~J. S. Afr./nst. Min. Metal/., vol. 92, no.7.Jul. 1992. pp. 185-193.

The influence of microstructural parameters on the yieldstress and fracture toughness of 3CR12 steel

by F. SIum*, N.R. Comins*, and S.l. Weissf

SYNOPSISThe effect of microstructure on the deformation and fracture properties of 3CR12 steel was investigated at differentstrain rates in the temperature range -196 to 200"C. The yield stress of the as-received steel, showing a bandeddual-phase microstructure, was much lower than that of the flake-like dual-phase microstructure produced by heattreatment. The heat treatment also resulted in an increase in the volume fraction of martensite. The fracturetoughness of the as-received 3CR12, on the other hand, was signifICantly higher than that of the heat-treatedmaterial. These changes in both yield stress and fracture toughness were attributed to the effect of microstructuralparameters on the constraints imposed on the deformation and subsequent failure of the ferritic phase in the as-received and heat-treated steels. The exceptionally Iow resistance to crack extension in a plane parallel to the roilingplane of the as-received material was ascribed to the delamination process, which is typical for controlled-rolledsteels. The yield stress and fracture toughness were found to depend on temperature and strain rate. Theappearances of the fracture surfaces were found to be consistent with the mechanical properties.

SAMEVAmNGDie effek van die mikrostruktuur op die vervormings- en falingeienskappe van 3CRI2-staal is teen verskillendevervormingstempo's, by temperature tussen -196 en 200"C, ondersoek. Die vloeispanning van die soos verkrygdestaal, wat 'n gebande tweefasemikrostruktuur getoon het, was baie laer as die van die flokkerigetweefasemikrostruktuur wat deur hittebehandeling geproduseer is. Hierdie hittebehandeling het ook 'n toename in dievolumefraksie van martensiet tot gevolg gehad. Die breuktaaiheid van die soos-verkrygde 3CR12, aan die anderkant, was heelwat hoer as die van die hittebehandelde materiaal. Hierdie veranderings in beide die vloeispanning endie breuktaaiheid is toegeskryf aan die invloed van die mikrostruktuur op sekere beperkings in vervorming engevolglike faring van die ferritiese fase van die soos-verkrygde en die hittebehandelde staal. Die uitsonderlik laeweerstand teen kraakvoortplanting in 'n vlak parallel tot die walsvlak van die soos-ontvangde materiaal is toegeskryfaan die delaminasieproses wat tipies vir beheerdgewalste staal is. Daar is gevind dat die vloeispaming en diebreuktaaiheid afhanklik is van die temperatuur en vervormingstempo. Daar is ook gevind dat die voorkoms van diebreukvlakke in ooreenstemming met die meganiese eienskappe is.

INTRODUCTION

Low-alloy dual-phase steels, which are characterized by aferritic-martensitic microstructure (although they maycontain three or more phases), have aroused great interest inrecent years since they show an attractive combination ofstrength, ductility, and initial work-hardening ratel-3. Amajor advantage of these dual-phase steels is that theirmechanical properties can be manipulated bymicrostructural modification. The mechanical properties aregenerally sensitive to both the volume fraction of martensiteand the morphology of the phases. The tensile properties ofmost dual-phase steels have been found to varyapproximately linearly with the volume fraction ofmartensite4-8, unlike their Iow-cycle fatigue behaviour,which does not follow a rule of mixture9. It has, however,also been shownlO-lz that the interaction between phasesduring straining has an effect on the deformation behaviourand that the law of mixture cannot be appliedunambiguously.

The locally produced chromium-containing 3CR12* steelis of particular interest for many applications since itcombines better corrosion resistance than that of mild steelwith the attractive properties of dual-phase steels, such as

.Division of Materials Science and Technology, CSIR, Pretoria.

t Department of Materials Engineering, TechniorHsraellnstitute ofTechnology, Haifa, Israel.

<C> The South African Institute of Mining and Metallurgy, 1992. SAISSN 0038-223Xj3.00 + 0.00. Paper received January 1991;revised paper received June 1992.

good ductility and high initial work-hardening rate13,14.Aprevious study has shown that the yield stress and impactenergy of 3CR12 steel depend on temperature and strainrate, and that the yield stress can be presented as a functionof a strain-rate parameterlS. In an earlier investigationl6, itwas found that this alloy also showed continuous yieldingand a three-stage work-hardening behaviour under tensileloading, similar to that reported in other Iow-alloy dual-phase steelg4,17,18.

