Mechanics, Materials Science & Engineering, September 2016
ISSN 2412-5954
Effect of Alternating Bending and Texture on Anisotropic Damage and Mechanical Properties of Stainless Steel Sheets V.V. Usov1, N.M. Shkatulyak1, E.A. Dragomeretskaya1, E.S. Savchuk1, D.V. Bargan1, G.V. Daskalytsa1 1
South Ukrainian National Pedagogical University named after K.D. Ushinsky, Odessa, Ukraine DOI 10.13140/RG.2.2.35491.04640
stainless steel.
Keywords:
ABSTRACT. Effect of alternating bending and the crystallographic texture on the anisotropy of damage and mechanical properties of stainless steel sheets X5CrNi18-10 at subsequent uniaxial tensile tests were studied. The symmetric tensor of damage of the second order D was used for the analysis of anisotropy damage of sheet material. The only one non-zero component of this tensor D at uniaxial tensile was determined by the defect of the Young's modulus from the mechanical test data. The value of D was found on relation . Here E0 and E and tested material, respectively. It was established the anisotropy of the damage and mechanical properties of steel sheets at uniaxial tensile tests of initial sheet as well of sheets after alternating bending. This anisotropy is caused by the texture that is formed in sheets of investigated steel as was showed by correlation analysis.
Introduction. Stainless steels are widely used in various fields of engineering: architecture, construction, transport engineering, medicine, food industry, energy [1]. An important role is played stainless steel in the petroleum refining [2]. This steel is practically irreplaceable in high-temperature nal forecasting of durability of materials refining industry remains an important problem in relation to the requirements of increasing the depth and quality of oil processing. In the operating conditions under the influence of alternating prolonged loading of equipment inevitably arise damage or irregularities of its working capacity even in the absence of defects in workmanship and compliance in the operation of regulatory requirements. Over last years has been proposed the calculation model of resource estimation of coilpipes furnace of pyrolysis with considering forming quasi-multilayer shell that is formed due the diffusion of carbon in surface layers of steel pipes 20CrNi23-18 at the furnace operation [3]. Articles [4-6] are focused on corrosion and protection from it. The above review shows that proposed methods of predicting damage of structural materials and residual life of process equipment usually are based on monitoring of mechanical properties, metal thickness, morphology and distribution of structural components and structural defects in the steel. It is known that final properties of steel and products depends on many factors such as the chemical composition and its distribution in thickness, metal structure (average size of grains and sub-grains deviousness their borders) [7], crystallographic texture [8], operating temperature, duration of thermal action etc. The emergence during the operation of equipment large number of different defects indicates that is implemented several mechanisms of damage accumulation in metals. In the same time certain characteristics such as crystallographic texture, damage, which could be used for monitoring of the structural condition of the steel rarely taken into account. The impact of above characteristics on corrosion [9] of structural materials requires a more detailed study in terms of degradation and forecasting of metal state. Not investigated also effect of alternating bending (AB) on the anisotropy of damage accumulation in the sheet metal under uniaxial tension. The alternating bending is usually applied before using of roll metal for the straightening of sheets, reducing residual stresses and imparting to the metal of optimal flat characteristics. During the straightening of the metal in him arise and accumulate uncontrollable micro defects, such as micro cracks, micro pores that are found already at tensile on 3-10% [10]. The MMSE Journal. Open Access www.mmse.xyz
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Mechanics, Materials Science & Engineering, September 2016
