resistance upset butt welding of austenitic to martensitic stainless steels

Materials and Design 31 (2010) 3044–3050

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Materials and Design

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Short Communication

Resistance upset butt welding of austenitic to martensitic stainless steels

Mahmood Sharifitabar, Ayyub Halvaee *

Faculty College of Engineering, University of Tehran, P.O. Box: 11365-4563, Tehran, Iran

a r t i c l e i n f o a b s t r a c t

Article history:Received 6 October 2009Accepted 15 January 2010Available online 20 January 2010

0261-3069/$ - see front matter � 2010 Elsevier Ltd. Adoi:10.1016/j.matdes.2010.01.026

* Corresponding author. Tel.: +98 21 61114104; faxE-mail address: halvaee@ut.ac.ir (A. Halvaee).

In this research, joining austenitic to martensitic stainless steels and effect of welding power on micro-structure and mechanical properties of the joint were investigated. Microstructure of the weld wasstudied using optical microscopy (OM) and scanning electron microscopy (SEM). Energy dispersive spec-troscopy (EDS) linked to SEM was used to determine chemical composition of phases and distribution ofchromium (Cr), nickel (Ni) and iron (Fe) at the joint interface. Microhardness and tensile strength testswere performed. Finally fracture surface of samples were studied by SEM. Results showed that an inter-layer composed of 80% ferrite and 20% martensite has formed at the joint interface and there were threedifferent zones in the heat affected zone (HAZ) of two steels. Different forms of austenite phase includingwidmanstatten austenite (Wc), allotriomorphic austenite (Ac) and intergranular austenite (Ic), deltaferrite (d-ferrite) and chromium carbide (Cr23C6) have formed in the HAZ of austenitic stainless steel.Fractography of tension samples indicated that in all samples fracture occurred in austenitic stainlesssteel HAZ. The strength and hardness of the joint increased and HAZ length decreased with increasingof welding power.

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1. Introduction

Resistance upset butt welding is carried out in the solid stateand two similar or dissimilar metals are jointed before the meltingtemperatures are reached. In this process, passing of electrical cur-rent through the joining face which resists against the current flow,leads to heating of metals up to the mushy state. Press of joiningfaces to each other causes atomic diffusion producing the joint.The most important factors are welding power, welding time andapplied pressure. The applied pressure causes deformation andjoining of the contact surfaces. Heat is generated through the elec-trical resistance of the material and the joint face [1–3]. There aretwo types of resistances namely contact resistance and bulk resis-tance. At the earlier stage of the welding, the contact resistanceplays the main role but gradually it decreases and the role of bulkresistance finds more importance [4,5]. In comparison with fusionwelding processes, the chemical composition and metallurgicalproperties are not significantly changed leading to better mechan-ical properties. Simplicity, welding speed, capability of remote con-trol and independence of welding quality from the operator skillare the other advantages of this process [6,7]. Therefore, resistanceupset welding is finding more application such as sealing of atomicwaste containers, welding of automotive parts and joining of stain-less steels and low carbon steels as well as super alloys and alumi-num alloys [4,8–10].

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Stainless steels are widely used in industries. Corrosion resis-tance of martensitic stainless steel is less than other classes ofstainless steels due to lower chromium and higher carbon con-tents. These steels are used when high strength and atmosphericcorrosion resistance are important. Their weld ability is low be-cause of the formation of non-tempered martensite and possibilityof hydrogen cracking in weld and heat affected zones.

Austenitic stainless steels are the main group of stainless steelsand show good corrosion resistance to many corrosive media [11].In order to combine these properties, these two groups of steelshave to be welded suitably. Solid state welding processes such asresistance upset welding are proper welding procedures. Similarjoining of austenitic stainless steels has been performed by thisprocess [2,9]. But this procedure has not been used to weld dissim-ilar steels such as austenitic to martensitic stainless steels. In thisresearch, resistance upset butt welding of X3CrNiCu19-9-2 (AISI304) austenitic stainless steel to X20Cr13 (AISI 420) martensiticstainless steel and effect of welding power on microstructure andmechanical properties of the joint have been studied. Also energydispersive spectroscopy (EDS) linked to the SEM was used to deter-mine the chemical composition of the phases and distribution ofCr, Ni and Fe at the joint interface. Finally fracture surface of ten-sion samples were studied by SEM.

