Article Influence of Microstructure on Strength and Ductility in Fully Pearlitic Steels
Jesús Toribio *, Beatriz González, Juan-Carlos Matos and Francisco-Javier Ayaso
Fracture & Structural Integrity Research Group, University of Salamanca, E.P.S., Campus Viriato, Avda. Requejo 33, Zamora 49022, Spain; [email protected] (B.G.); [email protected] (J.-C.M.); [email protected] (F.-J.A.) * Correspondence: [email protected]; Tel.: +34-980-545-000 (ext. 3659); Fax: +34-980-545-002
Academic Editor: Hugo F. Lopez Received: 24 October 2016; Accepted: 5 December 2016; Published: 15 December 2016
Abstract: This article deals with the relationship between the microstructure and both strength and ductility in eutectoid pearlitic steel. It is seen how standard mechanical properties and fracture micromechanisms are affected by heat treatment and the resulting microstructure in the material. The yield stress, the ultimate tensile strength and the ductility (measured by means of the reduction in area) exhibit a rising trend with the increasing cooling rate (associated with smaller pearlite interlamellar spacing and a lower pearlitic colony size), while the strain for maximum load shows a decreasing tendency with the afore-said rising cooling rate. With regard to the fracture surface, its appearance becomes more brittle for lower cooling rates, so that the fracture process zone exhibits a larger area with observable pearlite lamellae and a lower percentage of microvoids.
Keywords: pearlitic steels; heat treatments; strength; ductility; fracture surface; fracture micromechanisms
1. Introduction Pearlitic steels exhibit an increasing yield stress with lowering interlamellar spacing in such a manner that a Hall–Petch type equation can be fitted to reproduce the relationship between microstructure and strength [1–11]. However, in some materials, such as cold-drawn pearlitic steel wires, the orientation of microstructure (ferrite and cementite lamellae almost fully oriented in the drawing direction) makes the classical Hall–Petch fitting not applicable, as shown in [12], and only a modified Hall–Petch relationship between yield stress and pearlite interlamellar spacing is applicable, as demonstrated in a more recent paper [13]. Mechanical resistance in pearlite is governed by events taking place in ferrite, while at the same time the cementite lamellae act as barriers against dislocational movement and limit the slip distance in ferrite [11]. On the other hand, the ductility is more dependent on the prior austenite grain (PAG) and rises for smaller grain size [14–16]. The higher rate of strain hardening in pearlite could be attributed to the load transfer from ferrite to cementite, with the effect of constraint created by the harder phase (cementite) on the softer one (ferrite) being significant [3]. With regard to the influence of the interlamellar spacing, steels with thin and coarse pearlite have similar strain hardening coefficients [17]. While the coarse pearlite is deformed in a non-homogeneous manner (exhibiting localized plastic strain in the form of narrow slip bands), thin pearlite shows a much more uniform strain distribution [15,17,18]. In addition, when pearlitic microstructures are considered, plastic strain generates compressive residual stresses in the ferrite and tensile stresses in the cementite, in such a manner that the level of residual stresses is higher in steels with greater interlamellar spacing [10]. When one performs standard tensile tests, the fracture micromechanisms are controlled by physical processes taking place in those colonies containing pearlite lamellae parallel to the tensile axis, where the deformation occurs in narrow bands of locally intense shear stress [11].
Metals 2016, 6, 318; doi:10.3390/met6120318 www.mdpi.com/journal/metals Metals 2016, 6, 318 2 of 12 lamellae parallel to the tensile axis, where the deformation occurs in narrow bands of locally intense shear stress [11]. This paper deals with the microstructure (governed by the cooling rate), the mechanical Metalsproperties2016, 6 ,(strength 318 and ductility) and the associated fracture micromechanisms in pearlitic steels2 of 12 subjected to different heat treatments.
