Role of the microstructure on the mechanical properties of fully pearlitic eutectoid steels

This paper analyses how the microstructure of an eutectoid pearlitic steel affects its conventional mechanical properties (obtained by means of a standard tension test) and the associated micromechanisms of fracture. Results show how the yield strength, the ultimate tensile strength (UTS) and the ductility, increase as the continuous cooling rate rises, whereas on the other hand the strain at UTS decreases. The fracture surface exhibits more brittle features when the continuous cooling rate decreases, in such a manner that in the fracture process zone an increasing area appears where the pearlite lamellae can be detected and less regions of microvoids are observed.


INTRODUCTION
n pearlitic steel, hardness and yield strength increase with decreasing interlamellar spacing following a Hall-Petch type relationship [1][2][3][4][5][6][7][8][9][10][11].Resistance in pearlite is controlled by processes that occur in the ferrite, the main role of the cementite lamellae (at low stresses) being to limit the slip distance in the ferrite [11].In contrast, the ductility depends on the prior austenite grain, so that it increases with the decreasing size of such characteristic parameter of the material [12][13][14].The high work-hardening rate of pearlite is mainly attributed to the load transferred from the ferrite to the cementite, with a significant contribution due to the effect of the constraint of the hard phase on the ferrite [3].Steels with thin pearlite and coarse pearlite show similar strain hardening coefficients [15].The coarse pearlite is deformed in a non-homogeneous way, presenting localized plastic strain in narrow slip bands, while thin pearlite shows a much more uniform strain distribution [13,15,16].In pearlitic steel, plastic strain induces compressive residual stresses in the ferrite and tensile stresses in the cementite, so that the residual stress level is higher in steel with greater interlamellar spacing [10].Fracture in standard tensile test is determined by processes in the colonies having pearlite lamellae parallel to the tensile axis, where the deformation occurs in narrow bands of locally intense shear stress [11].This paper studies the influence of pearlitic steel's microstructure on conventional mechanical properties obtained with the standard tensile test.Moreover, an analysis of the different micro-mechanisms involved in fracture by simple tension of the steels has been carried out.

I EXPERIMENTAL PROCEDURE
he material used was steel with eutectoid composition and some alloying elements: 0.789% C, 0.681% Mn, 0.210% Si, 0.010% P, 0.218% Cr and 0.061% V.The steel, previously produced by hot rolling, was subjected to various heat treatments, consisting of heating the material in a furnace at 900°C (above the eutectoid temperature) for 1 hour and then cooling it in the closed furnace (PF); with the furnace partially open (PS); or outside the furnace, in the air, at room temperature (PA).The microstructure of the material was analyzed by metallographic microscopy and by scanning electron microscopy (SEM), after the metallographic preparation of small samples of steel by mounting, grinding and polishing (up to a mirror finish) and attack with Nital 4% (a mixture of 4 ml of nitric acid with 96 ml of ethanol) for several seconds to reveal the microstructure.Standard tensile tests (three for each material) were performed by using cylindrical specimens of 11 mm in diameter and 300 mm in length.The crosshead speed was 2 mm/min and an extensometer with a gage length of 50 mm was placed in the central part of the specimens.Finally, the fracture surfaces and longitudinal sections of the fractured specimens (after metallographic preparation and attack with Nital 4%) was observed through SEM.

Microstructural analysis
sing optical microscopy and scanning electron microscopy, the microstructure of pearlitic steels obtained by various heat treatments (Figs. 1 and 2) was observed.The diameter of the pearlite colony d C was measured by the intercept method [17], which consists of counting the number of grains intercepted by a straight line drawn at random along the micrograph, considering the line's randomness and the randomness of the metallographic cut by means of statistical methods.The average interlamellar spacing of pearlite s 0 was calculated by the method of the circular line [18], a variant to the linear intercept method, in which a circle of known length is drawn on the photograph, and the number of intersecting pearlite lamellae is counted (also taking into account the randomness).The transformation temperature, which in continuous cooling is determined by the cooling rate, controls interlamellar spacing.Decreasing the cooling rate (or increasing the cooling time) increases interlamellar spacing and the size of the pearlite colony (Tab.1).Interlamellar spacing is independent of the prior austenite grain size and the size of the pearlite colony [4].

