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Grain Refinement and Superplastic Behavior ina Commercial Bearing Steel*By Motohiro OKADE, ** Masaharu T OKIZANE* ** andOleg D. SHERB Y****Synopsis

An ultra-fine-grained SUJ-2 steel was obtained by applying one ofthermomechanicalrocessing outes developedn UHC-steel. This materialshowed the high elongation-to-failure values of 400 840 % at warmtemperature between 650 C and 730 C over a range of initial strainrates from 1 X 10_4 to 8 X 10_4 sec-1. The value of m 0.33 and theactivation energy of 195 kJ/mol was obtained or the deformationprocessin this range.

rom these results, it could be concluded hat grain boundarysliding isnot totally dominating the deformationprocess of the present material al-though superplastic low plays an important role in the range of the presenttesting conditions.I. Introduction

ince 1975 Sherby and his co-workers"2~ havedeveloped a variety of thermomechanical processingroutes which makes ultrahigh carbon (UHC) steelscontaining 1-'2.1 wt% carbon to have a very finestructure of spheroidized cementite particles in ferrite.The materials processed by these methods are of muchinterest as new steels since they are Superplastic atwarm temperature, and strong and ductile3~ atroom temperature. Furthermore, it has been alreadydemonstrated that this thermomechanical processingcan be applied to provide superplastisity for somecommercial high carbon steels such as 52 100 bearingsteel.4~In the present work, similar thermomechanicalprocessing was applied to a SUJ-2 commercial bearingsteel which i s most widely used in Japan. The pur-pose of this paper is to show the behavior and themechanism of superplastic flow of the processedSUJ-2 steel at warm temperature between 650 C and730 C (in the ferrite plus cementite temperaturerange).II. The Material and the Thermomechanicalrocessing

he SUJ-2 steel used in this work was provided inspheroidized-annealed condition in the form of 80 mmdiameter bars. The chemical composition of thesteel is shown in Table 1. The microstructure ofthis steel in the as-received condition is shown in

Photo. 1, which represents ferrite matrix of grain size10.'20 im with spheroidized cementite particles of0.5-2 pm. These bars were forged to billets havingdimensions 55 mm x 50 mm X 1 000 mm at tempera-ture around 1 100 C. The blocks of about 50 mmlength, cut from the billet were thermomechanicallyprocessed by the schedule shown in Fig. 1(a). Theblocks were air cooled to 650 C after heating at1 150 C for 1 hr, and then, isothermally rolled at650 C. This was carried out by rolling from thethickness of 55 mm to the final thickness of 4 mm(s, true strain, of -2.62) by repeated roll passes (16passes) of around 15 % reduction per pass with fur-nace reheats between passes. The rolled plates werethen thermal cycled with the schedule shown inFig. 1(b).he scanning electron micrographs of the speci-mens in the as-rolled and the thermal cycled condi-tions are shown in Photo. 2. In the as-rolled condi-tion, the structure mostly consists of fine-grainedferrite with fine cementite particles, but a smallamount of pearlite are still observed. This is nota suitable microstructure to expect superplastisity.However, such a pearlitic structure is fully removedby the thermal cycling step and the structure becomesalmost homogeneous. As shown in Photo. 2(b), thestructure in the cycled condition consists of fine-grained ferrite (average grain diameter N 0.8 pm) con-taining fine cementite particles (average particlediameter.0.3 pm).n addition to such homogenizing the thermalcycling is believed to convert low angle boundaries

Table 1. Chemical composition of the steel. (wt%)

Photo. 1. Scanning electroneceived condition. micrograph of the steel in the as-

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Originally published in Tetsu-to-Hagane, 67 (1981), 2710, in Japanese. English version received February 24, 1981.Graduate School of Science and Engineering, Ritsumeikan University, Tojiin-kita-machi, Kita-ku, Kyoto 603.Department of Mechanical Engineering, Ritsumeikan University, Tojiin-kita-machi, Kita-ku, Kyoto 603.Department of Materials Science and Engineering, Stanford University, Stanford, California, U.S.A.

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to high angle boundaries and thereby to enhance thesuperplastic properties.5~ In this work, the materialin the cycled condition was used for the followingtests.III. Experimental Methods

wo types of tests were performed using an Instron-type universal testing machine accompanied by aninfrared focussed radiation furnace. The tests werecarried out in a flowing gas mixture (90 % N2+10 % H2) in order to prevent the oxidation of thespecimens during testing. Figure 2 shows the dimen-sions of the tensile specimens used in the followingtests.1. Tensile Ductility Tests

o measure the tensile ductility, constant crossheadspeed tests to fracture were carried out at severaltemperatures between 650 C and 730 C over a rangeof initial strain rates from 1 to 8 x 10-4 sec-1.2. Change-in-strain-rate Tests6~