The aim of this paper is to discuss some aspects of theinfluence of microstructural parameters on the yield stressand fracture toughness of 3CR12 steel.

MATERIAL AND EXPERIMENTAL PROCEDURES

The investigated steel was supplied by ColumbusStainless in the form of 20 mm hot-rolled sheet. Thechemical composition is given in Table I. The as-receivedsteel was characterized by a banded ferritic-martensiticmicrostructure with elongated grains parallel to the rollingdirection. The microstructure is shown in Figures I and 2for the longitudinal and transverse directions respectively.Heat treatment for two hours at 850°C followed by oilquenching resulted in a more isotropic and coarse flake-likemorphology of martensite and ferrite as shown in Figure 3.In Figures 1 to 3 (nital etched), the ferrite appears light andthe martensite dark.

:j: 3CR12 is the tradenarne of a chromilJl'TH:ontaining corrosion-resistant steel produced by Columbus Stainless, previously MiddelburgSteel & Alloys (Pty) Umited, Middelburg, Transvaal.

Journal of the South African tlstitule of Mining and Metallurgy JULY 1992 185

Figure l~ptlcal micrograph of the as-received 3CR12 lteelln L-Torientation

Figure 2~ptlcal micrograph of the as-received material In SoTorientation

Tensile specimens having a gauge section of 60 by 15mm with a thickness of 5 mm were machined from the as-received steel plate in the longitudinal (L-T) and in thetransverse (T-L) orientations and from the heat-treatedmaterial showing the isotropic flake-like morphology. Theyield stress at 0,2 per cent offset was then determined attemperatures between -196°C (liquid nitrogen) and 100°C,and at strain rates of 10-4 and 10--1s--1,by use of a 100 kNservohydraulic testing machine. The yield stress at 0,2 per

cent offset is deemed as the intersection of the stress-straincurve with a line that is parallel to the elastic part of thecurve and that intersects the strain axis at 0,2 per cent

Single-edge notch-bend (SENB) specimens with athickness (B) of 18 mm and a width (W) of 36 mm wereprepared from the as-received and heat-treated materials.The specimens from the as-received material weremachined in the L-T, T-L, and through-thickness or short-transverse (S-T) orientations. Suitable extension studs were

186 JULY 1992 Journal of /he Sou/h Amcan Institute of Mining and Metallurgy

Rgure ~ptlcal micrograph of the heat-treated material

Table IChemical composition of Investigated 3CR12 stllel

Element Ti NiMn Sic Cr S FeN

Weight % 1,450,022 1,41 0,38

friction welded to the surfaces of 40 by 40 mm 3CRI2plates to facilitate the fabrication of the S-T specimens,with the machined notch in a plane parallel to the rollingplane (sheet surface) and normal to the rolling direction.The specimens were then loaded cyclically on theservohydraulic testing machine to grow a fatigue crack ofabout 3 mm from the machined notch. Fatigue pre-crackingof fracture-toughness test pieces is necessary to eliminatethe effect of the notch shape on the crack-tip stress field.The specimen preparation and fatigue pre-cracking, as wellas three-point bend fracture-toughness tests, wereconducted in accordance with the relevant ASTM standardspecifications19.

In the case of the heat-treated and S-T specimens,conditions were met for the direct determination of theplane-strain fracture-toughness parameter K1cby the offsetprocedure19. In accordance with this technique, the load,PQ' associated with a crack extension of about 2 per centwas determined from the intersection of the load-displacement trace and the 5 per cent secant line. This lineis defined as a line through the origin of the load-displacement curve with a slope 5 per cent less than theslope of the initial elastic part of the curve. Under small-scale yielding conditions, as manifested in the minimumspecimen size requirements, PQ was then directly used inthe calculation of K1c'These conditions were not satisfied atambient and higher temperatures in the case of the L-T andT-L test pieces machined from the as-received material.Thus, the fracture toughness was evaluated in terms of anelastic-plastic fracture parameter, the i-integral, employing

11,31 0,012 0,013 Balance0,69

the multi-specimen resistance curve method20. The criticalvalue for the i-integral, ic, was then converted to anequivalent critical stress-intensity factor by therelationship21

