Journal of Materials Processing Technology 83 (1998) 115–120

Effect of temperature and strain rate on the superplastic behaviourof high-carbon steel

M.M. Moshksar *, E. Marzban RadDepartment of Materials Science and Engineering, School of Engineering, Uni6ersity of Shiraz, Shiraz, Iran

Received 24 March

Abstract

The superplastic behaviour of 0.9% carbon steel has been investigated at intermediate temperatures of 650–710°C and strainrates ranging from 5×10−5 to 7×10−3 s−1. For this purpose, heat treatment was performed to reduce the grain size of the steel(1–5 mm). Microstructure evaluations were made by optical microscopy and scanning electron microscopy (SEM). The materialexhibits high tensile ductility (a total stain of 150–305%) and a high strain-rate-sensitivity exponent m (up to 0.68). It is concludedthat grain growth retards grain boundary sliding and superplastic deformation. © 1998 Elsevier Science S.A. All rights reserved.

Keywords: Strain rate; Superplastic behavior; High-carbon steel

1. Introduction

The ability to produce complex shapes by simpledeformation is the main reason of studying the super-plastic behaviour of materials. As reported [1], super-plasticity was introduced by Rosenhime et al. withZn–Al–Cu alloys and by Pearson with Pb–Sn eutecticalloy, as early as 1920. Investigation into the superplas-tic behaviour of materials was accelerated during the1960s and attracts much attention at the present time.Superplasticity in materials is produced by two distinc-tive mechanisms: (i) micrograin superplasticity; and (ii)transformation superplasticity. In the former, uncom-monly large tensile extension is obtained in fine-grainedmaterials at a temperature above the recrystalizationtemperature, while in the latter such deformation canbe produced by the repeated thermal cycling of materi-als through phase transformations while the sample isunder load [2,3].

Superplastic materials are characterised by their highstrain-rate sensitivity m and their tensile deformation iswell understood from studies on resistance to neckgrowth [4]. For high values of m, the local increase in

strain rate during the necking process generates suffi-cient local hardening to retard neck growth [5].

Fig. 1. The strain-rate dependence of: (a) flow stress; and (b) strain-rate sensitivity, m.

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M.M. Moshksar, E. Marzban Rad / Journal of Materials Processing Technology 83 (1998) 115–120116

Fig. 2. The microstructure of the steel used (echant: Nital 2%).

In superplastic materials, there is an S-shaped rela-tionship between the flow stress and the strain rate [6].As shown in Fig. 1, the behaviour can be divided intothree distinctive regions. Superplasticity with high de-formation to failure occurs over a rather narrow rangeof intermediate strain rates in Region II and there aredecreases in the elongation to failure both at low strainrates in Region I and at high strain rates in Region III[7].

The mechanism of deformation has been clarified ineach region and various models have been presented inthis respect [8]. The mechanisms of flow in the differentregions are interpreted in terms of transitions betweendifferent rate-controlling processes. Region I is associ-ated with fine and coarse grains at any selected stresslevel. Region II is due to the presence of very smallgrain sizes and superplastic materials, within this regionflow occurring by the movement of grain boundarydislocations and grain sliding. The mechanism of Re-gion III is creep deformation at high temperature thattakes place by the glide and climb of dislocations.

In the present investigation, the effect of tempera-tures over 650–710°C and strain rates in the range from5×10−5 to 7×10−3 s−1 were studied in respect at thesuperplastic behaviour of a micrograin 0.9% C steel.

2. Materials and experimental procedure

The composition of the steel used in the presentstudies is shown in Table 1. The steel has a pearliticstructure with a grain size of 5–15 mm. The microstruc-ture of the steel is shown in Fig. 2.

In order to produce a fine grain steel, the materialwas heat treated as shown in Fig. 3. Following this heattreatment, the size of the grains was found to bereduced to the range of 1–5 mm. Microstructure evalua-tions were made by SEM and optical microscopy meth-ods. It should be noted that at the end of heattreatment, the transformation of martensite to Alfa wasincomplete.

Load was applied to the sample while it was in anespecially constructed furnace for 15 min in a tempera-ture range of 650–710°C, during which the martensite-to-Alfa transformation was completed. The dimensionsof the samples used in the present study are shown inFig. 4.

Loading was applied by an electronically modifieduniversal testing machine (type ESC-20) with speciallyconstructed jaws to obtain the desired strain rate in therange of 5×10−5–7×10−3 s−1.

3. Results and discussion

3.1. Effects of strain rate on the 6ariation of the totalstrain with temperature

In order to study the effect of strain rate on the

Table 1Composition of steel (wt.%)

V WMn CrC Si

0.030.01 0.11.20.9 0.50

M.M. Moshksar, E. Marzban Rad / Journal of Materials Processing Technology 83 (1998) 115–120 117

Fig. 3. Heat-treatment procedure for the 0.9% carbon steel. W.Q.,water quench.

