J. Mater. Sci. Technol., Vol.25 No.2, 2009 181

Tensile and Impact Properties of Shielded Metal Arc Welded

AISI 409M Ferritic Stainless Steel Joints

K. Shanmugam, A.K. Lakshminarayanan and V. Balasubramanian†Centre for Materials Joining & Research (CEMAJOR), Department of Manufacturing Engineering, Annamalai University,

Annamalai Nagar 608 002, Tamil Nadu, India

[Manuscript received January 21, 2008, in revised form April 7, 2008]

The present study is concerned with the effect of filler metals such as austenitic stainless steel, ferritic stainlesssteel and duplex stainless steel on tensile and impact properties of the ferritic stainless steel conforming to AISI409M grade. Rolled plates of 4 mm thickness were used as the base material for preparing single pass buttwelded joints. Tensile and impact properties, microhardness, microstructure and fracture surface morphologyof the joints fabricated by austenitic stainless steel, ferritic stainless steel and duplex stainless steel filler metalswere evaluated and the results were reported. From this investigation, it is found that the joints fabricated byduplex stainless steel filler metal showed higher tensile strength and hardness compared to the joints fabricatedby austenitic and ferritic stainless steel filler metals. Joints fabricated by austenitic stainless steel filler metalexhibited higher ductility and impact toughness compared with the joints fabricated by ferritic stainless steeland duplex stainless steel filler metals.

KEY WORDS: Ferritic stainless steel; Shielded metal arc welding; Tensile properties;

Impact toughness

1. Introduction

Ferritic stainless steels constitute approximatelyone-half of the AISI type 400 series stainless steels.These steels contain 10% to 30% Cr along with otheralloying elements, notably Mo. Ferritic stainless steelsare noted for their excellent stress corrosion crack-ing resistance and good resistance to pitting andcrevice corrosion in chloride environments[1]. Theyare cheaper alternative to austenitic stainless steels.However, the heat of welding leads to grain coarsen-ing in the heat-affected zone and in the weld metal offerritic stainless steels because they solidify directlyfrom the liquid to the ferrite phase without any inter-mediate phase transformation[2]. It is therefore rec-ommended that these alloys are welded with a lowheat input and at high welding speeds[3]. While thesealloys have useful properties in the wrought condition,welding is known to reduce toughness, ductility, andcorrosion resistance because of grain coarsening andformation of martensite[4]. Welding these commercialgrades usually requires preheat and post weld heattreatment. For these reasons, the application of thisgroup alloys is limited. To overcome some of the dif-ficulties and to improve weldability, several standardgrades of ferritic stainless steels have been modified.Attempts have been made to grain refine the weldsof these steels by the addition of elements such as Tiand Al[5]. Studies have also been conducted to grainrefine ferritic stainless steel welds by electromagneticstirring[6] as well as through liquid metal chilling[7].Ferritic stainless steels can be welded by fusion weld-ing processes employing filler metal of matching com-position or austenitic stainless steel and Ni alloys.However, very low carbon is essential if sensitizationin the fusion zone is to be avoided, and the effectof filler metal dilution on weld structure and corro-sion resistance must be considered. In general, the

† Corresponding author. Prof., Ph.D.; Tel.: +91 4144 239734;E-mail address: visvabalu@yahoo.com (V. Balasubramanian).

ductility of ferritic stainless steel welds is reported tobe low due to large grain size of the fusion zone[8,9].

Enormous research work has been carried out onwelding of austenitic stainless steel, but the publishedinformation on ferritic stainless steels is very limited.Most of the researchers so far have concentrated onwelding aspects of austenitic stainless steels and du-plex stainless steels. Recently, ferritic stainless steelsare finding greater importance in structural applica-tions including fabrication of rail wagons[10]. How-ever, the only shortcoming of ferrtic stainless steels isthe poor notch toughness due to coarse grains in theweld fusion zone. Hence, the present work has beencarried out to choose the best filler metal to weld AISI409M grade ferritic stainless steel by studying the ef-fect of three different filler metals: austenitic stainlesssteel (ASS), ferritic stainless steel (FSS) and duplexstainless steel (DSS) on tensile and impact propertiesof welded joints.