Kc=.Ji:E,

where E is the Young's modulus.Charpy V-notch specimens having a cross-section of 10

by 10 mm and a length of 55 mm were prepared from theas-received and heat-treated steel for the determination ofthe yield stress and the fracture toughness at higher loadingrates. These tests were conducted at temperatures between-196 and 200°C, use being made of an instrumented impactmachine to record the impact load as a function of timeand/or deflection. The inertial loading peak22,23,resultingfrom the change in the momentum of the specimen as it isaccelerated from rest to the tup velocity, is reduced byimpacting the tup against a dampening lead pad glued tothe specimens. The increase in loading time by a factor ofabout 2 as a result of the testing of 'padded' specimens is ofno practical consequence, since an increase in the strainrate by a factor of 10 to lOO is required to producemeasurable changes in the mechanical properties of strain-rate sensitive materials24,

The impact or dynamic yield stress, O'yd,was determinedfrom standard Charpy V-notch specimens by a relationshipgiven by Server and Ireland:

Journal of;'e South A6ican Institute of Mining and Metallurgy JULY 1992 187

O'yd '" 51,55 Py (MPa),

where Py (in kN) is the general yield load obtained from therecords of load versus time2/>.

Fatigue pre-cracked Charpy specimens were used toobtain a measure for the impact fracture toughness, Kd' Forlinear-elastic fracture behaviour, the Kd values weredetermined from the fracture load in accordance with therelevant ASTM standard19.When the load-deflection tracesindicated that plastic deformation had occurred prior tofracture, the resistance to ductile-crack extension wasevaluated in terms oftheJ-integra120.

Since the impact test cannot be interrupted, multi-specimen or partial unloading compliance techniques areinappropriate for the evaluation of the dynamic test data.Strain-gauge instrumented specimens were therefore testedto determine the load, Pc, at the onset of crack extensionfrom records of load versus time and of strain versus time,as suggested by Kalthoff et alP. The critical value for theJ-integral, Jc corresponding to the onset of ductile-crackextension was then obtained from the area dermed by thecurve for load versus displacement up to the load, Pc fromthe well-known relationship20

AJe= B(W-a)

f(a/W) ,

where A is the area under the load-displacement curve, Band Ware the respective thickness and width of thespecimen, a is the total crack length (machined notch plusfatigue pre-crack), and the factor f(a/W), taken as 2 forquasi-static conditions, becomes about 1,2 for higherloading rates28. Jc was then converted to an equivalentstress-intensity factor, Kc, as shown above.

The test results were related to the appearance of thefracture surface and to the failure mode from fractographicexaminations of the tested specimens conducted in ascanning electron microscope (SEM).

RESULTS

Influence of Microstructure on YieldStress

The variation of quasi-static yield stress with temperatureis shown in Figure 4 for the banded dual-phasemicrostructure of the as-received steel and for the flake-likedual-phase microstructure of the heat-treated material. Inthe as-received condition, the yielding behaviour was foundto depend on temperature and was very similar for the L-Tand T-L directions, the yield stress being slightly higher forthe T-L orientation. The heat treatment, which produced acoarse flake-like dual-phase microstructure with a highervolume fraction of martensite, increased the yield stress(and ultimate tensile strength) by 30 to 40 per cent.However, the temperature sensitivity of the yield stress is,for all practical purposes, the same for both the as-receivedand the heat-treated material.

Figure 5 demonstrates the effect of temperature andstrain rate on the yield stress of the heat-treated steel. Theincrease in the yield stress with increasing strain rate anddecreasing temperature is evident from this diagram. It

1 200

CJ L-T ORIENTATION. T-L ORIENTATION0 !-EAT - TREATED

~~ 400I-rn

~ 200>=

rE :: 10-4 a-I

EACH RESUL T IS THE AVERAGEOF THREE TESTS

0-200 -100 0

TEMPERATURE ("C)

100

Figure 4-Dependence of yield stress (0,2 per cent offset) ontemperature for as-received (L-T and T-L orlentatlons) and

heat-treated 3CR12

should be noted that, for the instrumented impact tests atlow temperature, where fracture occurred in a brittlemanner, the general yield load was not resolved from theinertial loading peak, and dynamic. yield-stress data couldconsequently not be obtained for those temperatures.