2. ExperimentalThis paper deals Method with the microstructure (governed by the cooling rate), the mechanical properties (strength and ductility) and the associated fracture micromechanisms in pearlitic steels subjected to differentA eutectoid heat treatments. steel was used (chemical composition: 0.789% C, 0.681% Mn, 0.210% Si, 0.010% P, 0.008% S, 0.003% Al, 0.218% Cr and 0.061% V). After hot-rolling the steel to produce the base material, 2.different Experimental heat treatments Method were applied, consisting of heating the material in a furnace at 900 °C (above the eutectoid temperature) for 1 h and then cooling it in the fully closed furnace (FCF); in the partially A eutectoid steel was used (chemical composition: 0.789% C, 0.681% Mn, 0.210% Si, 0.010% P, opened furnace (POF); or outside the furnace, i.e., air-cooling (AC) at room temperature. 0.008% S, 0.003% Al, 0.218% Cr and 0.061% V). After hot-rolling the steel to produce the base material, Material microstructure was analyzed by scanning electron microscopy (SEM) using a JEOL different heat treatments were applied, consisting of heating the material in a furnace at 900 ◦C (above JSM-5610 LV (Jeol Ltd., Tokyo, Japan) after the metallographic preparation of small samples by the eutectoid temperature) for 1 h and then cooling it in the fully closed furnace (FCF); in the partially mounting, grinding and polishing (up to a mirror finish) and attacking with Nital 4% (a mixture of 4 opened furnace (POF); or outside the furnace, i.e., air-cooling (AC) at room temperature. mL of nitric acid with 96 mL of ethanol) for several seconds to reveal the microstructure. Material microstructure was analyzed by scanning electron microscopy (SEM) using a JEOL Standard tensile tests (three for each material) were performed by using cylindrical specimens JSM-5610 LV (Jeol Ltd., Tokyo, Japan) after the metallographic preparation of small samples by of 11 mm in diameter and 300 mm in length. The crosshead speed was 2 mm/min and an extensometer mounting, grinding and polishing (up to a mirror finish) and attacking with Nital 4% (a mixture of with a gage length of 50 mm was placed in the central part of the samples. Finally, the fracture 4 mL of nitric acid with 96 mL of ethanol) for several seconds to reveal the microstructure. surfaces and longitudinal sections of the broken samples (after metallographic preparation and attack Standard tensile tests (three for each material) were performed by using cylindrical specimens of with Nital 4%) were observed through SEM. 11 mm in diameter and 300 mm in length. The crosshead speed was 2 mm/min and an extensometer with3. Microstructural a gage length ofFeatures 50 mm was placed in the central part of the samples. Finally, the fracture surfaces and longitudinal sections of the broken samples (after metallographic preparation and attack with NitalThe 4%) different were observed microstructures through SEM. of pearlitic steels after distinct heat treatments (Figure 1) were observed by SEM (×2500). In all cases, two different microstructural levels can be defined: (i) the first 3.microstructural Microstructural level Features is the pearlite colony as a set of lamellae (ferrite and cementite) with a common orientation;The different (ii) the microstructuressecond microstructural of pearlitic level steelsis defined after by distinct ferrite/cementite heat treatments alternating (Figure lamellae.1) were observedThe pearlite by SEM colony (×2500). diameter In all cases, dC was two measured different by microstructural the linear intercept levels canmethod be defined: [19], consisting (i) the first of microstructuralthe account of level the number is the pearlite of colonies colony intercepted as a set of lamellae by a random (ferrite straightand cementite) line drawn with a along common the orientation;micrograph, (ii) considering the second the microstructural randomness level of both is defined the metallographic by ferrite/cementite cut and alternating the line and lamellae. using statistical methods. The interlamellar spacing of pearlite s0 (Figure 2) was calculated by the method The pearlite colony diameter dC was measured by the linear intercept method [19], consisting of the circular account line of the[20], number a variant of of colonies the linear intercepted intercept method, by a random in which straight a circle line of drawn known along length the is micrograph,drawn on the considering photograph, the and randomness the number of of bothintersecting the metallographic pearlite lamellae cut andis counted the line (also and taking using into account the randomness). statistical methods. The interlamellar spacing of pearlite s0 (Figure2) was calculated by the method In the matter of continuous cooling, the transformation temperature controls the microstructure of the circular line [20], a variant of the linear intercept method, in which a circle of known length is and is governed by the cooling rate. A decrease of such a rate (or, in other words, an increase of the drawn on the photograph, and the number of intersecting pearlite lamellae is counted (also taking into cooling time) makes the interlamellar spacing and the size of the pearlitic colony increase (Table 1), account the randomness). i.e., slower cooling produces coarser microstructures.
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Figure 1. Cont. Metals 2016, 6, 318 3 of 12 MetalsMetals 20162016,, 66,, 318318 33 ofof 1212
((bb))
((cc))
FigureFigure 1. 1. PearliticPearlitic microstructure microstructure in in steels steels cooled cooled using: using: ( ( (aaa))) airair air-cooling--coolingcooling at at room room temperature temperaturetemperature,,, AC;AC; ((bb))) partially opened furnace furnace,furnace,, POF; POF; ( (cc)) fully fully closed closed furnace, furnace, FCF. FCF.
FigureFigure 2. 2. InterlamellarInterlamellar spacing spacing of of pearlite. pearlite.