2. Experimental

The rolled plates of 4 mm thickness AISI 409Mgrade FSS were cut into the required dimension(300 mm×150 mm) by oxy-fuel cutting and grinding.The chemical composition of base metal is presentedin Table 1. The initial joint configuration was ob-tained by securing the plates in position using tackwelding. Single ‘V’ butt joint configuration was usedto fabricate the joints using shielded metal arc weld-ing (SMAW). All necessary care was taken to avoidthe joint distortion and the joints were made with ap-plying clamping fixtures. The welding conditions andprocess parameters used to fabricate the joints aregiven in Table 2. The soundness of the welded jointswas checked using ultrasonic testing.

The chemical composition of the weld metals wasobtained using a vacuum spectrometer (ARL-Model:3460). Sparks were ignited at various locations of theweld metal sample and their spectrum was analysed

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Table 1 Chemical composition of base metal and all weld metals (wt pct)

Material C Mn P S Si Cr Ni Ti Mo Cu FeBase metal (AISI 409M) 0.028 1.10 0.030 0.010 0.40 10.90 0.39 0.004 – – Bal.ASS (AISI 308L) 0.035 0.82 0.018 0.015 0.67 19.00 11.00 – 0.01 0.1 Bal.FSS (AISI 430) 0.075 0.70 0.017 0.014 0.60 17.00 0.40 – – – Bal.DSS (AISI 2209) 0.030 0.80 0.018 0.016 0.80 22.00 9.00 – 3.00 – Bal.

Table 2 Welding conditions and process parameters

Welding Polarity arc voltage Welding Welding speed Heat input Electrode Shielding gas Shielding gasmachine /V current/A /(mm/s) /(J/mm) diameter/mm flow rate/(l/min)

Lincoln, USA DCRP 25 120 4 625 3.16 Argon (99.99%) 14

Fig. 1 Dimensions of tensile and impact specimens: (a)unnotched tensile specimen, (b) notched tensilespecimen, (c) sub-size impact specimen. All di-mensions are mm

for the estimation of alloying elements. The chemi-cal composition of the weld metals in weight percentis given in Table 1. Microstructural examination wascarried out with a light optical microscope (MIL-7100,MEJI, Japan) incorporated with an image analyzingsoftware (Metal vision). The specimens for metallo-graphic examination were sectioned to the requiredsize from the joint comprising weld metal, heat af-fected zone (HAZ) and base metal regions and werepolished using different grades of emery papers. Fi-nal polishing was done using the diamond compound(1 µm particle size) in the disc polishing machine.The specimens were etched with 5 ml hydrochloricacid, 1 g picric acid and 100 ml methanol applied for10–15 s.

The welded joints were sliced using power hack-saw and then machined to the required dimensionsfor preparing tensile and impact test specimens. Twodifferent tensile specimens were prepared as shownin Fig. 1(a) and (b). The unnotched smooth tensilespecimens were prepared to evaluate transverse tensileproperties of the joints such as yield strength, tensilestrength and elongation. The notched specimens wereprepared to evaluate notch tensile strength and notchstrength ratio of the joints. Tensile test was con-ducted in 100 kN, electro-mechanical controlled uni-versal testing machine (UNITEK-94100, FIE-BLUESTAR, India). ASTM E8M-04 guidelines were fol-lowed for preparing and testing the tensile specimens.

Charpy impact specimens were prepared to the di-mensions shown in Fig. 1(c) to evaluate the impacttoughness of the weld metal. Since the plate thicknesswas small, sub-size specimens were prepared. Impacttest was conducted at room temperature using pen-

dulum type impact testing machine (ENKAY, India)with a maximum capacity of 30 J. The amount ofenergy absorbed in fracture was recorded and the ab-sorbed energy was defined as the impact toughness ofthe material. ASTM E23-04 specifications were fol-lowed for preparing and testing the specimens.

