Microstructure and mechanical properties of friction welded joints of a fine-grained hypereutectoid...

Journal of Materials Processing Technology 166 (2005) 63–70

Microstructure and mechanical properties of friction welded joints of afine-grained hypereutectoid steel with 4% Al

Niyazi Ozdemir∗, Nuri OrhanDepartment of Metal, Faculty of Technical Education, University of Fırat, 23119 Elazı˘g, Turkey

Received 11 September 2003; received in revised form 20 July 2004; accepted 20 July 2004

Abstract

In this study, joining characteristic of a fine-grained hypereutectoid ultra-high-carbon steel (UHCS) with thermo-mechanical cycle wasinvestigated by the friction welding process. The joining performances of UHCS/UHCS friction welded joints were studied, and the influencesof welding parameters (friction pressure, forging pressure, friction time, forging time and rotational speed) on the microstructure and mechanicalproperties of the welded joints were also estimated. The microstructural properties of the heat-affected zone (HAZ) were examined by opticaland scanning electron microscopy (SEM). The microhardness across and perpendicular to the interface were measured and the strength of thej xperimentalr ed, frictionp tion speed,f©

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oints was determined with tensile tests. The results were evaluated considering the microstructures formed during welding. The eesults indicate that each parameter has a little individual effect on the quality of the joint but the combined effect of the rotation speressure and friction time has a significant effect on the mechanical and metallurgical properties. Especially, by choosing rota

riction pressure and friction time properly, it is possible to increase the quality of joint.2004 Elsevier B.V. All rights reserved.

eywords: Solid-state welding; Friction; Friction welding; Hypereutectoid steel

. Introduction

Superplastic steels have a very fine-grained structureue to the carbides and nitrides of the small amount of thelloying elements added such as Al, V, Nb, Ti, Cr, B and Mo

1]. Superplastic materials are characterized by their hightrain-rate sensitivity index (m) and their deformation onesistance to neck growth[2]. Backofen[3] determined that aroper thermo-mechanical treatment led to superplasticity by

orming a structure composed of a homogenously distributedarbide network within the fine-grained ferritic matrix inltra-high-carbon steels (UHCS). These steels, which cannote readily formed at room temperature due to their high car-on content, became industrial materials after it was learned

hat they showed superplasticity at some certain temperaturesnd superplastic forming techniques were developed[3,4].oining this kind of steels has gained importance after their

∗ Corresponding author. Tel.: +90 424 2370000/6245.E-mail address:[email protected] (N.Ozdemir).

industrial applications as structural materials[5]. Thoughfine-grained hypereutectoid steels can be formed easilysuperplasticity temperature, the superplastic microstrudisappears and high carbon content causes crackingthey are joined with the conventional welding meth[4,6,7]. The main goal should be protecting the superplamicrostructure as much as possible and preventing crain the welding of superplastic high carbon steels. Sincestructure of HAZ completely changes during fusion weldfriction welding is one of the solid-state bonding meththat seems proper for joining UHC steels[8,9]. There ismuch research on the diffusion bonding of superplasteels but no study on the friction welding of UHC steelthe literature. Therefore, in this study, first we manufacta fine-grained hypereutectoid steel with thermo-mechaprocesses. Second, the possibility of joining UHCS, in suplastic state by friction welding was examined. This incluexamination of the joining characteristics of UHCS/UHfriction welded joints using different combinations of weing parameters. The microstructural changes in heat-aff

924-0136/$ – see front matter © 2004 Elsevier B.V. All rights reserved.oi:10.1016/j.jmatprotec.2004.07.095

64 N. Ozdemir, N. Orhan / Journal of Materials Processing Technology 166 (2005) 63–70

Table 1The chemical composition of the hypereutectoid steel by atomicspectrometry

Alloying element wt.%

C 1.18Si 0.15Mn 0.70P 0.025S 0.039Cr 0.029Ni 0.029Al 3.95Mo 0.20Ti 0.10

zone (HAZ) and the mechanical behavior of the joints wereexamined.

2. Materials and experimental procedures

2.1. Materials and specimen preparation

The material used in this work was a hypereutectoid steelmanufactured by induction melting and casting into the barsof 60 mm in diameter and 400 mm in length. The chemicalcomposition of the materials is listed inTable 1. The castmaterial was homogenized at 1100◦C for 150 min, then hotforged with a cross-sectional reduction of 94% at 850◦C fol-lowed by tempering at 650◦C. Finally, a heat treatment cyclewas performed comprising heating up to a temperature 30◦Chigher than the A1 (723◦C) temperature determined by DTAfor this steel and quenched in oil at 200◦C. This cycle wasrepeated 10 times in order to obtain a fine-grained microstruc-ture (less than 10�m), which is a prerequisite for superplasticbehavior.

F ; (4) sta ;( lve; (1

Table 2Friction welding parameters used in the present work

Specimenno.

