5. Fracture Surfaces
In general terms, from the macroscopic point of view, the fracture surface (Figure 4
and Figure 5
) exhibits an increasingly brittle feature when the cooling rate decreases or, in other words, when the cooling time increases. In the instant of fracture, the air-cooled steel develops localized strain in the form of necking; it is a type of failure which may be classified as moderately ductile (Figure 4
a and Figure 5
a). On the other hand, the steel cooled in the FCF reaches the instant failure and becomes cracked without even necking, in such a manner that the fracture is more brittle and exhibits a more irregular topography (Figure 4
c and Figure 5
c). Finally, in the middle case consisting of the steel cooled in the partially-open furnace, the fractographic aspect can be classified as intermediate between the other two steels analyzed (Figure 4
b and Figure 5
b), i.e., with almost no evidence of necking.
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 (FPZ). Later, it continues by means of an unstable propagation zone (UPZ) which advances following a radial direction towards the periphery of the sample and ends at the external ring (ER) in the form of a ductile shear lip oriented 45° from the radial direction of the wire. The FPZ with fibrous appearance, situated in the central area of the wire (of a lighter color in the photographs), is many times shifted from the center of the wire. The catastrophic propagation zone (UPZ) shows radial micro-cracking, which grows more winding with increasing cooling time, while the radial thickness of the fracture area associated with the ER decreases. Generally speaking, the roughness of the whole fracture surface rises with a lowering cooling rate (Figure 4
and Figure 5
). Finally, for the steel cooled inside a FCF, in some specific tests, secondary cracking also appears in a level different from that of the main fracture area (Figure 5
For the AC steel, the FPZ (Figure 7
a) at high magnification (×2500) shows the presence of regions formed by a ductile fracture mechanism consisting of microvoid coalescence (MVC) in which the microvoids (round and elongated) present various sizes, together with small regions where pearlite lamellae are observed. In the steel cooled inside the FCF, regions formed by pearlite lamellae observed in the AC steel are now more extensive (Figure 7
c), thereby indicating a more brittle fracture mode. Finally, the steel cooled inside the POF (Figure 7
b) represents an intermediate case between the more ductile AC steel (Figure 7
a) and the more brittle steel cooled in a FCF (Figure 7
In the UPZ, the fracture surface consists of mainly cleavage facets with some MVC regions among them (Figure 8
), in such a manner that the typical cleavage facets (containing river patterns which mark the direction and sense of fracture propagation) are surrounded by the areas of MVC. With regard to the effect of the heat treatment duration, the lower the cooling speed the greater the size of the afore-said cleavage facets (it is seen from Figure 8
a showing the smaller facets to Figure 8
c with the bigger ones, Figure 8
b representing an intermediate situation). The described phenomenon 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
The MVC regions between the cleavage facets are most abundant in the AC steel (Figure 8
a with more ductile fractography) than in steels cooled inside the furnace, either inside the POF (Figure 8
b) or inside the FCF (Figure 8
c 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.
According to previous research [5
], 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 discussed in previous paragraphs [24
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 Science (MEC; Grant BIA2005-08965), Ministry for Science and Innovation (MICINN; Grant BIA2008-06810), Ministry for Economy and Competitiveness (MINECO; Grant BIA2011-27870), Junta 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).
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.
Conflicts of Interest
The authors declare no conflict of interest.
- Hall, E.O. The deformation and ageing of mild steel: III Discussion of results. Proc. Phys. Soc. Sect. 1951, B64, 747–753. [Google Scholar] [CrossRef]
- Petch, N.J. The cleavage strength of polycrystals. J. Iron Steel Inst. 1953, 174, 25–28. [Google Scholar]
- Karlsson, B.; Lindén, G. Plastic deformation of eutectoid steel with different cementite morphologies. Mater. Sci. Eng. 1975, 17, 153–164. [Google Scholar] [CrossRef]
- Marder, A.R.; Bramfitt, B.L. The effect of morphology on the strength of pearlite. Metall. Trans. 1976, 7A, 365–372. [Google Scholar] [CrossRef]
- Hyzak, J.M.; Bernstein, I.M. The role of microstructure on the strength and toughness of fully pearlitic steels. Metall. Trans. 1976, 7A, 1217–1224. [Google Scholar] [CrossRef]