The fractured surface of the tensile and impacttested specimens was analysed using scanning electronmicroscopy (SEM, 6410LV, JEOL, Japan) at highermagnification to study the fracture morphology to es-tablish the nature of the fracture. Vicker′s microhard-ness testing machine (HMV-2T, Shimadzu, Japan)was employed for measuring the hardness across theweld with 0.5 kg load for 30 s.

3. Results

3.1 Tensile properties

Transverse tensile properties such as yieldstrength, tensile strength and percentage of elonga-tion, notch tensile strength (NTS) and notch strengthratio (NSR) of the joints were evaluated. In each con-dition, three specimens were tested, and the averageof results is presented in Table 3. The yield strengthand tensile strength of unwelded base metal are 359and 524 MPa, respectively. But the yield strengthand tensile strength of SMAASS (fabricated usingASS filler metal) joints are 205 and 255 MPa, respec-tively. This indicates that there is 51% reduction instrength compared to base metal strength. Similarly,the yield strength and tensile strength of SMAFSS(fabricated using FSS filler metal) joints are 210 and265 MPa, respectively, which are 49% lower comparedto base metal. However, the yield strength and tensilestrength of SMADSS (fabricated by DSS filler metal)joints are 225 and 285 MPa, respectively. Of the threejoints, SMADSS joints exhibited higher strength val-ues, and the enhancement in strength value is approx-imately 9% compared to SMAASS joints and 7% com-pared to SMAFSS joints.

The elongation of unwelded base metal is 12%.But the elongation of SMAFSS joints is 6.76%. Thissuggests that there is a 44% reduction in ductilitycompared to base metal. Similarly, the elongationof SMADSS joint is 8.35%, which is 30% lower com-pared to the base metal. However, the elongation ofSMAASS joints is 10.55%. Of the three joints, theSMAASS joints exhibit higher elongation (ductility)values, and the improvement in ductility is approx-imately 36% compared to SMAFSS joints and 21%compared to SMADSS joints.

The NTS of unwelded base metal is 577 MPa. Butthe NTS of SMAASS joint is 280 MPa. This reveals

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Table 3 Transverse tensile and impact properties of base metal and welded joints

Joint Yield strength Tensile strength Elongation Notch tensile strength Notch strength Impact Location/MPa /MPa /% /MPa ratio (NSR) toughness/J of failure

Base metal 359 524 12.0 577 1.10 22 –SMAASS 205 255 10.55 280 1.1 24 InterfaceSMAFSS 210 265 6.76 278 1.05 6 InterfaceSMADSS 225 285 8.35 325 1.14 18 Interface

that there is 51% reduction in NTS compared tobase metal. Similarly, the NTS of SMAFSS jointis 278 MPa and that of SMADSS joint is 325 MPa.Of the three joints, SMADSS joints exhibit higherNTS values, and the enhancement is 14% comparedto SMAASS joints and 15% compared to SMAFSSjoints. Another notch tensile parameter, NSR is aratio between tensile strength of notched specimenand unnotched specimen and it is found to be greaterthan unity (>1) for all the joints. This suggests thatthe joints are insensitive to notches, and they fall un-der “notch ductile materials” category. The NSR is1.10 for unwelded base metal, but it is 1.1 and 1.05for SMAASS and SMAFSS joints, respectively. Ofthe three joints, the SMADSS joints exhibit relativelyhigher NSR (1.14), and the improvement in NSR is3.5% compared to SMAASS joints and 7.9% com-pared to SMAFSS joints.

3.2 Impact toughness

Charpy impact toughness values of all the jointswere evaluated and they are presented in Table 3. Theimpact toughness of unwelded base metal is 22 J. Butthe impact toughness of SMAASS joints is 24 J. Thisindicates that there is 9% increase in toughness valuecompared to the base metal. Similarly, the impacttoughness of SMAFSS joints is 6 J, which is 73% lowercompared to base metal. However, the impact tough-ness of SMADSS joints is 18 J. Of the three joints,the SMAASS joints exhibit higher impact toughnessvalues, and the enhancement in toughness value isapproximately 25% and 75% compared to SMADSSjoints and SMAFSS joints, respectively.