Influence of Microstructure on Fracture Toughness

The variation of quasi-static fracture toughness withtemperature is shown in Figure 6 for the banded dual-phasemicrostructure of the as-received 3CR12 steel and for theflake-like dual-phase microstructure of the heat-treatedmaterial. The temperature dependence of the fracturetoughness of both steels is evident from this diagram. Asexpected from the yield-stress results given in Figures 4

HEAT -TREATED

1 000""as~,

tu 800(J)

u.~~ 600

0(Jj

ff3 400a:I-(J)

Cm 200>-

0 rE:: 10'" a-I

. rE:: 10-18-10 rE :: 102 8-1

EACH RESULT IS THE AVERAGEOF THREE TESTS

0-200 -100 0

TEMPERA TLJ=IE('C)

100

Figure 5-Dependence of yield stress (0,2 per cent offset) ontemperature at different strain rates for heat-treated 3CR12

188 JULY 1992 Journal of /he South African Institute of Mining and Metallurgy

140

. T-L ORENT A TION

~120 c L-T ORENT A TION

as 6 S-T ORIENTATIONa.. 0 HEAT-TREATED

~100Jl

~~80

~E:=

10-4.-1a 60....

~4O(J-<lE 20 6

EACH RESlL T IS THE AVERAGEOF TWO TESTS

00 100-200 -100

TaPERA TURE (.C)

100

~80

c L-T ORENT ATION 06 S- T ORENT ATK».I

t 0 tEAT -TREATED.....

"~fI) 60fI)

!t!:J:g E=

1O2s-1~40

wa::;)t-

~20IL

EACH FESlL T IS TI-E AVERAGEOF TWO TESTS

0200-200 -100 0 100

TEMPERAlURE (.C)Figure 6-Dependence of fracture toughness on temperature atIow strain rate for as-received (L-T, T-L, and SoT orlentatlons)and heat-treated 3CRI2. The data points for the L-T and T-Lorlentatlons at room temperature and 100°C representequivalent critical stress-Intensity factors derived from

Jolntegral tests

and 5, the resistance to crack extension is lower in theflake-like dual-phase microstructure than in the bandeddual-phase microstructure. The fracture toughness of theas-received steel was found to be very similar for the L-Tand T-L directions. However, the fracture toughness of theas-received 3CR12 in the SoT orientation, where crackpropagation occurred in a plane parallel to the rolling plane,was found to be 30 to 50 per cent lower than that of theheat-treated material.

The influence of microstructure on fracture toughnessdetermined at a strain rate of about 102 g-l, also referred toas impact or dynamic fracture toughness, is shown inFigure 7 as a function of test temperature. Similar to quasi-static loading conditions, dynamic fracture toughnessdepends strongly on microstructure, and is appreciablylower in the heat-treated steel, but is lowest when crackextension occurs in a plane parallel to the rolling plane ofthe as-received material.

Fractographic Observations

Scanning electron fractography indicated that thefracture-toughness results could be linked to particularfailure modes. Fully brittle fracture, where the fracturesurface was made up entirely of cleavage facets, wasassociated with low KIc values. Brittle-fracture surfacescontaining isolated regions of ductile tearing wereassociated with intermediate fracture-toughness properties.The fracture surface of specimens that showed a highresistance to cracking contained voids of up to 50 JlIDindiameter, which were linked by regions with fine dimples.At comparable levels of fracture toughness. the fracture-surface topography produced on Charpy and SENBspecimens exhibited similar features.

Changes in fracture topography with test temperature areshown in Figures 8 to 12 for pIe-cracked Charpy specimensin the as-received and heat-treated conditions. Themicrographs depict the regions immediately ahead of the

Figure 7-Dependence of Impact fracture toughness ontemperature for as-received (L-Tand SoTorlentatlons)and

heat- treated 3CR12

fatigue pIe-crack tip or the stretch zone, and the generaldirection of crack extension was from left to right

At a test temperature of -196°C, all the test piecesgenerally failed by transgranular cleavage, as shown inFigure 8 for a heat-treated specimen. It should be noted thatisolated areas of ductile tearing were found on the surfaceof L-T specimens. At room temperature, the fracturebehaviour remained fully brittle for the S-T microstructure.while the fracture surface of the heat-treated specimensshowed evidence of ductile tearing (Figure 9). In contrast,crack extension and subsequent failure in the L- Tspecimens were governed by the initiation, growth, andcoalescence of voids, as revealed by the ductile-fracturesurface illustrated in Figure 10.