In the matter ofTableTable continuous 1.1. MicrostructuralMicrostructural cooling, theparametersparameters transformation asas aa resultresult temperature ofof thethe coolingcooling controls system.system. the microstructure and is governed by the cooling rate. A decrease of such a rate (or, in other words, an increase of the CoolingCooling SSystemystem ACAC POFPOF FCFFCF cooling time) makes the interlamellar spacing and the size of the pearlitic colony increase (Table1), i.e., ddCC (μm)(μm) 1212 1515 1919 slower cooling produces coarser microstructures. ss00 (μm)(μm) 0.160.16 0.230.23 0.260.26 Table 1. Microstructural parameters as a result of the cooling system. 44.. MechaMechanicalnical PPropertieropertiess OnOn thethe basisbasis ofof loadloadCooling--displacementdisplacement System plotsplots afterafter AC thethe standardstandard POF tensiletensile tests,tests, FCF thethe truetrue stressstress––strainstrain curvecurve σσ––εε characteristiccharacteristic ofofd Ceacheach(µm) materialmaterial isis obtainedobtained 12 (Fig(Figureure 153).3). TheThe appearanceappearance 19 ofof thethe threethree mainmain curvescurves (one(one representativerepresentatives 0ofof(µ eachm)each heatheat treatment)treatment) 0.16 indicatesindicates 0.23 thatthat aa smallsmall 0.26plateauplateau cancan bebe observed,observed, afterafter thethe linearlinear elasticelastic part.part. TheThe sizesize ofof suchsuch aa pplateaulateau risesrises withwith thethe coolingcooling raterate appliedapplied duringduring thethe
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4. Mechanical Properties On the basis of load-displacement plots after the standard tensile tests, the true stress–strain curve σ–ε characteristic of each material is obtained (Figure3). The appearance of the three main curvesMetals (one 2016, representative6, 318 of each heat treatment) indicates that a small plateau can be observed,4 of after12 the linear elastic part. The size of such a plateau rises with the cooling rate applied during the heat treatment.heat treatment. The strain The strain hardening hardening portions portions of the of curvesthe curves (elastic–plastic (elastic–plastic regions) regions) are are quasi-parallel quasi-parallel for thefor three the analyzedthree analyzed steels. steels.
1200 Steel cooled 1000 AC steel using POF
800 Steel cooled using FCF 600
(MPa)
400
200
0 0.00 0.02 0.04 0.06 0.08 0.10
FigureFigure 3. 3.Stress–strain Stress–strain curves (standard t tensileensile tests). tests).
TableTable2 shows 2 shows the mechanical the mechanical properties. properties. The three The three pearlitic pearlitic steels producedsteels produced using three using different three heatdifferent treatments heat (coolingtreatments systems: (cooling AC, systems: POF andAC, FCF)POF and have FCF) the samehave the Young same modulus Young modulus (E~200 GPa),(E~200 but GPa), but while the yield stress σY, the ultimate tensile strength (UTS) σR and the reduction in area while the yield stress σY, the ultimate tensile strength (UTS) σR and the reduction in area percentage percentage RA rise with the increase of the cooling rate, the strain at UTS εR diminishes. RA rise with the increase of the cooling rate, the strain at UTS εR diminishes.
Table 2. Mechanical properties obtained by standard tensile tests. Table 2. Mechanical properties obtained by standard tensile tests.
Cooling System AC POF FCF Cooling SystemE (GPa) AC202 200 POF 203 FCF E (GPa)σY (MPa) 202650 560 200 441 203 σ Y (MPa)σR (MPa) 6501105 1055 560 965 441 σR (MPa) 1105 1055 965 εR 0.067 0.072 0.092 εR 0.067 0.072 0.092 RA (%)RA (%) 3333 20 20 14 14
In pearlitic microstructures, the yield stress is related to a critical stress necessary to shift dislocationsIn pearlitic in ferrite microstructures, between two the impenetrable yield stress cementite is related walls, to in a such critical a manner stress that necessary the interfaces to shift dislocationsbetween ferrite in ferrite and betweencementite two act impenetrableas barriers to cementitedislocational walls, movement in such a[11]. manner Such thata critical the interfaces stress betweenrises with ferrite the and refinement cementite of theact aspearlitic barriers microstructure, to dislocational i.e., movement a decrease [ of11 ]. pearlite Such a interlamellar critical stress risesspacing with leads the refinement to an increase of thein the pearlitic resistance microstructure, to glide, and a i.e., Hall a–Petch decrease type of relationship pearlite interlamellar [1,2] with spacingthe exponent leads to −1/2 an being increase able in to the be resistancefitted to describe to glide, the andrelationship a Hall–Petch between type the relationship yield stress [and1,2] the with theinterlamellar exponent − spacing.1/2 being The able Hall to–Petch be fitted expression to describe (with the the relationship exponent −1/2 between) sometimes the yield results stress in an and theinternal interlamellar frictional spacing. strength The of Hall–Petch ferrite [8] with expression a negative (with value, the exponent and this is− the1/2) reason sometimes why some results inresearchers an internal assume frictional that strength an exponent of ferrite −1 in [ 8the] with equation a negative is more value, adequate and this[4,5,7]. is the reason why some researchersWith assumeregard to that the an strength exponent of −the1 insteel, the in equation addition is to more the adequateimportant [4role,5,7 of]. the interlamellar spacing,With regardother factors to the can strength also be of inf theluen steel,tial, inamong addition them to the the pearlite important colony role size of theor the interlamellar possible spacing,existence other of a factors phenomenon can also of beprecipitation influential, hardening among them (or precipitation the pearlite strengthening) colony size or due the possibleto the existencealloying of elements a phenomenon [21]. The oflevel precipitation of the afore hardening-said effect usually (or precipitation depends on strengthening) the cooling strateg duey to in the alloyingcertain elementsmetals, such [21]. as The iron level–vanadium of the– afore-saidcarbon alloys, effect in usuallysuch a manner depends that, on when the cooling these materials strategy in are transformed from the γ to the α state, a banded dispersion of vanadium carbide is formed. As a certain metals, such as iron–vanadium–carbon alloys, in such a manner that, when these materials are result, the hardening is shown to be dependent on the transformation temperature because it defines the state of the dispersion [22]. In this way, the precipitation of dispersed vanadium carbides upon pearlitic transformation of the high-carbon vanadium steel can therefore be used as a plausible procedure of precipitation strengthening of pearlite [23].
