The real structure of α-Fe phase of rolled AISI 2205 duplex stainless steel after shot peening

M. Rušin, J. Čapek, K. Trojan

Department of Solid State Engineering, Faculty of Nuclear Sciences and Physical Engineering, Czech Technical University in Prague

rusinma3@cvut.cz

Duplex (stainless) steels are a family of grades that are used in areas such as automotive, aviation industry, civil engineering, and food storage. Two main phases are α-Fe and γ-Fe, mostly in a 1:1 ratio. Duplex steels exhibit better properties, such as corrosion resistance, compared to single-phase steels [1]. Post‑processing, e.g., Shot Peening (SP), is used to further improve the final mechanical properties of the steel. The main purpose is to generate compressive residual stresses in the surface and subsurface layers of the peened material. As crack initiation and propagation are reduced in a compressively stressed zone, SP provides a considerable increase in service life [2]. Apart from residual stresses, SP also influences other parameters of the real structure. Using X-ray diffraction techniques, the impact of SP intensity on crystallite size, residual stresses, and texture were analysed. The depth distributions of these parameters in α-Fe phase are described.

Three AISI 2205 rolled samples were used. One, which was not shot peened for reference, was labelled N, and two peened with pressure 1.5 bar and 7 bar were denoted P1.5 and P7, respectively. Rolling (RD), transversal (TD), and normal (ND) directions created their coordination system. The Empyrean and X’Pert PRO MDP PANalytical diffractometers with manganese and chromium X‑ray tube were used. In order to obtain the depth distributions of above-mentioned parameters, the samples were gradually electrochemically polished.

The crystallite size was calculated using the Scherrer formula analysed by {211} diffraction line. It was found that SP caused a reduction in crystallite size in subsurface layers. For example, crystallites were smaller for P1.5 sample by approximately 5 nm compared to sample N. With increasing distance from the sample surface, the crystallite size changed to values comparable to those for sample N, as shown in Fig. 1.

Figure 1. The depth distributions of the crystallite size of {211} diffraction line for N and P1.5 samples.

The same diffraction line was used to analyse the residual stress distribution using “” method assuming the bi-axial state of the residual stresses with respect to RD and TD axis. As expected, SP led to an increase in compressive residual stresses in near-surface regions. Their surface values were around 600 MPa, 550 MPa, and 400 MPa (both directions) for P1.5, P7, and N, respectively. Similar to the case of crystallite size, the greatest change was also found near the surface. As the depth increased, the values decreased to stresses comparable to sample N. The depth distributions of the compressive residual stresses in RD are shown in Fig. 2.

Figure 2. The depth distributions of the residual stresses in RD for all samples.

The orientation distribution function (ODF) calculated from the experimental pole figures obtained by analysis of {110}, {200}, and {211} diffraction lines was used for texture analysis. The MATLAB^TM MTEX toolbox programme [3] was used to calculate ODF. For N sample, the typical rolling texture of body-centred cubic materials was found. Crystallites were oriented along the incomplete α-fibre with a dominant {112}<110> texture component in all measured depths. After SP, the texture components changed. They differed not only with respect to peening intensity but also with respect to the distance from the surface for individual samples. Fig. 3 shows ODF in = 0° sections for each sample on the surface. The texture components predominantly changed their significant planes. The <110> direction remained preserved in most of the measured depths. It was also found that ODF are better described by particular texture components, rather than texture fibres.

Figure 3. The ODF in = 0° sections from surface for all samples. For sample N, α‑fibre represented by dashed line and {112}<110> texture component are shown.

1. TMR Stainless, Practical Guidelines for the Fabrication of Duplex Stainless Steel, London: IMOA, 2014.

2. J. Champaine, Shot Peening Overview, The Shot Peener, 2001.

3. F. Bachman, R. Hielscher, H. Schaeben, Texture Analysis with MTEX – Free and Open Source Software Toolbox, Solid State Phenomena, 160, (2010), 63-68.

This work was supported by the Grant Agency of the Czech Technical University in Prague, grant No. SGS22/183/OHK4/3T/14.