With increasing test temperature, the amount of ductiletearing increased in the SoT and heat-treatedmicrostructures, but cleavage facets were still noticeable onthe fracture surfaces of specimens tested at 100°C (Figure11). At a temperature of 200°C, fracture of the SoT andheat-treated specimens was entirely controlled by ductiletearing. These ductile-fracture surfaces exhibited in generaltwo characteristic features: (i) voids of 20 to 50 JlIDin size,and (ii) regions of finer dimples separating the larger voids.Figure 12 shows a typical example of ductile rupture in anS-T specimen tested at 200°C.

DISCUSSION

The tensile properties of dual-phase steels are generallydependent on both the volume fraction of martensite andthe morphology of the microstructure4-9. The yield stress,for instance, increases with increasing volume fraction ofmartensite, and has been reported to be higher in coarseflake-like morphologies6.

It is generally accepted4-6 that, in dual-phase steels,plastic deformation occurs flfSt in the ferrite and appears inthe martensite at high nominal stresses. Earlier studies on

Journal of /he South Atican Insftute of Mining and MetaIIutgy JULY 1992 189

Figure 8-Scannlng electron fractograph showing fully brittle fracture mode In a heat-treated Charpy specimen tested at -196"C. Thedirection of crack propagation along varlouscleavage planes Is Indicated by well-defined river patterns

Figure 9-Scannlng electron micrograph of the predominantly brittle fracture surface In a heat-treated Charpy specimen tested atroom temperature. Tear ridges between cleavage facets give evidence of local ductile failure (see arrow)

the tensile defonnation of 3CR12 in a SEM29 showed thefonnation of slip lines in the ferrite at nominal strains ofabout 2 per cent. These slip-line traces terminated atferrite-marten site boundaries since the propagation ofdislocations is obviously obstructed by the strongermarten site. At higher strains, dislocation pile-ups areexpected to occur at ferrite-martensite interfaces, resultingin the nucleation of voids at interfaces, and eventually in

slip-line fonnation in the martensite during necking. Thisobservation is in agreement with the results obtained byMarder4, but Zhang and Nutting30 reported that martensitealready begins to defonn plastic ally during stage 11 oftensile straining.

The increase in yield stress in the flake-like morphologyof the heat-treated 3CR12 (Figure 4) is in agreement withother observations6 and can be explained in tenus of the

190 JULY 1992 Journal of the South Ahican Institute of Mining and MeIaHurgy

FIgure lO-Scanning electron micrograph of ductile fracture surface In L-Tspecimen tested at room tel11'erature

Figure 11-Scannlng electron micrograph of an area In a heat-treated specimen revealing local failure by cleavage (bottom) andductile-tearlng (top). The test tel11'erature was l00"C

constraints under which the softer ferrite defonns. In thebanded dual-phase microstructure of the as-received 3CR12steel, the plastic deformation of the continuous ferriticphase (Figure 1) occurs under minimal constraints, and sliplines may propagate freely without being obstructed by thestronger martensite. Therefore, the yield stress of the as-received steel is much lower than the yield stress of theheat-treated material, where the plastic deformation in the

ferrite is more constrained by the surrounding marten site(Figure 3), requiring higher stresses for macroscopicyielding. This conclusion is in agreement with argumentsby Shen et a/.31,who reported that, with increasing volumefraction of martensite, the ferrite phase becomesincreasingly constrained by the martensite, resulting in areduction in its contribution to the total macroscopic strain.