Metals 2016, 6, 318 5 of 12 transformed from the γ to the α state, a banded dispersion of vanadium carbide is formed. As a result, the hardening is shown to be dependent on the transformation temperature because it defines the state of the dispersion [22]. In this way, the precipitation of dispersed vanadium carbides upon pearlitic transformation of the high-carbon vanadium steel can therefore be used as a plausible procedure of precipitation strengthening of pearlite [23]. The rise of the cooling rate used during heat treatment of a given steel produces a drop of the maximumMetals uniform2016, 6, 318 elongation attained in standard tensile tests, while at the same time increasing5 of 12 the elongation when the necking appears (i.e., at the instant of maximum strain localization during the The rise of the cooling rate used during heat treatment of a given steel produces a drop of the standardmaximum tensile uniform test). elongation attained in standard tensile tests, while at the same time increasing the elongation when the necking appears (i.e., at the instant of maximum strain localization during 5. Fracturethe standard Surfaces tensile test). In general terms, from the macroscopic point of view, the fracture surface (Figures4 and5) exhibits 5. Fracture Surfaces an increasingly brittle feature when the cooling rate decreases or, in other words, when the cooling time increases.In general In the terms, instant from of the fracture, macroscopic the air-cooled point of view steel, the develops fracturelocalized surface (Figures strain 4 in and the 5) form of exhibits an increasingly brittle feature when the cooling rate decreases or, in other words, when the necking; it is a type of failure which may be classified as moderately ductile (Figures4a and5a). On the cooling time increases. In the instant of fracture, the air-cooled steel develops localized strain in the other hand,form of the necking steel; cooledit is a type in theof failure FCF reacheswhich may the be instant classified failure as moderate and becomesly ductile cracked (Figures without 4a and even necking,5a). in On such the other a manner hand, thatthe steel the fracturecooled in isthe more FCF reaches brittle andthe instant exhibits failure a more and becomes irregular cracked topography (Figureswithout4c and even5c). necking, Finally, in in such the a middlemanner that case the consisting fracture is more of the brittle steel and cooled exhibits in a themore partially-open irregular furnace,topography the fractographic (Figures aspect 4c and can5c). be Finally, classified in the as middle intermediate case consisting between of the the other steel two cooled steels in the analyzed (Figurespartially4b and-open5b), i.e.,furnace, with the almost fractographic no evidence aspect ofcan necking. be classified as intermediate between the other two steels analyzed (Figures 4b and 5b), i.e., with almost no evidence of necking.
(a)
(b)
(c)
Figure 4. Top view of the fracture surfaces in the steels cooled using: (a) AC; (b) POF; (c) FCF. Figure 4. Top view of the fracture surfaces in the steels cooled using: (a) AC; (b) POF; (c) FCF.
