mmn

ara

Sa

acce 3

izatone

1 tehardened before the experiments were exposed to spheroidization annealing at 700 C temperature for ve dierent time periods as 4, 8,12, 24 and 48 h. For evaluation of cold forgeability with dierent spheroidization annealing periods, the hardness, percentage of reduc-tion in area in unaxial tension and reduction of area in notched tensile test were recorded. The results of reduction of area in notched

the ductility of the materials is one of the most important

perature for 820 h before cooling to room temperature,

spheroidized rapidly. When investigating the two pro-

initial ferrite-pearlite microstructure and initial martensiticmicrostructure on the spheroidization time periods. Whenevaluating the optimal spheroidization time periods, coldforgeability values determined by the reduction of area in

* Tel.: +90 264 2955856; fax: + 90 264 2955601.E-mail address: erdalk@sakarya.edu.tr

Materials and Design 29 (2

Materialsfactors. The materials should be ductile enough in ordernot to be damaged. Spheroidizing provides the needed duc-tility for cold forgeability [1,2]. These spheroidization treat-ments are long (1048 h). Due to longer process timecausing increase in material treatment cost, the shortesttime is aimed in industrial applications.

Usually, two types of spheroidization processes are usedin forging industry [1]. In the rst one termed intercriticalprocess, the spheroidization treatment consists of heatingthe steel into the intercritical temperature region (740760 C) for 26 h and then slowly cooling below the lowercritical temperature (700715 C) and holding at this tem-

cesses, it was observed that the spheroidization process isaccomplished in a shorter time by the subcritical processthan the intercritical process [1,3]. Since the intercriticalprocess destroys this normalized microstructure by heatinginto the intercritical region [1]. It is noticed in the literaturereview that the researchers using the subcritical processstudied with initial martensitic (hardened) microstructurestarting conditions [4,5] but it is also noticed that there isnot any study that consists of the comparative investiga-tion of the cold forgeability and spheroidization for thesetwo starting conditions.

The aim of this study is to investigate the inuence oftensile test showed that the highest performance is reached with 12 h spheroidization time after being hardened. 2006 Elsevier Ltd. All rights reserved.

1. Introduction

Cold forging has been widely used for forming of med-ium carbon steel machine parts. Medium carbon steelshave been usually softened by spheroidizing treatmentbefore applying cold forging processes. In this process,

as shown in Fig. 1(a). In the second one termed subcriticalprocess, the steel could be spheroidized by heating it to justbelow the A1 temperature for a long time, as shown inFig. 1(b). Initial microstructures were normalized for inter-critical process and subcritical process. This normalizedmicrostructure consists of a ne pearlite, which should beShort Com

Inuence of dierent initialof spheroidizatio

Erdal K

Department of Mechanical Engineering, Engineering Faculty,

Received 23 March 2006;Available onlin

Abstract

In the study, an investigation has been carried out for spheroidTwo dierent spheroidization processes were considered. The rstlong time. The second was the annealing of hardened steel under A0261-3069/$ - see front matter 2006 Elsevier Ltd. All rights reserved.doi:10.1016/j.matdes.2006.11.015unication

icrostructure on the processin cold forging

deniz *

karya University, Esentepe Campus, 54187 Sakarya, Turkey

epted 21 November 20060 March 2007

ion of medium carbon steel (AISI 4140) used in forging industry.was the annealing of normalized steel under A1 temperature for amperature for a long time. Specimens which were normalized and

www.elsevier.com/locate/matdes

008) 251256

&Design

ef 2 lnd0=d f 1where d0 and df are the initial diameter and the diameter atthe point of the fracture [9].

Other mechanical property measurements included theBrinell hardness (187.5 kg load and 2.5 mm diameter ball)and the reduction of area in tension tests on bars of9 mm diameter and 32.1 mm gauge length.

The changes in the microstructure of AISI 4140steel were monitored by optical microscopy in 500magnication.

Fig. 2. Notch tensile test specimen [9].

nd Design 29 (2008) 251256notched tension tests are taken into consideration. The

A1

Tem

pera

ture

Time

Time

Tem

pera

ture

A1

Fig. 1. Spheroidization cycles (a) A typical intercritical annealing cycle.(b) A possible subcritical annealing cycle.

252 E. Karadeniz / Materials aresults showed that subcritical method with starting condi-tion of martensitic microstructure provided the optimalcold forgeability in a shorter time.

2. Experimental studies

In this study, AISI 4140 steel was used. It contains0.41%C, 0.23%Si, 0.77%Mn, 1.08%Cr, 0.18%Mo, 0.01%Sand 0.02%P.

The specimens for spheroidization treatment were pre-pared from rods having a diameter of 18 mm and a lengthof 150 mm. All specimens were annealed at 870 C for45 min and then quenched in air. Then half of these speci-mens were annealed again at 870 C for 45 min and thenquenched in oil. These two groups of specimens exposedto two dierent types of heat treatment were annealed at700 C for 4, 8, 12, 24 and 48 h to spheroidization in anindustrial heat treatment furnace having 35 kW capacityand then cooled in the furnace.