As expected from the yielding behaviour, the fracture

Joumal of /he South Ahican Indtute of Mining and Metallurgy JULY 1992 191

Figure 12-Scannlng electron fractograph associated with ductile rupture In an S-T specimen tested at 200"C

toughness (crack-initiation toughness) of the heat-treated3CR12 steel is lower than the fracture toughness of the as-received material in the L-T and T-L orientations (Figures6 and 7). For instance, fractographic studies showed that, atroom temperature, cracks extend in the bandedferritic-martensitic microstructure of the as-received steelby a ductile rupture mechanism involving the nucleation,growth, and coalescence of voids after the development ofa stretch zone. Fractographic evidence was also found thatvoids nucleated at inclusions (Figure 12). This mechanismof ductile fracture is controlled by the stress-modifiedcritical strain criterion32, which states that crack extensionoccurs when the local equivalent plastic strain ahead of thecrack tip exceeds a critical fracture strain over acharacteristic distance. This critical strain is also a functionof the stress state, and the characteristic distance is relatedto microstructural features such as the spacing of voidnucleating particles. Tensile tests in the SEM29showed thatvoids were nucleated by cleavage of non-metallicinclusions and by de-cohesion at particle-matrix interfaces.These particles, after being analysed by a combinedenergy-dispersive X-ray and Auger-electron technique,were identified as titanium carbonitrides. Void nucleationin dual-phase steels has also been attributed to the de-cohesion at ferrite-martensite interfaces and the separationof fractured or adjacent martensite particles33.

The change in the micromechanism of failure bycracking in 3CR12 as a result of heat treatment can beexplained in terms of observations34.35 that the ferrite isconstrained by a continuous martensite phase. The more-continuous martensite in the heat-treated 3CR12 (Figure 3)imposes high constraints on the plastic deformation of theferrite, resulting in reduced crack-tip blunting. As aconsequence, the conditions for instability under load interms of a critical stress modeP6, i.e. the onset of brittle-

crack extension, are reached before the critical fracturestrain is attained, and the favourable micromode of failureis transgranular cleavage. Although plastic deformation andcrack nucleation occur fIrSt in the ferrite, it has also beenreported4 that, in a continuous martensite phase, which isless ductile than the ferrite, crack propagation is enhancedand occurs by transgranular cleavage as a result ofrestricted plasticity. This change in failure mechanism isillustrated in Figure 9, which shows the fracture surface ofa heat-treated 3CRl2 specimen tested at room temperature;the morphology of the predominantly brittle fracture isconsistent with the relatively low fracture toughness(Figure 6).

The very low fracture toughness obtained from the as-received specimens machined in the S-T orientation, wherecrack extension occurs in a plane parallel to the rollingplane, can be attributed to the process of delamination.Delamination or splitting37.38 occurs, even at roomtemperature, in a brittle manner, and is typical forcontrolled-rolled steels. It has been related to the presenceof a sufficient number of inclusions and a bandedmicrostructure, as well as to texture hardening, residualstresses, grain-boundary segregation, and pan caking ofgrains in a plane parallel to the rolling plane. The lowfracture toughness of 3CRl2 steel in the S-T direction isalso in agreement with reports39 that failure in the rollingplane occurs predominantly by separation of elongatedgrains facilitated by the presence of non-metallic particles.

It is important to note that the application of the 5 percent secant method19 produces unexpectedly low anderroneous values of fracture toughness for 3CR12 in the as-received condition. The primary object of the 5 per centsecant methOd19is the determination of the critical stress-intensity factor, Kc, corresponding to a load, PQ' at which apre-existingcrack in a test piece has grown by about 2 per

192 JULY 1992 Journal of /he South African Institute of Mining and Metallurgy

cent under small-scale yielding conditions. The relativelylow load determined for the as-received 3CR12 steel inaccordance with this procedure is a result of the material'scontinuous yielding behaviour. Hence, the low fracturetoughness obtained is not representative of the material'sresistance to crack extension.

CONCLUSIONS(1) The yield stress of the dual-phase 3CR12 steel in the

heat-treated state was found to be considerably higherthan that in the as-received condition. The increase wasattributed to changes in the microstructure that give riseto different constraint effects under which the ferritephase deforms.

(2) The values of fracture toughness obtained at differentstrain rates were found to be higher for the as-receivedcondition than for the heat-treated state. The more-continuous martensite phase in the heat-treated 3CR12steel imposes higher constraints on the plasticdeformation of the ferrite, resulting in a decrease in theresistance to crack extension.

(3) The fracture-surface morphologies were found to beconsistent with the results obtained from the three-pointbend and impact fracture toughness tests.

ACKN~EDGEMENTS

The authors thank Mr D. Hope for technical assistance,and Columbus Stainless, previously Middelburg Steel &Alloys (Pty) Limited, for providing the material used in thisinvestigation.

