Real structure of ferritic steel and ferrite phase of duplex steel after rolling
J. Čapek1, K. Trojan1, J. Němeček1, N. Ganev1
1Department of Solid State Engineering, Faculty of Nuclear Sciences and Physical Engineering, Czech Technical University in Prague
jiri.capek@fjfi.cvut.cz
Duplex stainless steels have high corrosion resistance in many environments, where the standard austenitic and ferritic steel is consumed and where their properties significantly exceed single phase steel. Duplex steels combine properties of both phases and due to two-phase microstructure, some properties are better than high-alloyed single-phase steel, e.g. abrasion resistance [1]. Thereby, smaller amount of material from duplex steel is necessary to manufacture function components. Duplex steels are due to austenite phase susceptible to mechanical reinforcement, i.e. local changes in mechanical properties of surface layers.
The importance of the texture resides in the anisotropy of most material properties. For this reason, the determination and subsequent interpretation of the texture in material engineering is very important. Moreover, texture analysis during the thermo-mechanical processing of materials provides information on basic mechanisms including deformation, recrystallization or phase transformation. The properties, which are influenced, are, for example, the Young's modulus of elasticity, Poisson number, hardness, strength, ductility, abrasion resistance, magnetic permeability, electrical conductivity etc. So called plastic anisotropy prefer to use a certain slide plane system during deformation. Texture is therefore used in the production of materials with specific properties [2]. Major deformation mechanisms responsible for the formation of ferrite and austenite rolling textures in duplex steels should be the same as in the single phase steels; however their contribution and significance are expected to change [3].
Generally, metals and alloys with a body centred cubic lattice (bcc) tend to form fibre textures. For bcc material, there are six characteristic fibres [4]. Most orientations are formed into two characteristic fibres of Euler space. During cold rolling, primarily, the α1 and γ fibre are created. The α1 fibre is characterized by crystallographic direction <110> which is parallel to rolling direction, e.g. {001}<110>, {112}<110>, {111}<110>. The γ fibre include crystallographic directions with {111} planes which are parallel to normal direction, e.g. {111}<110>, {111}<112> [2, 3, 5]. The values of texture components are significantly dependent on the structure (especially on the grain size and initial texture), chemical and phase composition [3].
The tested samples of plate shape were made of AISI 420 (ferritic) and AISI 318LN (duplex) type of stainless steel. The samples were rolled with 0, 10, 20, 30, 40, and 50% reduction of thickness. At the end, the samples were annealed in air laboratory furnace for 7 hours at 650°C in order to reduce residual stresses.
Using CoKα radiation, X'Pert PRO MPD diffractometer was used to sample analysis. Texture analysis was performed on the basis of the orientation distribution function (ODF) calculated from experimental pole figures recorded of three planes {110}/{220}, {200}, and{211} using MTEX software.
According to [3], after 40% of deformation the limited α1, ε, and γ fibres, see Fig. 1, may describe the rolling texture of ferrite. It is evident that initial texture (0% deformation) is nearly random. For higher degree of deformation, α1 and γ fibres are dominant components of rolling texture of ferritic steel. Ferritic steel exhibited the {001}<110>, {112}<110>, and {111}<110> orientations which are components of α1 fibre texture. Rotated cubic orientation {001}<110> is one of the typical components of ferrite rolling texture. All fibres assume the higher values of the ODF for increasing degree of deformation. Both fibres α1 and ε describing the final texture are very inhomogeneous.
Figure 1. Values of ODF, i.e. f(g), along α1, ε, and γ fibres of ferritic steel.
Nevertheless, it is necessary to expect that two-phase steel have different behaviour of the constituent phases in comparison with single-phase steel [6]. In most cases, there will be differences between the textures of two- and single-phase steels. According to [3], these values of fibers are not typical for ferrite phase in duplex steels.
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2. H. J. Bunge, Texture Analysis in Materials Science. London: Butterworth. 1982.
3. J. Ry, W. Ratuszek, M. Witkowska, Arch. Metall. Mater., 50.3, (2006), 495-502.
4. S. Suwas, R. K. Ray, Crystallographic Texture of Materials. Springer-Verlag. 2014.
5. H. Hu, Texture, 1.4, (1974), 233-258.
6. J. Čapek, K. Kolařík, Z. Pitrmuc, L. Beránek, N. Ganev, Material Structure, 23.4, (2016), 362-363.
This work was supported by the Grant Agency of the Czech Technical University in Prague, grant No. SGS16/245/OHK4/3T/14.