2. Experimental procedure

The chemical composition of X3CrNiCu19-9-2 (AISI 304) andX20Cr13 (AISI 420) steels are presented in Table 1. Fig. 1 shows

Table 1Chemical composition of two different stainless steels.

Euro norm name C Si Mn P S Ni Cr Cu Mo

X3CrNiCu19-9-2 0.02 0.32 1.54 0.037 0.015 8.05 18.33 2.31 0.3X20Cr13 0.19 0.62 0.77 0.031 0.02 0.19 12.72 0.15 –

Fig. 1. Microstructures of: (a) austenitic stainless steel and (b) martensitic stainless steel.

Table 2Welding conditions of samples.

Samplename

Current(A)

Voltage(V)

Weldingpower (V A)

Weldingpressure (MPa)

Weldingtime (s)

P1 1900 2 3800 1.01 0.5P2 1600 2 3200 1.01 1.1P3 1200 2 2400 1.01 1.6

Fig. 2. SEM microstructure of interface.

M. Sharifitabar, A. Halvaee / Materials and Design 31 (2010) 3044–3050 3045

the microstructures of these steels. Slipping bands and twins arevisible in the microstructures due to cold work prior to welding.

A pair of 100 mm long and diameter of 8 m steel rods wereclamped in the welding machine. Welding was performed under1.01 MPa pressure and 1200, 1600 and 1800 Ampere (A) of currentintensity. Primary and upset pressures were the same. Electricalcurrent was stopped using an automatic controller after weldingand samples were cooled in the air. During welding, the electricalpotential was measured by an avometer and welding power wascalculated using:

P ¼ VI ð1Þ

where P is the welding power (V A), I is the current intensity (A)and V is the electrical voltage (V) [4]. Welding time which is thepassing time of electrical current was also measured by the avom-eter. Welding parameters have been shown in Table 2.

Tensile test was performed according to ASTM-E8 standard atstrain rate of 5 mm/min while the joint interface was held in themiddle of clamping jaws. Longitudinal sections of samples wereprepared to study the microstructure using optical and scanningelectron microscopes. Energy dispersive spectroscopy was carriedout on the phases and transverse lines at the joint interface. Shaef-fler diagram was used for determination of phases that formed inweld interface [11]. A small triangular zone is observed in this dia-gram which represents ferrite + martensite structure. If the chem-ical composition of the weld joint falls into this zone, the weld willbe sound [12]. Balmforth diagram was used to estimate the quan-tity of phases [11]. Micro hardness test was performed along thewidth of the joint interface at 0.2 mm intervals. Finally the fracturesurfaces were studied by SEM.

3. Results and discussion

Macroscopic examinations showed that flash was formed in theaustenitic stainless steel side and martensitic stainless steel did notparticipate in the flash formation suggesting that deformation ismainly limited to austenitic stainless steel side. This has also beenobserved in joining ferritic to austenitic stainless steels [13].

Fig. 2 shows three apparent zones which are: (1) a 100–200 lmjoint interface, (2) heat affected zone of austenitic stainless steeland (3) heat affected zone of martensitic stainless steel. Fig. 3shows different areas of the HAZ of austenitic stainless steel insample P3. As shown in Fig. 3a, the structure adjacent to the jointinterface consists of different forms of austenite phase includingallotriomorphic (Ac), intergranular (Ic) and plate form widmanstat-ten austenite (Wc) and also lamellar d-ferrite among the austenitelayers [14]. In the one millimeter far from the joint area, austeniticgrains are fine and equiaxed and the ferrite has formed in austenitegrain boundaries (Fig. 3b).

As during welding, the temperature of the area adjacent to theweld interface has reached to above the ferrite–austenite portion

Fig. 3. SEM microstructures of different zones in the austenitic stainless steel HAZ in sample P3: (a) near the interface, (b) 1 mm far from the interface and (c) 3 mm far fromthe interface.

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of the Fe–Cr–Ni phase diagram, d-ferrite + liquid phases haveformed at high temperatures. But, because d-ferrite in austeniticsteels is not stable at room temperature [11], during cooling, solidstate transformation of ferrite to austenite occurs. Southwick andHoneycombe [15] concluded that the decomposition of d-ferriteto austenite occurs by two different mechanisms depending uponthe transformation temperature. At high temperature, the reactionoccurs by a diffusion nucleation and growth process whereas atlow temperature the austenite phase forms by a displacive mech-anism on a habit plane close to {1 3 3}d. It is believed that the wid-manstatten austenite grows by a displacive mechanism whereasallotriomorphic austenite is considered to be a reconstructivetransformation. Abtibol Menezes et al. [16] have reported that inbead on plate welding of two-phase ferritic–austenitic stainlesssteels, residual compressive stresses that have been formed nearthe ferrite to austenite transformation temperature also increasethe formation of widmanstetten austenite.