he specimen was pulled isothermally at a lowcrosshead speed until a steady state flow stress wasapproached. Then the crosshead speed was changedrepeatedly with only a small increment of strain ateach change. In such a manner the flow stress-strain rate relation was established at each deforma-tion temperature (650, 665, 680, 695, 710 and 730 C).Iv. Results and Discussion

ll of the specimens deformed to fracture are shownin Photo. 3. Elongation-to-failure obtained fromthese results are also shown in Fig. 3 as a functionof strain rate. At each deformation temperature,elongation-to-failure increases with decreasing strainrate except for the case at 730 C. At a given strainrate, elongation-to-failure increases with increasingdeformation temperature. A maximum elongation-to-failure value of 840 % is found at 730 C at initialstrain rate of 2.8x 10-4 sec-1. At lower strain rateof 1.4 X 104 sec-1, the value of elongation-to-failuredecreases to 800 %. This may be attributed to theoccurrence of grain growth due to the prolongedtesting time at such a high temperature. In fact, as

seen in Photo. 4, the specimen tested to failure at thistesting condition showed remarkable grain growth ingauge length.

he flow stress-strain rate curves obtained from theresults of the change-in-strain-rate tests at varioustemperatures were shown in Fig. 4. These resultsshow that the values of strain rate sensitivity exponent,m, is close to 0.33 (n ~ 3, where n is stress exponent)in almost the whole range of the experiments exceptfor the small region extending to high strain rates atlow temperature where the value of m is close to0.20 (n.5). The value of mN0.33 is comparativelylower than that obtained from the superplastic flowregion of UHC-steel (typically m O.5).6 However,the large elongation-to-failure values (400 N 840 %obtained in this study implies that superplastic flowplays an important role in the deformation processof the material.t is well known that the superplastic flow rate offine-grained materials, when grain boundary diffusionis rate controlling,7'10~ s given by the phenomenologi-cal equation:

p.f d3 E)Dgb a2 (1)where A-108, d is the grain size, b is Burgers vector,Dgb is the grain boundary diffusion coefficient in thematrix phase of the superplastic material, a is thecreep stress and E is the unrelaxed dynamic Young'smodulus. However, it is difficult to fit the presentflow stress-strain rate data to this equation since thevalue of the stress exponent is close to 3. Hence, theactivation energy for the plastic deformation processwas tentatively calculated from these data assuming

Fig . 1. Schematic diagrams for the thermomechanicalrocessing and the thermal cycling.

Photo. 2. Scanning electron microg raph of the specimens.

Fig. 2. Dimension of tensile specimens.

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Transactions ISIJ, Vol. 22, 1982 (145)

the relation5~n Q=K E exp -RT ..................(2)

where K is constant, n is the stress exponent for creep,Q is the activation energy for plastic flow, R is thegas constant and T is the absolute temperature.From Eq. (2), the activation energy for plastic flowis defined as

dln(~En) 3)Q=const-R 1 ..................(.Tor

d in n~ Et- - ...................... (4)1=011 R 1-The plots or determination f he activationenergycorresponding o Eqs. (3) and (4), using n=3, aregiven in Figs. 5 and 6 respectively. In these plots,the temperature dependenceof modulus,E, for thepresent steel is assumed to be the same as that fora-iron. The value of E for a-iron as a functionof

temperature was given by Koster.ll~ As can beseen in these plots, an average activation energy forthe deformation process is 195 kJ/mol. This is some-what higher than that for grain boundary diffusionof iron in a-iron (QgbN 170 kJ/mol)6} but lower thanthat for the lattice diffusion of iron in a-iron (QL N250 kJ/mol).6~ The result is seen to be consistentwith the fact that the obtained value of m (~ 0.33) issmaller than the expected value of m (~ 0.5) for idealfine-structure superplasticity. As seen in Table 1,the carbon content (0.75 wt%) of the present steel is

Research Article

Photo. 3.Specimens of the SUJ-2after testing to failure.

steel

Fig. 3. Elongation to fai lure as a function of strain rate.

Photo. 4. Scanning electron micrograph obtained fromauge length of the specimen tested to failure30C at initial strain rate of 1.4x 10_4 sec-1.

theat

Fig. 4. Flow stress-strain rateSUJ-2 steel.

relat ion for f ine grained

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comparatively low as a nominal composition (lwt%C-1.5 wt%Cr) of SUJ-2 bearing steel. This concen-tration of carbon will lead to a volume fraction ofcementite particles typically 25 % less than in a1 wt% C steel (about 12 vol% cementite). Thislow amount of cementite particles likely contributesto the value of m-0.33 observed for two reasons.First, ferrite grain growth is more likely to occurbecause of the reduced amount of cementite particlespresent. Evidence for this effect was noted; Photo.5 shows the 2 to 3 fold increase in grain size inthe gauge length during the change-in-strain-ratetests at 730 C. Grain coarsening will inhibit grainboundary sliding resulting in a decrease in thestrain-rate sensitivity exponent. Second, the reducedamount of cementite particles enhances slip creep(m