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occurrence and accumulation of micro defects indirectly reflected in changing of material properties, ulus defect, which can be used to measure the accumulation of damage in the metal [11]. In this paper, we investigate the effect of alternating bending and the crystallographic texture on the anisotropy of damage and mechanical properties of stainless steel sheets X5CrNi18-10 in the process of subsequent uniaxial tensile tests. Experimental Procedure. As initial material for investigation were used sheets of stainless steel X5CrNi18-10 of 1 mm thickness in delivery conditions after recrystallization annealing. Sheets diameter of 50 mm in the rolling direction (RD). The speed of movement of the metal during bending was about 150 mm / s, which corresponds to a strain rate of ~ 10-2 s-1. From initial sheets and sheets after bending on 0,5; 1; 3 and 5 cycles were cut three batches of samples for mechanical test in the direction (TD), and also samples for study of texture. Testing machine Zwick Z250 / SN5A with power sensor on 20 kN at room temperature was used for mechanical tests on a tensile of samples cut in the RD, DD, and TD. Samples for mechanical testing have had total length of 90 mm, the width of working part was of 12.5 mm. Values of mechanical properties were found by averaging the test results of at least three specimens in each direction. The X-ray method [12] with the construction of inverse pole Fig.s (IPF) was used for investigation of the crystallographic texture. On the diffractometer DRON-3m in the filtered Mo Ka radiation was performed the theta-2-theta scanning of sample without texture, as well as of samples after corresponding cycles of the AB. The scanning carried out from two opposed surfaces of sheets, as well and in the RD. These data were used for the construction of IPF ND and IPF RD, respectively. Samples were chemically polished to a depth of 0.1 mm for removing distorted surface layer before texture investigation. Sample without texture was prepared from the fine powder of studied steel after recrystallization. Composite samples in the form of glued each other strips wide of 3 mm cut perpendicular to RD were prepared for the texture investigation in the RD. The microscope Axioplan 2 of the firm KARL ZEISS was used for examine of the metallographic structure from end surfaces of samples cut in the RD and TD. A symmetric damage D tensor of the second order [13, 14] was used for the analysis of the anisotropy of sheet material damage. Only one nonzero component of the tensor D exists for the case of uniaxial stress. This nonzero component D is determined by the formula [13, 14]:
.
(1)
where and E are elastic modules of intact material and the current modulus determined at uniaxial tensile tests, respectively. Results and discussion. number of (Fig. 1, a, b) are typical for the rolling texture of FCC metals. Texture undergoes marked changes after various stages of the AB (Fig. 2, c-l). Fig. 3 shows appropriate microstructure. The presence of twins is seen in the initial sample (Fig. 3, a, b). The tendency to increase amounts of twins is traced with increasing number of the AB cycles (Fig. 3, c-l). Therefore, one should expect the development of twins orientations during the alternating bending, since the role of twinning is amplified at deformation of materials with low stacking fault energy [15], to which belongs the investigated here steel.
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Anisotropy of mechanical properties and damage D take place (fig. 1). Anisotropy coefficient k that was determined by relation (1) decreases with increasing number cycles of the AB. .
(2)
where F is the appropriate property. The minimum value of k is observed after 5 cycles of the AB (Fig. 1). The character of the tensile strength anisotropy does not change with increasing number cycles of AB. In all cases, the ultimate strength in the RD has a higher value than in the TD, and in the DD has intermediate value. There is probably manifested an effect of the mechanical texture, namely, the preferential elongation of the grains in the RD. Coefficient anisotropy of the tensile strength initially increases with increasing number of cycles, taking the value of 5.0 % in the initial sheet; 2.9 % after 0.5 cycle; 6.8% after one cycle, and then decreases to 4.1% after 5 cycles. Yield strength in RD exceeds its value in the TD in the initial sample. Coefficient of anisotropy k has made 3.6%. Anisotropy character changed after 0.5 cycle of AB. Yield strength in RD is smaller than in TD, and in a diagonal direction has intermediate value. Anisotropy ratio had decreased. Its value was 2.9%. A similar pattern of anisotropy persists after the one AB cycle. At the same time the anisotropy coefficient grew to 6.4%. Anisotropy character of yield stress is similar to him in the initial sample, and the anisotropy ratio decreased to 1.6% after 5 AB cycles. Absolute values of yield strength and tensile strength of the studied steel also are increased with increasing number of AB cycles, and reach a maximum after 1 cycle of AB. Absolute values of the strength properties of the investigated steel are decreased with further increase in the number of AB cycles. Elongation shows an opposite tendency with respect to the tensile strength (Fig.1).
