MICROSTRAIN ANALYSIS OF LOW CYCLE FATIGUE AUSTENITE STAINLESS STEEL SAMPLES BY NEUTRON DIFFRACTION

Yu.V.Taran1, J.Schreiber2, M.Vrana3, P.Lukas3, P.Strunz3, P.Mikula3

1 Joint Institute for Nuclear Research, Frank Laboratory of Neutron Physics, 141980 Dubna, Moscow Region, Russia, e-mail: taran@nf.jinr.ru
2 Fraunhofer Institute for Nondestructive Testing, EADQ, Krugerstrasse 22, D-01326 Dresden, Germany
3 Nuclear Physics Institute, 250 68 Rez near Prague, Czech Republic

Keywords: Residual stress, strain, neutron diffraction

Austenite stainless steels are widely used in engineering applications because of their high corrosion resistance and toughness. Internal residual stresses are usually introduced into steel components in the manufacturing process, mechanical and thermal treatment and external load during exploitation. Of special interest are the studies of such properties as creep, martensite phase formation, mechanical and thermal fatigue, fatigue failure due to cyclic load, plastic deformation and texture influence on elastic properties. Transformation from the austenite to martensite phase can be induced by both quenching and plastic deformation which belong to extensively studied effects. The austenite fcc-structure twists into the martensite tetragonal distorted lattice. The degree of distortion strongly depends on the carbon content and for low carbon steels, the tetragonal structure does not differ considerably from the bcc-lattice of -Fe.

In the present study, low carbon austenite steels with some content of martensite produced by plastic deformation during low cycle fatigue are the subject of our interest. A series of samples from austenite stainless steel X6CrNiTil8l0 (0.04 wt % C) with different degrees of low-cycle fatigue was prepared for experiments. The cylindrical samples (14 mm long and 14 mm in diameter) were subjected to a number of tensile-compressive loading cycles with a maximum plastic deformation 0.6% at the frequency 0.1 Hz. The experimentally determined number of cycles corresponding to sample failure is Nmax=750.

Recently, the samples were studied on the High Resolution Fourier Diffractometer (HRFD) at the IBR-2 pulsed reactor (Dubna) to determine residual macrostrain and phase volume fractions [1]. The SKAT Texture Diffractometer at the same reactor was also used to estimate the texture index [2]. The texture in both phases is found to be rather weak.

Although the HRFD resolution is very good, a rather complicated shape of diffraction peaks does not allow us to extract some additional structural information (microstrain/grain size) from the shape of diffraction profiles. Such profile analysis can be easily carried out if the instrumental peak profile is well described by the normal (Gaussian) distribution. A series of cycled specimens was then studied on the SPN-100 focusing diffractometer in NPI Rez [3] to obtain microstrain characteristics. The diffractometer is equipped with a sandwich type monochromator consisting of two bent crystal slabs of different cuts providing two different neutron wavelengths simultaneously. Thanks to employment of focusing a high luminosity and resolution are obtained (d/d=0.25 % at d=2 Ä).

The measurements were performed with the sample axis orientation perpendicular (the radial direction) and parallel (the longitudinal direction) to the neutron scattering vector. The gauge volume 2212 mm3 was determined by the 2 mm input and output slits both fixed close to the samples. Two austenite reflections 200 (=2.7 Ä, 2=97.6) and 311 (=1.65 Ä, 2=99.7) and two martensite reflections 110 (=2.7 Ä, 2=84) and 211 (=1.65 Ä, 2=90) were examined. The instrumental curves for both phases were approximated by the profiles measured from a pure -Fe standard specimen and a pure austenite sample (virgin state).

Transformed model fitting [4] was used to extract microstructural parameters from the measured profiles. The approach used is a modification of the procedure recently proposed for diffraction edge analysis [5] and takes into account the influence of the microstrain and the size of coherently diffracting blocks as well as the corrections for the instrument resolution. Modeling was performed in the reciprocal space and the microstrain contribution was treated according to a simple de Keijser's approach [6]. The calculation assumes nontextured specimens. Nevertheless, in principle, the procedure enables the introduction of texture effects. In fact, the fitted data were practically not sensitive to changes in the size of blocks within some realistic range and therefore, their size was fixed in all cases of final refinement.

Fig.1 shows the mean square microstrain <e2>1/2 vs. the fatigue degree N/Nmax. Practically no evolution of <e2>1/2 was observed in the measured samples. A similar stabilization effect after a certain number of cycles is noted in [7]. No influence of the sample axis orientation with respect to the scattering vector was found the martensite phase. A small effect observed in the austenite phase can be possibly explained by the fact that the mean square microstrain was calculated with respect to pure austenite with its profile used as an instrumental curve. Feeble difference in <e2>1/2 for different (hkl) reflections in the austenite phase indicates the orientational dependence of the substructure development.

Fig.1. The mean square microstrain vs. the fatigue degree.

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