The results obtained from tensile test are not accuratedue to neck formation [68]. Therefore, in this study,plastic deformation and strain values were measured bymeans of three dimensional notched tensile tests by usingspecimens having 2 mm notch radius shown in Fig. 2 in aZwick 1498 tensile test machine having 500 kN capacitywith 2 mm/min. tensile speed in room temperature. Thereduction in cross-section was measured with 0.01 mmsensitivity.

The fracture strain (cold forgeability) was taken as3. Results and discussion

3.1. Microstructure

In this study, two starting microstructures of AISI 4140steel were used for spheroidizing treatment. One of them isa normalized microstructure. This normalized microstruc-ture was realized when initially austenitized at 870 C for45 min and cooled in air to room temperature. The othermicrostructure of AISI 4140 steel is martenzitic. It was pro-duced when initially austenitized at 870 C for 45 min andquenched in oil to room temperature. Figs. 3 and 4 showthe microstructure of normalized and hardened AISI4140 steel, respectively.

Figs. 59 present the microstructures of AISI 4140 steelhaving dierent spheroidization time periods for normal-ized samples. In the initial period of spheroidizing treat-Fig. 3. Normalized microstructure of initially austenitized AISI 4140 steelat 870 C for 45 min and cooled in air to room temperature.

Fig. 6. Microstructure of AISI 4140 steel taken from spheroidizingtreatment after 8 h at 700 C for normalized specimen.

Fig. 4. Hardened microstructure of initially austenitized AISI 4140 steel at

E. Karadeniz / Materials and Dment up to 4 h, pearlites in the microstructure were coars-ened and ngerprint structure was disappeared. Over thisperiod, the degree of spheroidization continued to increaseup to 48 h and primer ferrite grains were seen in all thesamples for all periods. However, ferrite grain size wasbecoming ner with increase in time over 4 h. The sphero-idization process is too slow. Fig. 9 shows that it was notaccomplished completely at the end of 48 h.

Figs. 1014 present the microstructures of AISI 4140steel having dierent spheroidization time periods for hard-ened samples. Spheroidizing treatment of hardened AISI4140 steel showed that the microstructure of the sampleswas gaining a spheroidized microstructure with in a shorttreatment time. However, over a period of 12 h grain

870 C for 45 min and quenched in oil to room temperature.growth was seen clearly.

Fig. 5. Microstructure of AISI 4140 steel taken from spheroidizingtreatment after 4 h at 700 C for normalized specimen.esign 29 (2008) 251256 253On a comparative investigation of the two processescomparatively, it was observed that the spheroidizationprocess was accomplished within a short time by thespheroidization for hardened samples than the spheroidiza-tion for normalized samples, and homogeneously distrib-uted carbides were observed inside the hardened specimens.

3.2. Mechanical tests

Fig. 15 presents the variation of Brinell hardness withspheroidization time. For normalized and hardened sam-ples, it is clear from this gure that the hardness decreasessharply during the rst 4 h. Between 4 and 48 h of sphero-idization time, the drop in hardness rate is very less. There

Fig. 7. Microstructure of AISI 4140 steel taken from spheroidizingtreatment after 12 h at 700 C for normalized specimen.

nd D254 E. Karadeniz / Materials ais not much dierence between the normalized and hard-ened processes.

As a general approach, hardness test is a convenient testto investigate the cold forgeability behavior of a material.It can be said that spheroidization increases the forgeabilityand it shows rise trend with increasing spheroidizationtimes. On comparison of the two processes applied in theexperiments, it was found that the normalized sampleshave more advantages in all spheroidization time periods.

From the results of tensile test, the reduction of area inunaxial tension was found to increase much more rapidlywith spheroidizing process (see Fig. 16), because thespheroidization process enhances the ductility of steel: thisis valid and similar to the literature [1,10]. Hardened spec-

Fig. 9. Microstructure of AISI 4140 steel taken from spheroidizingtreatment after 48 h at 700 C for normalized specimen.

Fig. 8. Microstructure of AISI 4140 steel taken from spheroidizingtreatment after 24 h at 700 C for normalized specimen.esign 29 (2008) 251256imens show better ductility than normalized ones. This ismay be due to the homogeneously distributed carbidesinside the hardened specimens.

Table 1 presents the results of the notched tensile tests.Fig. 17 presents the variation in reduction of area withspherodizing time for normalized and hardened samples.

Fig. 17 shows the eect of spheroidization time on coldforgeability of AISI 4140 steel. Fracture strain, ef, values ofhardened spheroidized specimens increase up to 4 h sharplythen continue to increase with a lower rate up to 12 h. Afterthat these values start to decrease from this point. Thistendency is seen up to 48 h. Similarly the normalizedspheroidized specimens ef values increasing up to 4 h and

Fig. 10. Microstructure of AISI 4140 steel taken from spheroidizingtreatment after 4 h at 700 C for hardened specimen.