Mechanical properties
From the load-displacement records, resulting from standard tension tests, the true stress-strain curves σ-ε characteristic of each material have been calculated (Fig. 3).In them the existence of a small plateau is observed, following the linear elastic region (with the same slope for all the curves), whose size increases with the cooling rate used in the heat treatment.The elasto-plastic region shows a quasi-parallel curve for the steels analyzed.The conventional mechanical properties obtained from the standard tension tests are shown in Tab. 2, beside the ductility measured by the reduction in area percentage (RA).The three pearlitic steels (PA, PS and PF) have the same Young modulus (E ~ 200 GPa), but while the yield strength σ Y , the ultimate tensile strength (UTS) σ R and the reduction in area percentage RA increase with the increase of the cooling rate used for the thermal treatment of the material, the strain at UTS ε R decreases.The yield strength in the pearlite is consistent with the stress required to move the dislocations in ferrite between two impenetrable walls made of cementite, where ferrite-cementite interfaces act as barriers to dislocational movement [11].
The stress increases with the refinement of the interlamellar spacing leading to an increase in the resistance, so that the yield strength increases with decreasing interlamellar spacing following a Hall-Petch type relationship [1,2], with the exponent -1/2.This expression sometimes results in an internal frictional stress in the ferrite with a negative value, which is why some authors prefer to use the exponent -1 in this equation [4,5,7].
Increasing the cooling rate, used in the heat treatment of the steel, reduces the maximum uniform elongation attained in standard tension tests, while increasing the total elongation in the necking.Deformation (dislocational structure) has a size effect on interlamellar spacing [15].

Fracture surface
Macroscopically, the steels' fracture surface shows a more brittle appearance with a decreasing cooling rate (Fig. 4).While in the air-cooled steel a necking and cup-cone fracture is observed, steel cooled in the furnace broke without even a necking, and with a more brittle fracture and with a more irregular topography.For steel cooled in the partially-open furnace, the aspect is in between the other two steels analyzed.The fracture begins in the process zone, continues with an unstable propagation zone with a radial form to the wire surface, and ends at the external ring.The fibrous process zone, situated in the central area of the wire (of a lighter color in the photographs), is deviated from the center of the wire; to a greater extent as the cooling rate used in the treatment decreases.The catastrophic propagation zone shows radial micro-cracking, which grows more winding with increasing cooling time, while the external ring thickness decreases (ductile shear lip inclined 45°).The fracture surface's roughness increases with decreasing cooling rate (Fig. 4), and, in some tests, for the steel cooled inside the furnace, secondary cracking contained in the cross section of the wire was also observed (Fig. 4f).

Fractographic analysis
For the air-cooled steel, the process zone at high magnification shows the presence of microvoids (ductile fracture mechanism) of various sizes (round and elongated) along with small regions where pearlite lamellae are observed (Fig. 5a).
In steel cooled inside the closed furnace, regions formed by pearlite lamellae are more extensive than in the air-cooled steel, thus presenting a more brittle fractography (Fig. 5b).
In the unstable propagation zone, the fractography consists of cleavage facets, with microvoids regions among them (Fig. 6).
For air-cooled steel the size of these cleavage facets (Fig. 6a) is smaller than for steel cooled in the closed furnace (Fig. 6b), a phenomenon related to the size of the prior austenite grain.The microvoid regions between the cleavage facets are most abundant in the air-cooled steel (with more ductile fractography) than in steel cooled inside the furnace.The critical fracture unit has demonstrated to be a region where the ferrite (and cementite) of neighboring colonies share a common orientation {100}.The size of this orientation unit is controlled by the prior austenite grain size and can be calculated by measuring the extension of cleavage facets appearing on the fracture surface [5,19].

Fracto-metalographic analysis
The specimens' cuts along longitudinal sections allow the observation, in areas close to the fracture surface, of the microstructural damage that the material has undergone because of the plastic strain occurring during the standard tension test.In many of the colonies whose lamellae have an orientation close to the axial one of the wire (direction in which the load is applied in the standard tension test) the existence of abundant microcracking is observed (Fig. 7).Therefore, the fracture is determined by processes 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 [20].In this phenomenon the slip bands in the ferrite produce microcracking in the cementite plates, followed by tearing in the ferrite lamellae.
In the colonies with lamellae of thin thickness (air-cooled steel) microcracks with uneven appearance (with regards to the crack opening) and shorter in length than the colony's size are observed (Fig. 7a).On the other hand, the colonies with greater lamellae thickness (steel cooled inside the closed furnace), the inclined cracking is generally of greater length (even across the complete colony) and looks more uniform (Fig. 7b).