Metals 2016, 6, 318 6 of 12 Metals 2016, 6, 318 6 of 12
Metals 2016, 6, 318 6 of 12
(a)
(a)
(b)
(b)
(c)
Figure 5. Side view of the fracture surfaces in (the c) steels cooled using: ( a) AC; ( b) POF; ( c) FCF. Figure 5. Side view of the fracture surfaces in the steels cooled using: (a) AC; (b) POF; (c) FCF. Figure 6 shows a scheme of the different fracture regions in the steels. The origin of the failure Figure6 shows a scheme of the different fracture regions in the steels. The origin of the failure phenomenon is an internal area in the whole fracture surface: the so-called fracture process zone phenomenonFigure is6 shows an internal a scheme area inof the wholedifferent fracture fracture surface: regions the in so-called the steels. fracture The origin process of the zone failure (FPZ). (FPZ). Later, it continues by means of an unstable propagation zone (UPZ) which advances following Later,phenomenon it continues is an by meansinternal of area an unstablein the whole propagation fracture zone surface: (UPZ) the which so-called advances fracture following process azone radial a radial direction towards the periphery of the sample and ends at the external ring (ER) in the form direction(FPZ). Later, towards it continues the periphery by means of of the an sampleunstable and propagat endsion at the zone external (UPZ) which ring (ER) advances in the following form of a of a ductile shear lip oriented 45° from the radial direction of the wire. The FPZ with fibrous ductilea radial shear direction lip oriented towards 45 the◦ from periphery the radial of the direction sample ofand the ends wire. at the The external FPZ with ring fibrous (ER) in appearance, the form appearance, situated in the central area of the wire (of a lighter color in the photographs), is many situatedof a ductile in the shear central lip area oriented of the 45 wire° from (of a the lighter radial color direction in the photographs),of the wire. The is many FPZ with times fibrous shifted timesappearance, shifted fromsituat edthe in center the central of the area wire. of Thethe wire catastrophic (of a lighter propagation color in the zone photographs), (UPZ) shows is many radial from the center of the wire. The catastrophic propagation zone (UPZ) shows radial micro-cracking, microtimes-cracking, shifted from which the grows center more of the winding wire. The with catastrophic increasing propagationcooling time, zone while (UPZ) the radial shows thickness radial which grows more winding with increasing cooling time, while the radial thickness of the fracture of microthe fracture-cracking, area which associated grows with more the winding ER decreases. with increasing Generally cooling speaking, time, thewhile roughness the radial of thickness the whole area associated with the ER decreases. Generally speaking, the roughness of the whole fracture surface fractureof the fracturesurface arearises associated with a lowering with the cooling ER decreases. rate (Figures Generally 4 and speaking, 5). Finally, the for roughness the steel of cooled the whole inside rises with a lowering cooling rate (Figures4 and5). Finally, for the steel cooled inside a FCF, in some a FCF,fracture in surfacesome specific rises with tests a lowering, secondary cooling cracking rate (Figures also appears 4 and 5).in Finally,a level fordifferent the steel from cooled that inside of the specific tests, secondary cracking also appears in a level different from that of the main fracture area maina FCF, fracture in some area specific (Figure tests 5c)., secondary cracking also appears in a level different from that of the (Figure5c). main fracture area (Figure 5c).
FPZ FPZ
UPZ UPZ
ER ER
Figure 6. Scheme of the different fracture regions in the steels: fracture process zone, FPZ; unstable FigureFigure 6. 6.Scheme Scheme of of the the different different fracturefracture regionsregions in the steels:steels: fracture fracture process process zone zone,, FPZ FPZ;; unstable unstable propagation zone, UPZ; external ring, ER. propagationpropagation zone, zone UPZ;, UPZ external; external ring, ring,ER. ER.
ForFor the the AC AC steel, steel, the the FPZ FPZ (Figure (Figure 7a) 7a) atat highhigh magnification ( (××2500)2500) shows shows the the presence presence of of regions regions For the AC steel, the FPZ (Figure7a) at high magnification ( ×2500) shows the presence of regions formedformed by by a aductile ductile fracture fracture mechanism mechanism consistingconsisting of microvoidmicrovoid coalescencecoalescence (MVC) (MVC) in in which which the the formed by a ductile fracture mechanism consisting of microvoid coalescence (MVC) in which the microvoidsmicrovoids (round (round and and elongated) elongated) present present variousvarious sizes, togethertogether with with small small regions regions where where pearlite pearlite microvoids (round and elongated) present various sizes, together with small regions where pearlite lamellaelamellae are are observed. observed. In In the the steel steel cooled cooled inside inside the FCF, FCF, regions regions formed formed by by pearlite pearlite lamellae lamellae lamellaeobservedobserved are in in the observed. the AC AC steel steel In are theare now steelnow more more cooled extensive extensive inside the (Figure FCF, 7c), regions therebythereby formed indicating indicating by pearlite a amore more lamellae brittle brittle fracture observed fracture inmode.mode. the ACFinally, Finally, steel the are the ste now steelel cooled more cooled extensive inside inside thethe (Figure POFPOF (Figure7c), thereby 7b) representsrepresents indicating an an a intermediate moreintermediate brittle fracturecase case between between mode. thethe more more ductile ductile AC AC steel steel (Figure (Figure 7a) 7a) andand thethe moremore brittle steelsteel cooledcooled in in a a FCF FCF (Figure (Figure 7c). 7c).
