formation of deformation twinning in as-welded austenitic stainless steel weld metal has rarely been reported [ 111. Pan and Chen [ 111 suggested that the rapidly induced large shear stress impedes the movement of dislocations and the excitation of the fee slip system, therefore twinning becomes the major plastic deformation mode

(lathy morphology) results from the dissolutionof ferrite

during welding.

Fig. 3. Photographs of (a) spatters of each condition (M 1, M2, M3, M4 and M5): (b) appearance of all flat position welding.

148

M.T. Liao. W.J. Chen /Materials

Chemist? and Physics 55 (1998) 145-151

Table 1 Chemical composition and ferrite number of all weld metals Chemical ;;yryition

Chemical composition

c (%I 0.045 Si (%) 0.45 Mn (%) 1.92 Ni (%‘a) 9.15 Cr (%j 20.37 P (%I 0.029 s (%I 0.009 N (PPm) 0 @pm) Nickel equivalent ( NiEq) ’ Chromium equivalent (CrEq) b Ferrite number (FN)

Shielding gas Ml 90%Ar + 10%COz

M2 XO%Ar + 2O%CO,

M3 98%Ar + 2%COa

hi14 98%Ar + 2ROx

M5 93%Ar t 2%02 + 5%C01

0.050 0.38 1.61 9.4 19.3 0.022 0.007 378 251 12.91 20.05 7.35

0.070 0.39 1.60 9.4 19.0 0.022 0.007 361 245 13.5 19.77 5.75

0.040 0.44 1.73 9.4 19.3 0.022 0,007 412 332 12.67 20.15 8.5

0,030 0.43 1.67 9.4 19.5 0.022 0.007 385 155 12.34 20.34 9.9

0.050 0.39 1.63 9.5 19.5 0.022 0.007 373 264 13.02 20.27 8.45

’ Nickel equivalent (NiEq) = %Ni + 30 X %C + 30 X%-N-t 0.5 X %Mn. ’ Chromium equivalent ( CrEq) = %Cr + %&IO + 1.5 X %Si + 0.5 X %Nb

Fig. 6. Bright-field TEM micrograph of the Ml sample.

Fig. 5. Optical micrographs of the ferrite morphology in all deposited metals.

The ultimate tensile strength (UTS) values of deposited metal for each shielding gasare listed in Table 2. It appears that the effect of oxygen potential is not significant. The fracture surfaceof the M2 tensile specimenis shown in the SEM image of Fig. 8. The fracture surfacesof the remaining tensile specimens(Ml, M3, M4, and M.5) are the sameas M2. In Fig. 8, it is clear that the fracture-surfacemorphology is dimple rupture. The dimples are associatedwith impurity particles (inclusions), which are generally round and have various different sizes.The results of EDAX analysesindicated that the inclusions contained silicon, manganese,iron and chromium, as shown in Fig. 9. This indicated that the inclusions are silicon oxides and manganeseoxides, etc. The impactenergyversustesting temperaturefor all deposited metalsis plotted in Fig. 10,which shows that the impact energy decreaseswith decreasingtesting temperature.It is also shown that the energy difference is quite large at high testing temperatures,while it is much less at low testing temperature. These phenomena can be explained by the microstructures (Fig. 5) and fracture-surfacemorphologies

M.T. Licio, W.J. Chen /Mntrrials

Chemistry

149

and Physics 55 (1998) 145-151

ix-RAY:

I

FS:51 I MEMI:

0-20keV

ch

296=

57

cts

I

Fig. 9. Typical EDAX spectrum of the inclusions in Fig. 8.

Fig. 7. (a) Bright-held TEM micrograph of the deformation twins in Ml samples, (b) Diffraction pattern of [ lOI],,,,]] [ i4i].,,,,,,. Table 2 Tensile strength and elongation of all weld metals

Strength (.N mm-‘) Elongation (SC)

Ml

M2

M3

M4

M5

602 38.2

610 36

613 38

606 36

615 36.6

jl

Fig. 10. Effect of temperature on impact energy for all deposited metals.

Fig. 8. Fracture morphology (SEM image) of tensile specimen fractured at room temperature. The inclusion is indicated by the arrow. The EDAX spectrum of the inclusions is shown in Fig. 9.

(Fig. 11) of the weld deposits as follows. Fig. 11 shows the fracture morphologies of an M2 sample at testing temperatures of 25 and - 196°C. It is clear that the fracture-surface morphology of all deposited metals is dimple rupture at 25°C while it is dimple rupture with a few characteristic features

of cleavage at - 196°C. Fig. 5 shows that all the deposited metals contain delta-ferrite (vermicular and lathy ferrite) and austenite phases. It is known that the austenite has an fee structure and delta-ferrite a bee structure. FCCmetal has a high notch toughness which is almost independent of temperature. thus brittle fracture does not occur. On the other hand, the notch toughness of bee metal is strongly dependent on temperature [ 121, therefore brittle fracture is severe at low temperature. For bee metal at low temperature the fracture is cleavage, while at high temperature it is ductile rupture. Thus, the type of fracture of all the deposited metals is ductile rupture at high temperature and ductile rupture with a few cleavages at low temperature. It is also shown in Fig. 11 that inclusions were found in all deposited metals. Inclusions, which act as crack sites, will degrade the notch toughness property. In this study, the major inclusions are oxides such as silicon oxides, manganese oxides, etc. The number of oxides can be determined by the oxygen potential and it increases with increasing oxygen potential. The influence of oxygen potential on notch tough-

