CHANGE OF CRYSTALLOGRAPHIC ORIENTATION DISTRIBUTION DURING FORMING IN CARBON STEEL DEFORMED BY DEEP DRAWING

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3^ème Conférence Internationale sur

le Soudage, le CND et l’Industrie des Matériaux et Alliages (IC-WNDT-MI’12) Oran du 26 au 28 Novembre 2012,

http://www.csc.dz/ic-wndt-mi12/index.php 38

CHANGE OF CRYSTALLOGRAPHIC ORIENTATION DISTRIBUTION DURING FORMING IN CARBON STEEL

DEFORMED BY DEEP DRAWING

A.Boumaiza¹, R. harieche¹, N.Rouag²

1Laboratoire de Physique de Rayonnement et Applications, Université de Jijel, Ouled Aissa BP 98 Jijel 18000 Algérie

2Laboratoire Microstructures et Défauts, Faculté des Sciences, Université Mentouri Constantine 25000 Algérie

Corresponding author. E-mail: [email protected]

Abstract:

Advances in techniques for measuring individual crystallographic orientations have made possible to investigate the role of local crystallography during crack propagation in polycrystalline materials. The change in crystallographic orientation distribution during deformation by deep drawing in carbon steel has been investigated in order to understand the deformation mechanisms leading to crack propagation. The well-known strain rate dependence of the deformation behaviour was examined by SEM-EBSD (scanning electron microscopy/electron back scatter diffraction pattern) analysis. This tool was used particularly to characterize the various crystallographic parameters.

Evolution of grain boundary distribution during plastic deformation strongly depends of strain rate.

Fraction of low angle grain boundaries increased after deformation with high strain rate, presumably due to dislocation activity, while fraction of random boundaries was high in the specimen deformed with low strain rate, and one observes conservation of initial texture. Further, the intragranular misorientation, transgranular misorientation and local orientation are analyzed in relation with the accommodation process during plastic deformation.

Keywords: crack propagation, strain rate, grains boundaries, texture, EBSD.

1 Introduction

Industrial metals always undergo various external stresses during fabrication and under service conditions. When they possess a heterogeneous microstructure, the usual mechanical properties do not allow accurately determining the risks of cracking that often occurs throughout manufacturing and service. In the plastic deformation studies, the microstructure is a basic factor of predicting the mechanical material behaviour. The presence of microstructural inhomogeneities will certainly affect the agreement between predictions based on homogeneous deformation and the experimental observations [1,2].Crack propagation and stops is an important topic in the formability field [3,4]. It is often required in safety analysis to consider that a crack might be initiated and lead to its propagation.

It is important to know whether or not the material will be able to stop the crack before it goes through the entire structure, during its response to the external stresses. To answer this question, it is essential to consider the combination of “micro and macro” effects [5,6]. Undeniably, the

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http://www.csc.dz/ic-wndt-mi12/index.php 39 phenomenon of localization is a result of both the external solicitation (loading, piece geometry) and the microstructure properties. It is this duality that makes the success of the micro-macro approaches which are based on changes of dimension scales. Therefore, such approaches allow a more comprehensive study, closer to the material behaviour during use.

The global behaviour of industrial metals is essentially controlled by the local micro-stress concentrations. Strain localization that appears may locally lead to a strong decrease of the ductility and therefore to a material rupture. The EBSD technique (Electron Back Scattering Diffraction) allows the correlation between the microstructure characteristics and the distribution of plastic deformation and stresses [7].

EBSD is presently used predominantly in nearly all metallurgy research approaches and has become a common technique used in the characterization of polycrystalline materials [8]. Since its advent, the method has become increasingly useful in the analysis of grain size and distribution, grain boundary disorientations and texture analysis of materials. The EBSD technique makes it possible to extract both local and global information about the material structure; it allows a complete microstructure characterization from local measurements.

The initial texture of the material is an essential parameter related to its mechanical behavior. In the metallurgical considerations of formability, the texture analysis links the crystallographic characteristics of the material to stress distribution [9,10]. The strong drawing deformations are accompanied by a noticeable modification of the initial texture. The low carbon steels generally possess a  fiber texture {111}<uvw>, favourable to deep drawability [11]. In the present study, the investigated sheet possesses such fiber with a main component {111}<112>. After drawing, this fibre is kept with a change of the main component to bcc orientation deformation {111}<110>. The industrial problem is the initiation and propagation of cracks during forming. The average mechanical parameters of the sheet (Re=230 N.mm^-2, Rm=311 N.mm^-2, A%=36, HV= 140 Kg.mm^-2) are correct and do not allow an appreciation of the risk of cracking.

2 Experimental procedure

The material used in the present study is a primary recrystallized steel sheet of 1.5mm thickness, containing 0.07% C, 0.03% Si, 0.38% Mn, 0.025% P, 0.012% S, 0.02% Al, 0.007% N (wt%). The samples (1cm²) examined by EBSD are polished on standard emery papers up to grade 1200 and with the diamond paste up to 0.5µm. They are subsequently electropolished in a solution of: 25vol.

perchloric acid (d=1.67), 235 vol. acetic acid (d=1.05) and 250vol. monobutyl ethylene glycol ether (d

= 0.9), at (-5  -2) °C, using an applied voltage of 25V with 0,5A.