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22. MAN, J., and HOLZMANN,M. Measurement of fracture load and deter-mination of fracture toughness GIc in instrumented Charpy tests.I. Iron and Steel Inst., Feb. 1970. pp. 199-201.

23. SEIDL, W. Comparison of impact testing on Charpy V-notch speci-mens and WOL-IX specimens. Advances in Fracture Research.Proceedings of the 5th International Conference on Fracture (ICF5J,Cannes (France), 1981. pp. 347-354.

24. IRELAND,D.R Procedures and problems associated with reliable con-trol of the instrumented impact test. Proceedings of Symposium onImtrumented Impact Testing, Philadelphia (USA), ASTM-STP 563,1974. pp. 3-29.

25. SERVER,W.L., and IRELAND,D.R Non standard test techniques utiliz-ing the instrumented Charpy and!zod tests. Ibid., pp. 74-91.

26. SHECKHERD,J.W., KANOn.ASKI, M., and BAUER, AA Impact proper-ties of shock-strengthened type 316 stainless steel. Ibid., pp. 118-132

27. KALTHOFP, LF., WINKLER, S., BOHME, W., and KLEMM, W.Determination of the dynamic fracture toughness, KIc' in impact testsby means of response curves. Advances in Fracture Research.Proceedings of the 5th International Conference on Fracture (lCF5)Cannes (France), 1981. pp. 363-373.

28. BASSIM,MN. Private communication, 1987.29. BLUM, E, 1'ERBLANam, S.P., and HoPB, D. Direct observation in the

SEM of deformation processes in dual-phase 3CR12 steel.Proceedings of the 25th Annual Conference of the ElectronMicroscopy Society of Soulhern Africa. Potchefstroom. vol. 16. 1986.pp. 153-154.

30. ZHANo, Y.S., and NUIT1NO, J. High-strain defonnation of dual-phasesteel. Mater. Sci. Technol., vol. 2. Jun. 1986. pp. 564-570.

31. SHEN, H.P., LEI, T.C., and LIU, J.Z. Microscopic defonnationbehaviour of martensitic-ferritic dual-phase steels. Mater. Sci.Technol., voI. 2 Jan. 1986. pp. 28-33.

32. MACKENZIE, A.C., HANCOCK, J.W., and BROWN, D. K. On the influ-ence of state of stress on ductile failure initiation in high strengthsteels. Engng Fract. Mech., vol. 9. 1977. pp. 167-188.

33. STEINBRUNNER,D.L, MATLOCK, D.K., and KRAuss, G. Void fonna-tion during tensile testing of dual phase steels. Met. Tram., vo!. 19A.1988. pp. 579-589.

34. KUNIO, T., SHIMIZII, M., Y AMADA,K., and SUZUKI, H. An effect of thesecond phase morphology on the tensile fracture characteristics ofcarbon steels. Engng Fract. Mech., vol. 7. 1975. pp. 411-417.

35. JEONO, W.C., and KIM, C.H. Effects of transfonned ferrite growth onthe tensile fracture characteristics of a dual-phase steel. Met. Tram.,voL 19A. 1988. pp. 309-317.

36. RITCHm, R.O., KNOTT, J.F., and RICE, J.R. On the relationshipbetween critical tensile stress and fracture toughness in mild steel.J. Mech. Phys. Solids, vol. 21. 1973. pp. 395-410.

37. DEARoo, AJ. An investigation on the mechanism of splitting whichoccurs in tensile specimens of high strength Iow alloy steels. Met.Trans., vol. 8a. 1977. pp. 473-486.

38. BRAMPITT,B.L, and MARDER, A.R. Splitting behavior in plate steels.Toughness Characterization and Specifications for HSLA andStructural Steels, Proceedings of the 106th Meeting of AIME.Mangonon, P. L, Jr. (ed.). Georgia (USA), 1977. pp. 236-255.

39. KNurSEN, R.D., and HUTanNOS, R Occurrence of non-metallic inclu-sions in 3CRI2 steel and their effect on impact toughness. Mater. Sci.Techflol., vol. 4. 1988. pp. 127-135.

Journal of tie South Atican Institute of Mining and MetaJIu'9Y JULY 1992 193

Jewelpark to provide spark for jewellery industry

More than 200 representatives of the South Africanjewellery-manufacturing and related industties gathered atMintek in Randburg on 21st July, 1992 to attend apresentation on Jewelpark, a totally new concept geared toprovide impetus to the industry.