During solid state transformation of ferrite to austenite in thisregion, allotriomorphic austenite forms in ferrite grain boundaries,but the transformation across the entire grain is suppressed byhigh cooling rate of upset butt welding resulting low diffusion rateand low driving force due to low transformation temperature [11].This causes that residual ferrite transforms to widmanstatten aus-tenite by displacive mechanism. Compressive stresses duringtransformation in this welding process also encourage ferrite towidmanstatten austenite transformation.

The joining surface has the highest temperature during weldingand with increasing distance from the weld interface, temperature

decreases. At 1 mm far from the joint interface in 304 austeniticsteel side, the temperature rises up to austenite + d-ferrite regionin Fe–Cr–Ni phase diagram resulting to the formation of ferrite inaustenite grain boundaries. Because of the high cooling rate ofthe joint, the possibility of ferrite to austenite transformation islow and some ferrite will remain in grain boundaries. So the micro-structure of this region consists of austenite and d-ferrite [11]. Dy-namic recrystallization due to hot deformation also is one of themost important factors in decreasing the grain size in this region[17]. Presence of d-ferrite in austenite grain boundaries acts as acrack growth inhibitor and reduces the possibility of intergranularfracture [18] (Fig. 3b). Fig. 3c shows the precipitation of Cr23C6 car-bide particles in austenite grain boundaries in 3 mm far from theweld interface. This phase is formed due to increasing of tempera-ture of this region up to the precipitation temperature of chro-mium carbide (600–850 �C) during welding [11].

Fig. 4 shows the microstructure of HAZ in martensitic stainlesssteel in sample P1. During welding at high temperatures, themicrostructure adjacent to the weld interface is mainly austenite,but a little ferrite is formed in austenite grain boundaries. Duringcooling of the sample, austenite transforms to martensite, but fer-rite does not transform and retains in the microstructure. So micro-structure of this region consists of martensite and retained d-ferritein ambient temperature. The microstructures of areas 1 and 2 mmfar from the interface consist of non-tempered and tempered mar-tensite, respectively.

As seen in Fig. 5, the concentration of Cr, Ni and Fe graduallychanges in a distinct zone in sample P1. Presence of this zone

Fig. 5. EDS analysis of variation in alloying element concentration at the joint interface.

Fig. 4. Optical microstructures of different zones in the martensitic stainless steel HAZ: (a) near the interface, (b) 1 mm far from the interface and (c) 2 mm far from theinterface.

M. Sharifitabar, A. Halvaee / Materials and Design 31 (2010) 3044–3050 3047

shows that Cr and Ni have diffused to martensitic stainless steelwhile Fe has diffused to austenitic stainless steel. Also, becauseof high electrical contact resistance [4], formation of liquid poolat the joint interface during welding and dilution between mar-

tensitic and austenitic steels is the other reason for formationof this region. Fig. 6 shows the microstructure of interface andTable 3 presents the chemical composition of the phase shown inFig. 6. Cr and Ni Equivalents confirm ferrite–martensite structure

Fig. 6. SEM microstructure of interface in sample P1.

Fig. 7. Shaefller diagram [11].

Fig. 8. Effect of welding power on strength of the joint.

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in this region using Shaefller diagram (black spot in Fig. 7) andaccording to Balmforth diagram it consists of 80% ferrite and 20%martensite. As the composition of this area is in the triangular por-tion of Shaefller diagram, the interface is sound [12].

Fig. 8 shows the effect of welding power on weld strength. It isobserved that the weld strength increases with welding power. Allsamples have broken from the HAZ of austenitic stainless steel.This is because the heat generated at this area leads to the growthof secondary phases such as d-ferrite and carbide which is most of-ten (Fe, Cr)23C6 (Fig. 3a–c). As mentioned, d-ferrite inhibits thecrack growth and increases the weld strength [18]. On the otherhand, stress concentration field is formed around carbide particlesreducing ductility and strength of the samples. This raises theprobability of fracture in carbide precipitated area [19]. Increasingof the welding power decreases the welding time [20] and there-fore the possibility of chromium carbide precipitation in the grainboundaries. The role of contact resistance in generating heat in-creases with welding power due to decreasing in the welding time(Table 3) [4]. This raises the temperature gradient between thecontact surface and electrodes, so the cooling rate increases [10].This in turn reduces carbide precipitation.