Rotationspeed(rpm)

Frictionpressure(MPa)

Forgingpressure(MPa)

Frictiontime (s)

Forgingtime (s)

S1 1500 25 50 6 2S2 1500 30 60 6 2S3 1500 25 50 8 4S4 2000 25 50 6 2S5 2000 30 60 6 2S6 2000 25 50 8 4

Fig. 2. Schematic of the HAZ showing the fully plasticized (Zpl), partiallydeformed (Zpd) and undeformed (Zud) regions.

2.2. Friction welding tests

All the friction welding tests were carried out usinga direct-drive type friction welding machine, which wasdesigned and manufactured for this purpose (Fig. 1).The surface preparation of each sample before frictionwelding involved grinding to an 800-grit finish, followedby degreasing with acetone, which ensured grease- anddirt-free surfaces. Friction welds were obtained underdifferent parameter combinations (friction pressure, forgingpressure, friction time, forging time end rotational speed).The experimental conditions used are summarized inTable 2.

ig. 1. Experimental set-up: (1) electric motor; (2) clutch; (3) rotary jaw9) motor and gear pump; (10) hydraulic tank; (11) direction control va

bilizer; (5) stationary jaw; (6) hydraulic cylinder; (7) piston bearing; (8) inverter2) speed control valve.

N. Ozdemir, N. Orhan / Journal of Materials Processing Technology 166 (2005) 63–70 65

2.3. Examinations

The joints were cut perpendicular to the bond interfaces,using a low-speed saw, and the cross-sections of thesejoints were metallographically polished using 2�m ofdiamond paste as final polish and cleaned in an acetonebath. In order to study the microstructure and elementalanalysis of HAZ thus evaluating the quality of the joints,conventional characterization techniques such as scanningelectron microscopy (SEM), energy depressive spectrograph(EDS) and tensile tests were employed. Fractographywas also employed in the evaluation of the fracturedsurfaces.

3. Results and discussion

3.1. Microstructure

According to Midling and Grong[10], the contact zoneat the joint interface is divided into three zones: (1) the fullyplastized region (Zpl) where the deformation depend on acombination of the rotational velocity gradient and the ratesof axial and radial upsetting; (2) the partially deformed re-gion (Zpd); and (3) the undeformed region (Zud), as shownin Fig. 2. The SEM photos taken from the HAZ of the spec-imens manufactured in this work are given inFigs. 3–9,respectively.

Fig. 3. SEM micrograph of specimen S1 taken from HAZ and SEM m
icrographs of HAZ of the specimen S1: (a) Zpl; (b) Zpd; (c) Zud.


Fig. 4. SEM micrographs of HAZ of the specimen S2: (a) Zpl; (b) Zpd; (c) Zud.





Microstructural changes took place in the first and sec-ond regions. The weld region (forged region) became nar-rower as a result of viscous material being carried outwardswith the increase in the weld speed, forging and frictionpressures.

Fig. 3 shows the microstructure of S1 specimen in HAZ.In this specimen, the microstructure in the interface of thejoint consists of lath and plate martensite; the effect of theplastic deformation on the grain morphology can be seenin Fig. 3a. In the partially fine-grained region, the structurecomprises grain boundary cementite and tempered martensite(seeFig. 3b). In the third region, the original microstructureexists.

Fig. 4shows the microstructure of sample S2 in the HAZ.In the joint of the specimen S2 welded by increasing thefriction and forging pressures as far as 5 and 10 MPa, re-spectively, due probably, to the increased dislocation density,

of the s

plate martensite occurred in the interface[11] (Fig. 4a). Inthe second region, lath martensite is seen in a widespread ce-mentite network (Fig. 4b). S3 is the specimen that was weldedwith prolonged friction and forging times. In this joint, HAZextended due to the prolonged friction time. However, as aresult of the prolonged friction time and subsequent exten-sion in HAZ, the lath martensite seen in the first region existspartly in second region as well. Plate martensite also occurredin this region (seeFig. 5a).

The effect of the increase in speed can be observed in thespecimen S4 (Fig. 6). As the speed increases, the time forthe maximum temperature decreases and consequently, thecooling gradient is steep due to the heat rapidly absorbedby the adjacent region[11]. This leads to a fully mertensiticmicrostructure (lath and plate) in the first and second regions(Fig. 6a and b). There is a widespread cementite network inthe third region.

Fig. 8. SEM micrographs of HAZ
pecimen S6: (a) Zpl; (b) Zpd; (c) Zud.


Fig. 9. The hardness distribution in HAZ of the specimens S1, S2 and S3.

As the friction and forging pressures are increased, thestructure seen in the specimen S4 changes to a combination oflath martensite and cementite network in the fully deformedregion or interface in the specimen S5. The cementite networkis separated in the second region probably due to the higherforging pressure and the maximum temperature (just aboveA3 temperature) reached (Fig. 7). In the third region, theoriginal structure changed to a widespread cementite networkenveloping, probably upper bainit. The maximum decrease inlength (approximately 4.5 mm) happened in the specimen S6welded with the longest friction and forging times and underthe largest friction and forging pressures creating increasein metal viscosity and pressure. The forged region has thefinest grains among the other specimens. This is caused bythe higher temperature and relatively higher forging effect.Since the material was subjected to the maximum temperatureand pressure during the test, it was transferred out of theinterface by viscous forces. The microstructure are composedof lath martensite islands distributed among an extensive andwidespread cementite network (seeFig. 8). It is clear that themartensitic transformation remained incomplete. The forgingeffect produced a microstructure comprising a finer cementitenetwork with respect to the original structure, in the secondregion.