Fig. 1. Dependence of tensile strength, proof strength, uniform elongation and damage D on the number of AB cycles. MMSE Journal. Open Access www.mmse.xyz
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Mechanics, Materials Science & Engineering, September 2016
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Analysis of initial sample IPF (Fig. 1, a, b) showed that its texture consists of two limited axial components. The first component with the axis <110> parallel to the ND extends from {011} <100> up to {011} <112>. The second component may be characterized by an axis of <110> inclined toward
Fig. 2. Experimental IPF of the studied steel; (a, b) are the initial state, respectively IPF (ND) and IPF (RD); (c - l) are IPF (ND) after the alternating bending: (c, d); (e, f); (g, h); (k, l) are after 0.5; 1; 3; and 5 cycles of AB, respectively. MMSE Journal. Open Access www.mmse.xyz
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Mechanics, Materials Science & Engineering, September 2016
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The development of these two limited axial components is in the agreement with the Taylor prediction model on the base of the normal octahedral sliding [16]. In addition, there are the twinned orientations {113} <211> that were formed probably during the annealing [16].
Fig. 3. Microstructure of steel sheets: (a, b) are corresponded to the initial state; on (c - l) are shown states after the AB: (c, d), (e, f), (g, h), and (k, l) are shown microstructure after 0.5, 1, 3, and 5 cycles of AB, respectively. a, c e g k are filmed in the cross section perpendicular to the RD; b d f h l are filmed in a cross section perpendicular to the TD. MMSE Journal. Open Access www.mmse.xyz
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Mechanics, Materials Science & Engineering, September 2016
ISSN 2412-5954
Texture is undergoing significant changes after different stages of the AB. The following deformation model of samples was basis of the texture changes interpretation. Grains of metal in layers on the convex sheet side are exposed to the action of tensile stresses at bending of the sample in one direction (0.25 cycles). Meanwhile, on the concave side occur compressive stresses. In the strip are initiated shear deformations as a result of the action of opposite sign stresses. Direction of acting stresses is reversed when the strip bends in the opposite direction. Thus, in the metal strip arise alternating shear deformations, which lead to the formation of the shear texture components during the AB. In FCC metals are formed following components of shear texture: A - {111} <hkl>; B - {hkl} <110>; C - {001} <110>. Orientation {hkl} <uvw>, listed here indicate that the plane {hkl} coincide with the shear plane, and the direction <uvw> coincide with the shear direction [16]. Component B of shear texture formed after 0.5 cycle of the AB in the sample at one side of the sample [16]. The formation of shear bands in the rotated twin-matrix regions changes the orientations {332} <113> and {111} <110> near to {011} <100> and {011} <112> positions respectively in metals and alloys with the low stacking fault energy (SFE) [17]. On the corresponding IPF (Fig. 2, c) pole density <110> increased to 2.38 while in the initial sample it was 1.81 (Fig. 2, a). On the opposite side of the sample also take place twins orientations (Fig. 2, c). Component C of shear texture is formed in the sample after 1 cycle of the AB (Fig. 2, e, f). At the same time on the opposite side of the same sample (Fig. 6, f) orientations of the initial sample are observed (Fig. 2, a). Texture that is similar to the texture of the initial sample (Fig. 1, a) was formed at one side of the sample after three cycles of the AB (Fig. 2, g). Sufficiently intense component C of shear texture (Fig. 2, h) is present on the opposite side of this same sample. Texture on the one sample side after five cycles of the AB is characterized by orientations of C shear texture and by orientations of twins {113} <211> (Fig. 2, k). The area of increased pole density on the corresponding IPF is greatly expanded in comparison with the other samples probably due to the twinning [18]. The texture of same sample on the opposite side is characterized by orientations of twins (Fig. l, 2). In general, scattering of the texture had increased when considering of both surfaces