MT. Lao, W.J. Chen/Materinls

Chemistry and Physics 55 (1998} 145-151

decreased by increasing the CO, content (e.g., increasing oxygen potential) of the ArS COz mixtures with CO, percentage from 2 to 20%. Welding using the 8O%Ar+ 2O%CO, mixture has the lowest impact energy since it has the highest oxygen potential. At the lowest testing temperature ( - 196”C), the fracture mode of delta-ferrite is brittle cleavage, therefore the existence of delta-ferrite will promote crack growth. Thus, both delta-ferrite and oxide inclusions are detrimental to the notch toughness properties, and the energy difference is much less at low testing temperature. However, the delta-ferrite plays a much more important role at this temperature.

4. Conclusions

Fig. 11. Fracture morphology (SEM image) of impact specimen in M2 sample: (a) 25°C; (b) - 196%

zl51 14y”13-1221 lL w lo: 9-

“, 8

= 7 F6 5i 4n 3- _ i?2 M5 Ml Y3)14 2 l,l,l~‘l~,l, ,#/1,~,1/~,f/r11 0 1 2 3 4 5 6’7 6 9 101112131415 Oxidation Potent ia1(02+0. ‘i’Cos) Fig. 12. Effect of oxygen potential on impact energy.

ness at various temperatures is shown in Fig. 12. It is obvious that the notch toughness drops as the oxygen potential increases at high testing temperature, while it is insensitive to oxygen potential at low testing temperatures. From the above discussion, it is clear that the notch toughness of all the deposited metals is affected by the delta-ferrite and oxygen potential (oxide content). At room temperature, delta-ferrite is not detrimental to the notch toughness properties, therefore the notch toughness is strongly dependent on the oxygen potential, and higher oxygen potential results in a degradation of notch toughness. Thus, the impact energy is

The composition of shielding gas has a significant effect on the microstructure and mechanical properties of 304 stainless steel welds. The spatter rate increases when the CO1 content of the Ar + CO, mixtures increases from 2 to 2O%, and also increases with the oxygen potential. The increase of CO2 percentage of the gas mixtures will raise the C content in the weld deposits, therefore the ferrite number is decreased by increasing the amount of CO* in the Ar + CO, mixtures in the range 2 to 20%. Both vermicular and lathy ferrites are observed, and the ferrite volume fraction decreases as the COz level of the Ar + CO2 mixtures increases from 2 to 20%. The notch toughness of all the weld metals is affected by the delta-ferrite and oxygen potential. At room temperature, the notch toughness property is strongly dependent on oxygen potential. At - 196”C, both delta-ferrite and oxide inclusions are detrimental to the notch toughness properties, but the delta-ferrite plays a much more important role at this temperature. Twins in the austenite matrix of the weld metal are also observed.

Acknowledgements The authors wish to thank Goodweld Corporation for providing welding wire and Union Gas Corporation for providing premixed gas. The authors also wish to thank Dr L.C. Yang for his help in preparation of the manuscript.

References [ 11 N. Stenbacka, K.A. Person, Weld. J. 68 ( 1989) 41. [ 21 K.A. Lyttle, W.F.G. Stapon, Weld. J. 69 (1990) 21. [ 3 ] E. Folkhard, Welding Metallurgy of Stainless Steel, Springer-Verlag, Vienna, 1988. [4] J.A. Brooks, A.W. Thompson, Int. Mater. Rev. 36 (1991) 16. [51 P. Bilme, A. Gonzalez, CL. Lorente, M. Solari, Weld. Int. 10 (1996) 797. [6] R. Petersens, I. Ballingal, 0. Runnerstam, Weld. Rev, Int. (1993) 52.

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W.J.

Chen

/ Materials

Chemistv

[7] K.A. Lyttle, Shielding gases, in: Material Handbook of ASM, vol. 6, p. 64. [ 81 T. Kobayashi, T. Sugiyama, IIW Dot. XIZ-E-33-82, X11-B-25-82, 1982. [9] J.R. Fould, J. Moteff, Weld. J. 61 (1982) 189s.

and Physics 55 (1998) 145-151

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[ lo] R.W. Hetzberg, Deformation and Fracture Mechanics of Engineering Materials, John Wiley, New York, 1996, p. 116. [ 111 C. Pan, B. Chen, J. Mater. Sci. Lett. 14 ( 1995) 1758. [ 121 G.E. Dieter. Mechanical Metallurgy, McGraw-Hill, London, 1988, p, 476.