In the EDAX analyses, while C, Al, and Cr were detected,no carbide-forming elements added such as Ti and Mo wasd entsa hint

3

s ll thej ma-t turesf s, itc nsite( fric-t f them org-

Fig. 10. The hardness distribution in HAZ of the specimens S4, S5 and S6.

ing pressures are thought to contribute to the formation ofmartensite in the interface or the first region and partly in thesecond region.

Among the specimens, the highest hardness value wasrecorded for the S1 specimen (seeFig. 9). This hardness val-ues indicate a martensitic region just at the interface. As isknown well, the external stresses increase the dislocation den-sity and decrease the energy barrier for the nucleation for theloss of the coherency of the second phase precipitates. Thistriggers the formation of martensite[12].

It can be seen from theFig. 9 that the hardness gradi-ent is stepper in the specimen S2 due to the increase in thefriction and forging pressures and consequent more severeplastic deformation. Depending on the increase in the fric-tion and forging pressures, more mass is thought to be trans-ferred out of the interface thus narrowing the interface or firstregion. Increase in the friction and forging time caused theinterface to reach the temperatures much higher than S1 andS2 above A3 temperature. The larger martensitic region andsteeper hardness gradient into the main material support this;seeFig. 9 and microstructure photograph (Figs. 4 and 5).In the specimen S4, the speed is increased. It is seen fromboth the SEM photos and hardness distribution, the speed is

etermined in the weld region. This shows that these elemre dissolved within the matrix completely or placed wit

he cementite as a result of the careful heat treatments.

.2. Hardness

Microhardness test results are given inFigs. 9 and 10. It iseen that hardness was maximum at the interface for aoints and decreased at different rates toward the mainerial. The hardness values are consistent with the strucormed. From the hardness distribution and SEM photoan be said that the microstructure are entirely martelathe and plate) in the first and second regions. Theion and forging times and pressures affect the type oartensite formed in these regions. The friction and f
Fig. 11. Tensile test results of the superplastic material, S2 and S4.


Fig. 12. Micrographs of the tensile fracture surfaces of the specimens observed by SEM: (a) superplastic material; (b) S2; (c) S4.

not very much influential on the microstructural changes forthis steel since the hardness gradient is softer and the widthof the martensitic region is a bit wider than the other speci-mens. In the specimens S5 and S6, the highest hardness valueswere recorded around 550 HV (seeFig. 10). This indicatesa martensitic structure and micrographs from these regionssupport this. The fully martensitic region is approximately1 mm wide. In the third and partly second regions, there istempered martensite. As a whole, there are no big differencesin the hardness profiles of the specimens yet increased fric-tion time and friction pressure together with increasing speedwidened the deformed and transformed region.

3.3. Tensile test results

After friction welding, results of the cold tensile tests per-formed are given altogether inFig. 11. Before the welding,the specimens showed a relatively ductile manner in tensionbut after the welding the manner changed into a brittle one(seeFig. 11). There is no plastic deformation detected in thespecimens after the welding. This behavior can also be seenfrom the micrographs of the fractured surfaces (Fig. 12). Thefracture is dimple before the welding due to the fine grainsize (seeFig. 12a). After welding, it is seen that the modechanged to a mixed mode, mainly in brittle manner becauseoa

4

urea of theh rans-

ferred out of the interface region and consequently the regionwith higher hardness narrowed. In case of the lower pressuresand shorter friction times, the transformed region in the in-terface widened and a region composed of a fully martensiticstructure and local carbides occurred. This created a con-siderably hard and brittle interface. The maximum hardnesswas recorded with the longer friction times that broadenedthe HAZ.

The characteristic microstructure in the fully deformedregion is martensite. The friction and forging pressures andtimes determine the type of martensite.

The fine-grained hypereutectoid high carbon steels can bewelded with the friction welding but in the interface a fullymartensitic microstructure occurs causing the fine-grainedstructure, which is a prerequisite for the superplasticity ofthese steels, to disappear completely.

Acknowledgements

The authors would like to thank the University of Fırat forthe financial support of investigations. This work is sponsoredby the University of Fırat Research Fund under the projectNo. 479.

R

pidlyes.

te of

,

f martensite formed at the interface of the joints (Fig. 12bnd c).

. Conclusions

Generally, the increase in friction time, friction pressnd forging pressure are expected to increase the slopeardness profile. At higher pressures, more material t

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[8] R.Y. Tylecote, The Solid Phase Welding of Metals, Edward ArnoldLtd., London, 1968, pp. 1–150.

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Microstructure and mechanical properties of friction welded joints of a fine-grained hypereutectoid...

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