3.3 Hardness and microstructure

The hardness across the weld cross-section wasmeasured using a Vickers microhardness testing ma-chine, and the results are displayed in Fig. 2. Thehardness of weld metal (WM) region is greater thanthe HAZ region, but lower than the base metal (BM)region, irrespective of filler metals used. The hardnessof the SMAASS and SMAFSS joints in the weld metalregion is 254 VHN and 302 VHN, respectively. How-ever, the hardness of the SMADSS joints in the weldmetal region is 360 VHN, which is relatively highercompared to SMAASS and SMAFSS joints.

Microstructure of all the joints was examined atdifferent locations such as WM region, HAZ regionand WM-HAZ interface region as shown in Fig. 3.The joints fabricated by ASS filler metals primar-ily contain solidified dendritic structure of austenite(Fig. 3(a)–(c)) and the joints fabricated by FSS fillermetals contain solidified ferritic grains (Fig. 3(d)–(f)).Whereas, the joints fabricated by DSS filler metalscontain both solidified austenitic structure in the fer-

Fig. 2 Hardness profile across the weld

rite matrix (Fig. 3(g)–(i)). The HAZ region of thejoints invariably contains coarse ferrite grains irre-spective of filler metals used.

3.4 Fracture surface

The fractured surface of tensile specimens ofwelded joints was analyzed by SEM to reveal thefracture surface morphology. Figure 4 displays thefractographs of tensile specimens and impact speci-mens. The tensile and impact fracture surfaces ofSMAASS joints (Fig. 4(a) and (d)) and SMADSSjoints (Fig. 4(c) and (f)) show ductile fracture,whereas, SMAFSS joints (Fig. 4(b) and (e)) showbrittle fracture. An appreciable difference existsin the size of the dimples between SMAASS andSMADSS joints. Elongated and fine dimples are seenin SMADSS joints, whereas fine dimples are seen inSMAASS joints. Large numbers of fine dimples areseen in SMAASS joints compared to SMADSS joints.Since fine dimples are a characteristic feature of duc-tile fracture, the SMAASS joints have shown higherductility compared to all other joints (Table 3).

4. Discussion

In this investigation, an attempt was made to findout the best filler metal to weld AISI 409M gradeferritic stainless steel, and hence three different fillermetals: ASS, FSS and DSS were used to fabricatethe joints. From this investigation, it is understoodthat the tensile and impact properties of FSS jointsare influenced by the filler metals. The joints fab-

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Fig. 3 Optical micrographs of FSS weldments: (a) SMAASS joint-interface, (b) SMAASS joint-weld centre, (c)SMAASS joint-HAZ, (d) SMAFSS joint-interface, (e) SMAFSS joint-weld centre, (f) SMAFSS joint-HAZ,(g) SMADSS joint-interface, (h) SMADSS joint-weld centre, (i) SMADSS joint-HAZ

ricated using DSS filler metal exhibited higher yieldstrength and tensile strength compared to ASS andFSS filler metals. The joints fabricated using ASSfiller metal yielded higher ductility (percentage ofelongation) compared to DSS and FSS filler metals.The joints fabricated using ASS filler metal exhibitedhigher impact toughness compared to DSS and FSSfiller metals. The joints fabricated using DSS fillermetal recorded higher hardness in weld metal regionand HAZ region compared to other filler metals. Theabove variations in tensile, impact and hardness be-haviour of FSS joints are caused by the two importantcharacteristics of weld metal. They are: (1) chemicalcomposition of the weld metal (2) microstructure of

the weld metal.Cr is one of the most important alloying element

in stainless steels, which promotes the formation offerritic structure. It is also strong carbide former andhence increases the hardness and strength of the stain-less steels[9]. Of the three joints, SMADSS joint con-tains higher amount of Cr (2% higher than SMAASSjoints and 5% higher than the SMAFSS joints). Thismay be one of the reasons for the formation of ferriticstructure in the weld metal region of SMADSS joint(Fig. 3(h)).