To study the anisotropic mechanical behaviour in relation with the structural characteristics, we have considered the mechanical characteristic evolution on the rolling plane. Different samples were used for tensile tests up to fracture with different angles in respecting to the rolling direction (ranging from 0° to 90°, with a step of 15°). Tensile tests were conducted on a ZWICK universal testing machine

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http://www.csc.dz/ic-wndt-mi12/index.php 40 using constant crosshead speed 0.2cm/min. For the initial state, the Vickers micro-hardness was measured with 200g of loading. The macroscopic mechanical characteristics were obtained with the average of five measurements on the specimen.

The characterisation of the global matrix texture was performed by X-rays and locally completed by the EBSD technique near the crack. The misorientation profiles were drawn taking into account the point to point and the point to origin misorientation. The point to point misorientation is the misorientation detected between neighboring points along a line selected through the scan area. The point to origin misorientation gives the misorientation for each point on the scan line, relative to the first point. After selecting a point of origin in a given grain, the point to origin misorientation recorded over the area was used to draw in-grain misorientation maps.

3 Results and discussions

The effect of plastic anisotropy is clearly apparent in the deep-drawing test of cylindrical cups. In this test, the crack progress with a zigzag by a globally ductile rupture of the material with some secondary fissures around the main line figure 2(a). The geometry of the crack propagation is illustrated in figure 2(b).

Figure 1: (a)SEM image of the analysed crack; and (b) geometry of the crack propagation

Figure 2 shows the initial microstructure observed in section (RD-TD) for this carbon steel. In this section the grain size distribution was very heterogeneous and there are gatherings of small and big shared in clusters. These two populations react differently during the deformation. As a consequence, the distribution of stress will be heterogeneous in the clustered matrix [12,13]. The existence of these different populations can conduct to regions with alternating high and low dislocation densities during deformation by deep drawing.

a b c

d e

f g

(a) (b)

Crack propagation

A B

[u₂v2w2] [u₁v1w1]

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http://www.csc.dz/ic-wndt-mi12/index.php 41 Figure 2: Microstructure of the initial state of the steel sheet obtained by EBSD

Figure 3 shows the local orientation characterization performed by EBSD in the area around the crack. We can note the existence of a fiber texture {111}<uvw >, with a variation from <110> to

<112> for <uvw > orientation. This local characterization gives a similar component texture to the one obtained by global analysis.

Figure 4(a) shows a misorientation profile across the crack when passing to the areas identified by A and B. Except for the peak which corresponds to the crack position, the misorientation profile along line (1) between the neighbouring grains shows a perfect continuity of the observed evolution Fig 4(b). This continuity of the observed evolution clearly indicates that it is the same grain along the considered profile, on the two sides of the crack. The right side corresponds to orientation {111}

<112> whereas the left side includes orientations from {111} <112> towards {111} <110>. The peak misorientation around the distance 40-50 micron-meter takes 50°-55°, that this clearly demonstrates the change from {111}<112> to {111}<110>, ideally being 54.7°. The inverse pole figures corresponding to the points of profile line 1 are reported in figure 4(c). A detailed analysis of orientations inside grains shows that almost all of them show weak variations in their orientations (111)<110>.

(a) (b)

TD RD

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http://www.csc.dz/ic-wndt-mi12/index.php 42

Figure 3: (a) Pole figure 100, and (b) invese pole figures of the mixed fracture during deep drawing

Figure 4: (a) Crack propagation, (b) Misorientation profile across the crack showing a perfect continuity of the observed evolution, and (c) change in the inverse pole figure of the tensile direction during deformation.

There is a continuous variation of site orientations from {111}<112> towards {111}<110 >. This observation confirms that the observed crack is trans-granular and not inter-granular, as will let it expected by the globally ductile character of the considered material. The trans-granular crack mechanism cannot be explained by a simple cleavage in a soft steel sheet characterized by a fiber texture {111} <uvw >. It could be suggested that the apparently local brittle-mode of fracture is actually the result of grain reorientation. In deformation mechanism by slip, each grain undergoes a lattice rotation, it is certain that the grain reorientation from the initial orientation {111} <~112 >

towards the bcc deformation orientation {111} <110 > implies an important micro-constraint state, which is able to initiate a trans-granular evolution of the crack, if the slip planes present some sufficiently weak angles with the propagation direction. Figure 5 shows the global evolution of texture main component from initial orientation {111} <112> to {111}<110> for deformed samples by

A (1)

B

(a)

(c)

(b) Line (1)

50°-55°

112

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http://www.csc.dz/ic-wndt-mi12/index.php 43 tensile tests with different angles α of tensile axis rapport the rolling direction for all tensile axis (α=0°,45°,90°). An analysis of different tensile cracking samples is performed in order to confirm the possibility of fragile micro-zone existence in the considered soft steel.

Figure 5: Changes of the orientation densities in deformed samples after tensile tests as a function of the angle of tensile axis respect to the rolling direction.

Conclusions

This study considers the texture effect on the propagation and the arrest of cracks in carbon steel during forming. The micro-macro observations confirm the importance of the effects of the microstructure’s heterogeneity. The initiation of the crack is generally related to deformation incompatibilities of plastic origin. The misorientation profile permits to clearly show adjustment of the grain orientation from {111}<112> initial orientation towards the bcc deformation orientation {111}<110 >. This process can explain the possibility of transgranular crack propagation in a globally ductile material.

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F(g)

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