- Kavishe, F.P.L.; Baker, T.J. Effect of prior austenite grain size and pearlite interlamellar spacing on strength and fracture toughness of a eutectoid rail steel. Mater. Sci. Technol. 1986, 2, 816–822. [Google Scholar] [CrossRef]
- Ray, K.K.; Mondal, D. The effect of interlamellar spacing on strength of pearlite in annealed eutectoid and hypoeutectoid plain carbon steels. Acta Metall. Mater. 1991, 39, 2201–2208. [Google Scholar] [CrossRef]
- Modi, O.P.; Deshmukh, N.; Mondal, D.P.; Jha, A.K.; Yegneswaran, A.H.; Khaira, H.K. Effect of interlamellar spacing on the mechanical properties of 0.65% C steel. Mater. Charact. 2001, 46, 347–352. [Google Scholar] [CrossRef]
- Elwazri, A.M.; Wanjara, P.; Yue, S. The effect of microstructural characteristics of pearlite on the mechanical properties of hypereutectoid steel. Mater. Sci. Eng. A 2005, 404, 91–98. [Google Scholar] [CrossRef]
- Yahyaoui, H.; Sidhom, H.; Braham, C.; Baczmanski, A. Effect of interlamellar spacing on the elastoplastic behavior of C70 pearlitic steel: Experimental results and self-consistent modeling. Mater. Des. 2014, 55, 888–897. [Google Scholar] [CrossRef][Green Version]
- Dollar, M.; Bernstein, I.M.; Thompson, A.W. Influence of deformation substructure on flow and fracture of fully pearlitic steel. Acta Metall. 1988, 36, 311–320. [Google Scholar] [CrossRef]
- Toribio, J. Relationship between microstructure and strength in eutectoid steels. Mater. Sci. Eng. A 2004, 387–389, 227–230. [Google Scholar] [CrossRef]
- Toribio, J.; González, B.; Matos, J.C. Microstructure and mechanical properties in progressively drawn pearlitic steel. Mater. Trans. 2014, 55, 93–98. [Google Scholar] [CrossRef]
- Gomes, M.G.M.F.; de Almeida, L.H.; Gomes, L.C.F.C.; le May, I. Effects of microstructural parameters on the mechanical properties of eutectoid rail steels. Mater. Charact. 1997, 39, 1–14. [Google Scholar] [CrossRef]
- Bae, C.M.; Nam, W.J.; Lee, C.S. Effect of microstructural features on ductility in hypo-eutectoid steels. Scr. Mater. 1999, 41, 605–610. [Google Scholar] [CrossRef]
- Lewandowski, J.J.; Thompson, A.W. Effects of the prior austenite grain size on the ductility of fully pearlitic eutectoid steel. Metall. Trans. 1986, 17A, 461–472. [Google Scholar] [CrossRef]
- Izotov, V.I.; Pozdnyakov, V.A.; Luk′yanenko, E.V.; Usanova, O.Y.; Filippov, G.A. Influence of the pearlite fineness on the mechanical properties, deformation behavior, and fracture characteristics of carbon steel. Phys. Met. Metallogr. 2007, 103, 519–529. [Google Scholar] [CrossRef]
- Porter, D.A.; Easterling, K.E.; Smith, G.D.W. Dynamic studies of the tensile deformation and fracture of pearlite. Acta Metall. 1978, 26, 1405–1422. [Google Scholar] [CrossRef]
- Abrams, H. Grain size measurement by the intercept method. Metallography 1971, 4, 59–78. [Google Scholar] [CrossRef]
- Hu, X.; Houtte, P.V.; Liebeherr, M.; Walentek, A.; Seefeldt, M.; Vandekinderen, H. Modeling work hardening of pearlitic steels by phenomenological and Taylor-type micromechanical models. Acta Mater. 2006, 54, 1029–1040. [Google Scholar] [CrossRef]
- Li, J.; Sun, F.; Xu, W. On the evaluation of yield strength for microalloyed steels. Scr. Metall. Mater. 1990, 24, 1393–1398. [Google Scholar]
- Batte, A.D.; Honeycombe, R.W.K. Strengthening of ferrite by vanadium carbide precipitation. Met. Sci. J. 1973, 7, 160–168. [Google Scholar] [CrossRef]
- Izotov, B.I. Precipitation of disperse vanadium carbides at the interphase boundary upon the pearlitic transformation of a steel. Phys. Met. Metall. 2011, 111, 592–597. [Google Scholar] [CrossRef]
- Park, Y.J.; Bernstein, I.M. The process of crack initiation and effective grain size for cleavage fracture in pearlitic eutectoid steel. Metall. Trans. 1979, 10A, 1653–1664. [Google Scholar] [CrossRef]
- Miller, L.E.; Smith, G.C. Tensile fractures in carbon steels. J. Iron Steel Inst. 1970, 208, 998–1005. [Google Scholar]
Pearlitic microstructure in steels cooled using: (a) air-cooling at room temperature, AC; (b) partially opened furnace, POF; (c) fully closed furnace, FCF.
Interlamellar spacing of pearlite.
Stress–strain curves (standard tensile tests).
Top view of the fracture surfaces in the steels cooled using: (a) AC; (b) POF; (c) FCF.
Side view of the fracture surfaces in the steels cooled using: (a) AC; (b) POF; (c) FCF.
Scheme of the different fracture regions in the steels: fracture process zone, FPZ; unstable propagation zone, UPZ; external ring, ER.
Fracture process zone (FPZ) in the steels cooled using: (a) AC; (b) POF; (c) FCF.
Unstable propagation zone (UPZ) in the steels cooled using: (a) AC; (b) POF; (c) FCF.
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 the standard tensile test).
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, the vertical size represents the wire axis (direction of applying the load in the standard tensile test).
Miller–Smith mechanism [25
Microstructural parameters as a result of the cooling system.
Mechanical properties obtained by standard tensile tests.
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