Similarly, Ni is another important alloying ele-ment in stainless steels. Ni promotes the formationof austenitic structure and hence increases ductility

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Fig. 4 SEM fractographs of tensile and impact specimens: (a) SMAASS, (b) SMAFSS, (c) SMADSS, (d)SMAASS, (e) SMAFSS, (f) SMADSS. D—Dimples; QC—Quasi cleavage; C—Cleavage; FD—Fine dimples,ED—Elongated dimples

and toughness of the stainless steels[10]. Of the threejoints, SMAASS joint contains higher amount of Ni(2% higher than SMADSS and 10.6% higher thanSMAFSS). This may be one of the reasons for theformation of austenitic structure in the weld metalregion of SMAASS joint (Fig. 3(b)). Higher amountof Cr and very less amount of Ni in SMAFSS joint ledto the formation of ferritic structure alone in the weldmetal region.

Mo is also another important alloying element,which strongly promotes the formation of ferriticstructure in stainless steels. It is also carbide formerand hence increases the strength and hardness of thestainless steels[11]. Of the three joints, SMADSS jointcontains higher amount of Mo (higher than SMAASSand SMAFSS joints). Though the amount of Cr andNi in weld metal region of SMAASS and SMADSSjoints are approximately equal, the presence of higheramount of Mo in SMADSS made all difference. Higheramount of Mo suppressed the formation of austeniticstructure and promoted the formation of ferritic struc-ture in SMADSS joint and it acted as a balancingelement.

Though the C content in weld metal region is verylow compared to Cr, Ni, Mo, it also plays an impor-tant role in the formation of microstructure. Carbonis a strong austenite former and strongly promotesan austenite structure in stainless steels. It also hasstrong affinity towards Cr and Mo to form carbidesand hence it subsequently increases the strength andhardness of the stainless steels[12]. Of the three joints,SMAFSS joints contains higher amount of C (0.04%higher than SMAASS joint and 0.045% higher thanSMADSS joint). However, the presence of higherpercentage of carbon will enhance the brittleness ofthe weld metal and also will lead to the formation ofhigher volume fraction of chromium carbides in theweld metal region. Though the carbon is an austeniteformer, very less amount of Ni and higher amount ofCr suppress the formation of austenitic structure inthe weld metal region of SMAFSS joint.

The formation of austenitic structure will en-

hance the ductility (percentage of elongation) andsubsequently increase the impact toughness values[13].However, the formation of ferritic structure will en-hance the tensile strength[14]. The formation of car-bides such as chromium carbide, molybdenum carbidewill also be beneficial to enhancing the strength andhardness of stainless steels to some extent. But highervolume fraction of these carbides will lead to reduc-tion in ductility, strength and impact toughness[15].

Hence, the formation of fully austenitic structurein the weld metal region of SMAASS joint due tothe presence of higher percentage of Ni content ledto the enhancement in ductility and impact tough-ness properties. On the other hand, the formation offully ferritic structure and possible formation of largevolume fraction of carbides in the weld metal regionof SMAFSS joint due to the presence of higher per-centage of Cr and C (very less percentage of nickel)content led to the reduction in ductility and impacttoughness properties. However, the formation of fer-ritic and austenitic structure in the weld metal regionof SMADSS joint due to the presence of balanced per-centage of Cr, Ni and Mo content led to the higherstrength and hardness properties.

5. Conclusions

(1) Of the three filler metals used, the joint fabri-cated by DSS filler metal showing 9% and 7% highertensile strength compared to the joints fabricated byASS and FSS filler metals, respectively.

(2) Of the three filler metals used, the joint fabri-cated by ASS filler metal showing 75% and 25% higherimpact toughness compared to the joints fabricated byFSS and DSS filler metals, respectively.

Acknowledgements

The authors are grateful to the Department of Man-ufacturing Engineering, Annamalai University, Anna-malainagar, Tamil Nadu, India, for extending the facilitiesof Metal Joining Laboratory and Materials Testing Lab-

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oratory to carry out this investigation. The authors arevery grateful to Dr. G. Madhusudhan Reddy, Scientist-F, Defence Metallurgical Research Laboratory (DMRL),Hyderabad for his valuable suggestions, guidance and dis-cussion.

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