Fig. 4. Dimensions of the sample (mm).

s−1 were selected; Fig. 5(a)–(d) shows the results.As shown for all four of the selected strain rates, the

increase in temperature up to a particular value leads toan increase in strain to failure, after which it drops.With increased temperature, the movement of disloca-tions, diffusion and grain boundary sliding are per-formed more easily and the material is able to suffermore deformation before failure. The decrease in total

variation of the total strain to failure, constant strainrate tests were carried out. For this purpose, strainrates of 7×10−3, 1×10−3, 1×10−4 and 5×10−5

Fig. 5. Effect of strain rate on the variation of total strain with temperature: (a) 71×10−3; (b) 1×10−3; (c) 1×10−4; and (d) 5×10−5 s−1.

M.M. Moshksar, E. Marzban Rad / Journal of Materials Processing Technology 83 (1998) 115–120118

Fig. 6. The microstructure of failed samples (echant: Picral 0.1%).

strain at higher temperature at a given strain rate canbe attributed to grain growth. At low strain rates, thegrain growth starts at relatively lower temperaturesbecause the sample is kept longer at the test tempera-ture. This provides sufficient time for grain growth.Further increase in temperature accelerates graingrowth, which is the reason for the decrease in totalstrain. Fig. 6 shows the microstructure of a failedsample, in which grain growth is clear.

As shown in Fig. 5, with increased strain rate, themaximum value of the total strain before failure can beobserved at higher temperatures. This is because thehigher strain rate increases the velocity of grain

boundary sliding and cavitation. The increase in tem-perature also leads to higher diffusion and accommoda-tion of grain boundary sliding, which can retardcavitation and results in more deformation. However,grain growth is a function of time and with increasingstrain rate more deformation can be expected beforegrain growth.

Fig. 5 also shows that when decreasing the strain ratefrom 7×10−3 to 1×10−4 s−1, the maximum value ofthe total strain increases. Further decrease in strain ratereduces the maximum value of the total strain. Oneexplanation for this behaviour is a decrease in cavita-tion as strain rate decreases from 7×10−3 to 1×10−4

M.M. Moshksar, E. Marzban Rad / Journal of Materials Processing Technology 83 (1998) 115–120 119

Fig. 7. The effect of strain rate on strain-rate sensitivity, m : (a) 625°C; (b) 650°C; and (c) 680°C.

s−1, resulting in greater total deformation. Strain rateslower than 1×10−4 s−1 will result in maintaining thesample longer at high temperature before appreciablestrain develops, thus grain growth occurs and decreasesthe maximum total strain.

3.2. Effect of temperature on the 6ariation of stain-ratesensiti6ity with stain rate

The effect of the strain rate on the total strain can beevaluated by the value of the stain-rate-sensitivity expo-nent, m. The method suggested by Hedworth and Stow-ell has been applied for measuring m in the presentwork and the results of this research at temperatures of625, 650 and 680°C are presented in Fig. 7. As shownin this figure, with increasing stain rate, the value of mincreases initially, but then decreases. Thus, at each

temperature there is a maximum value for m at aparticular stain rate, the maximum value of m corre-sponding to Region II of Fig. 1. Fig. 7 also shows thatthe maximum value of m changes from 0.41 to 680°C.At the same time, the value of maximum m shifts tohigher strain rates at higher temperatures. This be-haviour is expected from these materials, as at greatertemperatures, the deformation mechanism operating inRegion I can operate at greater strain rates. The samekind of behaviour is expected for Regions II and III.

Although the increase in m with increase of tempera-ture from 625 to 650°C can be explained on the basis ofthe thermal activation mechanism of deformation, noclear explanation of the decrease in m with furtherincrease of temperature to 680°C can be given. Never-theless, the shift of the operating mechanism fromRegion I to Region III may be one explanation.

M.M. Moshksar, E. Marzban Rad / Journal of Materials Processing Technology 83 (1998) 115–120120

4. Conclusions

The superplastic behaviour of fined-grained 0.9%carbon steel prepared through heat treatment hasbeen investigated. The experiments were performedwithin the temperature range of 650–710°C and thestrain-rate range of 5×10−5–1×10−3 s−1. As thetemperature increases, the maximum values of thestrain-rate-sensitivity exponent m shift to greaterstrain rates. From the experiments carried out, themaximum strain-rate-sensitivity exponent of this steelwas 0.68 and the maximum superplastic deformationwas 305% at 650°C, with a strain rate of 1×10−4

s−1. For strain rates lower than 1×10−4 s−1, graingrowth causes a reduction in the maximum totalstrain.

Acknowledgements

Financial support by The Office of Research Coun-

cil of Shiraz University through grant number 74-EN-871-508 is appreciated.

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