of the sheet after 5 cycles of the AB (Fig. 2, k, l), as compared with the initial state of the sheet in Fig. 1, a. The above described anisotropy of the proof strength corresponds to the texture formed in samples. In the IPF (RD) of the initial sample (Fig. 1, b) there is a high pole density of <111>. This means that there is a significant volume fraction of crystals axis <111> of which coincides with the RD. In this case <112> and <110> crystals axis are oriented along the TD. The crystals that have axes <111> oriented along the applied stresses are characterized by of high flow stresses in comparison with other crystal orientations [18]. Number of crystals with axes <111> oriented along the RD is decreases, and in the TD is increases with increasing of AB cycles number from the 0.5 to 1 inclusive, due to the increasing of shear texture. Consequently, the proof strength in TD is becoming greater than in the RD. Increasing of the AB cycles number up to 5 leads not only to the development of shear texture component again to the formation of texture components similar to initial orientations but with more significant scattering. Initial character of proof stress anisotropy is restored, but its absolute value is decreased. Fig. 2 shows that orientations of <110> have the highest values of the pole density on IPF ND. Significant correlations of averaged through the direction of sheets at uniaxial tensile tests of values of ultimate strength
, proof stress
, relative uniform elongation
with values of pole density of <110>, averaged on both sides of sheets
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, and damages take place. The
Mechanics, Materials Science & Engineering, September 2016
ISSN 2412-5954
corresponding regression equations and approximations reliability coefficients by relations
are represented
(3) (4) (5) (6)
Summary. Effect of alternating bending and the crystallographic texture on the anisotropy of damage and mechanical properties of stainless steel sheets X5CrNi18-10 in the process of subsequent uniaxial tensile tests was studied. Texture of stainless steel X5CrNi18-10 of 1 mm thickness in delivery conditions after recrystallization annealing includes two limited axial components and twinning orientations {113} <211>. The first component with the axis <110> parallel to the ND extends from {011} <100> up to {011} <112>. The second component may be characterized by an axis of <110> inclined toward ND on approximately 600. It extends from ~ {112} <111> through {135} <211> up to {011} <112>. Various combinations of the original texture of rolling, components of shear texture {001} <110> and twinned orientations are formed in sheets during the alternating bending. The twinning role is enhanced at the increasing of number alternating bending cycles that is confirmed by metallographic data. Anisotropy of damage and mechanical properties take place in initial sheet as and in sheets after alternating bending. Anisotropy decreases with increasing of number alternating bending cycles. The minimal anisotropy was observed after 5 cycles of the alternating bending. Anisotropy is caused mainly by texture formed in steel sheets. Significant quadratic correlations take place between values of ultimate strength, proof stress, relative uniform elongation and damage, averaged through the direction of sheets at uniaxial tensile tests with values of <110> pole density averaged on both sides of sheets. References https://www.metalsupermarkets.com/most-common-uses-of-stainless-steel/ [Online]. Aviable: https://www.nickelinstitute.org/~/media/Files/TechnicalLiterature/RoleofStainlessSteelinPetroleum Refining_9021_.ashx [3] A. Chirkova, N. Makhutov, M. Gadenin, M. Kuzeev, V. Farkhutdinov experimental method for estimating degradation of mechanical characteristics of steels under the conditions of highInorg Mater, vol. 46, pp. 1688 1691, 2010, DOI: 10.1134/S002016851015015X [4] V. Mertinger, M. Benke, Sz. Engineering Failure Analysis, vol. 18, pp. 1675 1682, 2011, DOI: 10.1016/j.engfailanal.2011.02.003
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[5] G. Hughes, Materials of crude oil Refining: corrosion Problems and prevention [Online]. Aviable: http://www.dunand.northwestern.edu/courses/Case%20study/Gareth%20Hughes%20%20Materials%20of%20Crude%20Oil%20Refining.pdf Acta tehnica
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