Metals 2016, 6, 318 7 of 12
Finally, the steel cooled inside the POF (Figure7b) represents an intermediate case between the more ductileMetals 2016 AC, steel6, 318 (Figure7a) and the more brittle steel cooled in a FCF (Figure7c). 7 of 12
(a)
(b)
(c)
FigureFigure 7. 7.Fracture Fracture process process zonezone (FPZ)(FPZ) in the steels cooled using: using: (a (a) )AC; AC; (b (b) )POF; POF; (c ()c FCF.) FCF.
In the UPZ, the fracture surface consists of mainly cleavage facets with some MVC regions amongIn the them UPZ, (Figure the fracture 8), in such surface a manner consists that of mainlythe typical cleavage cleavage facets facets with (containing some MVC river regions patterns among themwhich (Figure mark8), the in suchdirection a manner and sense that theof fracture typical cleavagepropagation) facets are (containing surrounded river by patterns the areas which of MVC. mark theWith direction regard and to the sense effect of of fracture the heat propagation) treatment duration, are surrounded the lower bythe the cooling areas speed of MVC. the greater With regard the tosize the of effect the afore of the-said heat cleavage treatment facets duration, (it is seen the from lower Figure the 8a cooling showing speed the smaller the greater facets the to Figure size of 8c the afore-saidwith the bigger cleavage ones, facets Figure (it is8b seen representing from Figure an intermediate8a showing thesituation). smaller The facets described to Figure phenomenon8c with the may be related to the size of the PAG, considering the well-known relationship between the PAG size and the extension of the cleavage facet in steels [24].
MetalsMetals2016 2016, 6,, 3186, 318 8 of8 12 of 12
The MVC regions between the cleavage facets are most abundant in the AC steel (Figure 8a with biggermore ones, ductile Figure fractography)8b representing than in ansteels intermediate cooled inside situation). the furnace, The either described inside phenomenon the POF (Figure may 8b) be relatedor inside to the the size FCF of the(Figure PAG, 8c considering with a more the brittle well-known fractography). relationship In addition, between in thethe PAGlatter size case and, the the extensionfractographic of the appearance cleavage facet consists in steels of very [24]. large cleavage facets (at different levels) and thus MVC thin bands delimiting the boundaries of the facets.
(a)
(b)
(c)
FigureFigure 8. 8.Unstable Unstable propagation propagationzone zone (UPZ)(UPZ) in the steels cooled cooled using: using: (a ()a )AC; AC; (b ()b POF;) POF; (c) ( cFCF.) FCF.
According to previous research [5,24], the critical fracture unit in pearlitic microstructures is a regionThe MVC where regions the crystalline between structure the cleavage of ferrite facets (and are most cementite) abundant of neighboring in the AC steel colonies (Figure share8as with a morecommon ductile orientation. fractography) The size than ofin this steels orientation cooled unit inside is controlled the furnace, by the either PAG inside size and, the POFtherefore, (Figure can8 b) or inside the FCF (Figure8c with a more brittle fractography). In addition, in the latter case, the fractographic appearance consists of very large cleavage facets (at different levels) and thus MVC thin bands delimiting the boundaries of the facets. Metals 2016, 6, 318 9 of 12
According to previous research [5,24], the critical fracture unit in pearlitic microstructures is a region where the crystalline structure of ferrite (and cementite) of neighboring colonies shares a common orientation. The size of this orientation unit is controlled by the PAG size and, therefore, can be calculated by measuring the extension of cleavage facets appearing on the fracture surface as Metals 2016, 6, 318 9 of 12 discussed in previous paragraphs [24]. be calculated by measuring the extension of cleavage facets appearing on the fracture surface as 6. Fracto-Metallographicdiscussed in previous paragraphs Analysis [24]. The fracto-metallographic analysis consists of studying both the fracture propagation path 6. Fracto-Metallographic Analysis (fractographic analysis) and the internal microstructure of the material (materiallographic analysis), i.e., the aimThe is fracto to evaluate-metallographic the areas analysis of damage consists in the of middlestudying of both the ferrite/cementitethe fracture propagation lamellae path inside the pearlitic(fractographic colonies, analysis) so that and the the fracture internal process microstructure is seen of as the a consequence material (materiallographic of material microstructure, analysis), i.e., the aim is to evaluate the areas of damage in the middle of the ferrite/cementite lamellae inside in a sort of materials science approach. the pearlitic colonies, so that the fracture process is seen as a consequence of material microstructure, The experimental procedure is as follows: after cutting the fractured specimens along longitudinal in a sort of materials science approach. sections andThe their experimental metallographic procedure preparation, is as follows: it is possible after cutting to observe the fractur differented specimensfracture phenomena, along suchlongitudinal as the cracking sections paths, and their the metallographic areas of localized preparation, damage, it is shearpossible cracking to observe effects, different debonding fracture or delaminationphenomena zones,, such etc. as the In the cracking particular paths, case the of areas the ofsteels localized analyzed damage, in this shear paper, cracking in areas effects, close to the fracturedebonding surface, or delamination it is possible zones, to observeetc. In the the particular micro-structural case of the damagesteels analyzed that the in steelthis paper, has suffered in becauseareas of close the plasticto the fracture strain occurringsurface, it is during possible the tostandard observe the tensile micro test.-structural damage that the steel Inhas many suffered of because the colonies of the whoseplastic strain lamellae occurring have during an orientation the standard close tensile to the test. axial one of the wire (directionIn in many which of the the colonies load is whose applied lamellae in the have standard an orientation tensile close test), to the the axial existence one of ofthe abundantwire (direction in which the load is applied in the standard tensile test), the existence of abundant microcracking is observed (Figures9 and 10). microcracking is observed (Figures 9 and 10).