Fig. 11. Microstructure of AISI 4140 steel taken from spheroidizingtreatment after 8 h at 700 C for hardened specimen.

nd Design 29 (2008) 251256 255E. Karadeniz / Materials atheir value continues to increasing up to 48 h with a lowerspeed.

On looking at Fig. 17, we can conclude that hardenedspecimens used in subcritical spheroidization method reachhigher ductility values in 12 h in comparison to these ofnormalized ones, which cannot be shown equal value atthe end of 48 h. Hence, initially hardening and then subcrit-ical spheroidization method application give greater forge-ability values in a short time, so it is preferable in coldforging production from AISI 4140 steel with respect totime and cost.

Fracture strain, ef, values of hardened specimensdecrease from 24 h to 48 h in spheroidization processesdue to grain growth of spheroidized carbides inside thestructure.

Fig. 12. Microstructure of AISI 4140 steel taken from spheroidizingtreatment after 12 h at 700 C for hardened specimen.

Fig. 13. Microstructure of AISI 4140 steel taken from spheroidizingtreatment after 24 h at 700 C for hardened specimen.

Fig. 14. Microstructure of AISI 4140 steel taken from spheroidizingtreatment after 48 h at 700 C for hardened specimen.

150

250

350

450

550

650

0 10 20 30 40 50

Spheroidization time, h

Brin

ell h

ardn

ess

Normalized Hardened

Fig. 15. Hardness decrease during spheroidization of AISI 4140 steels.

0

10

20

30

40

50

60

70

0 10 20 30 40 50Spheroidization time, h

Perc

ent r

educ

tion

of a

rea

Normalized Hardened

Fig. 16. Change of ductility of AISI 4140 steel during spheroidization.

The notched tensile test is a useful tool in determiningthe eect of spheroidization times on forgeability.Here, the reduction in area is valid to give accurateresults.

The highest ductility values were obtained in hardenedspheroidized specimens in 12 h, however, these valuescannot be gained in normalized spheroidized ones in48 h. Therefore, hardened spheroidized specimens givemore accurate results than that of normalized ones.

Specimens that were initially hardened and then exposedto spheroidization process for 12 h give the best coldforgeability results for AISI 4140 steel.

References

[1] OBrien JM, Hosford WF. Spheroidization cycles for medium carbon0.5

0.6

0.7

ity

Normalized Hardened

Table 1The results of notched tensile tests

Spheroidiziation time periods (h) Fracture strain, ef

Normalized Hardened

0.16 0.024 0.43 0.538 0.45 0.5712 0.46 0.6024 0.50 0.5948 0.56 0.54

256 E. Karadeniz / Materials and Design 29 (2008) 2512560.3

0.4

Cold

forg

eabi

l4. Conclusion

It was proven that the spheroidization process recoversthe cold forgeability.

0

0.1

0.2

0 10 20 30 40 50Spheroidization time. h

Fig. 17. Change of cold forgeability of AISI 4140 steel duringspheroidization.steels. Metall Mater Trans A 2002;33A:125561.[2] Sarruf Y. Criteria and tests for cold headability. MSc Thesis, McGill

University, Montreal, Canada: 2000.[3] Karadeniz E, Ozgirgin MC, Findik F, Ogur A. Investigation of the

spheroidization of hypoeutectoid steels with dierent method. In: 6thDenizli materials symposium, proceedings, Denizli, Turkey: 1995.p. 1706.

[4] Zhou M, Clifton RJ. Dynamic ductile rupture under conditions ofplane strain. Int J Impact Eng 1997;19:189206.

[5] Ni H, Wang Z. Eect of pre-strain and mean stress on cyclic plasticdeformation response of iron-based alloys. Mater Sci Eng A2001;314:1223.

[6] Davidenkov NN, Spiridonova NI. Mechanical methods of testing.Analysis of the state of stress in the neck of a tension test specimen.Proccess ASTM 1946;46:114758.

[7] Cockcroft MG, Latham DJ. Ductility and the workability of metals. JInst Metal 1968;96:339.

[8] Ulvan E, Koursaris A. Metal formability in bulk deformationprocesses. J Metals 1983:206.

[9] Gelin JC, Oudin J, Ravalard Y. Inuence de quelqees parametresmetallurgiqves sur la rupture ductile des aciers a bas et moyencarbone. MES Revue de Metall 1984:16979.

[10] OBrien JM. Spheroidizing of medium carbon steels. Dissertation,University of Michegan: 2000.

Influence of different initial microstructure on the process of spheroidization in cold forgingIntroductionExperimental studiesResults and discussionMicrostructureMechanical tests

ConclusionReferences