Fig. 9a shows the hardness profile at various welding powers.Fig. 9b also shows the hardness measurement at the joint interface.It is observed that in HAZ of martensitic stainless steel the hard-ness gradually increases with increasing distance from the inter-face, reaching to its maximum in 1–1.5 mm away from theinterface and then reduces. The presence of ferrite phase reducesthe hardness near the welding interface (Fig. 4a) [11]. At 1 mmfar from the interface in sample P1, all carbides dissolve in austen-ite and grain growth occurs due to high temperature of welding.After cooling, this area becomes martensitic. As all carbon dissolvesin austenite and no annealing occur, the hardness in this area ishighest [11]. After this area due to lower temperature, a littleamount of martensite transforms to austenite so there is justnon-tempered martensite. This leads to carbide growth and hard-ness falls [11]. On the other hand, the hardness of austenitic stain-less steel HAZ does not change remarkably and slightly increases at1 mm away from the interface due to formation of d-ferrite and

Table 3EDS chemical analysis of the joint interface microstructure.

Element Si Ka Cr Ka Mn Ka

Weight% 0.52 ± 0.21 14.24 ± 0.36 1.02 ± 0

increases in welding power does not affect the hardnesssignificantly.

The hardness of martensitic stainless steel HAZ increases se-verely with welding power, as the welding time is reduced andthe cooling rate is increased. With increasing the cooling rate,probability of formation of non-tempered martensite and hencehardness increases. Also as the welding power increases, the max-imum hardness is observed in area closed to the interface, espe-cially for martensitic stainless steel. This shows that the HAZwidth decreases with welding power. As mentioned before,increasing of welding power leads to increasing of thermal gradi-ent between the contact surface and electrodes [4]. This in turn re-duces the length of area whose temperature is higher thanmartensite–austenite transformation temperature.

Figs. 10a and b show the fracture surface of sample P2. It is ob-served that the fracture is completely ductile. In general, the holeswhich are the main sources of ductile fracture nucleate heteroge-neously from the zones of less deformability. Therefore, the holesnucleate from second phases and particles preferably [19].Fig. 10a shows the holes on the fracture surface with different sizesand that large holes have probably nucleated from the areas ofchromium carbide precipitation. Fig. 10b also shows the chromiumcarbide precipitation at the bottom of the dimple.

Fe Ka Ni Ka Cu Ka

.3 81.32 ± 0.6 2.04 ± 0.33 0.86 ± 0.3

Fig. 9. (a) Effect of welding power on microhardness profile across the joint interface and (b) hardness measurement at the joint interface.

Fig. 10. SEM micrograph of fracture surface: (a) low magnification showing the formation of continuous dimples due to the precipitation of chromium carbide particles and(b) high magnification showing the chromium carbide particle in the bottom of dimple.

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4. Conclusions

1. The chemical composition of the weld interface betweenaustenitic and martensitic stainless steels is compatible to fer-rite + martensite portion of Shaefller diagram. According toBalmforth diagram, there are 80% ferrite and 20% martensitein the weld structure.

2. Delta ferrite is formed between widmanstatten plates and onaustenite grain boundaries 2 mm away from the weld interfacein the austenitic stainless steel HAZ, while chromium carbideparticles precipitate in grain boundaries in the area 3 mm farfrom the weld interface in sample P3.

3. In the martensitic stainless steel HAZ adjacent to the weldinterface, the microstructure is fully austenite at high tempera-ture, but little amount of d-ferrite may present on austenitegrain boundaries. During cooling, austenite transforms to mar-tensite and ferrite remains in the microstructure. In areas 1 and2 mm far from the weld interface, the microstructure consists ofnon-tempered and tempered martensite respectively.

4. The weld strength increases with welding power.5. With increasing the welding power, the area in which hardness

changes is restricted and the hardness rises severely in the mar-tensitic stainless steel HAZ.

6. The fracture of welded samples was ductile and all sampleshave failed in the austenitic stainless steel HAZ.

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