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Figure 9. Micro-damage in AC steels: (a) in the close vicinity of the main crack; (b) below the previous Figure 9. Micro-damage in AC steels: (a) in the close vicinity of the main crack; (b) below the previous area. In both micrographs, the vertical size represents the wire axis (direction of applying the load in area. In both micrographs, the vertical size represents the wire axis (direction of applying the load in the standard tensile test). the standard tensile test). In the case of colonies constituted by relatively fine pearlite (AC steel with thin interlamellar spacing and small colonies), microcracks with uneven appearance (with regard to the crack opening)
Metals 2016, 6, 318 10 of 12 and that are shorter in length than the colony size are observed (Figure 9). As a consequence, the Metalsfracture2016 behavior, 6, 318 in this case is more ductile because of the shortness of the afore-said microcracks10 of 12 acting as weak stress raisers and thus slight initiators of fracture.
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(b)
Figure 10. Micro-damage in steels cooled in a FCF: (a) in the close vicinity of the main crack; (b) below Figure 10. Micro-damage in steels cooled in a FCF: (a) in the close vicinity of the main crack; (b) below the previous area. In both micrographs micrographs,, the vertical size represents the wire axis (direction of applying the load in the standard tensiletensile test).test).
On the other hand, in the case of colonies constituted by relatively coarse pearlite (steel cooled In the case of colonies constituted by relatively fine pearlite (AC steel with thin interlamellar in a FCF with wider interlamellar spacing and greater colonies), the inclined cracking is generally of spacing and small colonies), microcracks with uneven appearance (with regard to the crack opening) greater length (even across the complete colony) and looks more uniform (Figure 10). As a and that are shorter in length than the colony size are observed (Figure9). As a consequence, the consequence, the fracture behavior in this case is more brittle because of the longer length of the afore- fracture behavior in this case is more ductile because of the shortness of the afore-said microcracks said microcracks (more or less aligned or quasi-parallel) acting as marked stress risers and thus strong acting as weak stress raisers and thus slight initiators of fracture. fracture-initiators. On the other hand, in the case of colonies constituted by relatively coarse pearlite (steel cooled in a Therefore, the fracture process is determined by physical events in the pearlite colony with the FCF with wider interlamellar spacing and greater colonies), the inclined cracking is generally of greater lamellae being parallel to the tensile axis, where the deformation occurs in narrow bands of locally length (even across the complete colony) and looks more uniform (Figure 10). As a consequence, the intense shear stress [11] according to the Miller–Smith mechanism [25], see Figure 11. In this fracture behavior in this case is more brittle because of the longer length of the afore-said microcracks phenomenon, the slip bands in the ferrite produce microcracking in the cementite plates, followed (more or less aligned or quasi-parallel) acting as marked stress risers and thus strong fracture-initiators. by tearing in the ferrite lamellae. Therefore, the fracture process is determined by physical events in the pearlite colony with the lamellae being parallel to the tensile axis, where the deformation occurs in narrow bands of locally intense shear stress [11] according to the Miller–Smith mechanism [25], see Figure 11. In this phenomenon, the slip bands in the ferrite produce microcracking in the cementite plates, followed by tearing in the ferrite lamellae.
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Figure 11. MillerMiller–Smith–Smith mechanism [[2525].
7. Conclusions On the basis of the experimental results presented in this pape paper,r, the following conclusions may be drawn:
(1) TheThe cooling cooling rate rate clearly affects the steel microstructure, so that quick cooling produces finerfiner pearlitic microstructures (smaller colonies and lower interlamellar spacing). (2) MaterialMaterial strength strength (represented (represented by by the yield stress and the ultimate tensile strength) increases when the heat treatment is applied by means of a rising cooling rate.