The event, which was opened officially by Mr GeorgeBartlett, the Minister of Mineral and Energy Affairs, is asequel to the seminar 'Jewellery-the road ahead'organized by Mintek in 1990 in response to theGovernment's lifting of the ad va/orum excise duty onlocal jewellery manufacture. On that occasion, the aim hadbeen to provide an open forum with the view to initiatingaction programmes for the stimulation of the local industry.The latest event can be viewed as the culmination of theefforts of a number of disparate individuals andorganizations over the past two years, all sharing thecommon motive of providing a 'kick-start' to set theindustry on the road to real growth.

Jewelpark will consist of a single integrated complex, tobe built at Mintek' s site, comprising a jewellery trainingschool, a 'hive' system for smaller manufacturers, large-scale manufacturing plants, and facilities that will enablejewellery manufacturers to relocate their existingoperations.

Advantages for such manufacturers would includeoperating in a secure, parklike environment (very differentfrom many existing operations); a safe, centralized energysupply; the savings arising from sharing of certainfacilities; a comprehensive support structure, includingconference and display facilities, and a secretariat;precious-metal handling, and bulk gold despatching andrefining operations; group buying advantages; andaffordable marketing, research, and feasibility studies.

In his presentation on the proposed training facility,educationalist Mr Pappie Moloto stressed the importance ofthe linking of educational and entrepreneurial skills, as wellas the provision of an ideal learning environment in having

10 classes with a maximum of 25 students per class.Formal training would cover a period of two years, and thestudents would be exposed to the. manufacturingenvironment at all stages. The course content would consistof personal and computerized modules, and there would beinteraction with and input from Italian and German expertsin the field. It is envisaged that the course could in time beextended on a countrywide basis.

Selection of potential students would be on a non-cultural, non-academic basis, the optimum age group being24-45 years. Because of the close and dynamicenvironment that would exist between training andproduction, job opportunities would be assured.

Mintek's motivation for spearheading the development ofthe local jewellery industry has been the critical need forthe downstream development of South Africa's mineralresources; this has become increasingly urgent in recentyears. It has also been perceived that South Africa needs torefocus its approach towards its gold resources, and moveaway from viewing them in purely fmancial terms.

It is felt that, now that the political climate is right, SouthAfrica needs both to learn from major producers such asItaly, Germany, the USA, and Japan, and to encouragethese countties to expand their operations to South Africa,where both labour and raw materials abound.

Jewelpark and similar entrepreneurial ventures willprovide the cement needed to reshape and rebuild theeconomic structure of the new South Africa, to enable it tooptimize its valuable mineral and human resources.

Issued by the Information and Communications DivisionMintekPrivate Bag X3015Randburg2125Telephone: 709-4111

For further information contact Mrs Pat Speedie.

The current success rate of students attending thelaboratory is claimed to be approximately 80 per cent. Thisis because most of the students have already gained someexperience on the mines, and are attending the Technikonas part of a career path to higher positions. Unfortunately,the present depressed state of the gold-mining industry ishaving an effect on the number of students enrolling, with adecrease in students sponsored by the mines. However, thisis partly offset by a marginal increase in private students.Although most of the students are white, the number ofblack students is growing each year.

EQUIPMENT

The present economic situation in the country hasrestticted funding from the Chamber of Mines for furtherdevelopment of the laboratory. In an effort to increase thespread of work conducted in the laboratory, Chris Viljoen

asked several mining companies and manufacturers to assistby donating either funds or equipment. Iscor Pretoria,Multotec, and Mineral Deposits responded to this appeal.Donated equipment from Iscor and Multotec were rebuiltinto pilot plants for heavy-medium separation and columnflotation. The column-flotation pilot plant differs fromthose available elsewhere in that it uses a co-current columnas opposed to the countercurrent column in common use.

With the existing equipment in the laboratory, project andresearch work can be done to raise funds for furtherdevelopment. Training can now also be offered viaworkshops and/or seminars held on or off campus in areassuch as ore preparation and hydrometallurgy. Over a periodof two years, projects have been conducted in the laboratoryon particle size analysis, gravity concentration, cyclosizing,

see page 215

194 JULY 1992 Joumal of fie South AfuIn Institute of Mining and MetaHurgy