(3) TheThe increase increase of of the cooling rate produces a decrease of the strain for maximum load (uniform elongation)elongation) and and a ann increase of the reduction in area (non-uniform(non-uniform elongation). (4) From the macroscopic point of view, fracture surfaces resemble a more ductile appearance as (4) From the macroscopic point of view, fracture surfaces resemble a more ductile appearance as the the cooling rate rises. cooling rate rises. (5) The fractography of the process zone (that associated with fracture initiation) consists mainly of (5) The fractography of the process zone (that associated with fracture initiation) consists mainly of a mixture of two fracture micromechanisms: microvoid coalescence (MVC) and localized areas a mixture of two fracture micromechanisms: microvoid coalescence (MVC) and localized areas where the pearlite lamellae are easily observed. where the pearlite lamellae are easily observed. (6) For coarse pearlite microstructures (those associated with slow cooling that produces bigger (6) For coarse pearlite microstructures (those associated with slow cooling that produces bigger colonies and higher interlamellar spacing), the fracture process is more brittle and more presence colonies and higher interlamellar spacing), the fracture process is more brittle and more presence of ferrite/cementite lamellae can be observed in the fracture process zone (with the associated of ferrite/cementite lamellae can be observed in the fracture process zone (with the associated lower percentage of MVC areas). lower percentage of MVC areas). (7) The macroscopic fracture mode (brittle or ductile) can be explained on the basis of the (7) microstructureThe macroscopic of the fracture pearlitic mode steel (brittleas a consequence or ductile) of canthe heat be explained treatment onapplied the basison it. In of fine the pearliticmicrostructure microstructures, of the pearlitic micro steel-damage as a consequenceduring the standard of the heat tensile treatment test takes applied place on in it.the In form fine ofpearlitic very small microstructures, microcracks, micro-damage thereby producing during a ductile the standard fracture tensile behavior. test takes On the place other in thehand, form in coarseof very pearlitic small microcracks, microstructures, thereby micro producing-damage a ductile happens fracture in the behavior. form of longer On the and other aligned hand, microcracks,in coarse pearlitic thus generating microstructures, a more micro-damagebrittle fracture happensbehavior. in the form of longer and aligned microcracks, thus generating a more brittle fracture behavior. Acknowledgments: The authors wish to acknowledge the financial support provided by the following Spanish Acknowledgments: The authors wish to acknowledge the financial support provided by the following Spanish Institutions: Ministry for Science and Technology (MICYT; Grant MAT2002-01831), Ministry for Education and Institutions: Ministry for Science and Technology (MICYT; Grant MAT2002-01831), Ministry for Education and Science (MEC; GrantGrant BIA2005-08965),BIA2005-08965), Ministry for Science and Innovation (MICINN; Grant BIA2008BIA2008-06810),-06810), Ministry for Economy and Competitiveness (MINECO; Grant BIA2011 BIA2011-27870),-27870), Junta Junta de de Castilla y León (JCyL; Grants SA067A05, SA111A07 and SA039A08) and the Spanish University Foundation “Memoria de D. Samuel Solórzano Barruso” (Grant 2016/00017/001).2016/00017/001). Author Contributions: J.T., B.G. and J.-C.M. conceived and designed the experiments; B.G., J.-C.M. and F.-J.A. Author Contributions: J.T., B.G. and J.-C.M. conceived and designed the experiments; B.G., J.-C.M. and F.-J.A. performed the experiments; J.T., B.G., J.-C.M. and F.-J.A. analyzed the data; J.T. wrote the paper. performed the experiments; J.T., B.G., J.-C.M. and F.-J.A. analyzed the data; J.T. wrote the paper. Conflicts of Interest: The authors declare no conflict of interest. Conflicts of Interest: The authors declare no conflict of interest. References References 1. Hall, E.O. The deformation and ageing of mild steel: III Discussion of results. Proc. Phys. Soc. Sect. 1951, B64, 1. Hall,747–753. E.O. [CrossRef The deformation] and ageing of mild steel: III Discussion of results. Proc. Phys. Soc. Sect. 1951, 2. B64Petch,, 74 N.J.7–753 The. cleavage strength of polycrystals. J. Iron Steel Inst. 1953, 174, 25–28. 3.2. Petch,Karlsson, N.J. B.;The Lindén, cleavage G. strength Plastic of deformation polycrystals. of J. eutectoid Iron Steel steelInst. 1953 with, 174 different, 25–28 cementite. morphologies. 3. Karlsson,Mater. Sci. B. Eng.; Lindén,1975, G.17 ,Plastic 153–164. deformation [CrossRef of] eutectoid steel with different cementite morphologies. Mater. Sci. Eng. 1975, 17, 153–164.
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